Gary Hall, Prairie Material Sales, Inc.
High strength concrete for high-rise buildings, post-tensioned parking structures, and overlays or special repair concrete for existing structures has been produced in Chicago for many years. Most major Chicago area ready-mixed concrete producers have quality control programs and procedures for production of such concrete. The Wacker Drive Reconstruction Project with its special concrete requirements, however, needed a much greater level of commitment of high quality materials, personnel, and time.
An extensive prequalification process determined the raw materials and concrete mix proportions to achieve the desired properties.* Some materials were locally available while others were brought in from out of state. The project specifications required a rigorous sampling and testing program. Samples of all admixtures and cementitious materials, along with their mill certifications were required weekly. Additional samples were also required for every bridge deck placement. Aggregate gradations, moisture contents, and absorptions had to be measured every time that HPC was produced, whether it was 1 or 2000 cu yd (0.7 or 1500 cu m). Materials that did not comply with project specifications were rejected.
To enable production of the quaternary mix, the two central mix plants at the production yard, located near downtown Chicago and less than 2 miles (3.2 km) from the project, were each set up to accommodate the four cementitious components of the mix. Mechanical adjustments for each cementitious material gate, as well as adjustments to the parameters of the batching software were necessary to compensate for the different flow characteristics of each material. The silica fume required close attention because, at an addition rate of only 27 lb/cu yd (16 kg/cu m), it could easily exceed the ± 1 percent tolerance. Even on a 9 cu yd (6.9 cu m) batch, the tolerance is only ± 2.4 lb (1.1 kg). Weighing times were increased considerably, due to each of the four cementitious materials having to hit its target weight before the next could be weighed. The batching tolerance should be reconsidered when the batch weight of a cementitious material is relatively small. An increase in mixing time of about 50 percent was absolutely necessary in order to ensure optimal uniformity of the HPC mix.
The mix was typically batched with a water-cementitious materials ratio of 0.36, producing an initial slump of 3 to 4 in. (75 to 100 mm). Job site addition of high-range water-reducer (HRWR) increased the slump to approximately 7 to 8 in. (175 to 200 mm). Job site control of air entrainment was a problem encountered early in the project. After some discussion, it was decided to introduce a portion of the HRWR into the mix at the time of batching in an effort to reduce the variability of air content after final jobsite addition of HRWR. This approach seemed to mitigate the variation of air content, but constant attention was necessary for the duration of the project. Frequent sampling was done at the plant, and each batch of concrete was tested by the contractor at the jobsite for slump, air content, and concrete temperature.
Prior to every major deck placement, a pre-pour meeting was held at the jobsite field office and required attendance by personnel from our operations and quality control departments. That portion of the meeting directly involving the ready-mixed concrete producer addressed the rate of delivery, routing of trucks, location of concrete testing stations, and type and configuration of placing equipment. The latter was important due to its impact on plastic air content of the concrete at the point of placement. At times, it was necessary to ship concrete exceeding the upper limit of the specified air content in order to be within specification at the point of discharge.
During the deck placements, a yard manager and quality control technician were required at the plant, while at least one operations and two quality control personnel were present on the jobsite. Air entraining and HRWR admixtures were at hand to provide field adjustment of mixes when required. Constant communication was always maintained between the plant and jobsite, allowing for reasonably quick adjustments to the mix or delivery rate. Due to the size of the deck placements, involvement and coordination of many people were required to make the placements go smoothly. An outstanding job was done by the City of Chicago, its engineering consultants, and contractors.
Production and delivery of HPC for the Wacker Drive Project was very challenging. Though representing only a small percentage of total concrete production at our plant serving downtown Chicago, the HPC mix required considerably more attention to materials testing and production. While at times difficult, it was not impossible. HPC is not only materials and finished product. It can also be considered an ideology that must be adhered to. As a result, the new Wacker Drive is an impressive structure that should not require reconstruction for many years to come.
*See HPC Bridge Views, Issue No. 19, January/February 2002.