Treatment for slab and beam subject to excessive deflection

Serviceability limit state (SLS) governs the functional and operational use of structural elements. These limit states include: crack due to fatigue, repairable damage, corrosion, fire, deflections/deformation and vibration in structure. Deflection control aims at not avoiding deflection completely because every loaded member must deflect but avoiding visible deflection which in other words is called excessive deflection. This implies that when deflection becomes visible in a house and makes the user uncomfortable, it is excessive. Checking for deflection ensures that the structure does not deflect excessively in a manner that would impair the appearance of the structure, cause cracking to partitions and finishes and affect the overall functionality and stability of the structure.

Deflection controls are usually paramount in slabs and beams. At the design stage of a structure, deflection is controlled by:

• Increasing the member depth.
• Use of compression reinforcement.
• Use of tension reinforcement.
• Making the members wider where this is possible.
• Decreasing the load on the member where this is permitted.
• Shortening the span of the member in case of beam.
• Stiffening the beam (Note: stiffness is the governing parameter for the deflection of structural members).
• Fixing the support of the beams or slabs.
• Prestressing the members.
• Revising the geometry of the structure.

You can find more on deflection control according to the Eurocode 2 HERE.

In a situation where the deflection becomes excessive due to increased loading, errors in design or ageing etc, sections of the beam or slab can be increased as a solution to the deflection problem through the following procedure:

• Erect telescopic steel props (see Figure 1) at the mid-span of each slab subject to deflection and tighten with jacks to reduce the slab loading on beams. Also remove the tiles or other finishes on the slab that contributes load.

• Remove the existing plaster of the beams and insert extra reinforcement at the bottom of the beam which should be tied to the supporting columns.
• Fill the holes surrounding the reinforcement inserted into the column with epoxy grout to ensure proper bond.
• Fix and anchor expanded wire mesh (see Figure 2) around the sides of the beam and apply epoxy bond coat to ensure proper bond between old concrete and polymer modified concrete.

• Apply polymer modified mortar to achieve the required thickness that would also offset existing deflected shape.
• Cure the mortar for 3 days with water and cure in air to achieve required strength.
• For the slab, remove all loose materials on the surface of the old slab.
• Drill holes of about 12mm diameter and 50mm depth into the old slab at interval of 500mm x 500mm.
• Insert steel bolts of 10mm diameter and 100mm length into the holes leaving 50mm bolt above the old concrete surface (see Figure 3).

• Fill the gap between the shaft bolt and surrounding hole with epoxy grout.
• Provide extra reinforcement over the supports because old reinforcement at supports is now neutral axis of composite section.
• Apply epoxy bond coat over the old concrete surface and cast C20 or higher concrete grade of 75mm thickness over the old concrete.
• The bolt or shear connector helps to prevent lateral slip between the top and bottom concretes
• Remove the telescopic props after curing the slab and beams up to 28 days

Note: the polymer mortar along with wire mesh forms and acts like ferrocrete trough. This provides additional flexural capacities to the section.

Watch live VIDEO of strengthening reinforced concrete slab after deflection using overlay method.

References

Bhedasgaonkar, B.V. and Wadekar, M.K. (1995): Repairs of beams and slab with excessive deflection. ICJ Vol. 69, No. 1 pp 47-50.

Cover Image Credit: www.cambria.co.uk