Robert J. Frosch, Purdue University
Many bridges in Indiana have cracks in their concrete decks. Cracking has occurred in both negative and positive moment regions of bridges, on both the top and bottom surfaces, and can appear before or shortly after the opening of the structure to live loads. Various crack widths and amounts of cracking exist in different bridge systems including decks on both concrete and steel girders. To determine the factors affecting transverse and longitudinal bridge deck cracking as well as to develop design recommendations that minimize or prevent these types of bridge deck cracking, a research study(1) was initiated by the Indiana Department of Transportation (INDOT). The research focused on the design and construction of new bridge decks and included bridges designed by both the empirical and traditional methods.
Research
The research involved the following five phases: field evaluation; instrumentation of a typical bridge; laboratory investigation to study the effects of shrinkage and restraint on cracking, including stay-in-place forms; effect of formwork type; and effect of bar spacing and epoxy thickness on crack widths and spacings.
Based on the research, transverse deck cracking is caused by restrained shrinkage of the concrete deck. Restraint is primarily provided by composite attachment to the girders. Longitudinal deck cracking typically occurs above the edge of the girders and is caused by a combination of factors including restrained shrinkage, flexural response, and the use of a metal angle along the girder flange to support stay-in-place formwork. The angle usually has a 3-in. (75-mm) high leg turned up into an 8-in. (200-mm) thick deck and forms a crack initiation location. Since reduction of restraint is not possible due to the economic advantages of composite construction, recommendations were developed to minimize deck cracking.
Recommendations
The following recommendations were made:
A minimum 7-day wet curing process should be used to reduce overall shrinkage strains.
Drying shrinkage of the concrete mix should be minimized. This can be achieved through concrete mix design and materials selection. For example, proper aggregate selection and gradation can produce mixes with lower shrinkage.
Concrete compressive strength should be minimized. Strengths higher than specified by design are not required and can exacerbate deck cracking. Higher compressive strengths require additional cementitious materials that produce concretes with higher shrinkage, a higher tensile strength that can increase the likelihood of reinforcement yielding, and a higher modulus of elasticity that provides a larger internal restraint against shrinkage.
Additional reinforcement above current practice is required to control crack widths in concrete decks. The total amount of reinforcing steel recommended(1) is:
where:
Ag = gross area of section, in.2
As = area of reinforcement in cross-section, in.2
f’c = specified compressive strength of concrete, psi.
fy = specified yield strength of reinforcement, psi.
The purpose for this quantity of reinforcement is to prevent yielding of the reinforcement that can result in uncontrolled crack growth. For a 4000 psi (28 MPa) compressive strength concrete with a 60,000 psi (414 MPa) yield strength reinforcement, this requirement results in a reinforcement percentage of 0.63.
Closer bar spacings are required to control early age bridge deck cracking. To produce maximum crack widths in the range of 0.016 in. (0.41 mm), a maximum bar spacing of 6 in. (150 mm) was found necessary when using current cover requirements and currently accepted epoxy thicknesses of 0.006 to 0.012 in. (0.15 to 0.30 mm).
Alternatives to stay-in-place metal deck forms should be considered. These forms resulted in concrete curling that can exacerbate cracking on the top surface of the deck, provide for a crack initiation location due to the pan shape, and prevent visual inspection of the bottom deck surface. Removable formwork with a flat surface eliminates these problems.
Support of formwork through the use of an angle with a leg turned into the deck should be discontinued. As an alternative, the angle can be turned down to eliminate this discontinuity.
The recommendations outlined above have been implemented in several bridges in Indiana. Some of these projects have been accompanied by companion research studies to evaluate the performance of bridge decks incorporating the recommendations.(2) These studies clearly indicate that the proposed recommendations are effective in controlling bridge deck cracking. Furthermore, these projects demonstrate that proper control of bridge deck cracking requires consideration of materials selection, reinforcement design, and construction procedures. It should be noted that all bridge decks in Indiana now require a 7-day wet cure. While alternative deck forming methods were considered desirable, the original construction technique has been maintained at the present time due to contractor familiarity and economic considerations.
Additional research studies are ongoing to provide refinements to the recommendations and provide extension of the recommendations when fiber reinforced polymer reinforcement is specified. Preliminary findings indicate that the maximum reinforcement spacing can be increased to 9 in. (230 mm). Additional field implementations are planned with a major project being the reconstruction of I-465 around the west side of Indianapolis. It is anticipated that the results of this research and field implementation program will be integrated into design and construction specifications to enable widespread application and provide high performance bridge decks that are capable of extended service lives with lower life-cycle costs.
References
- Frosch, R. J., Blackman, D. T., and Radabaugh, R. D., “Investigation of Bridge Deck Cracking in Various Bridge Superstructure Systems,” Joint Transportation Research Program, FHWA/IN/JTRP-2002/25, 160 pp.
- Frosch, R. J. and Aldridge, T. S., “High-Performance Concrete Bridge Decks: A Fast-Track Implementation Study, Volume 1: Structural Behavior,” Joint Transportation Research Program, FHWA/IN/JTRP-2005/11, 160 pp.