Though concrete relatively is a durable building material, yet it may suffer or develop damages during its life due to many reasons. Concrete suffers from damages during its production or during its service conditions requiring intermediate attention. The causes of damages many be as- (i) Applied and environmental loads exceeding the designed loads, (ii) Accidents and subsidence etc. Sometimes damages in a structure are caused by poor construction practices, error in design and detailing and construction over loads. The other causes may be drying shrinkage, thermal stress, chemical reactions, weathering, and reinforcement corrosion etc.
The damages in plastic or green concrete may develop due to plastic shrinkage and settlement cracking. Some times on removing or stripping off the forms many surface defects such as ridges, bulges, bolt-holes, honey combing etc., are noticed in the fresh concrete members. Such defects can be avoided to a great extent by using water tight and rigid form work in such a way that form work may be removed without the use of crow bar or tools. In addition to these defects, blow holes are developed during the concreting operation due to improper design of form work.
These holes are developed in the surface of concrete by water bubbles and trapped air against the face of the form work. These defects can be reduced by adopting form work slightly water absorbent and giving adequate compaction to the concrete. In case smooth surface is required or blow holes are very large, then these holes may be filled with 1:1 or 1:2 cement sand mortar. The sand used should be sieved through 300 or 600 micron sieve depending upon the smoothness of surface desired. Preferably crushed lime stone dust should be used in place of sand. The mortar should be rubbed over the affected area with a rubber face float and finally it should be rubbed with a smooth stone for a smooth finish.
The honeycombing consisting of groups of inter connected deep holes indicate the inadequate compaction or loss of matrix through joints of form work or between previously cast surface and form work. In such cases, the affected area is cut by saw to a depth of about 5 cms. The unsound material is chipped out upto the solid concrete. The surface is cleaned, washed and dried. After preparing the surface a bonding coat of neat cement is applied to all exposed surfaces and new concrete is filled and compacted.
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The repair of projections and bulges can be done by chipping off the concrete from the surface and then rubbing it with a grinding stone. The scouring of a vertical surface of a hardened concrete indicates a sign of excessive wet or harsh concrete, usually it is a superficial defect. If it is un-usually deep and cover to reinforcement is small, then measures to rectify it may be the same as that suggested for filling the blow holes. The finishing repair should be carried as soon as possible after removing the form work and concrete becomes too hard. In order to develop a good bond between the parent concrete and repaired work, there pair should be carried out within 24 hours of removing the form work.
If repair is not carried out properly, the newly placed concrete becomes loose and ineffective with the passage of time and finally gets detached from the parent concrete. The darker colour of the repaired patches can be corrected by adding 10 to 20% white cement in the mortar if the surface is of importance.
1. Voids in Concrete:
Cause ― Honeycombing–Inadequate compaction, loss of cement paste or grout.
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Prevention measures ― (i) Improve compaction, (ii) Reduce max size of aggregate and (iii) Prevent leakage of paste or grout.
Remark ― (i) Inject good resin mix and (ii) cut and repair
2. Blow Holes in form Faces of Concrete:
Causes ― Air or waste trapped against from work, inadequate compaction, unsuitable mix design, and use of undesirable release agents.
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Prevention measures ― Improves compaction, change mix design, use of absorbent form work and appropriate release agent.
Remark ― Fill with polymer modified fine mortar.
3. Cracks (in Horizontal Surface as Concrete Stiffens or soon after that):
Causes ― Plastic shrinkage, rapid drying of surface
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Prevention measures ― (i) Use air entrainment and (ii) Shelter during placing concrete.
Remark ― Seal or paint by brushing by cement or low viscosity polymer.
4. Crack from above Reinforcement or in Deep Lifts:
Causes ― Plastic settlement, concrete continues to settle after starting to stiffen.
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Prevention measures ― Change mix design, use air entrainment.
Remark ― Re-compact concrete of upper part while still plastic. Seal cracks after concrete has hardened.
5. Cracks in Thick Sections Developing as Concrete Cools:
Causes ― Check thermal contraction
Prevention measures ― Delay cooling till concrete has gained strength. Restraint to contraction should be minimized.
Remark ― Seal crack.
6. Erosion of Vertical Surface:
Causes ― Scouring, water moving upwards again from face
Prevention measures ― (i) Reduce water content and (ii) Mix design should be more cohesive.
Remark ― Apply polymer modified fine mortar.
7. Colour Variation:
Causes ― Variation in mix proportion, curing condition, materials characteristics of form face, vibration, leakage of water from form work
Prevention measures ― Prevent leakage from form work and ensures uniformity of all relevant factors.
Remark ― Adopt surface coating
8. Powdery Formed Surface:
Causes ― Surface retardation caused by sugar present in certain timbers.
Prevention measures ― Changes form material, seal surface of form work. Before other applications apply lime wash to form face.
Remark ― Generally no treatment required.
9. Rust Strain:
Causes ― (i) Pyrites in aggregates (ii) Rain streaking from un-protective steel (iii) Ends of wire ties turned out.
Prevention measures ― Avoid contaminated aggregates, protect exposed steel, clean form thoroughly, Turn ends of ties inwards.
Remark ― Clean with dilute acid or sodium citrate or sodium surface coating
10. Plucked Surface:
Causes ― Careless removal of form work, insufficient release agent
Prevention measures ― More care in removing form work and more care in application of release agent
Remark ― Patch as for spalled concrete or rub in fine mortar
11. Lack of Cover to Reinforcement:
Causes ― Displacement of reinforcement during placement of concrete or reinforcement fixed badly, inadequate tolerance in detailing.
Prevention measures ― Provide better support to reinforcement, more accurate steel fixing and greater tolerance in detailing.
Remark ― (i) Apply protective coat (ii) Apply polymer modification cement and sand rendering