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.
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.
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.
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.