2. 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.
3. 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.
4.
5. 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.
6. 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.
R emedial measures: Seal the cracks.
7. Heat of Hydration
Heat of hydration of
cement raises the
temperature of
concrete.
8. 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.
9. Surface texture defects
Blow 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.
10. 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.
11. 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.
R emedial measures: Application of some form of
paint or surface coating.
12. 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.
• R stains can be removed by treatment with
ust
dilute Oxalic or HCl (10% solution).
13. 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.
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.
