EFFECT OF TENSILE AND SHEAR REINFORCEMENT ON THE FLEXURAL BEHAVIOUR OF REINFORCED GEOPOLYMER CONCRETE BEAMS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 5, May 2017, pp , Article ID: IJCIET_08_05_110 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFECT OF TENSILE AND SHEAR REINFORCEMENT ON THE FLEXURAL BEHAVIOUR OF REINFORCED GEOPOLYMER CONCRETE BEAMS Rashmi Gopalan, Parthiban Kathirvel School of Civil Engineering, SASTRA University, Thanjavur, Tamilnadu, India ABSTRACT Geopolymer concrete (GPC) is the best alternative greener material to the existing cement concrete which is most expensive, resource and energy consuming. Geopolymer technology not only has the potential to reduce global greenhouse gas (GHG) emissions caused by ordinary Portland cement (OPC) production, it also has exceptional mechanical and durability properties. Until in recent times, the comprehension of structural GPC was exceptionally inadequate; moreover there is no prescribed codal provision for mix proportioning of geopolymer concrete. With the intention of overcome the limited resource of structural behavior of GPC, this work was proposed to assess the effect of tension reinforcement percentage (1.0, 1.5 & 2.0) and shear span/depth (1.9 & 2.9) on the flexural behaviour of reinforced alkali activated slag concrete beams with ground granulated blast furnace slag (GGBFS) as source material. The properties such as load carrying capacity, flexural strength, moment carrying capacity, deflection and ductility behaviour of six RGPC beams were experimentally evaluated using two-point loading and the results were compared with the existing codal provisions (BS 8110, BIS 456, ACI 318). The results infer that the experimental value shows superior performance than the predicted values prescribed by the codes. Key words: Sustainable construction, geopolymer concrete, flexural behaviour, shearspan and reinforcement percentage. Cite this Article: Rashmi Gopalan and Parthiban Kathirvel, Effect of Tensile and Shear Reinforcement on the Flexural Behaviour of Reinforced Geopolymer Concrete Beams. International Journal of Civil Engineering and Technology, 8(5), 2017, pp INTRODUCTION The excess emission of green house gases (GHG) by the human activities into the atmosphere may leads to global warming, in which carbon dioxide (CO 2 ) is the major treat leads to 65% editor@iaeme.com

2 Rashmi Gopalan and Parthiban Kathirvel of global warming. The cement manufacturing industries are the major contributors of CO 2 emissions, which accounts for around 6 % of total CO 2 emissions. Moreover, in the production of ordinary Portland cement (OPC) equivalent amount of CO 2 is emitted into the atmosphere (McCaffrey 2002, Davidovits 1994) [1,2] and the emission rate may increase by 50 % of its present state. These ecological impacts paved the way for a sustainable concrete, where the utilization of PC is totally replaced with the industrial byproducts which when reacts with an activator solution results in a polymerization product. Because of the chemical reaction that takes place in the polymerization process, Davidovits coined the term Geopolymer to represent these binders and has got wide applications as traditional cementitious binders (Duxson et al. 2007) [3]. Geopolymers exhibits a very high mechanical properties and high resistance to chemical attack and thermal attack (Hardjito & Rangan 2005) [4]. A wide research took place in Alkali Activated Slag as a substitute binder for cement concrete (Parthiban & Saravana Raja Mohan 2017, Puertas et al. 2000, Brough & Atkinson 2002) [5-7]. Many research works have been conducted to evaluate the flexural behavior of reinforced GPC beams (Al-Otaibi 2008, Parthiban & Saravana Raja Mohan 2016) [8,9]. Comparative studies based on provisions of structural codes ACI 318, EC2 and BS 8110 (Tabsh 2013, Mari et al 2014, Vollum 2009) [10-12], provide insight into the various approaches to codified structural design in various countries and point to what extent one code differs or agrees with another code with regard to the level of accuracy, factor of safety, complexity and other details (Hawileh 2009) [13]. This paper evaluates the influence of tensile reinforcement percentage and shear span-to-depth ratio on the flexural behavior of reinforced GPC beams. A total of six beams were cast with percentage of tensile reinforcement as 1, 1.5 and 2 % with corresponding shear span-to-depth ratio of 2 and 3. The properties such as ultimate load and moment carrying capacity, load-deflection behaviour, stiffness degradation, ductility characteristics and failure pattern at different stages were studied. In addition, the experimental results were compared with the existing provisions of codes such as BIS 456, ACI 318 and BS MATERIALS AND METHODS 2.1. Materials Ground granulated blast furnace slag (GGBFS) derived from steel plant was utilized as geopolymer source material (GSM) in the fabrication of geopolymer concrete. Table 1 details the chemical composition of GGBFS utilized in the present work. Sodium hydroxide (NaOH) in the form of flakes (99 % purity) and sodium silicate (Na 2 SiO 3 ) in solution form with a silica modulus of 2.5 were used to activate the geopolymerization. The fine aggregate (FA) and coarse aggregate (CA) were equipped with ASTM C33/C33M [14] and their moisture condition was observed to be in saturated surface dry condition. Fine (natural river sand) and coarse aggregates (crushed granite type) with a specific gravity of 2.81 and 2.61 respectively, and fineness modulus of 3.39 and 6.97 respectively were used. Table 1 Chemical composition of the source materials Oxide CaO SiO 2 Al 2 O 3 MgO SO 3 Fe 2 O 3 Na 2 O K 2 O GGBFS (%) From the chemical composition presented in Table 1, the basicity coefficient of GGBFS used in the study has been computed as 0.98 (less than 1), which categorizes the GGBFS used as acidic and best suited as a starting material for alkali activated slag binder. In addition, the ratio CaO/SiO 2 is 1.18 (0.5 to 2.0), Al 2 O 3 /SiO 2 is 0.56 (0.1 to 0.6) [15] and hydration modulus is 2.04 (greater than 1.4) [16] which makes GGBFS best suited for binder editor@iaeme.com

