Laboratory Tests of Foam Concrete Slabs Reinforced with Composite Grid

Laboratory Tests of Foam Concrete Slabs Reinforced with Composite Grid

Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 193 (2017) 337 – 344 International Conference on Analytical Models and ...

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Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 193 (2017) 337 – 344

International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures AMCM’2017

Laboratory tests of foam concrete slabs reinforced with composite grid Jacek Hulimkaa, Rafaá KrzywoĔa*, Agnieszka JĊdrzejewskaa a

Silesian University of Technology, Department of Structural Engineering, Akademicka 5, Gliwice 44-100, Poland

Abstract Nowadays foam concrete is commonly used as replacement of compacted soil to fill excavations, underground channels. There are also attempts to structural use of this material; among the examples are precast walls and slab foundations. The high degree of thermal insulation makes foam concrete a perfect material for use in the passive houses design. Unfortunately, foam concrete cannot be reinforced as easily as traditional concrete. Because of low resistance to concentrated stresses it is difficult to ensure sufficient bond of reinforcement. The idea of solid foam concrete slab presented in the paper incorporates bi-directional composite reinforcing mesh placed in tensile zone of the slab where transverse fibers are ensuring anchorage for fibers in main direction. This method of reinforcement, apart from the increase of bearing capacity, should reduce the risk of uncontrolled failure due to cracking. The paper describes the results of preliminary laboratory tests of nine slabs made of foam concrete reinforced with meshes made of carbon fibers. The results obtained at the initial stage show great potential of this method of reinforcement. There was a significant increase in capacity and less brittle failure mode. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Peer-review responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures. Concepts inunder

Concrete and Masonry Structures

Keywords: foam concrete, floor slab; composite reinforcement; carbon fibre grid

* Corresponding author. Tel.: +48-32-2372262; fax: +48-32-2372288. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures

doi:10.1016/j.proeng.2017.06.222

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1. Introduction Foamed concrete is a type of lightweight cellular concrete in which low density is achieved by increased volume of pores in the microstructure of the material obtained by introduction of technological foam. This type of concrete is also cement-based but it contains lightweight fine aggregates to further decrease the density of the product. Depending on its composition and production technology, foamed concrete can be obtained in a wide range of densities, starting from <300 kg/m3 to even up to 1900 kg/m3. Density of foamed concrete is a measure used for its qualification because it influences most of its major properties. An extensive characterization of foamed concretes and state-of-the-art review of their properties was presented by Narayanan and Ramamurthy [1], Ramamurthy et al. [2] and most recently Mugahed Amran et al. [3]. From these overviews it can be easily concluded that the most important property of foamed concrete is its thermal conductivity which makes the material a good thermal insulator. In general, thermal insulation properties of foamed concrete increase with decreasing density (increased air pores volume). However, at the same time, the mechanical properties of concrete decrease. Compressive strength of foamed concrete can range from <1 MPa for density of <300 kg/m3 to ~20 MPa for densities approaching limit value of 2000 kg/m3. Given this trade-off between strength and insulation properties, foamed concretes with low densities are very popular in use. Their applications are mainly geotechnical, where the strength of some MPa and corresponding stiffness of the material are sufficient to use it as a replacement of soil. Among many others, the examples of application of foamed concrete as a subbase layer in road construction [4] or industrial concrete floors [5] can be referred. Combining with good thermal properties, foamed concrete subbase was proposed by the Authors to be used in construction of sandwich foundation slabs in energy-efficient residential buildings [6, 7]. Nevertheless, it appears that foamed concretes with higher densities, thus higher values of mechanical properties, can be competitive with ordinary concretes in structural applications, especially when fibre reinforcement is applied [3], yet still providing beneficial thermal insulation properties. In the last few years several proposals of structural applications were made independently worldwide. All of them are related to panel elements subjected to either axial loading (walls) or flexural loading (one-way slabs). The first concept, proposed by Mugahed Amran et al. [8, 9], is a sandwich panel made of two foamed concrete wythes separated with polystyrene filling. This concept originates from non-bearing cladding systems in which prefabricated panels are made of concrete wythes separated with insulation material. The structural (concrete) components of the panel are joint together with steel shear connectors. These connectors ensure integrity and composite action of the panel as well as ductility of the element. Foamed concrete of ~1800 kg/m3 density and ~25 MPa compressive strength was used. Behavior of the panel under axial [8] and flexural [9] loading was investigated. The results of the experiments showed that the behavior of the foamed concrete panels was similar to what would be expected from reinforced concrete solid walls and slabs. Desirable increase in ductility in comparison to RC was reported in slabs. In both cases, the importance of steel connectors, which should ensure composite action of the panels, was strongly emphasized. It was concluded that the proposed foamed concrete sandwich panels have a potential to be used as an alternative to ordinary concrete walls and slabs, and can be designed with analogical approaches given that composite action can be assumed. Another concept comprises sandwich panels made by filling thin-wall corrugated steel forms with foamed concrete. Such panels were investigated to be used as either wall panels [10] or slab panels [11]. Corrugated steel was used as formwork but its main role was to increase ductility of the panel and prevent its otherwise brittle failure. Foamed concrete used in wall panels by Othuman Mydin and Wang [10] was of relatively low density (1000 kg/m3) characterized with cylindrical compressive strength of only ~5 MPa. However, the test results showed there is composite work of steel faces and concrete core, and proved that such parameters of concrete are sufficient to provide capacity of the sandwich panel which enables it to be used as a wall panel in low-rise buildings. Similar lowdensity foamed concrete (targeted density of 1000 kg/m3 with compressive strength of ~5 MPa) was used by FloresJohnson and Li [11] in production of corrugated steel–foamed concrete slab panels. In this case the importance of the bond between steel and concrete was also emphasized because it ensures required composite action of the panel.

