Advancements in electromagnetic interference shielding cementitious composites

Advancements in electromagnetic interference shielding cementitious composites

Construction and Building Materials 231 (2020) 117116 Contents lists available at ScienceDirect Construction and Building Materials journal homepage...

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Construction and Building Materials 231 (2020) 117116

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Advancements in electromagnetic interference shielding cementitious composites Dimuthu Wanasinghe a, Farhad Aslani a,b,⇑, Guowei Ma a, Daryoush Habibi b a b

Materials and Structures Innovation Group, School of Engineering, University of Western Australia, WA 6009, Australia School of Engineering, Edith Cowan University, WA 6027, Australia

h i g h l i g h t s  Existing concrete use in construction industry does not provide adequate shielding.  Addition of high conductive materials help to increase overall shielding.  Energy absorbing nanoparticles can mitigate the propagation of electromagnetic wave.  Synergetic effect of fibres and particles enhance shielding properties in composites.

a r t i c l e

i n f o

Article history: Received 11 March 2019 Received in revised form 27 September 2019 Accepted 29 September 2019

Keywords: Electromagnetic interference Shielding Cementitious composites Nanomaterials

a b s t r a c t With the advancement of modern technology, there has been a rapid rise in the electronic devices, and along with this growth, there has been an increased concern over the electromagnetic (EM) radiation emitted by these devices. Research into electromagnetic interference (EMI) shielding materials has been on the rise since it is known that the EM radiation generated artificially by a nuclear detonation is strong enough to destroy most modern electronic devices. Traditionally, metals have been used as the ideal shielding material simply due to their high shielding effectiveness (SE) that arises as a result of their high electrical conductivity. However, due to a few undesirable characteristics of these metallic materials such as the corrosion, there have been novel experiments into the development of other materials that can be used as an effective EMI shield. While some of these research work focuses on developing cementitious composites, others have focused on creating lightweight polymer-based shielding materials. This paper reviews such novel cementitious composite materials which have been developed to shield against EMI. The review emphasises the type of additives used in the fabrication of the composite giving rise to adequate SE as described in industrial standards. Ó 2019 Elsevier Ltd. All rights reserved.

Contents 1.

2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Theory of EMWs and shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.1. Open field method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.2. Shielded box method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.3. Shielded room method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.4. Co-axial transmission line method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Other characterisation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 EMI shielding cementitious shielding composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. SE of different types of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Carbon particles based composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4. Carbon fibre based composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

⇑ Corresponding author at: Materials and Structures Innovation Group, School of Engineering, University of Western Australia, WA 6009, Australia. E-mail address: [email protected] (F. Aslani). https://doi.org/10.1016/j.conbuildmat.2019.117116 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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3. 4.

2.5. Carbon nanotube-based composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Particle-based composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Hybrid composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. SE of common construction materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction An electromagnetic wave (EMW) is a form of energy that is commonly found in the atmosphere as visible light, ultraviolet radiation, and radio waves. EMWs have the ability to ionise the air and can be generated from natural sources such as lightning or can be generated by manmade instruments [1–4]. EMWs are most commonly generated at domestic level by many electronic devices such as microwave ovens or mobile phones. While many of these artificially generated EMWs such as radio waves are used for communication, some of these waves are created as a byproduct from many electronic devices during their operation [5– 7]. These EMWs can induce eddy currents in other electronic devices, interrupting the functionality of these devices. With the rapid advancement of electronic devices in recent times, such EMWs within the atmosphere have increased significantly, which has led to what is known as electromagnetic pollution. The disruption caused by EMWs in another electronic device, causing the second device to malfunction is known as the electromagnetic interference (EMI) [7–12]. While in most cases, this would be a harmless effect, in some, it can cause significant disturbances causing some devices to seize functionality altogether. Permanent damage caused to electronic devices due to EMI can even be lethal if the damage occurred to sensitive electronic devices within a hospital or electronic medical implants worn by people such as cardiac pacemakers [13–20]. Some of the research work carried out to measure the effect of EMI on human health have found that EMI can cause harmful effects on newborn babies and pregnant women

12 14 17 18 20 21 21 21 21

[21–24]. Prolonged exposure to EMWs is known to cause complex medical conditions within humans, such as cancer and heart problems [25]. In some instances, EMI can also be used as a weapon in warfare, which can be used to cripple electronic systems by artificially generated EMI [26–30]. These are some of the key reasons why shielding against EMI is sought. Due to the harmful effects of EMI and increased EMWs within the atmosphere, the necessity to measure the EMW intensity inside the buildings and to provide adequate shielding has also increased. The most significant threat of EMI is the crippling of electronic systems within a building which can come in the form of a High Altitude Electromagnetic Pulse (HEMP). HEMPs can be generated by detonation of a small nuclear device in the troposphere. As a result, most of the early research work relating to EMI and the amount of Shielding Effectiveness (SE) needed have been carried out by the military [26]. Accordingly, the US Department of Defense has identified the minimum shielding requirement for buildings, which is shown graphically in Fig. 1. Details of SE defined by the US Department of Defense are published in standard MIL-STD-188-125-1, which is accessible to the general public. The frequency range identified for shielding in this standard is from 1 kHz to 1.5 GHz. Based on these shielding requirements, there have been numerous designs for shielding enclosures employing a variety of materials [31–34]. However, in particular buildings such as hospitals, the need for SE is much more stringent. Since some of the electronic devices used within hospitals such as Magnetic Resonance Imaging (MRI) scanners generate high electromagnetic fields, it is essential that

Fig. 1. Minimum SE requirement defined by the US Department of Defense [46].

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

these fields do not interfere with the functionality of other electronic devices within the same building [35]. As a result, rooms containing such equipment must be lined with shielding materials that would prevent the leakage of EMWs out of the room. Traditionally, copper has been used as the ideal shielding material for such requirements. Apart from copper, steel and aluminium are also being used in hospitals for EMI shielding [36]. Although there have been no extensive research on the effect of such short burst of EM energy emitted from MRI scanners to humans, as a general precaution, it is advised that these shielding requirements must be met within hospitals [25,37]. Additionally, there are general rules and guidelines for usage of other electronic devices that produce EMWs, such as mobile phones and laptops within hospitals. The general public is advised to minimise the usage of these devices within hospitals in order to prevent malfunction of electronic equipment used in hospitals [38–41]. Research into materials for EMI shielding dates back to first nuclear tests conducted by the USA. During these tests, sensitive electronic devices and cables were shielded with metal enclosures, which prevented most of the damage caused by the EM energy released by the nuclear blasts [42]. Extensive research has been conducted on materials which provide shielding against EMI to find suitable ones for specific frequency ranges. Metals have been the most commonly used shielding materials since they are good conductors and create a Faraday cage upon encountering EM energy, thus shielding components within. Steel, copper, coppernickel alloy, tin, aluminium, and Mu-metals are the most commonly used metals for EMI shielding [43,44]. Despite their good shielding properties, metallic shields pose problems since they are heavy, bulky, and prone to corrosion [45]. Because of these drawbacks, there has been increased interest in new materials which can provide adequate shielding against EMI, light in weight, and easy to fabricate. These new studies have led to many promising novel materials that can act as effective shields against EMI. Many of these new materials are mostly cementitious or polymer composites, which have additional filler materials to improve their shielding properties. This paper summarises many of these novel cementitious composites that have been developed for EMI shielding.

1.1. Theory of EMWs and shielding

end of this spectrum, the waves have lower frequencies hence lower energies, and these progressively increase towards the higher end of the spectrum. When EMWs fall on to a material, some of the energy in the EMWs will be reflected while some will be absorbed by the material. The remaining EMWs will pass through the material to the other side. The phenomenon known as shielding is when the intensity of the EMWs passing through the material is reduced compared to incident waves by means mentioned above. The type of interaction EMWs have with material depends on many properties of the material as well as the frequency of EMWs. Three main interactions that can take place when an EMW falls on to material are illustrated in Fig. 2. Shielding effectiveness (SE) of the given material can be calculated using Equation (3), which compares the received power of the beam with the material present (P1 ) and the received power of the beam without the material present (P 2 ).

SE ¼ 10logðP1 =P2 Þ

ð3Þ

Reflection of EMWs from the surface of material occurs mainly due to the impedance mismatch between the incident EMWs and the surface of the material, which can be expressed mathematically by Equation (4), where f is the frequency, e is the electrical permittivity, and l is the magnetic permittivity [49];



SER ¼ 10log

rT 16f lr



c f

ð1Þ

The energy contained within EMWs can be calculated as per Equation (2), where E is the energy, h is the Planck’s constant (h = 6.62607  1034 J), and f is the frequency.

E ¼ hf

ð2Þ

For practical applications, EMWs have been divided into several categories based on the frequency and the energy within them. The EM spectrum is a visual representation of how EM waves are grouped based on their frequency or wavelength. At the lower

ð4Þ

rT is the total electrical conductivity of the materials which explains why materials with high electrical conductivity are also good reflectors of EMWs. Some of the EMWs that penetrate the surface of the material could undergo internal reflections within the material [50]. Multiple reflection of the EMWs occur when the material contains large specific internal surfaces. Composites which contain multiple fillers are known to have multiple reflection mechanism due to fillers having different dielectric properties, which can be calculated using the Maxwell equation. In monolithic materials that does not contain fillers, the skin depth also plays a vital role in the multiple reflection mechanism. Skin depth is defined as the depth of the material where the intensity of the incident field drops in to 1/e of the incident value [51,52]. Absorption of the EMWs within a material occurs due to the dielectric properties of the material and would result in the release of heat [49] during these internal reflections.

EMW is represented as two sinusoidal waves vibrating perpendicular to each other, consisting of electrical and magnetic energies. The behaviour of EMWs was first theorised by the Scottish physicist James Clerk Maxwell [47]. Like all other sinusoidal waves, EMWs are also characterised by the wavelength, which is the distance between two consecutive peaks or nadirs in the wave, or the frequency, which is the number of cycles occurring per second. Wavelength and the frequency of an EMW are related through Equation (1) where k is the wavelength, f is the frequency, and c is the speed of light in a vacuum which is 2.998  108 m/s.



3

Fig. 2. Possible interaction of EMWs with materials [48].

