Should you pay attention to slump, water-cement ratio, or total amount of water in a mix?

We’ve all heard the saying, “With concrete there are two guarantees: it will get hard and it will crack.” How hard it will get and how much it will crack has a lot to do with the amount of water and cementitious material used to make it. Measuring Water in Concrete is important!

Water has always been the ingredient in concrete that contractors use to make concrete easier to place—the only ingredient they have control over on the jobsite. But almost everyone knows that adding too much water is bad because strength is reduced and more shrinkage results, causing additional cracking. So how much is too much? What’s the right way to specify how much water should be in a mix?

When you read articles about concrete or sit in ACI committee meetings, you hear these three terms: slump, water to cement (w/c) ratio, and “total water.” The terms often are used interchangeably. The assumption is that they all mean the same thing—a valid reference as to how much water is in a mix. But if they amount to three ways to say the same thing, wouldn’t it be best to discard two terms and refer to only one?

Changes in concrete mixes

Two developments further increase the confusion about water. One is the introduction of superplasticizing admixtures, also referred to as high-range water-reducing admixtures (HRWA), because they change the amount of water needed to make concrete easy to place. This is especially the case for the more recently developed polycarboxylate HRWAs. With no addition of water, the flowability of concrete can be changed greatly.

This concrete looks like the amount of water in the mix may be excessive but adding high-range water-reducing admixtures will produce the same look. Knowing the w/c ratio, or the total water in the mix, is the only way to know for sure. Photo: Joe Nasvik

The other development is the increasing interest in well-graded concrete mixes. They require less cementitious material which, in turn, reduces the amount of water that’s needed. Well-graded mixes are designed with several sizes of aggregates to reduce the volume of the open voids between aggregates, as well as the total surface area of aggregates. The net effect is that it takes less cementitious material to coat the aggregate surfaces and glue them together than the more traditional gap-graded mixes that use fewer aggregate gradations.

When you combine a well-graded mix with higher doses of polycarboxylate HRWAs and viscosity modifying admixtures (VMA), self-consolidating concrete (SCC) mixes result that change placing requirements. Projects such as the Trump Tower in Chicago (see “Reaching New Heights in Chicago” in the June 2007 issue of CONCRETE CONSTRUCTION) placed polycarboxylate treated concrete at slumps in excess of what was specified, but at w/c ratios lower than what was specified. The results were a better finished product placed on a faster schedule.

Defining terms

Here are the three ways water is specified for concrete mixes and how each is useful in terms of understanding the impact that water has on concrete.

Slump. When you want to know how much water is in concrete, your first question is probably “what is its slump?” Of the three ways to determine the amount of water in a mix (slump, w/c, or total water), it’s the only test performed in the field to provide a quick answer. But there are many things wrong with this test. It’s imprecise and relative at best. It’s possible to get different slump readings from the same batch of concrete. The age and temperature of concrete affect the results as well. High concrete temperatures, where hydration is developing quickly, result in lower slump readings than concrete at lower temperatures. Also the slump of very fresh concrete is higher than concrete an hour old. In both cases the amount of water in the mix hasn’t changed. You can continually add water to maintain a constant slump (a common practice), but this practice forever alters the w/c ratio.

The other problem with testing slump to measure the water content of concrete occurs when adding water-reducing admixtures to the mix design. Slump readings change dramatically when there is no change to water content at all. In the case of SCC, inches of slump are completely irrelevant. “Spread” is the relevant term—how far the mix spreads out horizontally after the slump cone is pulled. SCC mixes generally have spreads between 18 to 30 inches.

The reason that the slump test still is worthwhile is that it provides workers in the field with estimates of place-ability and consistency between loads. That is the reason the test was developed originally. It was never intended to be a measure of concrete quality. Concrete with 5- to 6-inch slump readings generally is considered to be good for placement, however, concrete with 6- to 7-inch slump commonly is considered by placing crews to be more desirable.

Concrete placing crews must have concrete that can be placed efficiently and slump is the relevant test. Additionally for wall placements, higher slump concretes make good consolidation possible with fewer bug holes.

Water-cement ratio. It’s referred to as the “water to cement” (w/c) or occasionally as the “water to cementitious” (w/cm) ratio when pozzolans are included in a mix. But the normal reference is w/c, which includes all cementitious materials. The w/c ratio is calculated by dividing the weight of the water in a mix by the weight of cementitious material. This ratio usually is calculated when a mix is designed and it provides clues as to what the resulting compressive strength and durability of the mix will be. We know, for instance, that concrete with w/c ratios that fall between 0.40 and 0.55 generally is considered to be concrete with a proper amount of water. For the protection of reinforcement against corrosion, w/c ratios should be closer to 0.40. Concrete exposed to freeze/thaw conditions should be around 0.45. Interior flatwork mixes are generally between 0.47 and 0.55. This variance recognizes the differences that aggregate types and gradations have on a mix.

