Ten Myths About Concrete Construction

August 14, 2017

 

Another effort in the never-ending war on bad information and ingrained myths

 

Transcript:

Myths, fantasies, and fairy tales surround us at work or play.

Just like when you were a child and were told stories that made you believe in magic beans from Jack and the Beanstalk or the tooth fairy, the construction industry has a few tall tales of its own but it's time for a reality check against these popular but mistaken ideas.

 

Myth One: You can specify a concrete mix by the number of bags of cement

 

Homeowner: "Hi, I'd like to some concrete for my patio in my backyard. What do you guys recommend?" 

Concrete yard: "Well, actually, we'd recommend the six-bag mix for patios and we do have that. All I need is some more information from you and we can send it right out."

Fact: Mixes should be specified based on performance requirements, not just cement content

Specifying bags of cement without tying in the amount of water is no specification at all. One of the most important properties of a concrete mix is the water to cement ratio. Mixes need to meet performance requirements including a minimum compressive strength that's governed by the amount of water in the mix, not just the cement content.

 

Myth 2: Adding water to the mix is the only way to increase slump

 

'Veteran' construction worker: "What's your slump?"

'Rookie' construction worker: "Two and a half inches."

'Veteran' construction worker: "It's way too dry. I want 10 gallons."

Fact: There are many ways to increase the slump of concrete other than adding water

Adding just one gallon of water to one cubic yard of concrete can

  • Increase slump one inch
  • Decrease compressive strength 150 to 200 psi
  • Waste about one-quarter bag of cement
  • Increase shrinkage

So, how can slump be adjusted in the field without adding more water? Consider using a water reducer or super plasticizer at the site to increase the slump while maintaining the water to cement ratio.

Back at the plant, you can modify the proportions or gradations of aggregates. The grading and maximum size of aggregates influence cement and water requirements that directly affect the workability.

Measure and correct for variations and moisture content of aggregates throughout the work day. Also, monitor prolonged delivery and unloading time which can decrease the slump.

It is important to maintain high-quality control from truck to truck so that reasons for differences in slump can be identified. 

 

Myth 3: Curing concrete means letting it dry

 

Homeowner: "Thanks, it looks great. Is there anything else I need to do?"

Construction worker: "No, it'll be fine. Just let it dry out."

Fact: Concrete needs water to continue to hydrate and gain strength

As cement hydrates, new chemical compounds form in the fresh concrete. The new compounds are responsible for setting, and hardening, and strength properties of concrete.

Loss of water prevents continued hydration.

Curing maintains a satisfactory moisture content and temperature within the concrete to ensure that desired properties develop.

The longer the period of time that you cure concrete, the stronger and more durable it will become.

 

Myth 4: No visible bleed water and a 'Footprint Test' mean a thumbs-up for concrete finishing

 

Construction worker 1: Hey, is that slab ready? 

Construction worker 2 walks on the slab and signals with his hand that it is ready to finish.

Fact: These general finishing guidelines do not always prevent slab surface defects

Finishing concrete is an art. It takes the experience to know when to begin finishing operations.

The terms "over-finishing" and "premature finishing" haunt concrete finishers every time they tackle a fresh slab.

Improper finishing can cause surface defects, such as blisters, dusting, crazing, and delaminations.

Relying on the absence of the sheen of water on the surface to determine when bleeding has stopped may not be enough.

Depending on the concrete properties and environment, bleeding may still occur when it isn't visible. The bleed water may be evaporating as soon as it reaches the slab surface.

Bleeding must be completed throughout the slab thickness before finishing can begin.

The use of power finishing tools has changed the recommended indentation depth from 1/2 inch in the past, to 1/4 inch for walk-behind and riding power trowels, and 1/8 inch for heavier riders.

With higher floor flatness tolerances, required finishing operations may need to begin earlier than usual. Choosing the appropriate time to begin finishing operations takes good judgment and knowledge of materials being used.

 

Myth 5: Calcium Chloride is an antifreeze agent

 

Construction worker 1: "Hey, man, it's gonna get pretty cold tonight. You think we ought to cover this concrete with blankets and stuff?"

Construction worker 2: "We don't have to worry about that concrete, Johnny, it's got calcium chloride in it."

Fact: Calcium Chloride is an accelerator only, it is not an antifreeze agent

The concrete can still freeze if not properly protected. Concrete gains strength very slowly in low temperatures. The use of accelerators such as calcium chloride increases strength development at an early age.

However, the fresh concrete must be protected from freezing until the concrete has reached a minimum strength of 500 psi or significant strength reductions will occur.