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There are several methods developed for the measurement of the SE, giving emphasis to various parameters and standards. Most of these techniques use a vector network analyser (VNA) which generates a radio frequency signal and transmit it by using an antenna. EMWs that reflect and pass through the material can be measured by the same VNA [53,54]. The energy within the reflected and the transmitted signal can be used to calculate the SE of the material. Four main techniques that have been developed for the measurement of SE can be described as follows. 1.1.1. Open field method Also known as the free space method, this mode of measuring the SE comprises of dual antenna setup where the EMWs are transmitted from one while the transmitted EMWs are captured by the other. The shielding specimen which is placed between the two antennas should be placed 1 m, 3 m, 5 m, 10 m, or 30 m from the receiving antenna depending upon the standard used for the SE measurement [50,55]. This method is known to be a very realistic form of measurement of SE since the testing conditions are very similar to that of practical scenarios. 1.1.2. Shielded box method In the shielded box method, the sample is placed in an opening within a Faraday cage. The specimen is irradiated with EMWs from the antenna placed outside the box while the antenna inside measures the transmitted wave energy [50]. This method suffers from several drawbacks such as limitations in the range of EMWs that can be used and difficulty in achieving the required electrical contact between the specimen and the box. 1.1.3. Shielded room method This method has been developed to overcome the limitations of the shielded box method but remains to be one of the most complicated methods of measuring the SE. It comprises of an anechoic chamber with two antennas and the sample placed between the two antennas. One antenna transmits the EM signal onto the specimen while the other measures the intensity of the signal coming through the specimen [50]. Shielded room method has been described in MIL-STD-188-125-1 standard for the measurement of SE [46]. Another variation of the shielded room method is the reverberation chamber method, which has an enclosure which can be used to measure the SE of both small and large specimens. The specimen which needs its SE measure is kept inside the chamber attached to a small enclosure while the chamber is irradiated with EMWs of different frequencies [56,57]. The SE can be measured by the antenna, which is placed inside the smaller enclosure. Few of the key advantages of this method include repeatability, the ability to use a wider range of frequency, and the ability to irradiate the specimen with EMWs with different angles [58,59]. 1.1.4. Co-axial transmission line method Co-axial transmission line method is the most commonly used technique for the measurement of SE due to various advantages it provides such as the ability to measure the SE over a wide range of frequencies, repeatability of the testing, and the comparability of results tested at different facilities [60,61]. The specimen is kept within a sample holder while it is irradiated with EMWs from the VNA. Intensities of the reflected and transmitted EMWs are then measured by the same VNA. Since many new materials being investigated for EMI shielding properties are composites, it is often necessary to use other characterisation techniques to evaluate their mechanical and morphological characteristics. While having high SE values, materials used for EMI shielding applications are required to have sufficient strength

in order to carry physical loads applied during real-life applications. 1.2. Other characterisation techniques Apart from SE measurements, materials fabricated for EMI shielding should also undergo other characterisation techniques to understand their morphological, compositional, mechanical, and compositional properties. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analyses are the most commonly utilised morphological characterisation techniques mainly due to the high-resolution images they can provide. X-Ray Diffraction (XRD) and Energy-dispersive X-ray Spectroscopy (EDS) analyses can be used to analyse the composition of the materials. XRD can be used for qualitative and quantitative analysis of the material and have been used for a very long time due to its reliability. For the characterisation of polymeric materials, Fourier Transform Infrared Spectroscopy (FTIR) is the most commonly used technique. New developments in technology have made the characterisation of other materials also possible by using this method. Tensile test, 3-point bend test, and the compressive test can be utilised for the measurement of mechanical properties. Some of the materials, such as cementitious materials would require certain time periods such as 28 days before the final strength can be measured. 2. EMI shielding cementitious shielding composites 2.1. Introduction EMI is known to cause failures within many sensitive electronic systems on a regular basis with catastrophic losses [11]. Research work conducted on EM radiation and human health have shown that EM radiation emitted from most of the electronic devices can have a long-term adverse effect on human health [24,62]. Due to such reasons, shielding from EM radiation has attracted a lot of attention in the field of material development. Most of the existing material used in EMI shielding are metals with good electrical conductivity. Unfortunately, these metals have high specific weights and are also prone to corrosion. While metals have been able to satisfy shielding requirements, modernisation of electronic devices requires more lightweight, flexible, and corrosion resistant materials. As a result of these requirements, there has been an increase in the research conducted on EMI shielding of novel materials. This is reflected by the number of research publications on EMI shielding materials over the past few decades. Most of these publications have focused on using polymers as shielding materials since most polymers are light in weight and corrosion resistant. Some researchers have tried to formulate cementitious, woodbased, or even modified metallic materials to suit modern shielding needs. Naturally, cement-based materials are known to have a slight amount of conductivity owing to the ion transfer, which depends on the evaporable water content in the cement mixture. In this regard, the overall conductivity of cementitious materials is known to increase by having a porous microstructure with a significant degree of interconnectivity [63]. Based on the type of fillers used to enhance the SE, cementitious composites can be broadly classified, as shown in Fig. 3. This paper discusses some of the novel cementitious composites within this classification. Concrete is one of the most commonly used cementitious construction material in the world which has been investigated for its electrical conductivity and EMI shielding properties. Many of the past research into concrete have revealed that conductivity and SE depend on many factors, such as the type of additives, water to cement ratio, porosity, and fillers [64]. Several publications have

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

5

Fig. 3. Broad classification of EMI shielding cementitious composites.

shown that the addition of steel rebar can increase the overall SE of the concrete since it would add to the conductivity of the composite. Hyun et al. [65] have carried out several tests on concrete with and without the inclusion of rebar in order to measure their effect on the shielding effectiveness. Authors have found that concrete does possess a very small natural shielding property, especially at higher frequencies owing to relative permittivity and loss tangent values. The addition of rebar enables the concrete to have a higher attenuation of EMWs at lower frequencies. Further studies on the rebar and their size have yielded that with the increase of the rebar diameter and the decrease of the spacing between the rebar, the attenuation of the EMWs and the SE of concrete increase. Additionally, a double layer rebar structure has proven to have a lower transmission of EMWs than a single layer rebar structure. From these results, authors conclude that the addition of double layer rebar can reduce the transmission coefficient by up to 30– 60 dB when compared to concrete with single layer rebar. 2.2. SE of different types of concrete According to some of the published literature, different types of concrete that are already being used in the construction industry have shown to possess a varying amount of SEs. Koppel et al. [66] have conducted experiments to measure the SE of three different concretes. Natural fibre concrete pressed plate, aerated concrete, and high-performance concrete has been tested for their SE along with thirteen other most commonly used construction materials in this experiment. SE test has been carried out at 2.4 GHz in order to measure how much shielding each of these materials would provide in the Wi-Fi frequency band. Out of all the materials tested, high-performance concrete has provided the highest reflection coefficient and the lowest transmission coefficient, making it the best EMI shielding material out of the tested ones. In addition to this finding, authors have suggested that atmospheric factors, such as humidity can also affect the shielding properties of some the materials which require further investigations. For example, authors claim that aerated concrete might be able to produce higher SE when the atmospheric moisture content is increased because of the absorbed moisture within the concrete can increase the attenuation of EMWs. Apart from high-performance concrete, authors claim gypsum board and oriented strand board can be

used in EMI shielding applications since they have shown considerably lower transmission coefficients. Since the water content within the cementitious composite is a factor that can alter the SE, Chung and Kharkovsky [67] have investigated how water content can affect the EMW absorption of concrete. Additionally, authors have investigated the effect of coarse aggregates on the EMW absorption as well. While the primary objective of this experiment has been to measure the curing rate of concrete and mortar using EMWs, it has also provided valuable information into absorbance and reflection of microwaves during the curing period. The shielding characteristics have been measured over a frequency range of 8.2–12.4 GHz. Authors have observed that the electrical conductivity of the concrete and mortar both decrease with the ageing period, which is mainly due to the reduction of free water within the material. The rate of reduction of the conductivity was observed to be different in the two materials and theorised to be due to the addition of aggregates within the concrete. The reflectivity of EMWs from the specimens has shown similar behaviour to that of the conductivity. While authors have not measured the shielding properties of materials in details in this work, they conclude that conductivity can be used as an effective method in measuring the curing behaviour of concrete. While early research into EMI shielding materials have explored the possibility of using concrete as it is, Sato et al. [68] have investigated the possibility of modifying the surface of the concrete wall to mitigate the EMWs and enhance the shielding capabilities. They have used a triangular and sinusoidal wave-shaped structure for the concrete walls. The effects of wave-shaped concrete walls have been analysed theoretically using the Finite Difference-Time Domain (FD-TD) technique at the frequency of 2.5 GHz. To verify the accuracy of the theoretical calculations, actual concrete specimens constructed with these shapes have been tested using an experimental setup within 6–10 GHz frequency range. There have been slight discrepancies when the two results from the theoretical calculation and the experimental setup have been compared. Authors suggest the differences might be due to the differences of the relative permittivity, which would have occurred because of the possible inclusion of water within actual samples. Authors claim that the SE of the concrete walls can be further enhanced by adjusting the width and the depth of the grooves but suggest

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Table 1 Summary of SE in different types of concrete. No.

Type

Frequency

Specimens thickness

Effect of shielding

References

1

Steel reinforces concrete





[65]

2

2.4 GHz

50 mm/100 mm/13 mm

3

Natural fiber concrete pressed plate/Aerated concrete/High performance concrete plate, round shape/High performance concrete plate Plain concrete

The transmission coefficient approximately 30–60 dB lower in double-layered rebar, than the single-layer reflection coefficients, 0.03/0.11/0.38/0.35

8.2–12.4 GHz

250 mm

[67]

4

Plain concrete

6–10 GHz

10–30 mm

Varies greatly over the hydration period, an average value cannot be determined By adjusting the width and the depth of the grooves, reduction of reflection and transmission is found possible at the required wall thickness

extending the experiment to a three-dimensional analysis for better accuracy. Summary of SE produced by different types of concrete that has been discussed within this section is provided in Table 1. 2.3. Carbon particles based composites Since the SE of concrete has proven insufficient to be used as an effective barrier against EMI, the development of cementitious materials has focused on the addition of filler materials into the cement mix to enhance the SE. One of the earliest experiments in using filler materials to reflect EMWs has been conducted by Fu and Chung [69]. The main focus of the research has been to use conductive concrete as a guidance system in automatic highways. Carbon filaments having a diameter of 0.1 lm and length > 100 lm, has been the main filler added to the mix in order to enhance the EMW reflection properties. Different mixes containing 0.5 wt%, 1.0 wt%, and 1.5 wt% of carbon filament have been created to find out the mix with best reflective properties. Authors claim no aggregates have been used in any of the mixes. Before adding to the mix, carbon filaments have been surface treated with ozone gas to enhance the bonding between the filaments and the cement matrix. The SE tests have been carried out for 1 GHz frequency. Results from the SE tests have revealed that with the increase of the carbon filament content, the SE of the composites increases. The mix with the largest SE has been the mix containing 1.5 wt% carbon filament. The primary shielding mechanism has been identified as the reflectivity of EMWs. Authors further claim that the addition of carbon filaments have greatly enhanced the mechanical properties of the concrete as well. Many experiments focused on fabricating EMI shielding cementitious composites have looked into using fillers with high electrical conductivity to achieve the required level of shielding. Graphite is known to be an excellent electrical conductor and has been used in a variety of applications where high conductivity is sought. Hence, graphite is one of the preferred materials to be added to the cement mix to increase the conductivity and SE. Guan et al. [70] have analysed several cementitious mixes in literature where graphite have been added to improve the SE. They have found that the SE and the electrical conductivity of mixes increase with the increase of the graphite content. Authors claim that a 3 mm thick cementitious composite specimen containing 30 wt% graphite can produce a SE of about 10–40 dB over the frequency range of 200–1600 MHz. However, graphite being a brittle material can have a significant impact on the overall mechanical properties when added to a cement mix. Unfortunately, authors have not mentioned the change in mechanical properties that may arise due to the addition of graphite. Analysis of another experimental work shows that the creation of a graphite layer on top of the cementitious composite can also increase the overall SE of the composite. According to details in the experiment, the graphite

[66]

