Concrete Basics

4000 psi, 6 bag mix, fiber, low-chert ….
What Does This All Mean?

concrete-basicsConcrete is everywhere, we walk on it, drive on it, work and live on it. Concrete is literally the foundation to our civilization but like most people, concrete is not something you probably think about or care to learn. That is until it’s time to hire a concrete contractor to replace a driveway or install a new patio. What’s so complicated about it? It’s just concrete? Well, as one starts collecting estimates and talking with contractors, neighbors and friends, they start to realize there’s more to it than we know and what seemed like a simple and standard product to buy is not so simple.

Well don’t worry, we took the time to explain the basics of concrete construction that every home or business owner must be aware of in order to be an informed consumer. We hope this information will lead to a successful concrete project for you.

What Is Concrete?

Let’s start with the most common mistake people make and that’s calling concrete cement or using those terms interchangeably. Concrete is not cement and cement is not concrete. Concrete is a mixture of gray powder called “Portland Cement”, sand and gravel or crushed stone and water. So cement is an ingredient used to make concrete. It would be like calling cake, yeast. The “Portland Cement” reacts chemically with water and hardens over time – a process called “hydration,” Concrete doesn’t “dry;” it “cures.” Additives are generally included in the mixture to improve its strength, durability or workability.

Common concrete additives, also known as “admixtures”, to be aware of:

Water-reducers: Water reducers help with making the concrete more workable without needing to add more water to it. Whats wrong with water you ask? To much water added into the concrete will weaken the concrete. So using a water reducer helps to create stronger concrete.

Air-entraining: Air-entraing makes the concrete resistant to cycles of freezing and thawing, Air-entrained concrete additives, contain billions of microscopic air bubbles. These bubbles allow space for water to expand into during freezing tempatures. Without this space, the surface of the concrete will crack and pop. Air-entrained concrete is a must for all exterior concrete in order to withstand Illinois’s large temperature range over our four seasons.

Accelerators & Retarders: Sometimes concrete contractors need to speed up the curing processes or slow it down in order to combat extremely cold or hot weather. Accelerators and retarders are basically chemicals that could speed-up or slow down the time it takes for the concrete to harden. An experienced concrete contractor will know when and how to use them appropriately.

What does 4000 psi, 3500 psi, 3000 psi concrete mean and what should I use on a driveway?

4000 psi, 3500 psi and 3000 psi all refer to the compression strength of the concrete (resistance to downward force in pounds per square inch), usually at 28 days of curing. Ready-mix suppliers sell their concrete based on it’s rated psi and the higher the psi, the more it will cost. In general, 3000 psi is usually used for concrete walls and footings and 3500 psi is used for flat work such as floors and walkways. However, a good contractor who wants to avoid call-backs will use at least a 4000 psi mix for a driveway. Since a driveway will be exposed to vehicle traffic which can bring in salt in the winter, 4000 psi is a good strength to use. Most 4000 psi mixes if properly placed and finished under ideal conditionals will actually reach 4500-5000 psi, since most ready-mix suppliers over engineer their mixes to make-up for environmental and workmanship issues.

What does a six bag mix mean?

A six bag mix is a concrete mix that has six bags of portland cement in it. In general, the more bags of cement in a mix, the stronger the mix. Most mixes have 5 to 7 bags of cement. Some concrete contractors (usually the old timers) will order their concrete based on bags rather than psi. Because of advancements in concrete design, bags is no longer a practical way to identify concrete strength or quality. Most mixes now include fly-ash, a by-product of coal power production. Substituting a portion of the cement with fly-ash reduces cost but also increases strength and workability. Also, the water-to-cement ratio and air in the mix also plays a role in the strength. Ready-mix suppliers can produce a 4000 psi mix many different ways by changing the type and ratio of materials (cement, fly ash, air, water, admixtures). So the best way to order concrete is by psi not bags.

What is chert?

Chert is a type of stone found in aggregate deposits which are used in concrete. What makes chert problematic is that it’s very porous with a high capacity to absorb water. This means chert stones do not hold up to weathering like most stones and can expand and break during the winter. When chert ends up in concrete they can cause pop-outs to occur. A pop-out is a hole or indentation in concrete surfaces that are created when chert or other porous stones hold in moisture, freeze and then expand. If this happens at the top of the concrete slab it will cause the pop-out at the surface.

What is low chert concrete?

