What Is Concrete Made Of?

The four core ingredients

Almost all structural concrete is made from the same four ingredients. What changes between a garden path and a bridge deck is the proportion of each, the quality of the materials, and any admixtures added to tune its behavior.

• Cement — the binder, most commonly Portland cement, that reacts chemically with water
• Water — triggers the hydration reaction that hardens the cement paste
• Fine aggregate — typically sand, filling smaller voids and adding workability
• Coarse aggregate — typically crushed stone or gravel, providing bulk and much of the mix's compressive strength

Typical proportions by volume

Aggregate makes up the large majority of a concrete mix. The Portland Cement Association's reference text, Design and Control of Concrete Mixtures, describes normal concrete as roughly the following by volume. Exact numbers depend on the mix design, but the pattern is always the same: a relatively small amount of paste coating a large volume of stone and sand.

The water-cement ratio

The ratio of water to cement (by weight) is one of the most important variables in a concrete mix. A lower water-cement ratio generally produces a stronger, less permeable concrete, but also a stiffer, harder-to-work mix. Too much water weakens the finished concrete even though it makes the wet mix easier to pour and finish — a common on-site mistake.

Only part of the water is needed for hydration; the rest is there to make the mix workable. When that extra water eventually evaporates it leaves tiny pores behind, which is why a wet mix ends up weaker and more permeable. Most everyday mixes fall somewhere around 0.40 to 0.60, and building codes cap the ratio for severe exposures — ACI 318, for example, limits concrete exposed to freezing and thawing while frequently wet to a maximum of 0.45.

Worked example: a mix with 564 lb of cement per cubic yard at a water-cement ratio of 0.50 contains 564 × 0.50 = 282 lb of water — about 34 US gallons (282 ÷ 8.34). Add ten extra gallons at the truck and the ratio climbs to roughly 0.65, which can cost a large share of the design strength.

Fine and coarse aggregate

Aggregates are split at the No. 4 sieve (4.75 mm): material passing it is fine aggregate (sand), and material retained on it is coarse aggregate (gravel or crushed stone). ASTM C33 sets grading and quality limits for both.

A well-graded blend of sizes packs tightly, leaving fewer voids for the cement paste to fill, which improves strength and economy. The aggregate gradation article explains why, and the Sieve Analysis Calculator computes the grading curve and fineness modulus from lab sieve weights.

Admixtures

Beyond the four core ingredients, many modern concrete mixes include chemical admixtures — air-entraining agents (for freeze-thaw resistance), water reducers (allowing a lower water-cement ratio while keeping workability), retarders or accelerators (adjusting curing time), and others tailored to the project's needs.

• Air-entraining agents (ASTM C260) create billions of microscopic bubbles that give freezing water room to expand — essential for exterior slabs in cold climates.
• Water reducers and superplasticizers (ASTM C494 Types A and F) keep a mix workable at a lower water content.
• Retarders and accelerators (ASTM C494 Types B and C) slow down or speed up setting for hot or cold weather.
• Supplementary cementitious materials such as fly ash (ASTM C618) and slag cement (ASTM C989) replace part of the cement and can improve long-term strength and durability.

Nominal mix ratios for site mixing

Engineered ready-mix is proportioned by weight to hit a specified strength. For small, non-structural jobs mixed by hand or in a drum mixer, people often use a simple volume ratio of cement : sand : coarse aggregate, such as 1 : 2 : 3 or 1 : 2 : 4. These are convenient rules of thumb rather than a guaranteed strength, and they still depend on using the right amount of water.

Premixed bagged concrete does the proportioning for you. Use the yield printed on the bag to convert your volume into a bag count, since yield differs between products and bag sizes.

Why proportions matter for estimating

When you're calculating how much concrete you need with the BuildNumerics Concrete Calculator, you're calculating the total mixed-product volume — the cement, water and aggregate already combined. If you're mixing on-site from bags of cement plus separately purchased aggregate, you need each ingredient's quantity individually, which is a different (and more involved) calculation than ordering ready-mix.

That second calculation is what the Cement Calculator is for: it takes a concrete volume and a mix ratio and returns cement bags, sand and aggregate. Note that the dry ingredients occupy more volume than the finished concrete, because sand and cement fill the voids between the stones — ingredient calculations allow for this with a dry-volume factor.

Common mistakes

• Adding water to make the mix easier to place instead of using a water reducer or a higher-slump mix design.
• Using dirty or clay-coated aggregate, which weakens the bond between paste and stone.
• Omitting air entrainment for exterior flatwork in freeze-thaw climates.
• Assuming the finished volume equals the sum of the dry ingredient volumes.

Key takeaways

Concrete is mostly aggregate, bound by a thin coat of cement paste. The water-cement ratio has more influence on strength and durability than almost anything else you control on site. For how that translates into a strength rating, read Understanding Concrete Strength and Mix Basics.