Errors during casting 53
steel cages before casting. As soon as the concrete is in place, it is nearly
impossible to find out the real positioning and amount of the reinforcement. As illustrated hereafter, things can go wrong on different levels.
3.5.1 Wrong amount of reinforcement
It is straightforward that the designed amount of reinforcing steel has to
be positioned, as outlined in the reinforcement plans developed during the
design stage. Leaving out a few bars can have serious consequences such
as excessive cracking and even structural failure. Due attention should be
given to the anchorage and overlap zones in order to respect prescribed
designs.
In case of steel fibre reinforced concrete, the prescribed amount of fibres
has to be respected. During mixing, it has to be made sure that the right
amount is added and that the fibres are evenly distributed within the whole
batch. This brings us back to the issue of accurate proportioning and appropriate mixing (Sections 3.2 and 3.3 of this chapter). Nevertheless, statistical
variations of the fibre content in the finalized structural element will always
exist (Taerwe et al. 1997).
3.5.2 Wrong position of reinforcement
Steel reinforcing bars in structural concrete elements have to be positioned
where they can efficiently take over tensile loading, leaving the compressive loading mainly to be taken by the concrete itself. In this way, a perfect
symbiosis between steel and concrete is obtained, as has been shown in
innumerable structures worldwide. Although the principle seems not too
complicated, optimal solutions sometimes are not straightforward in the
case of complex structures. This, however, is beyond the scope of this
textbook.
In the following example, the consequences of a wrong positioning of
the reinforcement will be illustrated. Figure 3.3 shows the structural concept of a balcony, constructed as a cantilever slab rigidly connected to the
concrete skeleton of the building. When the balcony is loaded, it will bend
downward. Tensile stresses will occur in the upper part of the balcony,
compressive stresses in the lower part. Placing the reinforcing bars in
a central position, as suggested in the figure, will thus not provide an
adequate solution. This will not be an optimal position for carrying the
tensile loads. The right solution in this case is to place the reinforcing
bars towards the upper side of the cantilever slab leaving sufficient cover
thickness.
The given example is a well-known case, to which only few errors will
be made in practice. However, in less obvious cases these kinds of errors do
occur. As an example, it is remarkable to see that foundation slabs (although
steel cages before casting. As soon as the concrete is in place, it is nearly
impossible to find out the real positioning and amount of the reinforcement. As illustrated hereafter, things can go wrong on different levels.
3.5.1 Wrong amount of reinforcement
It is straightforward that the designed amount of reinforcing steel has to
be positioned, as outlined in the reinforcement plans developed during the
design stage. Leaving out a few bars can have serious consequences such
as excessive cracking and even structural failure. Due attention should be
given to the anchorage and overlap zones in order to respect prescribed
designs.
In case of steel fibre reinforced concrete, the prescribed amount of fibres
has to be respected. During mixing, it has to be made sure that the right
amount is added and that the fibres are evenly distributed within the whole
batch. This brings us back to the issue of accurate proportioning and appropriate mixing (Sections 3.2 and 3.3 of this chapter). Nevertheless, statistical
variations of the fibre content in the finalized structural element will always
exist (Taerwe et al. 1997).
3.5.2 Wrong position of reinforcement
Steel reinforcing bars in structural concrete elements have to be positioned
where they can efficiently take over tensile loading, leaving the compressive loading mainly to be taken by the concrete itself. In this way, a perfect
symbiosis between steel and concrete is obtained, as has been shown in
innumerable structures worldwide. Although the principle seems not too
complicated, optimal solutions sometimes are not straightforward in the
case of complex structures. This, however, is beyond the scope of this
textbook.
In the following example, the consequences of a wrong positioning of
the reinforcement will be illustrated. Figure 3.3 shows the structural concept of a balcony, constructed as a cantilever slab rigidly connected to the
concrete skeleton of the building. When the balcony is loaded, it will bend
downward. Tensile stresses will occur in the upper part of the balcony,
compressive stresses in the lower part. Placing the reinforcing bars in
a central position, as suggested in the figure, will thus not provide an
adequate solution. This will not be an optimal position for carrying the
tensile loads. The right solution in this case is to place the reinforcing
bars towards the upper side of the cantilever slab leaving sufficient cover
thickness.
The given example is a well-known case, to which only few errors will
be made in practice. However, in less obvious cases these kinds of errors do
occur. As an example, it is remarkable to see that foundation slabs (although