Question:
What can be done to reduce cracking in a concrete bridge deck?
Answer:
Research over the past 30 years has addressed the causes of cracking in bridge decks. The results of the studies provide guidance on modifications in materials, construction techniques, and design that will reduce the amount of cracking. Concrete shrinkage is a major cause of cracking in bridge decks. Shrinkage can be reduced by decreasing the volume of water and cement paste (cement and water) in the concrete. Workability can be enhanced even with reduced paste contents by using water-reducing and high-range water reducing admixtures. Optimized aggregate gradations can also help. With careful attention to the airvoid system, shrinkage-reducing admixtures can also play a role. Increased compressive strength often associated with high performance concrete increases cracking. In studies of bridge decks in Kansas,(1, 2) monolithic decks with average measured compressive strengths of 4500 psi (31 MPa) exhibited average crack densities of only 0.05 ft/ft2 (0.16 m/m2) compared with 0.15 ft/ft2 (0.50 m/m2) for decks with average measured strengths of 6500 psi (45 MPa).
During construction, settlement and plastic shrinkage cracks serve as locations for subsequent drying shrinkage cracks to form as the concrete ages. Settlement cracks that form over transverse reinforcing bars can be reduced using increased concrete cover, decreased bar size, and lower concrete slump. Plastic shrinkage cracks increase as the rate of evaporation from the concrete surface increases. Even when plastic shrinkage cracking is not specifically observed, conditions associated with high evaporation rates correlate with increased total cracking in a completed deck. Techniques such as windbreaks and fogging have had a positive impact, as has thorough curing. An excellent solution involves curing immediately behind the finishing equipment. Longer curing periods are often hotly debated because of the effect on construction time. However, they have the added advantage of increasing the amount of water that combines chemically with the cement. This reduces potential evaporation losses and associated drying shrinkage.
Data from the field demonstrates that bridges can be constructed that have far less cracking than occurs on many bridges today. A project is currently underway, through the University of Kansas, with the goal of constructing 20 bridge decks in several states using the best practices to achieve near crack-free bridge decks.
References
- Miller, G. G. and Darwin, D., “Performance and Constructability of Silica Fume Bridge Deck Overlays,” SM Report No. 57, University of Kansas Center for Research, Lawrence, Kansas, January 2000, 423 pp.
- Schmitt, T. R. and Darwin, D., “Effect of Material Properties on Cracking in Bridge Decks,” Journal of Bridge Engineering, ASCE, Vol. 4, No. 1, Feb. 1999, pp. 8-13.
Answer contributed by David Darwin of the University of Kansas. He may be contacted at [email protected] or 785-864-3827.