A K H Kwan

Semi-adiabatic Curing Test with Heat Loss Compensation for Evaluation of Adiabatic Temperature Rise of Concrete

Early thermal cracking has been a prevalent problem in many concrete structures. To mitigate such cracking problem, the adiabatic temperature rise of the curing concrete needs to be limited. In practice, curing tests in different forms of temperature rise evaluation test (TRET) are often employed to determine the temperature rise of concrete. However, due to heat loss to the surroundings, the results from TRET do not truly reflect the temperature rise of concrete under adiabatic condition. To address this issue, a semi-adiabatic curing test method with heat loss compensation to simulate the adiabatic condition was developed as presented in this paper. Using the test method, the adiabatic temperature rise and heat generation of ordinary Portland cement (OPC) concrete, pulverised fuel ash (PFA) concrete and condensed silica fume (CSF) concrete were measured. Based on the test results, design charts for estimating the adiabatic temperature rise of OPC, PFA and CSF concretes were devised.

Nonlinear multilevel analysis of reinforced concrete frames

Full range analysis of reinforced concrete (RC) members covering the post-crack and post-peak regimes is important for obtaining the deformation response and failure mode of structural members. When a RC member is subject to an increasing external load, the critical sections would exhibit cracking and/or softening. Due to stress relief effect in the proximity of crack opening and plastic hinging, unloading may occur at the adjacent regions. The variable stress states of discrete sections would lead to sectional variation of stiffness, which could not be accounted for by conventional structural analysis methods. In this paper, a nonlinear multilevel analysis method for RC frames whereby the frame members are divided into sub-elements and sectional analysis is utilised to evaluate stiffness degradation and strength deterioration is developed. At sectional level, the secant stiffness is determined from moment-curvature relation, where the curvature is evaluated based on both transverse displacements and section rotations of the frame member. Unloading and reloading behaviour of concrete and reinforcing steel is simulated. In implementing the multilevel analysis, secant iteration is performed in each step of displacement increment to obtain the convergent solution satisfying equilibrium. Numerical example of RC frame is presented to demonstrate the applicability and accuracy of the proposed nonlinear multilevel analysis method.

Effects of Using High-strength Concrete on Flexural Ductility of Reinforced Concrete Beams

The complete moment-curvature behavior and flexural ductility of reinforced concrete beams made of normal- or high-strength concrete have been analyzed by a newly developed theoretical method that uses the actual stress-strain curves of the materials and takes into account the strain reversal of the tension reinforcement in the analysis. It was found that as expected the flexural ductility decreases with the tension steel ratio and increases with the compression steel ratio. However, the variation of the flexural ductility with the concrete grade is quite complicated; at fixed tension and compression steel ratios, the flexural ductility increases as the concrete grade increases but at a given degree of being under- or over-reinforced, the flexural ductility decreases as the concrete grade increases. In order to better reveal the combined effects of the steel ratios and the concrete grade, the flexural ductility has been plotted against the flexural strength for different steel ratios and concrete grades. From the graphs plotted, it can be clearly seen that the use of a higher grade concrete could increase flexural ductility at same flexural strength, increase flexural strength at same flexural ductility or increase both flexural strength and flexural ductility.