Evaluation of surface strength of self-compacting concrete under shrinkage and magnesium sulfate using "twist-off" method

Concerning the importance of shrinkage in the concrete industry, much research is being performed on this subject. Since the self-compacting concrete has high consistency, many researchers believe that the shrinkage of self-compacting concrete should be more attended than the shrinkage of plain concrete. Sulfates are among the common reasons for concrete destruction in most areas of Iran especially in the southern areas, where the concrete is subjected to seawater (which includes sulfate compounds). Since the amount of drying is higher in the surface of a concrete piece than its depth and the durability of the concrete structure highly depends on the strength and permeability of its surface layer, this paper applied a “Twist-off” test (which shows proper sensitivity against the surface change of concrete) to investigate the effect of shrinkage and magnesium sulfate on the surface strength of self-compacting concretes. Then, the change in the length of samples was presented together with their weight drop. In the end, by using the “Twist-off” test, the effect of magnesium sulfate and the length change of samples on the surface strength was evaluated. The results represent that the plain concrete, compared to the self-compacting ones, shows more reduction in the surface strength due to the effect of drying shrinkage. The reduction in strength of plain concrete is 18.4% by shrinkage that is 1.34 times of the self-compacting concrete with 45% fly ash, 1.35 times of the self-compacting concrete with 35% fly ash, and 1.37 times of the self-compacting concrete with 25% fly ash. For the self-compacting samples curing in magnesium sulfate solution, the 3-day and 7-day surface strengths were reduced by the increase of fly ash percentage, while the 28-day surface strength was increased.