Concrete Materials and Mix Design
Understand what goes into concrete, how water changes its strength, and how to order the right mix and the right volume for a pour.
Table of contents
Concrete is the most used building material in the world, and most of what goes wrong with it is decided before the truck arrives. The right ingredients in the right amounts make concrete that lasts for decades. A few extra gallons of water, added on site to make it easier to place, can quietly take away much of that strength.
This guide covers what concrete is made of, how the mix is specified and how to order it. Structural mixes are specified by the engineer, and local code and the plans set the requirements for your job.
What You Will Be Able to Do
- Name the ingredients of concrete and what each one does.
- Explain why added water harms concrete and what to do when a mix is too stiff.
- Choose among the common cement types and admixtures.
- Read a mix order: strength, slump, aggregate size and air.
- Calculate cubic yards and order the right amount.
What Concrete Is
Concrete is a mix of cement, water, sand and stone. When water meets cement, a chemical reaction called hydration forms a hard paste that glues the sand and stone together. Concrete does not harden by drying out. It hardens by reacting with water, which is why keeping it moist after placing matters so much.
Do not confuse the words. Cement is the gray powder. Concrete is the finished product. Mortar is a mix of cement, lime and sand used to bond masonry.
Portland Cement
Portland cement is the usual binder. ASTM C150 defines the types you are most likely to see:
| Type | Use |
|---|---|
| Type I | General purpose |
| Type II | Moderate sulfate resistance, common as a dual Type I/II |
| Type III | High early strength, for fast form stripping and cold weather |
| Type V | High sulfate resistance for severe soils and water |
You may also see blended cements such as Portland-limestone cement (Type IL), which many plants now use in place of Type I. Fly ash and slag can replace part of the cement too. These can change how fast the concrete sets, so ask the plant.
Aggregates
Stone and sand make up about 60 to 75 percent of concrete by volume. Coarse aggregate is usually 3/4 to 1 inch at the largest. Sand fills the gaps between stones. Aggregate needs to be clean, hard and well graded. Clay, dirt and organic material weaken the bond.
Water
The water-cement ratio (w/c) is the weight of water divided by the weight of cement. It is the biggest single control on strength and durability. Lower ratios make stronger concrete.
| w/c ratio | Rough strength | Notes |
|---|---|---|
| 0.40 | About 5,000 psi | Stiff, very durable |
| 0.50 | About 4,000 psi | Common residential range |
| 0.60 | About 3,000 psi | Weaker and more porous |
These are rough values. The actual strength depends on the mix.
Why Added Water Is the Enemy
Water beyond what the mix needs does not stay in the concrete. It leaves behind tiny voids as it bleeds and evaporates. That means:
- Lower strength. A commonly quoted figure is a loss of about 200 to 400 psi for each extra gallon per cubic yard.
- More shrinkage. More cracking as the slab dries.
- Less durability. More permeable concrete, a weaker surface, and more scaling.
If the mix is too stiff to place, do not just add water from the hose. Ask the foreman, who can check the ticket. Many mixes have a small amount of water held back at the plant for adjustment, which is recorded on the ticket. A water-reducing admixture gives you more workability without more water.
Admixtures
Admixtures are chemicals added to change how concrete behaves.
| Admixture | What it does | Notes |
|---|---|---|
| Water reducer | More workable with less water | Standard in many mixes |
| Superplasticizer | Makes the mix highly flowable | For dense steel or tight forms |
| Air entrainer | Adds tiny air bubbles for freeze-thaw protection | Target about 5 to 7 percent air |
| Accelerator | Speeds setting in cold weather | Calcium chloride corrodes steel, so use non-chloride |
| Retarder | Slows setting in heat or for long hauls | Can raise form pressure |
| Fibers | Reduce plastic shrinkage cracks | Do not replace structural steel |
Air entrainment is needed for exterior concrete in freeze-thaw climates. Water freezing inside concrete needs somewhere to expand, and tiny air bubbles give it space. Without them, the surface will scale off after a few winters. Do not steel trowel air-entrained exterior slabs, as that can seal the surface and cause it to flake.
Calcium chloride is cheap and effective, but chloride corrodes embedded steel, so it is avoided in reinforced concrete.
