Bridges deicing salts

Corrosion of suspension bridge over Severn River, Pelham bridge in Lincoln, bridges in New York state and other areas containing deicing salts. This is an expensive problem that needs immediate attention... 

Bridge decks, parking structures, and other reinforced concrete structures lend themselves to cathodic protection, particularly when the concrete is contaminated with cathodic chlorides from deicing salts or a marine environment. Special techniques are required as described by Rog and Swait [71] NACE [72, 73] and Morgan [74]. 

In the case of highway bridges, the vast majority of the problems are caused by chloride migration into the concrete because of deicing salt application and marine exposure. On reaching the surface, the chloride ions cause the disruption of the passive film and create conditions favorable for accelerated corrosive attack on the... 

Surface Chloride Concentration The main source of chloride ions arises from salt added during road and bridge deicing. Sodium chloride (NaCl) and calcium chloride (CaCy have been traditionally used as deicing salts. In the United States, 282 kg of salt are used per lane-mile. Normally the number of apphcations varies from less than 8 to more than 16 per year depending on climate conditions. Annual salt estimates for various states are given in Table 12.1. Rebar surface can be exposed to the following... 

Structural collapses of reinforced concrete structures due to corrosion are rare. The author knows of two multi-storey parking structures in North America which have collapsed due to deicing salt induced corrosion (Simon, 2004). A prestressed concrete bridge collapsed in Wales dne to deicing salt attack on the strands hidden from investigation (Woodward and Williams, 1988), and another one in Enrope. Usually concrete damage would have to be well advanced before a strncture is at risk. 

Figure 4.3 Visual and delamination survey of a cross beam on a motorway bridge suffering from deicing salt ingress (developed view of beam). Courtesy of the Highway Agency.

XD3 Cyclic wet and dry Bridges, car park slabs exposed to deicing salts... 

Figure 4.2 Visual delamination survey on a cross beam on a motarway bridge in the UK suffering from deicing salt ingress (developed elevation of beam viewed in the direction of increasing bent numbers)...

The problem of corrosion of steel in concrete was first ascribed to stray current flows from trams and DC railway systems Hime, 1994). Once chloride, in the form of deicing salt, was identified as the major culprit (when trams disappeared but corrosion increased), an enterprising engineer in the California Department of Transportation (Caltrans) took a standard pipeline cathodic protection design and flattened it out on a bridge deck. 

Thare are several reasons for FBECR becoming the protective system of choice in the USA and Canada for reinforcing steel exposed to chloride attack. One is the reluctance to use waterproof membranes on bridge decks as they are difficult to install properly and to monitor both for correct installation and for performance after installation. The preference for a very low maintenance bridge deck led most state DOTs and the Federal Highway Administration (FHWA) to look for alternative protective systems on all bridges exposed to chlorides from the sea or from deicing salt. 

J2 Corrosion of steel reinforcing ban (reban) in a concrete bridge head, (a) The corrosion cell which arises due to water penetration and chloride pick-up from the concrete or from deicing salts, (b) The expansive corrosion products may cause cracking of the concrete. Tn severe cases, delamination results, (c) Impressed current, cathodic protection may be applied, (d) A flexible anode mesh may be installed (which is then covered with a concrete overlay)i Power usage is < 10 Wm" of the concrete surface. (Courtesy Raychem Ltd.)... 

Reinforced concrete electrochemical protection methods used against corrosion caused by contamination with chlorides or the carbonization process may be used for new structures, as well as those already used in soil, water, and atmospheric environments. The first practical CP application was realized in the U.S. A. for the protection of a bridge contaminated with deicing salts (Stratfull, 1974). 

Figure 9.22 Corroded overpass steel bridge above the most traveled lane of a two-lane highway, a telltale symptom of deicing salt corrosion.

Bridge deck in 1-696 highway near Detroit, Michigan 1995 lype 304 rebars. Exposure to winter deicing salts. 

Bridge deck in 1-295 highway near Trenton, New Jersey 1985 Carbon steel rebars with t5fpe 304 cladding. Exposm to winter deicing salts. If ends of clad products are exposed, these represent a galvanic corrosion risk. 

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