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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.


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.

Strengthening a slab by increasing its depth from bottom

Fig.1: Strengthening a slab by increasing its depth from bottom

Strengthening a slab by increasing its depth from top.

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.

Gopal Mishra

Gopal Mishra

Gopal Mishra is a Civil Engineer from NIT Calicut and has more than 10 years of experience in Civil Engineering and Construction. He is the founder of The Constructor.

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  1. Hi, kindly suggest and Help. Iam from Bangalore. Building dim 40X33 Feet, No of Cloumns 14. Pillars steel 16mm, beam 16,20&25 MM have been used. Nearly 4.5 Ton of steel used for Roof and Beams only. PROBLEM is during ground floor roofing heavy rain fall. Concrete ordered for M25 though it was recommended by architect for M20. When tested after 28 days results are scary 14 at SW corner only other places 17-18. Now by the time we have raised 2 more floors. Kindly suggest how to improve the structure and slab strength and viable speedy options. Or is it ok. Further curing was done for a week and after 3-4 days Cloumns was raised and after 25 days next floor roof was laid.
    I can also send drawings to email. I need help of structural or of expert. Since I am being taken for ride by architect, his structural engineer, and my close friend contractor. Repenting for giving work to close friend and who is showing true colours and extracting money for everything. Please suggest and help imd.

  2. Does the above technique suffice?…It needs to be done for the beam as well right?…Else how will the beam be able to take the load due to the additional layer of concrete on the slab?…..

  3. Fiber wrapping should be another method to add flextural strength of weak slab.

  4. Thank u Amalechi, for that innovative article.
    I shall try it next time, for sure!!!!

  5. I just to make sure that if allowable to construct foundation on top of prestressed concrete roof slab…? in view that existing condition of roof slab very bad where cracks and damaged. The worst thing the water can get through inside and then dripping to the ground floor. The leakages more worst especially during raining time. Another foundation constructed on top may lead to more leakages that cannot be control anymore in the future. Please assist me on this issue.

  6. Interesting post. Can you add insulation such as foil insulation under this new concrete layer and over the old?

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