An assessment of a practical implementation of the deemed-to-satisfy durability design and specification approach in the Swakopmund-Walvis Bay Freeway upgrade project in Namibia

Steel corrosion in reinforced concrete (RC) structures in the marine environment is a major concern for engineers and asset managers. This paper presents an assessment of the application of a deemed-to-satisfy durability design and specification approach on the newly constructed RC bridges on the Swakopmund-Walvis Bay Freeway upgrade in Namibia, with a desired service life of at least 100 years. The deemed-to-satisfy design and specification approach comprised a combination of South African recommendations for binder type, minimum binder content and w/b ratio, EN 206-1 recommendations for cover depth, and South African National Roads Agency Limited (SANRAL) durability index targets along with on-site concrete quality control guidelines on placement, compaction and curing. As-built concrete durability properties (i.e. water sorptivity, oxygen permeability and chloride conductivity) and cover depth were measured and used to check (using a Fickian-based service life model) if the deemed-to-satisfy specifications were adequate. The results showed that the as-built concrete durability properties surpassed the deemed-to-satisfy durability performance targets, while the on-site cover depth values showed significant inadequacies in some bridge elements. The service life estimations showed that the deemed-to-satisfy durability performance specifications were adequate. Considering the relatively high concrete quality achieved on site, it is clear that the 100-year service life could have been achieved more sustainably using a good balance of concrete quality and cover depth [e.g. a lower concrete quality (using less cement) or lesser coved depth] if a more rigorous performance-based durability design and specification approach had been used. Ultimately, the service life analyses using the as-built concrete model input parameters highlight not only the critical importance of both concrete quality and cover depth in achieving durable RC structures prone to steel corrosion, but also the inherent limitations of the deemed-to-satisfy durability design and specification approach.