4.13.1 Introduction

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 where concrete will not be able to meet its functional and performance requirements; however, concrete can suffer undesirable changes with time because of improper specifications, violation ofspecifications, or adverse performance ofits cement paste matrix or aggregate constituents under physical or chemical attack. Additional information related to environmental effects on concrete is provided in molten core concrete interaction (Chapter 2.25, Core Concrete Interaction).

Portland cement concrete durability is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process or deterioration.1 A durable concrete is one that retains its original form, quality, and serviceability in the working envi­ronment during its anticipated service life. The mate­rials and mix proportions specified and used should be such as to maintain concrete’s integrity and, if applicable, to protect embedded metal from corro — sion.2 The degree of exposure anticipated for the concrete during its service life, together with other relevant factors related to mix composition, work­manship, and design, should be considered.3 Guide­lines for production of durable concrete are available in national consensus codes and standards, such as American Concrete Institute (ACI) 3184, which have been developed over the years through knowledge acquired in testing laboratories and supplemented by field experience. Serviceability of concrete has been incorporated into the codes through strength requirements and limitations on service load conditions in the structure (e. g., allowable crack widths, limitations on midspan deflections ofbeams, and maximum service level stresses in prestressed members). Durability gen­erally has been included through items such as specifi­cations for maximum water-cement ratios, minimum cementitious materials contents, type cementitious material, requirements for entrained air, and minimum concrete cover over reinforcement. Requirements are frequently specifiedin terms of environmental exposure classes (e. g., chloride and aggressive ground environ­ments). Specifications in terms of service life require­ments (e. g., short <30years, normal 30-100years, and long >100 years) have only recently been developed, primarily through European standards.5

Water is the single most important factor con­trolling the degradation processes of concrete (i. e., the process of deterioration of concrete with time is generally dependent on the transport of a fluid through concrete), apart from mechanical deteriora­tion. The rate, extent, and effect of fluid transport are largely dependent on the concrete pore structure (i. e., size and distribution), presence of cracks, and micro­climate at the concrete surface. The primary mode of transport in uncracked concrete is through the cement paste pore structure (i. e., its permeability). The domi­nant mechanism controlling the rates ofwater penetra­tion into unsaturated or partially saturated concrete is absorption caused by capillary action of the concrete’s pore structure. To improve the durability of concrete, generally the capillary and pore size within the con­crete matrix should be reduced to a minimum.

Although the coefficient of permeability for con­crete depends primarily on the water-cement ratio and maximum aggregate size, it is influenced by the curing temperature, drying, cementitious materials content, and addition of chemical or mineral admixtures as well as the tortuosity of the path of flow. Concrete compressive strength has traditionally been utilized as an acceptance test for concrete, but it typically is not a good indicator of durability. Many structures have been fabricated with concretes having adequate 28-day compressive strength only to lose their func­tionality because they were facing an environment for which they had not been designed or because the concrete had not been placed or cured correctly.6

The safety-related concrete structures in nuclear power plants (NPPs) are designed to withstand load­ings from a number of low-probability external and internal events, such as earthquake, tornado, and loss — of-coolant accident. Consequently, they are robust and not subjected to high enough stresses during normal operation to cause appreciable degradation. In general, this has been the case, as the performance of reinforced concrete structures in NPPs has been good. (Operating experience is discussed in the next section.) However, as the NPPs age, degradation incidences start to occur at an increasing rate, pri­marily due to environmental-related factors. One — fourth of all containments in the United States have experienced corrosion, and nearly half of the con­crete containments have reported degradation related to either the reinforced concrete or post-tensioning

system.7 Although the vast majority of these struc­tures will continue to meet their functional and per­formance requirements during their initial licensing period (i. e., nominally 40 years), it is reasonable to assume that with the increasing age of the operating reactors there will be isolated examples where the structures may not exhibit the desired durability without some form of intervention.

Currently, the United States has 104 NPP units licensed for commercial operation, which provide about 20% of the electricity supply. As all but one of the construction permits for existing NPPs in the United States were issued prior to 1978, the focus for the existing plants has shifted from design to condition assessment. Here, the aim is to demonstrate that struc­tural margins ofthe plants have not eroded or will not erode during the desired service life due to aging or environmental effects. One of the key factors to maintaining adequate structural margins to protect public health and safety in the unlikely event of an accident is implementation of effective inspection and maintenance programs. An inspection program is important for identifying and characterizing any deg­radation that may be present in a timely manner. Once degradation has been identified, or its potential to occur established, a maintenance program is imple­mented to repair the degradation and arrest (as far as possible) the mechanism(s) causing the degradation. Proper maintenance is essential to the safety of NPP structures, and a clear link exists between effective maintenance and safety. Uncertainty in condition assessment can be assessed using probabilistic methods, which are also an essential ingredient of risk-informed management decisions concerning continued service of the NPP structures.