Law Research And Mechanism Analysis Of Shrinkage And Cracking On High Performance Concrete

The project studies on how to prevent and cure the cracking initiated by high performance concrete shrinkage and by the changes of circumstantial temperature and relative humidity. It also studies on how to evaluate the cracking grades and to predict the total shrinkage microstrains. It investigates with early-age plastic shrinkage and cracking slab test on high performance concrete, early-age autogenous shrinkage test on mortar and restrained and unrestrained drying shrinkage test high performance concrete. Early-age plastic shrinkage and cracking slab test is based on the suggesting test design of new Manual for Concrete Structure Endurance Design and Construction of China. This test design is proved to be a fast and correct way to evaluate the concrete cracking grade. Autogenous shrinkage test appliance improves on other researchers' invention. Autogenous shrinkage measures the volume changes of mortar. Restrained drying shrinkage is also called ring test. Restrained drying shrinkage initiates cracking which grows at the height direction of the ring specimen. So the crack width is measured at three different heights of the ring. Prism specimen is used in unrestrained drying shrinkage. Six sides of the prism are exposed to the drying circumstance. Axial shrinkage strain is measured and recorded. In order to theoretically explain the cracking initiated by shrinkage, it discussed the three necessary crack initiation conditions on smooth surface of non-linear brittle materials based on the crack initiation of linear elastic fracture mechanics. It also adopted the adhesive crack model to explain the process of crack initiation, development, stop and closure. It studied on the moisture loss, consumption and migration to explain concrete shrinkage mechanism. Shrinkage predict models were discussed and one of the predict model was used to predict the drying shrinkage strains compared the measured shrinkage strains in this project. Conclusively, the study on the shrinkage and cracking of high performance concrete shows the best prominent influencing factor is water binder ratio. With a high water binder ration, autogenous shrinkage is reduced and early-age cracking resistance capability is improved. Active mineral admixture and polypropylene fibers are better prominent influencing factor. Active mineral admixture efficiency varies with its species and fineness. Polypropylene fiber is much helpful to prevent early-age cracking of concrete. Shrinkage-reducing admixture and polypropylene fiber are widely considered as the best solution to prevent and cure early-age shrinkage and cracking of high performance concrete.