Nondestructive Tests of Thickness Measurements for Concrete ...

Nondestructive Tests of Thickness Measurements for Concrete Pavements Tests Really Work Jim Grove, Kevin Jones, Dan Ye, and Jagan M. Gudimettla In the past few years, considerable effort has been expended to compare pavement core measurements with nondestructive tests such as ground penetrating radar (GPR), impact echo, laser method, and probing (2–4 ). The primary intent of all these studies is to reduce the number of cores that are to be taken for pavement thickness measurements. The GPR and impact echo methods have limitations with freshly placed concrete. Most consider the accuracy of these two techniques not to be reliable or accurate enough for specification compliance determination. A new, nondestructive test that is fast, accurate, and independent of the maturity of concrete is needed. Magnetic pulse induction technology has gained popularity in the past 5 years to measure pavement thickness nondestructively. This technology has been used by one manufacturer to develop a testing device that is both accurate and reliable. The T2 is an easy-to-use, accurate, nondestructive, and relatively inexpensive device that is able to determine pavement thickness with the accuracy similar to core measurements for pavement thickness (AASHTO T148/ASTM C174). This paper describes that technique. It presents the experience of agency users, offers an example specification, and provides data from comparison studies aimed at determining the accuracies comparing the conventional coring techniques with the nondestructive testing (NDT) method.

Magnetic pulse induction can be used to determine the thickness of concrete pavement in a nondestructive manner. This induction provides the same level of accuracy and can save time and money for state agencies when compared with coring the pavement for thickness determination. The MIT-SCAN-T2 is a commercially available device that uses magnetic pulse induction to measure pavement thickness. The technology and operation of this device are described. Field experience from various states is provided. The accuracy and the repeatability, when compared with measuring core lengths, are good on the basis of data collected to date. The advantages of this nondestructive testing are presented. A specification developed by the Iowa Department of Transportation is included. The paper also includes a discussion of other issues that may be raised when this technique is used or there is ongoing work related to implementation.

AASHTO has recently released the software for a new mechanistic– empirical pavement design procedure (1). This new methodology incorporates traffic, climate, material, and design inputs into a software program to predict pavement performance during the pavement design life and determine the required pavement thickness. The software is used to predict the long-term performance of the pavement. But unless the pavement is constructed to meet or exceed the design thickness, the pavement may not provide the desired performance during its intended design life. Therefore measuring the thickness of the as-constructed pavement is an important acceptance activity for agencies. For years, most agencies have drilled cores from the new pavement as a sampling tool to determine compliance with the plans and specifications (AASHTO T148-97, ASTM C174M). This procedure is normally effective, but in cases in which a permeable base is used and the mortar fills some of the surface voids, the determination of the thickness is not as easy and the accuracy is reduced. Also coring is a destructive, expensive, and timeconsuming task. Today highway agencies do not have the personnel to core, inspect, and test these samples and patch core holes as they did in the past.

The Technology The T2 device uses magnetic pulse induction technology to measure the distance from a sensor to a metal reflector. The metal reflector is usually referred to as a “target” and is identified in this manner throughout the rest of the paper. While scanning, the T2 device generates a variant magnetic field that creates an eddy current in the target. The eddy current will generate an induced magnetic field inside the target, the intensity of which is detected by sensors from the T2 device. For a given type of target, the intensity of the induced magnetic field is determined primarily by the distance from the T2 device to the target. A calibration file, recording the relationship between the induced magnetic field intensity and the distance, is developed for each unique type of target produced by the manufacturer. The manufacturer-supplied targets are circular and made of 0.6-mm-thick galvanized sheet metal. Different size targets are available from the manufacturer depending on the thickness of the pavement to be measured.

J. Grove, FHWA and Global Consulting, Inc., Office of Pavement Technology, 2711 South Loop, Suite 4502, Ames, IA 50010. K. Jones, Iowa Department of Transportation, 800 Lincoln Way, Ames, IA 50010. D. Ye, Fugro Consultants, 8613 Cross Park Drive, Austin, TX 78754. J. M. Gudimettla, FHWA and Global Consulting, Inc., E73-105C, HIPT, 1200 New Jersey Avenue, SE, Office of Pavement Technology, Washington, DC 20590. Corresponding author: J. Grove, jim. [email protected].

