When making concrete it's important to use the correct concrete mixing ratios to produce a strong, durable concrete mix.
To make concrete there are four basic materials you need: portland cement, sand, aggregate (stone), and water. The ratio of aggregate to sand to cement is an important factor in determining the compressive strength of the concrete mixture.
A concrete mixture ratio of 1 part cement, 3 parts sand, and 3 parts aggregate will produce a concrete mix of approximately 3000 psi.
Mixing water with the cement, sand, and stone will form a paste that will bind the materials together until the mix hardens. The strength properties of the concrete are inversely proportional to the water/cement ratio.
Basically this means the more water you use to mix the concrete (very fluid) the weaker the concrete mix. The less water you use to mix the concrete (somewhat dry but workable) the stronger the concrete mix.
Accurate concrete mixing ratios can be achieved by measuring the dry materials using buckets or some other kind of measuring device. By measuring the mixing ratios you will have a consistent concrete mix throughout your entire project.
To produce a 3000 psi cubic yard of concrete (27 cubic feet) the concrete mixture ratio is:
1. 517 pounds of cement
2. 1560 pounds of sand
3. 1600 pounds of stone
4. 32  34 gallons of water
This mixing ratio will give you a concrete mix that is strong, durable, and good for most concrete projects. A cubic yard of concrete will fill an area 8 feet wide by 10 feet long by 4 inches thick, or 80 square feet @ 4 inches thick.
At 6 inches thick a cubic yard of concrete will fill an area 52 square feet and at 5 inches thick, it will fill an area that's 65 square feet.
To produce a cubic yard of 4000 psi concrete, you have to adjust the concrete mixing ratio to:
1. 611 pounds of cement
2. 1450 pounds of sand
3. 1600 pounds of stone
4. 33 35 gallons of water
As you can see a little more cement and a little less sand is required to produce this stronger concrete mix that is great for driveways, pooldecks, sidewalks, exterior patios, and commercial garages.
Knowing the weight of the materials and how much water to use should help you determine how much cement, sand, and stone to purchase to complete your project.
For estimating purposes, you can make about 1 cubic yard of concrete with 5 1/2 94pound bags of cement, 17 cubic feet of sand, and 18 cubic feet of gravel. (It takes about forty 80pound bags of prepackaged materials to make 1 cubic yard of concrete.)
If you need less than 1 cubic yard of concrete (or if readymix 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, 3 buckets of sand, 3 buckets of stone, 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 mixing ratios and requires only the addition of water to create fresh concrete.
These concrete mixture ratios are designed for the concrete to reach full strength at or around 28 days. Curing the concrete can be done by keeping it wet after the first day until the concrete is 7 days old. This is an important step to take for the concrete to achieve full strength by 28 days.
For more on concrete slabs see the information below:
POURING A CONCRETE SLABReturn from Concrete mixing ratios to Concrete slab
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