3 Effect of Tensile and Shear Reinforcement on the Flexural Behaviour of Reinforced Geopolymer Concrete Beams A combination of 10 and 12 mm diameter steel bars were used as tension reinforcement and 8 mm diameter bars were used as top reinforcement with 6 mm diameter rectangular links at various spacing were used as shear reinforcement. The steel bars were tested under uni-axial tension and the obtained results are detailed in Table 2. Diameter (mm) Table 2 Reinforcing steel properties under axial tension f y (MPa) f u (MPa) f b (MPa) Elongation (%) Note: f y = yield strength; f u = ultimate strength; f b = bearing strength Mix Proportioning The GPC mixes were prepared with a liquid-binder ratio of 0.50, slag content of 400 kg/m 3, NaOH concentration of 14 M, as the increase in the NaOH concentration results in improved strength properties [17] and alkaline ratio (ratio of Na 2 SiO 3 solution to NaOH solution) of 2.0. The detailed proportioning of the mixes used in the study is given in Table 3. Slag Table 3 Mix proportioning of the GPC mixes (Quantity in kg/m 3 ) Quantity of Materials in kg/m 3 sand coarse aggregate NaOH solution Na 2 SiO 3 solution Compressive Strength in MPa BIS 516 / BS 8110 ASTM C Test Setup The workability of the mix was verified with the help of slump cone test following ASTM C143/C143M [18]. The compressive strength at the age of 28 days curing was ascertained with the help of 100 mm cubes as per BIS 516 [19] and 100 mm x 200 mm concrete cylinders as per ASTM C39/C39M [20]. The moisture absorption capacity was ascertained with the help of 100 mm size cubes as per ASTM C642 [21]. A 4-point flexure mechanism was utilized to study the flexural behaviour of reinforced geopolymer concrete beams as prescribed by ASTM C1161 [22]. The beams of size 1.20 m x 0.10 m x 0.15 m simply supported over an effective span of 1000 mm were used with varying reinforcement details. Three percentages of tensile reinforcement (p t ) of 1.0 %, 1.5 % & 2.0 % were used. Two legged vertical stirrups of 6 mm diameter at varying spacing were provided as shear reinforcement to have shear span-depth ratios of 1.9 and 2.9. The geometry of the beam specimens used in the work is shown in Figure editor@iaeme.com

4 Rashmi Gopalan and Parthiban Kathirvel 3. RESULTS AND DISCUSSIONS Figure 1 Reinforcement Details of the Tested Beams 3.1. Load Carrying Capacity Figure 2 shows the loads taken by each beams at various levels of failure. It was observed that the load carrying capacity of the beams increases with the increase in the percentage of tensile reinforcement as well as with the increase in the shear span-depth ratio. The first crack load was observed to be in the range of 29 to 35 % of their ultimate loads and no significant variation was observed with the variation in the reinforcement. The load at first crack was observed to slightly reduce with the increase in the percentage of tension reinforcement. The yield load was observed in the range of 79 to 93 % of its ultimate load and increases with the increase in the percentage of tension reinforcement. Similar trend in the load carrying capacity was observed by Sharifi and Maghsoudi [23] in cement concrete beams with varying percentage of tensile reinforcement. The ultimate load carrying capacities of all the tested beams were in the range of 64.2 kn to 82.4 kn and found to increase with the increase in the percentage of tensile reinforcement and shear span-depth ratio. Figure 2 Load carried by the beams at various levels 3.2. Deflection Behavior The load-deflection behaviour of all the tested beams is shown in Figure 3. It has been observed up to the yielding of the reinforcing steel, that the load-deflection curve shows a linear variation except for the case of S1-2, where the slipping of the load leads to nonlinearity in its behaviour. The linear variation was observed to be more prominent with the increase in the percentage of tensile reinforcement. There was a shift in the nature of linearity editor@iaeme.com

