An RH-based non-uniform shrinkage prediction model considering the environmental humidity fluctuations and decorative layer sealing effect for steel-recycled aggregate concrete composite slabs

Steel-recycled aggregate concrete (RAC) composite slabs, a feasible component to apply RAC, can quickly consume construction and demolition waste, helping to protect the environment and promote sustainable development. However, the design of composite slabs is often influenced by long-term deflection caused by significant non-uniform shrinkage. At the microscopic level, shrinkage is primarily driven by relative humidity (RH). For composite slabs, moisture cannot escape from the underside due to steel decking, which leads to non-uniform RH and shrinkage throughout the slab. Furthermore, environmental humidity fluctuations and decorative layer installation can affect RH distribution, leading to a complex non-uniform shrinkage prediction problem. However, traditional regression models cannot predict the influences of environmental humidity fluctuations, decorative layer installation, and recycled aggregate (RA) properties. To solve this problem, this paper develops a theoretical model for predicting non-uniform shrinkage in steel-RAC composite slabs based on the RH model. A theoretical framework for linking RH to free shrinkage was derived, and mechanical equilibrium equations were used to consider the interlayer constraint effects in the specimen. To validate the proposed model, 16 tests were conducted for 270 days, examining autogenous shrinkage and non-uniform shrinkage under constant and fluctuating humidity conditions and the condition with decorative layer installation; 22 test results from five related studies were also used for validation. The proposed model can well predict the influences of environmental humidity fluctuations, decorative layer installation, and RA properties on non-uniform shrinkage development. Results indicate that the maximum discrepancy between the predicted and test shrinkage at the exposed surface is 14.8 % at the end of the test. This research quantifies the beneficial effects of decorative layers on the shrinkage of steel-RAC slabs, which can extend RAC's application and promote its sustainable development.