RoadScience: Corrosion — the enemy within
Early cracking in concrete bridge decks is a complex issue, and probably results from a combination of production and placement factors, such as high evaporation rate, high magnitude of shrinkage, the use of high slump concrete and excessive water in the concrete during mixing or placement, insufficient top reinforcement cover, insufficient vibration of the concrete, inadequate reinforcing details at joints, and the weight and deflection of the forms, according to researchers for the Kentucky Transportation Cabinet (the state’s DOT).
The alarm first was sounded in National Cooperative Highway Research Program (NCHRP) Report 380, Transverse Cracking in Newly Constructed Bridge Decks, by Krauss and Ernest Rogalla, S.E., both of Wiss, Janney, Elstner Associates, Inc., Northbrook, Ill.
Since then, complaints about cracking involving HPC decks have been abundant and widespread, although not all HPC decks have been troubled. In 2004 the Federal Highway Administration released a list of what 23 states were doing to address cracking in HPC bridge decks. The report concluded that careful curing is the secret to trouble-free HPC decks.
“One of the biggest problems that states are fighting right now is that almost all of these high-performance concretes have cracking issues,” Krauss said. “California, Illinois and a lot of other states are trying to figure out why their decks crack so much. As far as decks are concerned they haven’t had a lot of success preventing them from cracking because they are so strong and tend to be brittle. The cracking problem defeats the purpose of using the HPC.”
The increased risk of HPC early cracking is likely related to use of silica fume and their higher strengths, Krauss said. “High strength will make deck transverse cracking even worse,” Krauss said. “If one of the aspects of specifying HPC is increased durability, then one of the considerations for durability should be elimination of cracking. It’s not all just about permeability; it’s about balancing the permeability with workability, how well the concrete consolidates, how consistent will it be for the contractor. All these factors should go into considering HPC for a deck, not just its high strength.”
Krauss has researched the efficacy of sealing cracked HPC decks with high molecular weight methacrylate resin. “It’s an acrylic resin used as a topical treatment to flood the deck surface and seal the cracks,” Krauss told Better Roads. “If cracks occur in a bridge deck, we know the cracks will reduce the performance of that deck. The resin flows down into the cracks and bonds and seals them.”
Such cracks can be very hard to see on the top of the deck, due to its textured surface and grooves, which is a reason to spread the resin across the deck. “They’re hard to see and hard to follow,” Krauss said. “They will be more evident from the bottom of the deck due to water staining. When you have so many of these cracks, it doesn’t make much sense to chase each individual crack; it’s easier just to coat the whole surface.”
If an impervious deck is the first line of defense against chloride-induced corrosion, the last line will be coatings that keep the deicers off the rebar itself, and the most popular option there is epoxy coated reinforcing steel.
Epoxy coated rebar has become very widely used in recent decades. When it first appeared in the early 1970s, epoxy coating added 80 percent to 120 percent to the cost of uncoated reinforcement, according to the Concrete Steel Reinforcing Institute. But since then, the cost of epoxy-coated reinforcement has dropped significantly, CRSI says, especially when life cycle costs are calculated, including the avoidance of corrosion-induced cracks, spalls and potholes.
“There is definitely a lot of value in epoxy coating of rebar, especially for the price,” Krauss said. “The research and field performance has proven it to be an effective method of reducing corrosion of reinforcing steel.” The benefits? One of the major benefits, Krauss said, is that the rebar is being protected from chlorides, and at the same time, electrically isolating the bars from each other. “This dramatically reduces the corrosion rate over time,” Krauss said.
One way of avoiding the corrosion problem altogether is to use inert nonmetallic reinforcement that won’t corrode. Stainless steel rebar is another long-standing but expensive option. A less-expensive option ultimately may develop from the new generation of carbon fiber reinforced polymer (CFRP) bars that continue to undergo analysis in the lab and in the field.
In 2006 the Kentucky Transportation Cabinet released the report, Inspection and Evaluation of a Bridge Deck Reinforced with Carbon Fiber Reinforced Polymer Bars, by Choo Ching Chiaw and Issam E. Harik of the Kentucky Transportation Center at the University of Kentucky, which said that the cracks in the CFRP decks it studied were well below the accepted threshold.
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