Magazine

Cracking Up Not a Laughing Matter

By Drew Robb

Concrete cracks. That’s a fact of life. Some cracks might not need attention, while others could have serious structural consequences. But how can you tell the difference? Much of the time you can’t, and expert advice is required.
“It’s important to accurately assess structural cracks to determine what is significant and what is not,” said John Duntemann, a Principal at consulting engineering firm Wiss, Janney, Elstner Associates (WJE). “You have to properly identify the extent and cause of cracking before you can implement the right repair solution.”
Cracks in concrete might be caused by a number of factors, such as thermal expansion and contraction, sub-grade settlement, the loads being applied, and even earthquakes. This all adds up to internal and external stress that manifests as big and small cracks.
Parking garages, stadiums, airports and other structures heavily reliant on concrete are subject to cracking due to heavy loads, vibration, and temperature and weather shifts. Garages, for instance, are subject to concrete cracking because of not only applied load, but also structural movement due to thermal changes.
All require vigilance when it comes to cracking and structural integrity.
In many cases, however, there is uncertainty as to what to do about cracks. Pete Barlow, a Principal at Contech Services, which repairs, strengthens and waterproofs concrete structures, said his company is constantly being contacted by anxious building managers and owners.
“Many times a week, people send us photos of cracking at their facilities, asking for advice,” Barlow said.
Crack Classification
There are various methods of classifying cracks in concrete. One primary way is to split them into two groups: structural and non-structural.
Structural cracking affects the integrity of the building. Cracking in support beams, columns and load-bearing areas is of particular concern.
Non-structural cracks, on the other hand, are not detrimental to building integrity, though they may need to be addressed due to cosmetic reasons or to take proactive steps to prevent those cracks from growing over time and eventually reaching the structural stage. Tiny fractures also can lead to other challenges.
“Small cracks in below-grade foundation walls can allow water to migrate into the structure,” Barlow said.
Size of the gap is another consideration. Hairline cracks are generally not a problem, but it depends on where they are. A high frequency of them, though, could indicate an underlying stress issue that needs to be addressed.
And as the size of the openings increases, so should the concern. As a rule of thumb, cracks “0.015 of an inch or larger” should be investigated, Barlow said.
“The larger the width, the greater the likelihood you have lost aggregate interlock,” he said. “But repairs can be done on cracks down to 0.005 of an inch.”
A third issue is location/orientation. Cracks in random directions are of lower priority than those in beams, columns or other load-bearing areas.
“If one side of the fracture is offset from the other, that is often a bad sign,” Barlow said. “Other warning signs are closely spaced cracks or repetition of cracking at the same locations on each floor.”
Epoxy in Concrete Repair
In many cases, epoxy injection is used as the remedy for concrete cracks. Although there are many epoxy types and uses, high-performance epoxies manufactured for structural concrete bonding and crack repair are the focus here.
Duntemann gave the example of cracking that appeared in a cooling tower being constructed at a synthetic-fuel plant. Cracking developed at the ends of pre-stressed concrete beams. Demolishing the structure and starting over was not considered an option.
Management brought in WJE to find an alternative. “The beams were injected with epoxy and load-tested to verify their structural capacity,” Duntemann said.
In another case, the firm was retained to investigate large cracks in concrete caps that sat atop concrete pile foundations. “We developed a method of reinforcing the pile caps and injected an epoxy into the cracks to reconnect the fragmented caps,” Duntemann said.
Water, however, presented a further challenge – the epoxy had to deal with a saturated environment. WJE requested help from chemists at ChemCo Systems. They formulated an epoxy that could fill three-quarter-inch cracks and cure in the presence of water.
“You often run into challenging environments such as cracks full of water, cold temperatures or exceedingly wide cracks that require a customized solution,” Duntemann said. He pointed out that while many different types of epoxy are available, one size does not fit all. It requires the right combination of materials, chemistry and expertise, he said.
Summary
Building owners or managers noticing cracks in the concrete, are then advised to contact a structural engineer to assess the extent of the damage. He can help determine whether or not it is a significant issue.
“As building owners and managers are typically not engineers, they are advised to retain a licensed structural engineer who can evaluate concrete cracking,” Barlow said.
He also advised against a do-it-yourself approach for all but the most minor problems. In his view, too many factors are involved in the engineering and chemical formulation side to leave anything to chance.
“When a problem is identified, it’s important to retain professionals with experience solving these problems,” Duntemann said. “The correct solution requires a good understanding of the cause of that problem.”
For more information, contact ChemCo Systems VP John Bors at email@chemcosystems.com or visit www.chemcosystems.com.

Article Abstract from March, 2011




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