Sreenivas Alampalli and Frank T. Owens, New York State Department of Transportation
When the New York State Department of Transportation (NYSDOT) decided to improve the quality of its bridge decks, it developed a three-phase program to achieve the desired results. The first phase was to enhance the deck material itself, making it more durable for a longer service life. The second phase focused on optimizing bridge deck design. The third, and probably the most important phase, was to develop practical and effective construction practices to ensure realization of the material’s enhanced properties.
In Phase 1, the existing New York State bridge deck concrete mix was modified to enhance its durability characteristics. The new high performance mix takes advantage of a reduced water/cementitious material ratio and contains fly ash and silica fume substitutions for a portion of the portland cement. Designate Class HP concrete for Class H modified with Pozzolanic substitutions, this material has been specified as the standard for use on all structural slabs and approach slabs since April 1996. By June 1998, more than 80 bridge decks had been constructed using this more durable concrete.
Reports of material performance and workability of Class HP concrete were favorable, and theoretical estimates of service life were very encouraging. Based on an analytical model that assumed 3 in. (76 mm) of cover and the use of uncoated reinforcing steel, it was estimated that corrosion might be expected to commence after 62 years for Class HP concrete decks. This age is over 2.5 times longer than that for previous concrete mixes used in bridge decks. Late in 1998, a study was undertaken to quantify these enhanced performance characteristics.
Study Approach
All new and replacement bridge decks built using the HP concrete mix from 1995 through early 1998 were inspected. Number, length, and plan location of all non load-related transverse cracks were charted. Results were compared with the performance of concrete decks conforming to previous specifications. Crack initiation time and effects of staged construction on deck cracking were also investigated.
Survey Results
The results of the study revealed that 49 percent of the 84 inspected decks exhibited no cracking. Transverse cracking was found on 48 percent of the decks and longitudinal cracking on 44 percent. Forty percent of the bridge decks exhibited both transverse and longitudinal cracking. Although it might be expected that years-of-service would have a significant negative effect on deck condition, the data revealed no influence of deck age on deck cracking.
Staged lane construction is used to minimize disruption of traffic flow in New York State and the study examined possible influences of this type of construction on deck cracking. The results indicated that staged construction has no negative effects on the deck. Average transverse crack density of cracked HP decks was found to be 6.9 cm/m2, with a maximum density of 26.8 cm/m2 which is comparable to the lower end of crack densities referenced in earlier NYSDOT studies. Of these cracked decks, over fifty percent began cracking within 14 days of the concrete placement.
Inspectors were asked to qualitatively compare Class HP decks to previously specified concrete decks. More than eighty percent of the responses reported that HP decks performed about the same or better than the previously specified concrete in resisting transverse and longitudinal cracking. Based on a statistical analysis, cracking densities were found to be independent of superstructure type, material, span length, or support condition.
Conclusions
Results of this study indicated that performance of deck material has improved since the introduction of Class HP concrete for New York State bridge decks. “Performance” is measured in terms of increased crack resistance without compromise in workability, construction practices, or cost. It was reported, through visual inspection, that Class HP bridge decks cracked with less frequency, and exhibited narrower and shorter cracks than their non “high performance” counterparts. It was also observed that most cracks occurred within two weeks of the deck pour and were not influenced by staged lane construction.
Further Information
For further information about New York State Bridge Decks, see HPC Bridge Views No. 6 for a companion article or contact the author at (518) 457-5826 or [email protected].