Road Management Journal
Copyright © 1997 by TranSafety, Inc.
October 1, 1997|
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Iowa Field Study Documented Successful Heat-Straightening Repair of a Steel Bridge by In-House Personnel
Sealers Shown to Lengthen the Service Life of Concrete Bridges Exposed to Chloride
Large Trucks a Significant Factor in Major Freeway Incidents in Houston, Texas
Warnings Combined with Enforcement Can Reduce Speeding
Highway Safety Publications Catalog. Articles on Road Engineering,
Road Maintenance & Management, and Injury Litigation. Information and consulting for the Automobile and Road User,
as well as for law professionals in accident investigations.
Sealers Shown to Lengthen the Service Life of Concrete Bridges Exposed to ChlorideMany steel-reinforced concrete bridges in the United States are subject to corrosion from chloride ions. This corrosion is a more significant problem in chloride-rich coastal areas and in northern states with heavier snowfall, where roads are kept free of snow and ice with the use of chloride salts. Many concrete bridges in these areas become contaminated with chlorides, which in turn begin to corrode the reinforcing steel. The corrosion affects bridge components, including the deck, abutments, beams, cross-beams, diaphragms, piers, and piles.
Applying a sealer to the concrete can be an effective and initially inexpensive method of tackling this corrosion problem, thus increasing the service life of a reinforced concrete structure. However, not all sealers have an equal service life; some will require more maintenance costs and more frequent reapplications. To determine how long various sealers extend the service life of bridges affected by chloride corrosion, Jerzy emajtis and Richard E. Weyers examined generic sealer types typically applied to steel reinforced concrete bridges in the U.S. They concluded the service life of a sealer is affected by various factors, including environmental conditions, traffic wear, penetration depth, ultraviolet (sun) light, and exposure type (horizontal or vertical). Results of their investigation were reported in "Concrete Bridge Service Life Extension Using Sealers in Chloride-Laden Environments" (Transportation Research Record 1561).
Surface treatments, which include sealers and coatings, can effectively protect all concrete surfaces of a bridge, including the undersides of bridge components. Sealers and coatings are distinguished by their actions: sealers penetrate a structure's surface pores, while coatings form a thick film of up to 2 mm on the structure's surface. Concrete sealers fall into two main groups--pore blockers and hydrophobing agents. The latter penetrate the concrete to some degree, while pore blockers provide little penetration and form instead a thin film on the concrete's surface. Pore blockers are further distinguished by their ability to partially or fully fill the surface pores, a capability not shared by hydrophobing agents. This distinction means that pore blockers should only be used on substructure components and other areas not subject to traffic wear. The performance of a concrete sealer is affected by a number of factors, including coastal or inland location, average annual daily traffic (AADT), average annual snowfall (resulting in the use of deicing chloride salts), exposure to salt water, and the concrete's surface preparation for applying a sealer or coating.
Four sealers--two pore blockers and two hydrophobing agents--were studied. The pore blockers were a water-based epoxy (WBE) and a solvent-based epoxy (SBE); the hydrophobing agents were silane (SIL) and siloxane (SLX).
To serve as test surfaces, fifteen horizontal slabs and a wall surface were cast from the same batch of concrete. Three slabs were sealed with each of the four types of sealers, and three unsealed slabs functioned as controls. Five test sections on the wall were each sealed with one of the four sealer types, and one unsealed section served as a control. All test sections were sealed according to the manufacturer's recommendations.
Researchers also applied the four sealer types to two existing bridge decks near Blacksburg, Virginia. One bridge (Pepper's Ferry Bridge) had an AADT of 12,430 in 1990; the other bridge (I-81 bridge) had an AADT of 24,270 for the same year. The bridge decks were exposed to full direct sunlight, applications of deicer salt, and traffic wear.
The slabs, wall, and bridge decks were exposed to the outdoor chloride for 30 weeks (winter, spring, and summer).
Researchers recorded ultraviolet (sunlight) exposure times and traffic abrasion rates for the two bridge decks. They also recorded ultraviolet exposure times for the slabs and wall section--950 and 190 hours, respectively. Exposure times for the bridge decks were assumed to also be about 950 hours, since they had the same horizontal orientation and were within 10 km of the slabs. Traffic wear on the Pepper's Ferry Bridge could not be determined because the bridge had only been in use two years. The traffic wear rate on the I-81 bridge (in service for about 27 years) was 0.17 mm per year.
Based on the research, the wear service life for tested sealers on the I-81 bridge "will be slightly less than 9 years." Lower traffic-volume bridges may have a longer service life, and high-volume bridges a shorter service life. Research also showed that, considering wear only, "a maximum service life for hydrophobic sealers is about 7 years and about 10 years for bridge members not exposed to traffic wear." For the pore blocking sealers (WBE and SBE), the maximum service life is less than a year; visual observations of both bridge decks treated with these sealers confirmed this. On surfaces not subject to abrasion, service life (reapplication period) is about three years. For both pore blockers and hydrophobing agents, the ability to extend the service life of bridge components "would . . . be based on the chloride diffusion characteristics of a particular surface treatment."
Tables 1-4 show the estimated service lives and extended equivalent times for the four sealers and the control that corresponded to exposure conditions in New York, Pennsylvania, and Virginia. Extended equivalent time is the difference between the estimated service life of sealed and unsealed concrete.
Estimated Service Lives for Investigated Sealers Based on Diffusion Characteristics--Horizontal Exposure Condition
Estimated Service Lives for Investigated Sealers Based on Diffusion Characteristics--Vertical Exposure Condition
Service Lives Extension (Extended Equivalent Times) Based on Diffusion Characteristics for Investigated Sealers--Horizontal Exposure Condition
Service Lives Extension (Extended Equivalent Times) Based on Diffusion Characteristics for Investigated Sealers--Vertical Exposure Condition
Results validated the methods used in the study (measuring chloride diffusion through the sealer) as a rational way to estimate the corrosion protection service life of a sealer. Ultraviolet light and traffic wear degrade sealers. A sealer's performance is also influenced by exposure type (vertical exposure is "more severe" than horizontal exposure) and by the quality of the concrete used. Good quality concrete "significantly enhances the chloride diffusion life extension characteristic of sealed surfaces." Finally, the study recognized that while the sealers were effective to varying degrees, they will not always lengthen service life for 75 years. As such, "more field studies are needed to determine what sealer types provide the best protection for reinforced concrete elements."
Copyright © 1997 by TranSafety, Inc.