As concrete ages, changes in its properties will occur as a result of continuing microstructural changes (i. e., slow hydration, crystallization of amorphous constitu­ents, and reactions between cement paste and aggre­gates), as well as environmental influences. These changes do not have to be detrimental to the point that concrete will not be able to meet its performance requirements. Concrete, however, can suffer unde­sirable changes with time because of improper spec­ifications, a violation of specifications, or adverse performance of its cement paste matrix or aggregate constituents under either physical or chemical attack.

Concrete durability and the relationship between durability and performance, a review of the historical perspective related to concrete and longevity, a description of the basic materials that comprise rein­forced concrete, and information on the environmental factors that can affect the performance of NPP con­crete structures have been provided. Primary aspects related to management of aging of NPP concrete structures have been noted: degradation mechanisms, damage models, and material performance; assessment and repair (i. e., component selection, in-service inspection, nondestructive examinations, and remedial actions); and estimation of performance at present or some future point in time (i.e., application ofstructural reliability theory to the design and optimization of in­service inspection/maintenance strategies, and deter­mination of the effects of degradation on plant risk).

Several areas have been identified where addi­tional research would be of benefit to aging manage­ment of NPP concrete structures: (1) compilation of material property data for long-term performance and trending, evaluation of environmental effects, and assessment and validation ofnondestructive eval­uation methods; (2) evaluation of long-term effects of elevated temperature and radiation on concrete behavior; (3) improved damage models and accep­tance criteria for use in assessments of the current condition as well as estimation of the future condition of the structures; (4) improved constitutive models and analytical methods for use in determination of nonlinear structural response (e. g., accident conditions); (5) nonintrusive methods for inspection of thick-walled, heavily reinforced concrete structures and basemats; (6) global inspection methods for metal­lic pressure boundary components (i. e., steel contain­ments and liners of concrete containments) including inaccessible areas and backside of liner; (7) data on application and performance (e. g., durability) of repair materials and techniques; (8) utilization of structural reliability theory incorporating uncertainties to address time-dependent changes to structures to ensure that minimum accepted performance require­ments are exceeded and to estimate on-going compo­nent degradation to estimate end of life; and (9) application of probabilistic modeling of component performance to provide risk-based criteria to evaluate how aging affects structural capacity.