Jacek Hulimka et al. / Procedia Engineering 193 (2017) 337 – 344

The results of the before mentioned investigations are very promising. The literature overview shows that panels made of foamed concrete can be used in construction of low-rise buildings as an alternative to ordinary reinforced concrete where decreased self-weight and energy efficiency of the building are desired. However, it must be remembered that foamed concrete is still itself a very brittle material. Foamed concrete structural elements must comprise other materials which would act in a composite manner and enhance its ductility. The researchers strongly emphasize the challenges in ensuring this composite action. That is why there are also proposal of foamed concrete panel elements which relate to classical reinforced concrete. For example, Ikponmwosa et al. [12] investigated the use of solid foamed concrete slabs reinforced with bamboo shoots. Bamboo shoots were placed in the tensile zone in a way analogical to classical steel reinforcement. The problem was that the failure of slabs was brittle. Hence, the Authors of this paper proposed yet another concept of a slab. This concept involves a one-way solid floor slab made of foamed concrete reinforced in tensile zone with bi-directional carbon-fiber mesh. Investigation of its mechanical behavior was performed; the research was focused on determination of strength capacity, load–deflection relationship and ultimate load at failure along with a failure mode. A comparison was made to the behavior of ordinary concrete slabs with classical steel reinforcement. Preliminary laboratory tests were performed on nine slabs. The results obtained at the initial stage show great potential of this concept: there was a significant increase in capacity and less brittle failure mode. Moreover, application of bi-directional meshes allowed to marginalize the problem of concrete-to-reinforcement bond: fibers along main direction acted as main reinforcement while transverse fibers served as anchorage. Nomenclature fc,cube fc Ec

compressive cube strength of concrete compressive cylindrical strength of concrete secant modulus of elasticity

2. Outline of the experimental program The performed experimental program aimed to determine what effect composite mesh reinforcement has on the bending capacity of the foam concrete structural members. The research program contained 9 slab elements tested in three-point bending, including 3 reference slabs without any reinforcement, 3 slabs reinforced with basalt fiber grid and last 3 with carbon fiber grid. Investigations were completed by the necessary tests of material properties. 2.1. Test set-up All tested foam concrete specimens where made of the same concrete mixture. Applied mixture comprises Portland cement, a small amount of gravel, water, technical foam, fiber additive and superplasticizer. Volume weight of ready concrete was designed at 800kg/m3 and was individually controlled for each tested member. Concrete curing was performed in natural conditions, at temperature near 10°C. In the initial phase members were covered with styrofoam boards to reduce the loss of temperature and water. Figure 1 shows the scheme of tested specimens. To ensure the proper amount of reinforcement slab-type elements were chosen of section dimensions 120 × 350 mm, total length about 700 mm and span length 600 mm. Such configuration provides span-to-depth ratio equal to 5 which should effectively reduce the impact of the shear on the final results. As it is difficult to ensure proper bond of reinforcement due to brittleness of foamed concrete, specimens were reinforced with composite grid wherein the perpendicular fibers provide anchorage. Two types of grids were used: one based on relatively cheap basalt fibers and second one on more expensive, but also stronger carbon fibers. Table 1 shows comparison of the main properties of the applied meshes.

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Jacek Hulimka et al. / Procedia Engineering 193 (2017) 337 – 344 Table 1. Properties of reinforcing meshes.

Type of reinforcing fibers Grid geometry (longitudinal × transverse spacing) Supply form (roll width) Break load Elongation at break Tensile modulus of elasticity Other properties

Applications

BSC220.220.260.100 [13]

C-GRID®C50-2.36×2.36 [14]

Basalt continuous filament

Carbon fiber

25 mm × 25 mm

60 mm × 60 mm

1.0 m

1.2 m

>50 kN/m

>54.17 kN/m

2.5±1%

0.99%

86÷94 GPa

234.5 GPa

Non-corrosive, lightweight, outstanding mechanical bond with concrete

Resistance to chemically aggressive environment, dielectric, easy to install

Slabs on-grade, overlays, silos and concrete tanks, shotcrete, balconies, precast architectural concrete

Construction industry, reinforcement of mortars and non-load-bearing concrete

The properties of the hardened foam concrete were evaluated performing uniaxial compression tests at the same age as the main bending tests. Particularly the age was 54 days.

