1. Thermal Integrity Profiling
of Concrete Deep Foundations
Thermal integrity profiling is a technique used to assess the intactness of cast-in-place concrete foundations such as drilled shafts, bored piles, and auger-cast-in-place piles as well as diaphragm walls, barrettes, dams, etc. It makes use of the energy released from hydrating concrete and the resulting elevated temperature. This image shows a miniature data collection computer (left) and the down-hole thermal probe equipped with infrared temperature sensors (right).
Drilled Shaft Design & Construction Seminar
Gray Mullins, Ph.D., P.E.
Professor, University of South Florida

2. Outline Background What do TIP results show? Case Studies
Testing Equipment and Procedure
Analysis
Field Demonstration???

3. Visual Inspection (sometimes?)
Visual inspection is the most common mechanism to identify concrete related problems in above ground structures (e.g. honeycombing, cracking, or discoloration) that in turn allow the structure to be more thoroughly scrutinized. The discoloration shown (left) was the worst of an entire region of concrete that was disrupted just after initial gel due to delays in concreting.
Mullins, G., and Ashmawy, A. (2005). “Factors Affecting Anomaly Formation in Drilled Shafts,” Final Report, FDOT Project BC , March, 293 pp. (original source Dan Brown, P.E., Ph.D.)

4. Visual Inspection (but limited)
Visual inspection of shafts is far less common and limited to those areas intended for excavation. The severity of defects that can result from the blind nature of underground concreting processes is clearly shown right. This is particularly problematic when drill slurry is required to maintain the excavation stability as the entire concreting process is under the water table (under slurry). This example is particularly extreme whereby most integrity methods used today would detect it. However, when anomalies form outside the reinforcing cage or due to concrete contamination, visual inspection does not help. The thermal method discussed today provides that capability.
Note this shaft was not equipped with access tubes as shafts intended for secondary structures (high mast lighting or signage) were not required. This shaft and shafts like it spurred the FDOT to mandate access tubes in all shafts.
The left image shows a shaft that is visually intact but concrete quality was far below that intended (discussed later).
I – 4, Orlando, FL
Crosstown Expressway, Tampa, FL

5. This images shows a 6 ft diameter (100 ft long) shaft now supporting the I-35W replacement bridge in Minneapolis, MN. Note there are six access ducts which is in keeping with common integrity practice where one duct is required for every 1 ft of shaft diameter. Prior to the use of thermal methods, common practice used cross-hole sonic logging where only the concrete directly between the tubes could be accurately assessed. In essence, only the hexagonal region was tested. This leaves the most important concrete untested which is that forming the bond between the structural reinforcing cage and the geo-material. Further, this concrete (previously untested) also contributes the most to the moment of inertia to resist bending.
Gamma gamma logging (GGL) has a radial zone of detection that is inches from the centerline of the access tube.

6. I - 4 & SR 400

7. I - 4 & SR 400

11. Assuming full coverage inside the polygon formed by the number of tubes, CSL testing provides between 25 and 70% concrete coverage, but no coverage outside the cage (unless single hole sonic logging is used). GGL provides around 10% coverage of the cover and between 15 and 5% coverage overall.
Thermal integrity profiling allows the entire shaft to be evaluated.

12. Principles of Thermal Integrity Profiling

13. What do TIP results show?
Tests the entire volume of concrete
Cage alignment
Estimated shaft shape
Necks, bulges, or inclusions
Concrete cover
Quality of concrete

14. Campers adjust their seats around the campfire to find that sweet spot where it’s not too cold and not too hot. If your friends stood you up, blind folded you, gave you ear plugs and spun you around like you were preparing to hit a piñata and let you go, could you tell them were the fire was? Of course you could, in fact, if they walked you closer or farther from the fire you would also detect that as well. In this way, Thermal integrity profiling is very similar to a campfire.

15. It turns out that the reinforcing cage is very much in that same sweet spot when considering internal concrete temperature and conveniently offers a mechanism to monitor the shaft concrete health

16. If we use our campers as temperature detectors, and the entire ring of campers’ seats moved as a unit, increases and decreases in temperature would be detected by the campers.

