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Concrete In Australia : March 2014
54 Concrete in Australia Vol 40 No 1 FEATURE: GEOPOLYMERS fct = 0.2605*(fcmi )0.7129 (2) For mean in situ Compressive Strength (fcmi )=65.7MPa (geopolymer I N, 65 MPa), Uniaxial Tensile Strength (fct ) = 5.15 MPa (close to measured value of 5.3 MPa) 4.2 Flexural tensile strength The flexural tensile strength of concrete was determined by testing of 100 mm x100 mm x 350 mm concrete beams at 28 days of maturity according to AS 1012.11 . The data in Table 4 represents the average result from three samples. This data suggests that standard slumped (80-120 mm slump) geopolymer concrete possesses similar flexural strength to OPC concrete. Similar to the uniaxial tensile strength result, the super workable geopolymer concretes possess lower flexural tensile strength than slumped concrete. In Table 4, the average Flexural Strength Ratio (measured/ calculated) for standard slumped concrete is 1.58 and for super workable concretes is 1.37. The flexural tensile strength attained by geopolymer concrete is found to be higher than the calculated value from AS 3600. From AS 3600 (Clause 3.1.1 .3), Characteristics flexural Tensile Strength (f ’ ct.f ) can be estimated as: (f’ ct.f ) = 0.6* √fc ’ (3) For 65 MPa geopolymer I N concrete, f ’ c = 65 MPa, ie; (f’ ct.f ) = 4.8 MPa (compared to the measured value of 7.0 MPa). Using the above data, a graph can be plotted between measured flexural tensile strength (fct.f) and mean in situ compressive strength (fcmi ). Since super workable concrete behaves in a different way to standard slumped concrete, the graph considers standard slumped concretes only. The 28 days mean compressive strength and corresponding flexural tensile strength are plotted in Figure 3. A comparison with AS 3600 relationship is also presented here. Figure 3 can be represented by a relationship: fct.f =0.0421*(fcmi )1.2358 (4) For mean in situ Compressive Strength (fcmi )=65.7MPa (geopolymer I N, 65 MPa), Flexural Strength (fct.f ) = 7.4 MPa (close to measured value 7.0 MPa) 4.3 Modulus of elasticity The modulus of elasticity of different concretes was determined using the static chord modulus of elasticity method (AS 1012.17) at 28 days of maturity. Each data point in Table 5 represents the average result from three samples. The results show the similarity in modulus of elasticity of geopolymer concretes and OPC concretes. Following the previous trend, super workable concretes possess lower elastic modulus than slumped concrete. Mean modulus of elasticity (Ecj) calculated from AS 3600 (Clause 3.1 .2), For 65 MPa, geopolymer I Slumped concrete, Mean compressive strength (fcmi ) = 65.7 MPa and density= 2420 kg/m3 Ecj = ρ 1.5 (0.024 √fcmi +0.12) MPa; when fmci >40 MPa (5) Calculated modulus of elasticity (Ecj) = 37.4 GPa (similar to measured value of 37.0 GPa) According to ACI 318 - 08; Ec = 0.043ρ1.5 *√f’c (6) Calculated modulus of elasticity (Ec) = 41.3 (higher than measured value of 37.4) The above data and calculation suggest that AS 3600 can be applied to estimate the modulus of elasticity of both OPC and geopolymer concretes. Another noticeable factor in this relationship is that it is based on average mean in situ compressive strength rather than characteristic compressive strength (in ACI method), which is a practical approach. Figure 2: Relation of tensile strength and compressive strength. Uniaxialtensilestrength(fct)MPa Mean compressive strength (fcmi) MPa Measured value AS 3600 f' = 0.36*(f'c) 0.5 fct = 0.2605*(fcmi)0.7129 50-56 - Neupane.indd 54 50-56 - Neupane.indd 54 3/02/14 12:10 PM 3/02/14 12:10 PM