Effect of Aging Concrete on Seismic Performance of Shear Wall Structures, 20-R8090Printer Friendly Version
Inclusive Dates: 10/01/09 Current
Background - Concrete aging may cause deterioration of the mechanical properties of concrete and affect the seismic performance of existing reinforced concrete structures. Failure of aging infrastructure such as bridges, dams and nuclear power plants in the U.S. and other countries is a major concern. As concrete ages, properties change as a result of continuing microstructural processes (i.e., slow hydration, crystallization of amorphous constituents, and reactions between cement paste and aggregates). Under aggressive environments, degradation may accelerate. In addition, physical challenges including freeze/thaw cycling, thermal exposure/thermal cycling, abrasion/erosion/cavitation, fatigue or vibration, and corrosion of steel reinforcing rebars significantly contribute to the overall degradation of structural resistance. Several studies have focused on experiments to evaluate the potential effects of aging 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. For example, the component aging effects usually are not extrapolated to the entire structural system to evaluate system capacity. The objective of this research is to develop a reliability-based methodology to assess concrete aging in shear walls under seismic loading and its effect on the system performance.
Approach - This investigation will try to connect the physical mechanisms that take place during concrete aging (e.g., cracking, rebar corrosion, and spallation) and the deterioration of mechanical properties that directly affect the computation of the system capacity under external events, particularly seismic events. The effect on mechanical properties of concrete aging will be obtained from data collected from previous studies and from experimental tests carried out in this investigation. In addition, long-term effects of elevated temperatures on aging concrete and the material behavior will be evaluated experimentally. To obtain the variation in the system capacity caused by concrete aging, detailed numerical models of reinforced concrete components and models of selected structural systems will be developed. Several analytical methods used to generate fragility curves and ultimately the probability of structural failure will be evaluated for the first time by addressing the effects of concrete aging. Thus, the study will improve the methodology for evaluating performance of systems affected by concrete aging, particularly those under the effect of seismic events.
Accomplishments - Experiments were carried out to evaluate the effect of concrete aging on compressive strength. Concrete cylinders of 6-in. diameter by 12-in. length and 4-in. diameter by 8-in. length have been tested at different times to obtain the evolution of the concrete compressive strength with time. The compressive strength expected for the concrete mix at 28 days is 4,000 psi. The concrete cylinders were fabricated using the same lot of concrete mix and were cured in a standard moisture environment at 23 ± 2 °C [73 ± 3 °F] and 95 percent relative humidity. After 90 days, a subset of the 4- by 8-in. cylinders was moved to a laboratory oven to be cured at a temperature ranging from 90 to 95 °C [194 to 203 °F]. To date, cylinders of 4 by 8 in. and 6 by 12 in. have been tested at 28, 90, 180, and 270 days. In parallel to these tests, concrete core samples were obtained from existing structures and tested for compressive strengths. The laboratory test results to date showed that the compressive strength of concrete increases with time at early stages of exposure at room temperature, but decreases during elevated temperature exposures. Further tests and analyses of test data are in progress. Development of a concrete shear wall model and analysis under seismic ground motion is in progress. In addition, the project team is developing a structural system model for evaluating seismic performance.