Tim Chojnacki, Missouri Department of Transportation
In the fall of 1998, two companion bridges on Missouri Route 21 over Route M in Jefferson County were opened to traffic. The northbound bridge was constructed using prestressed concrete Igirders made of high performance concrete (HPC) with a design strength of 10,000 psi (69 MPa) and a release strength of 5500 psi (38 MPa). The southbound bridge was constructed using prestressed concrete I-girders made of conventional concrete with a design strength of 5000 psi (34 MPa) and a release strength of 4000 psi (28 MPa). The conventional bridge required six lines of Missouri Department of Transportation (MoDOT) Type 6 girders for a total of 24 girders, while the HPC required only five lines of the MoDOT Type 6 girders for a total of 20 girders.
The costs of fabrication and placement of the HPC girders per foot of bridge was 16 percent higher than for the conventional girders. Reduced maintenance and longer life are expected to offset this initial cost.
Concrete Properties
The HPC mix was designed by the fabricator to meet the requirements of the special provisions. The required compressive strength was 10,000 psi (69 MPa) at 56 days and the required chloride permeability measured in accordance with AASHTO T 277 was 1000 coulombs or less at 56 days. For improved quality control, the HPC specifications were written with tighter tolerances to minimize variations during construction. The coarse aggregate had to meet the Missouri Standard Specifications for pavement quality. The fabricator chose Plattin Limestone. The measured air content of the fresh concrete could be no less than the design air content, nor could it exceed that value by more than 3.5 percentage points. The slump could not exceed 8 in. (203 mm) and had to be within 2 in. (51 mm) of that stated in the approved mix design. The water/cement (w/c) ratio had to be within 0.02 of that specified in the approved mix design. To achieve the strength and permeability characteristics required on the project, the concrete contained 50 lb/cu yd (30 kg/cu m) of silica fume and 850 lb/cu yd (504 kg/cu m) of Type I cement.
The average compressive strengths of the HPC were 11,480 psi (79.2 MPa) at 7 days and 12,360 psi (85.2 MPa) at 28 days. The average compressive strengths of the conventional concrete were 6380 psi (44.0 MPa) at 7 days and 6850 psi (47.2 MPa) at 28 days. In both cases, release strengths were reached in 1 day and design strengths were reached in 3 days. The average chloride permeability of the HPC was very low at 110 coulombs while the conventional concrete had an average value of 3050 coulombs.
Both the HPC and the conventional concrete were subjected to freezing and thawing tests in accordance with ASTM C 666 Procedure B. The freeze-thaw resistance of the conventional concrete was excellent. However, the freeze-thaw resistance of the HPC was poor. This warranted additional investigation and a follow-up research study is currently underway.
All of the test specimens for MoDOT’s research were cured in a “match-cure” environment in an effort to better simulate the actual member curing temperatures. The temperature of test specimens was maintained within 3°F (1.7°C) of the internal girder temperature. A comparison was made of the compressive strength of the “match-cured” cylinders and member-cured cylinders used by the fabricator.
The compressive strengths at early ages were higher for “match-cured” cylinders than for member-cured cylinders and should be more representative of the actual girder strengths. At 56 days, the member-cured and match-cured cylinders had similar strengths.
The high compressive strength and low permeability achieved on this project have shown that HPC is a viable concept in Missouri, although further study into the freeze-thaw durability is needed. Locally available materials exist that can be used in the production of HPC.
What’s Next?
Missouri is pursuing additional HPC bridge projects. Currently, there is an InversetTM steel/concrete composite bridge being built over I-70 in St. Louis. This bridge incorporates HPC with a 7000 psi (48 MPa) design strength and low permeability concrete in the deck portion of the bridge units. A similar HPC mix will be used in a cast-in-place bridge deck on Route 291 near Kansas City in 2001. A bridge incorporating HPC in both the prestressed concrete I-girders and the deck will be built on U.S. Route 412 in Missouri’s Boot Heel in the near future.
Further Information
Chojnacki, T. M., “Determination of High Performance Concrete (HPC) Characteristics,” MoDOT RDT Report No. RDT99-008, September, 1999. For a copy of the report or for further information, contact the author at 573-526-4337 or [email protected]