Reinforced concrete beams need strengthening when the existing steel bars in the beam are unsafe or insufficient, or when the loads applied to the beam are increased. In such cases, there are different solutions that could be followed:
I-ADDING REINFORCEMENT STEEL BARS TO THE MAIN STEEL WITHOUT
INCREASING THE BEAM’S CROSS SECTIONAL AREA
This solution is carried out when the reinforcing steel bars are not capable to carry the stresses applied to the beam. The following steps should be followed:
1. The concrete cover is removed for both the upper and lower steel bars.
2. The steel bars are well cleaned and coated with an appropriate material that would prevent corrosion.
3. Holes are made, in the whole span of the beam under the slab, as shown in Fig.1, 15-25cm apart, a diameter of 1.3cm and extend to the total width of the beam.
Fig.1: Holes in the span of a beam
4. The holes are filled with an epoxy material with low viscosity and installing steel connectors for fastening the new stirrups.
5. Steel connectors are installed into the columns in order to fasten the steel bars added to the beam.
6. The added stirrups are closed using steel wires and the new steel is installed into these stirrups.
7. The surface is then coated with a bonding epoxy material.
8. The concrete cover is poured over the new steel and the new stirrups.
The previous steps are illustrated in Fig 2.
Fig.2- Strengthening a beam without increasing cross sectional area.
II-INCREASING BOTH THE REINFORCING STEEL BARS AND THE CROSS – SECTIONAL AREA OF CONCRETE
This solution is chosen when both the steel and concrete are not able to carry the additional loads applied to the beam. In such cases the following steps should be followed as in Fig 3.
1. Removing the concrete cover, roughing the beams surface, cleaning the reinforcement steel bars and coating them with an appropriate material that would prevent corrosion.
2. Making holes in the whole span and width of the beam under the slab at 15-25cm.
3. Filling the holes with cement mortar with low viscosity and installing steel connectors for fastening the new stirrups.
4. Installing the steel connectors into the columns in order to fasten the steel bars added to the beam.
5. Closing the added stirrups using steel wires and the new steel is installed into these stirrups.
6. Coating the concrete surface with an appropriate epoxy material that would guarantee the bond between the old and new concrete, exactly before pouring the concrete.
7. Pouring the concrete jacket using low shrinkage concrete.
Fig.3: Strengthening of beam by increasing the cross-sectional area and bars
III-ADDING STEEL PLATES TO THE BEAM
When it is required to strengthen the beam’s resistance against the applied moment or shear stress, steel plates are designed with the appropriate size and thickness.
Then those plates are attached to the beam as follows:
1. Roughing and cleaning the concrete surfaces where the plates will be attached.
2. Coating the concrete surfaces with a bonding epoxy material.
3. Making holes in the concrete surfaces and plates.
4. Putting a layer of epoxy mortar on top of the plates with a 5mm thickness.
5. Attaching the steel plates to the concrete using bolts.
The previous steps are illustrated in Fig 4.
Fig.4: Strengthening of Beam by adding steel plates
In some cases, it is needed to reduce the load on the beam that needs strengthening before implementing the previous steps, either partial or complete unloading.
This is made by putting steel beams on top or below the concrete beams, as shown in Fig5.
Fig.5: Reducing the load on the beam using steel beam.
The following photos were taken during strengthening an existing building; they present the practical method of implementing some strengthening techniques.