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8 Table 3. Projects and material types included in the field evaluation. Part Project Identification & Location Layer/Material Evaluated HMA Dense-Graded Base Mixture TH-23 Reconstruction Project; Granular Base Class 6, Crushed Aggregate A 1 Wilmar/Spicer Minnesota Class 5 Low Plasticity, Improved Soil with Gravel & Embankment Large Aggregate Particles I-85 Overlay Project; Auburn, 12.5 mm Stone Matrix Asphalt Mix; PG76- A 2 HMA Alabama 22 HMA Coarse-Graded Base Mixture; PG67-22 US-280 Reconstruction Project; A 3 Granular Base Crushed Limestone Base Opelika, Alabama Embankment Improved Soil; Aggregate-Soil Mix I-85 Ramp Construction Project; A 4 Embankment Low Plasticity, Fine-Grained Soil Auburn, Alabama Coarse-Graded 19 mm Base Mixture; PG64- HMA SH-130 New Construction Project; 22 A 5 Georgetown, Texas Coarse-Grained Aggregate/Soil; Improved Embankment Soil SH-21 Widening Project; Caldwell, High Plasticity Fine-Grained Soil with A 6 Subgrade Texas Gravel US-47 Widening Project; St. Clair, HMA Coarse-Graded Base Mixture B 7 Missouri HMA Fine-Graded Wearing Surface I-75 Rehabilitation Project, B 8 HMA Dense-Graded Binder Mixture; Type 3C Rubblization; Saginaw, Michigan HMA Coarse-Graded Base Mix; PG58-28 Crushed Gravel with Surface Treatment; B 9 US-2 New Construction; North Dakota Granular Base Class 5 Embankment Soil-Aggregate Mixture US-53 New Construction; Toledo, HMA Coarse-Graded Binder Mixture B 10 Ohio Granular Base Crushed Aggregate; Type 304 B 11 I-20 Overlay; Odessa, Texas HMA Coarse-Graded Mixture; CMHB B 12 County Road 103; Pecos, Texas Granular Base Caliche, Aggregate Base NCAT; Alabama Overlay, Section E-5, Wearing Surface with 45% RAP; PG67, no HMA Opelika, Alabama modifiers used. NCAT; Alabama Overlay, Section E-6, Wearing Surface with 45% RAP; PG76 with B 13 HMA Opelika, Alabama SBS. NCAT; Alabama Overlay, Section E-7, Wearing Surface with 45% RAP; PG76 with HMA Opelika, Alabama Sasobit. HMA PMA Mixture with SBS; PG76 NCAT; Florida; Structural Test B 14 HMA Neat Asphalt Binder Mix; PG67 Sections N-1 & N-2 Granular Base Limerock Base HMA Polymer Modified Asphalt Mix; PG76 (SBS) NCAT; Missouri; Structural Test B 15 HMA Neat Asphalt Binder Mix; PG64 Section N-10 Granular Base Crushed Limestone NCAT; Oklahoma; Structural Test B 16 Subgrade Soil High Plasticity Clay with Chert Aggregate Sections N-8 & N-9 NCAT; Alabama; Structural Test HMA Coarse-Graded Base Mix; PG67; Limestone B 17 Section S-11 Granular Base Crushed Granite Base CMHB Coarse Matrix, High Binder Content (mixture type term used by the Texas DOT specifications) PG Performance Grade PMA Polymer Modified Asphalt RAP Recycled Asphalt Pavement Field Evaluation of NDT Devices Identifying Anomalies and Physical Differences A standard t-test and the Student-Newman-Keuls (SNK) mean separation procedure using a 95 percent confidence level were used to determine whether the areas with anomalies or physical differences were significantly different from the other areas tested. Table 7 lists identification of the physical differences of the unbound and HMA layers within a project. The DSPA and GeoGauge are considered acceptable in identifying localized differences in the physical condition of unbound materials, while the PSPA and PQI were considered acceptable for the HMA layers.
