What Causes Concrete to Sink?
Concrete is a wonderful building material. It’s durable, strong, easy to produce and economical. Which is why it’s the most commonly used building material in the world for pavements. And as you know, concrete can crack, chip, crumble and sink. In this post, I’m going to focus on sinking and discuss the most common causes for sinking or settled concrete and also share some tips on reducing the chance your existing or new concrete will sink in the future.
Why is my concrete sinking?
When a concrete slab sinks or settles a noticeable amount over time, the reasons for it is either due to poor construction or water intrusion. Just like a chain is only as strong as it’s weakest link, concrete is only as good as the base it was built on. So concrete built over a weak base, overtime there is a good chance the concrete will start to sink. A weak base is one that was never compacted properly and/or the wrong material was used such as soft soils or loose aggregates. A common place you see sunken concrete is around the perimeter of your foundation. This is because when a foundation is constructed, the area is excavated a few feet wider to allow room for workers to install and remove the forms. This area is known as the over-dig area and if this area is not backfilled correctly it will settle over time and cause the concrete built over it such as the driveway, walkways and patio to sink. This is an extremely common issue to see in homes built around the Chicago area since correct back filling is time consuming and the homebuilder will be long gone by the time you start noticing the effects.
Another cause for sinking concrete is water intrusion. Water that is constantly intruding below the slab will overtime erode or wash away the soil or stone base. Clay rich soils will compound the effects of water intrusion and settling as the clay will freeze and expand during cold weather and shrink when it heats up creating voids. A common place we see water intrusion is where downspouts are discharging water right along the edge of a slab. Also a poorly graded yard, one where the water doesn’t flow away to the designed area such as open permeable area or storm drain and instead sits along the concrete can cause concrete to sink. Lastly, a leaky or broken water or sewer pipe can pose a problem if it’s below or near concrete.
How can I prevent my concrete from sinking?
If you have existing concrete and want to make sure it doesn’t sink or get any worse then here are some low cost easy things any homeowner with some time can do.
- Seal all open cracks and joints with a polyurethane or silicone caulk. Large gaps can be filled with a backer rod first and then caulked on top.
- Make sure downspouts are discharging at least five feet away from any concrete slab – the farther the better.
- Tightly pack any open sides of a slab with soil to ensure runoff water is not going below the concrete.
- If you are having new concrete installed, ensure the base is at least of 4” of gravel and gets compacted. Base should not move when you stomp your feet on it.