Kyle Schultz, Walsh Construction Company of Illinois
The use of high performance concrete (HPC) on the Wacker Drive Reconstruction project presented several special challenges to the general contractor. Some of these resulted from the concrete mix proportions while others resulted from the special provisions.
The first challenge was the erratic air content following the addition in the field of the high-range water-reducer (HRWR). During the first weeks of production, the addition of the HRWR would cause the air content to either increase or decrease, with no apparent pattern. Further, the required dosage of air-entraining admixture to achieve the specified 5.5 to 8.5 percent air content was not consistent from one day to the next. Not knowing which direction the air content was going following the addition of the HRWR would lead to rejected material. This problem was resolved by the concrete supplier trying different batching sequences with the admixtures, and deciding to dose the concrete with a third of the HRWR during initial batching. This provided a predictable loss of air content after the balance of HRWR was added on site. However, the day-to-day required amount of air-entraining admixture remained erratic throughout the project.
The second issue was what to do when the air content was too high. Typically, high air content of concrete can be reduced by additional mixing, or by stopping the truck’s drum completely for a time. However, the HPC on this project seemed reluctant to release entrained air regardless of these, or any other techniques. It was believed that the four cementitious materials made for a very sticky mix and the entrained air was unable to escape from the cementitious materials. This problem persisted throughout the project, and was the primary cause of rejected concrete.
The concrete adhered aggressively to the formwork and made it necessary to strip bulkheads and pull keyway formwork as soon as possible. Delayed stripping tended to require the use of jackhammers and labor intensive prying to remove the formwork.
The mix proportions made it difficult for smaller concrete pumps to move the HPC through horizontal pipes or flexible lines. The larger 150- and 170-ft (46 and 52-m) long truck-mounted booms did not experience the same problems as the smaller pumps. The HPC was also susceptible to high air loss during pumping. Boom configurations were selected to minimize vertical drops. All superstructure placements required a “double 90° bend” at discharge to generate a constant stream and minimize air loss. It also became necessary to equip boom lines with a reducer to minimize the air loss. Without a reducer or “double 90° bend,” air loss could be as high as 5 to 6 percent; with the reducer, air loss at point of placement was typically limited to 2 to 3 percent.
Despite these minor problems, the HPC at Wacker Drive was relatively easy to work with. The design required the use of a HRWR with a maximum slump of 8 in. (200 mm). Though intended to provide for maximum workability while maintaining a low water-cementitious materials ratio, the flexibility for a higher slump allowed for adjustments when placing concrete around critical areas such as post-tensioning anchors and under horizontal bulkheads, or utilizing different placement and finishing methods. This played an important part on the night of July 24, 2002. On that night, four separate crews placed the largest and most complicated section of bridge deck utilizing a 170-ft (52-m) long boom concrete pump, two back-out conveyors, and two truck conveyors simultaneously. Slumps were easily adjusted to provide maximum workability and productivity for each crew.
The HPC required specific curing procedures. As soon as possible, cotton blankets were placed on the freshly cast deck. Soaker hoses were then placed atop the blankets to keep the blankets saturated during the 7-day curing period. Finally, white polyethylene sheeting was placed on top of everything. The average bridge deck placement required about 4,500 sq ft (418 sq m) of blanket and polyethylene and 1800 ft (549 m) of soaker hose. During placement of bridge decks, weather conditions were monitored to calculate the rate of evaporation and minimize plastic shrinkage cracks. When conditions required, continuous fogging was maintained until conditions permitted its discontinuation or until the curing blankets were applied.
Though the start of HPC production was a little rough, we simply rode out the learning curve. After which, working with the HPC was no different from any other concrete. Everyone grew accustomed to the HPC with its special requirements and construction never seemed to be hampered due to the mix.