Criteria for and prediction of limit states of degradation of reinforced concrete structures

The past decade has seen significant advances in service life design of concrete structures and models for degradation processes are on the point of entering design standards. Application has so far been directed principally at new construction. The 'end of service life' criteria most commonly employed for carbonation or chloride penetration is the initiation of corrosion on the reinforcement. It is widely apparent, however, that many concrete structures continue to give adequate service even though corrosion has progressed well beyond a level sufficient to crack concrete cover. Indeed, many condition assessment protocols currently in use only record deterioration once cover concrete is cracked. Repairs to concrete structures are expensive in financial terms. Depending on the situation of the structure under consideration, they may also be environmentally costly. Unnecessary work should therefore be avoided. For a structure with a clearly defined useful life, the question might be: when can major maintenance be terminated without an ultimate limit state of degradation being reached? A range of possible limit states to be considered in apprising the need for repair and the various ways in which corrosion can impair structural capacity are discussed. The paper presents a scoping analysis, based on field data for chloride penetration and for corrosion rate, to examine the relative duration of different stages in the deterioration process. The analysis demonstrates that time to the ultimate limit state of degradation might typically be 5-10 times that to the initiation of corrosion.