In recent years, the Bridge Section of the Idaho Transportation Department (ITD) has observed increased cracking in concrete bridge decks. This increase in cracking has occurred in concrete bridge decks that have used designs that historically produced satisfactory performance. The cracking has occurred both in new decks and in decks that have been subjected to traffic for various periods of time.
To determine possible reasons for the increased cracking, researchers at the University of Idaho have performed a literature review of articles, papers, and standards focused on bridge deck cracking. In addition, the deck of a bridge for highway US 95 constructed over the South Fork of the Palouse River was instrumented with strain and temperature gauges. This project was the first in the State of Idaho involving the use of HPC for the bridge deck. The weather and concrete placement procedures were also monitored and material testing was performed on the deck concrete. The deck was placed in two stages; the first stage portion of the deck was constructed using the conventional Idaho Class 40A mix, while the second stage portion was constructed using a high performance concrete mix.
The results of the monitoring and testing on both stages of the bridge deck were then compared to the literature review to determine the cause of cracking in the deck. In addition, the report compares the concrete used in the two bridge decks to determine if the high performance concrete mix provided any improvement with respect to cracking. Finally, the report presents recommendations on how to reduce cracking. Results from the monitoring and testing of the stage 1 deck indicated that cracking in the concrete was mostly due to restraint of the deck by the girders and parapet wall. Uplift from skew and high heat of hydration temperatures were the main causes of tensile stress build up in the deck, compounded by the low creep and high modulus of elasticity of the concrete used. Results from the monitoring and testing of stage 2 indicated that cracking in the concrete was also mostly due to restraint of the deck by the girders and parapet wall. Reducing the cement content, adding fly ash to the mix, decreasing skew, and/or reducing deck restraint appear to be effective in reducing deck cracking.