To avoid problems while placing concrete in cold weather, you can maintain concrete temperature using enclosures, insulated forms, and curing blankets.

 

Myth 6: You can place concrete on frozen ground without any precautions

 

Construction worker 1: Hey, are we pouring today? The ground's pretty frozen.

Construction worker 2: Yeah, it's a go. The concrete will heat up the soil.

Fact: Precautions must be taken to protect concrete and prevent future soil problems in adverse weather conditions

Never place concrete on frozen ground. When the subgrade thaws, it may settle unevenly and cause cracking.

The difference in temperature between the frozen soil and warm concrete can cause rapid cooling of the concrete and may retard the rate of hardening.

Ideally, the soil temperature should be as close as possible to the concrete temperature when placed.

There are things you can do to thaw the ground and place concrete.

Never place pellets of calcium chloride on the frozen ground to thaw the surface.

 

Myth 7: Reinforced Concrete Won't Crack

 

Construction worker 1: How come is so much steel over there? 

Construction worker 2: They put the steel down there to keep the concrete from cracking.

Fact: Structural reinforcement does not prevent concrete from cracking due to volume changes

Volume changes caused by temperature and moisture cycles are a natural part of a concrete's life. Concrete restrained from movement may crack because concrete is weak in tension.

Many times, it is the rebar that causes the restraint and allows the cracking.

The structural reinforcement does not prevent cracking but instead holds the crack faces together. That process transmits the tensile stress from the concrete to the steel and allows concrete to withstand higher tensile loads.

 

Myth 8: Fresh Concrete that is flat and level will remain flat and level after hardening

 

Construction worker 1: I think it's a perfectly level flab and I'm very happy with it.

Construction worker 2: (smiles and nods, gives thumbs-up)

Fact: Concrete can change shape while settling

In addition to horizontal movements caused by changes in moisture and temperature in a concrete slab, changes in shape including curling often occur.

Curling is the lifting of the slab edges at joints and cracks. Curling can be caused by differences in moisture content and temperature between the top and the bottom of the slab.

 

Myth 9: Concrete is impermeable

 

Construction worker 1: So is this concrete gonna be dry enough to put the flooring down? 

Construction worker 2: Yes. Concrete's a very dense material. Nothing will get through it.

Fact: Even the densest concrete is somewhat porous

To say that something is as solid as concrete really means that it is as porous as a sponge.

Concrete is permeable. Moisture and other substances in the form of liquid and vapor can pass through it. Depending on just how porous the concrete is, that can take anywhere from a few hours to a few months to happen.

To make concrete less permeable and more watertight:

  • Use mix designs with a lower water to cement ratio
  • Uniform aggregate gradation
  • Chemical and mineral admixtures such as super plasticizers and silica fume
  • Provide a vapor retarder directly underneath the slab

 

Myth 10: The higher the concrete strength, the more durable the concrete

 

Lab technician: I have the result of your 28-day breaks. They average out at 6,550.

Contractor: Ah, that's great news, that makes my day. That concrete will make it through a lot of winters.

Fact: Compressive strength alone does not determine the concrete's durability

Although compressive strength is an important characteristic of concrete, other qualities can be even more important for concrete in harsh environments.

In general, the principal causes for deterioration in concrete are:

  • Corrosion of reinforcing steel
  • Exposure to freeze-thaw cycles
  • Alkali-silica reaction
  • Sulfate attack.

All of these problems start with exposure to moisture. Reducing concrete's permeability and an adequate air void system are the keys to durability.

 

"Continuing the education process, make sure that the suppliers are educated, number one.

That they then inform the contractors what the implications are as far as potential changes that they're asking for, and that the contractors and the engineers are specifying the right products that they also know what the implications are."

—Paul Tarvin, STS Consultants, Milwaukee, WI

 

"It just takes time to get rid of the myths. Education, repeated attempts. You can't just show somebody once or twice, you have show them several times.

And when they see maybe that can cost them money when you have a reputable contractor that you're dealing with, and he is taking responsibility for failures of concrete and he recognizes what his responsibility in it and that he can change that, then I think we'll see changes for the industry."

—Jesse Jacobs, contractor, Otto Jacobs Company, Lake Geneva, WI

 

If the myths were dispelled, we would have a totally improved industry. We would all prosper in the industry.

I think our customers would use our product rather than other products and it would be better for everyone all around.

Terry Alby, concrete supplier, Alby Materials, Waterford, WI

 

—This video is from the Portland Cement Association

 


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