Errors during casting 61
formwork should be guaranteed, especially in the case of very flowable
concretes like self-compacting concrete. As mentioned previously, too high
flexibility of formworks can contribute to formwork leaking. Furthermore,
incorrect use of poker vibrators can also cause movement of reinforcement
and formwork and can contribute to leakage of paste.
3.7.4 Wrong positioning of formworks
Some details in the position of the formwork can have consequences for
the mechanical and durability behaviour of the completed structure. Two
examples with different consequences are shown hereafter.
The first example considers the connection between a concrete column
and a rib floor as illustrated in Figure 3.12. As positioned here, the column
formwork is somewhat too tall and penetrates the area of the concrete
beam. As a result, the shear capacity of the beam near the supporting
column will be significantly reduced, increasing the risk of shear failure.
The second example illustrates what can happen when concrete floor
slabs are cast without making sure that the formwork shows a negative
deflection while casting. After striking the formwork and loading the slab,
the deflection will clearly increase. When the expected mechanical deflection of the slab in use is not (partly) compensated by a good positioning
of the supporting formwork before casting the slab, the final deflection
of the floor slab might visually become too high. In case of roof slabs, as
illustrated in Figure 3.13, this might lead to stagnation of rain water, and
increase the risk of ponding effects.
Figure 3.11 Surface defects and honeycombed concrete due to leaking formworks.
formwork should be guaranteed, especially in the case of very flowable
concretes like self-compacting concrete. As mentioned previously, too high
flexibility of formworks can contribute to formwork leaking. Furthermore,
incorrect use of poker vibrators can also cause movement of reinforcement
and formwork and can contribute to leakage of paste.
3.7.4 Wrong positioning of formworks
Some details in the position of the formwork can have consequences for
the mechanical and durability behaviour of the completed structure. Two
examples with different consequences are shown hereafter.
The first example considers the connection between a concrete column
and a rib floor as illustrated in Figure 3.12. As positioned here, the column
formwork is somewhat too tall and penetrates the area of the concrete
beam. As a result, the shear capacity of the beam near the supporting
column will be significantly reduced, increasing the risk of shear failure.
The second example illustrates what can happen when concrete floor
slabs are cast without making sure that the formwork shows a negative
deflection while casting. After striking the formwork and loading the slab,
the deflection will clearly increase. When the expected mechanical deflection of the slab in use is not (partly) compensated by a good positioning
of the supporting formwork before casting the slab, the final deflection
of the floor slab might visually become too high. In case of roof slabs, as
illustrated in Figure 3.13, this might lead to stagnation of rain water, and
increase the risk of ponding effects.
Figure 3.11 Surface defects and honeycombed concrete due to leaking formworks.