Data on the long-term performance of the reinforced concrete materials are of importance for demonstrat­ing the durability of the NPP concrete structures and in predicting their performance under the influence of pertinent aging factors and environmental
stressors. This information also has application to establishing limits on hostile environmental exposure for these structures and to developing inspection and maintenance programs that will prolong com­ponent service life and improve the probability of the component surviving an extreme event such as a loss-of-coolant accident.

Prior reviews of research conducted on concrete materials and structures indicate that only limited data are available on the long-term (40-80 years) properties of reinforced concrete materials.26 Where concrete properties have been reported for condi­tions that have been well documented, the results were generally for concretes having ages <5 years or for specimens that had been subjected to extreme, nonrepresentative environmental conditions such as accelerated corrosion or aging. Relatively few inves­tigations have been reported providing results on examinations of structures that had been in service for the time period of interest, 20-100 years, and they did not generally provide the ‘high quality’ informa­tion (e. g., baseline material characteristics and changes in material properties with time) that is desired for meaningful assessments to indicate how the structures have changed under the influence of aging factors and environmental stressors.

Limited data on the long-term performance of reinforced concrete materials reported in the litera­ture, results from concrete cores removed from NPPs, and specimens cast in conjunction with NPP facilities have been reported.78 As shown in Figure 6, these results generally show an increase in compres­sive strength (relative to the 28-day reference

strength) at a decreasing rate with age, but the data obtained from the literature were for concrete ages <50years and the nuclear plant data for ages <27 years. (Using results obtained from concrete cores removed from residential buildings and bridges, one reference indicates that although the concrete strength and modulus exhibit an increase with age, the ability of concrete to resist shear and torsion may decrease.79) Long-term laboratory results in the fig­ure for the Portland Cement Association (PCA) and University of Wisconsin (WIS) studies that attained the largest increases in strength were generally for concrete mixes having high water-cement ratios (lower reference compressive strength) and that had access to moisture for continued cement hydration. NPP concretes had higher reference compressive strengths and were essentially maintained under sealed conditions. With the availability of decommis­sioned NPPs and plant modifications requiring removal of materials, opportunities exist to obtain samples for use in providing an improved under­standing of the effects of extended exposure under the unique conditions found in NPPs. In addition to aging, areas of interest would be the effects of long­term thermal loadings at moderate temperature levels and effects of irradiation on load-bearing con­crete structures operating for >40 years. Additional applications of a concrete material sampling activity would be for assessment of construction quality, development of improved damage models, assess­ment and validation of nondestructive testing meth­ods, and evaluation of the performance of repair activities.

With respect to post-tensioning systems, current examination programs such as ASME Section XI Subsection IWL80 are adequate for determining the condition of the post-tensioning system materials and evaluating the effects of conventional degradation. Isolated incidences of wire failure due to corrosion have occurred. Leakage of tendon sheathing filler (ungrouted tendons) has occurred at a few plants but, except for the potential loss of corrosion protec­tion, the problem appears to be primarily aesthetic.81 Tendon forces at most plants are acceptable by a significant margin, but larger than anticipated loss of force has occurred at a few older plants. The hypothetical effect of reduced prestressing force and degradation of prestressing tendons (e. g., broken wires) has been investigated for a typical PWR post — tensioned concrete containment during a loss-of- coolant accident using finite-element analysis.56 (Results for the scenario investigated indicated that loss of prestressing force leads to increased concrete cracking at lower pressurization levels, complete fail­ure of selected hoop tendons can have a significant impact on the containment ultimate capacity, and failure of selected vertical tendons does not have a significant impact on ultimate capacity.) With the potential use of grouted tendon systems in some of the new reactor designs proposed for construction in the United States, improved guidance on in-service inspection of grouted tendons is desired. Other potential research topics related to post-tensioning systems include development of an improved relation­ship between the end-anchorage force measured by the lift-off test and change in mean force along the tendon length for unbonded tendons, as well as an assessment of the validity of using estimates of time-dependent loss of prestressing force based on limited-duration relaxation tests (e. g., 1000 h) and concrete creep results (e. g., 6 months): at a plant 60-year old, this involves application of time factors of 500 and 120, respectively.