Time-dependent deflection of conventional, self-compacting and lightweight concrete slabs

Abstract

Design codes usually define time-dependent deflection as a multiple of instantaneous deflection. Therefore, the ratio of long-term to instantaneous deflection is crucial in the design of concrete structures. The experimental results of three series of experiments on long-term deflection of conventional, lightweight and self-compacting concrete slabs under sustained loads are compared and discussed in this study. The slabs were subjected to loading at age 14 d after casting. The flexural deflection caused by two distinct levels of applied load, and the effects of the drying shrinkage and creep were monitored for 225 d for three pairs of simply supported one-way slabs. Comparative analysis of the mechanical properties of concrete types at 14 and 28 d is conducted and the creep coefficient and drying shrinkage strain in the mixtures are determined. Furthermore, individual effects of these parameters on the ratio of long-term to instantaneous deflection are discussed. The results obtained confirm that, in the existing time-dependent deflection models, the effects of loading age, load value and the environmental conditions should be included. In addition, the experimental ratio of the long-term to instantaneous deflection in slabs is far from the predictions of ACI 318-14, AS 3600-09, EN 1992.1.1 and CSA-A23-04 codes.