Concrete Petrography: An Essential Component of Cost-effective Decision Making for Infrastructure Renewal

With provincial and municipal infrastructure budgets as stretched as ever, and greater accountability demanded with respect to sustainability in rehabilitation programs, the salvage value of existing infrastructure is of critical importance. From a sustainable development perspective, there is an imperative to utilize as much as possible of existing infrastructure, and to minimize waste generation when undertaking renewals. A key factor in this process, especially with respect to transportation structures, is to decide how much of the existing concrete can be salvaged. This is not just a question of establishing existing condition but predicting future life for a complex construction material that may already be 50, or more, years old. Unfortunately, the practical evaluation of old concrete is not as simple as reviewing the results of a series of standard laboratory tests such as compressive, tensile or flexural strength, chloride profiling, and air voids content. Improving our ability to reliably determine the in-situ health of old concrete can support cost-saving engineering decisions to retain bridge piers and abutments while only replacing the deck; reline an existing tunnel rather than replace it; or to leave old concrete pavement in place beneath a multilane freeway. The techniques are varied and project-specific. They rely on an understanding of the components of structural concrete: steel, aggregates and cement paste, and how they interact and deteriorate. The evaluation techniques comprise destructive and non-destructive testing but with the essential component of concrete petrography. This latter technique can detect the early stages of destructive chemical reactions and may be used to determine to what stage such reactions have progressed and might continue to progress; the signs of freeze-thaw damage; the impact on concrete integrity from the corrosion of reinforcing steel; and other aspects. With this detailed knowledge, the appropriate remedial solutions can be identified, taking advantage of the vast array of effective modern specialty concrete repair products and techniques that are available. This paper will discuss the approaches to the condition evaluation of old concrete structures with a focus on concrete petrography and present some case studies to illustrate the benefits of an effective concrete health check before deciding on full reconstruction.