Operation of T2

Transportation Research Record: Journal of the Transportation Research Board, No. 2268, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 61–67. DOI: 10.3141/2268-08

The operation procedures of the T2 comprise two phases as described in the following paragraphs. 61

62 Transportation Research Record 2268

Before Concrete Placement Place targets (see Figure 1a) at desired locations on the surface of the base or subbase. Targets should be placed at least 3 ft away from any foreign metal (dowels or tie bars) to mitigate the influence of other metal objects.

rately. As will be discussed later, ensuring that the scanner is directly above the target is key to obtaining accurate measurements. 3. Perform the scanning over the target (see Figure 1d ). Once the general location of the target is determined, set the T2 approximately 10 in. away from the target, and scan over the target at a steady speed. A thickness measurement will be displayed once the T2 scans over a distance of roughly 3 ft.

After Concrete Placement Testing can be conducted as soon as the concrete can be walked on. In this phase, three easy steps are involved: 1. Assemble the device (see Figure 1b). The T2 is usually dismantled in two parts for storage with other accessories in a compact case for easy transport. 2. Locate the target (see Figure 1c). Although the approximate location of the target should be marked while the target is being placed, T2 has a built-in capability to locate the target more accu-

Field Performance The FHWA Concrete Pavement Technology Program (CPTP) and Mobile Concrete Laboratory (MCL) are national programs of research, development, and technology transfer that operate within the FHWA Office of Pavement Technology. During the past 5 years, the T2 was used and demonstrated at several field projects in various states across the country either by the CPTP or the MCL as part of the ongoing technology transfer activities.

Target

(a)

(b)

(c)

(d)

FIGURE 1 Operation of T2: (a) reflector placement, (b) T2 assembly, (c) reflector locating, and (d) scanning operation.

Grove, Jones, Ye, and Gudimettla

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• The core measurement test procedure and the nondestructive thickness procedure do not measure to the same point on the pavement cross section. The core measurement test procedure allows for some of the granular base material to remain on the bottom of the core. The nondestructive procedure is always measuring to the top of the granular base.

Iowa Department of Transportation In 2008 the Iowa Department of Transportation (DOT) took advantage of an equipment loan program sponsored by FHWA’s CPTP and evaluated the T2 on several projects (5). The results were encouraging, so in 2009 the Iowa DOT purchased two units and 100 targets. The manufacturer’s targets were not available in the United States for a reasonable price, so the Iowa DOT had 500 targets fabricated locally by using sheet metal meeting ASTM A653 CS, Type 2, G90, 24 gauge. A calibration equation was developed for these targets by Iowa. Further testing and evaluation was conducted on three paving projects. The results of the further testing were positive, so the nondestructive method was specified in place of coring for incentive and dis incentive on two projects in 2010. In addition, further comparative testing was done on two other projects. About 1,000 targets were used in 2010. On the basis of the positive results and feedback from inspectors and contractors, three more T2 devices were purchased, and a specification and test method were written for use in 2011. The specification was applied to four large paving projects. About 2,000 targets were used on these four projects. Some of the early observations follow:

Accuracy Table 1 shows a comparison between Scan T2 measurements and lengths of cores at 106 locations from 13 concrete pavement projects across the country. Eight of the projects shown in Table 1 were from Iowa (2010 construction season), and the remaining five were from other states where the T2 was demonstrated and used as part of the MCL and CPTP. For brevity, Table 1 shows only the average pavement thickness measurements using the T2 at each of the 13 projects. To validate the T2 measurements, cores were taken from the concrete pavement on top of the targets after T2 measurements were taken. Because the T2 can detect the exact location of the targets, it was easy to locate and core on top of the targets. The cores were later measured for their length by using nine-point measurements, which is typically done by most state DOTs (AASHTO T148-97). The average pavement thickness, based on core measurements, for each of the 13 projects is also shown in Table 1. Figure 2 shows the individual T2 versus core length measurements for all 106 locations shown in Table 1. Figure 2 shows excellent correlation between the T2 and core length measurements for a range of pavement lengths from 224 mm (8.8 in.) to 375 mm (14.8 in.). Table 1 shows that except for one project in Indiana (three data points), the difference between T2 and core length measurements was 2 mm or less. The average and standard deviations of the difference between the core and T2 measurements for all 106 locations were 1.3 mm and 1.3 mm, respectively.

