After placing, vibrating, and finishing concrete for a bridge deck, contractors sometimes notice cracks forming directly over the top-mat rebar. Most likely, these cracks are subsidence or settlement cracks, which form when fresh concrete settles around the rebar and separates, usually within 2 to 5 hours of concrete placement. The cracks can also appear over form ties, embedments, or other fixed items in the concrete. But on bridge decks these cracks are especially undesirable because they allow water and deicing salts to easily enter the concrete, increasing the probability of surface spalls caused by freeze-thaw cycles and rebar corrosion.
In 1975, researchers showed that increasing concrete cover over the top rebar effectively limited subsidence cracking. But putting the top bars deeper in the deck has a downside: As the distance of the bars from the surface increases, their ability to control the width of other deck cracking decreases. How can designers limit subsidence cracking without specifying excessive cover depths? To find out, The Aberdeen Group conducted additional research by adding synthetic fibers to concrete test specimens as a means of decreasing the required cover depth. Synthetic fibers have two effects on fresh concrete properties that should help reduce subsidence cracking: They reduce bleeding so solids in the concrete don't settle as much, and they increase the tensile strength.