Fig. 1. Test set-up scheme.

According to standard PN EN 12390-3:2009 [15], compressive strength, fc,cube, was evaluated on six single cubes with side dimensions of 120 mm, while compressive cylindrical strength, fc, and secant modulus of elasticity, Ec, were estimated testing six single cylinders with a diameter of 150 mm and height of 300 mm. The mean results can be found in Table 2. Table 2. Hardened foam concrete mechanical properties. Age of test

fc [MPa]

fc,cube [MPa]

Ec [GPa]

54

1.68

1,87

1.65

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2.2. Test outline Laboratory tests were executed in three-point bending test shown in Figure 1. All slabs were loaded until failure monotonically with loading speed of 0.05 kN/s. Force was applied using a hydraulic press with automatic recording of the applied force. To determine deflections, test stand was equipped with linear displacement transducers at three points along the slab’s length (at supports and in the middle of the span). Electrical strain gauges were glued on the reinforcement and on upper and bottom surface of each tested specimen according to Fig. 1. Due to the homogeneous structure of foam concrete, relatively short gauges were used with a base length of 10 mm. Additionally, the test was recorded using high resolution camera to allow optical analysis of strains and displacements, especially crack propagation and width. 3. Test results The evaluation of the flexural behavior was made by recording failure load and analysis of deformations during the test. Table 3 shows the summary of the achieved results for all slabs. Table 3. Selected tests results. Specimen

reinforcement

Density of foamed concrete

Failure force

Failure moment

Deflection at failure

[kN]

[kNm]

[mm]

[kN/m3] FC_1 FC_2

no

7.78

0.953

0.157

0.98

7.93

1.196

0.193

1.13

FC_3

7.78

1.269

0.204

1.39

BC_1

7.63

9.196

1.393

10.25

BC_2

7.98

7.926

1.203

6.04

BC_3

8.09

9.238

1.399

8.31

CC_1

7.67

11.792

1.783

7.46

7.74

10.746

1.626

7.91

7.44

9.957

1.508

6.57

CC_2 CC_3

Basalt mesh

Carbon grid

3.1. Failure load Analysis of failure load shows directly the effectiveness of the applied reinforcement. As it is visible in Table 3, the highest bending capacity was achieved in slabs reinforced with carbon grid. Mean failure force was equal in this case 10.831 kN. A slightly smaller failure force (mean 8.786 kN) was obtained in specimens reinforced with basalt mesh. Both of these results are impressive in comparison to unreinforced elements for which the failure force did not exceed 1.269 kN. Reinforcing grids did not break during the test. Differences in bearing capacities of basalt-reinforced (BC) specimens and carbon-reinforced (CC) specimens could be explained by greater modulus of elasticity of carbon fibers. As a result of smaller rotation of cross-section, the strain and consequently also the stress in the compressed zone of foamed concrete were smaller. An interesting observation can be made from destruction images of un-reinforced and reinforced slabs shown in Fig. 2. Specimens without reinforcement failed in a conventional manner, after cracking in the cross-section of the largest bending moment (Fig. 2a). Slabs reinforced with basalt and carbon grids also initially cracked in the central zone, but finally they failed due to inclined shear crack with delamination and slippage along the surface of the reinforcing grid. It is shown in Fig. 3. The transverse fibers provided effective anchorage, but at extreme load they acted like a knife cutting sample through the plane of the grid (Fig. 3b).

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Fig. 2. Typical failure modes of tested specimens: a) no reinforcement (FC3), b) reinforcing mesh (BC3).

Fig. 3. Delamination and slippage along the surface of reinforcing mesh: a) side; b) front of the specimen.

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3.2. Deformations Figure 4 shows comparison of mid-span deflection curves for all tested specimens. What is also presented in Table 3, the failure deflection of specimens reinforced with basalt grid was greater than the one in slabs reinforced with carbon grid. This is understandable and is the result of more than twice higher value of the elastic modulus of carbon fibers. The scale of difference in flexural stiffness is interesting, which for CC models is almost two times greater than for BC models. It shows much greater impact of the reinforcement parameters on the deformability of specimens made of foam concrete in comparison to these made of ordinary concrete. 14

12 FC1

Loading force [kN]

10

FC2 FC3

8

BC1 BC2

6

BC3 4

CC1 CC2

2

CC3

0 0

2

4

6

8

10

12

14

16

deflection [mm]

Fig. 4. Comparison of deflections of tested specimens.