[68]

coating is created by suspending graphite particles on water or alcohol. Once the water or the alcohol is evaporated, the graphite coating is applied over the composite. One key advantage of this method is the increased electrical conductivity and SE of the composite due to the interconnectivity of graphite particles. A 4.4 mm thick coating made by using this technique has been able to provide a SE of 22.3 dB at 1.0 GHz and 25.6 dB at 1.5 GHz respectively. Authors also mention that styrene-butadiene latex and silica fume can be used to disperse graphite within the composite that would expand the conducting network increasing the SE. Guan et al. [70] have also analysed published literature where carbon black (CB) particles have been added to the cementitious composite to increase the SE. A 10 mm thick cementitious composite specimen containing 3.0 vol% CB has been able to produce a SE of 6–8 dB when tested within 2–8 GHz frequency range. To improve the SE of the composite, authors have fabricated another mix containing CB and a secondary absorbent which has pushed the SE up to 15 dB. Authors claim that the addition of the secondary absorbent has decreased the conductivity of the material, improving the impedance matching between the specimen surface and the free space. This has resulted in a decrease in the reflection and an increase in the absorption of EMWs. The absorbing behaviour of the composites containing different CB content is shown in Fig. 4(a). A TEM micrograph of CB nanoparticles used in this experiment is shown in Fig. 7(a). Dai et al. [71] have used high-structure CB fillers to create an EMI shielding cementitious composite with high SE. Highstructure CB differs from low-structure CB mainly due to the higher number of branching and chaining within an aggregate. Apart from the branching differences, high-structure CB also shows very high electrical conductivity and high specific surface area. Authors have opted to use high-structure CB to fabricate a cementitious composite since high-structure CB is not a commonly used filler in the fabrication of cementitious composites even though it has been widely used in fabricating polymer composites with high electrical conductivity. Several cementitious mixes have been fabricated by varying the CB content so that their shielding properties and mechanical properties can be compared to the control mix containing 0 wt% CB. Shielding tests have been carried out for 8.0–18.0 GHz and 18.0–26.5 GHz frequency ranges. Conductivity tests, which have been carried out by using the four-probe technique, have shown that the conductivity of the cementitious composites increases with the increasing CB content. Compressive strength, on the other hand, has decreased with the increase of the CB content. The compressive strength of the composite had decreased at a rapid rate when the CB content was increased beyond 3 wt%. Authors theorise one possibility for the reduction in the compressive strength as the need for higher water content in the cement mix because of the absorption of a higher amount of water by CB due to their large specific surface area. Throughout the entire frequency range tested, a composite containing 2.5 wt%

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

7

Fig. 4. (a) The absorbing performance of carbon black added composite materials. 1#: 1.5 vol% CB, 2#: 3.0 vol% CB, 3#: 6.0 vol% CB, 4#: combined wave absorber [70], (b) the absorbing performance of CB mixed cementitious composites with different concentration of CB in the frequency range of 8–18 GHz (c) 18–26.5 GHz [71], (d) SE of cementitious composites containing carbonized peanut and hazelnut shells (CPS- carbonized peanut shells, CHS- carbonized hazelnut shells) [74], (e) EMI SE of cementitious composites containing GO [76], and (f) SE of the cementitious composite with 10 wt% GO and 2 wt% SF [77].

CB has shown a minimum reflectivity of 20.30 dB. Authors claim that the loss factor of the composite has increased due to the addition of CB, resulting in the increased absorption of EMWs. Absorption performance (reflection loss) of fabricated composites is shown in Fig. 4(b) and (c). SEM micrograph of the cementitious composite containing high-structure CB is shown in Fig. 7(b). Xie et al. [72] have investigated the EMI SE of CB mixed composite fabricated into a honeycomb structure. The CB has been coated on to a paper honeycomb structure which then filled with gypsum plaster. The particle size of the CB that has been used for the composite fabrication had ranged between 30 and 50 nm. Apart from having different CB contents, authors have varied the side length and height of the honeycombs to study the effect of the geometry parameters on shielding. EMI shielding tests have been carried out using arched reflecting testing method over the frequency range of 2–8 GHz. As expected, the results have shown that increasing the CB content yields a higher SE. Authors claim honeycombs with smaller side length, and larger height leads to composites having better SE since the EMWs tend to have a higher number of internal reflections, which leads to higher attenuation of EMWs. Even though this research has not fabricated a cementitious composite, the method described by the authors can be adopted in fabricating a cementitious composite. Yee and Jenu [73] have studied the SE and the permittivity of cementitious composites containing fine graphite powder (FGP). Authors have fabricated cementitious composite mixes containing 7.2 vol%, 9.6 vol%, and 12 vol% of FGP and compared the results with a control mix containing 0 vol% FGP. All the specimens had a thickness of 20 mm. The SE tests have been carried out within 50–400 MHz frequency range. Results from the shielding tests have shown that the addition of FGP increases the SE of the composites. However, authors have discovered that addition of FGP between 200 and 250 MHz frequency range decreases the multiple re-reflection loss, reducing the SE of the composite. Even with this reduction, increasing the filler content has increased the overall SE of the composite. The composite containing 12 vol% of FGP has

shown the highest SE. Authors claim the addition of FGP has not increased the conductivity of the composite drastically hence has not produced a high absorption loss at lower frequencies. In the hope of creating a cost-effective EMI shielding cementitious composite, Khushnood et al. [74] have used micro and nanoparticles of carbon derived from carbonised peanut and hazelnut shells as filler materials. Raman spectrometer analysis conducted on both shells has revealed that the chemical composition of both shells is almost equivalent to each other, and both shells contain a limited amount of carbon in their structure. SEM images of the carbonised shells have shown smooth textures, a feature that would minimise possible entanglement problems that might occur during the mixing process. Two composite mixes containing 0.2 wt% and 0.5 wt% of each carbonised shell have been created along with a control mix containing 0 wt% of carbonised shells. The distribution of the carbon nanoparticles within the composite can be seen in the SEM micrograph given in Fig. 7(c). Each composite has been tested within 0.2–10 GHz frequency range for their SE. As expected, SE has increased with the increase of the carbonised shells content. At the same filler content, the composite containing peanut shells has shown a slightly higher SE than the composite containing hazelnut shells. A maximum SE of 2–10 dB has been obtained by these composites for the tested frequency range. SE variation of composites containing the two type of carbonised nutshells is shown in Fig. 4(d). Cost comparison with commonly used conductive fillers have shown that these carbonised nut shells can achieve the same SE but at a fraction of the cost compared to most commonly used expensive fillers such as CB and carbon nanotubes. Authors mention they plan to use other agricultural wastes to investigate the possibility of manufacturing more cost-effective EMI shielding composites in the future. Waste material collected from the palm oil industry known as the Palm Oil Fuel Ash (POFA) is also used as a replacement for cement in cementitious composites manufacturing. POFA is classified as fly ash due to its chemical composition and currently being treated as landfill, which causes severe environmental pollution.

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Studies in the literature have shown that POFA can be used as a strength addition filler in concrete. Narong et al. [75] have investigated the possibility of using POFA as a low-cost filler in EMI shielding cementitious composites. X-ray fluorescence (XRF) and chemical analyses have confirmed that the POFA used in this study conforms to ASTM C618 standard. Different mixes of cementitious composites have been fabricated by adding a varying amount of POFA. The SE of the mixes has been measured using a transverse electromagnetic parallel plate method within 0.1–1.5 GHz frequency range. Authors have used Taguchi Grey method to optimise the mixes further to enhance their mechanical properties, and as a result, they have been able to achieve an increase of 45.90% in the 28 days compressive strength. The optimisation technique has also revealed the optimum POFA content to be 20 wt%. A composite mix with 20 wt% of POFA has been able to generate a SE of 6 dB within the tested frequency range. Authors claim the use of the Taguchi Grey method has resulted in improvement in the mechanical properties as well as in the SE compared to original mixes in the experiment. As shown in equation (4), the SE of a material increase with the increase of its electrical conductivity. Hence, many of the EMI shielding material research have focused on increasing the conductivity of the specimen to achieve a high SE value. To achieve high electrical conductivity, some researchers have used graphene oxide (GO) as a conductive filler in the fabrication of cementitious composites [107–108]. Apart from having high electrical conductivity, GO particles are known to have a large specific surface area. Additionally, the presence of defects and groups within GO particles can attenuate EMWs by increasing the number of internal reflections. Due to these many advantages, Zhao et al. [76] have used GO powder to fabricate a conductive cementitious composite for EMI shielding applications. In this research work, graphite powder has been subjected to modified Hummer’s method to obtain the required GO. Obtained GO has been added to the cement mix and then ultrasonicated to disperse them within the cement matrix. Several composite mixes have been fabricated in this manner by varying the GO content. Resultant composites have been kept for 28 days to achieve the required mechanical strength. After specimens have been cured for seven days, they have been subjected to EMI SE tests within 8.2–12.4 GHz frequency range. Apart from these characterisation techniques, the specimens have been subjected to SEM and XRD analyses as well. The SEM micrograph showing the emerging of hydration crystals in GO/cement composite is shown in Fig. 7(d). Mechanical tests have revealed the compressive and the flexural strength of the specimens increased with the ageing time similar to that of the control mix containing 0 wt% GO. However, the compressive and the flexural strength after 28 days of the composite containing 0.08 wt% GO has shown a slightly higher value than the control mix, indicating the addition of the GO had a positive impact on the mechanical properties. The EMI SE of composites shows an increase with the increase of the GO content up to 0.08 wt% then recedes to a lower value than the control mix, as shown in Fig. 4(e). Unfortunately, authors have not provided possible reasons for such fluctuations of SE of the composites. Following the finding that GO can improve the SE of cementitious composites, Mazzoli et al. [77] have tried to enhance the SE of the cementitious composite by adding GO with metal fibres. By the combination of these fillers, authors have expected to obtain an excellent conducting network within the composite that would enhance the EMI SE. The experiment has used GO microparticles mixed in with brass coated steel fibres (SF) as filler materials. To check the effect of each filler, different mixes containing none of the conducting fillers, GO only, SF only, and GO and SF has been fabricated and tested for their mechanical and EMI shielding properties. Distribution of GO particles within the concrete mix is shown in Fig. 7(e). The compressive strength of the mixes contain-

ing GO has not seen an increase that was observable in previous literature containing GO. Authors suggest that the loss of planarity of the particles during the mixture preparation due to their considerable large size may have caused the compressive strength not to improve as expected. On the other hand, the addition of SF has improved the flexural strength of the composites. EMI shielding properties of the mixes have been tested within 0.8–7.8 GHz frequency range. Results have shown that the SE of the composites increases with the increase of GO content. However, a more profound effect on SE is generated when SF and GO are added to the composite mix. Hence, the composite containing GO and SF has shown the best SE. Authors claim the composite containing 10 wt % GO and 2 wt% SF has a stable SE between 40 and 50 dB over the tested frequency range, as shown in Fig. 4(f). The high SE achieved by this composite is due to the extension of the conductive network within the composite because of the synergetic effect of the two fillers which has resulted in increasing the number of internal refraction and higher attenuation of EMWs. While some researchers have focused on fabricating EMW reflecting composites, others have tried to maximise the absorption and minimise the reflection of EMWs. Mostly in indoor environments, it is crucial to minimise the reflection of EMWs since reflection is most likely in such environments, and it can cause additional interference in vulnerable devices. For the EMWs to be absorbed by a material, it is necessary for the waves first to penetrate the material. However, most of the cementitious composites have a very compact structure making it difficult for the EMWs to travel inside the composite. The composite needs to have a certain amount of porosity to make sure the EMWs can go into the material. Lv et al. [78] have tried to maximise the EMW absorption of cementitious composites by increasing the porosity of the composite. Authors have used hollow glass microspheres (HGM) to create the porous structure necessary for the penetration of the EMWs. However, when the porosity of the composite is increased, the strength of the composite can be decreased. Therefore, there should be an optimum amount of porosity within the composite while maintaining sufficient strength to carry the applied load. To maximise the absorption of the EMWs while maintaining sufficient strength, authors have used graphene nano-platelets (GN) along with HGM in the composite mix. Several composite mixes comprising of the two fillers have been fabricated and tested for their SE and mechanical properties. The SEM analysis carried out to observe the morphology of the composite has shown a uniform distribution of GN and HGM within the concrete. The SE test has been carried out by using an arched anechoic chamber setup for the frequency range of 2–18 GHz. To analyse the effect of thickness on the SE, specimens with thicknesses of 10 mm, 20 mm, and 30 mm have been cast and tested by using the same test. Mechanical test results show that the compressive strength of the mixes varies with the addition of filler, but authors have failed to establish a relationship with the filler content and the compressive strength of the composites. Variation of the EMI SE of different composite mixes is shown in Fig. 8(a). From this graph, it can be seen that the absorption of the specimens undergoes improvement as the HGM content is increased. Variation of the GN has shown that there is an optimum level of GN that can be added to mix to achieve maximum absorption of EMWs after which additional GN would result in an adverse outcome for the SE. Analysis of the variation of the thickness of specimens has shown that the absorption of the EMWs cannot be controlled only by changing the thickness and depends on many factors, including the frequency of the EMWs. From obtained results, authors have concluded that for a composite containing 40 vol% HGM and 0.2 wt% GN the optimum thickness would be between 20 mm and 30 mm if it is being subjected for EMWs within the frequency range of 2–18 GHz.

