Evaluating Free Shrinkage of Concrete for Control of Cracking in Bridge Decks

Abstract

The effects of paste volume, water-cement ratio, aggregate type, cement type, curing period, and the use of mineral admixtures and superplasticizers on the free shrinkage of concrete are evaluated with the goal of establishing guidelines to reduce cracking in reinforced concrete bridge decks. Three concrete prisms were cast and tested in accordance with ASTM C 157 for each mixture up to an age of 365 days under controlled conditions of 23 ± 2°C (73 ± 3°F) and 50 ± 4 percent relative humidity. The work was organized in five test programs. The first program included mixes with water-cement ratios of 0.40, 0.45, and 0.50, and aggregate contents of 60, 70, and 80 percent, with Type I/II cement and Type II coarse-ground cement. The second program included the mixes with one of three coarse aggregate types, granite, limestone, and quartzite. The third program evaluated the effects of Class C fly ash, ground granulated blast-furnace slag, and silica fume as partial volume replacements for portland cement. The fourth and fifth programs were used, respectively, to evaluate the effect of curing period (3, 7, 14, or 28 days) and the use of different superplasticizer types and dosages. The results indicate that concrete shrinkage decreases with an increase in the aggregate content (and a decrease in the paste content) of the mix. For a given aggregate content, no clear effect of water-cement ratio on the shrinkage is observed. In general, granite coarse aggregates result in lower shrinkage than limestone coarse aggregates. A similar conclusion cannot be made with quartzite coarse aggregate, although in some cases shrinkage of concrete containing quartzite coarse aggregate was lower than that of concrete containing limestone. The use of partial volume replacement of portland cement by Class C fly ash without changing the water or aggregate content generally leads to increased shrinkage. The use of partial volume replacement of portland cement by blast furnace slag without changing the water or aggregate content can lead to increased early-age shrinkage, although the ultimate shrinkage is not significantly affected. An increase in the curing period helps to reduce shrinkage. The use of Type II coarse ground cement results in significantly less shrinkage compared to Type I/II cement. The use of superplasticizers in concrete appears to increase in shrinkage to a certain degree. The results, however, do not present a clear picture of the effect of superplasticizer dosage on shrinkage.