Concrete installation, and where it will crack

What is concrete installation?

Concrete installation is the forming, pouring, finishing and curing of a slab - a driveway, patio, walkway, garage or shed pad. Most of the work happens before the truck arrives: excavating, compacting a base and setting forms to a drainage fall. Concrete shrinks as it cures and it will crack somewhere. Control joints are how the installer chooses where, and curing is what determines how strong the surface ends up.

Signs you need concrete installation

• The existing slab has cracks wide enough to catch a heel or wide enough for weeds
• The surface is flaking, pitting or shedding a sandy layer - the top has lost its integrity
• Sections have sunk or heaved so panels no longer meet flush
• Water pools on the slab or runs toward the house instead of away from it
• You are adding something new - a patio, a garage pad, a walkway, an equipment slab
• A slab is cracked in a spiderweb pattern across most of its area rather than in one line
• Reinforcement is visible and rusting where the surface has broken away

Seeing something not on this list? Browse concrete problems by symptom →

How concrete installation actually works

1. 1

   Layout, excavation and drainage planning

   The area is marked out and dug to depth, and the fall is set so water leaves the slab and moves away from the building. On a driveway that fall has to work with the street and the garage threshold at the same time.

   Watch forNo slope designed in, or slope running toward the house. Water directed at a foundation by a new slab is a far more expensive problem than the slab it came with.

2. 2

   Base preparation and compaction

   A layer of crushed aggregate is placed and mechanically compacted in lifts. This is what actually carries the load - the slab spreads it, the base supports it. Soft or organic soil is dug out rather than built on.

   Watch forBase material dumped and raked rather than compacted with a plate or roller, and compaction done in one thick layer instead of several. Uncompacted base settles under load, and a slab bridging a void cracks in a way no amount of thickness prevents.

3. 3

   Forms, thickness and reinforcement

   Forms are set to line and grade, then reinforcement - wire mesh or rebar on chairs - is positioned so it sits within the slab rather than on the ground. Thickness is specified for the load: a walkway and a slab that will carry a vehicle are not the same.

   Watch forMesh laid on the base and "pulled up" during the pour. It rarely ends up where it should, which means it is doing nothing. Reinforcement supported on chairs before the pour is visible on site and easy to check.

4. 4

   Ordering the right mix

   The concrete is specified for the job - strength, and in freezing climates an air-entrained mix, which builds microscopic air voids that give freezing water somewhere to expand into.

   Watch forWater added to the truck at the site to make the mix easier to work. Extra water makes placement faster and the finished concrete measurably weaker and more prone to surface failure. It is the most common corner cut in the trade, and it happens in front of you.

5. 5

   Placing, screeding and floating

   Concrete is placed, struck off level with a screed, then floated to bring a workable surface up. Bull floating happens early; further finishing waits until bleed water has left the surface.

   Watch forFinishing worked while bleed water is still standing. Working that water back into the surface is a direct cause of the thin, weak top layer that flakes away a few winters later.

6. 6

   Final finish and edging

   The surface is given its texture - a broom finish for traction outdoors, a smoother trowel finish inside a garage - and edges are tooled so they do not chip.

   Watch forA slick trowelled finish on an exterior slab. It looks good on handover and is genuinely slippery when wet or icy. Broom finish exists for a reason.

7. 7

   Cutting control joints

   Joints are cut or tooled to a depth that creates a deliberate weak line, spaced according to slab thickness, so shrinkage cracking happens along them rather than randomly. Timing matters - too late and the slab has already cracked on its own.

   Watch forJoints too shallow, spaced too far apart, or cut a day later. Any of the three and the slab cracks where it wants to. A cracked panel between correctly cut joints is a different conversation from a crack across the middle of an uncut slab.

8. 8

   Curing

   Concrete gains strength through a chemical reaction that needs moisture, not through drying out. Curing means keeping moisture in - a curing compound, plastic sheeting, or wetting - for a period after the pour.

   Watch forA slab left to dry in the sun and wind with nothing on it. It will be walkable on schedule and permanently weaker at the surface than it should have been, and there is no fixing that later.

What does concrete installation cost?

Flatwork is normally priced per square foot, with thickness, reinforcement, access for the truck, and how much excavation and base work is needed driving the spread. Removing an existing slab is a separate line. Decorative finishes cost considerably more than a broom finish. See the Cost Guide.

See the full concrete installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →