The technology to lift concrete slabs has been around for many years. Highway slabs, airport runways, factory floors, etc. have been successfully raised for the past 40 years. However, until recently, the companies that did this work seemed to completely ignore the residential market. Now, in most cities, you can locate an individual who can magically raise a sunken or tilted concrete slab, floor or set of steps.
How is it Done?
The technology to lift concrete is centuries old. It is the same technology that is used to raise your car on a hydraulic lift at the car repair garage. Holes are drilled through the tilted concrete, usually two inches in diameter. A mixture of cement, sand, fly ash and water is then pumped into these holes under pressure.
This mixture has the consistency of a thick mortar. As it begins to flow beneath the slab, it exerts an equal pressure on every square inch of the concrete. Thus, if the pump produces just 50 to 100 pounds per square inch pressure (which isn't much) it can easily raise the slab.
You see, a concrete slab that is four inches thick only weighs 1/3 pound per square inch! Now do you see why it is so easy to raise a sunken or tilted slab?
Why did the Concrete Sink?
Good question!! There are all kinds of reasons. Usually, concrete slabs around your house tilt or sink because they were placed on fill dirt that was not compacted. This can happen just about anywhere.
For example, what about a sidewalk that is next to your foundation? How about a garage slab? What about a portion of a driveway or sidewalk that crosses a utility trench (one that contains a water, sewer, electric or phone line?) These situations almost always involve an excavation where dirt may have been simply dumped in after the work was complete.
Herein lies the problem. Buildings are built on dirt all the time. If done correctly, a house built on dirt will not settle or develop structural problems. The trick is to build the house on undisturbed dirt or soil that has good strength, or build it on compacted dirt that will support the structure.
How many times have you dug a hole and tried to put the same dirt back in? Have you ever been able to do it without much work? I didn't think so. When you dig dirt, you fluff it. That is the technical term. In other words, you introduce much more air space around each clump or dirt particle than it had in the ground.
So, if you or your builder simply dumps dirt into a hole, trench or alongside a foundation, it will always have too much air in it.
The process of compacting dirt simply removes this excess air. Many years ago, shepherds noted that the paths that sheep walked on were very firm. They figured out that the shape of the sheep's hooves did an excellent job of compacting the soil on the path. That is why that big round roller that you see behind a bulldozer is called a sheep's foot roller!
Mother Nature will compact dirt. But, she often takes a while to do it. The sewer trench in my front yard continues to settle even after 10 years. Gravity and rainwater continue to force out the air I dumped into the trench.
This type of settlement not only causes concrete to tilt inwards towards the foundation, but it also causes a majority of basement leak problems. People generally don't recognize the the slow process of compaction as performed by Mother Nature.
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