Methods of Repairing Active Cracks in concrete structures:
1. Drilling and Plugging through Crack:
One of the approximate methods would be to drill holes normal to cracks, fill them with a suitable epoxy or epoxy-mortar formulation and then place reinforcement bars (of predetermined sizes and lengths) in them to stitch across the cracks. The bars may be placed in the clean holes prior to filling the epoxy (so as to save loss of epoxy) but then great care is needed not to entrap any air.
2. Stitching of Concrete Cracks:
Stitching involves drilling holes on both sides of the crack and grouting in U-shaped metal units with short legs (staples or stitching dogs) that span the crack as shown in figure below:
Fig. 1: Stitching of concrete cracks
Stitching should be used when tensile strength has to be restored back across major cracks. Stitching a crack tends to stiffen the structure and the stiffening may increase the overall structural restrain, causing the concrete to crack elsewhere. Therefore, i t is necessary that proper investigation is done and if required, adjacent section or sections are strengthened using technically designed reinforcing methods. Because stresses are often concentrated, using this method in conjunction with other methods maybe necessary.
The procedure consists of drilling holes on both sides of the crack, cleaning the holes and anchoring the legs of the staples in the holes, with either a non-shrink cement grout or any epoxy resin-based bonding system. The staples should be variable in length, orientation, or both and they should be located so that the tension transmitted across the crack is not applied to a single plane within the section but is spread over an area.
3. External Prestressing:
The flexural cracks in reinforced concrete can be arrested and even corrected by the ‘Post –tensioning’ method. It closes the cracks by providing compression force to compensate for tensions and adds a residual compression force. This method requires anchorage of the tie-rods (or wires) to the anchoring device (the guide – bracket- angles) attached to the beam (Fig. 2).
Fig. 2: Post Tensioning Cracked Beam
The rods or wires are then tensioned by tightening the end-nuts or by turning of turnbuckles in the rods against the anchoring devices. However, it may become necessary in certain critical case to run at least an approximate stress-check to guard against any possible adverse effects.
4. Drilling and Plugging:
When cracks run in reasonable straight lines and are accessible at one end, drilling down the length of the crack and grouting it to form a key as shown in Fig. 3 could repair them.
Form key with precast concrete or mortar plugs set in bitumen. The bitumen is to break the bond between plugs and hole so that plugs will not be cracked by subsequent movement of the opening. If a particularly good seal is required, drill a second hole and plug with bitumen alone, using the first hole as a key and the second as a seal.
Fig. 3: Drilling and Plugging
A hole of 50 to 75mm dia depending on width of crack should be drilled, centered on and following the crack. The hole must be large enough to intersect the crack along its full length and provide enough repair material to structurally take the loads exerted on the key. The drilled hole should then be cleaned, made tight and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack.
The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. If water tightness is essential and structural load transfer i s not, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. If the key effect is essential, the resilient material can be placed in a second hole, the first being grouted.
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