- A crack is a complete or incomplete separation of concrete into two or more parts produced by breaking or fracturing.
- Cracks are one kind of universal problems of concrete construction as it affects the building artistic and it also destroys the wall’s integrity, affects the structure safety and even reduce the durability of structure.
Dangers & affects of cracks
- Carbon dioxide into the concrete through the cracks and speed up carbonation around the cracks, thus shortening the structure usage.
- The cracks in the concrete wall would cause the leakage of the building; it reduces the stiffness, durability and seismic performance of buildings.
- Cracks on the wall surface damage to the later rendering, will affect to the appearance.
Types of Cracks
- Structural cracks
- Non-structural cracks
- Based on width
Objectives of Maintenance
- To preserve in good condition buildings and services.
- When deterioration occurs due to any reason it is inevitable to restore it to its original standard.
- To make improvements whenever required.
- To sustain utility value.
Different methods of repair of cracks
- Non structural or surface cracks- They do not require elaborate measures as they prevail on the surface only. After removal of the rendering, cracks if observed in masonry are cut v shaped upto a depth of 12-30mm and filled with cement mortar 1:4 and then the surface is re plastered.
- Very heavy cracks- if there are very heavy cracks all over the external load bearing wall along with signs of settlement. The load carried by the wall is supported on props, the old wall is demolished part by part and then rebuilt either in brickwork or suitable RCC framework.
- RCC band- An active crack in progress may be arrested by providing an RCC band along the line of crack. RCC band helps in checking future cracking and provide adequate strengthened sections for brickwork. The brickwork has to be opened on either side of the crack upto a depth of 100 to 150mm or one third the thickness of the wall; the exposed surface is roughened and cleaned, the reinforcement mesh is placed and filled with good concrete. The procedure is repeated on both sides of the wall.
- Cracks over arch openings- In old buildings we can see cracks over arch openings. It indicates that the arch has reached the limit of its load transfer mechanism. If the cracks are not severe they are repaired by driving metallic wedges from below.
- Stitching- Stitching is done to repair cracks of brickwork.
- RCC column- Cracks appear due to difference in thermal coefficient of the two materials and can be prevented by inserting GI butterfly ties between RCC column and brickwork. The ties are provided at alternate layers of brickwork.
- Masonry Wall- In buildings having this problem and water entering from the crack and causing dampness of the wall, the external plaster on the outer face 100mm on either side of the crack may be taken out and re plastered after fixing chicken wire mesh over the crack. In all cases of junctions rendering should be one after fixing chicken wire mesh.
Repairs of cracks in RCC member of a structure
- Random cracks in structure exposed to weather- These cracks occur after many years of construction maybe 15 to 20 years; they are likely to be caused due to shrinkage from carbonation of concrete.
- Straight cracks in columns, beams and slabs- These cracks are parallel t reinforcement along with spelling of cover. Exposure of reinforcement may occur at places.
- Straight Cracks in RCC sun shades and balconies- These cracks are straight and along the length occurring at intervals of 3 to 5m.
- Straight cracks in RCC slabs of long open verandahs- These cracks may occur at intervals of 6 to 8 meters parallel to the reinforcement. They are caused due to shrinkage and thermal contraction. These are caused due to shrinkage and thermal contraction. These cracks are wider in winters.
Causes & Preventive measures
- Elastic deformation
- Thermal movement
- Chemical reaction
- Foundation movement and settlement of soil
- Poor construction practices
Technique to cure cracks
- Cracks in the RCC member- When we see cracks due to excessive bending moment, the load causing the cracks is released as far as possible and the member is strengthened by adding reinforcing steel with proper key and bonding with the old member is done.
- Cracks due to shear- These cracks are at 45 degree. To the axis of the member and are corrected by adding diagonal shear reinforcement in the form of stitching dowels.
- Cracks at support or at midspan bottom- They occur due to insufficient steel or insufficient provision of displacement of steel. They are corrected by adding of steel as required. They ends of the added steel are bent and inserted in the member by drilling.
- Pressure Grouting- This method is used when the concrete has become porous but has not decayed. To check this, holes are drilled in the member as per requirement and cement slurry and/or chemicals are grouted under pressure and forced in the holes.
- Cracks in Foundation due to settlement- These when detected is often repair, the foundation has to be redesigned with a wider base and the foundation has to b e taken on the soil having adequate bearing capacity. Cement slurry grouting is often used to increase the bearing capacity of soil.
- Load relieving techniques- The member can be prestressed externally by placing prestressing wires on both sides and then inducing tension.
Causes of Distress of Malfunctioning
- Corrosion of the reinforcing bars due to ingress of corrosion promoting chemicals or due to carbonation of relatively porous concrete.
- Distress caused in the structure due to attack of chemicals in chemical factories.
- Cracks formation due to alkali aggregate reaction, drying shrinkages or u earthquake damage or uneven settlements.
- Due to impact damage or sue to continuous erosion by water, liquids or solids.
- Damage due to
- Though it is impossible to guarantee against cracking yet attempts can be made to minimize development of crack.
- In case of existing cracks, after detail study and analysis of crack, after detail study and analysis of crack parameters, most appropriate method of correction should be adopted for effective and efficient repair or crack.