RoadScience: Corrosion — the enemy within
Enhanced Reinforcement. Steel rebar may be epoxy-coated, galvanized or clad in inert stainless steel to resist corrosion. The rebar itself may be of stainless steel, or, more recently, a carbon fiber polymer composite that will not corrode.
Concrete Additives and Mix Designs. Admixtures such as microsilica (silica fume) fill up the pores of the concrete, inhibiting migration of chloride-laden water. Low water-to-cement mix ratios help. And high-performance concretes (HPC) — which resist migration of chlorides in addition to boosting concrete compressive strength — have been used with some success, although unanticipated deck cracking linked to HPC has caused designers to rethink HPC in bridge decks in recent years.
Surface Sealers and Membranes. Largely impermeable dense and microsilica-enhanced concrete overlays, silane/siloxane sealers, methacrylate resin crack sealants, latex modified concrete overlays, and waterproof deck membranes keep superstructures protected from damaging chlorides.
Electrochemical Fixes. These include active cathodic protection of bridge structures and the removal of chlorides from them.
Maintenance Best Practices. Use of more expensive noncorrosive deicers, continuing repair of cracks, washing of decks, and periodic upkeep of drainage and bridge joints go a long way to protecting a bridge from corrosion.
Getting the deck right
“The principal mechanisms of bridge deck failure are surface deterioration and corrosion of the reinforcing steel, due to chloride ion intrusion,” write William S. Caires, principal, and Stanley R. Peters, P.E., senior engineer, for Centennial, Colo.-based Construction Technical Services, in their 2006 report to the Colorado Department of Transportation, Evaluation of Products that Protect Concrete and Reinforcing Steel of Bridge Decks from Winter Maintenance Materials.
If that’s true, then a robustly resistant deck surface provides the first line of defense against chloride penetration. A resistant deck will inhibit rapid chloride penetration and resist penetration from ponded meltwater over longer periods. But it also will have to be abrasion resistant, Caires and Peters write.
“Concrete mixes need to be developed to minimize drying shrinkage and permeability (high density, low shrinkage) as a second line of defense against chloride intrusion,” they say. “However, membranes appear to be the most effective method of preventing the direct intrusion of deicing chemicals through cracks that always occur.”
A popular deck barrier to deicers is a latex modified concrete (LMC) overlay. It’s used on busy highways as well to stand up to traffic. But LMC more often is placed as a thin bonded overlay on bridge decks and parking structures.
As LMC cures, the polymer forms internal plastic films which result in low permeability to chlorides, low modulus of elasticity (making it more flexible than conventional concrete), a bond that’s stronger than the substrate below, and high durability against abusive traffic loads. However, eventually LMC overlays will have to be replaced, and as strong as they are, they pose an exceptional challenge to contractors who will have to mill the aged LMC overlay in advance of a replacement.
“Latex modifiers help in the adhesion of the overlay to the deck, and also help reduce the permeability of the concrete,” says Paul D. Krauss, P.E., principal of Northbrook, Ill.-based Wiss, Janney, Elstner Associates, Inc. “LMC has been used for a lot of years, and there are a lot of decks that have had very good success, for example, in Chicago. But other agencies have had early-age plastic shrinkage crack problems.”
MORE FROM Featured Articles
- Obama signs memorandum to expedite infrastructure projects666 Views
- Florida’s Red Light Camera Game: G R E E N orange R E D392 Views
- Sydney uses water curtains to alert drivers to stop (VIDEO)389 Views
- Seattle tests bikes as disaster relief (VIDEO)380 Views
- FHWA deploys bridge-inspecting robots296 Views