Lifting sagging concrete slab a needed fix

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This article was published 19/06/2021 (1552 days ago), so information in it may no longer be current.

Question: I have an issue with some stucco cracking and the concrete slab outside my foundation wall has dropped in the last couple of years.

There is now a visible crack between the concrete and the foundation wall. With the recent heavy rains, I have some seepage into my basement.

I have read about mud jacking and poly jacking as possible options to repair the settling and seepage issues.

In Manitoba, what do you see as the benefits or disadvantages of each option? Thanks, Ed.

Answer: Lifting existing concrete slabs that have settled is a lower-cost alternative to replacement and often works well as long as the concrete is in good condition. Partial removal of the slab against your foundation should also be done at that time to allow for future regrading and prevent moisture intrusion into the foundation.

With the very dry weather of the last few years in our area, many homeowners, like yourself, are experiencing significant settlement especially at concrete sidewalks, driveways, and patios. Because the clay soil is so dry it is losing a substantial amount of its normal volume and shrinking. This is often first seen as a gap next to the house, between the soil and the foundation or concrete slabs in this area. Your home is one of many where this is occurring and the initial solution is quite simple.

First and foremost, you should start regularly watering the soil, yard, and vegetation around your house. Replenishing the lost moisture in the top portion of the soil will help prevent further shrinkage. Regular watering may also help retain the moisture much deeper down where you can’t normally reach. By keeping the upper levels moist, it can prevent the water contained within the lower soil from naturally evaporating into the dryer topsoil above. This may be the most important aspect of soil hydration, to prevent settlement of your foundation, if the deeper soil becomes too dry. For your purposes, it may immediately help prevent further sinking of your concrete slab.

Because it appears that the concrete slab has dropped beyond the point of no return, even with extreme soil watering, lifting it back into its original position may be a reasonable alternative to breaking it up and replacement. The main thing to consider is what the condition of the current concrete will be after lifting. If it is in generally good condition, with no serious surface spalling or deterioration, lifting may be worthwhile. Even if there are some cracks, it may be salvageable, as long as they are small enough not to cause the slab to pull apart when remediated. A skilled mudjacking crew should be able to straighten and even out most slabs on grade, even if they currently have some deflection between sections. If the concrete surface is in poor condition, or the cracks significantly spaced apart, lifting may be a poor use of upgrade funds.

The practicality of lifting your existing concrete should be easily determined by the estimator or crew from the construction company you consult. Mudjacking involves drilling small holes through the entire slab and pumping special clay underneath, which lifts it off the existing substrate. This type of clay mixture is resistant to shrinkage, even if the dry weather continues. After lifting, the holes are filled with concrete patching compound to smooth the surface. This type of service has been around for a few decades, with reasonable results. In my experience, it typically will hold up well for around 10 or more years, depending on several factors.

More recently, there have been several contractors that have been doing similar work to the mudjackers, but with expanding foam instead of clay. Many of these companies claim that their method is superior, because the foam does not deteriorate over time, unlike the mudjacking compound. Most of these claims are difficult to evaluate, due to the limited time these contractors have been in business. I have seen dozens of houses over my 20-plus years of home inspections that have had driveways, sidewalks, garage floors, and patios mudjacked and most of the homeowners are quite satisfied with the results. I have very limited experience with the Poly foam contractors and the few I have seen are typically involved with larger commercial projects.

So, whichever material you chose for lifting your slab, you should also do some alterations to the concrete prior, to prevent a reoccurrence of your current issues. One reason your foundation started leaking after the slab fell is due to the lack of access to improve the grading. I would suggest cutting the existing slab and removing the section for the first 30 to 60 cms near the foundation wall. This will allow you to build up the soil in that area, and easily improve the grading, if further erosion occurs. It will also leave easier access to the top portion of the foundation, should it require waterproofing or repairs due to rusted form ties, cracks, or other common defects that happen over time. Concrete slabs are not naturally waterproof and are not a good material to use for grading purposes. Even after lifting, the slab may settle over time, and you will be back to square one. If you leave a buffer of natural soil between the slab and the foundation, more soil or granular fill can easily be added to improve the slope away from the house, even if the slab drops again.

Lifting a concrete slab on grade outside a home may be a very reasonable choice, whether you chose a company that uses mudjacking or foam injection techniques. You must ensure your concrete is in good enough condition, beforehand, and should remove a portion that is immediately adjacent to the foundation wall to allow for further regrading, as needed.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (cahpi.mb.ca). Questions can be emailed to trainedeye@iname.com. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.