There’s no such a thing as the perfect concrete mix. There are many well-designed concrete mixes for a given application, but some are better than others.
You may think you have good concrete if it flows easily down the ready-mix truck chute, if it doesn’t scale and spall when winter comes, if cylinders taken at the jobsite turn out to have high compressive strength, or if it flows so freely in the forms that you don’t have to work hard to strike it off. You might be right, but the test of time will ultimately reveal how durable, and therefore how good, it really is.
Consider this definition of good concrete: It is the best possible compromise between plastic and structural properties, and durability. More specifically, what we want is concrete that:
There’s an adage in our industry: “Every task completed successfully leads to the successful completion of the next task.” Good concrete is the result of good equipment and good decisions at every step to create the concrete that arrives on your jobsite. As a concrete contractor your thoughts are focused on what comes down the chute. Here’s a story about what went on before the concrete got to the jobsite.
Starting at the quarry
Good concrete depends on good aggregate. Coarse aggregate can be as much as 75% of the volume in a mix. The best stone for making concrete is hard, durable particles with moderate absorption. The worst aggregate used to make concrete is friable, weakly cemented sandstones with high absorption. Where concrete is exposed to freeze/thaw cycles, lightweight cherts, ochres, shales, and weakly cemented materials such as conglomerate must be avoided.
ASTM C33 is a good guide for the maximum acceptable quantities of deleterious materials but even Class 5S aggregate for architectural exposure in severe weathering regions contains some frost-susceptible substances—crushed bedrock can avoid some of these problems. DOT specifications for bridge decks often require the best quality aggregate locally available—aggregate is very expensive to haul so your concrete depends largely on what’s available nearby.
But even if your region has the very best aggregate, what happens at the quarry determines how good it will really be as a concrete constituent.
Unstable material. It’s rare that all locations in a quarry produce the same quality aggregate. There are areas of higher and lower quality, so quarry management must constantly test to find the right quality material. Often there is a percentage of deleterious material or “chert”—a soft material which absorbs large amounts of water causing the aggregate to break apart or popout of the surface in freeze/thaw climates. ASTM C33 limits the amount of deleterious material permissible in aggregate but most aggregate has some present—even a little can cause problems, especially for decorative concrete.
Gravel or crushed rock. Gravel is mined as a mix of coarse and fine particles and is used in about half the concrete mixes in the U.S. Its edges are rounded due to abrasion from when it was being deposited. Crushed rock is blasted from solid rock deposits and crushed, the most common rock being limestone. There are areas of the country where only gravel or crushed stone are available.
Crushing. Different types and ages of crushing equipment are in use, the most common being jaw crushers, which tend to produce flatter more angular aggregate compared to conical crushers, which produce cubical stone shapes. Cubical shaped aggregate is the best for concrete.
Washing. In the mining and crushing process, aggregate gets mixed with fine particles (dust), some adhering tenaciously to the rock. This can create problems for good concrete, such as reducing bond to the cement matrix and higher water demand, so all coarse aggregate should be carefully washed. Aggregate containing as little as 0.5% clay fines can cause problems.
Pile management. When aggregate is piled, natural segregation occurs. To produce the best concrete, material handlers must work piles so as to deliver uniform gradations and moisture content.
Sand. Fine aggregate (sand) is added to a concrete mix to fill the spaces between large aggregate. Fine aggregate is also a blend of several sizes and the most common problems occur when some sizes are missing. Not all sand makes good concrete. For instance, beach sand doesn’t work well because its particles are too smooth and round, making it more difficult to achieve a good bond with cement paste. Sand must also be washed to eliminate dust that can reduce bond and make the concrete “sticky.”
Proportioning concrete mixes
Given the need to use what’s available locally, the first decision is whether your aggregate will be gap-graded or well-graded. Most mixes are gap-graded, meaning that certain aggregate sizes are purposefully left out, usually by having only one size of coarse aggregate. Well-graded mixes have some proportion of aggregate at all sizes from coarse to fine. Well-graded mixes have a higher percentage of aggregate than gap-graded mixes, meaning there is less cement paste and thus less water needed. This helps to produce high-strength concrete with less shrinkage, better workability, and higher durability. Think about it—the aggregate can’t shrink, only the paste, so if there’s less paste, there’s less shrinkage.
