Cracking of concrete caused by drying shrinkage adversely affects the durability of the structure. In the past research results, it is effective to add urea to concrete as a way to reduce drying shrinkage. However, in ready mixed concrete factory in Japan, adding various admixture materials to concrete will increase equipment costs and labor costs because of the increase in the number of input items, the management problem and putting problem when mixing.
In this research, in consideration of the above problems, we attempted to develop a cheaper and simpler method to apply or infiltrate the urea solution on the surface of concrete after demoulding as a method to reduce drying shrinkage. As a result, it was possible to reduce drying shrinkage by spraying urea solution on the concrete surface. It was also found that when the amount of urea solution in the concrete is higher, the drying shrinkage is lesser.
This is considered that the urea solution entering into the concrete crystallizes in the gaps in the process of drying, and the expansion pressure of the crystallizing resists the drying shrinkage. It can also be considered that the water holding ability of urea prevents excessive drying of concrete.
Furthermore, in order to prevent elution of urea inside the concrete due to rainwater, Na2SO4 reacting with remaining unreacted chemical components of concrete was mixed to the solution. And it was confirmed that effect of using the mix solution of urea and Na2SO4 was not lost if exposed in the rain