3.3. Other benefits of reinforcement One of the major drawbacks of the foam concrete is its large shrinkage. For this reason, the production of elements of large dimensions is associated with a high risk of cracking. The use of composite grid reinforcement effectively, though not completely, reduces the occurrence of shrinkage cracks. The least number of cracks was found on samples reinforced with carbon mesh which could be associated with its lowest deformability. What is important, the use of reinforcing mesh prevents brittle separation of cracked element and retains the ability to transfer tensile forces. 4. Conclusions There is no doubt that internal composite reinforcement has beneficial effect on foam concrete specimens under flexure. Mean failure load of elements reinforced with basalt grid is more than 7 times higher than in elements without composite reinforcement, while for carbon grids that factor reaches almost 9.

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Transverse grid fibers create effective anchorage for fibers in main direction. It is proved by failure which occurs as a result of delamination and slippage in the plane of the reinforcing mesh. The composite grid effectively prevents the growth of flexural cracks. Because of the greater modulus of elasticity, flexural stiffness of specimens reinforced with carbon grid is greater than the stiffness of specimens reinforced with basalt grid. The composite grid may be a remedy to the greatest disadvantage of foam concrete which is its susceptibility to shrinkage cracking. It not only limits the occurrence of such cracks, but also very effectively sews existing ones. Paper shows only preliminary studies of specimens made of relatively weak type of foam concrete. The study will be continued with the use of higher density foam concrete and other types of reinforcing grids, such as glass and cheap PP geogrids.

Acknowledgements Authors want to acknowledge AKCES BK Sp. z o.o., Czechowice Dziedzice, Poland for the technical assistance in production of samples.

References [1] N. Narayanan, K. Ramamurthy, Structure and properties of aerated concrete; a review, Cement Concrete Comp. 22 (2000) 321–329. [2] K. Ramamurthy K., E.K. Kunhanandan Nambiar, G. Indu Siva Ranjani, A classification of studies on properties of foam concrete, Cement Concrete Comp. 31 (2009) 388–396. [3] Y.H. Mugahed Amran, N. Farzadnia, A.A. Abang Ali, Properties and applications of foamed concrete; a review, Constr. Build. Mater. 101 (2015) 990–1005. [4] M. Decký, M. Drusaa, K. Zgútová, M. Blaško, M. Hájek, W. Scherfel, Foam concrete as new material in road constructions, Procedia Engineering 161 (2016) 428–433. [5] M. Kadela, M. Kozáowski, Foamed concrete layer as sub-structure of industrial concrete floor, Procedia Engineering 161 (2016) 468–476. [6] J. Hulimka, R. KrzywoĔ, A. Knoppik-Wróbel, Use of foamed concrete in the structure of passive house foundation slab, Proc. 7th International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures AMCM2011, Cracow 2011. [7] J. Hulimka, A. Knoppik-Wróbel, R. KrzywoĔ, R. Rudišin, Possibilities of the structural use of foamed concrete on the example of slab foundation, Proc. 9th Central European Congress on Concrete Engineering CCC2013, Wrocáaw 2013. [8] Y.H. Mugahed Amran, A.A. Abang Ali, R.S.M. Rashid, F. Hejazi, N. Azizi Safiee, Structural behavior of axially loaded precast foamed concrete sandwich panels, Constr. Build. Mater. 107 (2016) 307–320. [9] Y.H. Mugahed Amran, R.S.M. Rashid, F. Hejazi, N. Azizi Safiee, A.A. Abang Ali, Response of precast foamed concrete sandwich panels to flexural loading, Journal of Building Engineering 7 (2016) 143–158. [10] M.A. Othuman Mydin, Y.C. Wang, Structural performance of lightweight steel–foamed concrete–steel composite walling system under compression, Thin Wall Struct. 49 (2011) 66–76. [11] E.A. Flores-Johnson, Q.M. Li, Structural behaviour of composite sandwich panels with plain and fibre-reinforced foamed concrete cores and corrugated steel faces, Compos. Struct. 94 (2012) 1555–1563. [12] E. Ikponmwosa, C. Fapohunda, O. Kolajo, O. Eyo, Structural behaviour of bamboo-reinforced foamed concrete slab containing polyvinyl wastes (PW) as partial replacement of fine aggregate, Journal of King Saud University – Engineering Sciences (2015) IN PRESS. [13] Technical Data Sheet BSC220.220.260.100 FGMW0019, Technical Data Sheet, 4th January 2014, Incotelogy GmbH, Germany. [14] Technical Data Sheet C-Grid reinforced C50-2.36x2.36 Carbon Fiber Reinforcing Grids for Concrete Structures, March 2010, B&R Bulding Materials, Arendonk, Belgium. [15] PN EN 12390-3:2009 Testing hardened concrete. Compressive strength of test specimens.