• The targets needed to be nailed down to keep them from being blown off the grade or moved by the concrete placement. • Locating the targets on or near an even station made placing, finding, and referencing the target locations easier. • The targets needed to be located at a spacing that would discourage the up and down adjustment of the paver. • Marking the pavement at the target center location made taking repeat readings simpler. • The project inspectors said the nondestructive thickness method was less time-consuming than the effort spent locating, witnessing, hauling, preparing, and measuring concrete cores.

TABLE 1 Average T2 and Core Length Measurements

State

Project

Iowa Iowa Iowa Iowa Iowa Iowa Iowa Iowa Washington Alabama New York Indiana North Carolina Total count

US-34 US-63 I-29 US-20 I-35 US-20 I-35 US-30 I-5 I-59 I-90 I-465 I-540

Note: SD = standard deviation.

Number of Locations

Average Core Lengths (mm)

Average T2 Measurements (mm)

8 10 8 8 10 10 10 10 6 10 4 3 9 106

272 275 282 277 291 270 303 267 236 285 326 355 314

273 275 281 276 291 269 301 266 235 285 324 361 315

Difference Between Average Core Lengths and Average T2 Measurements (mm) −1 0 1 1 0 1 1 1 1 0 2 −6 −1

SD, Core Lengths (mm)

SD, T2 Measurements (mm)

6.8 12.3 12.0 15.1 13.9 15.1 12.2 4.0 10.0 6.7 4.3 2.1 39.8

6.8 12.9 12.4 15.3 13.7 15.3 12.5 4.0 9.8 7.0 4.6 2.8 41.4


400

MIT Scan T2 Measurements (mm)

380

y = 1.0308x -9.0521 R2 = 0.9968

360 340 320 300

Iowa

280

Washington State

260

Alabama

240

New York Indiana

220 200 200

North Carolina 220

240 260 280 300 320 340 360 380 Core Thickness - 9 Point Measurement (mm)

400

FIGURE 2 Pavement thickness measurements taken with T2 and cores from pavement.

Repeatability

destructive tests, T2 measurements can be taken at any maturity level of the concrete. 3. Ease of use. The T2 is very easy to use and does not require user interpretation, unlike some of the other nondestructive technologies. One person can operate and store hundreds of thickness measurements in the device. 4. Lower cost. The cost per measurement (including the cost of the equipment and targets on the long run) is significantly cheaper than taking cores. According to conversations with DOT personnel, the cost of taking cores is about $90 to $110 per core. The cost per measurement with the T2 is less than $20 (including the target). Because of the low cost per measurement, measurements can be taken at more locations, which will yield a statistically more robust measure of pavement thickness. 5. Nondestructive. The need to cut cores on new pavements is eliminated, thereby reducing the need to patch core holes, which may require additional maintenance in later years. 6. Grinding and overlay. If the existing concrete pavement contains targets underneath and it is then diamond ground or overlaid, the thickness of the pavement after the diamond grinding or overlay can be measured accurately. 7. Base material. The accuracy of the device is independent of the type of base material. When the base material has properties similar to those of concrete, other technologies may not provide results that are as accurate. The target defines the bottom of the pavement and eliminates the problem of mortar penetrating into a

The repeatability of the T2 was checked by the Iowa DOT by measuring the same targets with two different T2 devices (T2-A and T2-B). Table 2 shows the average and standard deviation of T2 measurements taken by using two T2 devices at four projects in Iowa. A total of 388 targets were placed in four projects, and measurements were taken by using both T2 devices. Because the primary objective was only to check the repeatability of the T2, cores were not taken at these locations. Figure 3 plots the pavement thickness measurements at the 388 targets from both devices. Figure 3 indicates that the repeatability of the T2 is excellent irrespective of the thickness of the pavement. According to Table 2, the average difference in pavement thickness measurements between the two T2 devices was 1 mm or less. The average and standard deviation of the difference between the two T2 devices for all 388 locations were 0.9 mm and 0.9 mm, respectively. Advantages of the T2 1. Faster measurements. Once the approximate locations for the targets are known, finding the exact location and measuring pavement thickness take less than 3 min per location. 2. Maturity of concrete. Measurements with the T2 can be taken as soon as the pavement can be walked on. Unlike some other non-

