How It's Made: Concrete

Concrete is a mixture of cement, water, and aggregate (crushed stone and sand) that hardens into a solid slab. A concrete surface is made by preparing the site, setting forms, adding reinforcement, placing and finishing the mix, and curing it so it gains strength. The steps below describe how that process generally works. For a sense of the finished work, see concrete driveways.

What goes into concrete?

Cement is the binder. When it is mixed with water, it reacts chemically and hardens around the aggregate. The aggregate makes up most of the volume and gives concrete its strength. Some mixes also include admixtures, which are chemicals added to change how the concrete behaves. One common type is an air-entraining agent, which creates tiny air bubbles in the mix. Those bubbles give freezing water room to expand, which helps concrete resist damage from freeze-thaw cycles, a real concern in Ohio winters.

How is the site prepared?

Preparation begins with excavation to the required depth, then compacting the native soil (the subgrade) so it will not settle. A layer of crushed stone, called the base, is placed and compacted on top. Forms, usually made of wood or metal, are set to define the edges and thickness of the slab. The grades are set during this stage so the surface will shed water rather than hold it.

How is concrete reinforced?

Reinforcement is steel placed inside the slab to help control cracking. It is commonly steel bar (rebar) or welded wire mesh. The reinforcement is held in the middle of the slab's thickness by small supports, so it is not resting on the ground where it would be less effective. Joints are also planned at this stage. Control joints are grooves or saw cuts that encourage the slab to crack in straight, planned lines rather than randomly. Expansion joints separate the slab from fixed structures such as walls or curbs.

How is concrete placed and finished?

Concrete is delivered in a ready-mix truck and placed quickly, because it begins to set within a limited time. It is leveled with a straight board called a screed, which is pulled across the forms. A bull float, a wide flat tool, smooths the surface and brings a thin layer of paste to the top. Edges are rounded with an edger, and the surface is then finished. A broom finish, drawn lightly across the surface while the concrete is still workable, leaves a texture that improves traction. The right finish depends on how the surface will be used.

Why is curing important?

Curing means keeping fresh concrete moist and at a suitable temperature so the chemical reaction between cement and water can continue. Concrete that dries out too fast will be weaker and more likely to crack. In cold weather, fresh concrete must be protected from freezing until it has gained enough strength to resist damage. Foot traffic and de-icing salts should also be kept off new concrete for a period of time, as the contractor will explain.

How does Ohio weather affect concrete?

Ohio's freeze-thaw cycles are the main weather challenge for concrete. Water that gets into the surface can freeze, expand, and break off small pieces over time. The protections are a well-drained base, proper air entrainment in the mix, joints placed where cracks are expected, and a surface that sheds water. De-icing salt can also accelerate surface damage, so the choice of products matters.

Frequently asked questions

Why does concrete crack?

Concrete shrinks slightly as it cures, and the ground beneath it can move. Joints are designed to control where cracks form. A small crack is not always a failure, but it should be looked at so the cause can be identified.

Can concrete be poured in winter?

It can be, with cold-weather practices such as protecting the concrete from freezing and adjusting the mix. Planning matters, and the contractor should review the weather forecast before the pour.

Why does some concrete have a rough surface?

The texture is intentional in many cases. A broom finish gives better traction than a smooth trowel finish, which is why it is common on walks and driveways.

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