But there can be problems with knowing what the true w/c ratio is for concrete on the jobsite. This includes unaccounted moisture levels in the aggregates used (moisture meters are not that precise) and the amount of water left in a ready-mix truck drum when it’s cleaned can be as much as 10 gallons.

Another problem with judging the quality of a mix by depending on its w/c ratio is that the amount of cementitious material can be adjusted upward or downward with the corresponding addition or deletion of water, while the w/c ratio remains the same. The concrete performance characteristics can change greatly and the w/c ratio won’t provide any information about that.

The architect wanted to see all the detail in the form boards and the rough spacing between them in this narrow column. Using self-consolidating concrete with a low w/c ratio met all expectations and solved placing problems. Photo: Jack Gibbons

On the jobsite, most construction workers don’t understand the relevance of w/c ratios, but they do understand why slump is important.

Total water. This refers to the total amount of water required for a concrete mix. There is beginning to be more references to total water as the measure for specifying water amounts. Factors that influence the amount of water needed include the following:

• Aggregate sizes and shapes
• Well-graded versus gap-graded mixes
• Total cementitious amounts, including types of cement
• Admixtures

Generally, good concrete has between 29 to 33 gallons of water per cubic yard. Strength and durability decreases as water exceeds these amounts. Specifying the total amount of water for a mix is very important for well-graded mixes that require less cementitious material for performance.

In the field the only way of Measuring Water in Concrete is by performing a microwave test, which is slow. It’s probable that concrete already will be in place before the test is completed. Also, most technicians are neither trained nor equipped to perform this test.

Using each method

Performing slump tests should only be used to provide information to the concrete contractor about the placeability of concrete. It also provides useful information about the consistency between loads of concrete because it doesn’t accurately define how much water is in a mix.

Whether you should think more in terms of w/c ratios or the total amount of water in concrete isn’t as clear as it is with testing slump. It’s probably best to be aware of both and how they change in relation to each other with different concrete mixes. Here are some thoughts about the relevance of each measure when you want concrete to have certain characteristics.

Shrinkage and curling. It’s important to have concrete for industrial and commercial floor construction that is resistant to shrinkage and curling over time. Reducing the amount of cementitious material and water is a primary way to achieve that goal. Concentrating on the total water in the mix is a good method.

Durability. Several things that can affect the durability of concrete, but water content is a central concern. Water not required for hydration occupies space in fresh concrete that later becomes a void when the concrete is hard. The voids reduce the strength properties of concrete. Specifying w/c ratios is a good way to address durability issues. Under ideal conditions, a w/c ratio of approximately 0.25 is all that’s needed for hydration so anything over that is considered “water of convenience.”

Flowable concrete. When concrete must consolidate well in forms or fill highly congested steel-reinforced beams and columns, SCC-type mixes are important. Designing these mixes with water measured by w/c ratios provides relevant information.

Compressive strength. Determining the amount of water that’s best for a specified mix can be evaluated by either w/c ratios or total water.

Water often is referred to as the cheapest admixture for making flowable concrete. But look at what small additions of it does to hardened concrete strength. Photo: Jack Gibbons

Finishability. The amount of fines, cementitious material, and water in concrete all play a part in how well it can be finished. If there isn’t enough water, concrete can become sticky and hard to finish. Contractors experienced with low-shrinkage mixes think about total water requirements for their concrete. When several floor mixes are compared to each other, w/c ratios and total cementitious content may be more helpful.

Closing thoughts

In the past, it has been said that 33 gallons of water was needed to make concrete. But today with changes in technology and mix designs, the old rules no longer apply. Mixes can have as little as 29 gallons (27 gallons with rounded gravel aggregates) or more than 33 gallons to produce good concrete for an application. We used to think that the placeability of a mix stood at one end of a continuum and strength at the other. But this isn’t necessarily the case.

Today, the best concrete results from the interaction between specifying engineers and contractors. Engineers should specify the qualities that are important, such as strength and durability, and contractors should work with their ready-mix producers to develop the mixes that will meet the specification. In terms of water, it’s important that all parties understand the ways to measure it and which way provides the most useful information for the job at hand. Measuring Water in Concrete still remains the most important part of the process.

Jack Gibbons

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