Durability specifications in the South African standards
(SANS 2005) and the new European standards
(BS EN 2004), for example, follow the prescriptive
concept and are of the ‘recipe’ type, setting limits on
w/c ratios, cement contents, and compressive strength
for different exposure classes. The design for durability
includes the correct choice of exposure class
and compliance with material requirements, concrete
cover specifications, and curing procedures.
However, durability is a concept that incorporates
material properties, processing technology and environmental
exposure conditions and, as such, it cannot
easily be assessed through intrinsic material properties.
The prescriptive approach ignores, to a large extent,
the different performance of the different cement types
and of the mineral components added to the cements
or to the concrete itself, as well as the influences of onsite
practice during the construction process.
Performance concepts, on the other hand, are based
on quantitative predictions for durability from exposure
conditions and measured material parameters. The
resistance of the structure, measured through durability
parameters of the actual concrete used, is compared
against the environmental load, taking the influences
of time into account. On this basis, the probability of
damage occurring to the structure during its lifetime is
calculated using appropriate deterioration models.
Different levels of sophistication may be applied to
performance-based design for durability, including the
use of durability indexes, the application of analytical
deterioration models, and full probabilistic methods. At
the moment, various approaches are being developed
worldwide, however yet with very limited application in
real projects. Of course, the added challenge of assessing
such models against long-term structural performance
remains as an important future research need.