In New York it's pronounced “strenth.” To concrete people it's compressive, flexural, tensile, or shear strength. To others it may mean muscle strength, the strength needed to rip a telephone book in two or open a jar, the strength to lift objects, stamina, the mental strength of deliberation or tolerance. Whatever it is, strength is usually confusing unless adequately qualified.

Strength is a measure of the resistance to a force. Concrete strength is usually expressed in terms of pounds per square inch (psi) whether compressive, flexural, tensile, or shear. In concrete, the term compressive strength can bring confusion. Was it determined using 2- or 4-inch formed or saw-cut cubes? 3x6-, 4x8-, or 6x12-inch cores or cylinders? Cast beams of varying sizes? How were they tested? After storage under water or in a fog room? After conditioning in sealed plastic bags? After air drying for a given period (at one of a variety of temperatures)? After onsite curing? All of these methods are allowed, and each represents concrete strength that is significant for a particular purpose. For example, to determine if you got what you ordered (the mixture you are paying for), the strength you want is usually the compressive strength of samples cast in the field, conveyed to the laboratory, moist cured at 70° F, tested at 28 days, and compared with the design strength requirement, which engineers call f'c (f prime sub c).

If you want more realistic in-situ (field) strength at 28 days after concrete placement, cylinders cured at the jobsite are preferable. If you want to determine “true” concrete strength, cores tested at the “actual” moisture condition of the concrete will provide that—but there is still the question of what core size yields results that are most representative of the concrete in the structure.

Generally, all other things being equal, specimen configuration and size dictate the comparative strength. For example, as the ratio of cylinder length to diameter decreases or as specimen size decreases, strength becomes higher. However, there are unexplained exceptions. So the word strength alone doesn't do it.

Cylinders left in the environment of the concrete they represent will be much closer to the real field strength than specimens cured in a fog room. The time to strip forms or apply stress for prestressing are best determined by control cylinders cured alongside the parent concrete because the parent concrete must be strong enough to withstand forces created by those manipulations. Time dictates when forms can be removed and reused, and when prestressing beds are reused—the more often, the lower the costs. It's economics in action. Economics plays a major role and is equal to, but most of the time does not overshadow, the engineering aspects.

Sometimes strength is sacrificed for specific objectives like sound attenuation and fire resistance. That's where cellular (highly air-entrained) concrete that has greatly reduced compressive strength is used—300 to 1000 psi as measured by compressive testing of 3x6-inch cylinders moist cured for 28 days after heating at 140° F for three days. Thomas Edison invented cellular concrete in the early 1900s using aluminum powder (still used today). The aluminum reacts with alkalies released when portland cement hydrates and creates hydrogen gas bubbles that are trapped in the concrete, ending up as air voids. Edison used this material as insulation in the cast-in-place concrete homes he engineered and constructed.

With cellular concrete, the lower the strength the better, because concrete's sound attenuation and reduced heat flow properties become progressively better. In contrast to the typical minimum strength requirement for conventional concrete, a strength cap is sometimes needed for cellular concrete. Since the air voids provide the acoustical and thermal properties, as air content decreases, the density, unit weight, and strength increase at the expense of sound attenuation and thermal conductivity. So we can see that higher strength is not always better—also true for durability.

Engineering evaluations of low strengths may require repair or, as a last resort, concrete removal. That removal or repair usually is based on the strength of samples taken from the field concrete. For concrete slabs on grade, it is based on compressive strength; for airport runways it is based on flexural strength; for overlays, shear or tensile strength. The engineers decide what is acceptable or unacceptable, and their opinions may differ.

Bernard Erlin is president of The Erlin Company (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 47 years.

William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.