Durability and service life 13
1.4.4 cement type
In order to characterize the durability of concrete, it is not sufficient to
know the water/cement ratio and the cement content. The cement type is
also very important with regard to concrete durability. As an example, concrete based on blast furnace slag cement or containing silica fume will provide a better resistance against chloride penetration. In the case where the
slag content of the binder is at least 60%, the chloride diffusion coefficient
is reduced by a factor of ten in comparison with Portland cement-based concrete with a similar water/cement ratio of 0.5 (Neville 1995). On the other
hand, blast furnace slag-based binders with such high slag contents can
be more sensitive to carbonation. Carbon dioxide will not be sufficiently
bound in the concrete cover and the pore system will not be blocked with
calcium carbonate, in contradiction to what happens in Portland systems.
This illustrates the difficulties involved in defining durability requirements
solely based on cement content and water/cement ratio. A similar remark
can be formulated towards strength as a governing parameter.
Besides chloride penetration and carbonation, the nature of the cementitious materials is also relevant for other degradation mechanisms. Furthermore, the selection of cement type can be influenced by requirements
concerning early age strength development (depending on the casting temperature), heat of hydration, shrinkage, etc. Puzzolan or other additions can
be very advantageous in some cases. On the other hand, it should also be
mentioned that in case of puzzolan or other slowly reacting additions, the
concrete is more sensitive to deficient curing. Casting and curing conditions
generally also influence concrete durability, as will be explained further on.
With the example of chloride penetration and carbonation, the importance of the right choice of cement type or binder system is illustrated. Not
every cement type is adequate for every application. Well-known is the choice
of a high sulphate resistant cement or a low alkali cement in some specific
conditions. A durable concrete requires a motivated choice of cement type
or binder system, duly considering the cement chemistry (Hewlett 1988).
1.4.5 strength
Durability (and sustainability) of concrete structures nowadays is an important issue. The due attention to durability is quite in contrast with earlier
ideas that only concrete strength is of importance, implicitly considering that
a strong concrete will automatically perform as a durable concrete. To illustrate the older idea that concrete is inherently durable, reference is made to
the British Standard Code of Practice CP 114 valid in 1948: “No structural
maintenance should be necessary for dense concrete constructed in accordance with this code” (Neville 1995). It is clear that the sole or at least main
attention to concrete strength has led to serious durability issues in the past.
1.4.4 cement type
In order to characterize the durability of concrete, it is not sufficient to
know the water/cement ratio and the cement content. The cement type is
also very important with regard to concrete durability. As an example, concrete based on blast furnace slag cement or containing silica fume will provide a better resistance against chloride penetration. In the case where the
slag content of the binder is at least 60%, the chloride diffusion coefficient
is reduced by a factor of ten in comparison with Portland cement-based concrete with a similar water/cement ratio of 0.5 (Neville 1995). On the other
hand, blast furnace slag-based binders with such high slag contents can
be more sensitive to carbonation. Carbon dioxide will not be sufficiently
bound in the concrete cover and the pore system will not be blocked with
calcium carbonate, in contradiction to what happens in Portland systems.
This illustrates the difficulties involved in defining durability requirements
solely based on cement content and water/cement ratio. A similar remark
can be formulated towards strength as a governing parameter.
Besides chloride penetration and carbonation, the nature of the cementitious materials is also relevant for other degradation mechanisms. Furthermore, the selection of cement type can be influenced by requirements
concerning early age strength development (depending on the casting temperature), heat of hydration, shrinkage, etc. Puzzolan or other additions can
be very advantageous in some cases. On the other hand, it should also be
mentioned that in case of puzzolan or other slowly reacting additions, the
concrete is more sensitive to deficient curing. Casting and curing conditions
generally also influence concrete durability, as will be explained further on.
With the example of chloride penetration and carbonation, the importance of the right choice of cement type or binder system is illustrated. Not
every cement type is adequate for every application. Well-known is the choice
of a high sulphate resistant cement or a low alkali cement in some specific
conditions. A durable concrete requires a motivated choice of cement type
or binder system, duly considering the cement chemistry (Hewlett 1988).
1.4.5 strength
Durability (and sustainability) of concrete structures nowadays is an important issue. The due attention to durability is quite in contrast with earlier
ideas that only concrete strength is of importance, implicitly considering that
a strong concrete will automatically perform as a durable concrete. To illustrate the older idea that concrete is inherently durable, reference is made to
the British Standard Code of Practice CP 114 valid in 1948: “No structural
maintenance should be necessary for dense concrete constructed in accordance with this code” (Neville 1995). It is clear that the sole or at least main
attention to concrete strength has led to serious durability issues in the past.