Home and Garden | Mixing Concrete on Site - Masonry & Concrete

Concrete is a semifluid mixture of portland cement, sand (fine aggregate), gravel or crushed stone (coarse aggregate), and water. Whether you mix it yourself on site or have ready-mix concrete delivered, the ingredients must be present in the correct proportions. The amounts of the various ingredients are dictated by the quantity of concrete needed. Use the table in Figure 1, "Concrete Proportions by Volume," to determine the proportions of cement, sand, gravel, and water required. For estimating purposes, you can make about 1 cubic yard of concrete with five 94-pound bags of cement, 14 cubic feet of sand, and 21 cubic feet of gravel. (It takes about forty 80-pound bags of prepackaged materials to make 1 cubic yard of concrete.) Note that the volume of the combined ingredients is about one-third less than the sum of the volumes of the individual ingredients. For example, if you use 1 cubic foot of cement, 2-1/4 cubic feet of sand, 1-1/2 cubic feet of gravel or crushed stone, and 1/2 cubic foot of water, you get a combined total volume of 5-1/4 cubic feet. But the mix will actually produce 3-1/2 cubic feet of concrete (or two-thirds of the combined total).

Air-Entrained Concrete

Maximum Size
Coarse Aggregate, Inches

Number of Parts per Ingredient
Cement	Sand*	Coarse Aggregate	Water

3/8

2-1/4

1-1/2

1/2

2-1/4

1/2

3/4

2-1/4

2-1/2

1/2

2-1/4

2-3/4

1/2

1-1/2

2-1/4

1/2

Concrete without air

Maximum Size
Coarse Aggregate, Inches

Number of Parts per Ingredient
Cement	Sand*	Coarse Aggregate	Water

3/8

2-1/2

1-1/2

1/2

2-1/2

1/2

3/4

2-1/2

1/2

2-1/2

2-3/4

1/2

2-1/2

1/2

Note: 7.48 gallons of water equals 1 cubic foot. One 94-lb. bag of portland cement equals about 1 cubic foot.
* "wet" sand sold for most construction use.
The combined finished volume is approximately two-thirds the sum of the original bulk volumes.

If you need less than 1 cubic yard of concrete (or if ready-mix is not available) you can mix your own on site either by hand or using a concrete mixer. To make smaller batches of concrete, use the same proportions, but with smaller quantities, substituting buckets for cubic feet. (For the mix proportions given previously, you'd use 1 bucket of cement, 2-1/4 buckets of sand, 1-1/2 buckets of gravel, and 1/2 bucket of water.) For any batch size, the most important thing is to keep the proportions of the ingredients the same. You can double or triple the batch size simply by doubling or tripling the number of buckets of each ingredient you add to the mix.

For very small projects, such as setting a mailbox post or doing repairs, you may want to purchase a packaged concrete mix. Such a mix combines cement, sand, and gravel in the correct proportions and requires only the addition of water to create fresh concrete.

The most commonly available sizes are 40-, 60-, and 80-pound bags. The 40-pound bag makes about 1/3 cubic foot of concrete. A 60-pound bag makes about 1/2 cubic foot, and an 80-pound bag about 2/3 cubic foot. While convenient, prepackaged mixes are too expensive to use for all but the smallest projects. If you use premixed bags, you can pick them up yourself from the supplier or spend a little extra and have them delivered. If you pick them up yourself, make sure none of the bags has already hardened. However you get the concrete to your house, be sure to store it up off the ground (such as on a pallet) in a dry location. If the bags are stored outdoors, cover them with plastic sheeting or a waterproof tarp. If you live in a humid climate, buy the concrete no more than one or two days before you intend to use it.

