Title: Effect of Mixture Composition on Restrained Shrinkage Cracking of Self-Consolidating Concrete Used in Repair

Author(s): Soo-Duck Hwang and Kamal H. Khayat

Publication: Materials Journal

Volume: 105

Issue: 5

Appears on pages(s): 499-509

Keywords: drying shrinkage; restrained shrinkage; self-consolidating concrete; shrinkage-reducing admixture; stress relaxation; tensile creep

DOI: 10.14359/19980

Date: 9/1/2008

Abstract:
Given its mixture design, which often involves the incorporation of a relatively high content of cement paste and low volume of coarse aggregate, self-consolidating concrete (SCC) can inherently exhibit greater risk of drying shrinkage and restrained shrinkage cracking compared with conventional concrete with normal slump consistency. An experimental program was undertaken to investigate the influence of various mixture parameters on shrinkage cracking of highperformance SCC designated for structural repair. The parameters included the type of high-range water-reducing admixture (HRWRA), type and content of synthetic fibers, dosage of shrinkage-reducing admixture (SRA), as well as the use of hybrid fibers (HF). An instrumented ring-type setup (ASTM C1581) was used to evaluate cracking potential. Test results indicate that an increase in fiber volume can lead to a decrease in cracking potential of concrete, regardless of the fiber type. On average, the increase in synthetic fiber volume from 0 to 0.25% and 0.25% to 0.50% led to an approximately 40% increase in the elapsed time before restrained shrinkage crack initiation. SCC made with a high concentration of SRA (and no fibers) developed 40% lower drying shrinkage after 56 days of drying and 2.4-folds longer time before cracking compared with similar concrete prepared without any SRA. The combined use of SRA and either synthetic fibers or HF at a dosage rate of 0.25% or 0.50% is quite effective to design high-performance SCC of low cracking potential. Values of drying shrinkage after 7 and 56 days of drying at 50% relative humidity (RH) and modulus of elasticity at the beginning of drying of the restrained shrinkage setup (3 days of age) can be used to estimate the elapsed time to crack initiation due to restrained shrinkage. For a given level of drying shrinkage, an increase in cracking potential can occur when the concrete develops higher elastic modulus at the time of initiation of drying. Similarly, the cracking potential increases with the extent of drying shrinkage deformation for concrete of a given elastic modulus.