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I am using forms to build a decorative wall. The instructions say I need 38 liters total per form. The form ends up producing a wall section 200 centimeters long x 50 centimeters high.

How do I calculate how much sand, cement and water will produce 38 liters of concrete?

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A concrete mixture ratio of 1 part cement, 3 parts sand, and 3 parts aggregate will produce a concrete mix of approximately 3000 psi.

http://www.everything-about-concrete.com/concrete-mixing-ratios.html

In your case:

38l / 7 = 5.42l. This is one "part" of your total volume.

5.42l * 1 = 5.42l cement
5.42l * 3 = 16.26l sand
5.42l * 3 = 16.26l aggregate (gravel)

Obviously it's not common to mix concrete in such precise quantities. You'll probably use larger, round volumes such as shovels-full or 60 lb. bags as your "parts". For example, add 1 shovel of cement, 3 shovels of sand, and 3 of gravel to a mixing bin. Repeat this until you have the desired quantity of concrete.

Regarding water volume, that depends on the "slump", or flow quality, of the mix desired. Typically you slowly mix in water until the concrete is workable. Too much water weakens the resulting concrete and makes finishing more difficult.

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