One of the most common observations we come across inspecting homes in Colorado are cracked foundations, most commonly made of poured concrete, concrete masonry units or brick. This issue is also a higher priority item a potential buyer may consider when deciding to go forward with or walk away from the purchase of a home. The recommendation of a Structural Engineer usually tends to insight panic or fear in a potential buyer, but it shouldn’t be that way. If anything, it should help build confidence in the choice to purchase or walk away from the home ownership opportunity. In this series, I’d like to go through common foundation types, potential defects related to each, and how to know if you really should be concerned about any defects that are found. This is Part 1 of a 3-part series.
Slab-on-Grade Construction
Slab-on-grade construction is the least common foundation type we see at inspections due to Colorado’s expansive soils, and a demand for square footage without having to just build up (which is why basements are very common here). Just because slab-on-grade foundations aren’t used as frequently doesn’t mean they can’t work if designed properly.
Concrete slabs are literally poured “on-grade”, on top of a prepared bed of sand, gravel, insulation, or a combination of these. There are three types of slab systems:
Monolithic: A monolith in nature is a solid stone structure. The term monolithic is used to describe a system that is one solid piece of material. Monolithic slabs are once piece of concrete and include the footers or stem walls. The entire foundation and interior slab are poured at the same time.
Supported: A supported slab is comprised of two separate pieces of concrete; one being the foundation, and one being the interior slab. The interior slab extends onto or into the foundation wall where its edges are supported by the footers or stem walls.
Floating: A floating slab is not supported around the edges by the foundation wall, but literally is poured inside the foundation walls and rests or “floats” on a bed of sand, gravel or insulation. The slab is not connected to the footers or stem walls.
All of these systems may incorporate different types of reinforcement like rebar or tension cables. All of these systems also incorporate control joints when built recently. Older slabs may not have control joints, as these methods of controlling cracking were not implemented until after the mid-century. Control joints are incorporated to allow the slab places to crack and potentially shift in pre-determined sections rather than just allowing the slab to crack and settle or heave at random. They are also put in place around posts and load-bearing members so that any movement of the slab won’t affect the structure of the home. That being said, these systems are designed for possible movement.
We frequently come across slabs that are heaving, settling, and cracked. This is very common in Colorado. Concrete will most always crack whether it be through curing and shrinkage or outside forces such as hydrostatic pressure. Given the expansive soils here in Colorado, concrete will frequently crack and then either settle down or heave up. Unless the slab is monolithic and physically connected to the foundation, this slab movement is independent of the foundation and usually poses few structural issues.
Outside of regular curing and shrinking, cracking and movement should be evaluated by a qualified professional. Frequently the culprit is expansive soils. Expansive soils are triggered by wet conditions and then subsequent drying out. The soils swell and then shrink. If a home is designed well, exterior grading should slope away from the home, typically 6 inches within the first 10 feet. This is done to direct water away from the foundation. If drainage from the roof is not installed well and the slope around the home is poor, water can and will concentrate around the perimeter of the home and soak the ground down to the foundation. In the case of a slab-on-grade foundation, the water only has to soak down far enough to wet the soils around and under the slab for problems to occur. It is important to make sure proper drainage around the home is maintained to limit potential cracking and movement of the foundation and concrete slab.
If a slab has cracked, heaved or settled, steps can be taken to manage the effects. If dealing with simple shrinkage cracking, appropriate sealants can be injected to seal the cracks. If concrete has settled severely, mud-jacking can be used to lift the concrete back into place. Additional concrete can also be added to depressions to bring the slab surface back to level. If heaving has occurred, slabs can be ground down to try and level. Care should be taken with this method, however, as thinning out of the slab can weaken it. In extreme cases, the slab may need to be removed and a new slab poured. Before any of these steps are taken to repair a slab or surrounding foundation, it is important to consult a professional concrete contractor, foundation contractor or structural engineer. Permitting may be required for major repair or renovation.
If you are in need of a professional opinion regarding a concrete slab or foundation, check out our Preferred Trade Partners & Professionals List.