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Have you ever been at a concrete placement when someone said, “How about adding some water to that load?” But is it acceptable to add water on site? Water addition to a load of concrete may or may not be acceptable depending on the parameters that need to be met. ASTM C94, “Specification for Ready-Mixed Concrete,” states the following regarding water additions:

If the desired slump or slump flow is less than specified, and unless otherwise stated, obtain the desired slump or slump flow within the tolerances stated in [applicable sections] with a one-time addition of water. Do not exceed the maximum water content for the batch as established by the designed mixture proportion. A one-time addition of water is not prohibited from being several distinct additions of water provided that no concrete has been discharged except for slump or slump flow testing. All water additions shall be completed within 15 min. from the start of the first water addition. The drum shall be turned an additional 30 revolutions, or more if necessary, at mixing speed to ensure that a homogenous mixture is attained.

This article will give you a better understanding of how performance of the concrete may be affected by water additions.

Slump and water addition

Concrete contractors will frequently add water to the load prior to or even during the unloading process to increase the slump and improve the workability. The rule of thumb is: One gallon of water will increase the slump of one yard of concrete by 1 inch. This is only a rule of thumb, though; conditions like temperature and air content will change the amount of water needed to increase concrete slump.

An important point in ASTM C 94 is that water should not be added after any significant quantity of concrete has been discharged from the mixer because the quantity of concrete being adjusted is uncertain as is the impact of the water addition on the concrete’s properties. ASTM C 94 permits the measurement of slump and air content from a preliminary sample from the initial portion of the discharge so that adjustments for slump and air can be made to a full load of concrete.