It is widely recognized that service life can be more economically extended with robust and durable initial design and construction than by future rehabilitation. To address limited future operating budgets, bridge owners are focusing increasingly on life-cycle costs and asset management, rather than just initial construction cost.
International Federation for Structural Concrete Model Code for Service Life Design
One established service life design methodology that is built on a broad base of experience and that resides in the public domain is the International Federation for Structural Concrete (fib) Model Code for Service Life Design (fib Bulletin 34, 2006). This methodology has recently been implemented in ISO 16204:2012 Service Life Design of Concrete Structures.
The methodology was developed through the publicly funded DuraCrete research project and was validated by a consortium of universities and consulting firms to offer a reliability-based approach similar in principle to modern limit states design-based structural design codes.
The fib methodology provides a rational probability-based approach to service life design of concrete structures subject to corrosion, a major deterioration mechanism for bridges, tunnels and marine structures.
This state-of-the-art durability design methodology has been implemented on the New NY (Tappan Zee) Bridge and the Abraham Lincoln Bridge between Kentucky and Indiana. The approach is analogous to structural design for which durability-related loads and resistances are assessed and quantified probabilistically, considering element specific exposures and material properties.
For example, durability loads include surface chloride concentrations and ambient temperature, and durability resistances include concrete cover and permeability.
Based on the fib methodology, durability requirements can be quantified and measured to verify that the required materials and properties are achieved. Concrete compressive strength is measured as a means of verifying the structural performance; we can now do the same for the durability requirements.
The concrete permeability is represented by the concrete chloride migration coefficient, and is determined by the test NTBuild 492 Chloride Migration Coefficient from Non-Steady State Migration Experiments.
In contrast to other concrete durability tests, the NTBuild 492 can be efficiently implemented as part of the construction quality control and assurance processes. Similar to concrete compressive strength tests, the NTBuild 492 is performed on standard concrete cylinders at 28 days after casting and the test has a 24-hour duration. The test provides a direct measurement of the concrete resistance to chloride penetration and the measured value is used directly in service life calculations.
This fib methodology goes beyond traditional “deemed-to-satisfy” rules and subjective durability requirements, providing a documented and validated probabilistic limit states design approach to service life. The contractor can demonstrate that service life requirements are met and the owner can be confident that their asset will not deteriorate prematurely. This approach benefits all parties involved on the project.