Working With Concrete

Concrete is a mix of cement, sand, and gravel (aggregate), and water.  A typical concrete ratio is 1:2:4. One part cement to two parts sand to four parts aggregate.


Cement
Cement is time-sensitive.  Be sure to store the cement in a dry location, protected from rain and potential flooding.  Store the cement in plastic bags, stacking vertically to get as much of the cement off the ground as possible.  The cement can pull moisture from the ground.  Older cement can still be used, but more cement is needed for the same equivalent strength.  This is outlined in the gravity flow handbook.

Sand
Avoid using beach sand when making concrete.  The shape of the grains of sand are rounder and reduce the overall strength of the concrete.  Instead, look for river sand clean of debris/silt or purchase sand from a local distributor.  Be careful storing sand as a heavy rain can wash it away.  Store on tarps or in sacks with an adequate cover.

Water
Use freshwater as saltwater can affect the curing of the concrete.  The appropriate amount of water is such that the concrete is smooth and plastic.  It should not appear runny nor so dry that it crumbles.


After mixing concrete, always cover the concrete with a plastic sheet to slow the curing process.  This will help strengthen the concrete.  Keep the concrete moist by adding water for the first 12 hours.