Concreto

Air entrainment and concrete
Some things every contractor should know about a combination that assures longer-lasting and more attractive concrete

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veryone likes a short cut. It’s just plain tempting. That’s why men once searched for a means of tra n s f o rm i n g base metals into gold. It seemed so much easier to make it than earn it. In the area of concrete construction, where muchdepends on k n ow - h ow, patience and careful w o rk m a n s h i p, it was only natural that short cuts should be offered in the form of admixtures. 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. Even this admixture is no wond e r- w o rk e r, howe ve r. It is notintended to replace correct procedures in mix design or concreting. Rather, it complements these quality concrete practices to produce a finished product that will be more durable and satisfactory to all concerned. Let’s take a closer look at this pre-eminent admixture.

This photomicrograph of air entrained plastic concrete suggests how minute bubbles of air function as very small ball bearingswhich make the concrete more workable than a non-air entrained mix.

What air does
The introduction of air into concrete mixes has some pronounced effects on the characteristics of both the plastic and the hardened concrete. In fresh concrete, the tiny air bubbles act as a lubricant in the mix which improves its workability and increases its slump. Also, in a sense, the bubbles function as athird aggregate. Because of their small size, the bubbles act as fines, thereby cutting down the amount of sand needed. Because air entrainment affords increased slump, it is possible to decrease the amount of water to get higher strengths without affecting workability. Less water means less drying shrinkage —al-

ways a desirable feature. Bleeding in concrete is cut approximately in half byentrained air. This reduces considerably the adverse effects of a higher water/cement ratio at the surface of slabs and of laitance forming on concrete surfaces. Air also produces stickier, more cohesive concrete; as a result, less segregation is experienced and more attractive surfaces are achieved. In hardened concrete, the outstanding attributes of air entrainment are the enhanced weatherabilityand resistance to scaling afforded. Damage to concrete by f re ezing and thawing is caused when enough ice forms in the capillaries to create a pressure greater than the tensile strength of the cement paste. This disrupts the capillary walls. Where enough air voids are present, howe ve r, hyd ra u l i c pressure created by freezing forces the remaining water into the air

voids. These air voids inair entrained concrete act thus as a safety valve by giving the water a reservoir into which to flow. When thawing occurs, compressed air in the voids forces the water back into the capill a ri e s, thereby freeing the voids for use again during the next freeze. Test sections of pavement in New York State built with and without air entrainment showed dramatically the value of air in exposed work.After 14 years of exposure to the rigors of this northern climate and frequent salting, the non-air-entrained concrete was badly scaled; in some cases 100 percent of the surface was affected. The air entrained pavement, laid next to the other sections, showed absolutely no scaling or D-cracking. During October 1942, 94 columns cast of concrete made with seve ra l types of cements and va ry i n gamounts of air entrainment were

set on the exposure rack at Treat Island, Maine. At the end of 1 year (188 freeze- thaw cycles) 10 percent of the columns had failed; none were air entrained concrete specimens. After 5 years of exposure (663 freeze-thaw cycles) 26 percent of the columns had failed; again, no air entrained specimens had failed. Application of deicers to pavements and other...