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Table 2 Summary of carbon powder based cementitious composites. No.

Shielding material

Frequency

Specimens thickness

Effect of shielding

References

1

Carbon filaments

1 GHz

3.6–4.4 mm

[69]

2

Carbon/Metal/Ferrite/Fly ash

3

Carbon black

2–8 GHz/2.45 GHz/75–100 GHz/ 1.0–1.5 GHz 8.0–18.0 GHz and 18–26.5 GHz

10 mm/-/30 mm/ 4.3 mm 30 mm

29 dB higher than the transmissivity 6–8 dB/8 dB/7–9 dB/4 dB

[71]

4

Carbon black (CB) coated paper honeycomb

2–8 GHz

5

Graphite Fine Powder

50 MHz–400 MHz

Variable thicknesses 20 mm

Reflectivity < -20.30 dB and < 10 dB Reflection loss ~ 10 dB

[73]

6

Carbonaceous nano/micro inerts [peanut shell and hazelnut shell] Palm Oil Fuel Ash (POFA) Graphene oxide (GO) graphene oxide particles + straight brass-coated steel fiber Graphene nano-platelets (GN) and hollow glass microspheres (HGM)

0.2–10 GHz



2.4 dB additional shielding at 360 MHz 2–10 dB

0.1 and 1.5 GHz 8.2–12.4 GHz 0.8–7.8 GHz

120 mm 5 mm 2.5 m and 0.8 m

6 dB 11–16 dB 25–50 dB

[75] [76] [77]

2–18 GHz

10, 20, and 30 mm

8.2 dB

[78]

7 8 9 10

Summary of composite mixes containing carbon powder that has been analysed in this section is provided in Table 2. Reflection loss of several specimens is plotted together for comparison in Fig. 5. For this comparison, maximum reflection loss shown by each composite mix within the tested frequency range has been plotted. The composite containing GN and HGM has shown three sharp peaks of 34, 37, and 20 dB at about 6, 10.5, and 15 GHz respectively. 37 dB is the maximum reflection loss shown by any composite containing carbon particles. However, the same composite does not maintain the reflection loss throughout the entire frequency range. Composite with 0.5 wt% carbonised peanut shells which contain 93.77% carbon, has shown the minimum reflection loss, which is about 0 to 7 dB. Composites containing 0.5 wt% CB has a very similar reflection loss characteristic to that of the composite with CPS, which leads to the conclusion that a cementitious composite with carbon particles alone can show only a limited increase in their reflection loss. Addition of higher percentages of carbon particles without another filler has not been investigated by authors since previous literature have shown that a high amount of carbon particles in the cementitious composites can increase the brittleness and the cost of the composites. Composites with CPS and carbon particles show an increasing SE

50 45 40 35

-10

30

-15

SE (dB)

Reflection loss (dB)

[74]

0.5% CPS [74] 0.08 wt% GO [76] 2 wt% SF + 10 wt% GO [77]

0.5 wt% CB [71] 0.5 wt% CB [71] 9 mm honeycomb height + 0.6% CB [72] 0.5% CPS [74] 0.2% GN + 60% HGM [78]

-5

[72]

with the increase of the frequency up to 18 GHz and starts to decrease again. Addition of a secondary absorber along with CB can improve the reflection loss characteristics of the composites. This composite with CB and secondary absorber shows two reflection loss peaks of about 18 and 14 dB at 4 and 6 GHz respectively. In general, the reflection loss of the composite with CB and secondary absorber is higher than composite with CPS. However, the addition of the secondary absorber has not helped to maintain a uniform reflection loss within the tested frequency range. Altering the geometry of the composite has shown an effect on the reflection loss as the composite with CB and honeycomb geometry shows slightly higher reflection loss than the flat specimen. The honeycomb with a height of 9 mm has shown two reflection loss peaks of about 18 and 16 dB at 3 and 7 GHz. Increased multiple reflections EMWs undergo when they encounter the hexagonal honeycombs has been the main reason for the increased reflection loss. Total SE of three composite mixes described in this section is plotted in Fig. 6. The comparison shows the addition of only carbon particle to cementitious composite imparts only a very small SE. Carbonised peanuts and graphene oxide particle added composites show very similar behaviour with the increasing frequency.

3 vol% CB to SiO2 + Secondary absorber [70]

0

[70]

-20 -25

25 20 15

-30

10

-35

5

-40

0 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28

Frequency (GHz) Fig. 5. Variation of reflection loss of carbon particle based cementitious composites.

0

1

2

3

4

5 6 7 8 Frequency (GHz)

9

10 11 12 13

Fig. 6. Total SE of carbon particle based cementitious composites.

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D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

Composite with CPS shows an overall SE of about 2–10 dB within 0.2–10 GHz frequency range. The composite with GO shows an overall SE of about 11–15 dB within 8.2–12.4 GHz frequency range. Both composites with CPS and GO shows an increasing SE with the frequency. However, given that composite with CPS has 0.5 wt% of particles and the composite with GO contains only 0.08 wt% of particles, it can be concluded that GO can impart larger SE than carbon particles in cementitious composites. The third composite with 2 wt% SF and 10 wt% GO has shown a SE, which is almost six times higher than the other two composites. The superior SE of the third composite is created due to the synergetic effect of steel fibres and carbon particles, which helps to extend the conducting network within the composite. A SE of 33–48 dB has been generated by the composite with SF and GO within 0.8–8.4 GHz frequency range. The composite has been able to maintain its high SE throughout the tested frequency range. From all these results it can be concluded that addition of carbon particles alone is insufficient to create an adequate SE within a cementitious composite and addition of secondary fillers such as steel fibres is necessary to achieve an adequate SE. 2.4. Carbon fibre based composites As mentioned in the previous sections, many of the researchers have used carbon-based fillers for the fabrication of composites requiring high electrical conductivity. Out of these carbon-based fillers, carbon fibres (CF) have been used in composite fabrication for several decades, mainly due to their lower cost of manufacturing compared to other filler materials such as carbon nanotubes. To measure the impact of CF, Zhang and Sun [79] have fabricated cementitious composites consisting of the varying amount of CF. Moreover, test results that were obtained for CF mixed composites have been compared with composites containing steel fibres. Each composite has been fabricated by using the mould cast method. After the composite has been cast, they have been left for 28 days to achieve their strength. Specimens have been tested for the SE by using the shielded box technique for the frequency range of 8– 18 GHz. SE test results have shown that SE increases with the increase of CFs and steel fibres in each composite type. However, the reflectivity of EMWs from the two composites does not show similar behaviour to that of the overall shielding. When the steel fibre content is increased in the composite, the reflectivity of EMWs increases, but when the CF content is increased, it reaches a maximum reflection value and gradually decreases. Authors claim this behaviour is mainly because of the impedance mismatch that would occur when the CF fraction is increased. The maximum amount CFs added to the composite has been limited to 1% due to their cost. The overall SE of the two composites with the fibre fraction is shown in Fig. 8(b) and (c). From these results, it can be seen that with the increase of the fibre content of both fillers, the overall SE is increased. Authors suggest that the addition of a secondary wave absorber, such as ferrite would be able to increase the overall SE even further. While there has been an increased number of research on EMI shielding cementitious composites in the past few decades, only a very small number of experiments have been conducted on these composites when subjected to environmental conditions. Since most of the cementitious composites are used in outdoor applications, Wang et al. [80] have studied how the SE of cementitious composites containing CFs varies with freezing and thawing cycles. The main reason behind this research has been to understand how the SE of CF containing cementitious composite varies when it experiences expansion and contraction due to being exposed to below freezing temperatures during the winter and high temperatures during the summer. Moreover, when cementitious composites experience such temperature fluctuations, there is a

possibility of crack development in the composite that would lead to an increment of moisture content within the composite. Both of these factors can result in a change in the mechanical properties and SE of the composite. For their experiment, authors have used cementitious composites containing 0.2%, 0.4%, 0.6%, and 0.8% CFs. SEM micrograph showing the distribution of the CFs within the fabricated composites is given in Fig. 7(f). Each of the fabricated composites has been subjected to 50 freezing, and thawing cycles to study their effect and results have been compared with the results of the control mix which contained 0% CFs which has undergone the same number of freezing and thawing cycles. SE tests have been carried out within 2–18 GHz frequency range. Results obtained from SE tests have revealed that the freezing and thawing does not change the SE when there is no CF within the composite. Regardless of being subjected to freezing and thawing, the SE of composites has increased with the increase of the CF content. Additionally, authors have observed that the porosity of the composite decreases with the increase of the CF content but start to increase when the CF content is 0.8%. Freezing and thawing of the composites containing CFs have shown that after composites have undergone these cycles, the EMW reflection of the composites increases while the absorption decreases at high frequencies. Variation of the absorption loss of the composite containing 0.8% CF before and after the freezing cycles is shown in Fig. 8(d). After analysing all the test results, authors conclude that CF can be added to cementitious composite undergoing freezing and thawing to enhance composite’s SE and mechanical properties. However, CF content should not be increased beyond the optimum level as it is detrimental to the composite. Addition of CF has shown to increase the SE of cementitious composites but below the required values in industrial standards. To overcome this limitation, some researchers have combined a secondary filler with CF to boost the SE of the composite. Ferroferric oxide (Fe3O4) is such an additive that has been mixed in with CF to create superior EMI shielding composites. Fe3O4 is a form of iron oxide that occurs naturally as magnetite which possesses soft magnetic properties [81,82]. Additionally, ferroferric particles are also known to have high surface energy and a large specific surface area. Particles of Fe3O4 can be added to cementitious composites to provide shielding against EMI and to increase the strength. Liu et al. [83] have researched the effect of CF along with Fe3O4 on the SE of cementitious composites. Fe3O4 nanoparticles prepared by a solvothermal method with manually cut CFs having lengths between 3 and 5 mm have been used for the fabrication of composites. Several composite mixes have been fabricated using 1 wt%, 3 wt%, and 5 wt% of the particles while maintaining the CF content at 0.4%. Fabricated specimens have been subject to SEM, XRD, and SE characterisation to find out the distribution of fillers within the composite and amount of shielding the composites could provide within 8.2–12.4 GHz frequency range. The thickness of the specimens subjected to SE tests has been 7 mm. SEM images of specimens obtained at different magnifications are shown in Fig. 14(a) and (b). Images clearly show the distribution of CFs within the composite. Fe3O4 particles are also visible in the SEM images as small flakes. Overall, SEM images confirm that the CFs and Fe3O4 particles have been distributed well within the composite. SE results have shown that with the increase of the Fe3O4 content, the SE of the composites increases. The composite containing 5% Fe3O4 has been able to produce a maximum SE of 29.8 dB within the measured frequency range with the absorption being the main shielding mechanism. EMI shielding effectiveness of fabricated composites containing varying amount of Fe3O4 nanoparticles is shown in Fig. 8(e). Authors believe the synergetic effect, illustrated in Fig. 9, of the two fillers, is the main reason for the enhanced SE of the composite. The EMWs entering the composite undergoes a high number of multiple reflections and scattering due to the combination of the fillers, increasing

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11

Fig. 7. Microstructure of several EMI shielding concrete (a) TEM micrograph of graphite to be added to cement mix [70], (b) SEM micrograph showing CB within the concrete mix [71], (c) SEM micrographs of cement mix with carbonized nanoparticles [74], (d) SEM image of GO within cement mix [76], (e) SEM image showing GO microparticles distributed with cement mix [77], (f) SEM image of CF dispersed within cement [80].

