M. Myint Lwin, Federal Highway Administration and Henry G. Russell, Henry G. Russell, Inc.
n recent years, many bridge owners have adopted the use of lower permeability concretes in bridge decks to reduce chloride penetration and extend the time before corrosion of the reinforcement begins. At the same time, more owners have reported an increase in the amount of cracking in concrete bridge decks. This issue of HPC Bridge Views, as well as the next issue, focuses on the experiences of several state Departments of Transportation in dealing with bridge deck cracking.
The problem of cracking is complex and, in some situations, cracking cannot be avoided. Nevertheless, cracks can be minimized by the careful selection of materials, proper design details, and appropriate construction practices. This article addresses some of the critical factors that may have contributed to bridge deck cracking, and identifies practices to reduce cracking.(1)
Low permeability concretes are generally achieved through the use of a low water-cementitious materials ratio and supplemental cementitious materials. The use of these materials frequently results in concretes having higher concrete compressive and tensile strengths, higher moduli of elasticity, and less creep. Although the tensile strength is higher, the higher modulus of elasticity and lower creep have led to an increase in the amount of cracking, which then provides the chlorides with an easier path to the reinforcement. As a result, the increase in the number of cracks offsets the benefits of the low-permeability concrete between the cracks. As stated “We have managed to get excellent concrete between the cracks!”(2)
It is important to design a concrete mix that balances the need for low permeability with the need to minimize cracking. It may not always be desirable or even essential to specify the lowest possible permeability value for bridge deck concrete. A range of 1500 to 2500 coulombs per AASHTO T 277 along with a top cover of 2.5 in. (64 mm) and a water-cementitious materials ratio in the range of 0.40 to 0.45 have been effective for many decks.
Construction practices can have a major impact on the likelihood of cracking. In a survey of 45 bridge agencies in 2003, the most effective strategies to control cracking were identified as fogging during placement of the fresh concrete and adequate curing of the hardened concrete.(1) The majority of owners specified a continuous water-saturated cover for a minimum of 7 days. Curing is particularly important when supplemental cementitious materials are used because of the tendency for less bleed water on the surface.
Other practices that can reduce cracking are:
- Decrease the volume of water and cementitious paste consistent with achieving other properties
- Use the largest practical maximum size aggregate
- Use aggregates, when locally available, that result in lower concrete shrinkage
- Use the smallest transverse bar size and minimum spacing that are practical
- Avoid high concrete compressive strengths
- Design the concrete mix to produce a low
- modulus of elasticity and high creep • Implement surface evaporation requirements and use windbreaks and fogging equipment, when necessary, to minimize surface evaporation from fresh concrete
- Apply wet curing immediately after finishing and cure continuously for at least 7 days
- Apply a curing compound after the wet curing period to slow down the shrinkage and enhance the concrete properties.
We encourage readers to let us know about other practices and innovative methods that have been used successfully for controlling cracking in concrete bridge decks.
References
- Russell, H. G., NCHRP Synthesis of Highway Practice 333: Concrete Bridge Deck Performance, National Research Council, Transportation Research Board, Washington, DC, 2004, 101 pp.
- Concrete Cracking Workshop, Northwestern University, Aug 18-19, 2005, http://acbm.northwestern.edu/ccw.html.