Question: We have just begun production on a precast bridge deck that calls for production of thousands of match cast deck segments sized approximately 3.5x7x2 meters. Our operation uses the freshly cast segment as a forming unit. We plan to accelerate the cure for the first two hours of the 12-hour curing process. Thus each segment would undergo curing twice, once as a fresh cast, then as a match piece. Does reheating precast segmental units cause any harmful effects on durability or other concrete properties?

Answer: We know of no harmful effects on durability. In some cases, however, thermal gradients can warp the match forming unit and cause a permanent deformation in the freshly cast segment. Significant deformation is likely to occur when the segment's wing-tip to wing-tip width (w) is very large compared with the segment length (L).Researchers at the University of Texas point out that the warped segments may also result in overly thick joints and, in extreme cases, can cause cracking of the segments (Ref. 1). Their study suggested that w/L ratios exceeding 6 could present similar problems.To minimize warping, the entire match forming segment should be exposed to the same curing environment as the segment being cured (Ref. 2). However, this is often impractical since both pieces may not fit in the curing area.When both pieces don't fit in the curing area, keep the exposed part of the match forming segment as warm as possible during curing. Curing blankets and plastic sheeting may help in warmer climates. It will also help to orient the casting beds so exposure to the sun can keep the free face warm. Morning castings allow the rising ambient temperatures to warm the match forming segments as well.References1. Carin Roberts-Wollmann, John Breen, and Michael Kreger, "Temperature Induced Deformations in Match Cast Segments," PCI Journal, July/August 1995, pp. 62-71.2. Recommended Contract Administration Guidelines for Design and Construction of Segmental Concrete Bridges, American Segmental Bridge Institute, March 1995, pp. 76-77.