RoadScience: Corrosion — the enemy within
The aggregates used in the overlay differ in that they are said to absorb and store liquid deicing chemicals that are applied to the surface. “These chemicals have the potential to reduce, and in some situations prevent, the accumulation of ice and snow on the overlay and thereby reduce the number of snow- and ice-related accidents,” Sprinkel says.
Backing away from HPC
High-performance concrete is durable concrete because its strength and impermeability to chloride penetration makes it last much longer than conventional PCC. It’s an engineered concrete made up of the classic elements of water, portland cement and fine and coarse aggregates, but with added components.
In HPC, materials and admixtures are carefully selected and proportioned (“optimized”) to form high early strengths, high ultimate strengths and high durability beyond conventional concrete.
HPC provides enhanced mechanical properties in precast concrete structural elements, including higher tensile and compressive strengths, and heightened modulus of elasticity (stiffness). In frost-prone regions the benefits of HPC are great. The enhanced durability of HPC helps it resist penetration of chloride-laden snow and ice meltwater. This results in longer life for the reinforcing steel within, and a reduction in spalling, cracking and associated repairs.
The Strategic Highway Research Program (SHRP, 1987-1993) studied the efficacy of HPC in bridge structures as a way to quell corrosion of rebar from meltwater from deicing salt. The first HPC structural designs were constructed in the mid-1990s. HPC began migrating to pavements in the late 1990s, and in 2004 it had fully penetrated PCC pavement construction.
HPC durability criteria include air void structure, low permeability, proper water content of fresh concrete, and low susceptibility to cracking. Some industrial “waste” materials of a few decades ago now are integral elements of this new engineered concrete. These admixtures, such as coal fly ash, microsilica and ground granulated blast furnace (GGBF) slag, add both strength and durability to the concrete, and enhance its marketability as an environmentally friendly product.
While HPC continues to be used in bridge superstructures, substructures, and in high-rise buildings, it’s fallen into disfavor for bridge decks due to unanticipated early cracking.
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.”
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