EARLY-AGE CRACKING IN CONCRETE BRIDGE DECKS IN DELAWARE: LABORATORY ASSESSMENT OF BRIDGE DECK MIX DESIGNS

Abstract

Early age cracking in concrete bridge decks remains a persistent challenge for many Departments of Transportation (DOTs). They contribute to premature deterioration of decks, reducing their overall service lives. To minimize cracking, DOTs have employed several strategies including reducing the cementitious content of concrete mixes, extending moist curing durations, incorporating fibers, shrinkage reducing admixtures, and specifying maximum drying shrinkage limits. This study involved an experimental program to assess five mix designs used in bridge decks in Delaware. The primary objective was to evaluate and compare the performance of the mix designs, with emphasis on shrinkage behavior and cracking potential. The experimental program involved conducting both unrestrained and restrained shrinkage tests to characterize the drying shrinkage and cracking potential of concrete mixes used by Delaware DOT. In addition, compressive strength, splitting tensile strength, and modulus of elasticity tests were conducted to evaluate their mechanical properties. Cracking potential was quantified as the ratio of stress induced by restrained shrinkage to concrete’s splitting tensile strength. The test variables included total cementitious material content, percent of ground granulated blast furnace slag (GGBFS) replacement, water/cementitious material ratio (w/cm) and the type of shrinkage reducing admixture (SRA). Experimental results showed that increasing GGBFS replacement level reduced the compressive strength and modulus of elasticity by 32% and 25% respectively, at early age. Increased GGBFS replacement also reduced drying shrinkage by 9%, 2.6%, and 1.3% following 7-, 14- and 28-day moist curing durations respectively. The cracking potential was not reduced with increased GGBFS dosage as mixture with increased GGBFS exhibited higher cracking potential and further cracked under laboratory testing. SRAs on the other hand significantly reduced drying shrinkage by 31% at 7 days and 30% at 28 days with no significant reduction at 14 days. With SRAs, the cracking potential was reduced to values below 1, and mixtures incorporating SRAs never cracked under laboratory testing. Extending the moist curing durations from 7 to 14 and 28 days decreased the magnitude of shrinkage by 12% and 28%, respectively, for high cementitious mixtures (705 lb/yd3), and by 10% and 30% respectively, for low cementitious mixtures (612 lb/yd3). Laboratory findings indicate that the potential of cracking is reduced with lower cementitious content, and with incorporation of shrinkage-reducing and shrinkage-compensating SRAs in concrete. To mitigate early-age cracking, the study therefore recommends limiting the total cementitious content to 612 lb/yd3, and incorporating shrinkage-reducing and shrinkage-compensating SRAs into the concrete mix. A 14-day moist curing duration is preferred over a 7-day period, as it effectively reduces the magnitude of drying shrinkage. Finally, the study recommends limiting the maximum drying shrinkage to 300 µɛ at 28 days, following 7 days of moist curing.