A cold joint in a concrete slab occurs when the first layer of concrete sets before the next layer is placed, resulting in a lack of intermixing between the layers. Causes of cold joints can include delays in concrete placement, poor workmanship, and improper surface preparation. Question: Difference between a contraction joint, isolation joint, expansion joint, construction joint, an. The delayed placement prevents full integration and knitting between the concrete batches and might. A cold joint in concrete construction is a plane of weakness that forms when new, wet concrete is poured against concrete that has already begun to harden. Time to break down the details.