Cracking in Concrete and Types
Cracking in Concrete and Types
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.
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. Crackingis more common in summer but may occur during winter.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.
Routine Vertical Pile Load Test Step by Step
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.
