Timothy P. Schmidt, Alfred Benesch & Company
The Chicago Department of Transportation (CDOT) retained Alfred Benesch & Company to provide construction management services for the Wacker Drive Viaduct Reconstruction Project. Our duty was to oversee the work of the contractors and the resident engineering (RE) consultants retained by CDOT on each of the three contracts used for the project. The RE consultants performed the quality assurance activities. For the high performance concrete (HPC), the general contractor and their concrete supplier performed the specified quality control activities.
The HPC used on the project possessed many good properties. The maximum slump of 8 in. (200 mm) after the addition of a high-range water-reducer (HRWR) produced a very workable mix that was easily pumped. A high degree of workability was essential for good consolidation of the concrete in areas of the bridge deck with highly congested reinforcing steel, posttensioning ducts, and post-tensioning anchors. The concrete had little segregation during pumping. We achieved the minimum compressive strengths of 4200 psi (29 MPa) for post-tensioning at about 3 days and 6000 psi (41 MPa) at 28 days with few problems.
One aspect of the HPC specifications found to be very effective was the use of cotton mats and soaker hoses for concrete curing. For standard bridge decks in Illinois, the specifications require installation of wetted burlap and plastic sheeting for the curing materials. The contractor is then required to keep the burlap wet during the curing period. Our specification allowed the contractor to install the cotton mats dry and saturate them with water immediately after installation. The contractor found the dry installation much easier to perform. Soaker hoses and plastic sheeting were then placed over the cotton mats. The cotton mats absorbed and retained a significant amount of water. It appeared that, even if the soaker hoses were not operating, the mats had absorbed enough water that they would always be wet. This specification saved both inspection time and potential confrontations with the contractor about installing and
keeping the burlap wet.
When placement of HPC for the substructure units began, we found a high degree of variability in the air content of the plastic concrete. This occurred on days when small volumes of concrete were placed. There were instances when large fluctuations in the air content were found after the addition of the HRWR. Occasionally, truckloads were rejected due to high air content. After several months, the QC/QA team decided to add some of the HRWR during mixing at the concrete plant. The balance of the HRWR was then added on the jobsite. This practice helped to “condition” the load of concrete and decreased the amount of fluctuation in the air contents. Even after instituting the practice of conditioning the loads, the concrete still required a high level of QC/QA effort to ensure proper air content. Fortunately, the problem of fluctuating air content did not plague us during the large deck placements. We attributed the decreased fluctuations to the fact that on days of large deck placements, the concrete supplier’s plant produced concrete almost exclusively for the Wacker Drive Project.
In the future, we suggest using less HRWR for the substructure concrete. Substructure placements generally do not need a high slump concrete. The volume placed is also generally less than that for a deck placement. It appears that the amount of chemical admixtures in a concrete mix is directly proportional to air content fluctuations. On days when smaller volumes are placed, the expected decrease in air content fluctuations would save both time and money.
Despite the challenges of controlling air contents, we found the HPC to be a very workable mix with good strength characteristics. Based on our long-term test data, we believe that the HPC placed on the Wacker Drive Viaduct Reconstruction Project will perform well over its intended minimum 75-year service life.
After reflecting on the project history, we feel the key element to the success of our project was the teamwork of all involved in producing, placing, and testing the HPC. Without the critical teamwork, we would have moved from fixable minor, problems, to major, irreparable ones. The QC/QA team required a learning curve to work past our normally adversarial relationships and to work together effectively. The key to our relationship was our shared goal: to provide our client with the best HPC product possible.