The Influence of Chemical Admixtures on Restrained Drying Shrinkage of Concrete

Abstract

A consequence of drying shrinkage is intrinsic cracking due to some form of restraint. In thick sections of concrete, drying from the surface causes differential shrinkage and such internal restraint can be responsible for surface cracking because of the induced tensile stress. When thin drying concrete members are restrained externally, a time-dependant failure is likely unless drying shrinkage is minimized. Besides drying shrinkage, the potential for cracking depends on tensile creep and tensile strength or tensile strain capacity and such properties are not normally measured in the laboratory. The possible effects of chemical admixtures on the forgoing properties is also largely unknown. The current research is investigating the role of tensile creep in relieving the tensile stress induced by fully restraining the drying shrinkage of concrete with and without chemical and mineral admixtures. All the relevant properties contributing to the time-dependant strength are being measured using bobbin-shaped specimens previously developed for uniaxial creep determination. The present paper presents the findings for concretes with and without plasticizer and a new shrinkage reducing admixture. While the plasticizer has little influence on properties, the shrinkage reducing admixture significantly lowers the strength, elastic modulus, free drying shrinkage and creep. When restrained from the age of seven days all the concretes failed between 4 and 13 days, the concrete with the shrinkage reducing admixture failing at the lowest stress but after the longest time.