In late 2009, the Echo Wash and Valley of Fire bridge decks were constructed in the Lake Mead National Recreation area in Nevada. Within six months, in early 2010, both decks exhibited considerable transverse cracking, with some cracks extending through the complete deck thickness. Similar cracking was observed in the Snake River bridge deck in Wyoming. This report details the results of a two-pronged approach to investigating the causes of such cracking. First, for the Nevada bridge decks, similar materials to those used in the bridge deck construction were obtained and mortars were prepared and evaluated for chemical shrinkage, autogenous shrinkage, and drying shrinkage. Second, cores from all three bridge decks were obtained and analyzed using both optical microscopy and scanning electron microscopy to assess air contents, paste and aggregate volume fractions, and the overall nature of the concrete microstructure. In all three cases, the concrete mixtures had been batched at a considerably lower water-to-cementitious materials ratio (w/cm) than that in the approved concrete mixture proportions, with reductions from the specified w/cm of approximately 0.40 to w/cm of 0.35 (Snake River) to 0.31 or 0.32 (Nevada). In the laboratory mortars, w/cm was therefore varied between 0.30 and 0.40 to examine the influence of this variable on shrinkage and cracking tendencies. The observed effects of w/cm on shrinkage and cracking were actually small, while petrographic analysis revealed that the in place concrete cores exhibited a w/cm of 0.4 or slightly higher, as the concretes had been retempered with additional water after being batched. This retempering produced an extremely inhomogeneous microstructure with dramatically high air contents that ultimately contributed greatly to the observed cracking.
Citation: NIST Interagency/Internal Report (NISTIR) - 7841Report Number:
NIST Pub Series: NIST Interagency/Internal Report (NISTIR)
Pub Type: NIST Pubs
Air content, autogenous shrinkage, bridge deck, early-age cracking, petrography, retempering.