Control Joints in Cast In Situ Concrete Slabs and Its Application

Concrete cracks due to shrinkage, changes of temperature and humidity and other reasons may impair the structural integrity and durability of concrete structures. To relieve the tensile strains and prevent unexpected cracks, one strategy is to intentionally set joints (namely control joints) at appropriate locations within the concrete members. In this paper, the validity and the major influencing factors of the control joints were studied. Firstly, in total eighteen slabs, including sixteen with control joints and the other two without control joints, were constructed and tested. Different concrete mix proportions, joint locations and depths, as well as the reinforcement ratios were considered. The concrete slabs were tested for 9 months with the complete process of the initiation and development of the concrete cracks in the slabs recorded. Secondly, for the purpose of numerical analysis, material properties of concrete were tested, using which numerical simulation and parametric study were conducted. It was found that the initiation and development of cracks in concrete slabs may be simulated by means of numerical modeling techniques. Cracks in concrete slabs can be to a large extent effectively regulated and the random occurrence of cracks can be prevented. Finally, the generated knowledge and the modeling technology were applied into a real project with tight restrictions on concrete cracks. By properly setting control joints according to the numerical simulation results the cracks were well controlled.