UNIT – V REPAIR, REHABILITATION AND RETROFFITNG OF STRUCTURES

REPAIRS TO OVERCOME LOW MEMBER STRENGTH: These guidelines have the following objectives; To indicate appropriate methods of repair and restoration taking into account the building type and the type of damage. To recommend methods of seismic strengthening to upgrade the strength of the building in line with the requirements of the seismic-zoning map of India (IS : ) and Earthquake Resistance Codes (13 : ) and (18: ).

CONCEPTS OF REPAIR, RESTORATION AND RETROFITTING: REPAIR: It consists of actions taken for patching up superficial defects, re-plastering walls, repairing doors and windows and services such as the following : Patching up of defects as cracks and fall of plaster and re-plastering if needed. Repairing doors, windows and replacement of glass panes.

Checking and repairing electrical connections, gas connections, plumbing, heating, ventilation Rebuilding non-structural walls, chimneys, boundary walls. Relaying cracked flooring at ground level and roofing sheets or tiles. Redecoration work (White or colour washing etc.) It would be seen that the repairing work carried out as above does not add any strength to the structure.

RESTORATION : This includes actions taken for restoring the lost strength of structural elements of the building. This is done by making the columns, piers, beams and walls at least as strong as originally provided as follows: Removal of portions of cracked masonry walls and piers, and rebuilding them in richer mortar. Use of non-shrinking mortar will be preferable. Addition of reinforcing mesh on both faces of the cracked wall, holding it to the wall through spikes or bolts and then covering it suitably with micro-concrete or 1:3 cement -coarse sand plaster.

Injecting neat cement slurry or epoxy like material, which is strong in tension, into the cracks in walls, columns, beams etc. If the structural restoration is properly executed, the structure will be as strong as before the-earthquake. It is also possible to strengthen a structure to take increased vertical loading, if required.

SEISMIC STRENGTHENING (RETROFITTING) It will involve actions for upgrading the seismic resistance of an existing building so that becomes safer under the occurrence of probable future earthquakes. The seismic behavior of existing buildings is affected by their original structural inadequacies, material degradation due to aging and alterations carried out during use over time. The complete replacement of such buildings in a given area is just not possible due to a number of social, cultural and financial problems. Therefore, seismic strengthening of existing undamaged or damaged buildings is a definite requirement.

Seismic strengthening structural restoration and cosmetic repairs may sometimes cost upto 25 to 30 per cent of the cost of rebuilding although usually it may not exceed 12 to 15 per cent. Hence justification of strengthening work must be fully considered from cost point of view. The main items of seismic strengthening could be some or all of that following actions: Modification of roofs, Substitution or strengthening of floors,

Modification in the building plan, Strengthening of walls including provision of horizontal and vertical bands or belts, introduction of header stones in thick stone walls, and injection grouting etc., Adding to the sections of beams and columns by casing or jacketing etc., Adding shear walls or diagonal bracings, Strengthening of foundations if found necessary (but very difficult and expensive).

STRENGTHENING OF FOUNDATIONS Seismic strengthening of foundations before or after the earthquake is the most involved task since it may require careful underpinning operations. Some alternatives are given below for preliminary consideration of the strengthening scheme. Introducing new load bearing members including foundations to relieve the already loaded members. Jacking operations may be needed in this process. Improving the drainage of the area to prevent saturation of foundation soil to obviate any problems of liquefaction which may occur because of poor drainage. Providing apron around the building to prevent soaking of foundation directly and draining off the water.

Adding strong elements in the form of reinforced concrete strips attached to the existing foundation part of the building. These will also bind the various wall footings and may be provided on both sides of the wall. To avoid digging the floor inside the building, the extra width could be provided only on the outside of external walls. The extra width may be provided above the existing footing or at the level of the existing footing. In any case the reinforced concrete strips and the walls have to be linked by a number of keys, inserted into the existing footing.

CRACKING Cracking, like corrosion of reinforcing steel, is not commonly a cause of damage to concrete. Instead, cracking is a symptom of damage created by some other cause. All Portland cement concrete undergoes some degree of shrinkage during hydration. This shrinkage produces multidirectional drying shrinkage and curing shrinkage cracking having a somewhat circular pattern. Such cracks seldom extend very deeply into the concrete and can generally be ignored.

Plastic shrinkage cracking occurs when the fresh concrete is exposed to high rates of evaporative water loss which causes shrinkage while the concrete is still plastic. Plastic shrinkage cracks are usually somewhat deeper than drying or curing shrinkage cracks and may exhibit a parallel orientation that is visually unattractive.

Thermal cracking is caused by the normal expansion and contraction of concrete during changes of ambient temperature. Concrete has a linear coefficient of thermal expansion of about 5.5 milli inch per inch per degree Fahrenheit (°F). This can cause concrete to undergo length changes of about 0.5 inch per 100 linear feet for an 80 °F temperature change.

Thermal cracking can also be caused by using Portland cements developing high heats of hydration during curing. Such concrete develops exothermic heat and hardens while at elevated temperatures. Cracking is also caused by alkali-aggregate reaction, sulfate exposure, and exposure to cyclic freeze-thaw conditions, as has been discussed in previous sections, and by structural overloads as discussed in the following section. Successful repair of cracking is often very difficult to attain.

The selection of methods for repairing cracked concrete depends on the cause of the cracking. First, it is necessary to determine if the cracks are "live" or "dead." If the cracks are cyclically opening and closing, or progressively widening, structural repair becomes very complicated and is often futile. Such cracking will simply reestablish in the repair material or adjacent concrete. If reflective cracking is intolerable, the repairs must be designed as separate structural members not bonded to the old existing concrete.