5 Effect of Tensile and Shear Reinforcement on the Flexural Behaviour of Reinforced Geopolymer Concrete Beams to polynomial variation after the yielding of the tensile reinforcement as the load increases. The deflection of all the tested beams was found to increase with the increase in the percentage of tensile reinforcement, thereby improving the ductile nature of the beam. Similar behaviour was observed with the increase in the shear span-depth ratio. The deflection of all the tested beams was found to be in the limiting range prescribed by the codal provisions. (a) a/d = 1.9 (b) a/d = 2.9 Figure 3 Load Deflection Pattern of the Reinforced GPC Beams Table 4 shows that comparison between the experimental deflection at midspan region at ultimate load carrying capacity and their corresponding deflection predicted as per BIS 456 [24], ACI 318 [25] and BS 8110 [26]. The experimental deflections were found to have a correlation with BIS 456 predicted values, whereas the experimental deflection values were found to be more compared with the ACI 318 and BS 8110 predicted values. The spandeflection ratio of the tested beams were found to be reduce with the increase in the percentage of tensile reinforcement and shear span-to-depth ratio and can be improved with the increase in the depth of the section [27]. Mix Reference Table 4 Deflection of the beams test results under ultimate moment Experimental Deflection, δ exp (mm) Theoretical Deflection, δ theo (mm) Deflection Ratio (δ exp /δ theo ) BIS 456 BS 8110 ACI 318 BIS 456 BS 8110 ACI 318 S S S S S S Ductility Behavior The ductility behaviour of the beams was assessed using the displacement ductility ratio as detailed in Table 5, which is the ratio between the displacements at ultimate load level to the load at which the steel yields, which is greatly influenced by the deformation of the beams. The ductile nature of the tested beams was found to decrease with the increase in the load as the load progress mainly owing to the rapid decline in the stiffness due to the flexural cracks editor@iaeme.com

6 Rashmi Gopalan and Parthiban Kathirvel formation. It was observed that the ductility ratio of the reinforced geopolymer concrete beams increases with the increase in the tensile reinforcement percentage as well as with the increase in the shear span-to-depth ratio. This infers improved ductility behaviour of the RGPC beams with the increase in the tensile reinforcement percentage and shear span-todepth ratio resulting in improved energy absorption characteristics of the beams without the increase in the critical failure. Mix Reference Table 5 Displacement ductility results of the tested beams Yield Stage Ultimate Stage Displacement Load (kn) Deflection Δ y (mm) Load (kn) Deflection Δ u (mm) Ductility Ratio, Δ u / Δ y S S S S S S Stiffness Degradation The stiffness degradation curves of the tested beams are shown in Figure 4. It was observed that the curves show a linear variation up to the first crack load and later on the curves shows a non-linear variation with the increase in the load. The stiffness values of the tested beams were observed to diminish with the increment in the load after reaching the yield state, which is mainly owing to the flexural hinge formation. Lesser stiffness values were observed with the increase in the tensile steel percentage and shear span-to-depth ratio. (a) a/d = 1.9 (b) a/d = 2.9 Figure 3 Stiffness Degradation Curves of the Tested Beams 3.5. Moment Carrying Capacity Table 6 details the comparison between the experimental moment carrying capacity of the tested GPC beams with their equivalent values predicted from the codes. It has been observed that the moment carrying capacity of the tested beams increases with the increase in the percentage of tensile reinforcement [23] and shear span-to-depth ratio, as it is directly influenced by the load carrying capacity of the beams. The moments arrived from the editor@iaeme.com

