Tom Koch, North Carolina Department of Transportation
The first application of high performance concrete (HPC) by the North Carolina Department of Transportation (NCDOT) was on dual bridges on U.S. 401 over the Neuse River just north of Raleigh, NC. The project consists of two 4-span structures with precast, prestressed concrete girders made continuous for live load with a cast-in-place concrete deck. Each structure consists of two, 92-ft (28.0-m) long spans of AASHTO Type IV girders and two, 57-ft (17.5-m) long spans of AASHTO Type III girders. Use of 10,000 psi (69 MPa) HPC in the girders and 6000 psi (41 MPa) HPC in the deck allowed the designer to reduce the number of girder lines from six to five. Both northbound and southbound structures used HPC. The southbound structure was instrumented by NC State University (NCSU) researchers.
Once the project had been chosen, a team of NCSU researchers and NCDOT personnel met to establish target performance criteria for the HPC. It was agreed that the material characteristics of strength, modulus of elasticity, shrinkage, creep, and chloride permeability were the most important and the team established target values for these characteristics.
The girders were cast in October 2000. The fabricator was responsible for the concrete mix design, which used a 0.30 watercementitious materials ratio, 900 lb/cu yd (534 kg/cu m) of Type I/II cement, and 50 lb/cu yd (30 kg/cu m) of silica fume. An initial requirement for a maximum slump of 6 in. (150 mm) was changed to 8 in. (200 mm) when the first casting had severe honeycombing that resulted in rejection of the girder. Cylinders for strength and modulus of elasticity tests were match cured to ensure that the test results accurately reflected the girder concrete properties.
The test results for all characteristics showed that with the exception of modulus of elasticity and chloride permeability, the girder concrete properties met the target values. The concrete strengths averaged 10,500 psi (72.4 MPa) at 28 days, compared to the specified value of 10,000 psi (69 MPa). The measured creep of the girder concrete after 120 days of loading was 0.25 and 0.21 millionths/psi (36 and 30 millionths/MPa) for the Type III and Type IV girders, respectively. This met the performance criteria of 0.31 to 0.21 millionths/psi (45 to 30 millionths/MPa). The measured shrinkage after 120 days was slightly above the target value of 400 millionths; the researchers felt that this was due to the relatively high cement content.
The modulus of elasticity had an average measured value of 5100 ksi (35 GPa) compared with a target value of 6000 to 7500 ksi (41 to 52 GPa). The rapid chloride permeability test values for the Type III and Type IV girders were 3700 and 4557 coulombs, respectively, at 56 days. The target value was between 800 and 2000 coulombs. The values had reduced to 2500 and 1250 coulombs, respectively, by the concrete age of 90 days.
Test results for the bridge deck concrete, which was designed by the ready-mixed concrete supplier for a concrete compressive strength of 6000 psi (41 MPa) at 28 days, were varied. The mix design required 587 lb/cu yd (348 kg/cu m) of Type I/II cement and 175 lb/cu yd (104 kg/cu m) of Class F fly ash. Test cylinders for the northbound structure had an average strength of 7150 psi (49.3 MPa). Cylinders for the southbound bridge, however, did not achieve the required strength. Results from three of the five sets of cylinders with an average strength of 5700 psi (39.3 MPa) were accepted in anticipation that the strengths would achieve 6000 psi (41 MPa) at 56 days. Results from the other two sets, one with an average strength of 4100 psi (28.3 MPa), were well below the required strength. It is unclear why the deck concrete for the southbound structure had low strengths.
Long-term monitoring of the deck and girders began at completion of the construction phase of the structure. This monitoring will take place over a three-year period and will measure girder stiffness and deflection, creep and shrinkage effects, and thermal effects. A live load test will be conducted prior to opening the bridge and again 12 months later to assess any changes in overall bridge performance due to service and thermal loads.
North Carolina’s need for girders and bridge decks with low permeability in corrosive environments ensures that the use of HPC bridges will increase. In conjunction with this effort, North Carolina plans to increase the use of 0.6-in. (15.2-mm) diameter strands in the girders to more fully utilize the high strength attributes of HPC.
Further Information
For more information, contact the author at 919-250-4046 or [email protected]