Adding Water to Concrete
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.
Compressive strength and durability
When water is added to a load of concrete in excess of the design w/cm, the following performance characteristics of the concrete will be negatively affected:
Compressive strength is lowered
Resistance to freeze-thaw cycles is reduced
Resistance to damage from sulfates in soil and water is lessened
Permeability increases which reduces durability
The ability to resist corrosion of reinforcing steel is reduced
Figure 2 and Table 1 (from Table 6.3.4, ACI 211.1-91) indicate a typical relationship between w/cmand compressive strength but just as important may be the effect on the other performance characteristics of concrete related to durability. The American Concrete Institute in ACI 318, “Building Code Requirements for Structural Concrete,” uses the w/cm ratio as the primary concrete mixture parameter to achieve the minimum durability requirements for concrete in buildings. It states “the licensed design professional shall assign exposure classes based on the severity of the anticipated exposure of structural concrete members for each exposure category.”
ACI 318 then goes on to require that concrete mixtures comply with the most restrictive requirements for each exposure class according to a table which includes a maximum w/cm ratio and a minimum specified compressive strength (f’c). ACI 318 also includes minimum requirements of other parameters such as air content, cementitious materials types and limitations regarding the types, the use of calcium chloride admixtures, maximum water-soluble chloride ion content in concrete expressed as percent by weight of the cement and other related provisions.
While ACI 318 requires a maximum w/cm for durability, it has a companion specified compressive strength, f’c, for each level of w/cm. This is a recognition that w/c cannot be reliably measured and verified for conformance to the requirement, while compressive strength can (with test cylinders). ACI 318 provides commentary related to the requirement that concrete mixtures must be proportioned to comply with the maximum w/cm and the other requirements based upon the anticipated exposure classification of the structural concrete, as follows:
Maximum water-cementitious material ratios (w/cm) of 0.40 to 0.50 that may be required for concretes exposed to freezing and thawing, sulfate soils or waters, or for corrosion protection of reinforcement will typically be equivalent to requiring f’c of 5000 to 4000 psi, respectively. Generally, the required average compressive strengths, f’cr, will be 500 to 700 psi higher than the specified compressive strength, f’c.
The idea here is that specifying a compressive strength that will achieve the desired durability will automatically ensure that the maximum w/cm is not exceeded. ACI 318 goes on to caution the designer not to specify a w/cm and a compressive strength that are inconsistent, say w/cm of 0.45 and f’c of 3000 psi. Going back to Table 1, if you need a 0.45 w/cm mix for durability, you should specify concrete with a compressive strength closer to 5500 psi. Since the usual focus during inspection is on strength, a 3000 psi concrete mixture might result in concrete with a w/cm that is higher than desired and therefore a lower durability concrete.
Care is taken to proportion concrete mixes that will achieve desired performance characteristics of compressive strength and resistance to damage from freezing and thawing, exposure to sulfates, and corrosion.
Contractors and concrete producers must understand that these performance characteristics are vulnerable when additions of water are made above the design limitations. The project’s specified design strengths should closely relate to proven concrete performance at the maximum permitted w/cm.