Why Concrete Cracks in the Bay Area (and What Prevents It)

Concrete Will Move — the Question Is How Much

Every concrete slab moves a little over its lifetime. It shrinks as it cures, it expands and contracts with temperature, and it responds to whatever the ground beneath it is doing. A few fine hairline cracks are normal. Wide cracks, uneven edges and sections that sink or lift are a different story, and they almost always trace back to what happened before the concrete was poured.

If you understand the handful of things that cause serious cracking, it becomes much easier to judge a contractor's plan for your driveway or patio.

Expansive Soil: the Number One Cause

In the Bay Area, the number one cause of concrete cracking is expansive soil moving under the slab. Expansive soils contain clay that swells when it gets wet and shrinks as it dries out. Over wet and dry seasons, that movement pushes up on a slab in some spots and leaves it unsupported in others. Concrete is very strong under compression but much weaker when it is bent, so uneven support is exactly the condition that breaks it.

You can't stop soil from reacting to moisture. What you can do is put a stable layer between the soil and the slab, reinforce the concrete, and manage the water that drives the movement in the first place. How much of each is needed depends on your soil, slope and use.

Base Rock and Compaction

Base rock is the layer of compacted aggregate that sits between the native soil and the concrete. It spreads loads, gives the slab even support, and helps separate the concrete from moisture changes in the ground below. The subgrade under it needs to be graded and compacted too; loose fill will settle later no matter what goes on top.

Skipping or skimping on base rock is a common shortcut in the industry. It saves time and material on the day of the job, and it usually doesn't show up until months or years later, when the slab starts cracking or settling. When you compare estimates, ask each contractor how they plan to prepare the subgrade and base.

Rebar Versus No Reinforcement

Reinforcement doesn't stop concrete from cracking, but it holds the slab together when it does, so a crack stays tight instead of opening up and letting the sections shift apart. Unreinforced slabs have nothing to keep pieces aligned once a crack forms.

Reinforcement only works if it ends up inside the concrete rather than lying at the bottom of the pour. At A·K we use rebar only for reinforcement, never wire mesh, and we place it inside the slab where it can do its job.

Control Joints

Control joints are the straight lines tooled or saw-cut into a slab. They create planned weak points so that when concrete shrinks and moves, it cracks along those lines instead of wandering across the surface. Joint spacing and layout depend on the size and shape of the slab, and a well-planned pattern makes the inevitable movement far less noticeable.

Where a slab meets a house, a wall or another fixed structure, an isolation or expansion joint lets the two move independently.

Drainage

Because water is what drives expansive soil, drainage is part of crack prevention. A slab should be sloped so water runs off the surface and away from the house. Downspouts, planters and irrigation that dump water along the edge of a slab keep the soil beneath it cycling between wet and dry. On some properties grading is enough; others need a broader drainage plan. Again, that depends on your soil, slope and use.

What to Look for in a Plan

A good concrete plan covers all five of these: an understanding of the soil, a properly compacted base, reinforcement placed inside the slab, a sensible joint layout, and a clear path for water. If an estimate is vague on any of them, ask questions before you sign.

A·K Landscaping & Concrete has been licensed (CSLB #823077) and based in Martinez since 1991, and our own crews handle every step. If you're planning a new driveway or patio, we're glad to come out and give you a free estimate.