Back about 50 years ago there were some common “rules of thumb” about concrete. One was that 3-day strengths were about two-thirds of 7-day strengths, and 7-day strengths were about two-thirds of 28-day strengths.
Things are different today. Most portland cements are much finer than they were then, and they now contain much more gypsum. Concrete made today without supplementary cementitious materials (fly ash, ground granulated blast-furnace slag, silica fume) gains strength more quickly—the two-thirds might well be replaced by nine-tenths.
The strength is more difficult to assess when supplementary cementitious materials are used. For example, with fly ash, strength gain will usually be slower over a much longer period because of the slower reaction of the fly ash—so 60-day strength gains are used to gauge what was determined in 28 days. When ground granulated blast-furnace slag is used, temperature plays an influencing role in strength gain because of its effect on the speed of the chemical reactions that provide the strength. During warm summer temperatures, 3-day strengths may be equivalent to 7-day strengths, and 14-day strengths equal to or exceed what would be expected for 28-day strengths. At winter temperatures, 7-day strengths may be equivalent to 3-day strengths because of the sluggish chemical reactions.
Although Type F fly ashes slow strength development, they also do many good things, including lowering concrete costs, increasing resistance to water penetration (decreased permeability), increasing resistance to aggressive chemicals, and slowing and reducing heat development in mass concrete. Fly ash is a “patriotic” material because it extends the use of portland cement, which consumes a lot of energy in the making, and making use of this major waste product improves the environment.
Ground granulated blast-furnace slag is also a patriotic material for the same reasons as fly ash. Nowadays, concrete mixtures are often designed with two (ternary) or three (tertiary) supplementary cementitious materials. How patriotic can you get?
We recall a concrete foundation placed decades ago during low winter temperatures using a 25 percent fly ash replacement of the portland cement. This same mixture had worked well at summer temperatures, but when placed at winter temperatures, the concrete crumbled and fell away when forms were removed after a week. Why? A straight portland cement mixture would have developed enough heat during hydration to warm the concrete sufficiently to allow hydration to continue. But the portland cement diluted by the fly ash greatly curtailed heat development. A lesson learned the hard way shouldn't let you forget—but it still happens.
Although 28-day strength requirements are usually the controlling specification, unexpected things can sometimes happen. For example, unacceptable 28-day strengths can result because of cold days, excessive air contents, badly handled test specimens, poor testing procedures, and “accidental” over-dosages of admixtures. In many cases, petrographic studies and chemical analyses permit reasonable prognostications of future strength gain in cases of low strength. We recall a second-floor cast-in-place, thickened edge beam that someone called about because the concrete failed to harden. We found an overdose of a water reducer, which acts as a retarder when used in excessive amounts—more isn't always better. The soft concrete was well put together except for its non-strength. We told the contractor that if he kept the concrete curing, it would not only reach the intended strength but probably would be even stronger than it might have been under more normal circumstances. He did and it was. That edge beam now serves at the entrance to a major public building in Washington, D.C.
Strength “requirements,” of course, is a subject to itself. House foundation concrete, for example, may have a code requirement of 2000 psi (at 28 days). But the foundation will never carry a load greater than a few hundred psi. That looks like a significant strength safety factor; however, there are other things to consider so that 2000 psi may be adequate in strength but inadequate when environmental requirements enter into the picture. In fact, requirements other than strength usually do dictate.
(The topic of this column was suggested by reader Rick Seymour. Thanks, Rick!)
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 47 years.