Say no to crack

Follow these suggestions from the Portland Cement Association to minimize random cracking on your next concrete pour.

by Paul Markgraff

Owner complaints lead to rework, and rework costs money. One of the most common complaints contractors hear is about cracked concrete.

However, concrete is supposed to crack. As it transforms from a liquid into a solid, it shrinks and cracks. It's natural behavior of the material.

Contractors can’t prevent concrete from cracking, no matter how hard they try. But, random cracking can be minimized.

“That’s why we put joints in concrete,” says Terry Collins, concrete construction engineer for the Portland Cement Association (PCA). “We know it’s going to crack, and joints provide a weakened plane or planned location for the concrete to crack in such a manner that the concrete will perform consistently and structurally.”

Focus on random cracking is important to understand because every piece of concrete in a building has cracks, says Collins. But there are strategies contractors can use to reduce random cracks that cause customer complaints.

Shrinkage?
Shrinkage is a fact of life when it comes to concrete. When contractors place concrete, there is a certain amount of water beyond what is required for cement hydration. This water lubricates the mixture and helps the contractor place, consolidate and finish the slab.

“Because the water is the lightest material in the mixture, it rises to the top,” says Collins. “The water that comes to the surface and collects there is called bleed water. That water evaporates into the air.”

Direct sunlight, high temperatures, low relative humidity and wind accelerate the rate of bleed water evaporation. When water evaporates from the surface faster than bleed water can accumulate, the surface begins to dry and crust.

“As it crusts, it creates a surface tension across the entire mass of concrete,” says Collins. “This shrinkage is exhibited as a series of cracks running parallel to each other on the concrete surface.”

Contractors can use three strategies to combat shrinkage on a concrete slab. The first is fogging or misting. Contractors can apply moisture at the surface of the concrete to satisfy that rate of evaporation while the hardening of the concrete below comes in line with the surface, so the concrete is consistently set through its depth.

“Misting can be done with something as simple as a hand sprayer,” says Collins. “If it’s a very large surface area, you may need something like a pressure sprayer to reach all sections of the slab.”

Contractors must treat the water they spray like they would treat regular bleed water. You can’t trowel the surface while it is covered in water, or you will add energy to the system. This will change the water/cement ratio at the surface and weaken the concrete, resulting in more cracks.

Contractors can also cover the surface with plastic sheets, which protect the moisture content of the concrete from rapid evaporation. This can slow the rate of evaporation to acceptable levels.

“When you pull the plastic, you need to allow that sheet of water to come off,” says Collins. “You’ll remove a light film of cement paste when you do this, but it’s minimal.”

The third strategy involves using chemical evaporation retarders, such as concrete films. These are spray-applied and formulated to evaporate more slowly than water. The water in the chemicals will eventually evaporate. During drying, the chemicals turn into a protective film that minimizes plastic shrinkage and cracking.

Don’t settle for less
There are two types of settlement cracking. The first results from the irregular settling of the base material that supports the concrete which can cause unusual stress points in the concrete above and eventually causes the slab to bend.

“As it bends, the concrete goes into tension at the base. Under the load, and at the weakest point, it will crack,” says Collins. “At some distance from the load, the settling creates surface tension in the concrete because the load-bearing soil beneath it is resisting the bend.”

Concrete can stand substantial vertical pressure. It takes 4,000 psi to crush a concrete test vessel, but it only takes 8 percent to 10 percent of 4,000 psi to overcome concrete’s tensile strength, or its horizontal strength, under vertical pressure.

“Good preparation of the sub-base that supports concrete can minimize this type of cracking,” says Collins. “The concrete’s job is to transfer the load to the larger area of soil underneath it. Most of the time, this type of cracking results from poor compaction of the base material before concrete pouring.”

Contractors may be surprised to learn that the density of the soil compaction is not the major factor of base support. Rather, the consistency of the soil compaction makes the most difference when trying to prevent this type of cracking.

The other type of settlement cracking involves steel reinforcement and placement. As the concrete bleeds and the extra water in the mix rises to the surface, heavier particles in the concrete descend to take that water’s place. In a steel-reinforced slab, this dynamic pulls some of the concrete away from the steel reinforcement, creating a small void between the rebar and the concrete. This type of settling sets up circumstances under which rebar can oxidize and concrete will crack.

“Good consolidation will minimize this problem,” says Collins. “When contractors shake the slab with an external or internal vibrator, the concrete starts behaving like a liquid. Heavier particles settle and lighter particles rise to the surface. This is called liquefaction, and air and water rise to the surface rapidly under these circumstances.”

Crack inducement
Cracks are going to happen, so what better way to minimize random cracking than by inducing these cracks intentionally before they surface accidentally?
In general, contractors should put joints in concrete slabs at a distance of 24 to 30 times the thickness of the slab. Then, cracks will end up where you placed the weakened plane. For example, if you have a 4"-thick slab, you would multiply 4" by 24 to get 96", or 8'. This means contractors should place a joint once every 8' when laying slab.

With larger, 1" coarse aggregate, contractors can extend the distance between joints to 36 times the thickness of the slab.

“Regardless of the thickness of the slab, you should not exceed 15' between joints unless you are going to provide load-transfer devices such as dowels or dowel plates,” says Collins.

A time and a place
Contractors must also be aware of the raw materials and weather patterns of the area.

Concrete is usually made of locally available materials. For example, in central Nebraska, concrete tends to have high sand content because they have miles and miles of sand, but not as much stone, Collins says.

When contractors use sand in the cement mixture, they are using a very small aggregate with a high amount of surface area. More cement paste and water are needed to coat every single grain of sand. This increases the potential for cracking because more water has been added to the system.

“If you have an even gradation of aggregates – meaning large particles, medium particles and small particles – then you have a very small void content,” says Collins. “You don’t need as much cement paste, and as a result, you don’t need as much water. Bad gradation, small aggregate size and high volumes of sand increase the water demand and the potential for shrinkage.”

Concrete placed during hot midday temperatures contracts as it cools overnight. A 40 F swing in temperature between day and night – not uncommon in many regions – would cause a 0.03" contraction in a 10' length of concrete. This will create cracks if the concrete is restrained in any way.

“You should take a look at the high and low temperatures and time your concrete placement for the middle of that window,” says Collins. “In a warm-weather climate, you may not want to place concrete at 8 a.m., because the outdoor temperature will hit its peak just as you finish your pour.”

An early-evening pour can work well because temperatures usually don’t swing as much overnight.
“Varying the time of the pour to hit the middle of the temperature swing can really help minimize cracking due to shrinkage,” says Collins. “However, this can be problematic for a contractor, because he or she has to pay his workers overtime if they come in at night to do this kind of job.”

Published in the March/April 2008 issue of Contractor Tools and Supplies magazine.

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