Concrete’s effectiveness depends on its ingredients and consistency. You don’t want a mixture that shrinks or becomes brittle; nor do you want it to be runny. There will be four basic materials you need in your mix: Portland cement, sand, aggregate and water.
Adding water will form a paste that will bind the materials together until the mix hardens. The strength of the concrete is inversely proportional to the water/cement ratio. In other words, the more water you use to mix the concrete, the weaker the concrete mix. The less water you use to mix the concrete, the stronger the concrete mix. A mix with little water and more concrete mix will be dryer and less workable but stronger.
But of course the water makeup isn’t the only consideration. The sand and the aggregate help to reduce the cost and also limit the amount of shrinking that happens to the concrete as it cures. In order to produce a strong, resilient concrete mix, you need to get the ratio of aggregate to sand to cement right. Consider the following formulas as you mix your concrete:
One standard recipe calls for one part of cement to two parts of sand to four parts of gravel. This results in a C20-rated concrete mix, which means the concrete will be of medium strength. Concrete is rated on a system that indicates the strength of the mix after it’s cured for approximately a month.
To make the concrete stronger, add more cement or less sand. The closer you bring the ratio to an even one-to-one of sand to cement, the stronger the rating becomes. This principles works in the opposite direction as well.
If you want to get a little more technical, some concrete experts recommend going for 26 percent sand, 41 percent gravel, 11 percent cement and 16 percent water. The lacking 6 percent volume is air entrainment. Air entrainment is an admixture added to the mix during production to assist the mix in resisting the damaging effects of freeze-thaw cycles. This admixture is required in all concrete exposed to exterior elements. Overall this makes a good general purpose mix for foundations and other structures.
While Portland cement is the standard for concrete mixtures, the type of sand you use may vary. Unwashed beach sand creates a mixture that isn’t quite as strong as products made with sand that’s been cleaned. Clean sand tends to produce a more high-quality product.
You can achieve an accurate mixing ratio by using buckets or other measuring devices to get the right quantity of each ingredient for your mixture. Getting the right ratios throughout the process means getting consistent mix throughout your whole concrete project.
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