Concrete contractors can specify low chert mixes. This means the quarry where the aggregates came from were tested and found to have no more than 2% chert. However, keep in mind that does not mean your concrete will have no more than 2% chert. Since the 2% is an average of the whole quarry not each truck load that leaves. Despite this, low chert is still worth it. Make sure you ask your concrete contractor where he buys his concrete and if it’s a low chert mix. If he doesn’t know, that’s not a good sign.

Can I get chert free concrete?

Chert free concrete is available in certain parts of the country but not in the Chicago area. If a contractor claims he is using chert free concrete, he is either lying or misinformed. All quarry’s around Illinois contain a certain level of chert. This means pop-outs, which are unattractive, are acceptable in concrete to a certain degree.

Does all concrete crack?

There are two kinds of concrete: the kind that is cracked and the kind that is going to crack. If a driveway contractor tells you his concrete driveways won’t crack, be skeptical, because it’s just not true. Although concrete has very high compression strength, it has very little tensile strength (resistance to being pulled apart). Because concrete is made from water, it shrinks as it cures, at a rate of about 1/16th of an inch for every 10 feet, there is strong tension pulling the cement slab. Because the whole slab can’t move over the ground, it can pull apart in the middle, causing a crack. Despite all the advancements made in building materials, there is still no such thing as crack-free concrete. Hairline cracks, which can be unsightly, are no cause for concern and common to see in new concrete despite how well the workmanship and material quality.

What Other Factors Cause Concrete to Crack?

In addition to shrinkage, there are four other main causes that contribute to cracking:

  • Expansion and contraction due to temperature changes
  • Concentrated heavy loads
  • Poor un-compacted sub-base conditions
  • Rapid water loss prior to finishing the concrete after it is poured.

All of these causes can be minimized with proper care and installation. A properly compacted sub-base of the proper thickness under the concrete is crucial. In Illinois’s cold winter climate, only air-entrained concrete should be used. Heavy vehicles, such as garbage trucks should be kept off the slab, especially near the edges.

While cracks cant be completely avoided they can be managed. Control joints, as they are called, weaken the concrete slightly at the surface, so that when the concrete cracks it will likely follow the control joint and not be noticed. These control joints can be created in two ways. The concrete can be cut in straight lines, roughly ¼ of the depth of the slab. Hand-troweled control joints are another option, also often seen on sidewalks. The number and spacing of these control joints depends on the specific installation.

Will steel or fiber reinforcement stop cracking?

As noted above, all concrete cracks however the right reinforcement can reduce cracking and also help hold the concrete together after a crack occurs. Steel wire mesh, which is usually a 6″ x 6″ grid is the most common and it helps with holding the concrete together after a crack is formed. It’s important that the wire mesh is raised up while the concrete is being poured. Steel that lays at the bottom of a slab is worthless. Fibers on the other hand help reduce early stage cracking, known as plastic shrinkage cracking. These are the most common cracks that form. They are caused when the concrete is curing too fast. The fibers can help minimize these but after curing the fibers really don’t do anything. Its best to use both steel and fibers. There are fibers that can used to replace steel wire mesh but they do cost more and leave the concrete looking hairy until they wear away. If your contractor only uses fiber ask which type of fiber they are using. Only “macro” fibers should be used to replace steel wire.

Best Practices For Strong, Long Lasting Concrete

There is no mystery when it comes to doing quality concrete work. Below are best practices that the American Concrete Institute and most experienced concrete professional recommend.

  • Make sure concrete is poured over a solid base. The code is most often 4″ of compacted gravel but more may be needed if the sub base is soft or wet. Many contractors skip on the gravel because it can be time consuming to dig out soil but it’s worth it.
  • A driveway slab should be no less then 5″ in thickness. Many villages require only 4″. The extra inch will greatly increase the strength of the slab.
  • Use 4000 psi concrete with a water reducer and low chert aggregates.
  • Fiber and steel reinforcement should be used together or a high dosage of Macro fibers.
  • Concrete finishing should be done after all the water on top of the surface (bleed water) is gone. Early finish will cause surface problems later. Steel trowels should never be used on exterior concrete. Floating and brooming is best.
  • Joints should be installed no less than every ten feet in a driveway and 3′-5′ for walkways.
  • For the best protection against salt damage, concrete should be sealed with a professional grade penetrating sealer. The cheaper sealers found at big box stores are not as effective.

Your Choice of A Concrete Contractor Is Important

To make sure your new concrete driveway gets off to a great start, you need to make sure the contractor who builds it has the experience needed to take all the factors above into consideration and the integrity to follow the best practices in building your driveway. Shortcuts and cutting corners during in construction will only lead to problems. Choose your contractor wisely for the best results and check out “How to find a good contractor.”

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