Jefferson City, Mo. — A new bridge spanning the St Mary’s River on Highway 50 just east of Jefferson City, Mo., is being used to test innovative concrete mixes side by side under real world conditions in a series of research projects being conducted by the Missouri University of Science & Technology and Missouri DOT. One test, to which County Materials Corporation contributed its experience, will assess the relative performance of girders constructed from two different mixes of self-consolidating concrete (SCC) alongside a traditional concrete mix. This will be the first bridge in Missouri to contain SCC.
SCC is a type of concrete that, when poured, is considerably more flowable than traditional concrete mixes. This confers a number of advantages:
SCC requires little to no use of mechanical vibrators to reduce entrapped air in the concrete during placement. It thus requires a smaller crew size, which leads to considerable savings in production costs.
When used to fabricate precast or prestressed concrete structures, SCC flows more readily between and around the steel substructure. A more elaborate steel framework is possible, which can reduce girder size and result in a stronger, more durable product.
Because SCC mixes contain less entrained air, fewer bubbles and “bug holes” appear in the finished product for a smoother, more aesthetically pleasing surface.
Most SCC mixes require an increase in the proportion of fines and a reduction of coarse aggregate compared to traditional concrete. This raises concerns that the materials’ shear resistance may be adversely affected, and that stiffness may be compromised. Researchers hope that the results of testing on this bridge will prove the materials’ merit and clear the way for the use of SCC on future transportation projects.
The three-span Highway 50 bridge contains three sets of girders fabricated by County Materials’ Bonne Terre, Mo., plant. The girders for the first span are made from a standard SCC mix. The second set of girders is manufactured with a high-strength SCC. This mix contains a higher percentage of cement for a stronger, more durable product with a compressive strength of up to 14 KSI. The third span is constructed using a standard Missouri DOT approved concrete mix design and will function as the control in the experiment. The bridge decking was also supplied by County Materials.
Sensors embedded in the girders and bridge deck will allow researchers to monitor the materials’ performance over time, and provide real-world data about their relative strength as well as detailed information on the bridge’s internal stresses.
“There have been very few projects in the United States that have used a high-strength self-consolidating concrete. It’s not so easy to translate laboratory tests into real-life estimates of service life. A project like this is really important because (the different types of concrete will) all be exposed to the same environmental conditions. Over time we’ll directly get to see exactly how much more durable these other concrete materials are,” says Dr. John Myers, the Missouri S&T professor who heads the research. Myers adds that using the high strength self-consolidating mix allowed the span length to increase by 20 percent, and is expected to reduce maintenance costs over the life of the structure. “Most of the current bridges (in Missouri) have a service life of 25-50 years. We’re hoping to double that.”
Preliminary research began in the fall of 2013. Since the success of an SCC mix is very dependent on the particular materials used, numerous mixes using local aggregates were tested, and the minimum amount of coarse aggregate for the high strength SCC mix was determined before County Materials’ Bonne Terre team formulated the final mixes used in the project. By March, 2013, County Materials had manufactured two large scale prototype girders. These underwent extensive laboratory testing before the mixes were approved for use in the bridge. The bridge is expected to be completed by late summer, 2014. Testing is expected to continue for years.
Missouri S&T graduate students and MoDOT personnel worked alongside County Materials staff at the Bonne Terre plant during fabrication of the girders and placement of the sensors. “It’s been a very positive experience for us as the research team. (County Materials has) always been very accommodating at the plant. It’s been a very positive relationship from the research perspective, integrating what we need to do with the folks in Bonne Terre,” comments Dr. Myers. “They were all willing to help out and do anything we needed,” agrees Shannon Inman, senior materials inspector of the materials division of MoDOT.
Both Myers and MoDOT were pleasantly surprised at the project’s cost-effectiveness. Even at the experimental stage, the cost to build the structure was only slightly higher than standard construction. “We’re very confident that as (the technology becomes more widespread), the cost will go down,” says MoDOT research engineer Jen Harper, adding that “Missouri has been working hard to find more (ways) to save taxpayers money.” If the results of the Highway 50 bridge experiment come out as expected, switching to SCC could be one way to do just that.