Many people think of concrete as a durable, long-lasting material; however, the past 40 years has shown it might be otherwise. Governments in Canada, the U.S. and Europe have spent billions of dollars repairing and restoring aging structures built in the mid-20th century that have not met their service life expectations. Engineers have warned this is only the beginning.
Owners and designers are now paying more attention to service life expectations during the design phase of projects. Service life expectations are being stated in defined terms, typically of 75 to 100 years. Many specifications and contracts have a definition of what “service life” is and how it will be measured. Designers are trying to assess the effect of various design options on service life and are making design decisions to optimize service life performance.
The aspect that has, perhaps, not been fully recognized is the cooperative, fully-integrated approach of all players in the design, construction and maintenance process that is necessary to actually achieve the optimum balance between service life, functional performance, initial cost and maintenance costs.
Service life is often defined as the period of time in which a structure performs its function without unforeseen or extraordinary maintenance or repair. So, when considering service life, it is critical to define the level of maintenance the owner is prepared to accept. This is important when considering the trade-offs between initial cost and future maintenance costs.
A typical example of this is a decision to use corrosion-resistant rebar, which is now available in various forms. The increase in initial cost is significant, which is problematic for many owners who face budget restrictions. On the other hand, the savings in maintenance costs for bridge decks or marine structures can be substantial.
Accordingly, the expectations of owners with respect to maintenance responsibilities must be established and the life cycle cost-benefit analysis of these design options must be given appropriate attention at the design stage of a project.
Predicting service life
Service life of a concrete structure depends on many variables, some of which are environmental and, as a result, beyond the control of the designers and owners.
So, when setting out to design such a structure, how can service life be predicted?
Fortunately, there are tools to assist designers in this task. Numerical modelling is now used to predict time to corrosion of rebar, which is the primary mechanism of concrete deterioration. This model allows design options to be tested under various environmental conditions. Then, rational decisions can be made to improve durability at reasonable cost. In another mode, the time to first repair can be estimated for various scenarios, allowing a comparison of initial costs and maintenance costs. More sophisticated models are under development that will allow similar analyses to be made of other forms of concrete deterioration such as alkali-aggregate reactivity and sulfate attack.
Achieving service life
Today, 100-year service life of concrete structures can be achieved with relative ease on paper. But achieving it in the real world requires an integrated approach.
Modern design standards now address the service life objectives and have been adopted into building codes and specifications. Designers are using the tools at their disposal to improve durability and reduce maintenance requirements. Materials suppliers are developing corrosion-resistant rebar, supplementary cementing materials to improve concrete properties and admixtures to stem other deterioration processes. Owners are more cognizant of the importance of maintenance and the trade-offs they can make between initial cost and maintenance costs. And contractors have developed quality control procedures to improve construction practices (curing, cover thickness, crack control) that will improve durability.
The critical point is all the actors have a role to play in achieving these extended service life expectations.
Neil A. Cumming, P.Eng., is executive vice-president of Levelton Consultants Ltd.