|FHWA > Engineering > Pavements > Concrete > Full-Depth Repairs|
The concrete mixture design should be based on the available lane closure time. Nowadays, State highway agencies often want to open full-depth repairs as soon as possible to alleviate traffic congestion. The shorter the time available before opening to traffic, the more rapid the strength gaining and curing of the concrete must be, and also the more expensive the concrete becomes.
Typical full-depth repairs operations utilize concrete mixes containing 390-502kg/m3 (658-846 lbs/yd3) of either cement Type I or Type III cement. A set-accelerator is frequently used to permit opening in 4 to 6 hours. Without the accelerator, these mixes allow opening in 12 to 72 hours. The use of proprietary concrete mixes are necessary to achieve opening times in as little as 2 hours. Using insulating blankets ( or boards) during the first few hours after placement also can improve the strength development of any mix. Regardless of the mix design used, the concrete mixture for full-depth repairs should have the following properties:
Mixes using Type III cement may require slightly more mix water than a similar mix with Type I portland cement. However, too much extra water may cause the concrete to suffer from high shrinkage during curing. A water-reducing admixture will disperse cement particles and reduce the water necessary for workability.
Calcium chloride (CaCl2) or another accelerating chemical admixture is recommended for use as accelerator in the patching concrete, provided that it is added as specified. It should be noted that initial set may occur within 30 minutes on warm days, therefore, use only 1% of calcium chloride by weight of cement when air temperature exceeds 27° (80°). Up to 2% is acceptable in lower temperatures. For on-site mixing, add calcium chloride in liquid form to the mixer before other admixtures are added (except the air-entraining admixture). When using calcium chloride, considerations should be given to the remaining service life of the adjacent pavement and weather dowel bars and reinforcing steel are coated.
If calcium chloride or other accelerating admixture are being added at the plant and the concrete consistently arrives at the site too stiff, then the calcium chloride should be added at the site. If , after the addition of calcium chloride at the site, the concrete is still too stiff, the ready-mix plant operator should be notified to increase the slump an appropriate amount, up to 150 mm (6 in).
Chemical admixture may be added to the concrete at the batch plant if the air temperatures are moderate ( less than 20°[68°F]) and the batch plant is less than 15 minutes from the project site. Non chloride accelerators are recommended for CRCP and JRCP full-depth repairs.