MEASURES TO ALLEVIATE SHRINKAGE PROBLEMS IN LARGE REINFORCED CONCRETE PODIUM STRUCTURES

摘要

Shrinkage of a concrete structure as the concrete dries after hardening could lead to substantial shortening movement and, if the movement is restrained, serious cracking of the concrete structure. This is a common problem in Hong Kong. If a large podium is structurally connected to two or more core walls supporting tower blocks above the podium, the comparatively stiff core walls will prevent the shortening of concrete structures and shrinkage stresses will develop. In many cases, the shrinkage cracks formed have been causing aesthetic, water leakage and durability problems. To minimise the effect of shrinkage, it is a common practice to adopt stage construction by providing late-cast strips to be cast later. In this case, various structural members may have different ages, and hence different strengths and tendencies to shrink. The effects of stage construction on the long-term response of reinforced concrete podiums are investigated by the finite element method using the step-by-step time integration and based on the conventional creep coefficients, time-dependent concrete moduli and shrinkage strains. Investigation into various problems of a typical podium arrangement is also carried out. Factors affecting the effectiveness of late-cast strips including the optimum time lag between the first-cast area and late-cast strips, and the locations and number of late-cast strips are evaluated. The tensile stress criteria of concrete are used to predict possible cracking of concrete. Results show that stage construction is an effective measure to reduce the risk of shrinkage cracking. The maximum shrinkage stresses are reduced because a significant part of shrinkage movements have taken place relatively freely before various parts are stitched together. The above cases are analysed by using the shrinkage models suggested by the MC-90 Code, the ACI-209 Code, BS5400 and the Hong Kong Code, which are based on different material parameters. An approximate shrinkage model for concrete with shrinkage reducing agent is also proposed. To investigate the effectiveness of shrinkage reducing agent, the shrinkage behaviour of a typical podium is analysed. Finally, some design recommendations including the arrangement of late-cast strips and the possible methods to mitigate the adverse effects of shrinkage of concrete are made.