17. Likewise, if the campers could not move their chairs, larger or smaller fires would be easily detected with little effort.

18. Hydration Energy
The energy produced by hydrating concrete can be enormous; for example, a single concrete truck containing 9 cubic yards has the equivalent energy of lbs of TNT depending on the exact mix design. It of course is released over a period of days and not split seconds. If you slice through the shaft an any location…

19. Single Shaft Heat Signature
Higher Temperature
Elevated Soil Temperature
…you will find a temperature distribution that looks very much like this (click) where elevated temperatures extend into the surrounding soil as well.

20. How Much Energy is in a Drilled Shaft?

21. Effect of Cement Content on Temperature
6 ft shaft
Effect of Cement Content on Temperature
This graph shows the effect of cement content on the core temperature of a six foot diameter shaft.
Cement content affects the internal temperature, but has less effect on the time to peak temperature.

22. Effect of Mix Design on Temperature
4 ft shaft
Effect of Mix Design on Temperature
This graph shows the effect of cement content on the core temperature of a six foot diameter shaft.
Cement content affects the internal temperature, but has less effect on the time to peak temperature.

23. Effect of Shaft Diameter on Temperature (How long can tests be performed?)
Shaft diameter affects the internal temperature measured at the reinforcing cage (access tubes) as well as the time at which peak temperature occurs. This graph also shows an optimal time to test which is after peak temperature.

24. Field Testing and Equipment
Method A (probe method)
Lower a thermal probe down dry access tubes
Monitor depth of probe with encoder wheel
Run one test near peak temperature
Method B (thermal wire method)
Install thermal wires beside or in lieu of access tubes
Attach a data collector to each thermal wire
Data is collected at user prescribed intervals (e.g. 15 min)

25. Equipment (Method A) Thermal Probe Infrared Sensors
The equipment used in Method A consists of a thermal probe (32 mm diam x 125 mm long) on a long lead wire (top left) that is equipped with four miniature infrared sensors (top right). A digital encoder on a depth wheel assembly tracks the position of the probe as it is lowered down the access ducts (bottom left) and a data acquisition computer is used to record the temperature and depth measurements.
Depth Encoder Assembly
Data Acquisition System

26. Field Testing (Method A)

27. Equipment (Method B)
The equipment used for Method B consists of pre-manufactured thermal wires with temperature sensors evenly spaced along the entire length. One thermal wire is installed for every foot of shaft diameter (same spacing and plurality as the TIP access tubes). A data collector (shown right) is connected to each wire which stores the data a user defined time intervals (e.g 15 minutes).

28. What should I see in the thermal profiles?

29. Convection to air
Conduction to soil
Cage Diameter
Excavation Diameter

30. Convection to air
Conduction to soil
Cage Diameter
Excavation Diameter

31. Convection to air
Conduction to soil
Cage Diameter
Excavation Diameter

33. Effects of Alignment and Shaft Radius
Temperature (F)
Radial Position (ft)
Shaft Radius (ft)

34. Field Observations
Little to no cage eccentricity (all tubes same temp throughout)
Probable location of water table at ft (causes sloughing until slurry is fully in place)
Clean top and toe signature (approximate 1 diameter temperature roll-off top and bottom)
Good Shaft

35. These shafts show typical cage offset results where opposite sides of the cage show warmer or cooler measurements depending on position relative to the center of shaft (center is warmer; edge is cooler). The graph on the right was from a cage that was pushed up by concreting approximately 5 ft and then pushed back down into the already placed concrete.

36. How does TIP estimate shaft shape, radius, and cover?

37. Concrete Yield Plots
Depth change per truck
Volume per truck
Convert to avg diam or radius per truck

38. The recorded depth to top of concrete between trucks is useful to produce an estimated shaft diameter as a function of depth. When compared the average temperature profile a clear correlation is seen. Recall, by averaging the measurements from all tubes at each depth, the effect of cage offset from the center of heat is removed showing only the shape of shaft.

39. Temperature to Radius Conversion
The bell shape relationship between temperature and radius shown earlier is strongly linear in the region near the cage plus or minus inches.

