Strengthening of RCC beams
The need for strengthening a reinforced concrete beam or a number of beams in a structure is usually caused by problems due to degradation of characteristics of materials with time, reduction in cross section, corrosion, wrong initial design or the increase in the load demand on the building when its utilized for a new purpose other than it was intended to. These problems may lead to the existing steel bars in the beam to become unsafe or insufficient. In such cases, there are a number of solutions to be applied to make them safe or sufficient enough to bear the load.
The following are some of solutions used:
Addition of steel bars to the main reinforcement without increasing the cross section of the beam:
This process is done when the reinforcement in the beam is not capable to carry the stresses applied to it. In this process, firstly the concrete cover is removed from both upper and lower steel bars. Then the steel bars are cleaned well and they are coated with a specific material which would prevent corrosion. Holes of 13mm diameter are made along the whole span of the beam under the slab with a spacing of 150-250m.
These holes are extended to the total width of the beam and are filled with an epoxy material with low viscosity. The steel connectors are installed for fastening the new stirrups. In order to fasten the steel bars added to the beam, steel connectors are installed into the columns. The added stirrups are closed using steel wires and the new steel is installed into these stirrups. The surface is then coated with a bonding epoxy material. The concrete cover is poured over the new steel and the new stirrups.
Addition of steel bars and increasing the cross section area of the concrete:
This process is done when both the steel and the concrete are not capable to bear the stresses on the beam due to the additional loads. The steps involved here are almost same as the above procedure which is as follows:
Removing the concrete cover and after cleaning the reinforcement steel, coating it with the material which will prevent the corrosion. Making holes the whole span of beam and to the full width under the slab with a spacing of about 150-250mm. These holes are filled with cement mortar with low viscosity. Steel connectors are installed to fasten the new stirrups. And in order to fasten the steel bars added to the beam steel connectors are installed in the columns.
The added stirrups are closed using steel wires and new steel is installed in these stirrups. Then the concrete surface is coated with a specific material which ensures the bond between the old concrete and new concrete just before pouring of concrete which is a low shrinkage concrete.
Adding steel plates to the beam:
When there is a need to strengthen the resistance of beam against the applied moment or the shear stress, steel plates are attached to the beams after they are designed with the appropriate size and thickness. The procedure is as follows:
First the surface of concrete where the plates are supposed to be attached, is cleaned. Then that surface is coated with a bonding epoxy material. Holes are made in the concrete surface as well as in the plates and a layer of epoxy mortar of thickness of 5mm is placed on top of plates. These steel plates are then attached to the concrete with the help of bolts.
There are some cases where it’s necessary to reduce the load on the beam which needs strengthening before the above procedures are implemented. This reduction of loading can either be partial or complete unloading. This is possible by placing steel beams on top or below the concrete beam.