# How to make concrete

Make concrete by combining portland cement, sand, coarse aggregate, and water, then mixing until every particle is coated and the batch looks uniform. A common site proportion is 1 part cement to 2 parts sand to 3 parts gravel by volume, with enough water to reach a workable 3 to 5 inch slump.

## Short version

- Four ingredients: cement, fine aggregate (sand), coarse aggregate (gravel or crushed stone), and water.
- The 1:2:3 by-volume proportion is a site rule of thumb, not a verified strength design.
- Water is the variable that ruins batches — more water means more workability and less strength.
- Mix until color and texture are uniform, then place within the working time of the batch.
- Curing is part of making concrete. A well-proportioned batch left to dry out still ends up weak.

## What are the four ingredients in concrete?

Concrete has four ingredients, and each one has a published material specification behind it. Cement is the only one that reacts; the aggregates are inert filler that make up roughly three-quarters of the volume.

| Ingredient | Specification | What it does | Rough share |
|---|---|---|---|
| Portland or blended cement | ASTM C150, ASTM C595 | Reacts with water to bind everything together | 1 part |
| Fine aggregate (sand) | ASTM C33 | Fills the voids between the coarse stone | 2 parts |
| Coarse aggregate (gravel, crushed stone) | ASTM C33 | Provides bulk and most of the volume stability | 3 parts |
| Mixing water | ASTM C1602 | Drives hydration and sets workability | ~0.5 by weight of cement |
| Admixtures (optional) | ASTM C494, ASTM C260 | Adjust set time, workability, or air content | dosed per the supplier |

Aggregate quality matters more than most people expect. ASTM C33 covers grading, deleterious substances, and soundness — that is why clean, washed, well-graded stone produces stronger concrete than the dirty pit-run gravel that costs half as much. Organic material and clay coatings on aggregate interfere with the cement paste bonding to the stone.

For the cement itself, Type IL portland-limestone cement under ASTM C595 is now approved by all 50 state DOTs and the District of Columbia, and it produces up to about 10% less carbon dioxide than ordinary portland cement. It is a drop-in choice for most residential work.

## What is the difference between cement and concrete?

Cement is one ingredient; concrete is the finished material. Portland cement is a fine gray powder produced by burning limestone and clay in a kiln and grinding the result with gypsum. On its own it is a binder, not a building material.

Concrete is what you get when cement, water, sand, and stone are combined and the cement hydrates. The reaction between cement and water forms crystals that lock the aggregate together, and that reaction is what gives concrete its strength. It is not drying — hydration needs water present to continue, which is why fresh concrete gets kept damp rather than allowed to dry.

Two related terms come up constantly. Mortar is cement and sand with no coarse aggregate, used to lay block and brick. Grout is a flowable cement mixture used to fill cores and voids. Neither one is concrete, and neither is a substitute for it in a slab or a footing.

Getting the vocabulary right saves money at the supply yard. Asking for "a yard of cement" when you need a yard of concrete is one of the more expensive mix-ups in residential construction.

## What mix proportions should you use?

Use 1 part cement, 2 parts sand, and 3 parts coarse aggregate by volume for general site work — a widely used rule of thumb that produces serviceable concrete for small non-structural jobs. Measure with the same container for every ingredient so the ratio actually holds.

Understand what that rule does not give you. A by-volume proportion has no verified compressive strength attached to it, because strength depends on the water-cement ratio, the aggregate grading, the cement type, the air content, and how the batch is cured. Ready-mix suppliers proportion by weight against a tested mix design and can certify a strength; a wheelbarrow batch cannot.

Adjust for exposure rather than guessing. ACI 318 requires air entrainment for exposure classes F1, F2, and F3 — freeze-thaw conditions — and site-mixed concrete without an air-entraining admixture will not meet that. Exterior flatwork in a freezing climate is exactly the case where hand-mixing is the wrong choice.

*General information, not engineering advice. Structural work should be designed by a licensed engineer.*

Keep hand-mixed concrete on jobs where failure is cheap: a fence post, a mailbox base, a small equipment pad, a repair patch. Footings, slabs, walls, and anything carrying a structure belong in a designed mix.

## How much water does a batch need?

Add the least water that still lets you place and consolidate the batch. Water-cement ratio is the strongest single lever on concrete strength, and typical structural mixes fall roughly between 0.40 and 0.60 by weight of cement. Lower ratios give higher strength and lower permeability; higher ratios give an easier pour and a weaker slab.

Judge workability by slump rather than by feel. Typical flatwork runs a 3 to 5 inch slump, which looks stiff to anyone used to soupy concrete. A batch that flows like pancake batter is over-watered, and the extra water leaves behind capillary pores as it evaporates — that is where the lost strength goes.

Remember that aggregate carries water of its own. Sand stockpiled outdoors after rain can hold several percent free moisture by weight, which is enough to move a batch two inches of slump. Wet the aggregate down evenly and cut the mix water when the sand is visibly damp.

If a batch stiffens before you finish placing it, use a water-reducing admixture rather than adding water. Retempering with a hose is the single most common way a good proportion turns into weak concrete.

## How do you mix and place a batch?

Mix in a rotating drum mixer whenever the batch is larger than about two cubic feet, and reserve hand mixing in a tub or wheelbarrow for patches. Load in this order: about half the water, then the coarse aggregate, then the cement, then the sand, then the rest of the water as needed.

Mix until the batch is uniform in color and no dry pockets show at the bottom, usually three to five minutes of drum time after the last ingredient goes in. Undermixed concrete has cement-rich and cement-poor zones that show up later as scaling and surface dusting.

Place within the working time of the batch. In hot, dry, windy weather that window closes fast, and concrete that has begun to stiffen should be discarded rather than rewetted. Consolidate as you place — rod it or vibrate it against the forms so trapped air escapes and the mix makes full contact with any reinforcement.

Then cure it. NRMCA's Concrete in Practice guidance calls for a minimum of 3 to 7 days of moist protection, and the specified strength is verified at 28 days under ASTM C39. Curing is not an optional finishing step; it is the second half of making concrete.

## When should you buy bagged mix or ready-mix instead?

Buy a bagged mix for anything under about one cubic yard, and order ready-mix above that. Bagged concrete mix already contains proportioned cement, sand, and aggregate, so the only variable left is water — which removes the most common way a site batch goes wrong.

Bag yields come from manufacturer technical data sheets and vary by formulation. As published figures, an 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 50 lb bag about 0.375 ft³. One cubic yard therefore takes roughly 45 bags at 80 lb — about 3,600 pounds of material to move by hand.

Order ready-mix when you need a certified strength, air entrainment, a large monolithic placement, or all of the above. Ready-mixed concrete is specified under ASTM C94, which sets what the supplier must deliver, including strength, slump, and air content tolerances.

The dividing line is really about consequences. Mixing your own is fine when nobody is relying on a strength number; the moment a design or an inspector is involved, buy concrete that comes with test data.

## Common mistakes

- **Adding water to make placing easier** — every extra gallon raises the water-cement ratio and permanently lowers strength and durability.
- **Using dirty or unwashed aggregate** — clay coatings and organic material block the paste from bonding to the stone, so the concrete crumbles at the aggregate interface.
- **Hand-mixing exterior flatwork in a freeze-thaw climate** — site batches have no entrained air, and ACI 318 exposure classes F1 through F3 require it.
- **Measuring by shovelful instead of by container** — shovel loads vary enough to move a 1:2:3 proportion well outside its intended range.
- **Skipping the cure** — a correctly proportioned batch that dries out in the first 3 days loses surface strength and dusts and scales later.