Damage occurring during construction Cast in-situ concrete structures are hardly ever built under ideal conditions so defects may occur as the concrete is being cast or very soon afterwards. Causes may be: Unsuitable or defective construction materials Unsuitable construction methods/poor workmanship Failure to appreciate hazards associated with a particular structural form or with prevailing weather conditions. The majority of construction defects can be made good, but demolition & rebuilding of affected member may be an economical solution sometimes.
CracksCracks that occur soon after concrete hasbeen placed are often ascribed vaguely to‘shrinkage’, but true drying shrinkagethroughout the section of a concrete membermay take months to become significant.Plastic shrinkage cracks in concrete: Cracks appearing in an exposed concrete surface very soon after it has been finished/in some cases before finishing is complete. Caused by rapid drying of concrete surface while body is still plastic.
CracksUsually discontinuous, seldom extend to afree edge.Typically diagonal in unreinforced slabextending upto 300 mm or so.Prevention: Sheltering the surface fromwind and sun during construction &covering it immediately after finishing.Use of air entrainment may be helpful.Remedy: Sealing cracks against ingress ofwater by brushing in cement or low-viscosity polymers.
CracksPlastic settlement cracks: Concrete may continue to settle especially in deep sections, after it has started to stiffen, and anything that obstructs this movement, such as reinforcement, may act as a wedge so that a crack forms immediately over the obstruction. Settlement of concrete will usually cause a void to form below the obstruction. Cracks also form on vertical surfaces. Precautions: By change of concrete mix design and use of air entrained concrete. R emedial measures: Seal the cracks.
Heat of Hydration Heat of hydration of cement raises the temperature of concrete.
Heat of HydrationIn thick sections with rich mixes the temperaturerise may be quite considerable.As concrete cools it will try to contract.If contraction is restrained concrete will be put totension, leading to cracks. These are ThermalContraction Cracks.Precautionary and Remedial measures: The risk ofthermal cracking can be reduced by minimizingrestraint to contraction and by delaying cooling untilconcrete is strong enough to resist the stressesinduced. If cracks do form then, remedial measuresare similar.
Surface texture defectsBlow holes: R esult of bubbles of air or water trapped against the face of the formwork. R emedial measures: Finest sand (600 microns or 300 microns sieve) with cement 1:1 or 1:2 and a polymer admixture. Mortar is rubbed on the surface with a rubber-faced float subsequently rubbed down with a mortar block.Honeycombing: Inadequate compaction or loss of grout through joints in the formwork leads to formation of voids which penetrate deeply and form interconnected voids.
Surface texture defects R emedial measures: Cut out the affected concrete and repair the surface. Injecting a low- viscosity resin into the concrete.Scouring of concrete surface: It is a sign of excessively wet or harsh concrete. It is caused due to water moving upwards against the work of the formwork Prevention: Change of mix design.
Colour VariationColour variation always occurs.Colour variation may result form a number offactors, including concrete mix design, formworksurface texture, variations in curing conditions,formwork absorbency or stiffness, vibration andreleasing agent, leakage of water through joints informwork.R emedial measures: Application of some form ofpaint or surface coating.
Other surface blemishesRust coloured stains: Causes: • Contamination of the aggregates with pyrites (Iron Sulphide). • Also caused if unprotected reinforcement is left projecting from partly completed R.C. work. Remedy: • Cutting out the offending particles and to make good the surface. • R stains can be removed by treatment with ust dilute Oxalic or HCl (10% solution).
Lack of CoverDue to displacement of reinforcement while concreteis being placed.As a result there is less cover than desired on at leastone face of concrete.R emedial measure: Increase cover by building out the face of the concrete with a rendering. A polymer-modified cement and sand mix may be used as mortar.