It took about a year for North Carolina DOT (NCDOT) and general contractor Lanford Brothers Company to find the right project to test a new type of concrete bridge deck overlay: very-early-strength Rapid Set® Cement dosed with an admixture that makes very-low-permeability concrete pavement -- less than 1,000 coulombs at 28 days.
The U.S. Route 64 bridge over the Tar River in Tarboro was part of a multiple-bridge overlay contract the state had awarded to Lanford, a third-generation bridge repair contractor based in Roanoke, Va. Most bridges were to receive an epoxy overlay. Damage to the eastbound U.S. 64 bridge, however, required a thicker, more robust solution. To minimize driver inconvenience, the deck would receive a Rapid Set® Latex-Modified Concrete (RSLMC) overlay that meets the state’s compressive strength requirement of 3000 psi in three hours.
Lanford Brothers, which has installed RSLMC overlays for three decades using its fleet of mobile volumetric mixers, had another idea: Apply the state’s RSLMC specifications to an overlay made with very-early-strength Rapid Set® Cement and Liquid Low-P™ admixture, and see how it performs.
Expanding Contractor Versatility
The admixture turns Rapid Set®, a calcium sulfoaluminate (CSA) cement that’s much less permeable than portland cement, into ultradurable concrete that reaches structural strength in less than three hours. Contractors can add the liquid to a mobile volumetric mixer’s admixture tank and easily switch from producing standard Rapid Set® cement concrete to producing an overlay that provides the same benefits as latex and polyester polymer concrete (PPC) without the hassle and/or expense.
Like Rapid Set® cement concrete, only water is required to activate the mix. Ten fluid ounces of Liquid Low-P™ are required for every 100 pounds (45.4 kg) of cement. That’s roughly 66 ounces of liquid to produce 1 cubic yard of concrete compared to the 24.5 gallons of latex required to make 1 cubic yard of LMC overlay.
“We definitely used much less Liquid Low-P™ than latex, so it was definitely less expensive,” says Lanford Vice President Brett Dietrich, who oversaw the installation.
Rapid Set® provides the additional benefit of greater versatility than portland cement in terms of sand and aggregate that can be used in a concrete mix. It contains little to no tricalcium aluminate (C3A), the component that makes portland cement susceptible to sulfate attack.
All overlays are intended to stretch a public agency’s maintenance budget by protecting steel-reinforced concrete steel from corrosion-inducing water and chlorides. A Rapid Set® Liquid Low-P™ overlay gives public agencies an overlay option that’s as effective as but easier and less expensive than latex and polyester overlays.
Placing a New Mix
Lanford Brothers installed the Rapid Set® Liquid Low-P™ concrete overlay over the bridge’s three lanes in two placements, each comprising 1.5 lanes and each taking about six hours: one at night and the other during the day with temperatures in the 90s. The bridge remained open to traffic during the entire project.
North Carolina requires hydrodemolition for surface preparation. Then, Lanford crews used the same mixing, placing, curing and finishing techniques honed over years working with Rapid Set® latex-modified concrete overlay: spreading and leveling the mix with a Bid-Well roller paver, hand troweling the integral screed’s edges, finishing with a burlap drag, and keeping the new concrete moist with wet burlap covered in plastic for three hours.
“Even though they were working with a new mix, our crew didn’t have much of a learning curve because of the concrete’s similarity to the Rapid Set® latex overlays they’ve been doing for years,” says Ken Lanford, president and grandson of the company’s founder. “We hope to hear soon about doing a similar project for Virginia. As general contractor, we suggested the state try the mix to see how it compares to latex.”
“We’re offering this admixture for the contractor and/or producer’s convenience and the public agency’s need for durability,” says Tracy Johnston, the CTS Cement Manufacturing Corp. engineering sales representative who collaborated with Lanford and NCDOT Preservation & Repair Staff Engineer Tim Sherrill, PE, on the successful installation.
