James T. Pappas, Delaware Department of Transportation
The Delaware Department of Transportation (DelDOT) first discussed the use of high performance concrete (HPC) with the Federal Highway Administration early in 1996. Based on these discussions, DelDOT initiated trial projects with HPC specifications. Although these were formal HPC specifications, DelDOT had been utilizing ground granulated blast-furnace slag (GGBFS) and other pozzolans in concrete since the late 980s to take advantage of the improved workability, protection against alkali-silica reactivity, and lower permeability.
The first contract incorporating the HPC specifications was for ramp widening at U.S. 202 to I-95 southbound in Wilmington. This structure was bid and constructed in late 1996. To date, DelDOT has constructed five projects with the HPC specifications and two more projects are in the design phase. Constructed bridges include three bridges in Frederica, one bridge in Little Creek, and one bridge in New Castle County. Bridges being designed include one in Milton and Churchman’s Road bridge over I-95 in Wilmington.
The benefits that our designers look for when specifying HPC include increased durability, lower concrete permeability, and higher compressive strengths. The latter allows for longer spans, thereby reducing substructure costs.
Specifications that were created for the HPC concrete are shown in the table. For concrete Classes A and D (cast-in-place), most producers have chosen to meet the specifications by using a 7 percent addition of silica fume to the specified minimum cementitious materials content of 705 lb/cu yd (418 kg/cu m). For concrete Classes B, B/Slipform, and Precast, producers and suppliers have used GGBFS at 50 percent of the specified minimum cementitious materials content of 564 lb/cu yd (335 kg/cu m) to meet the specifications.
The DelDOT specifications require the producer to cast trial batches of the proposed mixtures at least 28 days prior to incorporation into the project. Chloride permeability and compressive strength tests are then made on field-cast cylinders.
There have been a few problems associated with the HPC. The most serious problem was on one bridge in Frederica. The contractor was unfamiliar with HPC and had difficulty with finishing and curing the concrete bridge deck. The first attempt at placing the deck surface resulted in numerous random cracks with widths up to 1/8 in. (3 mm). This required removal of approximately 1-1/2 in. (38 mm) of the top surface of the original deck and subsequent replacement. The cause of cracking was determined to be the contractor’s inability to start curing the silica fume concrete soon enough. The contractor treated the HPC in the same way as conventional concrete. The lesson learned in this experience was that when placing deck concrete containing silica fume, the contractor must “underfinish and overcure” the concrete.
Aside from this major problem, the other problems associated with HPC have been relatively minor. The success of a project depends upon proper design, proportioning, mixing, placing, consolidation, and curing of the concrete. DelDOT realizes that one of the keys for successful implementation of HPC is education of the concrete supplier and placement contractor. Consequently, pre-pour meetings are mandatory for construction personnel, materials personnel, contractors, and concrete suppliers.
Further Information
For further information, contact the author at 302-760-2400 or [email protected]