New Pile Quality-Assurance Method Improves Underground Picture
Nine cubic yards of wet concrete will release the energy equivalent of 400 lb of TNT as it cures. Researchers at the University of South Florida are now partnering with industry to deliver a new device and software to monitor the heat distribution in curing cast-in-place concrete foundations, such as drilled shafts and bored piles, as a new quality-assurance tool.
The method, called Thermal Integrity Profiling (TIP), is the product of 15 years of research and recently has been taken to market by Pile Dynamics Inc. The Cleveland-based firm recently released a field-ready testing device that is cheap enough for mass distribution. The Florida Dept. of Transportation tested it during construction of the Judge S.S. Jolley bridge, Marco Island, Fla., and gave it glowing reviews.
“We’ve been impressed with the method,” says Larry Jones, geotechnical engineer for FDOT, “largely because it gives us the ability to evaluate a portion of the shaft inside the drill-shaft cage and outside as well.”
The device measures and infers the absence of intact concrete by noting cool regions—necks or inclusions—in its thermal mapping readout. The presence of additional concrete is registered by warm regions—over-pour bulging into soft-soil strata or voids—says Gray Mullins, a USF professor who helped develop the software. Anomalies both inside and outside the reinforcing cage disrupt the normal temperature signature for the entire shaft.
The TIP system can be set up in two ways. One method runs a thermal probe through standard tubes installed in the rebar cage before the pour—the same tubes used for probe tests. The other technique features thermal wiring installed in the rebar cage and embedded in the pile. Data is transmitted to the surface continuously as the concrete cools.
Mullens says the TIP system has advantages over other pile-integrity test methods, such as Cross Sectional Logging (CSL) and Gamma-Gamma, because unlike those methods, which can only check discrete areas, TIP scans the entire shaft.
"Each [method] has its limitation," he adds.
The CSL method measures portions of a shaft inside the metal reinforcement cage, but it cannot "see" outside the cage. The Gamma-Gamma method monitors a limited radius around each tube used to run the test, but the number of tubes that can be run is limited. California currently requires the Gamma-Gamma test, but it uses nuclear radiation and requires licenses for transport and handling of the equipment.
Mark Gaines, construction engineer with the Washington State DOT, used the Gamma-Gamma method last year and says it was “a bit time consuming” and involved several steps. “We poured water in tubes run down with the reinforcement cage and then poured concrete, evacuated the water and ran the probe down and up the shaft and then refilled the tubes with water.”
Timing is also critical when using the probe method, Mullins says. Testing must be performed within a limited window to get results before the concrete cools. “You want to do the tests when the shafts are hot enough for a good signal. This is usually a day or two after casting. Large-diameter shafts might be a week,” says Mullins.
Gaines says that he is attracted to the idea of using thermal wiring that stays embedded in the pile because he thinks the process would be much quicker. “The thermal wires begin measuring thermal levels immediately,” says Gaines.
After testing TIP, both Gaines and Jones recommended the WSDOT and FDOT, respectively, switch to the new integrity method on all upcoming projects. However, the WSDOT and FDOT balked, and both agencies have called for the use of TIP in conjuncture with CSL, noting that TIP is a patented process and not very many firms offer the test—it’s too new.
Foundation & Geotechnical Engineering, Plant City, Fla., partnered with Pile Dynamics Inc. to commercialize the TIP method.