TABLE 2 Average Thickness Measurements Taken With Two T2 Devices

Project US-63 Dallas County I-29 I-29 South

Number of Target Locations

Average T2-A Measurements (mm)

Average T2-B Measurements (mm)

Difference Between Average T2-A and Average T2-B Measurements (mm)

SD, T2-A (mm)

SD, T2-B (mm)

20 79 83 206

274 208 285 277

273 208 284 277

0 0 1 0

13.41 6.87 9.88 12.67

13.59 6.82 9.65 12.91


65

335 315

T2 - B (mm)

295

Y = 0.9983X R2 = 0.9984 STD Error = 1.17

275 255 235 215 195 175 175

195

215

235

255 T2 - A (mm)

275

295

315

335

FIGURE 3 Pavement thickness measurements taken with two T2 devices.

granular or permeable base, a problem that can make core length determination difficult. 8. Accuracy. As described in the preceding section, T2 results are very accurate (within 2 mm) when compared with the accuracy of cores.

there should be sufficient data to establish a PWL specification for future projects on a trial basis. T2 Target Studies CPTP Study

Implementation Activities The first Iowa DOT specification using the nondestructive thickness testing was a slight modification to the current specification with core drilling. Retaining much of the current specification and test procedure made the adjustment to the nondestructive method easier for the contractor staff and the DOT inspection staff. The major modifications were as follows: 1. The random test location method is changed. – The targets are located at 50-m or 200-ft intervals. – The target offset location from centerline is stratified random. – The target test location is random. 2. Three repeat readings are to be taken. If the difference between any of the readings is more than 3 mm, two additional readings are to be taken. If the two additional readings are within 3 mm of any of the first three readings, the measurement is valid for that location. If not, it is noted that the location is not valid, and the next target location not originally chosen for testing is selected. 3. When a thin area is identified, the target is retested. If retesting confirms a thin area, a core is drilled to confirm. If the core measurement confirms a thin area, core drilling continues to determine the extent of the thin area. 4. An adjustment to the current thickness incentive and disincentive levels was applied to specification. Testing had shown that the pavement cores over granular base consistently measured longer than the NDT results. An example of that testing is shown in Figure 4. 5. Independent assurance testing is done by checking a minimum of 10% of the locations with a different T2 and a different tester not associated with the project verification testing. The nondestructive thickness determination method would fit well into a percent within limit (PWL) specification. After 2012,

A target study was carried out under the CPTP to assess the feasibility of domestic substitutes for targets (6). Findings of that study are summarized as follows: 1. Domestic targets are able to produce repetitive measurements. 2. Orientation of square reflectors has a negligible impact on measurements (Figure 5). 3. As the scanning path wanders away (shown as the dashed lines in Figure 5) from the centroid of a reflector, the thickness measurements increase (Figure 6).

FIGURE 4 Cores taken on dense graded versus open graded bases. (left column: lot data without transformation; right column: modified Box–Cox transformation using golden section search algorithm).


Path traversing the center of the target

Path wandered from the center of the target

(a)

(b)

FIGURE 5 Square reflector orientation and scanning path: (a) parallel scanning path and (b) diagonal scanning path.

Iowa DOT Work with Targets To establish a consistent source of targets, the Iowa DOT contacted several metal fabricators that had the equipment and the interest to fabricate the targets as needed. Rather than trying to match the European standard for galvanized sheet steel, a readily available commercial grade of galvanized sheet steel was tested. The surface area, shape, galvanizing thickness, and base steel properties all have an influence on the T2 readings. ASTM A653 Type 2 commercial grade, G90 galvanized sheet steel from two Midwest steel mills produced similar responses on the T2 readings. The target thickness does not seem to be a critical property. To stay consistent with the manufacturer’s targets, a 24-gauge steel was used, which was a readily available gauge thickness and closely matched the manufacturer’s target thickness of 0.6 mm.