How a Mix Is Specified
When you order from a ready-mix plant, you need these details.
| Item | What it means | Common value |
|---|---|---|
| Strength | Compressive strength at 28 days, in psi | 3,000 to 4,000 psi for houses |
| Slump | How much wet concrete sags in a test | 4 inches for flatwork, 5 to 6 for walls |
| Aggregate size | Largest stone | 3/4 inch |
| Air content | Percent of air | 5 to 7 percent outdoors in freeze-thaw areas |
Strength. Residential footings and foundation walls are commonly specified at 3,000 psi. Under the International Residential Code, the minimum can be as low as 2,500 psi where there is no weather exposure, and up to 3,500 psi where weathering is severe. Exposed flatwork such as driveways and garage slabs commonly needs more strength and air. The plan and code decide, and engineered work can go much higher.
Design strength is the strength the concrete should reach at 28 days. That is the standard age for testing. Concrete keeps gaining strength after that, more slowly.
Slump measures the workability and consistency of fresh concrete before placement. It does not measure strength directly. When a mix order says slump, it is telling the plant how flowable to make the mix. In the test, a cone is filled in three layers, each rodded 25 times. The cone is lifted, and the drop in the concrete's height is the slump. Tests follow ASTM C143 and must be run by a qualified person, usually within a few minutes of sampling.
Testing technicians also cast cylinders, which are crushed at 7 and 28 days to prove strength. Do not move or bump them.
Figuring Volume
Ready-mix is sold by the cubic yard, which is 27 cubic feet.
Formula: length (ft) x width (ft) x thickness (ft) / 27 = cubic yards.
Worked example. A slab is 20 feet by 20 feet by 4 inches thick.
- Convert thickness: 4 / 12 = 0.333 feet.
- Multiply: 20 x 20 x 0.333 = 133.3 cubic feet.
- Divide: 133.3 / 27 = 4.94 cubic yards.
- Add 5 to 10 percent for waste and uneven grade: about 5.25 to 5.5 cubic yards.
Running short in the middle of a pour creates a cold joint, which is a weak seam between old and new concrete. Ordering slightly high is cheaper.
For footings, multiply the cross section by the total length. A footing 16 inches wide and 8 inches deep and 120 feet long is 1.33 x 0.67 x 120 = 107 cubic feet, or 4 cubic yards.
Bagged Concrete
For a small pour under about 1 cubic yard, bags can make sense.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 pounds | About 0.6 cubic feet | 45 |
| 60 pounds | About 0.45 cubic feet | 60 |
Add water slowly, following the bag. Mix until the color is even and no dry pockets remain, usually 3 to 5 minutes in a mixer. Place it within about 30 minutes. Store bags off the ground and under cover. A damp bag that has lumps should not be used.
Reading the Delivery Ticket
Every truck comes with a ticket. Check it before the first chute of concrete comes out:
- Is this the correct mix? Strength, slump, air and aggregate should match the order.
- How old is the load? Batch time is on the ticket. Many specifications limit concrete to about 90 minutes from batching, but the spec for your job controls this. Hot weather shortens the practical time.
- How much water was held back? Any water added on site must be recorded and approved.
If the load is too old or the mix is wrong, stop and call the foreman before it goes in the forms.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Adding water to make placing easier | Weaker, more cracked, less durable concrete |
| Ordering exactly the calculated volume | Running short mid-pour and a cold joint |
| Skipping air in freeze-thaw climates | Surface scaling after a few winters |
| Using chloride accelerator with rebar | Corroded steel and concrete that spalls |
| Ignoring batch time on the ticket | Stiff concrete that cannot be finished |
Check Your Understanding
1. The driver says the load is stiff and offers to add 10 gallons. The truck holds 8 cubic yards. Do you agree?
Check with the foreman first. That water is more than a gallon per yard and will reduce strength. Only approved water within the mix's limits should be added.
2. You must strip forms quickly in cold weather. Which cement type helps?
Type III, high early strength. Ask the foreman or the plant, and never strip early without approval.
3. How much concrete do you need for a 12-foot by 30-foot slab that is 6 inches thick?
12 x 30 x 0.5 = 180 cubic feet. 180 / 27 = 6.67 cubic yards. Order about 7.
4. A driveway in Minnesota needs a mix. What must the order include?
Air entrainment, with a target of about 5 to 7 percent. Without it, freeze-thaw damage will scale the surface.
5. What does a 4-inch slump tell the plant?
How workable the mix should be. It does not specify strength.
Key Takeaways
- Concrete hardens through a reaction with water, not by drying.
- Lower water-cement ratio means stronger, more durable concrete.
- Never add water without approval. It costs strength and adds cracks.
- Concrete reaches design strength at 28 days.
- Exterior concrete in freeze-thaw climates needs entrained air.
- Check the ticket before the first chute comes out.