| Principal Investigators David Darwin Deane E. Ackers Professor of Civil Engineering and Director of the Structural Engineering and Materials Laboratory JoAnn Browning Professor of Civil, Environmental, & Architectural Engineering |
POOLED FUND STUDY FOR THE CONSTRUCTION OF CRACK-FREE CONCRETE BRIDGE DECKS |
| Graduate Research Assistants
Research Links KU Mix Quick Start Guide 2.2-1 SM Report 94 "Development and Construction of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Construction Methods, Specifications, and Resistance to Chloride Ion Penetration" SM Report 97 "Lightweight Aggregates as an Internal Curing Agent for Low-Cracking High-Performance Concrete" |
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. Research Objectives: 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.
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| Phase I Sponsors
City of Overland Park, Kasas Delaware Department of Transportation Federal Highway Administration Idaho Transportation Department Indiana Department of Transportation Kansas Department of Transportation Michigan Department of Transportation Minnesota Department of Transportation Mississippi Department of Transportation Missouri Department of Transportation Montana Department of Transportation New Hampshire Department of Transportation North Dakota Department of Transportation Oklahoma Department of Transportation South Dakota Department of Transportation Texas Department of Transportation Wyoming Department of Transportation |
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| PHASE II | |