The fabricator used a computer-controlled laser cutter to cut the targets to a 300 mm +/−0.1 mm diameter. Each new shipment of targets is checked for coating thickness and dimensions and then tested with the T2 at preset depths. The G90 galvanized coating is much thicker than the coating on the manufacturer’s targets and produces a slightly greater thickness reading, but a consistent response on the T2. Other Issues • Because the targets are placed before the concrete is placed, it is often asked whether there is a tendency by contractors to adjust operations to increase the thickness of the pavement at the target locations. This is not an issue, because pavement smoothness is also a specification item and any sudden changes to the thickness of the

275 270 265

Averaged Depth (mm)

260 255 250 245 240 235 230 M-1 12S-1 Parallel 13S-1 Diagonal

225 220 0

1

2

11S-1 Parallel 12S-1 Diagonal 14S-1 Parallel

3 4 Offset from the Centerline (in.)

FIGURE 6 Effect of wandered path on thickness measurement.

5

11S-1 Diagonal 13S-1 Parallel 14S-1 Diagonal 6

7


pavement at the target locations may not comply with the smoothness requirement. In addition, more targets can be placed than required, and targets can be selected at random at a later time for pavement thickness measurement. The advent of stringless paving, in which the paver grade is controlled by digital models, makes it even less likely that random thickness adjustments could or would be attempted. • As mentioned elsewhere in the paper, using the target defines the bottom of the pavement and eliminated the problem of mortar penetrating into open graded bases. The Iowa DOT testing had shown that the pavement cores over granular or open graded bases consistently measured longer than the T2 results as a result of mortar penetration.

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• In addition to Iowa, other states such as Minnesota are also evaluating the T2 and have plans to adopt this technology in their specifications. • There is also an effort to develop an AASHTO standard test method for using this technology. References

Ongoing Efforts

1. Mechanistic–Empirical Pavement Design Guide, Interim Edition: A Manual of Practice. AASHTO, Washington, D.C., 2008. 2. Allison, G. W., G. C. Whited, A. S. Hanna, and H. G. Nasief. Evaluation of Probing Versus Coring for Determination of Portland Cement Concrete Pavement Thickness. In Transportation Research Record: Journal of the Transportation Research Board, No. 2152, Transportation Research Board of the National Academies, Washington D.C., 2010, pp. 3–10. 3. Morous, G., and E. Erdogmus. Use of Ground Penetrating Radar for Construction Quality Assurance of Concrete Pavement. Final report. NDOT Project Number P307. Nebraska Department of Roads, Lincoln, 2009. 4. Maser, K. R., J. Holland, R. Roberts, J. Popovics, and A. Heinz. Technology for Quality Assurance of New Pavement Thickness. Presented at 82nd Annual Meeting of the Transportation Research Board, Washington D.C., 2003. 5. Jones, K. B., and T. Hanson. Evaluation of the MIT Scan T2 for Nondestructive PDD Pavement Thickness Determination. Office of Materials, Iowa Department of Transportation, Ames, 2008. 6. Ye, D., and S. Tayabji. Tech Brief: Determination of Concrete Pavement Thickness Nondestructively using the Magnetic Imaging Tomography Technique. Concrete Pavement Technology Program, FHWA, U.S. Department of Transportation, 2009.

• There are currently efforts to identify U.S. sources for targets for easy access and further reduction in their cost.

The Portland Cement Concrete Pavement Construction Committee peer-reviewed this paper.

Conclusions The magnetic pulse induction technology offers a faster, easier, nondestructive, and significantly cheaper way to measure pavement thickness compared with the traditional method of taking cores. On the basis of the data shown for the T2 device, which uses the magnetic pulse induction technology, the accuracy of this technique is within 2 mm of the core test results for a wide range of pavement thicknesses. The repeatability, which is an important criterion for any test method, is excellent. Clearly there are many advantages compared with other nondestructive pavement thickness measuring techniques.