Cracking in Concrete

Random cracking in concrete is not desirable, it can make
your concrete look ugly and lead to structural weakness
of the concrete.
Reinforcement and joints are used to control
cracking. Bad cracking leaves the reinforcement
exposed to air and moisture, which may cause it
to rust and weaken concrete.
See CHAPTER 11 Joints in Concrete and
See CHAPTER 17 Reinforced Concrete
TYPES OF CRACKS
Two types of cracks happen in reinforced concrete:
PRE-SETTING CRACKS Cracks that happen BEFORE concrete hardens,
while it is still workable.
HARDENED CRACKING Cracks which happen AFTER concrete hardens.
PRE-SETTING CRACKS
Pre-setting cracks are cracks which form during placing, compaction and finishing caused
by movement of concrete before it is dry.
There are three types of pre-setting cracks:
PLASTIC SETTLEMENT cracks
PLASTIC SHRINKAGE cracks, and
Cracks caused by MOVEMENT OF THE FORMWORK.
Pre-setting cracks can be prevented by looking for them as they happen, while the
concrete is still setting.
If they are detected early on they can be easily fixed by re-compacting, re-trowelling or
re-floating the concrete surface.




Plastic Settlement Cracks
When do they form? They form soon after
concrete is placed, while it is still plastic. They
get bigger as concrete dries and shrinks and
tend to follow the lines of reinforcement.
Prevention
Revibrate the concrete.
Re-trowel the surface.
Look for cracks as the concrete is
setting. At this stage they can easily
be fixed.
Plastic Shrinkage Cracks
When do they form? On very hot days or in low humidity and moderate winds. Cracking
is more common in summer but may occur during winter.
See CHAPTER 12 Hot and Cold Weather Concreting
Plastic shrinkage cracks appear in lines,
roughly parallel or in a crazed haphazard
way. They are usually 300–600 mm long but
may be between 25 mm and 2 m in length.
Prevention
Dampen the subgrade and forms and protect
concrete from the wind.
Keep all materials cool on hot days.




Place, compact and cure as quickly as possible on hot days so concrete won’t dry out.
Once the concrete has been compacted, screeded and floated apply a uniform spray film
of EVAPORATIVE RETARDANT (Aliphatic Alcohol) to prevent rapid loss of surface
moisture, then continue with finishing.
Try to place at the cooler times of the day.
Repair Cracks may be closed by reworking
the plastic concrete.
Formwork Movement
If formwork is not strong enough it may bend or bulge. Formwork movement may happen
at any time during placement and compaction.
Prevention Make sure formwork is strong.
If the concrete collapses, strengthen the formwork and re-vibrate the concrete.
Thermal Shock
Applying cold water, as curing, over concrete on a hot day can result in cracks from the
sudden contraction.
Prevention Use warm water.
CRACKS AFTER HARDENING
Cracks after hardening may be caused by drying shrinkage, movement or settling of the
ground, or placing higher loads on the concrete than it was designed to carry.
Little can be done with cracks after hardening. Careful and correct placement helps
prevent serious cracking after hardening.
Only uncontrolled cracks are a possible problem. Cracks at control joints or controlled by
steel reinforcing is expected and acceptable.