CHAPTER 4. SEISMIC PERFORMANCE OF REINFORCED CONCRETE
4.7 Case studies representing non uniform corrosion 131
Some RC columns are located in more aggressive areas, such as highway bridge columns right next to the road shoulders or those at the basement of a building. Often time these columns are exposed to an uneven amount of intrusive chemicals that can accelerate the corrosion. For example, for the bridge columns right next to the highway shoulders, more exposure to deicing ice is expected as they are closer to the splash zone of the traffic zone and more likely to get in touch with the salt-laden snow that snowplows pile next to them. In such situations, the bottom region of the column is exposed to a more surface chloride content and is more likely to have corrosion initiated faster and propagate in a higher speed. A similar situation could be observed in the columns at the basement of a building, such as at the parking garage, where there is a higher likelihood of salty water exposure. On the other hand in a seismic regions, these columns are expected to undergo large earthquake induced demand and the additional corrosion created at the bottom region –where most of the time the plastic hinges are exposed to form- will jeopardize the seismic performance of the columns and the structure as a whole. To investigate the behavior of the column when it experiences inconsistent levels of corrosion throughout its cross section and length. Two case study columns that represent the above situations are presented in this section.
Case I is when both sides of the lower one third of the column height at 20 years is exposed to more chloride and hence experiences a higher corrosion rate (Case I-1: i = 5μA/cm2 and Case I-2: i= 10μA/cm2) and the rest of the column is subjected to a corrosion rate of
2μA/cm2. Case II is when only one side of the lower one third of the column height at 20 years experiences a higher corrosion rate: (Case II-1: i = 5μA/cm2 and Case II-2: i = 10μA/cm2). The rest of the column is subjected to a corrosion rate of 2μA/cm2 (Figure 4-14). Nonlinear time history analysis is performed for these four cases and the results compared with the column at 0, 20 and 50 years are shown in Figures 4-15-16.
Under EQ 2%, in Case I-1- 20 years when both sides of the column lower section are under a higher corrosion rate of 5μA/cm2, the capacity of the column is reduced to 76.2% of the column capacity at 20 years with uniform corrosion and the maximum relative lateral displacement is 2.1 times as large as that of the column at 20 years with uniform exposure. If comparing the seismic response of the column in Case I-1 and at 50 years, it can be seen that the capacity in Case I-1 is similar to that in 50 years and the maximum relative lateral displacement in Case I-1 reaches 92.3% of that at 50 years. Therefore, it can be known that when the critical lower section of the column experiences a higher corrosion rate, the seismic capacity of the column is significantly reduced and the column undergoes a much greater maximum lateral displacement indicating higher extent of plasticity and damage. A similar trend is observed in Case I -2 when the critical lower section experiences a much higher corrosion rate of 10μA/cm2 at 20 years. The capacity of this 20 year old column in Case I - 2 is lower than that of the column at 50 years. This may alert designers, inspectors and maintenance teams to pay special attention to the regions of the structural components that are expected to perform during a seismic event requiring certain protection measures or special designs for these locations.
It can be known from Case II that when only one side of the column lower section experiences a higher corrosion rate, the seismic capacity of the column is decreased and the
greater maximum relative displacement indicates higher extent of plasticity and damage compared to the column at same age undergoing a consistent corrosion level, although the columns with both sides of the base are subjected to that higher corrosion rate perform even worse. The reason is that under both EQ 10% and 2%, in Case I-2 and II-2, many of longitudinal steel elements at the base already reach their ultimate strains (observed from FE results), the longitudinal reinforcement of the columns are expected to rupture with large areas of concrete spalling at the base, resulting complete failure in those columns. Even for Case I- 1 and Case II-1 under EQ 10% and 2%, the maximum normalized steel strains are very high (0.8-0.9). Some of longitudinal steel elements at column base experience such high strains. The longitudinal reinforcement of these columns are very likely to rupture. Simultaneously, noticed from FE results, the steel strains of the spirals at the column bases in the four cases are very high, compared to the columns experiencing the consistent corrosion rates at 20 and 50 years, particularly for Case I-2 and Case II-2 under EQ 10%, as well as all the four cases under EQ 2%. In Case I-2 and Case II-2 under both EQ 10% and 2%, some of the spiral steel elements at the base already reach their ultimate strains. The bottom region of the spirals will rupture, leading to failure of the confinement in plastic hinge zone of the column. In Case I-1 and II-1 under EQ 2%, some of the spiral elements at the base experience high strains. The maximum normalized steel strains of the spirals are 0.6 and 0.7, respectively. In Case I-1 and II-1 under EQ 10%, although the spiral strains at the bottom are lower than that under EQ 2%, they are still much higher compared to the column experiencing the uniform corrosion rate at 20 years or even 50 years. Therefore, it can be known that when the critical section of the column experiences a higher corrosion risk, the column tends to fail due to lack of confinement at the critical location caused by corrosion-induced reinforcement degradation. Such columns are in
a critical situation and special attention must be drawn to them to prevent failure during seismic events.