Fig. 8. (a) Reflection loss characteristics of cementitious composites containing GN and HGM [78], (b) SE of steel fibre reinforced cementitious composites, (c) SE of carbon fibre reinforced cementitious composites [79], (d) Variation of the absorption loss of the composite containing 0.8% CF before and after the freezing cycles [80], and (e) EMI shielding effectiveness of composites containing CF and Fe3O4 nanoparticles (0%, 1%, 3%, and 5%) [83].

the SE. Authors claim that this type of composite can be investigated further to have even better SE. Details of cementitious composites containing CFs discussed within this section is provided in Table 3. Comparison of the composites consisting of CF and also a composite with steel fibres is shown in Fig. 10. Composite with 1% CF has shown a SE of about 30–45 dB within the tested frequency range, and its SE has been increasing with the frequency. Comparatively the composite with 0.4% CF and ferrite has shown a SE which is about half of that of the composite with 1% CFs. Even though ferrite is known to increase the SE of cementitious composites, it can be seen that a considerable amount of CF is necessary to impart high SE in these

composites. However, the addition of higher percentages of CF increases the overall cost of the composite. The composite with steel fibres has been able to generate a SE of about 55–70 dB within its tested frequency range. The SE of the composite with steel fibres is almost twice the SE of the composite with CF. one reason for the increased SE of steel fibre composite is the higher percentage of fillers, which is 3% compared to 1% CF in the other composite. The high cost of the CF has been the main reason to limit the amount of them added to the composite. Findings from these works can be used for future experiments where a higher percentage of CF along with other fillers to cement mix to create composites with higher SE.

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Fig. 9. Possible interaction of EMW and cement composite containing CF and Ferroferric oxide nanoparticles [83].

Table 3 Summary of SE in carbon fibres based cementitious composites. No.

Shielding material

Frequency

Specimens thickness

Effect of shielding

References

1

8–18 GHz

30 mm

30–50 dB/20–40 dB/reduced with fiber fraction

[79]

2

Steel fiber/carbon fiber/synthetic polyvinyl alcohol (PVA) fiber Carbon fiber

2.0–18.0 GHz

10 mm

[80]

3

Fe3O4 nanoparticles/CF

8.2–12.4 GHz

7 mm

12.5 dB to 4.9 dB After freezing–thawing cycles, the reflectivity increases 20–27 dB/reflection 2–2.5 dB

3% steel fiber [79] 1% CF [79] 0.4% CF + 5% Fe2O3[83]

70 65 60

SE (dB)

55 50 45 40 35 30 25 2

3

4

5

6 7 8 9 Frequency (GHz)

10

11

12

13

Fig. 10. Comparison of overall SE of carbon and steel fibres based cementitious composites.

2.5. Carbon nanotube-based composites A carbon nanotube can be viewed as a roll-up of graphene layer into a tubular form. If the tube consists of only one such structure, it is known as a single-walled carbon nanotube (SWCNT). Compared with SWCNT, multi-walled carbon nanotube (MWCNT) can be seen as a tube comprising of several rolled up graphene layers [84]. MWCNTs have been increasingly used in many of the composites investigated for SE owing to their extremely high electrical conductivity. Results from most of the composites with MWCNTs

[83]

have shown that there is a promising future for these composites in EMI shielding applications. Hence, increasing the volume fraction of MWCNT increases the overall SE of the composite. However, one of the critical limiting factors for using carbon nanotubes as a filler in composite fabrication has been their extremely high manufacturing cost. One of the earliest research in MWCNT/cementitious composite for EMI shielding has been conducted by Micheli et al. [85], where MWCNT in powder form has been added to the cement mix to fabricate the composite. The primary objective of their experiment has been to fabricate a cost-effective composite that would provide adequate shielding within the mobile frequency band of 0.8– 8 GHz. In their research, 3 cm thick composite specimens containing 0 wt%, 1 wt%, and 3 wt% of MWCNTs have been fabricated and tested for SE using the shielded box method. SEM image of the fabricated composite showing the conductive filler is shown in Fig. 14 (e). The fibrous appearance of the SEM image indicates the distribution of MWCNTs within its structure. The SE of the composites has increased with the increase of the conductive filler content, as shown in Fig. 12(a). The SE of the composite having 3 wt% MWCNTs has been able to produce a SE about 10–35 dB within the tested frequency range, which is the largest SE out of all the fabricated mixes. Taking a step further from their previous research authors have fabricated an EMI shielding cementitious composite for 1.7– 2.6 GHz frequency range [86]. In this experiment, authors have used a layered composite, as illustrated in Fig. 11. One of the main reasons for the fabrication of a layered composite has been to reduce the cost of fabrication by cutting down the MWCNT content. Two composites have been fabricated in this manner, one containing 1 wt% MWCNTs (M2) and the other containing 3 wt% MWCNTs (M3). High-resolution SEM micrographs, shown in

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

Fig. 11. Three-layered cementitious composite having MWCNT [86].

Fig. 14(c) and (d), taken from the two composites show the dispersion of the nanotubes within the cement matrix. Image of M2 specimen show clustering of the nanotubes while the image of M3 shows the well-dispersed MWCNTs making up a conductive network. Hence, from the morphological characterisation, it can be concluded that the electrical conductivity and the SE could be high in the M3 composite due to the formation of the conductive network. SE test has been carried out using a waveguide apparatus and shielded box method for the frequency ranges of 0.75– 1.12 GHz and 1.7–2.6 GHz. Apart from actual measurements of the SE, authors have relied on a mathematical model to predict the SE of the composites as well. When shielding results were analysed, authors have found that a 30 mm thick M3 specimen had the highest SE, which has reached a maximum of 80 dB at 2.6 GHz. Overall SE of the same specimen within the tested frequency range has been about 50 dB, as shown in Fig. 12(b) [87]. This SE is claimed to be a very high value by authors for a cementitious composite containing MWCNTs since SE of the composite with the same filler in literature have recorded lower values. Authors claim SE values obtained for these composites could be further improved by utilising a nanoparticle-based secondary EMW absorber that could be integrated into the composite. However, authors have not mentioned the variation of the mechanical properties of the fabricated composites. Even though MWCNT is an excellent conductor, SE generated by composites with only these fillers is insufficient compared to standards in practice. In order to enhance the SE of cementitious composites with MWCNTs, the addition of a secondary filler has been researched in many literature. In one of such experiment, Nam and Lee [88] have created several cementitious composite specimens by varying the fly ash (FA) and silica fume (SF) content while keeping the MWCNT content at 0.6%. Prior research done on SF and MWCNTs has shown that SF is a good dispersion agent of MWCNTs.

13

This has been the primary purpose for the addition of SF in this experiment. The composite specimens have been fabricated by replacing cement with 0%, 25%, 50%, and 75% of fly ash. In each composite mix, the SF content has been varied by 0 wt%, 10 wt%, 20 wt%, and 30 wt%. Each mix has been cast on plastic moulds having the dimensions of the coaxial transmission line used in SE testing. For all the specimens, SE has been tested within 1–18 GHz frequency range. Apart from SE characterisation, specimens have been subjected to SEM for morphological analysis and EDS for identification of elements within the composite. SEM micrograph of the composite containing 20 wt% SF and 75% FA is given in Fig. 14(f) which shows the distribution of MWCNTs mixed with other additives. EDS analysis has been able to identify components which are typically present in cementitious composites. The composite containing 20 wt% SF and 75% FA has been able to generate the maximum SE out of all the composite mixes, which is about 5– 55 dB as shown in Fig. 12(c). Authors state that these findings correlate well with values in literature. Furthermore, authors theorise that the MWCNTs distribute optimally at this SF content, increasing the SE of the composite. One of the constituents in FA is Fe2O3, which is a soft magnetic material that could enhance the SE of a composite when added as a secondary additive. As a result of the FA replacing cement in this experiment, the SE has increased with the increasing FA content. From this work, it is clear that proper distribution of MWCNTs combined with secondary EMW absorber could enhance the SE of the composite. Summary of the SE provided when different forms of carbon nanotubes are added to cementitious composites is provided in Table 4. Summary analysis of cementitious composites containing CNTs plotted in Fig. 13 clearly shows that the increase of the CNT content can increase the SE of the composites. Composite with 0.6 wt% MWCNTs, silica fumes, and fly ash fabricated by Nam and Lee in their experiment work has the lowest SE out of all the specimens. Throughout the tested frequency range, it has shown a SE of 1– 55 dB. All the specimens show an increasing SE with the increasing frequency. Both experiments conducted by Micheli et al. having 3 wt% MWCNTs in the composite shows slightly higher SE than the composite fabricated by Nam and Lee. Composite fabricated by Micheli et al. have shown a SE of 10–35 dB within the testing frequency range. Even though MWCNT is an excellent conductor, its high-cost limits the amount that can be added to the composite. To overcome this problem, Micheli et al. have fabricated a layered composite with 3 wt% of MWCNTs. Since it was constructed to be a layered composite, it has been able to produce a SE of about 60– 80 dB. Apart from the absorption of EMWs from the composite, the change in the impedance from layer to another would have contributed to higher SE since it can result in higher multiple reflections within the composite. From the comparison, it can be concluded that out of the analysed composites containing MWCNTs, the layered composite is the best composite for EMI

Fig. 12. (a) Variation of the SE of the fabricated specimens with the CNT content [85], (b) Variation of EMI SE of M3 specimens with different thicknesses [87], and (c) EMI SE of composites containing 20% SF and varying amount of FA [88].

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Table 4 Summary of the SE in carbon nanotube-based cementitious composites. No.

Shielding material

Frequency

Specimens thickness

Effect of shielding

References

1 2

Carbon nano tubes (MWCNT) Carbon nano tubes

0.8–8 GHz 2.6 GHz

15 dB around 2 GHz and up to 30 dB at 8 GHz 12 dB (3 cm thick) 80 dB (30 cm thick)

[85] [86]

3

Carbon nanotubes

60–80 dB

[87]

4

Multi-wall carbon nanotube (MWCNT) and fly ash (FA)

1.7– 2.6 GHz 1–18 GHz

30 mm 25  12 cm2/ 11  5.5 cm2 5 cm 10.0 mm

8.0~–57.1 dB

[88]

3 wt% MWCNT [85] 3 wt% MWCNT [86] 3 wt% MWCNT + concrete layered [87] 20 wt% SF + 75% FA[88]

80 70 60

SE (dB)

50 40 30 20 10 0 0

2

4

6

8 10 12 Frequency (GHz)

14

16

18

Fig. 13. Comparison of overall SE generated by carbon nanotube-based cementitious composites.

shielding. For future experiments, multi-layered composites with higher CNT contents can be explored for their SE. 2.6. Particle-based composites Many experimental results in literature have shown that Fe3O4 particles can enhance the SE of cementitious composites. However, the SE of the composite also depends on the distribution of the particles throughout the entire composite. Which means there should be an effective method to distribute the particles within the entire

matrix to obtain a high SE from the composite. To minimise this problem, He et al. [89] have used nano-Fe3O4 fluids to fabricate cementitious composite for EMI shielding. Co-precipitation method has been used to obtain the Fe3O4 liquid used in this experiment. The prepared liquid has been mixed into the cement mix with the other constituents, cast, and left for 28 days until the required mechanical strength is achieved. The amount of Fe3O4 present in the composite mixes has been varied by 3 wt%, 5 wt%, and 7 wt% to assess its impact on SE. EMI shielding properties of these specimens have been tested using the arched testing method for the frequency range of 8–18 GHz. Results from the shielding tests have shown that the absorption of EMWs is the primary form of shielding in these composites. Out of the different composite mixes fabricated one containing 5 wt% Fe3O4 has shown the best SE throughout the entire frequency range. Comparison of the results obtained in this experiment with literature values has been carried out to assess the impact of using liquid Fe3O4 instead of its powder form. The reflection loss values from the comparison have shown that the liquid form of Fe3O4 has far superior SE compared to their traditional powder counterparts, as shown in Fig. 15 (a). Authors claim this could be because Fe3O4 can disperse well within the entire mix since it is already in liquid form whereas dispersion would be difficult if it were in powder form. EDS tests conducted on the specimens have shown that Fe3O4 is distributed well within the entire composite. Authors also claim that the nanoFe3O4 magnetic fluid has accelerated the hydration of the cementitious composites leading to better early age compressive strength. Apart from creating high SE, one of the critical challenges in fabricating EMI shielding cementitious composites is to make it cost effective since most of the high conductive fillers that are used in these composite fabrications are expensive, increasing the entire

Fig. 14. (a) and (b) SEM images of the cementitious composite containing CF and Fe3O4 particles [83], High magnification SEM images of 1 wt% (c) and 3 wt% (d) MWCNT reinforced material bulk morphology [86], SEM images showing the microstructure of (e) conducting concrete containing MWCNT [85], (f) composite containing MWCNT [88], and (g) SEM micrograph of the cementitious composite containing 40% EAFS [90].

