How to mix concrete?

Typical thickness

4 inches for residential flatworkslab

Typical strength

3ASTM C39 at 28 days

Common mix

3,000–4,000 psi21–28 MPa residential

Test age

28 daysASTM C39

Short version

• Concrete is cement plus water plus aggregate. Cement is the binder, roughly 10–15% of the mix by volume.
• The cement and water react in a process called hydration — concrete hardens by chemical reaction, not by drying.
• Strength is specified in pounds per square inch and tested at 28 days under ASTM C39.
• Residential work is usually 3,000 to 4,000 psi; exterior flatwork exposed to freezing needs 4,000 psi with air entrainment under ACI 332.
• Mortar and concrete are not interchangeable: mortar has no coarse aggregate and is meant to bond masonry units, not carry structural load.

What is concrete actually made of?

Concrete is four ingredients: portland cement, water, fine aggregate (sand), and coarse aggregate (gravel or crushed stone). By volume the aggregate dominates — typically 60 to 75 percent of the mix — with cement paste filling the spaces between the stones and gluing them together.

The cement is the reactive part. When water is added it begins hydration, a chemical reaction that forms interlocking crystals and progressively stiffens and strengthens the paste. Most modern mixes also contain admixtures in small doses to control workability, set time, or air content, and many contain supplementary cementitious materials such as fly ash or slag cement in place of some of the portland cement.

What is the difference between cement and concrete?

Cement is an ingredient of concrete, the way flour is an ingredient of bread. Portland cement is a fine gray powder, manufactured by burning limestone and clay at high temperature and grinding the result. On its own it does nothing structural.

Concrete is the finished material: cement, water, sand and stone combined. The everyday phrase “cement driveway” is technically wrong — the driveway is concrete. The distinction matters when you buy material, because a bag labeled portland cement contains no aggregate and will not make a slab on its own.

How is concrete strength specified and measured?

Concrete strength is specified as compressive strength in pounds per square inch and verified by breaking cylinders in a press. The test method is ASTM C39, using specimens cast and cured under ASTM C31, and 28 days is the standard age at which the specified strength must be reached.

Typical values follow the exposure. Interior slabs and footings in mild climates are commonly 3,000 psi. Sidewalks, patios, garage floors and residential driveways are typically 3,500 to 4,000 psi, and ACI 332 requires 4,000 psi with air entrainment for garage floors and exterior flatwork exposed to freezing or deicing chemicals. Structural columns and commercial members run 5,000 psi and above under ACI 318 design.

Does the answer change by state?

People search this question with a state attached — a bucket among them — and the underlying material behaves the same everywhere. What changes is exposure and jurisdiction: freeze-thaw cycles decide whether air entrainment is required, deicing salt raises the exposure class, and frost depth, permits and the adopted code edition are all set locally.

Requirements are set by your local building department, not by any national table. See the state pages for cost bands, exposure classes and the authority that governs your address.

Common mistakes

• Using “cement” and “concrete” interchangeably when buying material — a bag of portland cement has no aggregate and will not make a slab
• Assuming concrete hardens by drying — it hardens by hydration, which needs water present — drying it out early makes it weaker
• Substituting mortar for concrete — mortar has no coarse aggregate and is not formulated to carry structural load