Can I Repair Cracked Concrete?

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 to 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. Several factors will govern the decision to go ahead with the crack repair, and regrettably, there’s no straightforward answer. However, we should take all of these factors into account.

IDENTIFICATION

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? Or if it is, is it shallow? If it’s not, get the 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 is 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 than 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 subgrade, 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?

REPAIR

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. We consider under 7mm a narrow crack and over, a wide crack.

Decorative/non-structural concrete.

1. Hairline cracks. < 1mm. These can be repaired with a vinyl concrete patching compound applied to 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 a 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 and require special tools & materials, and experience to get them right. The process involves seeing out the crack, filling it with 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

CONSIDERATIONS

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 are 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 are retained. Therefore the patch is not a hidden repair but a feature.

IN CONCLUSION

Remediation of concrete cracks is a 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.