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

cost of the composite. To overcome this drawback, several researchers have tried to enhance the SE of the composite while trying to keep the fabricating cost low by using more costeffective fillers. In one of these experiments, Ozturk et al. [90] have investigated the possibility of using electric arc furnace slag (EAFS) in the fabrication of cementitious composite for EMI shielding. EAFS is a by-product that is created in the steel production. Analysis conducted in previous literature has shown that EAFS has the required chemical and physical characteristics to be used as an aggregate in the fabrication of cementitious composites. Authors have chosen to use EAFS fine aggregates, having an average diameter of 4 mm in their research. Six different mixes containing 10%, 20%, 40%, 60%, 80%, and 100% EAFS replacing sand have been created along with a control mix for comparison. The flexural and compressive strength of the composite has been measured after 7, 14, and 28 days. Morphological characterization has been carried out using an SEM analysis. SE of each composite has been measured using the open space method in 3–18 GHz frequency range. Mechanical property analysis has shown that all the mixes have better flexural and compressive properties than the control mix. However, mechanical properties show an increase with the increasing EAFS content up to 40% and reduce thereon. The mix containing 40% slag has shown an increase of 30% in its compressive strength compared to the control mix. SEM image obtained for the mix containing 40% slag is shown in Fig. 14(g). This image shows how the slag particles are distributed within the cement matrix, minimizing the empty space within the composite. Authors believe the inclusion of slag in the cement mix has resulted in a better interlock between the cement paste and the slag granules. SE tests on the composites have shown that the SE increases with the increasing slag content with the mix containing 100% EAFS having the largest SE as shown in Fig. 15(b). The mix having the optimum mechanical properties has shown an overall SE of about 15–20 dB within the tested frequency range. Authors theorize that the high SE obtained when the slag content is increased mainly due to the high iron content within the slag and better interlock between the slag and the cement paste. Authors believe that these composites could be used for potential EMI shielding applications with further improvements. In some of the literature focused on fabricating EMI shielding cementitious composites, authors have reported an observable drop in mechanical properties. The presence of a porous microstructure, which helps the attenuation of the EMWs and increases the SE is the main reason for this reduction. To overcome the deterioration of mechanical properties in EMI shielding composites, Lu et al. [91] have opted to use calcined clay pellets consisting of nano-TiO2 powder. For additional SE, another mix containing the clay aggregates and 30 wt% manganese zinc ferrite powder has also been investigated. Authors have used two reference composite mixes consisting of gravel aggregates and haydite.

15

SE tests of specimens have been carried out in an anechoic chamber within 8–18 GHz frequency range. Evaluation of the mechanical properties has shown the composite with the gravel aggregates has the best compressive strength while the least compressive strength has been reported by the composite with the haydite. Composite consisting of clay/nano-TiO2 aggregates has shown a moderate 28-days compressive strength, which is higher than the minimum required value in the industry. The SE has been highest in the composite consisting of clay/nano-TiO2 and manganese zinc ferrite powder. This composite has been able to show a reflection loss of about 9 to 12 dB. Authors believe the improved magnetic properties of the composite imparted by the addition of fillers is the reason for its improved SE. The composite containing gravel aggregates has shown the lowest SE. Composite with haydite has shown the second lowest SE. Variation of the SE of each composite containing different fillers is shown in Fig. 15(c). Authors believe the porous structure of haydite may have given better SE to that composite compared to the composite with gravel aggregates. Findings in the literature on powder mixed cementitious composites have shown promising results in having a good SE. In an attempt to enhance the SE of powder mixed cementitious composites, Pretorius and Maharaj [92] have experimented in using ferrimagnetic MnZnFe2O4 powder and MnO4 magnetic powder. Several specimens have been fabricated by varying the ferrimagnetic and magnetic powder content in composite mixes. Authors have attempted to fabricate a cementitious composite suitable for indoor applications and aimed at shielding mobile frequency bands and Wi-Fi frequency band. For the SE measurement authors have used the open field measurement technique within the frequency ranges of 824–894 MHz, 890–960 MHz, 1.71–1.88 GHz, 1.86– 1.99 GHz, and 2.4–2.484 GHz, which are known as GSM850, GSM900, GSM1800, GSM1900, and Wi-Fi bands respectively. Results from the shielding test have revealed that the SE of both types of composites increases with the filler content. However, the composite containing the ferromagnetic powder has shown better SE than the one containing the magnetic powder. As a result, authors have analysed the SE of the composite containing the highest amount of ferromagnetic powder (5 wt%) in detail. The composite has been able to produce SEs of 8.5–9 dB, 3.5–5 dB, and 4.75–5.75 dB for GSM 850–900, GSM 1800–1900, and Wi-Fi frequency band ranges. Even though the SE produced by this composite is not extremely high, authors believe that it can be applied as a plaster to existing indoor walls or can be made into tile form that can be used to shield homes against external EMIs. While the addition of magnetic particles is beneficial for the SE of the cementitious composites, they also tend to increase the weight of the composite due to their high densities. Hence, careful control of the particle volume in the composite mix is needed to keep the overall density of the composite at a desirable level. To

Fig. 15. (a) Effect on SE of cementitious composites containing different forms of Fe3O4 [89], (b) The shielding effectiveness of the cementitious composite specimens with various EAFS aggregates ratios [90], and (c) Reflectivity of composite specimens containing gravel, haydite, and functional aggregates [91].

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

5 wt% nano-Fe2O3 fluid [89] Nano TiO2/clay + 30 wt% MnFe2O4 [91] -5

60 vol% EPS (20 mm) [93]

-10 Reflection loss (dB)

overcome this drawback, Guan et al. [93] have used expanded polystyrene (EPS) beads to make a lightweight cementitious composite for EMI shielding. EPS have been selected for this experiment due to their low density, high specific strength, and low water absorption properties. Previous literature show that cementitious composites containing EPS have already been fabricated to be used as lightweight concrete or thermally insulating concrete. Since EPS is light in weight, they tend to float on top of the cement mix. To prevent this, authors have pretreated the EPS beads with acetone and then rinsed with a polyvinyl alcohol solution to make them hydrophilic. EPS beads with 1 mm and 3 mm diameters have been used in this experiment. Different composite mixes have been created by adding 40 vol%, 50 vol%, and 60 vol% of EPS beads from both sizes. Each of these mixes then has been mould cast to specimens with thicknesses of 10 mm, 20 mm, and 30 mm. Fabricated specimens have been tested for their SE in the arched chamber testing method within frequency ranges of 8–12 GHz and 12– 18 GHz. The addition of EPS beads has drastically reduced the density of the cementitious composite with the larger EPS bead mixed specimens having the lower densities. The composite containing 60 vol% of EPS beads with the diameter of 1 mm had shown the best the SE when it was mould cast to a thickness of 20 mm as shown in Fig. 19(a). This composite is reported to have a reflection loss of 8.17 dB to 15.27 dB within 8–18 GHz frequency range. Although EPS does not possess magnetic properties to absorb EMWs, they can scatter the waves when they fall on to the cement coated bead surfaces, generating a shielding effect. Although the SE provided by the EPS bead mixed cementitious composite is not extremely high, authors believe there could be potential future applications to it due to its low density. Summary of SE in particle filler added cementitious composites is provided in Table 5. Comparison of reflection loss values of three composites consisting of TiO2/clay/MnZnFe2O4, EPS, and nano-Fe2O3 fluids is shown in Fig. 16. Both composites consisting of EPS and a mix of TiO2/clay/MnZnFe2O4 have shown similar SE characteristics with the first composite showing reflection loss about 8 to 14 dB while the second showing a reflection loss of about 10 dB throughout the tested frequency range. While the addition of EPS has reduced the density of the composite, it has not contributed to the SE greatly. The reflection loss of the composite with TiO2/clay/MnZnFe2O4 also has not proven to have high SE. On the other hand, the composite with nano-Fe2O3 fluids has shown better SE with the reflection loss varying between 8 to 35 dB, with a large reflection peak at 17 GHz. Unfortunately, authors have not provided reasons for the high reflection peak at this frequency but state the higher reflection loss is mainly due to the better dispersion of nano-Fe2O3 fluids within the composite. The SE variation of two cementitious composite mixes containing 40% and 100% EAFS replacing sand is shown in Fig. 17. From the plots, it can be seen that the SE of the cementitious composite increases with the EAFS content. The composite with 100% EAFS

-15 -20 -25 -30 -35 8

9

10

11

12 13 14 Frequency (GHz)

15

16

17

18

Fig. 16. Comparison of reflection loss of three particle based composites.

100% EAFS/sand [90] 40% EAFS/sand [90]

100

80

60 SE (dB)

16

40

20

0 4

6

8

10 12 14 Frequency (GHz)

16

18

Fig. 17. SE variation EAFS based composites.

has shown the best SE with its SE varying between 8 and 88 dB. However, authors have chosen the mix with 40% EAFS as the better mix since the composite with 100% EAFS has shown lower compressive strength than industry requirements. The SE shown by the composite with 40% EAFS has been about 2–30 dB. Since EAFS is known to contain magnetic ferrite, it can contribute to the SE of the cementitious composite. Since the SE produced by EAFS alone is insufficient, there is a possibility for future experiments where EAFS can be combined with fibre fillers to achieve higher SE.

Table 5 SE of particle-based cementitious composites. No.

