48 Damage to concrete structures
3.3 InaPProPrIate MIxIng
The aim of mixing is to obtain a good and homogenous dispersion of all
constituent materials within the concrete. The impact of mixing for mortar and concrete can be very significant and can affect the final properties
(Williams et al. 1999, Chang and Peng 2001, Vickers et al. 2007). It should
be kept in mind that concrete is a granular material, composed of a sometimes complicated combination of particles of different sizes ranging from
micrometre to centimetre. Mixing sequences are very important, especially
in case of mixes with high amounts of fine particles such as self-compacting
concrete (Chopin et al. 2004, RILEM TC 188-CSC 2006). The adequate
loading and mixing procedures are dependent on the mixer type,and should
be determined on an individual basis, e.g. by some batching experiments
(Jézéquel and Collin 2007).
When the mixing time is not long enough, especially the finer powder
materials (cement and additions) will not be dispersed in a proper way
and a good interaction between the fines and the admixtures might not
be obtained. This can lead to insufficient workability or to insufficient
strength development. On the other hand, when mixing time is too long,
high air contents could be introduced into the mix, leading to a reduction of strength properties. In normal conditions, an optimal mixing time
exists, depending on the type of concrete and the type of mixer.
Especially in the case of coloured concrete using pigment powders, a
good mixing procedure should be determined in order to perfectly distribute the pigments into the material. Failing to do so can result in unacceptable colour variations on the final surface.
It is clear, however, that a completely uniform distribution of all particles and constituent materials of concrete can be achieved only in theory. Full homogeneity would require a perfectly uniform distribution
of all components, not dependent on the sample size considered. It is
commonly taken for granted that mixers produce homogenous concrete,
because it is usually impracticable to test the entire batch (De Schutter
et al. 2008). In reality, concrete mixers deliver fresh concrete of varying degrees of ‘homogeneity’ or ‘uniformity’ (Bartos and Cleland 1993),
which is then practically impossible to improve during the concrete construction process.
3.4 aggressIve substances WIthIn the MIx
While concrete durability is normally verified with respect to potential environmental actions, it is clear that degradation of concrete can also occur
due to aggressive substances mixed into the concrete itself. In this case, the
‘attack’ comes from within the concrete, and not from the environment.
3.3 InaPProPrIate MIxIng
The aim of mixing is to obtain a good and homogenous dispersion of all
constituent materials within the concrete. The impact of mixing for mortar and concrete can be very significant and can affect the final properties
(Williams et al. 1999, Chang and Peng 2001, Vickers et al. 2007). It should
be kept in mind that concrete is a granular material, composed of a sometimes complicated combination of particles of different sizes ranging from
micrometre to centimetre. Mixing sequences are very important, especially
in case of mixes with high amounts of fine particles such as self-compacting
concrete (Chopin et al. 2004, RILEM TC 188-CSC 2006). The adequate
loading and mixing procedures are dependent on the mixer type,and should
be determined on an individual basis, e.g. by some batching experiments
(Jézéquel and Collin 2007).
When the mixing time is not long enough, especially the finer powder
materials (cement and additions) will not be dispersed in a proper way
and a good interaction between the fines and the admixtures might not
be obtained. This can lead to insufficient workability or to insufficient
strength development. On the other hand, when mixing time is too long,
high air contents could be introduced into the mix, leading to a reduction of strength properties. In normal conditions, an optimal mixing time
exists, depending on the type of concrete and the type of mixer.
Especially in the case of coloured concrete using pigment powders, a
good mixing procedure should be determined in order to perfectly distribute the pigments into the material. Failing to do so can result in unacceptable colour variations on the final surface.
It is clear, however, that a completely uniform distribution of all particles and constituent materials of concrete can be achieved only in theory. Full homogeneity would require a perfectly uniform distribution
of all components, not dependent on the sample size considered. It is
commonly taken for granted that mixers produce homogenous concrete,
because it is usually impracticable to test the entire batch (De Schutter
et al. 2008). In reality, concrete mixers deliver fresh concrete of varying degrees of ‘homogeneity’ or ‘uniformity’ (Bartos and Cleland 1993),
which is then practically impossible to improve during the concrete construction process.
3.4 aggressIve substances WIthIn the MIx
While concrete durability is normally verified with respect to potential environmental actions, it is clear that degradation of concrete can also occur
due to aggressive substances mixed into the concrete itself. In this case, the
‘attack’ comes from within the concrete, and not from the environment.