While ready-mixed concrete can be easier for those without home improvement know-how, hand mixed concrete can be better for some jobs. All concrete is made of sand, aggregate and gravel, and hand-mixing concrete can allow you to add ingredients such as pigments or decorative gravel. When mixing concrete by hand, it is important to get the right mix of dry and wet ingredients.
The dry ingredients in most concrete are 3 parts gravel or coarse aggregate, 2-1/2 parts sand and 1 part Portland cement. All of these ingredients are available in individual bags where ready-mix concrete is sold. When mixing concrete by hand, you must first mix all dry ingredients with a small shovel or hoe in a large, clean container like a wheelbarrow or large plastic bucket. Make sure the fine dry ingredients like sand and cement are thoroughly mixed and the gravel is incorporated. Always reserve a small amount of mixed dry ingredients to add back into the concrete if you find you have added too much water.
One additive used in commercial concrete mixing is called fly ash. This is a fine particle aggregate like cement that helps the concrete mix and flow. Other additives include plasticizers that make the concrete flow, chemical additives to prolong drying time and powdered pigments to color concrete. Decorative stones or crushed glass can also be mixed into concrete to give it a unique look.
When it comes to mixing concrete, always use the least amount of water possible to make the cement into a cohesive material. More water makes concrete more prone to cracking and shrinking and gives the concrete less strength when compressed. To find the right ratio of cement to water, you must calculate the weight of the water and the weight of the cement. As a minimum, concrete needs a 20 percent ratio of water to cement, or 1 pound of water for every 4 pounds of cement used in the mix. Most commercial concrete suppliers use a 35 percent to 40 percent ratio.
Once your dry ingredients are mixed, make a well in the center of the dry concrete. Slowly add water into the middle of the dry ingredients, and pull in the dry ingredients from the side to mix. Keep working the water into the cement until it keeps its shape and there is only a little water visible. When perfectly hydrated, concrete should hold ridges made in it with a trowel or hoe.
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