Business Articles - On the Job

Ordering Ready-Mix Concrete

The right admixtures can help you get stronger, more durable concrete — and make placement easier, too

by Don Kincaid

I’ve been producing concrete for more than 30 years at my company’s batch plant in Iowa. By now, I know just about what to expect from any mix my plant produces. If you know what you want to accomplish, I can tell you which concrete mix is appropriate.

Concrete in general is a pretty simple material, with just four main ingredients: cement, water, sand, and stone. Those elements mostly determine its characteristics. But most concrete also contains a variety of admixtures, which have an important effect on the results.

For best results, you should use admixtures where they’re appropriate. It’s important to understand all their effects because they can change the way concrete behaves. You may have to adjust your concreting practice to fit the characteristics of the mix you’ve ordered.

The Pozzolans
Most mixes we produce today include either fly ash or slag, which are types of pozzolans. Fly ash is a byproduct of coal-burning power plants, while slag is a byproduct of steel smelting. Finely ground, they can substitute for 15% to 50% of the cement in a mix. As waste products, they’re a lot cheaper than cement, and added to the mix they produce better concrete. They plug pores and strengthen the concrete matrix. Their rounded shape also lubricates the mix, allowing you to use less water.

Pozzolans reduce the heat of hydration, which can lengthen set times. That’s good in hot weather, when concrete sometimes sets too fast, but it can be inconvenient in cooler weather. Some water-reducing admixtures also have a retarding effect — the combined effects of a pozzolan and a water-reducer can stretch out your schedule by a couple of hours. Your ready-mix supplier can usually predict the set time for any mix design, so if it’s a concern, ask.

Air Entrainment
During mixing, all concrete will trap some air in the form of big bubbles. Those big bubbles aren’t good — we vibrate the concrete to get rid of them. But the tiny, well-spaced bubbles we call entrained air are desirable, especially in concrete that will be exposed to freezing weather, such as sidewalks, driveways, and garage slabs.

There is such a thing as air-entraining cement, but usually we just use an air-entraining admixture, aiming to get 4% to 6% entrained air by volume. The tiny bubbles make the concrete more workable and reduce the need for water, but, more important, those air-filled voids in the hardened concrete act as a safety valve when water in the concrete freezes and expands. The bubbles also block the flow of bleed water from the concrete during placement, which helps maintain good curing conditions and reduces the formation of bleed channels. That reduces the permeability of the end product.

When to avoid it. There are times when you don’t want air entrainment. I don’t recommend it for a finished floor indoors. If you’re going to try for a smooth, hard, finished surface using a steel trowel, air entrainment can be inconvenient. It makes the concrete sticky and causes pickups on the trowel, especially in the last couple of passes across the floor. Worse, it can fool the finisher about when it’s time to get on the slab and start troweling, due to the reduced bleed characteristics of air-entrained concrete.

Timing is everything. The reason for troweling a slab is to “tighten up” the top layer of cement. What’s happening is that as the cement particles hydrate and the little arms of crystal grow and intertwine, we are compressing and densifying the surface by driving air and water out of it, just like squeezing a sponge. We are destroying the air void system in the top surface. The idea is to let those particles interlock better and make that surface denser and less permeable.

But you have to wait for the right moment to start that. If you start when there’s bleed water on the surface, you’ll drive the water down into the top layer and actually weaken it instead of strengthening it. So you wait to start until after all the bleed water has evaporated off the surface and the slab is no longer shiny.

With air entrainment, bleed water escapes more slowly. On a hot, dry day, it might evaporate faster than it bleeds, and the surface will look dry even though there’s still a lot of water rising through the slab. If you steel-trowel too early, you’ll tighten up the top surface and trap the rising bleed water under it. Now you’ve created a water-saturated weak layer right under your finished surface. That could bring blisters and scaling problems in service (see Figure 1).

Figure 1. An air-entraining admixture can help prevent freeze-thaw damage such as the spalling shown (left). But entrained air slows the rise of bleed water, which can fool the finisher into starting to steel-trowel the surface before the bleed water has fully escaped. Trapping rising bleed water can create a weak layer just beneath the surface and lead to surface scaling (right).

You can do a good job finishing a slab with an air-entrained mix if you’re careful, but it takes experience to learn to judge the moment. I tell people to carry a plastic trash can lid with them. If you lay that lid down on the slab for five minutes to block evaporation, and there’s moisture under it when you pick it up, you still have bleeding going on and it’s too early to start finishing that slab.

While I avoid air entrainment for indoor slabs that will get a trowel finish, it’s critical for outdoor slabs that will see freezing temperatures. Still, it’s better not to polish those outdoor slabs with a trowel. You’ll get a tougher surface if you just bull-float and broom-finish — and then, of course, cure properly and seal the surface.

Water Reducers
There is a whole set of admixtures that reduce the amount of water needed in a batch. They act to break up clumps of cement particles so water molecules can reach the cement. Although they’re all water reducers, they fall into different classes because they don’t act the same. Some are used often and some seldom, but they all can be useful in the right situation.

Low-range water reducers. Type A, or low-range, water reducers lower the water requirement of a batch by 5% to 8%. This admixture is practically universal today — I use it in almost every mix I send out. The reason is economy: If somebody orders a 4,000-psi concrete, I can reduce my cement requirements by nearly half a bag and still get that strength by using a Type A water reducer. Lowering the water-cement ratio improves the concrete; with a water reducer, I can reduce both the cement and the water and keep the ratio the same. It saves on cement without sacrificing quality.

In fact, there’s a quality benefit: With less water and less cement, there’s less shrinkage and less potential for shrinkage cracking. Type A water reducer is cheap and effective, so even if you don’t ask for it, you’ll probably get it.

Superplasticizers. Type G, or high-range, water reducers, also called superplasticizers or just supers, reduce water requirements by as much as 30%. They can add five or six inches to the slump of a batch without increasing the water-cement ratio; or you can reduce the water and pour at a low slump, and drastically boost strength.

Super is expensive and you need to use a high dose of it, so it’s uncommon in residential work. It’s more often used in making extra-high-strength concrete for engineered projects like dams and high rises. But supers can come in handy on a small job, too. If you want flowing concrete that just streams into place without the rock separating out, superplasticizer gives you that. You’d use it to put the concrete around densely packed reinforcement. It’s also very helpful when you have to pump concrete.

Superplasticizers are added at the site (Figure 2). At the plant I would batch you a mix at about a two-inch slump; then the driver would add a measured dose of superplasticizer on site. That would take the slump up to six or eight inches or more in about three or four minutes (Figure 3).

Figure 2. Because the effects wear off quickly, the ready-mix driver must add superplasticizer directly to the truck just moments before starting the pour.

Figure 3. Concrete that has been proportioned for a 2- or 3-inch slump (top) can be knocked down to a 6-inch slump with the addition of superplasticizer (middle), while still keeping the high strength and durability that come from a low water-cement ratio. Or concrete proportioned for a 6-inch slump can be knocked down to a 9- or 10-inch slump (bottom), making the material highly flowable and almost self-leveling while still achieving normal strengths.

Don Kincaid
This article has been provided by JLC-Online is produced by the editors and publishers of The Journal of Light Construction, a monthly magazine serving residential and light-commercial builders, remodelers, designers, and other trade professionals.

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