It's possible that you as a contractor will never have to face up to the problems that air entrainment can cause when it's either too high or too low in concrete. For you, reading this article will serve as background information. Hopefully this information also will cause you to think more proactively to resolve problems involving air entrainment before concrete is placed.
Air entraining admixtures can be troublemakers, but necessary ones. By a wide margin, they create more problems for ready-mix producers, contractors, and owners than all the other admixtures. Almost everything influences them: ambient and concrete temperatures, the time it takes a ready-mix truck to travel from the batch plant to the jobsite, mixing time, the configuration of the mixing drum and the condition of the mixing blades, the water-cement ratio of a mix, the type of portland cement used, and other admixtures (especially polycarboxylate superplasticizers), to name a few.
Owners might accept scaling in plain concrete but not for colored decorative concrete. This failure probably is due to low air entrainment.
When entrained air is too high, contractors can experience problems when they finish it, especially when troweling machines are used. Too much air also can cause problems when contractors stamp decorative patterns in concrete. Concrete installed outdoors in freeze/thaw climates without adequate air entrainment is especially susceptible to scaling during the first or second winter. Placing concrete with too much or too little air entrainment becomes the contractor's problem. Even if tests eventually determine that others are responsible, the concrete contractor is invariably the one to face an upset owner, experience cash flow issues, and possibly have to remove and replace work at their own expense. With this in mind, measuring and adjusting air entrainment when concrete (especially first loads) arrives on the jobsite becomes a way to manage risk.
Air and what it does
In simplest terms, air entraining admixtures are surfactants, such as soap. They interact with the alkalinity of concrete to produce huge volumes of small spherical air bubbles that typically range from 0.006 to 0.008 inch in diameter. Ideally they are spaced no more than about 0.008 inches apart or less. When air entrainment is added to concrete, there is some effect on slump, and compressive strength is reduced by about 500 psi in a standard 6-bag (564-pound) mix.
There are two kinds of air in concrete: entrapped and entrained. Entrapped air is a result of the mixing process. As concrete is being mixed about 1.5% air becomes entrapped. The bubbles are shaped irregularly and have no effect on the concrete's durability because they are too large, easily seen by the eye. Entrained bubbles are the opposite. They are spherical, are best viewed under a microscope, and provide durability under freeze/thaw conditions.
When you measure air in concrete, the percentage reading represents the total amount of air—entrapped and entrained air. For example, if the reading on an air meter is 6% air, it means that approximately 1.5% of the total is entrapped air and 4.5% is entrained air. Though readings are reported always as percentages, the percentage number isn't what really counts. It's the size of the entrained bubbles and the space between them that is important. This usually is determined by petrographic analysis of hardened concrete. So percentage numbers provide approximate results. Different mixes, depending on aggregate sizes and the amount of cement powder in a mix, have different percentage requirements. For example, one mix might need 6% air while another needs only 4%. In each case, the bubble size and spacing should be the same. When a pressure test isn't performed correctly, the error usually results in higher readings.
Air entraining admixtures are used for three primary reasons. They always are specified for concrete exposed to freeze/thaw conditions. Occasionally, they are used to prevent bleeding—water coming to the surface of freshly placed concrete (remember, water is required to make bubbles). They also are used to reduce unit weights of concrete, especially for lightweight aggregate mixes, reducing unit weight beyond what the aggregates can provide.
Types of air entrainment
There are two general classifications today—wood-derived acid salts (Vinsol resins) and synthetic resins. Vinsol resins have been on the market the longest and many ready-mix producers continue to use them because they are familiar with them. They work especially well with very low water-cement mixes, developing good bubble structures—and are ideal for road pavements. But when slump exceeds 6 inches, including placing slumps when superplasticizers are added, entrainment readings begin to decrease. Percentages also decrease the longer concrete mixes are in the truck. After an hour in the truck, concrete can lose as much as 1% air (0.25% every 15 minutes) and more admixture should be added.
Synthetic resins have exotic generic names such as “fatty acids,” “gum resins,” and “tall oils” and have been in the marketplace for only a few years. These admixtures can produce smaller bubble sizes spaced closer together than Vinsol resins, offering greater durability under freeze/thaw conditions. But they also entrain more air as slumps increase, and can cause readings to be excessive. For example if a wet load arrives on a jobsite with specified 6% air entrainment, or if your crew adds water to make 6-inch or higher slumps, air entrainment readings can increase to 20% and even higher, compromising other properties of the concrete. You should measure the air before placing concrete under these conditions.