Defects in Concrete

Some defects are obvious only to a trained eye,
others, such as cracking, are obvious to anyone.
Some common defects, their causes and how to
prevent and repair them are explained below.
If in doubt please consult an expert.
COLOUR VARIATION
Difference in colour across the surface of concrete. May appear as patches of light and dark.
Causes Uneven or variable curing conditions.
See CHAPTER 10 Curing
Applying a different brand or type of cement to the surface as a 'drier'.
Prevention Use an even concrete mix when placing, compacting and finishing and keep
concrete evenly moist. Do not use driers.
Repair Many colour variations from workmanship
will be permanent. To hide the variation a SURFACE
COATING can be applied.
Rectification of colour variation from stains is a very
difficult operation and may need repeated gentle
treatments with a weak acid.
See CHAPTER 15 Removing Stains from Concrete.
CRAZING
A network of fine cracks across the surface of concrete.
Causes Crazing is caused by minor surface shrinkage
in rapid drying conditions. (ie Low humidity and hot
temperatures, or alternate wetting and drying.)
Prevention Finish and cure concrete correctly.
See CHAPTER 9 Finishing Concrete and
See CHAPTER 10 Curing Concrete
Repair Repair may not be necessary because crazing will not weaken concrete. If the
crazing looks too bad then a surface coating of a paint or other overlay sealer can be
applied to cover and/or minimise the effect of the cracks.
See CHAPTER 13 Surface Finishes on Concrete




DUSTING
A fine powder on the concrete surface which comes off on your fingers.
Causes Finishing before the bleed water has dried.
Also finishing during the rain.
See CHAPTER 9 Finishing Concrete
Not curing properly, or the surface is drying too quickly.
See CHAPTER 10 Curing Concrete
Concrete subject to severe abrasion or of too low a grade for the end use.
Prevention Let any bleed water dry up before trowelling or in cold conditions remove the
water. Cure correctly.
See CHAPTER 10 Curing Concrete
Protect concrete from drying out too quickly in hot or windy conditions.
For harsh conditions use a stronger concrete.
Repair As previously detailed, dusting concrete surfaces result from inadequate attention
to prescribed placement and finishing practices namely, the addition of excess water,
working in of bleedwater, the inadequate compaction and curing of concrete. Where
surface dusting is minimal the application of a surface hardener can be beneficial. If the
surface is showing significant wear distress it is essential to remove all loose material by
grinding or scrapping the surface to a sound base and then applying a suitable topping.
RAIN DAMAGE
The surface has bits washed away or many small dents.
Causes Heavy rain while concrete is setting or rainwater
being allowed to run across the concrete surface.
Prevention Cover the concrete if it is raining or it looks
like it might rain. Don’t lay concrete if it looks like it might rain.
Repair If the concrete has not hardened and damage is minimal the surface can be
refloated and re-trowelled taking care not to overwork excess water into the surface.
See CHAPTER 9 Finishing Concrete
If the concrete has hardened it may be possible to grind or scrape the minimal amount of
the surface layer and apply a topping layer of new concrete or a repair compound. This
may not always be possible and should only be done with expert advice.




SPALLING
When the slab edges and joints chip or break
leaving an elongated cavity.
Causes Edges of joints break because of heavy loads or
impact with hard objects. As concrete expands and contracts
the weak edges may crack and break.
Entry of hard objects, such as stones, into joints may cause spalling when the concrete
expands.
Poor compaction of concrete at joints.
Prevention Design the joints carefully. Keep joints free from rubbish. Keep heavy loads
away from the joints and edges until they have properly hardened.
Ensure proper compaction.
Repair For small spalled areas: scrape, chip or grind away the weak areas until you reach
sound concrete, making sure you brush the old concrete clean of any loose material. Then
refill the area with new concrete or repair mortar. Compact, finish and cure the new patch
carefully. Care should be taken that all joints be maintained and not filled.
For large spalled areas: seek expert advice
EFFLORESCENCE
A white crystalline deposit sometimes found on
the surface of concrete soon after it is finished.
Causes Sometimes mineral salts are dissolved in
water. If water with dissolved mineral salts collect
on the concrete surface as water evaporates salt
deposits are left on the surface.
Excess bleeding can also result in efflorescence.
Prevention Use clean, salt-free water and washed sands. Avoid excessive bleeding.
Repair Remove efflorescence by dry brushing and washing with clean water. Do not use
a wire brush.Wash with a dilute solution of hydrochloric acid.



HONEYCOMBING
When too much coarse aggregate appears on the surface.
Causes Poor compaction, segregation during placing or
paste leakage from forms. A poor concrete mix with not
enough fine aggregate causing a rocky mix.
Prevention Use a better mix design. Take care during
placing concrete to avoid segregation. Compact concrete
properly. Good watertight formwork.
Repair If honeycombing happens only on the surface it can be rendered. If honeycombing
happens throughout the concrete it may need to be removed and replaced. The surface
may require rendering. Rendering means to cover the surface with a layer of mortar.
BLISTERING
Blisters are hollow, low profile bumps
on the concrete surface filled with
either air or bleed water.
Causes They are caused when the fresh concrete surface is sealed by trowelling while
trapping air or bleed water under the surface. This may particularly occur in thick slabs or
on hot, windy days when the surface is prone to drying out.
Prevention After placing, screeding and floating leave the concrete as long as possible
before trowelling, which seals the surface. Cure to prevent evaporation.
If blisters are forming delay trowelling as long as possible and take steps to reduce
evaporation.
Repair Grind off the weakened layer to an even finish.