40. Convert Temp to Radius
Good agreement with yield plot information
Check tube / cage cover

41. Individual Tube Positions
Outer Radius of Cage
Individual Tube Positions
Check tube / cage cover
Three tubes touch side walls others with reduced cover
As a check of correctness, the volume of shaft from the predicted radius should be checked against that volume placed.
Design Radius of Shaft

43. Effects of Alignment, Radius and Time
Temperature (F)
Radial Position (ft)
Shaft Radius (ft)

53. X axis (independent)
Y axis

62. How do I distinguish between normal end effects and an anomaly?

63. Soil temp
The normal temperature reduction near the bottom (and top) of the shaft due to both radial and axial effects (shown in blue) causes a tanh(z) trend with depth where the inflection occurs at the boundary between the shaft and soil. Immediately after concrete placement, a perfectly squared off temp distribution exists that transitions into the tanh trend.

64. The normal temperature reduction near the bottom (and top) of the shaft due to both radial and axial effects (shown in blue) causes a tanh(z) trend with depth where the inflection occurs at the boundary between the shaft and soil. Immediately after concrete placement, a perfectly squared off temp distribution exists that transitions into the tanh trend.

65. The normal temperature reduction near the bottom (and top) of the shaft due to both radial and axial effects (shown in blue) causes a tanh(z) trend with depth where the inflection occurs at the boundary between the shaft and soil. Immediately after concrete placement, a perfectly squared off temp distribution exists that transitions into the tanh trend.

66. The normal temperature reduction near the bottom (and top) of the shaft due to both radial and axial effects (shown in blue) causes a tanh(z) trend with depth where the inflection occurs at the boundary between the shaft and soil. Immediately after concrete placement, a perfectly squared off temp distribution exists that transitions into the tanh trend.

67. The red dots show the fitted tanh curve that ranges between the average tube temperature and the soil temperature with the inflection point at the shaft / soil interface (shaft bottom). By knowing this relationship, the temperature can be corrected for end effects to show what the temperature would be if it were not nearing the bottom. This allows the toe shape to be more correctly predicted.

68. Comparison: FGE
These graphs show the effect of correcting for end effects both at the top and bottom. The graph on the left shows temperature after it has been converted to radius without correcting for end effects. On the right, the same data has been corrected.

69. Permanent Casing (straight shaft)
Unlike CSL or gamma density logging, it is best to analyze thermal data by looking at the entire shaft and not just one or two tubes at a time. Looking at this sample 4ft shaft with four tubes, we see the average (shown in black) is fairly constant through the upper permanent casing region; a large bulge where the temperature is highest; and a slight reduction near an O-cell. We also see normal end effects.
Bulge (more concrete)
O-Cell (less concrete)

70. T-1
T-3
Comparing opposite tube sets, the direction of cage movement can be determined; Tubes 1 – 3 show north-south movement.

71. T-4
T-2
Tubes 2 – 4 show east-west movement.

72. Similarly, with the local radius from all tubes and depths, a 3-D rendering of the as-built shaft can be produced.

73. Resolving the N-S and E-W movements into the vector solution, the maximum cage movement and the direction in which it moved can be determined.

74. What is the TIP zone of detection?
The zone is limited only to the size of anomaly that can be detected.
Large anomalies are detected by multiple tubes (or wires)
Smaller anomalies are only detected by the closest tube or wire
A 10% cross sectional inclusion is detected by multiple tubes (or wires)

75. In this case study, two levels of anomalies were tied to the outside of the cage. Both encompassed 10% of the cross section but in two different formations. The uppermost was split into two regions whereas the lower was lumped on one side. The upper anomaly was strongly detected by Tube 3 and to a lesser degree by Tube 5. The lower anomaly was more pronounced in Tube 1.

76. Summary TIP measurements: Tests the entire volume of concrete
Identify cage alignment
Estimated shaft shape
Show necks, bulges, or inclusions
Determine concrete cover
Quality of concrete

77. Questions?

78. Cement hydration is a highly exothermic process!
Portland Cement
Water
Hardened Cement
Heat
Cement hydration is a highly exothermic process!