Chapter: 1

Damage occurring during construction

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 materialsUnsuitable construction methods/ poor workmanshipFailure 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.

Cracks

Cracks that occur soon after concrete has been placed are often ascribed vaguely to ‘shrinkage’, but true drying shrinkage throughout the section of a concrete member may 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.

Cracks

Usually discontinuous, seldom extend to a free edge.Typically diagonal in unreinforced slab extending upto 300 mm or so.Prevention: Sheltering the surface from wind and sun during construction & covering it immediately after finishing. Use of air entrainment may be helpful.Remedy: Sealing cracks against ingress of water by brushing in cement or low-viscosity polymers.

Cracks

Plastic 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.Remedial measures: Seal the cracks.

Heat of Hydration

Heat of hydration of cement raises the temperature of concrete.

Heat of Hydration

In thick sections with rich mixes the temperature rise may be quite considerable.As concrete cools it will try to contract.If contraction is restrained concrete will be put to tension, leading to cracks. These are Thermal Contraction Cracks.Precautionary and Remedial measures: The risk of thermal cracking can be reduced by minimizing restraint to contraction and by delaying cooling until concrete is strong enough to resist the stresses induced. If cracks do form then, remedial measures are similar.

Surface texture defects

Blow holes:Result of bubbles of air or water trapped against the face of the formwork.Remedial 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

Remedial 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 formworkPrevention: Change of mix design.

Colour Variation

Colour variation always occurs.Colour variation may result form a number of factors, including concrete mix design, formwork surface texture, variations in curing conditions, formwork absorbency or stiffness, vibration and releasing agent, leakage of water through joints in formwork.Remedial measures: Application of some form of paint or surface coating.

Other surface blemishes

Rust 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.•Rust stains can be removed by treatment

with dilute Oxalic or HCl (10% solution).

Lack of Cover

Due to displacement of reinforcement while concrete is being placed.As a result there is less cover than desired on at least one face of concrete.Remedial 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.