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.

How much water is right?

First, let’s discuss what water’s role is in the process of cement hydration. The cement (and cementitious materials like fly ash) in the concrete needs water to hydrate and form calcium-silicate-hydrate (C-S-H) which is the crystalline glue that holds the concrete together. The water is chemically bound (consumed) during the reaction with the cement at approximately 25 pounds of water to every 100 pounds of cement. Therefore, it could be said that a water-to-cement ratio, w/c (or water-to-cementitious materials ratio, w/cm) of 0.25 is needed.

But that’s not all of the water that is needed. Additional water becomes physically bound between the cement hydrates. So to have enough water to enable complete hydration of the cement, approximately 20 more pounds of water is needed for every 100 pounds of cement. Combined, this means you’ll need 45 pounds resulting in a w/cm of 0.45. Other studies have shown that an approximate ratio of 0.40 was necessary for complete hydration of the cement. But concrete rarely gets the benefit of complete cement hydration because of the lack of physical access of the water to the inner unhydrated cement particles.

But the reality is that lower w/cm values enhance the strength and durability of concrete even though not all of the cement may have been hydrated. The reason is that with more water in the mixture comes greater dispersion of the ingredients which means less bridging of the C-S-H crystals can take place. The resulting concrete is therefore less dense, lower in strength, and higher in permeability (resulting in lower durability).

The dilemma that exists between wanting lower w/cm values, which result in a denser concrete, and having enough water in the concrete mixture for adequate workability and to optimize hydration is explored further in an insightful article entitled “Curing and Hydration - Two half-truths don’t make a whole,” written by Ken Hover in the L & M Concrete News in 2002. In that article, Dr. Hover states that the solution to the problem is:

  • Restrict mixture water content to bring the cement grains close together, and
  • Apply effective curing measures to minimize water loss, and whenever possible water-curet o externally provide the water needed to sustain hydration.
  • In order to determine the correct amount of water as part of the design process, the Portland Cement Association’s “Design & Control of Concrete Mixtures” states that a properly proportioned concrete mix should have acceptable workability when fresh; durability, strength, and uniform appearance when hardened, and be economical to make.