In the past, you may have heard that the best concrete is made with all-portland cement—fly ash being a way to make cheaper concrete. In actuality, the reverse is true—pozzolans or supplementary cementitious materials (SCM), such as fly ash, slag, or silica fume, work in combination with portland cement to produce concrete that is superior to what portland cement can achieve on its own.
Recently, in the spirit of producing sustainable concrete mixes (lower carbon footprint), specifiers will often require a 20% or higher replacement of portland cement with fly ash. This is a backward way to design a mix and you shouldn’t buy into that kind of reasoning. Rather, work with your ready-mix producer to design your concrete so the advantages of each ingredient are optimized. The goal should be a durable concrete with fresh properties suited to the installation methods.
In the good old days, there were negative attitudes about using admixtures, too. Contractors feared that admixtures were a way to reduce cost and sacrifice placeability. To contractors, water was the magic admixture—add water to extend freshness and to make the concrete easy to strike off. This is still sometimes the approach for residential concrete work where testing and supervision almost never happen.
But admixtures are essential to make good concrete. For instance, everyone involved with concrete in northern climates knows air-entrainment (created by using air-entraining admixtures) is vital for exterior concrete to prevent scaling and spalling due to freeze/thaw. But trouble results when this admixture produces bubbles of the wrong size or when the spaces between the bubbles are too great. Discuss the use of polycarboxylate superplasticizers, viscosity modifying admixtures (VMA), and set-retarding/accelerating admixtures with your ready-mix producer. Think of admixtures as smart concrete management tools.
Good concrete depends on the right blends of quality ingredients, but the actual making of the concrete is very important, too. You need good equipment to get consistent quality. It’s important that the producer has good control of water additions and can get the proper blending of the materials.
Ready-mix producers make concrete by two different methods: placing dry ingredients, water, and admixtures into a ready-mix truck that mixes on the way to a jobsite; or by using central mix plants to premix concrete before it’s placed into the truck for transport.
When concrete is mixed in the truck, the condition of the blades, or fins, is important because they do the mixing. When concrete is very important to a project, inspectors often inspect a truck’s drum, certifying the truck for use on the jobsite. Worn blades won’t always achieve complete mixing.
Central mixing plants make the best concrete because the fins in the drum are arranged for optimal mixing, moving concrete toward the back of the drum to achieve better blending action. That means that the central mixer can’t spin the drum backwards to discharge the concrete into the truck, so it must be tilted so concrete pours out by gravity into the truck.
Good concrete at the plant is the result of dispensing the right amount of each material in the right sequence into the mixer. But when truck drivers leave water in the drums from the previous wash-out, the next load is compromised.
Every ready-mix company should have a quality control program and at least one full-time person responsible for it. You can’t produce good concrete without this step. Unless you monitor and test each material used to make concrete and check the product just before placement, you can’t know if you are meeting the requirements and being consistent.
Testing at the ready-mix plant should include:
Contractors often regard the jobsite testing companies representing the owners with disdain and suspicion. If the concrete doesn’t measure up, they lose money—either by underuse of labor or battles related to payouts afterward. Without testing, good concrete can be compromised. Make the jobsite inspector’s life as easy as possible—provide good locations for testing and places to position curing boxes (see “Managing Concrete Test Data,” June 2011). The test results will be better and you’ll have fewer problems with out-of-tolerance concrete.
Testing on the jobsite typically includes:
Like many ready-mix producers, Cemstone, located in Mendota Heights, Minn., often assigns quality control staff to monitor concrete placements on key projects, or on smaller projects when issues arise. They test alongside the owners’ testing company and have the authority to quickly respond when loads fall outside certain parameters. They also start each day by checking loads as they leave the batching facilities and again at the jobsite to ensure that the concrete arrives in good condition.
