Thermal Integrity Profiling (TIP)
The method, Thermal Integrity Profiling, applies the heat generated by curing concrete to assess the quality of cast-in-situ concrete shafts by measuring the temperature via digital sensors attached to the rebar cage.
TIP-testing provides information about:
- The overall shape of the shaft – both inside and outside the rebar cage.
- The extent of imperfections, anomalies or defects and their impact
on shaft cross section properties and rebar cage concrete cover.
- The amount of lateral shift of the cage within the shaft.
The temperature measurements are performed via digital sensors by installing cables attached to the rebar cage prior to casting the shaft, during which cable- and shaft-information are registered.
The temperature is logged automatically via logging devices attached each thermal wire cable and placed safely above ground level. The temperature decreases radially from the center of the shaft towards the surrounding soil; hence, the digital sensors should be located in an equal distance from the center of the shaft to ensure radial symmetry of the temperature measurements. Consequently, a perfect cylindrical shaft will show a nearly uniform temperature distribution throughout its length, except at the top and bottom where not only radial, but also longitudinal heat losses occur and cause a so-called roll-off zone in the temperature profile.
Hence, a local temperature increase will indicate a bulge or an increased concrete-cover, and a local temperature decrease will indicate a necking or a reduced concrete cover. Differences in temperature between diametrically opposite thermal wires indicate a lateral shift of the cage relative to the shaft.
The measurements are performed until the peak temperature has been reached which normally will occur between 24-72 hours after concrete placement depending on the geometry of the shaft and concrete mixture.
Analysis and reporting
The temperature measurements are extracted from the logging devices and uploaded for analysis to evaluate the integrity of the shaft within a relatively short time frame after concrete placement.
The measured temperatures vary with the shaft diameter, distance to the center of the shaft and, concrete cover and time of measurement. Based on the obtained temperature readings and the distribution of the as-built concrete volume along the longitudinal axis, the effective radius of the shaft, the lateral shift of the cage relative to the shaft and the concrete cover are calculated. The accuracy of the radius calculation is dependent upon the accuracy of the applied concrete volume.
The radius is best evaluated approximately around the time of peak temperature; however, local defects are better observed during the early curing stage around half the time of peak temperature (Piscsalko et al., 2015; Piscsalko et al., 2016).
A preliminary evaluation of the results from the analysis can be forwarded as soon as, the analyses are performed. The final report is subsequently prepared including the supporting appendices. The integrity of the shaft is evaluated based on the stipulated project verification criteria.
NOTE: Thermal Integrity Profiling is currently not covered by Danish or European standards.
ASTM D7949-14 Standard Test Methods for Thermal Integrity Profiling of Concrete Deep Foundations. https://www.astm.org/d7949-14.html
Mullins, A. G., (2010) Thermal Integrity Profiling of Drilled Shafts, DFI Journal – The Journal of the Deep Foundations Institute, 4:2, 54-64 https://doi.org/10.1179/dfi.2010.010 Mullins, G., & Piscsalko, G. (2012). Thermal integrity profiling: an innovative technique for drilled shafts. The Magazine of Deep Foundation Institute, 51-54. https://www.pile.com/wp-content/uploads/2017/03/DFI_MAY_JUN2012_pg51-54.pdf
Piscsalko, G., Cotton, D., Belardo, D., & Likins, G. (2015). Using Thermal Integrity Profiling to Confirm the Structural Integrity of Foundation Applications. In IFCEE 2015 (pp. 2607-2616). https://ascelibrary.org/doi/abs/10.1061/9780784479087.242
Piscsalko, G., Likins, G. E., & Mullins, G. (2016, October). Drilled shaft acceptance criteria based upon thermal integrity. In Proceedings of the DFI 41st Annual Conference on Deep Foundations, New York, NY, USA (pp. 12-14). https://www.grlengineers.com/wp-content/uploads/2016/10/drilled-shaft-acceptance-criteria-based-upon-thermal-integrity-1.pdf