Concept of repairing and strengthening of RCC structure

1.1: Concept of repair of RCC structure

             In the present time, most of the structure is made up of reinforcement concrete or plain cement concrete.  Whatever be the constituent of concrete the structure is subjected to an extremely aggressive environment and adverse climate as a result of these conditions the durability of the material and the whole structure get reduced.

               In practice, it is not always possible to guarantee the load-bearing capacity of the concrete by providing proper design, conscious engineering, good workmanship and following the required standard because structure inhered some inbuilt defect or acquire some defect during the lifetime. A defect existing in a structure caused damage making the structure unsafe to use. The probable damage is due to various reasons and of various nature. These defects need to be repaired systematically through inspection.

1.2: Physical examination of common defects and damages

           The common defects of a distressed structured are visible and can be as certain by visual examination of the symptoms which appears on the surface.

1.3: Symptoms of defect and damages and their causes

1)Active Cracks 

a)vertical cracks

         Due to excessive movement of the structure,

b)Inclined cracks

          Due to shear or tension

2)Dormonent cracks

a)Vertical or inclined cracks

          Due to a temporary overload

b)Separating cracks extending throughout the member.

           Due to shrinkage or temperature stress.

c)Cracks at the change in the shape of the structure.

          Due to the lack of control points.

d) Cracks at the change of cross-section

           Due to local stress concentration.

e)Dormonent surface cracks

         Due to the plastic setting, poor curing, loss of surface water.

3)Swelling of concrete

         Due to the presence of alkaline concrete

4)Spalling of concrete

         Due to excessive, compressive stress

5)Discolouration 

          Due to chemical attack, fungus growth, rusting of steel, excessive temperature etc.

6)Erosion of concrete surface

           Due to abrasion, chemical attack, permeable concrete etc.

7)Rusting of concrete

         Poor and permeable concrete cover.

8)Yielding

         Due to overloading.

9)Excessive deflection of the member

         Due to poor design.

1.4: Repairs of cracks

            Cracks appear in concrete due to various reason, through which moisture enters and corrodes the reinforcing bars. The reinforcement when corroded expands loosing the bond with concrete. The concrete along with plaster peeled up causing damage to the member.

1.5: Methods of repair

1)Cracks in RCC structure

      Cracks appear in RCC members due to excessive bending moments. These cracks appear in the tensile zone of the members and go on increasing until the member fails. To correct this defect the members shall be free of the excessive load causing bending moment as far as possible. Then strengthen the member by providing adequate reinforcement with proper bonding with the old member.

2)Cracks due to excessive shear

       These cracks are at 45 degrees to the axis of members. They are wider at the lower fibre and appears in the compressive zone.

        These may be corrected by adding diagonal shear reinforcement in the form of stitching.

3)Cracks in slab

       These cracks are generally at the support or at the bottom of the mid-span, develop due to the provision of insufficient steel or reinforcement or displacement of steel.

          These defect may be corrected by the addition of steel as required. The ends of the added steel bars are bend at 90 degrees.

4)Pressure Grouting

         This method is adopted when the concrete has become porous but has not decayed.

           To correct this problem made drills on the surface of the concrete member. Diameter and depth of the drill are fixed as per required according to the thickness of the member and the quality of the concrete to be tested. Cement slurry or chemicals are grouted under pressure into the pores of the concrete.

5)Cracks in the foundation due to settlement

         Cracks when detected in the foundation they may be beyond repair. The foundation may have to be replaced if it is beyond repair in such case either the foundation has to be redesign with wider base and foundation have to be taken down on the soil having the adequate bearing capacity. The safe bearing capacity of soil may be increased by chemical grouting or cement slurry grouting for stabilisation of the soil.

6)Modifying the structural system

The structure may be redesign and the stress in the damaged member may be released with redesigning.

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