46 Damage to concrete structures
cubic meter of concrete. Suppose that the concrete should contain 700 kg
dry sand per cubic meter. Consider now the situation that the sand to be
used is wet with a water content of 6%. When the concrete plant ignores
the water content in the sand, 700 kg wet sand is proportioned per cubic
meter of concrete. This means an actual proportioning of 700/1.06 = 660
kg dry sand per cubic meter, while at the same time 40 kg extra water has
been added as part of the wet sand. This brings the total water content to
175 + 40 = 215 kg per cubic meter, and the water/cement ratio rises from
0.5 to 215/350 = 0.61. As a consequence, different concrete properties will
be obtained concerning workability, strength development, and durability.
The example is straightforward and most concrete plants will not make the
error of ignoring water content in aggregates. However, related errors more
often occur due to wrong determination of the water content (e.g. due to badly
calibrated moisture sensors or due to measurements of non-representative
samples) or due to deviations in the weighing equipment. Care should also be
taken to avoid extra water addition within the truck mixer during transport
after arrival at the concrete plant because the amount of added water cannot
be accurately controlled. During transport of the freshly mixed concrete, care
should also be taken to avoid rain water coming in. In case of sensitive mixes
like self-compacting concrete, this extra (rain) water could lead to segregation
of the mix, as experienced in at least one case known to the author.
Deviations in proportioned amounts of aggregates can also lead to modified concrete properties. In general, it can be said that proportioning errors
of fine aggregates tend to have larger consequences than proportioning
errors of the coarser aggregates. This can be illustrated by the information
given in Table 3.1, showing the effect of variations in grading curves of
aggregates on the obtained concrete properties (Taerwe 1996).
Due to the very small quantities in which they are proportioned, deviations in the dosage of admixtures like accelerators and plasticizers can have
major effects. On the one hand, small absolute errors on their dosage easily
Table 3.1 Influence of variations in grading curves of aggregates
on the obtained concrete properties
Particle size
Considered variation
Effect on concrete properties
15 mm
±20%
No significant effect
5 mm
±15%
Slight impact on workability
of fresh concrete
2 mm
±10%
Modified workability of
fresh concrete and
strength of hardened
concrete
0.2 mm
±10%
Important effect on fresh
and hardened concrete
properties
cubic meter of concrete. Suppose that the concrete should contain 700 kg
dry sand per cubic meter. Consider now the situation that the sand to be
used is wet with a water content of 6%. When the concrete plant ignores
the water content in the sand, 700 kg wet sand is proportioned per cubic
meter of concrete. This means an actual proportioning of 700/1.06 = 660
kg dry sand per cubic meter, while at the same time 40 kg extra water has
been added as part of the wet sand. This brings the total water content to
175 + 40 = 215 kg per cubic meter, and the water/cement ratio rises from
0.5 to 215/350 = 0.61. As a consequence, different concrete properties will
be obtained concerning workability, strength development, and durability.
The example is straightforward and most concrete plants will not make the
error of ignoring water content in aggregates. However, related errors more
often occur due to wrong determination of the water content (e.g. due to badly
calibrated moisture sensors or due to measurements of non-representative
samples) or due to deviations in the weighing equipment. Care should also be
taken to avoid extra water addition within the truck mixer during transport
after arrival at the concrete plant because the amount of added water cannot
be accurately controlled. During transport of the freshly mixed concrete, care
should also be taken to avoid rain water coming in. In case of sensitive mixes
like self-compacting concrete, this extra (rain) water could lead to segregation
of the mix, as experienced in at least one case known to the author.
Deviations in proportioned amounts of aggregates can also lead to modified concrete properties. In general, it can be said that proportioning errors
of fine aggregates tend to have larger consequences than proportioning
errors of the coarser aggregates. This can be illustrated by the information
given in Table 3.1, showing the effect of variations in grading curves of
aggregates on the obtained concrete properties (Taerwe 1996).
Due to the very small quantities in which they are proportioned, deviations in the dosage of admixtures like accelerators and plasticizers can have
major effects. On the one hand, small absolute errors on their dosage easily
Table 3.1 Influence of variations in grading curves of aggregates
on the obtained concrete properties
Particle size
Considered variation
Effect on concrete properties
15 mm
±20%
No significant effect
5 mm
±15%
Slight impact on workability
of fresh concrete
2 mm
±10%
Modified workability of
fresh concrete and
strength of hardened
concrete
0.2 mm
±10%
Important effect on fresh
and hardened concrete
properties