Shielding material

Frequency

Specimens thickness

Effect of shielding

References

1

Nano-Fe3O4 magnetic fluid

8–18 GHz

20 mm

[89]

2 3 4

Electric arc furnace slag TiO2 MnZnFe2O4/MnO4

[90] [91] [92]

5

Expanded Polystyrene (EPS)

3–18 GHz 8–18 GHz 824–894 MHz (GSM850); 890–960 MHz (GSM900)/1.71–1.88 GHz (GSM1800); 1.86–1.99 GHz (GSM1900)/2.4–2.484 GHz (WiFi) 8–18 GHz

10 dB (9.5 GHz) and < 15 dB (6.3 GHz) 15 dB – 20 dB (40% of filler) ~ 7.5 dB 8–9 dB/~4 dB/5.5 dB

8.17 dB to 15.27 dB

[93]

10 mm 20 mm

Various thicknesses

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

2.7. Hybrid composites Addition of metals into the cement-based composites has been an attractive method to boost the electrical conductivity, SE, and strength of the composites. Metal powders do not provide a huge advantage in creating a good EMI shielding composite because of their high density. Fibres, on the other hand, can be used effectively since they can create an excellent conducting network within the composite [70]. Different metal fibres have been added to cementitious mixes to enhance the SE and steel is one of the most attractive options because of its high strength, good conductivity, and low cost [94,95]. One of the earliest experiments on metal filler mixed cementitious composite had been conducted by Shi and Chung [96]. In this experiment, standard paperclips have been added to the composite mix in order to fabricate a cementitious composite with magnetic shielding properties. Zinc plated steel paperclips with a diameter of 0.079 cm, a length of 3.18 cm, and a width of 0.64 cm have been used in this research. Two different mixes have been produced by adding 3 vol% and 5 vol% of paperclips along with a control mix. Magnetic shielding properties of mould cast specimens have been tested by using a solenoid on one side of the specimen while a detector on the other side measured the magnetic field passed through the specimen. Authors report that the addition of paperclips did not affect the compressive strength of the specimens. However, paperclips have had a dramatic improvement on the magnetic shielding properties. Authors report that the specimens containing 5 vol% of paperclips were able to produce a shielding effect similar to that of a cementitious composite containing a steel mesh. Authors believe the high shielding properties of the cementitious composites containing discontinued paperclips is due to the intertwining tendency of the paperclips that aids in the enhancement of the electrical conductivity of the composite. Ogunsola et al. [97] have simulated the SE of steel fibre mixed cementitious composite assuming the composite is a heterogeneous mixture of cement, sand, aggregates, water, air, and steel fibres. Furthermore, steel fibres are assumed to be cylindrical in shape, identical in size, and uniformly distributed within the specimen. Each steel fibres is calculated to have a diameter of 0.5 mm and a length of 30 mm. The electromagnetic pulse has been calculated as a uniform plane wave Gaussian pulse. The simulation that has been carried out for a frequency range of 0–4 GHz has shown the SE of the composite increases with the addition of the steel fibres and composite with a thickness of 30 mm can have a SE of about 7–9 dB within the simulated frequency range. Unfortunately, authors have not conducted actual tests to verify the accuracy of the simulations. Since the addition of steel fibres is known to enhance the conductivity of cementitious composites, Yehia et al. [98] have used two different types of steel fibres to investigate their effect on conductivity and SE. One type of steel fibres has been straight while the other type was not. Both fibre types have been randomly distributed within the composite mix. Fly ash has been added to the composite mixes to boost the SE. Unfortunately, authors have not mentioned the amount of fly ash added to each composite mix. For the comparison purpose specimens containing no steel fibres and specimens containing steel fibres with a steel mesh have been fabricated and tested under the same conditions. The SE for mould cast specimens has been tested by using the open field method within the frequency range of 0.3–11 GHz. Compression tests conducted on the specimens have shown that there is no significant change in the compressive strength due to the addition of steel fibres. The conductivity and the SE of the specimens have seen a dramatic improvement by the addition of the steel fibres. The type of steel fibres has not affected the SE or the conductivity of the specimens. Specimens have been able to produce a SE up to

17

50 dB within the tested frequency range. Comparison with the composite consisting of a steel mesh has shown no change in the SE. Authors believe that because of the high SE, this composite has shown it has a vast potential to be used in EMI shielding applications in the future. Khalid et al. [99] have tried to develop a steel mixed cementitious composite to replace the existing carbon laced polyurethane composite as the EMW absorbing material used in anechoic chambers. The existing polymeric composite is known to be an effective EMW absorber; hence, it is the preferred material inside the anechoic chamber since no EMWs should leak out of the room. However, due to the high cost of the polymeric composite, the fabrication cost of the anechoic chamber is also high. Author’s primary objective in this research has been to come up with a costeffective material that would perform equal or better than the existing polymeric composite. Cementitious composite mixes consisting of steel fibres with different aspect ratios, petroleum coke (20 vol%) with different particle sizes, and synthetic graphite powder (2 vol%) have been fabricated in this experiment to find the optimum mix. Specimens have been mould cast into flat and pyramidal shapes for testing. SE tests have been carried out by using the open field technique for the frequency range of 1–5.5 GHz. Existing polymer composite has a SE of 50 dB for this frequency range. The newly tested pyramidal shaped cementitious composite has shown a SE of 65 dB for the same frequency range. Comparison of EMI SE of flat and pyramidal shaped specimens is shown in Fig. 19(b). Cost analysis conducted by the authors has revealed that the cementitious composite is lower in cost compared to the existing polymer composite. However, it is still expensive than cementitious composites used in construction applications. Expensive fillers added to enhance the SE of the cementitious composite has been the main reason for the increase of the cost. However, authors believe that the cost of these fillers would reduce in the future due to improved manufacturing processes, thus reducing the cost of the cementitious composite. While many of the metal filler incorporated cementitious composites have used steel fibres to impart high electrical conductivity, Yao et al. [100] have evaluated the SE of cementitious composites with nickel (Ni) fibres. Additionally, authors have analysed the effect of different dispersing agents on the electrical conductivity of the composites as well. While steel is lower in cost, readily available, and increase the SE and strength of the composite, authors claim the addition of Ni fibres can have higher electrical conductivity within the composite. Ni fibres with an average diameter of 8 lm and an average length of 6 mm have been used in this research. Three different dispersing agents, namely Methylcellulose (MC), hydroxyethyl cellulose (HEC), and sodium carboxymethylcellulose (CMC), have been used to disperse the Ni fibres within the composites. Different mixes have been synthesised by adding 1 vol%, 3 vol%, 5 vol%, 7 vol%, and 9 vol% of Ni. The electrical conductivity has been measured using the fourprobe technique while the SE has been measured using the coaxial transmission line method for 1–1500 MHz frequency range. The electrical conductivity is shown in Fig. 19(c) has increased up to 0.4 wt% of Ni and then reduced with the increase of Ni content. Authors believe the poor dispersion of the Ni fibres is the cause for the reduction of the electrical conductivity as none of the dispersant work well when the Ni content becomes significantly high. SE showed in Fig. 19(d), has increased with the increase of the Ni content, which signifies that the EMI shielding mechanism has taken place due to both reflection and absorption of the EMWs. From the electrical conductivity results, authors conclude that MC is the best dispersant that can be used for Ni fibres in a cementitious matrix. Even though the SE produced by these composites in not extremely high, authors believe these composites can be improved further to have better SE. However, authors have

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D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

Table 6 SE summary of hybrid cementitious composites. No.

Shielding material

Frequency

Specimens thickness

Effect of shielding

Reference

1 2 3 4 5 6

Steel fibres Steel fiber/mesh Fly ash + petroleum coke + synthetic graphite + steel fibers Nickel fiber Steel fibres, carbon powder, and taconite Steel fibres, carbon powder, and taconite + wire mesh structures

0.5–4 GHz 0.3–11 GHz 1–3 GHz 1 MHz–1500 MHz 10 kHz–1 MHz 10 kHz–1 MHz

30 cm 0.500 , 100 , and 200 2.5 cm 6 mm 12 in. 3.5 ft

7–9 dB ~50 dB for both ~65 dB 19.85–24.48 dB >80 dB 40–120 dB

[97] [98] [99] [100] [101] [103]

not measured the variation of the mechanical properties of specimens with the Ni content. In one of the most recent advances in creating EMI shielding cementitious composites, Krause et al. [101] have synthesised a mix containing steel fibres, carbon powder, and taconite which is a mineral rock containing iron. Even though originally, this mix has been developed to be used for deicing of pavements, it has been investigated for SE due to its high electrical conductivity [102]. EMI shielding testing of the specimens has been carried out according to the requirements described in MIL-STD-188125-1 standard [46]. The same mix has been used to create a large cube-shaped structure with a steel mesh and tested for SE as well. The results of the cube structure have shown that it can have a SE of 40–120 dB in the frequency range of 10 kHz–1 MHz which conforms with MIL-STD-188-125-1 standard requirements as shown in Fig. 19(e) [103]. This cementitious composite mix has shown superior shielding qualities to that of other mixes developed so far. SE summary of cementitious composites containing metal fillers is provided in Table 6. Variation of many of the composites analysed in this section is provided in Fig. 18. It is evident from this analysis the addition of Ni fibres to cement mix could not generate a high enough EMI shielding as the SE produced by this composite has been about 20 dB. Creation of a conducting network within the composite can increase its SE and has been demonstrated when steel fibres have been mixed with petroleum coke and graphite powder. This composite has been able to produce a SE of about 50–80 dB within the tested frequency range. While this is a high value of SE, even higher SE has been achieved when steel fibres, carbon powder, and taconite is added to the cement mix. The SE produced by this composite has been about 40–150 dB. The main reason for the generation of such a high SE from this composite has been due to the reflection and absorption of EMWs by various fillers within the

Steel fiber + petroleum coke + graphite powder [99] 9 vol% Ni fibers [100] Steel fibers + carbon powder + taconite [103] MIL-STD

140 120

SE (dB)

100 80 60 40 20 0 10-5

10-4

10-3 10-2 Frequency (GHz)

10-1

100

Fig. 18. SE variation of hybrid cementitious composite.

composite. When this mix is cast with a wire mesh, the entire structure has been able to exceed the shielding requirements stated in MIL-STD-188-125-1. The composite has shown a slight drop in its SE at about 25 GHz, which has not been explained by the authors. Even so, this is the only cementitious composite mix that is known to have higher SE than the specified values in MIL-STD188-125-1. For future work, this mix can be used with further developments to achieve even higher SE without the inclusion of steel wire mesh. 2.8. SE of common construction materials Apart from cementitious composite mixes discussed in above sections, there has been a considerable amount of literature published on the EMI SE of other construction materials as well. In one such research Büyüköztürk et al. [104] have calculated the complex permittivity of most commonly used construction materials by using transmission coefficient and time difference of arrival (TDOA) information in free-space measurement method. Values obtained from this mathematical model have been verified by experimentally derived values from the open field measurement technique. The results have shown a good correlation between the theoretical and experimental values. The experiment has revealed that materials used in this study, which are Teflon, Lexan, Bakelite, and Portland cement concrete have SEs of 2.28 dB, 3.74 dB, 7.25 dB, and 5.77 dB respectively within 8–18 GHz frequency range. Although authors believe this technique can be used as an in-situ method for the measurement of the permittivity and the SE of materials, further testing would be required to assess the accuracy of the method. With the increased demand for faster communication methods, the use of higher frequency EMWs is on the rise since higher frequency EMWs are good at faster data transfer. Choi et al. [105] have measured the SE of conventional construction materials when they are subjected to millimetre wave frequencies. Glass, tile, plasterboard, particleboard, marble, wood, and concrete have been tested in this experiment. The SE testing has been carried out by using the open field measurement technique in the frequency range of 13–28 GHz. Authors have opted to test these materials in such high-frequency band as they believe with the development of technology such as high-frequency EMWs would be used in the field of communication. The experiment has evaluated the behaviour of each material when they are subjected to EMWs from different incident angles. Results from this experiment have shown that for the reflection loss, there is no change due to the incident angle. However, the amount of SE changes with the incident angle during the transmission loss. While glass has shown the poorest transmission and reflection losses, wood has shown the best performance for the reflection loss while concrete for transmission loss. Authors believe that a more comprehensive study on materials and atmospheric conditions are necessary since millimetre wave frequencies tend to get affected by weather conditions. High-frequency EMWs, such as terahertz waves can be utilised for indoor communication purposes since the attenuation due to the atmospheric condition can be minimised within indoor envi-

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

19

Fig. 19. (a) Effect of thickness on EMW reflection loss 4#: 60 vol% EPS (20 mm), 7#: 60 vol% EPS (30 mm), 8#: 60 vol% EPS (10 mm) [93], (b) Comparison of EMI SE of flat (CC25) and pyramidal (CC-P) shaped cementitious composite specimens [99], (c) Electrical conductivity and (d) EMI SE of Ni fibre added cementitious composites [100], and (e) EMI SE of the cementitious composite mix with a wire mesh which was developed by Nguyen et al. [103].

ronments. Because of this reason, Kokkoniemi et al. [106] have investigated the possible interactions of EMWs in the terahertz frequency range with construction materials found in homes. Aluminium, glass, plastic, hardboard, and concrete have been tested within 100 GHz to 4 THz in this research. The reflectivity of the EMWs from each material has been measured with the change of the incident beam. Results obtained have shown that reflectivity of terahertz EMWs increases with the smoothness of the material surface, and as a result, glass and plastic materials have shown the best reflectivity. Another critical observation made by authors is that with the increase of the frequency, the scattering of the sig-

nal becomes high, which makes it difficult to distinguish lowintensity EMWs from the noise. However, authors claim that the tested materials have enough reflectivity of terahertz EMWs to be successfully used in indoor applications. Summary of findings of this research as well as two previous literature in this section is summarised in Table 7. All the literature on EMI shielding cementitious composites have shown that without adding high conductive and wave absorbing additives, it is challenging to achieve high SE values required by industrial standards. Many of the additives used to enhance the SE of the cementitious composites have shown that

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D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

Table 7 Summary of SE of common constructional materials. No.

