Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
59 This chapter was prepared by Ray Brown and Haley Bell of the U.S. Army Corps of Engineers Engineering Research and Development Center (ERDC). Introduction The lightweight deflectometer (LWD) is a nondestructive testing (NDT) device that provides a structural evaluation of pavement by using a drop weight and one to three sensors (Figure 6.1). The drop weights for the LWD are selected from 22, 33, or 44 lb, and the loading plate diameter can be adjusted to 3.8, 7.8, or 11.8 in. The weight is dropped from an adjustable height onto a rubber buffer located on top of a load cell. The results are presented as a plot of the time history of the load cell and the geophones. Peak deflection (recorded in mm) and surface modulus measurements of each sensor are recorded. Peak deflection is recorded in mils, and a mil is one thousandth of an inch. Round 1 testing took place in October 2009. The LWD was used to test hot-mix asphalt (HMA) samples in the National Center for Asphalt Technology (NCAT) laboratory and in field test sections on the NCAT Pavement Test Track. The purpose of this round of testing was to determine whether the LWD could identify sections with delamination at warmer temperatures. For the Round 2 testing conducted in February 2010, the LWD was tested only on test sections constructed on the NCAT test track. The purpose of the Round 2 testing was to evaluate the potential for using the LWD to identify delamination at cooler temperatures. The drop weight used for all LWD testing was 22 lb, and the spacing of the three geophones was approximately 6 in. The 7.8-in.-diameter load plate was also used. During Round 1, at least three tests were conducted at each test point on the laboratory slabs and on the test track. During Round 2, LWD testing was conducted at every other test point on the test track, resulting in approximately half the number of data points collected from the Round 1 testing. The LWD data were collected, inspected, and input into Excel spreadsheets. The waveforms from the raw data were evaluated for their shape and smoothness. If the waveform was not satisfactory, then the data were not included in the analysis. Measured deflection was the only data used for analysis, specifically Accelerometers D1 and D2, which were the two accelerometers closest to the weight. During both rounds of testing, Accelerometer D3 did not appear to provide much information; the measured deflections for D3 were close to zero in the field and in the laboratory. A closer inspection of D3, after the Round 2 evaluation was completed, proved that the accelerometer was not working properly during Round 1 and Round 2 testing. Laboratory Testing During the Round 1 laboratory testing with the LWD, the condition of the pavements was unknown. The measured deflections for the two slabs are shown in Tables 6.1 and 6.2. The laboratory results indicated that there were edge effects around the perimeters of each slab. This was indicated by a general increase in deflection of 20% to 100% around the edges. Because of the edge effects and the transition between two different sections in each slab, only four measured points for the two slabs were considered worthy for analysis. For Slab 1, the test locations used were Points 2-2 and 6-2 (highlighted in yellow in Table 6.1). For Slab 2, the test locations used were Points 9-2 and 13-2 (highlighted in yellow in Table 6.2). On the basis of information provided in Tables 6.1 and 6.2, it appeared that the section of Slab 1 represented by Point 2-2 was fully bonded because it had less measured deflection than the other three test areas did. Points 6-2 and 9-2 had the most measured deflection and are therefore likely to be the ones that had delamination. Point 13-2 appeared to be in the middle of the measurements, so it is likely that this area had simulated stripping. C h a p T e r 6 Controlled Evaluation of Lightweight Deflectometer
60 The assumptions were confirmed after analysis with a layout of the pavement structures and their deficiencies pro- vided by the NCAT personnel. Point 2-2 was assumed to be fully bonded, and the layout showed that the condition of this area of Slab 1 was not delaminated. Points 6-2 and 9-2 were assumed to be debonded on the basis of measured deflections. Figure 6.1. LWD testing on HMA pavement. Table 6.1. Slab 1 LWD Laboratory Test Deflection Measurements (mil) Slab 1 D1 D2 Location 1 2 3 1 2 3 1 7.97 5.37 4.90 6.97 4.54 4.25 2 6.09 4.20 4.47 5.68 3.54 3.89 3 5.98 3.57 4.63 3.42 4.42 5.85 4 5.98 4.19 5.20 5.85 3.98 5.32 5 7.01 5.23 5.97 6.36 4.59 5.29 6 7.97 6.70 6.42 6.93 5.75 5.65 7 11.23 9.28 7.90 9.49 7.87 6.80 Table 6.2. LWD Laboratory Test Deflection Measurements for Slab 2 (mil) Slab 2 D1 D2 Location 1 2 3 1 2 3 8 7.43 6.67 12.77 6.37 6.00 11.72 9 6.22 6.18 11.18 5.35 5.75 10.65 10 6.02 5.90 10.50 5.68 5.41 9.32 11 6.69 5.69 8.35 6.71 5.39 7.02 12 7.50 5.36 6.25 7.01 5.25 6.39 13 7.97 5.39 5.46 8.12 5.25 5.19 14 8.96 5.77 5.24 8.69 5.47 4.93 Figure 6.2. LWD testing on NCAT test track. The layout of the deficiencies of Slabs 1 and 2 showed that there was debonding at depths of 2 and 4 in., respectively. Point 13-2 was assumed to have stripping, which was also confirmed after analysis. Field Testing The pavement temperature at the beginning of the Round 1 tests was approximately 52°F, and the temperature at the end of the tests was approximately 70°F. For Round 2 testing, the pavement temperature ranged from 47°F to 56°F. Fig- ure 6.2 shows the LWD testing on the test track. Significant differences were evident between the magnitude of the data collected on the test track and the data collected in the labora- tory. The data collected at the track showed the deflections to be almost always less than 1 mil, whereas the laboratory measured deflections were generally above 4 to 5 mils. This
