Don Theobald, Gulf Coast Pre-Stress, Inc.
To most fabricators, the thought of producing high strength, high performance concrete (HPC) can be very intimidating. When the term HPC became the buzzword several years ago, I couldn’t imagine Gulf Coast Pre-Stress, Inc. producing concrete with strengths greater than 7000 psi (48 MPa) with our materials.
In January 1997, the Louisiana Department of Transportation and Development announced plans for their first HPC project. The Charenton Canal Bridge replacement, located in St. Mary Parish, was well within our market area. The products would be AASHTO Type III girders and 14-, 24-and 30-in. (355-, 610-, and 762-mm) square piles, each designed with 10,000 psi (69 MPa) concrete. The release strength for the girders was initially 6000 psi (41 MPa) but was later increased to 7000 psi (48 MPa) to demonstrate that higher strengths could be achieved. The project was scheduled for bidding in the first quarter of 1998.
Management immediately made a commitment to begin necessary research to successfully produce HPC with as many of our present raw materials as possible. The strategy was to decide on materials and proportions, and then batch laboratory trial mixtures. Next, we scheduled two separate 3 cu yd (2.3 cu m) batches to cast full-size specimens of AASHTO Type III girders. With the help of our cement supplier, several thermocouples were placed in the members to determine time-temperature relationships for steam- and naturallycured specimens.
The Louisiana Transportation and Research Center (LTRC) announced that they would be making several laboratory trial mixtures in preparation for the Charenton Canal project. I made every effort to be at LTRC each time a new trial mixture was batched. I wanted to get firsthand experience with the plastic concrete. I learned that HPC, with high percentages of Class C fly ash, had good workability. The concrete was very cohesive, even with large doses of high-range water-reducing admixtures. Concrete slumps as high as 9 in. (230 mm) demonstrated uniformity.
Larry Niceley, our quality control manager, and I began analyzing our materials. Our cement is a Type III so we had to be concerned about heat of hydration due to the increased cement content of the mix. We were of the opinion that the use of Class C fly ash at 30 percent of the total cementitious material content would help reduce internal temperatures. Maximum internal concrete temperature for the project was specified as 160°F (71°C). Our sand, consisting of natural quartz with a fineness modulus averaging 2.6, appeared adequate. Another concern was coarse aggregate. Previous experience indicated that our washed river gravel was too smooth to provide the bond needed for strengths over 10,000 psi (69 MPa). We considered using crushed washed gravel, but realized there would still be a percentage of smooth edges. We eventually found a source of good quality washed crushed limestone that was stockpiled in the neighboring state of Alabama. We chose to use a 1/2-in. (13-mm) nominal maximum size aggregate because of the necessity for high mortar-aggregate bond. The final pieces to the material puzzle were chemical admixtures. We experimented with three types but eventually chose our present water reducing and high-range water-reducing admixtures for economical reasons.
After several weeks of laboratory trial batches, the 28-day strength test for our reference mix was in excess of 14,000 psi (97 MPa). By June, we were ready to cast full-scale girder specimens. We cast two specimens on separate days in the middle of June and a third in July. With successive tests, the 28-day strengths showed noticeable increases. The first cast had an average compressive strength of 10,820 psi (74.6 MPa) with the second and third casts averaging 11,760 and 12,210 psi (81.1 and 84.1 MPa), respectively.
Lessons Learned
Several lessons were learned from our year-long research in preparation for the Charenton Canal project. Scanning thermometers to monitor internal concrete temperatures and a reliable cylinder match-cure system are a must. Converting to neoprene caps and 4×8-in. (102×203-mm) cylinder specimens from traditional sulphur caps and 6×12-in. (152×305-mm) cylinders was a necessity. Providing additional moisture control devices at the batch plant proved extremely valuable. And of course, executing a very thorough quality control plan was essential for our successful completion of the Charenton Canal project. In production, our release strengths ranged from 7620 to 9850 psi (52.5 to 67.9 MPa) and our 56-day strengths from 10,500 to 12,020 psi (72.4 to 82.9 MPa).