Number 6

Concrete Crack Control

Because ready mixed concrete is delivered in a fluid state then subsequently becomes a solid, the chemical and environmental influences on the change may sometimes causes cracking.

It is very unusual that these cracks cause any loss of strength but they provide a negative prospective to the end users of the product. The placing and the curing of the concrete has the single biggest influence on the quality of the end product.

The following table outlines the important information regarding the cause, and control of cracking in Allied Ready Mixed Concrete.

Crack Type	Plastic	Thermal	Drying
When Seen	Same Day.	First Few Days.	Weeks, months later.
Where they Appear	Random, no pattern and short in length.	Along the weak dimension of the slab i.e. box outs, gully traps.	As for thermal cracking.
Cause	Rapid evaporation of water from the slab surface.	The chemical reaction of cement hydration causes heat in the slab. The difference in temperature between the slab and the atmospheric temperature causes different expansion/ contraction rates, therefore cracking.	Long term drying of concrete causes shrinkage, any restraint to the slab shrinking causes cracking.
Prevention	Lower the evaporation from the slab. (See Concrete Curing Tip)	Reduce the restraints to slab movement/ contraction e.g. lower bay widths and pour dimension.	Correct location of saw cuts and construction joints. Properly positioned and use of free movement joints.
Once Cracked (Assess what the crack will do)	Leave as they are, better not to fill.	Will act as a saw cut, leave till the end of the project. Joints can then be filled, edges protected.	As for thermal cracking.

Allied Concrete staff are more than happy to help you with any problems or enquiries.

For more information or assistance, please don't hesitate to call. Your call will be automatically connected to our nearest plant. (Calls from mobile phones will be directed for Auckland or Christchurch.)

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DISCLAIMER: Allied Concrete has endeavoured to present the best possible information. However we accept no responsibility for the application of the principles discussed.