Concrete Basics

Concrete 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, 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 / admixtures are:

Water-reducers: Help with making the concrete more workable without needing to add more water to it.  What's wrong with more 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: 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 temperatures.  Without this space, the surface of the concrete will crack and pop.

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 a 5 Sack Mix mean? 

A 5 Sack Mix is a concrete mix that has 5 bags of Portland cement in it. In general, the more bags of cement in a mix, the stronger the mix.  Most mixes have 4 to 6 bags of cement. We make concrete based on bags rather than psi because our loading system is based on bag mixes and/or weight.  The water-to-cement ratio and air in the mix, also, play a role in the strength. 

What Sack Mix should I use?

In general

• a 4 Sack Mix is usually used for concrete walls and footings.
• A 5 Sack Mix is used for flat work such as floors and walkways.
• A 6 Sack Mix is used for driveways, which will be exposed to vehicle traffic.

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 not true. Although concrete has very high compression strength, it has very little tensile strength (resistance to being pulled apart). Since concrete is made from water, it shrinks as it cures, at a rate of about 1/16th of an inch per every 10 feet. There is strong tension pulling the concrete 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 is.

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 uncompacted 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 with the accurate thickness under the concrete, is crucial. Heavy vehicles, such as garbage trucks should be kept off the slab, especially near the edges. 

While cracks can't 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, 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 helps hold the concrete together after a crack occurs.

Steel wire mesh, which is usually a 6" x 6" grid, helps with holding the concrete together after a crack is formed.  It is 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.

Fiber, on the other hand, helps 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 fiber can help minimize these, but after curing the fibers really don't do anything. It's best to use both steel and fiber. There is fiber that can be used to replace steel wire mesh, but they 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 professionals 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 4" in thickness.
• Fiber should be used to reduce early stage cracking.
• 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. Using a bull float and/or broom is best. 
• Joints should be installed no less than every ten feet in a driveway and 3'-5' for walkways.