14 Damage to concrete structures
The dubious role of strength as a durability indicator can be further illustrated by studying some historical information. The general applicability
of concrete as a construction material significantly increased in the twentieth century thanks to extensive laboratory research coupled with increasing practical experience. New cements have been developed with increased
chemical resistance. It was gradually realized that the quality of the concrete
determines the resistance against degradation. In this respect, quality of the
concrete was often interpreted in a too limited way as the strength of the
concrete. However, a strong concrete is not automatically a durable concrete.
Some situations can be found where a stronger concrete can show more degradation than a weaker concrete, when the latter is based on a more resistant
binder system. The correlation between strength and durability might hold
more or less for concretes based on the same cement type, but it certainly
does not hold when comparing concretes with different binder systems.
Some interesting additional arguments can be obtained by studying
the historical evolution of cement properties. Typically, cement strengths
have increased during the last century (Neville 1997, De Schutter 2001).
Due to the increase in cement strength, a concrete with characteristic cube
strength of 32.5 N/mm² could be obtained in 1984 with a water/cement
ratio of 0.57, while in 1970 a water/cement ratio of 0.50 was needed.
Because of the required workability level, the water content per cubic meter
of concrete remained about the same, while the cement content could be
significantly reduced maintaining the same concrete strength level. Neville
(1997) concludes that the cement content per cubic meter of concrete could
be reduced by 60 to 100 kg, leading to an increase in water/cement ratio
of 0.09 to 0.13. It is clear that, in spite of a similar strength, the concrete
will show an increased porosity and permeability, and thus will be more
prone to degradation mechanisms like carbonation and chloride penetration. Furthermore, due to the faster strength development at early age,
formworks could be removed faster. This further increases the risk of inadequate curing conditions, which negatively influences the final quality of
the concrete, including durability performance (see next section).
1.4.6 casting and curing
The durability of concrete structures not only depends on the intrinsic
durability performance of the concrete mix, but to a large extent also on
the real casting and curing conditions. The difference is sometimes referred
to as the difference between ‘labcrete’ (concrete cast and cured in perfect
laboratory conditions) and ‘realcrete’ (concrete cast and cured in real conditions, on site). Three major aspects are important in this context: compaction, cover thickness, and curing.
Traditional concrete needs to be compacted when poured into the formwork. Uncompacted concrete contains a high percentage of voids, leading
The dubious role of strength as a durability indicator can be further illustrated by studying some historical information. The general applicability
of concrete as a construction material significantly increased in the twentieth century thanks to extensive laboratory research coupled with increasing practical experience. New cements have been developed with increased
chemical resistance. It was gradually realized that the quality of the concrete
determines the resistance against degradation. In this respect, quality of the
concrete was often interpreted in a too limited way as the strength of the
concrete. However, a strong concrete is not automatically a durable concrete.
Some situations can be found where a stronger concrete can show more degradation than a weaker concrete, when the latter is based on a more resistant
binder system. The correlation between strength and durability might hold
more or less for concretes based on the same cement type, but it certainly
does not hold when comparing concretes with different binder systems.
Some interesting additional arguments can be obtained by studying
the historical evolution of cement properties. Typically, cement strengths
have increased during the last century (Neville 1997, De Schutter 2001).
Due to the increase in cement strength, a concrete with characteristic cube
strength of 32.5 N/mm² could be obtained in 1984 with a water/cement
ratio of 0.57, while in 1970 a water/cement ratio of 0.50 was needed.
Because of the required workability level, the water content per cubic meter
of concrete remained about the same, while the cement content could be
significantly reduced maintaining the same concrete strength level. Neville
(1997) concludes that the cement content per cubic meter of concrete could
be reduced by 60 to 100 kg, leading to an increase in water/cement ratio
of 0.09 to 0.13. It is clear that, in spite of a similar strength, the concrete
will show an increased porosity and permeability, and thus will be more
prone to degradation mechanisms like carbonation and chloride penetration. Furthermore, due to the faster strength development at early age,
formworks could be removed faster. This further increases the risk of inadequate curing conditions, which negatively influences the final quality of
the concrete, including durability performance (see next section).
1.4.6 casting and curing
The durability of concrete structures not only depends on the intrinsic
durability performance of the concrete mix, but to a large extent also on
the real casting and curing conditions. The difference is sometimes referred
to as the difference between ‘labcrete’ (concrete cast and cured in perfect
laboratory conditions) and ‘realcrete’ (concrete cast and cured in real conditions, on site). Three major aspects are important in this context: compaction, cover thickness, and curing.
Traditional concrete needs to be compacted when poured into the formwork. Uncompacted concrete contains a high percentage of voids, leading