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9 Table 4. Construction defects exhibited on some of the field evaluation projects. Unbound Materials and Layers; Embankments No construction defect was observed in any of the Parts A and B projects. As listed in Table 5, however, there were All projects differences in the condition of the base materials and embankments that were planned to ensure that the NDT devices would identify those differences. HMA Mixtures Truck-to-truck segregation observed in some areas. Cores were taken in these areas, but some of the cores disintegrated during the wet coring process. US-280 HMA Base In addition, a significant difference in dynamic modulus was found between the initial and supplemental sections included in the test program. The supplemental section was found to have much higher dynamic modulus values. This difference was not planned. I-85 SMA Overlay No defects noted. TH-23 HMA Base No defects noted. No defects noted during the time of testing, but there was controversy on the mixture because it had been exhibiting checking during the compaction process. Changes were made to SH-130 HMA Base the mixture during production. The change made and the time that the change was made were unclear relative to the time of the NDT evaluation. US-47 HMA Base The mixture was tender; and shoved under the rollers. Portions of this mixture were rejected by the agency in other US-47 Wearing Surface areas of the project. No defects noted, but mixture placed along the shoulder was I-75 HMA Base, Type 3-C tender. No defects noted, but portions of this mixture were rejected by I-75 HMA, Type E3 & E10 the agency in other areas of the project. US-2 HMA Base Checking and mat tears observed under the rollers. US-53 HMA Base No defects noted. I-20 HMA CHMB Base No defects noted. NCAT Alabama HMA RAP; No defects noted on any of the test sections. with & without modifiers NCAT South Carolina HMA No defects noted. Base NCAT Missouri HMA Base No defects noted. NCAT Florida PMA Base No defects noted. NCAT Florida HMA Base, Checking and mat tears observed under the rollers. no modification Estimating Laboratory Measured Moduli Laboratory measured modulus of a material is an input parameter for all layers in mechanistic- empirical (M-E) pavement structural design procedures, including the MEPDG. Resilient mod- ulus is the input for unbound layers and soils, while the dynamic modulus is used for all HMA layers. The values determined by each NDT modulus estimating device (DCP, DSPA, PSPA, GeoGauge, and deflection-based devices) were compared to the moduli measured in the labo- ratory on test specimens compacted to the density of the in-place layer. Different stress states were used for determining the resilient modulus of unbound layers, while different frequencies at the in-place mat temperature were used to determine the dynamic modulus of the HMA layers. None of the NDT devices accurately predicted the modulus values that were measured in the laboratory for the unbound materials and HMA mixtures. However, all of the modulus estimat- ing NDT devices did show a trend of increasing moduli with increasing laboratory measured moduli.
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10 Table 5. Physical differences in the unbound materials and soils placed along some of the projects. Project Unbound Sections Description of Differences Along Project Identification No planned difference between the points Area 2, No IC Rolling SH-21 Subgrade, tested. High Plasticity Clay; With intelligent compaction (IC) rolling, the Caldwell, Texas Area 1, With IC Rolling average density should increase; lane C received more roller passes. Prior to IC rolling, Lane A (which is further Lane A of Sections 1 & 2 I-85 Embankment, from I-85) had thicker lifts & a lower density. Low Plasticity Clay; After IC rolling, the average density should Auburn, Alabama All Sections increase & the variability of density measurements should decrease. Construction equipment had disturbed this TH-23 Embankment, area. In addition, QA records indicate that this South Section Lane C Silt-Sand-Gravel area has a lower density--prior to final Mix; Spicer, acceptance. Minnesota Area with the higher density and lower water North Section Lane A content--a stronger area. SH-130, Improved Embankment, No planned differences between the areas All Sections Granular; tested. Georgetown, Texas Curb and gutter section; lane C was wetter than Section 2 (Middle Section) the other two lanes because of trapped water TH-23, Crushed Lane C along the curb from previous rains. The water Aggregate Base; extended into the underlying layers. Spicer, Minnesota Section 1 (South Section) Area with a higher density and lower moisture Lane A content; a stronger area. Records indicate that this area was placed with US-280, Crushed higher water content and is less dense. It is also Stone Base; Opelika, Section 4 in an area where water (from previous rains) Alabama accumulated. Table 6. Different physical conditions (localized anomalies) of the HMA mixtures placed along projects within Part A. Project HMA Sections Description of Differences Along the Project Identification TH-23 HMA QA records indicate lower asphalt content in this Section 2, Middle or Base; Spicer, area--asphalt content was still within the Northeast Section Minnesota specifications, but consistently below target value. Section 2, Middle; QA records indicate higher asphalt content in this area, I-85 SMA All lanes but it was still within the specifications. Overlay; Auburn, This part or lane was the last area rolled using the Alabama Lane C, All Sections rolling pattern set by the contractor, and was adjacent to the traffic lane. Densities lower within this area. Initial Test Sections, Segregation identified in localized areas. In addition, defined as A; Section QA records indicate lower asphalt content in this area 2, All Lanes of the project. Densities lower within this area. Supplemental Test US-280 HMA Sections near crushed Segregation observed in limited areas. Base Mixture; stone base sections, Opelika, Alabama defined as B. IC Roller Compaction Effort Higher compaction effort was used along Lane C. Section, Defined as C. SH-130 HMA Base Mixture; All Sections No differences between the different sections tested. Georgetown, Texas
