Thermal Integrity Profiling: An Innovative Technique for Drilled Shafts T he durability of drilled shafts relies heavily on the thickness and quality of the concrete cover around the steel reinforcing cage. Until recently, this concrete cover went largely untested as non-destructive test methods could not test this region or were severely limited in the detection capability. Further, the concrete cover contributes significantly to the moment of inertia resisting bending moments (at least on the side in compression) and is imperative to proper rebar bond/development length. A relatively new test, known as Thermal Integrity Profiling (TIP), is capable of detecting the presence (or absence) of intact concrete both inside and outside the reinforcing cage, thus providing a 100% scan of the shaft. The method was developed in the mid 1990s at the University of South Florida, Tampa, and has been used commercially since 2007. The test measures the internal temperature of the shaft, which is elevated by the cementitious materials present, and which react exothermically during hydration. The temperature rise from hydration energy has historically been considered an undesirable side effect that has been well studied in an effort to combat thermal-induced cracking. As high- strength concrete has been used more often, the associated higher cement content has caused higher internal temperature. As an example of this effect, Figure 1 shows the modeled core temperature versus time relationship for three, 6 ft (1.8 m) diameter shafts constructed with 2.7 ksi, 4.5 ksi, and 9 ksi (18.6 MPa, 31.0 MPa and 62.0 MPa) concrete with cement contents of 430, 600 510 kg per m) of concrete, respectively. No flyash or slag was used in these example mixes. The presence of flyash or slag in the mix and 860 lbs per cubic yard (255, 356 and 3 performed near the peak temperature (after hydration has completed), but can be conducted several days afterward depending on shaft size and mix design. When considering the 4.5 ksi (31.0 MPa) kg/m ), elevated shaft temperatures above 125ºF (52ºC) persist for 5 or 6 days. As a rule of thumb, TIP can be performed up to D days after concreting (where D is the shaft diameter in feet) and as early as 8 to 12 hours after concreting (depending upon shaft mix (Figure 1), 600 PCY or 356 3 ISSUE : INNOVATION SPECIAL center, measuring temperature at opposite sides of the cage are equally affected; one is hotter and the other is cooler. The average of both represents the temperature at the average location of the reinforcing cage. The individual temperature readings will indicate any cage eccentricity, but the average temperature will still allow for the determination of necks and bulges within the shaft. Note that the gradient for the various shaft sizes is similar at the location of the cage. This is dependent on the time Figure 1. The effect of cement content on core temperature of a 6 ft (1.8 m) dia. shaft shaft diameter and concrete mix), thus expe- diting the continuation of construction. The internal temperature distribution design can drastically change the time to peak temperature (approximately 24 hrs in Figure 1) up to 50 or 60 hours. Retarders further delay the time to peak temperature. Thermal Integrity Profiling is intended to be within the shaft is bell shaped as shown in Figure 2. Larger diameter shafts develop the highest core temperatures but vary little as the shaft size exceeds 6 ft (1.8 m). Thermal Integrity Profiling measures the temperature at the radial location of the reinforcing cage where the gradient is highest. As a result, the measured temperature is highly sensitive to the cage alignment and subtle offsets are easily detected; in this case, a change of 3.5ºF (1.9ºC) equates to 1 in (25 mm) of cage offset. Therefore, when the cage is off of testing and mix design, but is affected very little by shaft diameter. In this way, the local radius of the shaft is indicated by increases or decreases in temperature whereby the radius (or cover) is equally and oppositely higher or lower than that on the opposite side of the shaft when the cage is eccentric. As the gradient AUTHORS: Gray Mullins, Ph.D., P.E., Professor University of South Florida, and Technical Director, Foundations & Geotechnical Engineering, LLC George Piscsalko, P.E., Vice President Pile Dynamics, Inc. DEEP FOUNDATIONS • MAY/JUNE 2012 • 51
