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Can I Repair Cracked Concrete?

Concrete crack repair - Perfect Concrete Care

by Daniel Green

One of the oldest construction materials known to man, concrete is durable and long lasting but unfortunately, doesn’t cope well with torsional loading. This almost always results in cracks, from hairline fractures only a few mm deep to full blown tectonic plates inches apart. Can you personally repair the cracks? Of course you can. With enough time and money anything is achievable. But do you want spend hours schooling up on all the variables and potentially wasting hundreds of dollars making tiny errors that result in reworks anyway? Now there’s another story.

Before we delve into the pros and cons of you repairing your own cracked concrete, let’s dig a little deeper. Unfortunately, this may raise more questions than it answers. The decision to go ahead with crack repair will be governed by several factors, and regrettably, there’s no straightforward answer. However, all of these factors should be taken into account.


  1. The scale of the crack.

How long is it – does it run from one side to the other or does it just cut off a corner of the slab? Is it quite noticeable? How deep is it – is it only a few millimetres or does it go all the way to the sub grade? How wide is it – can you barely slide a piece of paper in or can you fit a whole finger? Is it between every contraction joint or just on one piece? How long has it been since the pour?

If the crack is small, short & shallow, if full curing has taken place and the client would rather spend their money elsewhere, chances are you’re better off leaving it to fend for itself. Opening it up to complete costly and invasive repairs is likely counterproductive.

2. The location of the crack.

Is the crack in a structural member of the construction like a beam or a column? Or it is on something innocuous like a footpath? If it is on the structural member – is it a hairline crack? If it is, is it shallow? If it’s not, get dimensions as above. If it’s on a footpath or in a carport, what will be the maximum load?

3. The cause of the crack.

This may be hard to determine but it’s one of the most important factors in the decision to repair. There are scores of reasons:

  • Substandard material. Poor grade sand or the wrong sand can cause inconsistencies in the final product, which can lead to cracking. If this is the case and the crack is in a structural member then it needs to be broken apart and repoured.
  • Poor workmanship. Having a mix that is too wet, too dry or improperly screeded can cause hollow spots under the surface which, when loaded up, can crack. Again, if this is the case and the crack is in a structural member then it is unsafe.
  • Amateurish preparation. Improper groundwork and/or formwork can also cause hollow spots under the slab which can fracture. If this is decorative concrete or something harmless like a footpath, and the cracks don’t compromise the integrity of the slab then it’s probably safe to leave it. Check with your engineer.
  • Bad design. Slabs that feather out to narrow edges tend to crack. In this situation it’s best to break the slab out, redesign it and repour it or it will just continue to worsen.
  • Unideal ambient conditions. Completing the pour in temperatures that are too hot or too dry can lead to inconsistent curing, which results in cracking. Depending on its end use, this may need reworking.
  • Organic interference. In the fight between manmade structures and natural ones, Mother Nature is always the winner. Sometimes, if the cause it learned early enough and the organic material [likely roots] are destroyed, the slab may subside to the point where it’s still functional. If the tree cannot be removed or the roots are sizeable then further chemical and mechanical measures may need to be taken. This will attempt to prevent further damage as the roots mature again.
  • Excess weight. Loading up the slab beyond its design capabilities. If the concrete will continue to be overloaded in its life course then redesigning it and redoing it will likely be the outcome.
  • Concrete cancer. Whilst not technically cracked, cancer delineates the slab from the inside out. There are some fixes that can be less invasive as others but generally, concrete cancer is a break-out and redo scenario. You can read more about concrete cancer here: link.
  • Natural disasters. Flooding can remove the sub grade, resulting in cracking. Earthquakes wreak havoc on concrete as well. Often, these are so violent that the slab will need to be demolished.
  • Age. Like everything, concrete has a lifespan. It might be time to demolish it.

4. Services.

Are there any services running through the slab that may be compromised by the crack/s? If the concrete movement continues to its natural conclusion, will it take out power, data, water or gas and therefore create a more complex [and dangerous] situation?


Now that you’re across the factors involved in creating the fractures in your slab, let’s take a look at the best options for repair. The proverbial line in the sand here is 7mm. Under 7mm is considered a narrow crack and over is considered a wide crack.

Decorative/non-structural concrete.

  1. Hairline cracks. < 1mm. These can be repaired with a vinyl concrete patching compound applied into the crack and then worked. The process is quite involved, so to get it right a concrete care specialist should be engaged.
  2. Narrows cracks. 1mm – 7mm. These are best filled with an elastomeric masonry crack filler that can be applied with caulking gun. However, the gap is best filled with a foam backing rod first. Additionally, the correct repair material should be used or the repair will likely fail.
  3. Wide cracks. > 7mm. These repairs are time consuming, require special tools & materials and experience to get right. The process involves veeing out the crack, filling it with an appropriate patching material and working it until it can be left to cure.

Structural concrete.

This is where it gets quite involved and a professional should definitely be engaged. With complex and complicated causes & repair options – some of which include UHP [40k psi] hydro-demolition and crack injection – this field possesses some very niche knowledge that could save you thousands of dollars.

Link: Crack injection.


No repair will ever have as much integrity or beauty as the original pour. The nature of concrete rectification dictates that there will be visual scarring in the form of non-matching materials. There’s just too many factors that decide the colour of a slab – factors that cannot be matched. Therefore, some options are:

  1. Coloured concrete. If the crack is professionally remedied and the final finish is flush with the existing material then colouring the slab will render the repair unnoticeable.
  2. Decorative concrete. The repair can be hidden within a complex design, much like tattoo artists can overlay an old tattoo with a new one. Again, this requires a professional finish.
  3. Featuring the repair. The design language of some construction/remedial works means that the imperfections caused by the building’s lifespan area retained. Therefore the patch is not a hidden repair but a feature.


Remediation of concrete cracks is complicated and complex task that requires a high level of experience. It’s one of those jobs that often costs less to contract out than to attempt yourself.

For concrete patching solutions in Greater Sydney Jaro is on 0414 902 112. If you’re in Wollongong then Marco is on 0414 830 134.

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