The real reasons concrete cracks in Los Angeles - expansive clay soil, fast curing in the dry heat, tree roots, and seismic movement - and which cracks are cosmetic versus which mean a slab is failing.
Some concrete cracking is normal and some is a warning, and in Los Angeles the difference usually comes down to the soil under the slab. Concrete is strong in compression and weak in tension, so anything that bends it - ground swelling, ground shrinking, a root, an earthquake - pulls it apart somewhere. The question is never really “why did it crack” but “is this crack cosmetic or is the slab moving.” Here is how to read yours.
Much of the Los Angeles basin and the San Fernando Valley sits on clay-rich soil that swells when it absorbs winter rain and shrinks through the dry summer. That yearly rise and fall lifts and drops whatever sits on it. A slab poured on a thin, poorly compacted base with no real joint plan has no way to ride that movement, so it cracks - classically straight across the middle of a driveway or diagonally off a corner. This is the crack that matters, because it is structural: the slab is being bent by the ground, and the crack will keep working open and closed with the seasons. The California Geological Survey maps the region's soil and seismic hazards, and expansive soil is exactly why a properly built LA slab needs the right base thickness, steel, and control joints.
Concrete does not dry, it cures - a chemical reaction needing water and time. In LA's dry heat, a slab whose surface flashes off too fast in the afternoon sun ends up with fine surface cracking (crazing) and can dust or spall later. These are usually cosmetic and are a finishing-and-curing problem, not a structural one, but they are a tell: a slab that was rushed and not kept damp through its first week was probably rushed elsewhere too.
The straight lines you see cut or tooled into a good slab are control joints, and they are there on purpose. Concrete is going to shrink slightly as it cures and move with the seasons, so we give it planned lines of weakness to crack along, out of sight in a groove. A crack that follows a control joint is the slab doing exactly what it was designed to do. A crack that ignores the joints and wanders across the field is the one to look at.
LA's mature ficus, pepper, and other street and yard trees are famous for lifting driveways and walkways. A root growing under a slab jacks it up, cracking it and creating a trip hazard. Separately, a slab poured over fill that was never properly compacted will settle unevenly and crack as one side drops - common where a patio or addition slab went in over a filled-in area without the base being built right.
Earthquakes get the blame, but a properly engineered, reinforced foundation is built to ride a quake as one stiff unit - that is what the rebar and tie-downs do. Seismic movement can crack under-reinforced or unpermitted concrete, but it is far down the list compared to the slow, relentless work of expansive soil.
Not sure which kind you have? Our Los Angeles concrete repair team will look at it and tell you honestly whether it is a seal-and-forget crack or a slab that is moving.
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