Placing on the jobsite
Good concrete is the result of a partnership between a quarry, the ready-mix producer, and the contractor placing the concrete. But responsibility for the final quality and durability falls on the contractor. They decide when to pour, control how long it takes to unload a truck (the age of a load has an enormous impact on results), and the amount of water and admixtures added to a load onsite—perhaps the most critical factor affecting quality.
Concrete contractors decide whether or not to pour concrete based on weather conditions. Weather can be the biggest risk of all: rain, freezing conditions, and combinations of wind, low relative humidity, and temperature can ruin concrete, whether it was good or bad coming down the chute. Many contractors don’t know when surface crusting or excessive drying conditions are present so they aren’t prepared to make decisions that could save their work. Consider using a handheld weather instrument to record jobsite conditions to help you know when you’re in the danger zone defined in the famous Menzel/NRMCA nomograph to estimate evaporation potential.
Some ready-mix producers, such as Cemstone, work with their customers to help them manage concrete placements through the use of admixtures and mix changes. For instance, if it’s known that a placement will take a long time, retarders can be added to keep concrete fresh and reduce slump loss. If it must be hauled long distances, using admixtures to initially retard and then accelerate set once it arrives on the job can make the difference. Meet with your ready-mix QC personnel before the job to work out these details.
Concrete can succumb to an amazing number of illnesses. Viewed this way, making good concrete is more of a miracle than a routine procedure. But most of the variables can be controlled so concrete is used in almost every structure built. It’s the second most commonly used material on the planet, next to water. Good concrete is the result of good planning involving everyone in the chain.
Kevin MacDonald is the vice president of engineering services for Cemstone, Mendota Heights, Minn.
When concrete is very important to a project, the ready-mix QC staff often work together with the owner’s testing company to check the concrete. The QC staff can quickly call order corrections from the job as testing indicates.
If you want to make bad concrete it’s not too hard. You don’t need to follow all of the steps listed below—just one or two of them will do the trick. But here’s a list you can explore to find a way to fail:
Step 1: Fail to plan. If you have a plan, then you will have the proper equipment, labor, and materials ready at the time of construction to make good concrete. Failing to plan will result in poor finishing, poor grade control, and a mad dash to cure—or forgetting to cure all together. And of course the hottest, windiest days create the best cracks (as does adding the most cement you can get into the mix). Letting the truck sit on the job for a couple hours helps, too.
Step 2: Order the wrong concrete.Don’t consider the environment that the concrete is in or the weather conditions during placement. Air entrainment and a water-cement ratio below 0.45 might be good for skyscrapers, but sidewalks aren’t the Taj Mahal. Well-constructed concrete pavements resist deterioration for years to come, interfering with repeat business. Sure they might not use the same installer but if everyone pulls together, there can be replacement work for all. In cold weather climates, choose the wrong aggregates. Weak and porous ones will cause popouts, while concrete with hard, durable aggregate or 100% crushed bedrock will stand up for years.
Step 3: Use dihydrogen oxide.Once the concrete shows up, it probably isn’t ready for placement. The best finisher’s helper has a foreign sounding name—dihydrogen oxide, but you probably know it by its nickname “water.” Concrete with lots of water runs easily down the chute, fills the forms, and practically levels itself—add as much as you can. It slows the set when it’s hot out and floats and trowels easier in any weather. If there isn’t enough water, concrete won’t bleed and without excessive bleeding, it’s really hard to float water into the surface, unless you spray a lot of it on the surface before you finish. This helps make the surface susceptible to freezing and abrasion damage, faster wear, and scaling.
Step 4: Avoid curing at all cost.Tell your concrete sales representative you want healthy concrete—not sick concrete that needs to be cured. Relate the one time you didn’t cure and it was fine, so obviously curing isn’t needed. Especially if you are pouring 6- to 8-inch slump concrete; there’s plenty of water in there already. We all know there are lots of other great ways to make bad concrete. If you come up with any new ones, let us know.