Material

Frequency

Specimens thickness

1

Teflon/Lexan/Bakelite/ GFRP/concrete Glass/Tile/Plasterboard/ Particleboard/Marble/Wood/ Concrete Aluminium/glass/plastic/ hardboard/concrete

8–18 GHz

6 mm/6 mm/6 mm/1.5 mm/50 mm

2

3

13 GHz– 28 GHz 100 GHz– 4 THz

2

2

2

600  610 mm /600  400 mm /400  400 mm / 1300  600 mm2/400  248 mm2/900  840 mm2/ 625  385 mm2 –

they can have a direct influence on the mechanical properties as well. Most of the experiments on EMI shielding cementitious composite fabrication have focused on creating an excellent conductive network within the composite mix, which has proven to be an effective method of increasing the overall SE. However, it has been shown from many experimental works that the SE of these composite mixes not only depend on the type of additive but also the thickness of the composite and the frequency of the EMWs. 3. Summary Analyses carried out in this paper shows that the SE of the cementitious composite can be improved with the addition of conductive fillers. However, the cost of high conductive fillers limits the amount of these fillers that can be mixed into the composite. Hence, the SE achieved with those composites is insufficient to meet the minimum requirements defined in standards. Reflection and absorbance tests conducted on many specimens show that a composite with an excellent conducting network can reflect EMWs

Effect of shielding

References

2.28 dB/3.74 dB/7.25 dB/5.77 dB

[104]

4 dB/8.5 dB/11.5 dB/10.5 dB/7 dB/ 17 dB/5 dB

[105]

Strong specular reflection in lower frequencies, but high scattering as frequency increases

[106]

and increase the SE. Many fibrous fillers are known to create good conductive networks within the composite while particle fillers are known to absorb and reflect EMWs. The combination of fibres and fillers are known to create an excellent conductive network and also to increase the attenuation of EMWs within the composite. However, to achieve this property, the fillers need to be dispersed well within the composite. For the proper dispersion of fillers within the composite, it is essential to use good dispersing agents. Since the composite might contain more than one type of filler, it would require careful experiments to determine the best dispersion medium. Properties of the cementitious composite with the best shielding properties that have been discussed in each section in this review have been summarised in Table 8. The distribution of SE in these composites with frequency is shown in Fig. 20. It is crucial for the cementitious composite designed for EMI shielding to have high mechanical properties since many of these composites will be used in load-bearing applications. Many of the cementitious composite containing fibres have shown that the addition of fibres helps to enhance the mechanical properties

Table 8 Summary of cementitious composites with the best shielding properties in each category. Primary filler

Secondary fillers

SE

Frequency range

Thickness

References

GO particles Carbon fiber MWCNT Nano-Fe3O4 magnetic fluid Steel fibers

Straight brass-coated steel fiber – (Three layered structure with concrete middle layer) – Carbon powder, taconite, and wire mesh structures

25–50 dB 20–40 dB 60–80 dB 5 to 35 dB (Reflection loss) 40–120 dB

0.8–7.8 GHz 8–18 GHz 1.7–2.6 GHz 8–18 GHz 10 kHz–1 MHz

2.5 m 30 mm 50 mm 20 mm 3 ft

[77] [79] [87] [89] [103]

Fig. 20. Distribution of SE of some of the best mixes analysed in this paper.

D. Wanasinghe et al. / Construction and Building Materials 231 (2020) 117116

of the cementitious composite. However, many carbon particles added to cementitious mixes show that high percentages of carbon particles lead to brittle composites. Therefore, when combining multiple fillers to achieve high SE, it is imperative to measure how each filler contributes to mechanical properties. From the graphical representation of SE distribution given in Fig. 20, it is clear that many of the mixes fall well below the required SE value. While some mixes have performed well in frequencies above 1 GHz, many of the mixes have not been able to achieve the requirement set for lower frequencies. Within 10 MHz–1 GHz, the composites need to have a SE above 80 dB. However, unfortunately, none of the mixes that have been tested within this frequency range has been able to produce such high SE values. Only one mix consisting of steel fibres, carbon powder, and taconite cast around a wire mesh structure has been able to produce higher than the minimum required SE within 1 kHz–10 MHz frequency range. Another mix containing CNTs has also shown promising results in the same frequency range. For a composite to meet the minimum shielding requirements specified by the MIL-STD-188-125-1 standard, it is essential to maintain an average SE above 80 dB within 1 kHz–1 GHz frequency range. So far, there has been no cementitious composite mix that has been able to meet this requirement. However, the information gathered from these mixes could be used in future research to fabricate cementitious composites that would exceed the expected requirements. 4. Conclusions Due to the rapid advancement of the electronic industry and shortcomings of metallic shielding materials, the need for novel EMI shielding materials is on the rise. To address this growing demand, many types of research have been conducted to find suitable alternatives. This paper has analysed various cementitious composites that have been developed to replace existing shielding materials. The analysis of different types of concrete that are being used in the industry currently shows inadequate shielding properties. However, high strength concrete show much better EMI shielding properties compared with other types. Additionally, the inclusion of steel reinforcement shows an increase in the EMI SE of the concrete. Many of these novel composites have been aimed at increasing the electrical conductivity of the composite by incorporating high conductive fillers. Nano/microfibers and nano/microparticles are some of the most commonly added fillers to cementitious mixes to increase their EMI shielding properties. From the analysis of powder-based cementitious composites, it could be seen that the GO-based composites have much higher shielding capabilities that composites containing other forms of particles. The high conductivity of the GO and the high surface area of small particles help to improve the SE by improving the overall electrical conductivity and the multiple reflections of EMWs within the composite. Out of other forms of particles, magnetic nano-Fe3O4 fluid based composite has shown superior SE due to better dispersion of the particles within the composite. However, the SE of this magnetic fluid incorporated composite is still below the SE of GO-based composites. Analysis of the fibre-based cementitious composites shows that the increase of the SE of the composite is attributed to the overall conductivity of the composite. Hence, composites containing high conductive fibres show good SE. However, most of the high conductive fibres such as carbon fibres and MWCNTs are extremely expensive and increase the overall cost of the composite. There has been no cost-benefit analysis conducted on these composites to-date. Experimental results have shown that the addition of one filler to enhance the SE of cementitious composites is insufficient and would require additional fillers. The composite containing steel

21

fibres, carbon powder, and taconite has been able to generate high SE compared with all the other composites reviewed in this work. However, even this composite has shown better SE than the standard required only when it is cast around a steel mesh. The review of all these novel materials shows that only a handful of them can achieve adequate SE levels as defined in standards. Moreover, to achieve the required SE, many of these new materials require a high-volume fraction of fillers, making them expensive. This leaves room for further research into the development of new materials and processes that can eventually lead to EMI shielding cementitious composite that is cost effective and has adequate shielding properties. Declaration of Competing Interest All authors declare that have no conflict of interest. Acknowledgment The authors would like to acknowledge the support of the Australian Research Council Discovery Project (Grant No. 608DP180104035). References [1] P. Nikita, V. Kevin, H. Mateo, Electromagnetic Radiation – Chemistry LibreTexts, Chem. Libr. (2015) 1 (accessed September 12, 2018) https:// chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_ Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_ Chemistry)/Spectroscopy/Fundamentals_of_Spectroscopy/Electromagnetic_ Radiation. [2] L.V. Keldysh, Ionization in the field of a strong electromagnetic wave, Sov. Phys. JETP 20 (1965) 1307–1314. [3] A. Ishimaru, Electromagnetic Wave Propagation, Radiation, and Scattering: from Fundamentals to Applications, John Wiley & Sons, 2017. [4] G. Goubau, F. Schwering, On the guided propagation of electromagnetic wave beams, IRE Trans. Antennas Propag. 9 (1961) 248–256. [5] H. Kim, B.-W. Min, A study on EMI generation from a capacitive touch screen panel, (2017) 344–346. doi: 10.1109/APEMC.2017.7975501. [6] H.W. Markstein, Shielding electronics from EMI/RFI, Electron. Packag. Prod. (1991) 31. https://www.engineeringvillage.com/share/document.url?mid= inspec_base903890503&database=ins. [7] J.P. Hertel, I.D. Flintoft, S.J. Porter, A.C. Marvin, Measurement of EMI on network cables due to multiple GSM phones, IEEE Trans. Electromagn. Compat. 42 (2000), https://doi.org/10.1109/15.902305. [8] A.K. Singh, A. Shishkin, T. Koppel, N. Gupta, A review of porous lightweight composite materials for electromagnetic interference shielding, Compos. Part B Eng. 149 (2018) 188–197, https://doi.org/10.1016/ j.compositesb.2018.05.027. [9] S. Chen, T.W. Nehl, J.-. Lai, X. Huang, E. Pepa, R. De Doncker, I. Voss, Towards EMI prediction of a PM motor drive for automotive applications, in: Eighteenth Annu. IEEE Appl. Power Electron. Conf. Expo. 2003. APEC ’03, 2003: pp. 14–22 vol.1. doi: 10.1109/APEC.2003.1179170. [10] V. Kraz, A. Wallash, The effects of EMI from cell phones on GMR magnetic recording heads and test equipment, Electr. Overstress/Electrostatic Disch. Symp. Proc. 2000 (IEEE Cat. No.00TH8476). (2000). doi: 10.1109/ EOSESD.2000.890050. [11] L. Palisek, L. Suchy, High power microwave effects on computer networks, in: EMC Eur. 2011 York, 2011: pp. 18–21. [12] W. Radasky, E. Savage, Intentional electromagnetic interference (IEMI) and its impact on the US power grid, Meta 1–3 (2010). [13] J. Dr. John S. Foster, M.E. Gjelde, D.W.R.G. (Chairman), D.R.J. Hermann, M.H. (Hank) M. Kluepfel, U. (Ret. . Gen Richard L. Lawson, D.G.K. Soper, J. Dr. Lowell L. Wood, D.J.B. Woodard, Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, 2008. http:// www.empcommission.org/docs/A2473-EMP_Commission-7MB.pdf. [14] Y. Kawamura, T. Hikage, T. Nojima, K. Fukui, H. Fujimoto, T. Toyoshima, Experimental estimation of EMI from electronic article surveillance on implantable cardiac pacemakers and implantable cardioverter defibrillators: interference distance and clinical estimation, Trans. Japanese Soc. Med. Biol. Eng. 50 (2012) 289–298. https://www.engineeringvillage.com/ share/document.url?mid=cpx_522190cd148d1d1a31cM7ce710178163125& database=cpx. [15] T. Nojima, Y. Tarusawa, A new EMI test method for electronic medical devices exposed to mobile radio wave, Trans. Inst. Electron. Inf. Commun. Eng. B (2001) J84-B. https://www.engineeringvillage.com/share/document.url? mid=inspec_base906877742&database=ins.

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