The ABCs of Sinking Concrete

Tuesday, December 30th, 2014 by Benjamin Bates

Sinking concrete slabs can be a major problem and can cause serious damages to a home or building and its value. Some of the most common causes for slab settlement include drying and shrinking of soil under a slab, poorly compacted fill soils and washout of soil. This can lead to floor cracks, tripping hazards, pooling water and other cracking and settling throughout a slab or building.

Each of these conditions create a void under the concrete slab. If the concrete is not strong enough to span the void, the slab will crack, break and settle into the void.

The ABCs of Sinking Concrete - Image 1

There are typically four options for home and business owners who want to address this issue. Of these four, one is being used on highways, airport runways and a variety of other commercial applications.

Your first option is to replace the concrete. This of course does not solve the actual problem (voids in between the concrete and soil), so the likelihood of reoccurring settlement is great. The new concrete will not match the older concrete and this is a fairly disruptive method. The area cannot be walked or driven on for some time.

The second option is to use re-leveling grout. This tends to add additional weight, making the problem worse. Additional grout may dislodge if it does not bond well to the existing slab and again, it does not address the actual problem.

Up next is mudjacking. This has been a popular method for some time but is becoming far less popular, mainly due to its failure rate. Looking at this concept openly, here’s what to expect. The holes in which the grout is injected into are roughly 2” wide and that grout -which is used to attempt to lift the concrete- is so heavy (140lbs per cubic foot), that the psi level in which it’s injected into those holes tends to make it shoot out from other holes, causing a mess. The cure time is typically 1-2days, the material can erode and it of course is so heavy, it can actually make the problem worse.

The final option is to use foam…yes, foam. Very similar to the popular spray foam insulation, this denser foam is injected under the sinking concrete, fills the voids and lifts the slab back up neat and tight. Here’s how. Small 5/8” holes are drilled through the concrete and the light (2lb per cubic foot) polymers are injected under the slab which start expanding to fill the voids. Quick reaction time of the foam allows for a perfect lift most times. The commercial applications listed in the third paragraph are of course using this method. The polymers are so expansive it allows for a high capacity lift for interior or exterior applications.

The ABCs of Sinking Concrete - Image 2

About the author
Benjamin Bates lives in the Chatham-Kent area and has been involved in the basement, crawl space and foundation industry since 2008. He has a passion for improving homes the right way and knows what it takes to waterproof, encapsulate or stabilize homes in Ontario. He assists with over 900 projects per year.

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