61 result is not too surprising, because it is not possible to construct the small sections to be as stiff as can be done at the track. There were significant differences within the LWD data collected along the test section on the test track. Similar trends were seen with the Round 1 testing in October and the Round 2 testing in February. However, the average measured deflections from Round 2 testing were slightly lower, as shown in Figure 6.3. The measured deflections from Round 1 were approximately 80% higher than the measured deflections from Round 2, particularly in Sections 5 through 10. This result was expected because of the lower pavement temperatures. The pavement from Stations 0 to approximately 1+00 was backfilled with reclaimed asphalt pavement (RAP), which explains the higher measured deflections of Sections 1 through 4 compared to Sections 5 through 10. Figures 6.4 through 6.7 present the results of Tests 1 through 4 at each station for Rounds 1 and 2 testing. Fig- ures 6.4 and 6.5 are the results of D1 and D2, respectively, for Round 1 testing, while Figures 6.6 and 6.7 are the results of D1 and D2, respectively, for Round 2 testing. The amount of testing was reduced by approximately 50% during Round 2 to eliminate unneeded data points. Test 1 is the outside of the lane near the pavement shoulder, and Test 4 is the inside of the lane near the centerline. There is significant scatter among the lanes with the D1 measurements for both rounds of testing; however, there is little scatter among the four lines of data with the D2 measurements. The 10 sections were also analyzed separately by using the information provided in the test track layout. The data points were analyzed to determine the differences in mea- sured deflection between each of the sections by using the various methods to simulate delamination and to compare the measured deflections of a fully bonded area to a debonded area. The measured deflections for each type of simulated delamination and bonded area were compared to each other for Sections 7 through 10. Sections 1 through 6 were left out of the comparison because there was concern about the effect of the backfilled RAP material on deflections in these sec- tions. Figures 6.8 and 6.9 show the results of the comparisons for Round 1 and Round 2, respectively. As stated earlier, the analyses revealed the same conclusions for both rounds of testing. As shown in Figures 6.8 and 6.9, the fully bonded areas provided similar deflection measurements to those from areas with RAP and baghouse dust. Also, simu- lated debonding using paper had much higher deflections compared to the baghouse dust, RAP, and fully bonded areas. It seems as though using the paper created loss in bond and possibly some loss in friction between the two layers, while the baghouse dust may have resulted in a higher bond than when the paper was used. The higher measured deflections with paper may be an indication of little friction between the layers under loading, resulting in more relative movement at the interface between the two layers. As the load from the LWD is applied, the HMA layers are able to slide along the paper, resulting in increased movement and deflection. Some friction most likely exists within the delaminated areas simulated with baghouse dust because of the texture provided by the baghouse dust. Summary On the basis of laboratory and field testing results, the LWD appears to be able to show differences in deflection between the various sections. It is difficult to identify for sure what causes this change in deflection, but certainly a section that is delaminated should have more deflection than a section that is not delaminated. Although the LWD can detect changes in the pavement structure, it is doubtful that it can be used to identify what causes the change in deflection and the depths at which the delaminations occur. The research team used an LWD on the test sections con- taining good bond and delamination. Since the LWD equip- ment can be included at no additional cost to the project, it is expected that the LWD equipment will continue to be used to identify delamination. Additional analysis approaches will be evaluated for the LWD to improve its ability to identify delamination.
62 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 D ef le ct io n (m ils ) Station (ft) D1 - Round 1 Testing 1 2 3 4 1 432 1098765 Figure 6.4. D1 measurements for Round 1 on each test location on test track. Figure 6.3. D1 average deflection measurements from Rounds 1 and 2 testing on test track. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 D ef le ct io n (m ils ) Station (ft) D1 Round 1 Testing Round 2 Testing
63 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 D ef le ct io n (m ils ) Station (ft) D1 - Round 2 Testing 1 2 3 4 1 432 1098765 Figure 6.6. D1 measurements for Round 2 on each test location on test track. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 D ef le ct io n ( m ils ) Station (ft) D2 - Round 1 Testing 1 2 3 4 1 432 1098765 Figure 6.5. D2 measurements for Round 1 on each test location on test track.
64 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 D ef le ct io n (m ils ) Station (ft) Average Deflection per Simulated Delamination - Round 1 fully bonded (D1) RAP (D1) paper (D1) bag house dust (D1) 7 1098 0.32-0.39 0.32-0.45 0.26-0.36 0.43-0.60 0.30-0.38 0.31-0.33 0.41-0.67 0.36-0.43 Figure 6.8. Average deflections of each delamination type in Round 1 from Sections 7 through 10. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 D ef le ct io n (m ils ) Station (ft) D2 - Round 2 Testing 1 2 3 4 1 432 1098765 Figure 6.7. D2 measurements for Round 2 on each test location on test track.
65 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 D ef le ct io n (m ils ) Station (ft) Average Deflection per Simulated Delamination - Round 2 7 1098 fully bonded (D1) RAP (D1) paper (D1) bag house dust (D1) 0.21 - 0.36 0.22 - 0.34 0.19 - 0.43 0.20 - 0.39 0.21 0.32 - 0.46 0.22 - 0.30 0.21 - 0.31 Figure 6.9. Average deflections of each delamination type in Round 2 from Sections 7 through 10.
