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CHAPTER 1

INTRODUCTION

The Federal-aid highway system contains over 235,700 bridges. The deterioration of an ever increasing number of the bridge roadway surfaces (decks) is a major and still growing problem facing State and Federal governments. The Federal Highway Administration--the agency responsible for the Federal-aid highway system--estimates that $6.3 billion is required to restore the surfaces on Federal-aid system bridges, of which $1.7 billion is for Interstate system bridge decks.

This problem originated in the mid-1950s when highway departments adopted a "clear pavements" policy of heavily salting roadway surfaces to improve winter traffic conditions. With repeated applications, the chlorides penetrate the concrete surface, and eventually reach the reinforcing steel. Corrosion of the steel will occur when the chloride interacts with the moisture and oxygen in the concrete. cause the rusted steel occupies much more space, pressure is exerted on the concrete, and eventually potholes develop. (See fig. 1.)

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Since the late 1950s, highway officials have become increasingly concerned about the growing number of prematurely deteriorated concrete bridge decks. Bridge decks are intended to provide relatively maintenance-free service for about 40 years. However, the Federal Highway Administration found that unprotected bridge decks receiving deicing chemicals require major repair within 5 to 10 years, and often must be replaced after only 15 years of service.

As a result, in the late 1960s, the American Association of State Highway and Transportation officials requested the Transportation Research Board of the National Academy of Sciences to prepare a state-of-the-art report on the concrete bridge deck deterioration problem. This report, issued in 1970, summarized earlier research study efforts which identified corrosion of the reinforcing steel as the primary cause of bridge deck deterioration.

RESEARCH PROGRAM OVERVIEW

The Federal Highway Administration's research and development program encompasses varied activities designed to achieve solutions to the urgent highway system problems.

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In July 1971, the Federal Highway Administration began a major research effort to develop solutions for the bridge deck problem with the initiation of an in-house laboratory testing program. The objective of this program was to determine the relative time-to-corrosion of reinforcing steel embedded in concrete slabs fabricated with various concrete mixes, construction techniques, and special additives. The intent here was to evaluate the majority of solutions then being suggested as methods of reducing the chloride permeability of concrete.

The study showed that certain techniques were not effective in limiting chloride intrusion, and that even excellent quality concrete (similar to that used in typical field construction) provided little protection against corrosion. Hence, it was concluded that special techniques were required to prevent early deterioriation of bridge decks. The study further demonstrated that application of certain techniques could significantly limit the intrusion of chlorides even after hundreds of daily salt applications. For example, research has been successful in developing several methods for protecting new bridges. These include:

Five sys

--Membrane systems with asphalt overlays.
tems, four sheet-type and one liquid-type, have
been designated as acceptable items for new con-
struction. These systems are intended to seal
the concrete against chloride intrusion.

--Epoxy-coated reinforcing steel bars.

The epoxy

coating protects the reinforcing steel bars from
the chlorides, moisture and oxygen; thus, corrosion
is prevented.

--Low water-cement ratio, dense concrete overlay system and the latex modified concrete overlay system. These special concretes slow the penetration of chlorides into the concrete, preventing critical levels of chlorides from reaching the reinforcing steel, thus, corrosion is prevented.

--Internally sealed concrete using wax beads. Federal
Highway Administration officials told us that this
is a promising new technique.

In May 1974, the Federal Highway Administration issued a notice to its field offices adopting several membrane systems as "nonexperimental" items for new construction. March 1976, the epoxy-coated reinforcing bars and the two

concrete overlay systems were also adopted as "nonexperimental" items for new construction. For States to qualify for Federal aid, the Federal Highway Administration now requires them to apply a protective system when constructing new bridge decks likely to receive deicing salt applications.

Early research also focused on developing techniques to determine the condition of existing bridge decks. In February 1974, the Federal Highway Administration's report stated that the only way to be assured of permanent repair was to remove all concrete in a potentially destructive condition, and then prevent future deicing salts from reaching the reinforcing steel. Bridge decks in a potentially destructive condition were defined as those containing a specified level of chlorides, referred to as the "corrosion threshold."

The Federal Highway Administration recognized that the removal of the contaminated concrete might not be feasible because of the high cost. Therefore, research also pursued the development of technologies which could stop the deterioration of decks without having to remove the contaminated concrete. The Federal Highway Administration supported the development and evaluation of technologies to neutralize the effect of the chlorides in the concrete. Research was primarily directed toward controlling the chlorides, rather than the other factors required to support corrosion, such as moisture and oxygen, because this was deemed the more feasible approach.

Pending development of these technologies, the Federal Highway Administration in April 1976 authorized an experimental procedure to protect bridge decks. This procedure is to place one of the membrane or overlay systems used for new construction over chloride contaminated decks.

Concerning the overall research program, Federal Highway officials, in responding to our report, said that since the late 1960s the program has been outstanding in the efforts to identify the causes of deterioration and has provided research leadership in the field of corrosion in concrete.

Because of the large potential saving of protecting, rather than replacing, the current inventory of structurally sound bridge decks, we reviewed the progress being made in applying protective systems to these existing bridges and the cost effectiveness of these measures. Chapter 2 discusses the current magnitude of the bridge corrosion problem. Chapter 3 discusses those decks containing levels of chloride below the level at which active corrosion of the

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