498 ACI Materials Journal/September-October 2008 ACI MATERIALS JOURNAL TECHNICAL PAPER ACI Materials Journal, V. 105, No. 5, September-October 2008. MS No. M-2007-269 received July 20, 2007, and reviewed under Institute publication policies. Copyright © 2008, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including authors’ closure, if any, will be published in the July-August 2009 ACI Materials Journal if the discussion is received by April 1, 2009. 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 high- performance 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. Keywords: drying shrinkage; restrained shrinkage; self-consolidating concrete; shrinkage-reducing admixture; stress relaxation; tensile creep. INTRODUCTION Properly designed self-consolidating concrete (SCC) can completely fill all spaces within the formwork under its own weight (high deformability) and pass among various obstacles and narrow spacing in the formwork without blockage (high passing ability) while maintaining adequate resistance to segregation, bleeding, and surface settlement (stability). These characteristics have led to a recent increase in specifying SCC for structural repair in areas presenting special difficulties to casting and consolidation, such as bottom sides of beams, girders, and slabs. 1,2 Despite the increasing use of SCC, it is important to tailor- design the concrete mixture to take into account its higher potential for shrinkage cracking compared with ordinary vibrated concrete. Although limited in numbers, studies on the cracking potential of SCC revealed that such concrete can develop higher shrinkage compared with conventional concrete (CC), given its greater volume of cement paste and relatively low coarse aggregate content. 3 Hwang and Khayat evaluated the cracking potential of SCC made with two mixture design approaches and three blended binder types. Instrumented shrinkage ring test results were compared with those of high-performance concrete (HPC) and CC mixtures. The two SCC mixture design approaches undertaken to secure proper stability involved low water-cementitious mate- rial ratio (w/cm) concrete of 0.35 as well as SCC with 0.42 w/cm and viscosity-enhancing admixture (VEA). The former concrete was prepared with a blend of CSA Type GU cement (similar to ASTM C150 Type I cement), 30% Class F fly ash, and 5% silica fume, by mass of cementitious materials. The second type of SCC was made with ternary binder containing 25% Class F fly ash and 5% silica fume. The HPC was prepared with 0.35 w/cm and the same ternary cement, and the CC was proportioned with 0.40 w/cm and a blend of 92% CSA Type GU and 8% CSA Type GUb-SF. Test results revealed that SCC mixtures made without any fiber reinforcement and shrinkage-reducing admixture (SRA) have relatively higher cracking potential than the HPC and CC mixtures. For example, SCC exhibited shorter cracking time (restrained shrinkage ring test) of 4 to 9 days compared with 10 to 14 days for the CC and HPC mixtures, when the polycarboxylate (PC)-based high-range water-reducing admixture (HRWRA) was incorporated. This is mainly due to higher drying shrinkage of the SCC compared with the HPC and CC mixtures. The authors pointed out that relatively high tensile creep coefficient (ratio of creep strain to elastic strain) for the SCC was not enough to compensate for the large difference in shrinkage between SCC and either HPC or CC mixtures. Recently, various types of fibers and SRAs have been used to mitigate the cracking potential in concrete. Grzybowski and Shah 4 evaluated the efficiency of different types and volumes of fibers on the reduction in shrinkage cracking potential of concrete using ring-type specimens. Concrete mixtures were proportioned with steel fibers measuring 25 mm (1 in.) in length and 0.4 mm (0.016 in.) in diameter as well as 19 mm (0.75 in.) fibrillated polypropylene fibers. The contents of the steel fibers were 0.25, 0.5, 1.0, and 1.5%, by volume. These values were 0.1, 0.25, 0.5, and 1.0% for the polypropylene fiber. Test results revealed that a steel fiber content of 0.25% can substantially reduce the crack width Title no. 105-M58 Effect of Mixture Composition on Restrained Shrinkage Cracking of Self-Consolidating Concrete Used in Repair by Soo-Duck Hwang and Kamal H. Khayat