Practical implementation of the durability index performance-based design approach

The paper provides a summary of findings from a study that was done to evaluate practicality of Durability Index (DI) performance-based specifications that have been implemented on a large scale in a major infrastructure project involving bridge and other structures in Gauteng Province, South Africa. The practicality of the approach was evaluated by considering: the magnitude and variability in DI test values (Oxygen Permeability Index, OPI, and water sorptivity) and cover depths; applicability of the DI tests in laboratories to determine if correct execution of test procedures is done, in addition to its application on site where samples are obtained; and response of the industry to this recently implemented approach which was assessed from reviewing responses to a questionnaire that was sent out to resident engineers (REs). From the statistical analysis, it was observed that the average values of the durability parameters (OPI, sorptivity and cover depth) in all cases complied with the limiting minimum values. However, for some structures, a high proportion of individual DI test results failed to comply with limiting values. The variability observed in the coefficient of permeability (k) and sorptivity values was high. For cover depth readings the variability measured on structures cast in situ was considerably higher than for precast elements. A mixed perception of the effects of this approach was obtained among engineers; some indicated that it had no effect on construction practices while others felt that, as a result of the performance-based project specifications, stricter controls had been placed in execution of construction practices. From this study it was concluded that the DI performance-based approach offers a potentially useful and practical way to perform quality control of cover concrete. However, certain limitations have to still be overcome to fully reap the potential benefits of this approach. These include high variability in test results (sorptivity, coefficient of permeability), poor quality of test panels, poor communication between site staff and laboratories, difficulties in transport of test panels.