Effect of Expansive Agent and Temperature on the Quality of Pre-cast Concrete Products

Pre-cast concrete products are sometimes manufactured in two cycles per day production with one mold for the purpose of productivity improvement. In such cases, to secure the early strength, which is required at demolding, it is necessary to increase the steam-curing temperature. However, when this is done, the issue of temperature-related cracking becomes a concern. In addition, self-compacting concrete to which ground-granulated blast-furnace slag has been added is increasingly being used in consideration of the environment surrounding a plant and the operating environment. Admixing with an expansive agent prevents cracking due to autogenous shrinkage. However, when an expansive agent is admixed with concrete, the possibility also exists that the high-temperature curing required for two cycles per day production may increase the likelihood of cracking. In this study, the cause of cracking of large-sized pre-cast concrete products with high amounts of expansive agent was investigated and the products of one cycle per day production compared to those of two cycles per day production. The results obtained indicate that, in contrast to one cycle per day production, high-temperature steam curing and early demolding in two cycles per day production promote thermal stress cracking. In addition, unexpected cracking can occur along the main reinforcement resulting from excessive expansion due to inappropriate curing of the expansive agent.