7 Effect of Tensile and Shear Reinforcement on the Flexural Behaviour of Reinforced Geopolymer Concrete Beams experiments were observed to be more compared to the predicted values of the codes. The capacity ratio, which is the ratio between the experimental and the theoretical moment carrying capacity of the beams are also been detailed in Table 6. The capacity ratio was found to reduce with the increase in the percentage of the tensile reinforcement and increases with the increase in the shear span-to-depth ratio. Mix Reference Experimental Moment, M exp (knm) Table 6 Moment carrying capacity of the beams Theoretical Moment, M theo (knm) Capacity Ratio (M exp /M theo ) BIS 456 BS 8110 ACI 318 BIS 456 BS 8110 ACI 318 S S S S S S Failure Mode and Cracking Pattern All the beams were subjected to diagonal tension failure and there is no shear-compression failure was examined which results in adequate shear reinforcement of GPC beams. The pattern and the initiation of the cracks were observed to be analogous for all the beams tested. A typical failure mode and crack pattern of a tested beam is shown in Figure 5. As expected, the first cracks were developed at the constant bending zone, the number and the depth increases with the increase in the load. At initial loading conditions, flexural cracks were initiated at the mid portion of the beam and steadily extended towards the support section at initial loading condition. Similar observation was made by Kragh-Poulsen et al. [28]. With the increase in the load, diagonal cracks were observed to develop from one of the support. When they stabilize, these cracks enlarged into a principal diagonal tension crack and extended towards the point of loading. The failure of the beams in this case occurred due to the splitting in the longitudinal compression zone near the point where the load is applied, and splitting along the horizontal tensile reinforcement near the beam end. No spalling of compression concrete was recorded after the beams reach the ultimate load. Figure 5 Typical crack pattern of a tested beam 4. CONCLUSIONS This paper details an experimental study on the effect of the percentage of tensile reinforcement and shear span-to-depth ratio on the flexural behaviour of reinforced editor@iaeme.com

8 Rashmi Gopalan and Parthiban Kathirvel geopolymer concrete beams. From the results obtained from the experiments conducted and the discussions, the following conclusions can be made: The ultimate load carried by the beams increases with the increase in the percentage of tensile reinforcement and shear span-to-depth ratios. The first crack load and the load at yielding were observed to be in the range of 29 to 35 % and 79 to 93 % of their ultimate loads. The deflection at ultimate load carrying capacity increases with the increase in the percentage of tensile reinforcement and shear span-to-depth ratio results in improved energy absorption capacity of the beams. The experimental deflections were found to be more than the theoretical deflections prescribed by the codes. The span-to-deflection ratios of all the tested beams were observed to be inside the permissible limits prescribed by the codes. The ductility behaviour improves with the increase in the percentage of tensile reinforcement and shear span-to-depth ratio. The ultimate moment carrying capacity of the beams were observed to be more than the corresponding values prescribed by the codes and found to increase with the reinforcement percentage and shear span-to-depth ratio. The failure of all the beams was found to be due to diagonal tension failure and no shearcompression failure, resulting in adequate shear reinforcement. ACKNOWLEDGEMENTS The authors acknowledge the Vice Chancellor, SASTRA University, Thanjavur, India for providing the facilities to carry out the work and the encouragement in completing this work. REFERENCES [1] McCaffrey, R. Climate change and the cement industry. Global Cement and Lime Magazine (Environmental Special Issue), 2002, pp [2] Davidovits, J. Global warming impact on the cement and aggregates industries. World Resource Review, 6(2), 1994, pp [3] Duxson, P., Fernandez-Jimenez, A., Provis, J. L., Lukey, G. C., Palomo, A. and van Deventer, J. S. J. Geopolymer technology: the current state of the art. Journal of Materials Science, 42(9), 2007, pp [4] Hardjito D, Rangan B V. Development and properties of low calcium fly ash based geopolymer concrete. Research report GC1, Curtin University of Technology, Perth, Australia, [5] Kathirvel Parthiban and Kaliyaperumal Saravana Raja Mohan. Influence of recycled concrete aggregates on the engineering and durability properties of alkali activated slag concrete. Construction and Building Materials, 133, 2017, pp [6] Puertas, F., Martnez-Ramirez, S., Alonso, S. and Vazquez, T. Alkali Activated Flyash/Slag Cements: Strength Behaviour and Hydration Products. Cement and Concrete Research, 30(10), 2000, pp [7] Brough, A. R. and Atkinson, A. Sodium Silicate based Alkali-Activated Slag Mortars: Part I. Strength, Hydration and Microstructure. Cement and Concrete Research, 32(6), 2002, pp [8] Al-Otaibi, S. Durability of Concrete incorporating GGBS activated by Water-Glass. Construction and Building Materials, 22(10), 2008, pp [9] Parthiban Kathirvel and Saravana Raja Mohan Kaliyaperumal. Influence of recycled concrete aggregates on the flexural properties of reinforced alkali activated slag concrete. Construction and Building Materials, 102, 2016, pp editor@iaeme.com

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