Cracking Tendency of Bridge Deck Concrete

Early-age cracking can adversely affect the behavior and durability of bridge deck concrete. Cracking of hardening concrete occurs when the induced tensile stress exceeds the tensile strength of the concrete. The development of in-place stresses is affected by the shrinkage, modulus of elasticity, coefficient of thermal expansion, setting characteristics, restraint conditions, stress relaxation, and temperature history of the hardening concrete. Tensile strength increases as the hydration of the cementitious system progresses. Rigid cracking frame (RCF) testing techniques capture the combined effects of modulus of elasticity, creep and relaxation, coefficient of thermal expansion, thermal conductivity, autogenous shrinkage, and tensile strength on the cracking potential of a mixture in a specific application. This paper describes an experimental evaluation of the effect of supplementary cementing materials, water-to-cement ratio (w/c), and placement temperature conditions on the early-age cracking tendency of bridge deck concrete through the use of RCF testing techniques. Specimens were tested under temperature conditions that match those in an 8-in.-thick bridge deck to explore early-age cracking mechanisms. The laboratory testing program revealed that the placement temperature and curing temperature significantly affected the time to cracking of all the mixtures. Use of either fly ash or ground-granulated blast-furnace slag was effective in reducing the heat generation and rate of stiffness development in bridge deck concretes and thus in significantly reducing restraint stresses and delayed the occurrence of cracking at early ages. A decrease in w/c resulted in increased stresses, and it accelerated the occurrence of cracking at early ages.