Abstract
It is empirically known that using shrinkage reducing agent（SRA）and/or expansive additive（EX）is effective in reducing shrinkage cracks in concrete structures. However, in order to control cracks more rationally, it is necessary that reduction effect of SRA and EX in shrinkage cracks are quantitatively evaluated for design values. In this paper, shrinkage crack characteristics were compared between concrete using SRA or EX solely, or in combination SRA and EX, and plain concrete by investigating crack distribution in uniaxially restrained specimens in which a deformed steel bar embedded. Furthermore, shrinkage crack control effects of SRA and EX were evaluated by determining relationship of drying shrinkage strain measured in the unrestrained specimens with the cracking coefficient calculated from the number of cracks and crack width. As a result, compressive strength and modulus of elasticity of concrete using SRA and EX cured under temperature of 20℃ and relative humidity of 60％ after wet curing at 7 days were equal to or greater than those of plain concrete. The drying shrinkage strain of concrete using SRA or EX was found approximately 10-20％ less than that in plain concrete. SRA and EX were obviously effective in reducing the number of shrinkage cracks and the cracking coefficient. It were also suggested that the crack control effect of SRA could be approximated quantitatively as the decrease in drying shrinkage strain, and that of EX could be as the sum of the decrease in drying shrinkage strain and the amount of expansion-induced strain of about 100-150×10－6. Further the effect of combined use of SRA and EX could be evaluated as the sum of their individual effects.
