Bridge Deck Cracking - Phase I
Pooled Fund Study for the Construction of Crack-Free Concrete Bridge Decks
Research Project Statement:
Cracks in concrete bridge decks provide easy access for water and deicing chemicals that shorten the life of the deck. Both materials increase the effects of freeze-thaw damage, while the deicing chemicals lead to higher concentrations of chlorides, and subsequently, corrosion of reinforcing steel. Measurements taken on bridges in Kansas show that dense, high quality concrete can significantly slow the penetration of chlorides to the level of the reinforcing steel. However, measurements taken at cracks show that the chloride content of the concrete can exceed the corrosion threshold at the level of the reinforcing steel by the end of the first winter. The formation of cracks, thus, significantly lowers the effectiveness of other techniques that are used to increase the life of a deck.
Research, some of which dates back over 30 years, has addressed the causes of cracking in bridge decks in North America. The research includes three detailed bridge deck surveys carried out by the University of Kansas during the past decade. The results of the studies provide specific guidance on modifications in materials and construction techniques that will reduce the amount of cracking in bridge decks: Settlement cracks, transverse deck cracks that form immediately over reinforcing bars, can be reduced with increased cover, decreased bar size, and decreased concrete slump. Shrinkage cracks can be reduced by decreasing the volume of water and cement, and maintaining an air content above 6%. Optimized aggregate gradations can be used to minimize the cement paste constituent of concrete, and workability can be enhanced at reduced paste contents using water reducers and superplasticizers. Increased compressive strength, normally associated with high-performance concrete, often has a negative impact on cracking. During construction, 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 are also associated with increased total cracking in the completed deck, due to movement of plastic concrete. Techniques such as wind breaks and fogging have had a positive impact on the problem, as has thorough curing of the concrete. The surveys by the University of Kansas demonstrate that, in general, cracking increases with increased age. However, concretes cast in different eras exhibit significantly different amounts of cracking — decks in Kansas cast between 1983 and 1987 average less than half the crack density of bridges cast since 1990. This observation is taken as evidence by some of the effect of the progressively finer gradations of Portland cement that have been produced to provide higher early strengths, but that also produce concretes with a greater tendency to shrink.
In spite of this accumulation of knowledge, only a small number of these findings have been used to implement changes in bridge deck design and construction procedures. In specific cases, on-site observations indicate that it is possible to develop nearly crack-free bridge decks, if “best practices” are followed. Even with these few successes, most bridge decks exhibit significant cracking, exposing the reinforcing steel to deicing chemicals and subsequent corrosion and increasing the degree of saturation, which increases the impact of freeze-thaw cycles. However, the current level of understanding offers the potential of constructing bridge decks with minimum cracking on a routine basis.
The purpose of this study is to implement the most cost-effective techniques for improving bridge deck life through the reduction of cracking. The work involves cooperation between cement companies (including the development of coarser, low-shrinkage cement), contractors, designers, and state agencies. The following tasks are being used to achieve this objective.
- Develop a detailed plan to construct bridge decks with minimum cracking by incorporating “best practices” dealing with materials, construction procedures, and structural design. This step will involve the cataloging of available techniques and meeting with department of transportation personnel from multiple states, as well as other experts, to select the procedures to be used and the bridge types to which they will be applied.
- Work with state DOTs, designers, contractors, inspectors, and material suppliers to modify designs, specifications, contracting procedures, construction techniques, and materials to obtain decks exhibiting minimal cracking. In this step, for example, contractor pay items may need to be changed to ensure that the concrete is fully consolidated, plastic shrinkage is minimized, and high-quality long-term curing is used. Concrete mixes with low cement contents and cements with low shrinkage characteristics will be evaluated. Also included in this task is a cost-benefit analysis to determine the feasibility of new construction and/or design provisions. Results of this task will help guide the final implementation scheme.
- Select bridges to be constructed using “best practices,” and pre-qualify designers and contractors in application of the techniques. Twenty bridges, 10 in northeast Kansas and 10 in other participating states, will be constructed using the new techniques. Researchers from the University of Kansas and state DOT personnel will work closely with designers and contractors to achieve the desired results. Pre-qualification of designers and contractors will include workshops sponsored by the University of Kansas to help educate and train engineers in implementing the “best-practices” identified in Tasks 1 and 2.
- Carry out detailed crack surveys on the bridge decks, three months, six months, one year, two years, and three years after construction. The surveys will be done using techniques developed at the University of Kansas that involve identifying and measuring all cracks visible on the upper surface of the bridge deck. Initially, surveys will be done by the University of Kansas. As the project progresses, teams from outside of the State of Kansas will be trained in the survey techniques.
- Correlate the cracking measured in Task 4 with the environmental and site conditions, construction techniques, design specifications, and material properties and compare with earlier data. Similar data from participating states, where it exists, will be incorporated in the analysis. Final cost estimates will be compared with potential benefits.
- Document the results of the study. A final report will be prepared and disseminated to participating states regarding the findings of Tasks 1-5. Recommendations for further implementation and/or studies will be discussed in a final presentation of results to a committee of the Pooled-Fund Study participants.
- Develop a training program to assist the participating states in implementing the findings of the study. The program will consist of workshops to be held at the representative state DOT offices. These workshops will be individually coordinated with each participating DOT.
State departments of transportation expend significant effort and resources on the construction of durable reinforced concrete bridges and bridge decks. Existing data indicates that specific modifications to construction procedures, materials, and design details will significantly reduce the degree of cracking in bridge decks, and thus reduce exposure of reinforcing steel to the corrosive effects of deicing chemicals and decrease freeze-thaw damage. Of the two, corrosion is by far the greater problem. The project will provide a mechanism for combining ideas from research and practice to develop a comprehensive strategy for the construction of bridge decks. If successful, the result will be a major reduction in bridge deck cracking, an improvement in durability, and an increase in the useful life of bridges. A great deal is known about the factors that affect cracking in bridge decks — it is time to implement that knowledge.