Analysis of Thermal Stress and Drying Shrinkage of CFRD Concrete Slabs

摘要

This report describes the possibility of cracks caused by thermal stress and drying shrinkage in CFRd concrete face slabs. Time variation of thermal stress and drying shrinkage after slab placing was calculated by two-dimensional FEM analysis,and the possibility of their occurrence,differences according to placing period,and effectiveness of admixtures are discussed. The FEM model consisted of foundation bedrock,a rockfill dam body,and concrete face slabs. The slope gradients of a rock-fill dam body are 1:1.8 and 1:1.9 on the upstream slope and downstream slope respectively. The slab thickness is variable and designed to satisfy thickness t(m) with t=0.3+ 0.003h (h is the water pressure on the slab). The analysis results were evaluated by the crack index defined as the ratio of concrete tensile strength to maximum principal tensile stress calculated for each age. The following are the results of the analysis. (1) The minimum crack index produced by thermal stress is sufficiently higher than 1.75 which is the criterion for prevention of cracking,even in the case marked by the harshest conditions:using moderate heat cement and placed on July 1.Consequently the face slabs of the model dam are adequately safe from thermal cracking. (2) The thermal stress shows that it is beneficial to pour concrete from late fall to spring;but relatively disadvantageous to do so in the summer or early autumn.But the maximum tensile stress is manifest in the winter,several months after placing,and the tensile strength at that time is sufficiently large to prevent thermal cracking. (3) Replacing cement with fly ash is an effective way to prevent thermal cracking. (4) According to the results of analysis considering drying shrinkage in addition to thermal stress in a case where part of the cement has been replaced with fly ash,the minimum crack index in the July 1 and October 1 placing cases,which are cases with the disadvantageous thermal stress,is sufficiently higher than 1.75 which is the criterion for prevention of cracking. Appropriate curing is thought to be effective and important to ensure sufficient safety in a case where the impact of drying shrinkage is considered in addition to that of thermal stress.