Hard evidence

Troubleshoot concrete problems before the pour to build your case for quality concrete

“If you are diagnosing concrete problems after the pour, it’s too late to do much about it,” says Luke Snell, director of the Concrete Construction Resource Unit of the School of Engineering at Southern Illinois University/Edwardsville.

Snell, a civil engineer by trade, has devoted his life’s work to helping contractors around the world achieve better results with concrete. His work has even taken him to Mongolia to help develop more practical ways to place concrete that can meet the challenging temperature extremes of the region.

“The best time to head off concrete problems begins with pre-design meetings followed by more specific meetings as the pour date gets closer,” he says.

Good rapport between the general contractor, subs and specialty contractors is key to an effective, well-run job. “Trial lawyers and experts really like it when contractors develop an ‘I gotcha’ attitude. The better informed everyone on the jobsite, the better the process will go,” he says.

The planning process
Front-end planning heads off surprises. “I have talked with contractors who had concrete trucks show up at the gates of a plant, only to be stopped because the truck drivers had not completed the safety orientation required of any person on the property. I have also seen this happen with tool and supply deliveries, where a whole jobsite was stopped because the driver had to meet the training requirement. If you know this in advance, plan for it,” he says.

As the pour date nears, members should meet again and walk through the pour. “When everyone can see what is planned and how it may affect other parts of the job, it’s a great way to head off problems before they happen,” he says.

Test right
Contractors spend a great amount of time selecting the right subcontractors and suppliers, but considerably less time selecting a concrete testing laboratory. Few realize that a testing laboratory could have a huge impact on the completion of a project. Snell offers these test lab selection tips:

1. Go to the testing facility and check it out. “Check out the curing room. Is it working? The concrete is supposed to be held at 72 F at 100 percent humidity. Is this documented? Require that it is,” he says.

2. Is the testing equipment capable of dealing with your concrete pours? Do you pour high-strength concrete? Can the facility adequately test your cylinders?

3. Can the facility pull rebar with enough force for accurate tensile strength readings if this testing is required?

4. Are current ASTM standards on file? “I have been involved with some testing facilities where the standards on file were more than 10 years old. Access to the latest standards is important in assuring the tests are conducted to the most recent specifications,” he says.

Watch the batch plant
“Concrete is a ‘just-in-time’ manufacturing process. Batch plants work with varying materials and produce several types of mixes on-the-go,” says Snell.

Many factors can affect the batch plant’s ability to replicate concrete loads precisely. “Material quality, weight, moisture content, temperature of materials and air content can cause mix variations. Also, the plant may incorrectly produce the concrete mix needed for the job. It may be difficult to see the differences on the day of the pour. That’s why testing is important,” he says.

The day of the pour
If the pour has been well-planned, the only variable should be the weather, which could make even the best-planned pour a disaster. High temperature, low humidity and wind can greatly affect the concrete curing process.

“When concrete cures too quickly or loses too much moisture from the surface during the curing process, plastic shrinkage is likely,” he says. The shrinkage can manifest itself as crazing or alligator cracking or as conventional cracks early in the life of the concrete. “Over time, the cracking will allow water and de-icers to penetrate and break up the concrete,” he says.

Snell says a free, easy-to-use computer program can be downloaded to estimate the likelihood of plastic shrinkage of curing concrete. Go to www.siue.edu/CCRU/research.htm and click on the Curing link. It will download a DOS-based program that factors wind speed, concrete temperature, relative humidity and ambient air temperature to figure the danger of plastic shrinkage. Many batch plants use this program to alert customers that plastic shrinkage is a real threat on the pour about to be completed.

“Plastic shrinkage is most common in hot, arid areas of the country, such as Colorado,” says Snell. However, if the contractor knows conditions are right, several steps can be taken to prevent it from occurring. “It’s a matter of controlling the variables to reduce the vulnerability of the curing concrete.

“Screens can be set up to protect the poured slab from wind. Although this will work, it’s rather impractical on large jobs. A fogging mist that raises the relative humidity over the concrete can work; so can using cold water or ice at the batch plant to bring down the temperature of the mix,” he says.

Pulling the core samples
One of the most difficult dilemmas facing a concrete contractor occurs when the test of a cylinder sample doesn’t meet standards. “The contractor is the first on the firing line, but is hardly ever the responsible party. It’s up to the contractor to prove it’s not something he or his crew has done. Fortunately, this is rather simple,” Snell says.

Batch plants keep very good records, which is your first place to look. “Review the batch tickets and assure that the proper mix was delivered. A 300-yard pour is substantial, but the batch plant is mixing many other loads besides yours. Is it possible that one wrong batch was delivered to the jobsite?

“Under the ASTM C 94 standard, contractors can request that water be added to the mix on-site up to a certain amount, usually to the point where is affects the ability to get a 2” to 4” slump. Drivers should record this and it should be a part of the batch ticket,” he says.

Pulling test cylinders is supposed to be a random process, but seldom is. This could set the operation up for compliance when they know testing will take place and slacking off when they know samples won’t be pulled.

“To avoid this, develop a system that assures random pulls. Don’t allow the testing agency to show up every morning at 10 a.m. to pull samples. Make sure they take samples at varying times so no one knows when the concrete will be tested,” he says.

Make sure at least three samples are pulled from each batch being tested. That allows the lab to test one cylinder at seven days to assure it is on its way to its correct compressive strength. The concrete should be two-thirds and 75 percent of its 28-day compressive strength.

The second and third samples would be used to check compressive strength at 28 days. “If samples are pulled from the same wheelbarrow and show a big difference in results, it’s likely there is a testing problem,” he says.

To further pinpoint testing problems, Snell has developed a free, easy-to-use computer program that works off of the cylinder test data. Sorting the data by technician, the program can verify if any testing technician has a statistical variance that may indicate improper testing procedures. You can download the Microsoft Excel spreadsheet at www.siue.edu/CCRU/research.htm. 

Post-pour testing
If the seven-day test produces questionable results, other tests can be completed on-site to verify – or refute – the initial lab report.

“Non-destructive testing can give you another indication of the concrete’s strength,” says Snell. The rebound hammer, which measures the surface hardness of the concrete through the rebound/recoil action of a plunger hitting the concrete, gives an indication of the surface hardness which can be used to determine the strength.

Pulse velocity meters are another non-destructive diagnostic tool. They fire an ultrasonic longitudinal pulse into the concrete and the wave it produces gets measured and calculated as an estimate of the concrete strength. However, researchers report ±20 percent accuracy, even under the best conditions.

“The size and type of aggregate, cement, wet or dry conditions, age of the concrete and weight of the concrete will affect the values of these non-destructive tests,” says Snell. “All of these methods require that you must be able to touch the concrete you are testing. That in itself may cause some safety or practicality issues,” he says.

Published in the January/February 2004 issue of Contractor Tools and Supplies magazine.

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