IntroductionFailure of structures was almost non-existent in the past. Structures were generally known for their durability, soundness, and stability. But Due to lack of performance of building structures in last few decades, there has been a growing interest in the field of durability and service life of structures.
Degradation and deterioration of structures caused by physical and chemical damage results in the decrease in performance with time, physical damage occurs due to fire, abrasion or expansion and contraction stresses while chemical damage occurs due to harsh environment. Lack of durability of concrete structures or initiation of cracking has been caused mainly due to exposure to harsh environment, which results in degradation of structures as shown in fig. 1.
Figure 1: degraded concrete structures
Several researchers have performed studies to indentify the causes of deterioration of RC structures, Wang and Liu (2010) identified change in bond strength, loss of concrete cover in tensile zone and/or reduction of concrete cover in compressive zone as the major cause of deterioration of RC structures. Crack growth due to corrosion products expansion has been considered as an important factor for the durability of structures by Benin et al. (2010). According to Mitra et al. (2010) repair and maintenance planning of concrete structures is based on the conditional states of concrete categorized by the assessment of conditions such as rusting and cracking, delamination, loss in steel section, workmanship, carbonation and chloride contents. From the analysis performed by Bastidas-Arteaga et al. (2008), the failure probability of bridge girders depends highly on the corrosion rates, surface chloride concentration and the traffic frequency. Ganjidoost et al. (2010) studied the sustainability and durability of concrete structures in corrosive environmental conditions mostly in marine environments; they proposed that W/C ratio must be between 0.3 and 0.5 to resist against corrosion and permeability. According to Song and Saraswathy (2007) corrosion of rebars is the major deterioration process, and reviewed the methods for monitoring the corrosion of reinforced concrete structures including electrical, electrochemical, harmonic, ultrasonic pulse velocity, X-ray and visual methods. Melchers et al. (2007) considered the influence of corrosion and its initiation coupled with applied loads, and impact of internal damage for determining structural deterioration of R.C. Beams under saline environment corrosion. Berto et al. (2007) proposed that main effects of corrosion are bond deterioration, reduction in steel cross sections, cover spalling, and concrete damage or cracks. Song and Kwon (2007) considered carbonation as the major cause of deterioration and found during carbonation process permeability of concrete changes due to change in capillary porosity. Durham et al. (2007) inspected several precast concrete bridges to identify the causes of deterioration, information collected includes concrete deterioration, environmental humidity, reinforcing steel corrosion, asphalt wearing surface, drainage and bridge site photos. And concluded that deterioration was mainly due to longitudinal cracks (caused corrosion of steel bars), and flexure cracks (overloading of live-loads). Chong and Low (2005) analyzed the defects in construction facilities at both construction and occupancy (after 2 to 6 years) and found that main causes of defects are design, workmanship, material, lack of protection and maintenance. According to Amleh and Mirza (2004) chloride content, quality of concrete cover and electrical resistivity of concrete have significant effects on the rebar corrosion. Dias and Jayanandana (2003) performed experiments to measure depth of carbonation, concrete cover, chloride content and sulfate attack, for the assessment of durability. Karokouzian et al. (2003) studied several swimming pools and found that ASR as the main cause of cracking and deterioration. According to Snathanam et al.(2001) sulfate attack is one of the major cause of deterioration and estimating the remaining service life of structures exposed to sulfate attack is important in order to develop repair and maintenance schedule.
From the above literature it has been recognized that, Strength as well as environmental conditions to which structure is exposed over is also important for the service life of RC structures. So, it is important to understand various deterioration mechanisms of RC structures. Major deterioration mechanisms of RC Structures identified are: