Four summers ago, crews for a Pacific Northwest highway contractor headed to a jobsite outside Portland, Ore., to rehabilitate a bridge deck by placing a ¾-inch polyester polymer concrete (PPC) overlay on both lanes.
Once onsite, they found grade differences of up to 3 inches. At roughly $3,000 per yard, leveling the lanes with PPC would far exceed the project’s budget. Roger Langeliers Construction (Langeliers) proposed that the bridge owner, Washington County, overlay the second lane with a new but less expensive material – very-early-strength (VES) concrete made with ASTM C1600 Rapid Set® Cement dosed with Liquid Low-P™ liquid admixture – and compare the overlays’ permeability and abrasion resistance.
Wary of investing taxpayer dollars in a new material, the county consulted with Oregon DOT Statewide Structural Materials Engineer David Dobson, PE. Having substituted the mix for PPC to fast-track structural overlays on two high-traffic bridges, he reported the overlay was viable. In fact, he’d approved its use on a third bridge.
So, the contractor placed a PPC overlay on one lane of the southwest Murray Blvd. bridge in Beaverton and the Rapid Set® VES, very-low-permeability cementitious mix in the other. Nation’s Mini-Mix, which supplied the concrete via an onsite volumetric mixer equipped with admixture tanks, added 10 ounces of admixture for every 100 pounds of cement (roughly half a gallon of liquid for every cubic yard of concrete).
“The mix isn’t as slushy as silica fume overlays and, because of the product’s consistency, you don’t get surprises,” says Langeliers Division Manager Sean Doran. “It goes down like a standard bridge overlay, but return to service is way sooner and you get structural strength. Polyester doesn’t provide structural value, whereas the Rapid Set® mix does. The service life of both is going to be about 20 years, so we’re going to get a really good comparison.”
Overlay Options for Reinforced Concrete Bridge Decks
The Holy Grail of reinforced concrete bridge deck maintenance is a VES, very-low-permeability structural overlay for fast reopening and long-term protection of embedded steel. Each option available to state and local transportation departments and contractors involves a compromise.
Moisture-intolerant polyester is fast but not structural, a potential health hazard, requires priming, and not an option when hydrodemolition is specified for surface preparation. Latex-modified concrete (LMC) is structural and can be accelerated, but requires bringing large volumes of heavy liquid to the jobsite. Silica fume is less expensive and structural but too slow for fast-track bridge deck rehabilitation.
According to the Federal Highway Administration’s Long-Term Bridge Performance (LTBP) Program, asphalt is the most popular overlay option with state transportation departments. That’s not surprising given it’s fast, easy, and the least expensive option. However, it doesn’t last as long as concrete.
LMC is the second-most-popular overlay nationwide. While permeability is very low, 24.5 gallons of latex must be transported to the jobsite for every cubic yard of overlay placed.
Epoxy polymer concrete is the third-most-popular overlay, although many public agencies consider the material a preventive coating rather than an overlay.
Polyester is eighth on the list of top 10 overlays that states use, but most don’t consider PPC a structural overlay. In addition to requiring a specialized paver, PPC shouldn’t be placed in temperatures under 50 degrees Fahrenheit, is flammable, and doesn’t tolerate extreme heat well, as when a car catches fire.
How Cement Type Affects Overlay Durability
In addition to the above, pavement engineers and maintenance supervisors seeking a fast-track structural concrete overlay have another choice, one that affects durability: whether to specify concrete made with ASTM C150 Type III portland cement or ASTM C1600 calcium sulfoaluminate (CSA) cement.
High-early-strength mixes reach structural strength in three days, but require additives, admixtures, and/or supplementary cementitious materials (SCMs). Portland cement concrete also doesn’t use all its mix water, promoting the shrinkage cracking that creates entry points for water and chlorides.
CSA was developed to address this issue. It is not portland cement. It uses virtually all mix water, increasing concrete durability by greatly minimizing shrinkage. It doesn’t contain the mineral phase tricalcium aluminate (C3A) that makes portland cement concrete susceptible to sulfate attack. It’s also much less prone to alkali-silica reaction (ASR) – even mitigating ASR in many instances – expanding the range of aggregate that can be used in a concrete mix.
To enhance concrete durability, public agencies are beginning to add a shrinkage requirement to concrete overlay specifications. In fast-track applications, only concrete made with CSA cement meets opening strength deadlines and shrinkage requirements.
Another Weapon in the Anti-Corrosion Arsenal
Speed, strength, and durability are why almost all 50 states, many cities, and other public agencies use Rapid Set® CSA cement for emergency repairs and rehabilitation projects that must be completed quickly to minimize public inconvenience. For example, many use Rapid Set® Cement for LMC overlays (RSLMC) because they can be reopened to traffic in three to four hours.
Produced by CTS Cement Manufacturing Corp., Rapid Set® is the key component in multiple products formulated for the convenience of public agencies. DOT Cement, for example, is air-entrained for long life in freeze-thaw regions. So is DOT Concrete Mix, which contains a corrosion inhibitor and fibers in addition to 3/8-inch top-size aggregate.
Low-P™ Cement was developed as a VES, very-low-permeability (“low-P” stands for “low permeability”) structural overlay alternative to LMC. Like all Rapid Set® products, only water is required to produce concrete that can be quickly reopened to traffic in hours rather than days. Chloride ion permeability, a key characteristic for protecting rebar from corrosion caused by the chlorides in deicers, is less than 1,000 coulombs per ASTM C1202.
More recently, CTS introduced Liquid Low-P™, a liquid admixture that provides the same benefits as Low-P™ Cement.
A contractor that’s installed Rapid Set® LMC overlays for three decades proposed using Rapid Set® Cement dosed with Liquid Low-P™ for a structural overlay project in North Carolina. The concrete mix met the state’s requirement of 3000 psi compressive strength in three hours.
“We used much less Liquid Low-P™ than latex, so it was less expensive,” says Lanford Brothers Vice President Brett Dietrich, who oversaw the installation. “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.”
On the other side of the country, Oregon is adding the admixture to its HES concrete (HESC) overlay specification as a requirement for permeability reduction.
