Reinforced concrete structures almost from the time of construction can start to deteriorate in one form or the other as a result of exposure to the environment (e. g., temperature, moisture, and cyclic loadings).77 The rate of deterioration is dependent on the compo­nent’s structural design, materials selection, quality of construction, curing, and aggressiveness of envi­ronmental exposure. Termination of a component’s service life occurs when it no longer can meet its functional and performance requirements, it becomes obsolete, or the maintenance costs become excessive.

Primary mechanisms (factors) that, under unfavor­able conditions, can produce premature deterioration of reinforced concrete structures include those that impact either the concrete or steel reinforcing materi­als (i. e., mild steel reinforcement or post-tensioning systems). Degradation of the concrete can be caused by adverse performance of either its cement-paste matrix or aggregate materials under chemical or phys­ical attack. In practice, these processes may occur concurrently to reinforce each other. In nearly all physical and chemical processes influencing the dura­bility of concrete structures, dominant factors involved include the transport mechanisms within the pores and cracks and the presence of water. Chemical attack may occur in several forms: efflorescence or leaching; attack by sulfate, acids, or bases; delayed ettringite formation; and alkali-aggregate reactions. Physical attack involves the degradation of concrete due to external influences and generally involves cracking due to exceeding the tensile strength of concrete, or loss of surface material. Physical attack mechanisms for concrete include salt crystallization, freezing and thawing, thermal exposure/thermal cycling, abrasion/ erosion/cavitation, irradiation, fatigue or vibration, biological attack, and settlement. Degradation of mild steel reinforcing materials can occur as a result of corrosion, irradiation, elevated temperature, or fatigue effects, with corrosion being the most likely form of attack. Post-tensioning systems are susceptible to the same degradation mechanisms as mild steel reinforce­ment plus loss of prestressing force primarily due to tendon relaxation and concrete creep and shrinkage. Of these, corrosion and loss of prestressing force are the most pertinent from the perspective of NPP dura­bility. Additional information on durability of NPP reinforced concrete structures is available.34