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7C h a p t e r 2 Reinforced concrete structures such as bridge decks and pil- lars, highways, and other infrastructure facilities experience loss of integrity over time because of poor initial quality, damage from deicing salts, overloading, freezeâthaw cycle- induced stresses, fatigue, and, above all, corrosion of rebars (Figure 2.1). According to the Federal Highway Administra- tion (FHWA), the cost of repairing and replacing deteriorat- ing highway bridges in the United States is approximately $100 billion (Lemieux et al. 2005; El-Safty 2008). The differ- ent kinds of deterioration observed in reinforced concrete structures are outlined in the following sections. The most frequent deterioration phenomena identified by Bien et al. (2007) are the following items: â¢ Corrosion; â¢ Carbonation; â¢ Alkali-silica reaction; â¢ Crystallization; â¢ Leaching; â¢ Oil and fat influence; â¢ Salt and acid actions; â¢ Creep; â¢ Fatigue; â¢ Influence of high temperature; â¢ Modification of founding conditions; â¢ Overloading; â¢ Shrinkage; and â¢ Water penetration. Common Deterioration types in Bridge Decks The deterioration of steel-reinforced concrete structures can be caused by the corrosion of steel or degradation of con- crete. These deterioration processes are complex and often prompt one another. Among all the deterioration phenom- ena listed above, four deterioration mechanisms are of the highest concern to bridge engineers and are the focus of this project. Those include the following: â¢ Rebar corrosion; â¢ Deck delamination; â¢ Vertical cracking; and â¢ Concrete degradation. Each of the deterioration mechanisms is briefly discussed in the following sections. Rebar Corrosion Reinforcing steel embedded in concrete is naturally protected from corrosion by the high alkalinity of the cement-based materials and an adequately thick concrete cover. High alka- linity can cause the formation of a passive and noncorroding protective oxide film on the steel surface. ACI 222R-01 (2001), which was reapproved in 2010, describes the process of metal corrosion in concrete. During this process, concrete allows electrolytic conduction and the flow of ions from anodes to cathodes. Once the oxide film is destroyed, an electric cell is formed along the steel or between steel bars, and the electro- chemical process or corrosion begins. Some areas along the bar become anodes discharging current in the electric cell, and iron goes into the solution with oxygen. Other steel areas receive current resulting in the formation of hydroxide ions known as cathodes. A major contributor to this problem is chloride diffusion. Chlorides are derived primarily from the application of roadway deicing salts. Corrosion of steel rein- forcement in a bridge deck can directly reduce the structural capacity of the deck. Furthermore, the corrosion process can cause internal stresses, cracking, delamination or surface fracture planes, and eventually spalling in concrete at, or just above, the level of the reinforcement (Figures 2.2 and 2.3). The two most common steel corrosion processes are chloride-induced pitting corrosion and carbonation. Bridge Common Defects of Concrete Bridge Decks
8engineers can often visually distinguish the two corrosion types. The locally confined, chloride-induced pitting corro- sion leaves blackish rust marks, whereas red or brownish rust stains indicate carbonation-based corrosion. The rate of cor- rosion is dependent on numerous factors, including the com- position of the metal, as well as humidity, temperature, water pH, and exposure to pollution and salt. Wet and dry cycles accelerate the corrosion process. Studies have shown that the corrosion rate is highest during the spring season and lowest during the winter. These rates can vary by a factor of about four or five during the year (Smith and Virmani 1996; Page et al. 1996). Deck Delamination Delamination or horizontal cracking caused by corrosion of embedded reinforcing steel is a serious form of deterioration in concrete bridge decks. Reinforcing steel expands as it corrodes. Such expansion may create a crack or subsurface fracture plane in the concrete at or just above the level of the reinforcement, as illustrated in Figure 2.4. Delamination may be localized or may extend over a substantial area, especially if the concrete cover is thin. It is possible that in a given area, delamination can occur along different planes between the concrete surface and the reinforcing steel. Delamination is not visible on the concrete surface; however, if repairs are not made in a timely fashion, the delamination progresses to open spalls. With continued corrosion, this process will even- tually affect the structural integrity of the deck. Vertical Cracking In addition to rebar corrosion, many other factors can cause cracking in bridge decks. These factors include plastic shrink- age, hydration heat, ambient temperatures, geometric con- straint as the deck concrete cures, traffic load, and freezeâthaw Figure 2.1. Deck deterioration. Figure 2.2. Corrosion process.
9Figure 2.3. Corroded rebar in an excavated deck (top) and extracted core (bottom left), and delamination in a drill hole in a deck (bottom right). Figure 2.4. Delamination observed in extracted cores.
10 cycles. The progression of rebar corrosion can further exag- gerate these cracks. Vertically oriented cracks and load-related cracks will be the primary focus of the validation testing. Concrete Deterioration A reduction in concrete strength or modulus is considered to be a form of concrete degradation. It may be the result of micro- cracking and macrocracking and other phenomena, such as alkaliâsilica reaction (ASR), delayed ettringite formation (DEF), plastic shrinkage, and freezeâthaw cycles. Each of the phenomena is briefly described in the following paragraphs. The alkaliâsilica reaction is a reaction between reactive sil- ica phases in aggregates and alkali hydroxides in the concrete pore solution. This reaction produces a silica gel that swells in the presence of water, causing internal and external cracking. The expansion of concrete resulting from ASR can cause two main problems: (1) the deformation of the structure, thereby impairing the serviceability, and (2) the development of a crack network through the structure (Figure 2.5). Delayed ettringite formation is perceived as a form of inter- nal sulfate attack. DEF is believed to be a result of improper heat curing of the concrete where the normal ettringite forma- tion is suppressed. The highly concentrated sulfate in the pore (a) (b) Source: ASR photograph courtesy of Dr. Moon Won, Texas Tech University. DEF damage photograph courtesy of Texas Department of Transportation Bridge Division. Source: Photographs courtesy of Dr. Ken Maser. Figure 2.5. (a) ASR and (b) DEF damage.
11 liquid may eventually react with calcium- and aluminum- containing phases of the cement paste to form the hydrated calcium sulfoaluminate mineral, ettringite. The formation of ettringite causes the concrete to expand, and empty cracks (gaps) may form around aggregates (see Figure 2.5) Plastic shrinkage (volume reduction) can cause cracks in concrete. These cracks often occur on plane structures such as deck slabs, with no preferential crack orientation. Freezeâthaw can increase the hydraulic pressure in con- crete. The concrete will rupture once the pressure exceeds the tensile strength of the concrete. The exposure of concrete to repeated freezeâthaw cycles will ultimately cause extensive deterioration in the form of cracking, scaling, or crumbling. Overlay Debonding Some old bridge decks are overlaid with asphalt concrete or portland cement concrete (PCC). In such bridge decks, the overlay can debond from the existing concrete decks. Once an overlay debonds, moisture and chlorides may enter the debonded region, further promoting deterioration. If repairs are not made, debonded overlay regions can eventually dete- riorate into open spalls, which affect the ride quality of the deck and compromise the structural integrity of the deck. Furthermore, bonded and debonded overlays contribute to the complexity of the analysis of NDT methods and may impede their effectiveness.