Why has my concrete cracked?

Why Has My Concrete Cracked?

The causes, prevention and how to repair cracked concrete.

Found Cracks In Your Concrete?

If you are reading this then the chances are you one of many that have experienced cracking concrete. Your initial assumption may be that something was wrong with the concrete that was delivered. When actually, the main causes of concrete surface cracking is usually lies with the aftercare and initial preparation.

Concrete Cracking Causes

Ineffective curing or no curing process at all are the most common causes of cracking concrete, let us explain further.

What Is Plastic Shrinkage?

Plastic shrinkage is when the rate of surface moisture evaporation occurs quicker than the rate of moisture absorption within the concrete mix.

The surface of the concrete dries out too quickly whilst the concrete strength is lower, which then causes plastic shrinkage cracks to appear.

How To Avoid Plastic Shrinkage

The simplest solution to reducing the risk of plastic shrinkage is reducing the water content. This will then cause its own issues for example, decreased workability.  

Avoiding plastic shrinkage in concrete takes a few simple steps and Total Concrete & Screed has different products available to reduce plastic shrinkage.

Plasticiser; An admixture that is introduced to the concrete mix which increases the flow of concrete without adding more water. Plasticiser is an approbate admixture for all kinds of concrete uses from concrete floor slabs to footings.

Concrete fibres; These hair like fibres reduce concrete cracking during the curing stage. They are designed to absorb the moisture and slowly release this moisture throughout the mix. These fibres provide a higher shatter and impact resistance to the ready mix concrete. 

A combination of both the concrete fibres and plasticiser is a perfect match to reduce plastic shrinkage.

Initial Preparation

We have put together a step by step guide to help you get ready for your concrete delivery. Outlined in this guide is the initial preparation needed. Ground prep is key, we advise compacting the ground and adding a waterproof membrane so moisture is not lost.

Concrete Aftercare

Weather conditions when the concrete is laid is also a factor when it comes to plastic shrinkage in concrete. Humidity and wind speed can play a part increasing the risk of surface cracks. If you are faced with these conditions, there’s no need to cancel your concrete delivery but you may want to take extra precautions.

  • Cover your ready-mix concrete with a waterproof membrane, this will reduce the water evaporation in the mixture. 
  • Provide additional shading to your concrete to avoid direct sunlight. 
  • Create windshields around the concrete to reduce air movement and loss of water.

Dealing With The Cracks

You may notice the cracks in the concrete within hours of the concrete being laid, however they might not be visible until the next day. Although plastic shrinkage cracks occur they are not considered to affect the structural integrity of the slab.

With this in mind, if you are using concrete for a shed base, these cracks will not affect the strength. However if the purpose is for a pathway or driveway which exposes the surface to wear, the durability of the surface may be jeopardised.  

At the earliest point, brush dry cement into the cracks. The residual water within the concrete will activate the cement and seal these cracks. If these cracks are left without repair and open to the elements, through the process of freeze thaw they will lead to further damage. 

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