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The techniques are described below among with other restoration measures. Small cracks
If the cracks are reasonably small (opening width=0.075mm), the technique to restore the original tensile strength of the cracked element is by pressure injection of epoxy. The procedure is as follows.
The external surfaces are cleaned of non-structural materials and plastic ports are placed among the surface of the cracks on both sides of the member and are secured in place with an epoxy sealant. The centre to centre spacing of these ports may be approximately equal to the thickness of the element. After the sealant has cured, a low viscosity epoxy resin is injected into one port at a time, beginning at the lowest part of the crack in case it is vertical or at one end of the crack in case it is horizontal.
The resin is injected till it is seen flowing from the opposite sides of the member at the corresponding port or from the next higher port on the same side of the member. The injection port should be closed at this stage and injection equipment moved to next port and so on.
The small the crack, the higher is the pressure or more closely spaced should be the ports so as to obtain the complete penetration of the epoxy material throughout the depth and width of member. Larger cracks will permit larger port spacing, depending upon width of the member. This technique is appropriate for all types of structural elements- beams, columns, walls and floor units in masonry as well as concrete structures. Two items should however be taken care of in such type of repair:
In the case of loss of bond between reinforcement bar and concrete, if the concrete, if the concrete adjacent to the bar has been pulverised to a very fine powder, this powder will dam the epoxy from saturating the region. So it should be cleaned properly by air or water pressure prior to injection of epoxy.It has been stated that cracks smaller than 0.75mm may be difficult pressure inject. So cracks smaller than this should not be repaired by this method.Large cracks and crushed concrete
For cracks wider than about 6mm or for regions in which the concrete or masonry has crushed, a treatment other than injection is indicated. The following procedure may be adopted:
The loose material is removed and replaced with expansive cement mortar, quick setting cement or gypsum cement mortar.Where found necessary, additional shear of flexural reinforcement is provided in the region of repairs. This reinforcement could be covered by mortar to give further strength as well as protection to the reinforcement.In areas of very severe damage, replacement of the member or portion of member can be carried out. In case of damage to walls and floor diaphragms, steel mesh could be provided on the outside of the surface and nailed or bolted to the wall. Then it may be covered or plastered or micro-concrete.Fractured, excessively yielded and buckled reinforcement
In the case of severely damaged reinforced concrete member, it is possible that the reinforcement would have buckled, or elongated or excessively yielding may have occurred. This element can be repaired by replacing the old portion of steel with new steel using butt welding or lap welding.
Splicing by overlapping will be risky. If repair has to be made without removal of the existing steel, the best approach would depend upon the space available in the original member. Additional stirrup ties are to be added in the damaged portion before concreting so as to confine the concrete and enclose the longitudinal bars to prevent their buckling in future.
In some cases it may be necessary to anchor additional steel into existing concrete. A common technique for providing the anchorage uses the following procedure:
A hole larger than the bar is drilled. The hole is filled with epoxy, expanding cement, or other strength grouting material. The bar is pushed into the place and held there until the grout has set.
Fractured wooden members and joints
Since wood is an easily workable material, it will be easy to restore the strength of wooden members, beams, columns, struts and ties by splicing additional material. The weathered or rotten wood should first be removed. Nails, wood screws or steel bolts will be most convenient as connectors. It will be advisable to use straps to cover all such splices and joints so as to keep them tight and stiff.
If the cracks are reasonably small (opening width=0.075mm), the technique to restore the original tensile strength of the cracked element is by pressure injection of epoxy. The procedure is as follows.
The external surfaces are cleaned of non-structural materials and plastic ports are placed among the surface of the cracks on both sides of the member and are secured in place with an epoxy sealant. The centre to centre spacing of these ports may be approximately equal to the thickness of the element. After the sealant has cured, a low viscosity epoxy resin is injected into one port at a time, beginning at the lowest part of the crack in case it is vertical or at one end of the crack in case it is horizontal.
The resin is injected till it is seen flowing from the opposite sides of the member at the corresponding port or from the next higher port on the same side of the member. The injection port should be closed at this stage and injection equipment moved to next port and so on.
The small the crack, the higher is the pressure or more closely spaced should be the ports so as to obtain the complete penetration of the epoxy material throughout the depth and width of member. Larger cracks will permit larger port spacing, depending upon width of the member. This technique is appropriate for all types of structural elements- beams, columns, walls and floor units in masonry as well as concrete structures. Two items should however be taken care of in such type of repair:
In the case of loss of bond between reinforcement bar and concrete, if the concrete, if the concrete adjacent to the bar has been pulverised to a very fine powder, this powder will dam the epoxy from saturating the region. So it should be cleaned properly by air or water pressure prior to injection of epoxy.It has been stated that cracks smaller than 0.75mm may be difficult pressure inject. So cracks smaller than this should not be repaired by this method.Large cracks and crushed concrete
For cracks wider than about 6mm or for regions in which the concrete or masonry has crushed, a treatment other than injection is indicated. The following procedure may be adopted:
The loose material is removed and replaced with expansive cement mortar, quick setting cement or gypsum cement mortar.Where found necessary, additional shear of flexural reinforcement is provided in the region of repairs. This reinforcement could be covered by mortar to give further strength as well as protection to the reinforcement.In areas of very severe damage, replacement of the member or portion of member can be carried out. In case of damage to walls and floor diaphragms, steel mesh could be provided on the outside of the surface and nailed or bolted to the wall. Then it may be covered or plastered or micro-concrete.Fractured, excessively yielded and buckled reinforcement
In the case of severely damaged reinforced concrete member, it is possible that the reinforcement would have buckled, or elongated or excessively yielding may have occurred. This element can be repaired by replacing the old portion of steel with new steel using butt welding or lap welding.
Splicing by overlapping will be risky. If repair has to be made without removal of the existing steel, the best approach would depend upon the space available in the original member. Additional stirrup ties are to be added in the damaged portion before concreting so as to confine the concrete and enclose the longitudinal bars to prevent their buckling in future.
In some cases it may be necessary to anchor additional steel into existing concrete. A common technique for providing the anchorage uses the following procedure:
A hole larger than the bar is drilled. The hole is filled with epoxy, expanding cement, or other strength grouting material. The bar is pushed into the place and held there until the grout has set.
Fractured wooden members and joints
Since wood is an easily workable material, it will be easy to restore the strength of wooden members, beams, columns, struts and ties by splicing additional material. The weathered or rotten wood should first be removed. Nails, wood screws or steel bolts will be most convenient as connectors. It will be advisable to use straps to cover all such splices and joints so as to keep them tight and stiff.
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