45
Chapter 3
Errors during casting
DOI: 10.1201/b12914-3
3.1 IntroductIon
After a perfect design, things can go wrong during casting of a concrete
structure. The concrete constituents can be badly proportioned or the mixing procedure can be inappropriate. By ignorance or by accident, aggressive substances could even be mixed into the concrete. On the construction
site, reinforcement could be badly positioned or in insufficient quantities.
The concrete could be badly vibrated or concrete vibration could even be
omitted. On the level of the formworks, problems can occur due to insufficient strength or stiffness and due to leakage. When demoulding, surface
damage could occur due to inadequate preparation of the formwork
surface. After demoulding, curing measures could be insufficient. A more
detailed overview of possible errors during casting is given in this chapter.
3.2 Errors durIng proportIonIng
A concrete plant has to take great care when proportioning the constituent
materials following the recipe as determined during the mix design. The
cement and water content, of course are of major importance, but also an
accurate dosage of fine and coarse aggregates has to be guaranteed. Even
more sensitive to variations, possibly with serious consequences, are the
admixtures like accelerators and plasticizers. Typically, special concrete
mixes containing sometimes carefully designed combinations of binder
powders and admixtures, as is the case for self-compacting concrete, can
be very sensitive to errors during proportioning. The generally increased
sensibility of self-compacting concrete to mix variations is well-reported in
previous documents (RILEM TC 188-CSC 2006, De Schutter et al. 2008).
A basic issue is to carefully control the water content of the concrete
mix, taking into account the actual water content of the aggregates.
In order to show the dramatic consequences when not doing so, consider a
conventional concrete mix containing 350 kg cement and 175 kg water per
Chapter 3
Errors during casting
DOI: 10.1201/b12914-3
3.1 IntroductIon
After a perfect design, things can go wrong during casting of a concrete
structure. The concrete constituents can be badly proportioned or the mixing procedure can be inappropriate. By ignorance or by accident, aggressive substances could even be mixed into the concrete. On the construction
site, reinforcement could be badly positioned or in insufficient quantities.
The concrete could be badly vibrated or concrete vibration could even be
omitted. On the level of the formworks, problems can occur due to insufficient strength or stiffness and due to leakage. When demoulding, surface
damage could occur due to inadequate preparation of the formwork
surface. After demoulding, curing measures could be insufficient. A more
detailed overview of possible errors during casting is given in this chapter.
3.2 Errors durIng proportIonIng
A concrete plant has to take great care when proportioning the constituent
materials following the recipe as determined during the mix design. The
cement and water content, of course are of major importance, but also an
accurate dosage of fine and coarse aggregates has to be guaranteed. Even
more sensitive to variations, possibly with serious consequences, are the
admixtures like accelerators and plasticizers. Typically, special concrete
mixes containing sometimes carefully designed combinations of binder
powders and admixtures, as is the case for self-compacting concrete, can
be very sensitive to errors during proportioning. The generally increased
sensibility of self-compacting concrete to mix variations is well-reported in
previous documents (RILEM TC 188-CSC 2006, De Schutter et al. 2008).
A basic issue is to carefully control the water content of the concrete
mix, taking into account the actual water content of the aggregates.
In order to show the dramatic consequences when not doing so, consider a
conventional concrete mix containing 350 kg cement and 175 kg water per