This photomicrograph of plastic air entrained concrete suggests how minute bubbles of air function as very small ball bearings, which make the concrete more workable than a non air-entrained mix (October 1959).
Admixtures are known from as far back as the ancient Greek and Roman civilizations, which used pozzolans mixed with lime to produce hydraulic cement. The Romans added a little imagination to the mix, having observed that substances such as blood, milk, and lard would improve workability by entraining air. Thousands of years later, the April 1976 issue of Concrete Construction lamented that “general acceptance of admixtures has been slow, partly because many are waste products [and] some were introduced without proper purification or control.”
That purported reluctance, however, did not deter the industry's innovative spirit. Over the past half-century, this magazine has reported on many admixture developments.
Admixtures specially formulated for use in pumped concrete lubricate the mix and facilitate its passage through the pumplines (February 1972).
October 1959—Air entrainment. “Of all the many products put on the market in the last few decades, one has risen head and shoulders above the others in its effectiveness. It is, of course, the air-entraining agent.” Three decades later saw the dawn of a better bubble. ... April 1988. A new, modified cocamide DEA air-entraining agent is reported to avoid problems found with traditional vinsol resin products, such as reduced concrete strength and fluctuations in air content. The new admixture improves the air void system by producing bubbles with thicker walls.
Flowable fill can be treated with an admixture to reduce water requirements up to 50% and increase material yield up to 30% (August 1993).
Thanks to the introduction of chemical pumping aids, pumping concrete has become economically attractive. Now, concrete can be pumped farther, faster, and higher, on the hottest and coldest days. In fact, “Concrete admixtures have met with such success in recent years that some ready-mixed concrete producers will now guarantee the pumpability of their products.”
December 1987—Foaming agent. Preformed foam, commonly used to make lightweight cellular concrete, is found to improve the properties of conventional concrete as well. The foaming agent improves workability and corrects problems caused by poor sand grading.
August 1992—Prepackaged admixtures. Engineers are beginning to approve the use of new bagged, dry-powder admixtures for onsite addition. The admixtures come in premeasured doses in water-soluble bags that are simply tossed into the hopper of a ready-mix truck and dissolve in the fresh concrete.
August 1993—Extended set-control systems. Chemical admixtures introduced a few years earlier can extend the setting time of concrete for hours or even days.
SCC is so fluid that its consistency is measured by slump flow rather than by slump. A technician lifts the slump cone and measures the diameter of the spread (January 2002).
July 1995—Midrange water reducers. A new class of water-reducing admixtures reduce the water-cement ratio and enhance hydration, and improve concrete strength, slump, and workability. These products also provide more predictable set times, and speed pumping and placement.
Although both of these specimens have approximately the same slump, the concrete on the left contains 1 1/3 gallons less water per bag of cement than that on the right due to the use of a water-reducer (July 1966).
July 1996—Shrinkage control. “At last, a liquid admixture that's designed to reduce materials' shrinkage due to drying.” Said to be the first of its kind for chemically altering the basic mechanism of shrinkage without adding expansive materials to the concrete, the admixture reportedly reduces 28-day concrete shrinkage by 50% to 80%, with reductions in ultimate shrinkage of 25% to 50%.
Concrete of 1 ½ inch slump (left) reached 8½ inch slump after a superplasticizer was added (March 1978).
As early as July 1966, Concrete Construction recognized that the use of calcium chloride, a low-cost, widely used accelerator, could contribute to corrosion of reinforcing steel or other metals embedded in concrete. In the early 1980s, the American Concrete Institute proposed stricter limits on the chloride used in concrete. This move, reported in the May 1982 issue, created an uproar, and the flurry of reader response prompted the editors to devote the entire October 1982 issue to the controversy. A summary follows.