Andrew Maybee, CPI Concrete Products, Inc
Today’s precast, prestressed concrete product manufacturers are meeting owners’ requirements with high performance concrete (HPC). As more and more projects appear that specify the use of HPC, it becomes increasingly apparent that the use of HPC in precast, prestressed concrete bridge components is adding value to the end product. On State Route 840 in Dickson County in Tennessee, the Tennessee Department of Transportation (TDOT) specified HPC on two bridges. Use of HPC was incorporated into both the bridge substructures and superstructures. This article focuses on the fabrication of the HPC bridge girders for the superstructures.
Both HPC bridges use AASHTO/PCI 72-in. (1.83-m) deep bulb-tee girders (BT-72). HPC allowed the engineers to design the longest single-piece BT-72 girders used to date in Tennessee, at a length of 156 ft (47.5 m). These recordsetting BT-72 girders were successfully delivered to the jobsite in September 1999. The delivery of all HPC girders for this project was completed in April 2000. Lateral stability of the long-span girders was of some concern. CPI engineering staff evaluated these conditions and worked closely with the trucking company to provide safe delivery of the girders. Higher strength concretes tend to help when stability is a concern. The concrete strength specified for the HPC girders was 10,000 psi (69 MPa) at 28 days. The HPC specifications also included a minimum cementitious materials content of 658 lb/cu yd, (390 kg/cu m), a maximum water-cementitious materials ratio of 0.43, and a permeability of less than 2500 coulombs at 28 days.
The approach that CPI Concrete Products, Inc (CPI) took for developing HPC was to try and utilize existing concrete materials. Materials that are readily available offer the fabricator an economical solution that can be passed along to the owner. Four years ago, CPI saw high strength and high performance concrete projects on the horizon. We knew that our existing materials were of high quality. We were also aware that our current concreting practices resulted in a very low water-to-cementitious materials ratio and a high strength concrete. We began an in-house concrete research program to test various combinations of aggregates, cementitious materials, and chemical admixtures in an effort to develop HPC. Upgrades to our quality control laboratory were also made to facilitate our research program. This included the purchase and installation of a 500,000 lb (2.22 MN) compression testing machine. After many trial mixes, we achieved a range of possible solutions. Full-scale testing in our central-mix batch plant was a must to assure us that our HPC could be replicated. High quality materials, computerized batch controls, continuous moisture compensation, and well-trained, dedicated personnel were the real keys to our success.
For this project, our trial batches resulted in concrete strengths averaging 9000 psi (62 MPa) in 16 hours and 11,000 psi (76 MPa) at 28 days. Other trial batch testing showed rapid chloride permeability test results of 580 coulombs at 67 days. We were pleased with the project results and learned that with normal materials, but stringent controls on batching and curing, CPI could produce the required HPC. An additional step to evaluate the early strength gain of the HPC was to monitor the internal curing temperature of the concrete and then match cure the test cylinders in the laboratory. CPI used this system to match cure test cylinders and to control the heat curing cycle on the casting bed. During production, the average concrete compressive strengths of 4×8-in. (102×203-mm) cylinders at 28 days were 11,040 psi (76.1 MPa) for match-cured specimens and 11,030 psi (76.1 MPa) for specimens cured alongside the girders.
CPI engineering and production personnel assisted Vanderbilt University faculty and staff in instrumenting four of the HPC girders. Girders were outfitted with strain gages, thermocouples, and camber measuring devices. Coordination of tight fabrication schedules with the installation of these instruments was tricky, but the information gathered from this research project will help TDOT assess the value of HPC and continue to build bridges that use this technology. The future of the prestressed concrete industry lies in the ability of prestressed concrete girder fabricators to meet the continuing demands for high quality products using HPC. Fabricators can and will meet these challenges through teamwork with owners, engineers, contractors, and production personnel.
Further Information
For further information, the author may be contacted at [email protected] or 901-775-9880.