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Concrete is very strong under compression, but relatively weak under tension. Concrete may need metal reinforcement to counter that, and a firm support beneath it. If the support beneath the slab is poorly constructed (soil not adequately compacted), the weight of the slab may compress the soil, and the slab will tilt. Portions may sag into the space created, and crack. Some soils, known as "shrink/swell" soils, will expand or contract as the water content changes- during very dry periods, some of the soil supporting the slab will contract, causing the slab to tilt. Movement of earth due to slippage or earthquakes will also cause tipping or sagging. Nearby construction may cause sagging, tilting or cracking, such as blasting, excavation, pile driving, or vibratory soil compaction. Concrete is mixed to different recipes- a mix that contains too much sand/ not enough Portland cement will be weak- as will a slab too thin for the weight load placed on it. You will need an experienced engineer to examine YOUR structure, and determine the causes of your problem. In SOME cases, a sagging slab may be respired by pumping concrete grout under the slab to push it back to a normal position.

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Q: Why homes built on a concrete slab begin to sag?
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