Many strengthening techniques are available depending on the purpose needed from strengthening. Some of those techniques are explained in details in the following sections.
Why do we strengthen or retrofit buildings?
Increase in the applied loads.
Mistakes or unsafe design.
R.steel corrosion or insufficient number of bars
Cracks in concrete or stress less than design stress.
The settlement in the foundation is more than the allowable.
STRENGTHENING OF R.C.SLABS
In some cases, and due to increasing the applied loads on slabs or their unsafe design, or corrosion of the reinforcing steel bars, or cracks in the slabs, one of the following solutions should be made:
1. If the slab is unable to carry the negative moment and the lower steel is sufficient, upper steel mesh should be added with a new concrete layer.
2. If the slab is unable to carry the positive moment or when the dead load (that will be added to the slab)is much less than the live load carried by the slab , a new concrete layer on the bottom of the slab should be added.
In order to implement the previous solutions, the following steps should be made as shown in Fig 1and Fig 2:
1. Removing the concrete cover.
2. Cleaning the reinforcing steel bars using a wire brush or a sand compressor.
3. Coating the steel bars with an epoxy material that would prevent corrosion.
4.If a high percent of corrosion was found in the steel bars, a new steel mesh, designed according to the codes’ requirements, must be added.
5.The new reinforcing steel mesh is then installed and fastened vertically to the slab of the roof and horizontally to the surrounding beams ,using steel dowels.
6. Coating the concrete surface with an appropriate epoxy material that would guarantee the bond between the old and new concrete.
7.Before the epoxy dries, the concrete is poured with the required thickness .Additional materials that would lower the shrinkage should be added to the concrete.
Fig.1: Strengthening a slab by increasing its depth from bottom
Fig.2: Strengthening a slab by increasing its depth from top.
There are also some other techniques used for strengthening a reinforced concrete slab such as:
Increasing the shear bearing capacity of the slab by adding steel plates strengthened by vertical screw bolts.
Strengthening of the slab by post stressed reinforcement.
Adding steel beams.
In case of hollow slabs, reinforced concrete is added inside the holes of the slab.