PROJECT BASICS

Project: Moderate
Estimated Project Time: Less than 30 minutes for small amounts
Estimated Project Cost: $4.00 per 60-lb. dry mix bag of concrete
Start Tips: Make a shallow depression in the center of your dry mix, then pour in the water
Safety Tips: Wear long sleeves, long pants, rubber boots, heavy waterproof gloves, and safety goggles
Recommendation: Do-it-Yourself

INSTRUCTIONS

Mixing concrete by hand
Mixing concrete by hand is hard work. While you can mix the ingredients in a wheelbarrow, it's usually easier to mix them on a clean, flat surface, such as an old sheet of plywood, or in a mortar box (also called a concrete barge). When mixing concrete by any method, make sure to protect yourself by wearing long sleeves, long pants, rubber boots, heavy waterproof gloves, and safety goggles. If concrete accidentally spatters on your skin, wash it off immediately, as it causes burning irritation.

Fig. 1

1) Test water content of the sand
The sand used to make the concrete should be wet, but not too wet. As illustrated in Figure 1, sand with the correct amount of water will hold a ball shape without wetting your hand, very wet sand will make a ball but "leak" water, while dry sand will crumble and not hold a shape. If the sand is too dry, wet it thoroughly using a garden hose the day before beginning work. Always cover your sand pile with a sheet of plastic when you're not working, to prevent the sand from drying out.

Fig. 2

2) Measure the ingredients
For mixing purposes, you can measure proportions by the bucket, by the shovelful, or with a measuring box, like the one shown in Figure 2. A measuring box is built to hold exactly 1 cubic foot of dry ingredients, as opposed to a mortar box in which wet and dry ingredients are combined. Use the same amount of each ingredient for each batch. Careful measuring ensures correct proportions. Place the materials in layers on top of each other, beginning with the gravel, then the sand, and finally the cement.

Fig. 4

4) Add the water
Whether you use a premix bag or a combination of separate ingredients, the water used to make the concrete must be clean enough to drink. Start with a gallon of water and keep track of the amount you use so that you can add the same amount to subsequent batches. Make sure to mix all ingredients thoroughly, scraping any unmixed cement and aggregates from the sides and bottom of the box or pile. The concrete mix should be an even color and have the same consistency throughout.

If mixing on a flat surface, make a shallow depression in the center of the dry mix using your hoe; then pour in a little water (Fig. 4). Add the water slowly. Mix thoroughly by pulling dry material from the edges into the water. Continue to add water until the mix reaches the proper consistency--not crumbly, not sloppy. If using a mortar box, place the dry materials so they fill about two-thirds of the box (from one end), leaving the rest of the box empty (on the side nearest the forms). Add water to the empty end, then pull the dry materials into the water, mixing them together as you go. Continue adding water in small amounts while turning over the mix until it reaches the proper consistency.

Fig. 5

5) Test the mix
You can tell if the concrete has too little or too much water by using the blade of your hoe or shovel to make ridges in the concrete. If the mix is too dry, you won't be able to make distinct ridges; if the mix is too soupy, the ridges won't hold their shape (you'll also notice water seeping out around the edges of the pile). In a proper mix, the ridges will hold most of their shape. (Fig. 5)

Using a power mixer
If you don't want to mix your concrete by hand (or if your project uses air-entrained concrete or an air-entraining admixture) you can rent a small concrete mixer with a capacity ranging from 1/2 to 6 cubic feet. Electric mixers are quieter and simpler to operate than gasoline-powered mixers, but you must have access to an electrical outlet. If an extension cord is required, make sure the wire gauge is heavy enough to handle the ampere draw of the motor. The size of the concrete batch is usually only about 60 percent of the total capacity of the mixer to allow room for proper mixing without spilling. Never load a mixer beyond its maximum batch size.

Be sure to use the correct proportions of concrete ingredients. For best results, follow this procedure: with the mixer stopped, add all the coarse aggregate and half the water. Start the mixer; then add the sand, cement, and remaining water. After all ingredients are in the drum, continue mixing for at least three minutes or until the ingredients are thoroughly mixed and the concrete has a uniform color.

Thoroughly clean the mixer as soon as you have finished using it. Place water and a few shovelfuls of coarse aggregate into the drum while it is turning, to scour the inside of the mixer. Then dump out the water and gravel, and hose out the drum.