60 Damage to concrete structures
Another typical case is the inclusion of small steel leftovers (e.g. binding
wire) which have fallen onto the formworks of slabs. When the formwork
is not properly cleaned before casting the concrete, these steel parts will
remain in the concrete near the surface, quickly leading to brown discolouring of the surface due to corrosion.
Mud and other dirt on the formwork, especially in case of slabs, can also
lead to reduced concrete quality when it is intermixed with the concrete
during casting, or to reduced bonding between different concrete batches
or layers when the mud is enclosed in between. In winter, entrapment of ice
can impair the final concrete quality.
3.7 ProbleMs WIth forMWorKs
3.7.1 Insufficient strength of formworks
Fresh concrete can exert considerable pressure on formwork, especially
when the (vertical) casting speed is high. To a certain extent, pressures are
close to hydrostatic pressures, levelling off later on due to setting effects. In
the case of self-compacting concrete, which can be considered as a liquid
and which enables very high casting speeds, pressures can be hydrostatic
all the way up. Self-compacting concrete also enables new casting techniques, pumping the concrete under pressure into the formwork, bottomup (De Schutter et al. 2008, Tichko et al. 2010). In that case, pressures can
locally even exceed the hydrostatic pressures. In case the formworks have
insufficient strength, serious accidents can happen during casting. As the
safety of workers is then at risk, due attention should be given to safe formworks able to withstand the high casting pressures.
3.7.2 too flexible formworks
More regularly occurring problems are related to too high deformability of
formworks. When formworks are too flexible, displacement of the formwork panels can lead to leaks. More importantly, it can lead to incorrect
geometry of the concrete element, and to concrete floors which seem to
have very high deflections. Although this seldom leads to serious structural
problems, from an esthetical or psychological point of view the resulting
situation can be unacceptable.
3.7.3 leaking formworks
When formworks are leaking, water and fine particles (cement, powders,
fine sand) can be drained away, leaving unacceptable surface defects and
honeycombed concrete as shown in Figure 3.11. A good tightness of the
Another typical case is the inclusion of small steel leftovers (e.g. binding
wire) which have fallen onto the formworks of slabs. When the formwork
is not properly cleaned before casting the concrete, these steel parts will
remain in the concrete near the surface, quickly leading to brown discolouring of the surface due to corrosion.
Mud and other dirt on the formwork, especially in case of slabs, can also
lead to reduced concrete quality when it is intermixed with the concrete
during casting, or to reduced bonding between different concrete batches
or layers when the mud is enclosed in between. In winter, entrapment of ice
can impair the final concrete quality.
3.7 ProbleMs WIth forMWorKs
3.7.1 Insufficient strength of formworks
Fresh concrete can exert considerable pressure on formwork, especially
when the (vertical) casting speed is high. To a certain extent, pressures are
close to hydrostatic pressures, levelling off later on due to setting effects. In
the case of self-compacting concrete, which can be considered as a liquid
and which enables very high casting speeds, pressures can be hydrostatic
all the way up. Self-compacting concrete also enables new casting techniques, pumping the concrete under pressure into the formwork, bottomup (De Schutter et al. 2008, Tichko et al. 2010). In that case, pressures can
locally even exceed the hydrostatic pressures. In case the formworks have
insufficient strength, serious accidents can happen during casting. As the
safety of workers is then at risk, due attention should be given to safe formworks able to withstand the high casting pressures.
3.7.2 too flexible formworks
More regularly occurring problems are related to too high deformability of
formworks. When formworks are too flexible, displacement of the formwork panels can lead to leaks. More importantly, it can lead to incorrect
geometry of the concrete element, and to concrete floors which seem to
have very high deflections. Although this seldom leads to serious structural
problems, from an esthetical or psychological point of view the resulting
situation can be unacceptable.
3.7.3 leaking formworks
When formworks are leaking, water and fine particles (cement, powders,
fine sand) can be drained away, leaving unacceptable surface defects and
honeycombed concrete as shown in Figure 3.11. A good tightness of the