Effect of Aging Concrete on Seismic Performance of Shear Wall Structures, 20-R8090
Inclusive Dates: 10/01/09 – 10/01/11
Background — Concrete aging can cause deterioration of the mechanical properties of concrete and affect the seismic performance of the existing reinforced concrete (RC) structures. The consequences from failure of aging infrastructure (e.g., bridges, dams, and nuclear power plants) in the United States and other countries are a major concern. As concrete ages, its properties change as a result of continuing microstructural changes (i.e., slow hydration, crystallization of amorphous constituents, and reactions between cement paste and aggregates); however, under aggressive environments, the degradation process may accelerate. In addition, physical challenges, including freeze and thaw cycling, thermal exposure and thermal cycling, abrasion, erosion or cavitation, fatigue or vibration, and corrosion of steel reinforcing rebars, are significant contributors to the overall degradation of structural resistance. Several experiments reported in the literature have evaluated the potential effects of aging and temperature on the mechanical material properties (e.g., compressive strength and elastic modulus). The effect of long-term degradation of concrete, however, has not been comprehensively addressed in seismic performance evaluations. In this research, investigators developed a methodology to assess the effect of concrete aging on the seismic performance of structures in which RC shear walls are the main lateral resistance components.
Approach — A large number of studies have evaluated the effect of high temperatures on concrete, but data regarding concrete performance under long-term, high-temperature exposure are limited and do not take into account the strength gain that concrete may have experienced at room temperature. Moreover, previous data do not consider widespread use of admixtures, such as plasticizers. The compressive strength of concrete cylinders was tested at 28 days and then at regular 90-day intervals over 24 months for a moist-cured specimen. Additionally, cores were exposed to 90 to 95 °C [194 to 203 °F] over the same period, and compressive strength was tested to study exposure of concrete to temperature. Petrographic analysis was performed to assess the deterioration of concrete at high temperatures. Concrete core tests were performed on samples taken from existing buildings. Additionally, threshold chloride levels for localized corrosion of carbon steel rebar material have been studied in three types of simulated concrete pore solutions.
A new approach was developed to generate fragility curves of structural systems that account for aging of concrete. The structures evaluated in this project consist of representative systems commonly used in the nuclear industry, which include thick, RC shear walls as the main lateral resistance component. Structural response was evaluated for concrete aging factors caused by loss of steel area, changes in concrete compressive strength and loss of concrete area caused by cracking and spalling. The separation of variables method used in seismic probabilistic risk analysis for nuclear power plants was used to develop fragility curves for the non-degraded and degraded shear wall components. The variability of parameters for fragility of non-degraded and degraded shear wall components were concrete strength, stiffness, and strain at ultimate strength, and yield strengths of steel. The probability of unacceptable performance or seismic failure was computed by convolving the fragility and hazard curve.
Accomplishments — The compressive strength of concrete mixes containing admixtures to accelerate strength development was tested at standard moisture content and elevated temperatures close to the boiling point of water. The results indicated that long-term compressive strength of concrete cured in a standard moisture environment is not significantly affected by the presence of water admixtures. The water-reducing admixture in the concrete contributed to early strength development because it accelerated the cement hydration; however, the long-term compressive strength has not been significantly affected after 1.5 years, and the concrete strength has stabilized with a gain of 10 to 15 percent with respect to compressive strength at 28 days. The effect of continuous exposure to temperatures resulted in a loss of compressive strength of less than 10 percent. Petrographic analyses suggest that deterioration of concrete compressive strength at these temperatures was mainly caused by physicochemical changes in the cement paste. The localized corrosion of carbon steel experiments demonstrated that if the chloride concentration is greater than certain pH levels, high-corrosion rate is estimated to cause a 20-percent reduction of the rebar cross-sectional area.
The seismic performance evaluation used experimental data on the corrosion of steel reinforcement and variability of concrete parameters as a function of time. To obtain the variation in the system capacity caused by concrete aging, detailed numerical models of RC shear walls were developed using the finite element method. The deterioration effects of RC are accounted for in the material properties of concrete and steel rebar in the shear wall model for evaluation of fragility curves. Failure probability increases with aging of RC because the increase in the concrete compressive strength with time does not overcome the degradation of the seismic performance caused by concrete cracking and corrosion of steel rebars. Aging effects increase the probability of failure of RC shear walls subjected to lateral loading. The increase is about four times of magnitude when all sources (demand and capacity) of variability are considered.