Characteristics of Grade 60 and Grade 72.5 Re-bars in Pakistan
In Pakistan, quality assurance of re-bars is practically non-existent and there is no generally accepted testing method for testing of re-bars. Steel re-bars of Grade 60 are exclusively used in Pakistan; however production of high strength re-bars has recently started. In this study, thirty samples of steel re-bars of Grade 60 and thirty samples of Grade 72.5 were tested using ASTM standard testing protocol to obtain the stress-strain curves and comparisons were made between the experimental results of Grade 60 and Grade 72.5 re-bars. It can be concluded that Grade 72.5 steel shows less deformation capacity as compared to Grade 60 steel. For Grade 60 re-bars, the yield strengths of all individual re-bars were much higher than the prescribed minimum yield strength, i.e. 60 ksi. For the Grade 72.5 rebars, about 50 % of the re-bars did not reach the prescribed minimum yield strength of 72.5 ksi. The experimental curves were then compared with an idealized stress-strain curve developed by using a fractional equation. It is recommended that instead of producing Grade 72.5 ksi reinforcing bars, the traditional ASTM Grade 75 reinforcing bars be produced.
Complete Triaxial Stress-Strain Curves for Concrete
A method to predict the ascending and the descending parts of stress-strain curve of concrete subjected to triaxial compressive stresses is presented. The proposed analytical stress-strain relationship depends on the values of the three principal stresses and strains at the ultimate compressive strength. A criterion to predict the triaxial compressive strength is proposed, which is sensitive to all three stress invariants. To enable predictions of the post-peak stress-strain relation, experiments were conducted on concrete confined by steel tube. Concretes with different compressve strengths were confined by steel tubes of varying thicknesses. The predicted stress-strain curves compared favorably with the available experimental data on uniaxial, biaxial, and triaxial compression.
Shear Transfer in Reinforced Concrete
Presents a study of shear transfer in reinforced concrete, that is, the transfer of shear across a plane, such as at the interface between a precast beam and a cast-in-place slab. Thirty-eight pushoff specimens were tested, some with, some without a pre-existing crack along the shear plane. The shear-friction theory was found to give a conservative estimate of the shear transfer strength of initially cracked concrete. A method is presented for the calculation of shear transfer strength in initially uncracked concrete, based on the Zia envelope to Mohr circles representing failure conditions for concrete.