CONCLUSION
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⏮ Discussion and Recommendations for Correction Factor
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This chapter used a numerical finite element analysis to examine the influence of a range of factors on the compressive strength determined through the standard masonry prism tests. The following conclusions were drawn:
– The effect of the friction caused by the end platens in the compression testing machine should not be ignored. It was shown that in the models with frictionless ends the measured strength is the same for specimens regardless of their h/t ratios. On the other hand the effect of friction on increasing the prism strength can be considerable and is a function of h/t ratio.
– The result from the FEM of different sizes of masonry prisms revealed that the compressive strength is not only a function of the thickness but also highly dependent on the length of the prism. In the current masonry codes the effect of the length of the prisms is not considered. As the length-to-thickness ratio increases, the codes’ tendency to over-predict the strength becomes more significant. The length-to-thickness ratio is hence an influential parameter in determining the strength of prisms and should be incorporated in the standards’ strength correction factors in the future.
– For prisms having equal transverse dimensions, the stress-strain behavior is a function of h/t ratio if this ratio is less than three. In the example presented, in a half-block prism with the cross sectional dimension of 190 mm 190 mm, the load capacity of three-course block and five-course block was approximately the same. This indicates that for h/t > 3.0 the frictional force due to the machine platen does not affect the strength. In the other example presented, in a full-block prism with the cross sectional dimension of 390 mm 190 mm, the strength decreased for h/t ratios of up to 5.0.
– The correction factors for concrete and clay masonry prisms are not compatible in the ASTM C1314 (2003) standard. For brick prisms the correction factor is normalized by the strength of a prism having an aspect ratio of 5.0, which seems to represent a realistic strength as the strength seems not to be affected by the machine platen effect. However, according to ASTM C1314 (2003), for concrete masonry prisms the strength is normalized by the prism having a h/t ratio of two. This small aspect ratio seems to be influenced by the support confinement and results in a strength over-prediction of about 20–25%, which is quite considerable and will lead to an unconservative design. To provide a compatible measuring system to obtain the compressive strength independent of the confinement imposed by the machine platen, it is recommended to normalize the correction factors for concrete masonry based on a height-to-thickness ratio of 5.0, the same way as considered in other masonry codes and considered in ASTMC 1314 (2003) for brick masonry. To set the h/t = 5.0 as referenced in ASTM C1314, the standard expressions need to be checked and the empirical formulations in the standard need to be revised. For instance the concrete masonry modulus of elasticity E = 900f 0 m new should be altered to 737f 0 m new in which f 0 m new is the compressive strength of a concrete prism having an aspect ratio of 5.0.
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⏮ Discussion and Recommendations for Correction Factor
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