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11 Table 7. Success rates of the NDT devices for identifying physical differences or anomalies. Success Rates, % NDT Gauges Included in Field Evaluation Unbound Layers HMA Layers Ultrasonic DSPA & PSPA 86 93 Steady-State Vibratory GeoGauge 79 --- Impact/Penetration DCP 64 --- Deflection-Based LWD & FWD 64 56 Non-Nuclear Density EDG & PQI 25 71 GPR Single Air-Horn Antenna 33 54 To compensate for differences between the laboratory and field conditions, an adjustment procedure was used to estimate the laboratory resilient modulus from the different NDT tech- nologies for making relative comparisons. The adjustment procedure assumes that the NDT response and modulus of laboratory prepared test specimens are directly related and propor- tional to changes in density and water content of the material. In other words, the adjustment factors are independent of the volumetric properties of the material. Table 8 lists the adjustment ratios for the unbound layers included in the field evaluation (Parts A and B), while Table 9 contains the ratios for the HMA layers. The adjustment ratios Table 8. Unbound layer adjustment ratios applied to the NDT moduli to represent laboratory conditions or values at low stress states. Adjustment Ratios Relating Resilient Moduli, ksi Laboratory Moduli to NDT Values Project Identification Laboratory Predicted Geo Measured with LTPP DSPA DCP LWD Gauge Value Equations Fine-Grained Clay Soils I-85 Low- Before IC Rolling 2.5 10.5 0.154 .0751 0.446 0.39 Plastic Soil After IC Rolling 4.0 13.1 0.223 0.113 0.606 0.39 NCAT; OK High Plastic Clay 6.9 19.7 0.266 0.166 0.802 --- SH-21, TX High Plastic Clay 26.8 19.6 1.170 0.989 3.045 2.78 Average Ratios for Fine-Grained Clay Soils 0.454 0.336 1.225 Embankment Materials; Soil-Aggregate Mixtures South Embankment 16.0 15.7 0.696 0.367 1.053 3.13 TH-23, MN North Embankment 16.4 16.3 0.735 0.459 0.863 3.13 US-2, ND Embankment 19.0 19.5 1.450 0.574 0.856 --- SH-130, TX Improved Soil 35.3 21.9 1.337 1.029 1.657 1.43 Average Ratios for Soil-Aggregate Mixtures; Embankments 1.055 0.607 1.107 Aggregate Base Materials Co. 103, TX Caliche Base --- 32.3 1.214 --- 1.436 --- NCAT, SC Crushed Granite 14.3 36.1 0.947 0.156 --- --- NCAT, MO Crushed Limestone 19.2 40.9 0.747 0.198 --- --- Crushed Stone, Middle 24.0 29.9 0.851 0.303 0.725 1.69 TH-23, MN Crushed Stone, South 26.0 35.6 0.788 0.235 0.560 1.69 US-53, OH Crushed Stone 27.5 38.3 1.170 0.449 0.862 --- NCAT, FL Limerock 28.6 28.1 0.574 0.324 0.619 --- US-2, ND Crushed Aggregate 32.4 39.8 1.884 0.623 1.129 --- US-280, AL Crushed Stone 48.4 49.3 1.010 0.244 0.962 1.04 Average Ratios for Aggregate Base Materials 1.021 0.316 0.899 Overall Average Ratios for Processed Materials 0.942 0.422 1.084 NOTES: 1. The adjustment ratio is determined by dividing the resilient modulus measured in the laboratory at a specific stress state by the NDT estimated modulus. 2. The overall average values listed above exclude those for the fine-grained clay soils.
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12 Table 9. HMA layer adjustment ratios applied to NDT modulus values to represent laboratory conditions. Dynamic Ratio or Adjustment Factor Project/Mixture Modulus, ksi PSPA FWD I-85 AL, SMA Overlay 250 1.055 0.556 TH-23 MN, HMA Base 810 1.688 NA US-280 AL, HMA Base; Initial Area 650 1.407 3.939 US-280 AL, HMA Base; Supplemental Area 780 1.398 2.516 I-35/SH-130 TX, HMA Base 1,750 5.117 3.253 I-75 MI, Dense-Graded Type 3-C 400 0.919 NA I-75 MI, Dense-Graded Type E-10 590 0.756 NA US-47 MO, Fine-Graded Surface 530 1.158 NA US-47 MO, Coarse-Graded Base Mix 420 0.694 NA I-20 TX, HMA Base, CMHB 340 0.799 NA US-53 OH, Coarse-Graded Base 850 1.275 NA US-2 ND, Coarse-Graded Base, PG58-28 510 1.482 NA NCAT AL, PG67 Base Mix 410 0.828 NA NCAT FL, PG67 Base Mix 390 0.872 NA NCAT FL, PG76 Base Mix 590 1.240 NA NCAT AL, PG76 with RAP and Sasobit 610 1.3760 NA NCAT AL, PG76 with RAP and SBS 640 1.352 NA NCAT AL, PG67 with RAP 450 0.881 NA Overall Average Ratio 1.128 2.566 NOTES: 1. The adjustment factor or ratio was determined by dividing the dynamic modulus measured in the laboratory for the in-place temperature and at a loading frequency of 5 Hz by the modulus estimated with the NDT device. 2. The laboratory dynamic modulus values listed above are for a test temperature of a loading frequency of 5 Hz at the temperature of the mixture when the NDT was performed. 3. The overall average adjustment factor excludes the SH-130 mixture because it was found to be significantly different than any other mixture tested in the laboratory; which has been shaded. were determined for the areas without any anomalies or physical differences from the target properties. · Unbound Layers. The GeoGauge and DCP provided a reasonable estimate of the laboratory measured values (average ratios near unity), with the exception of the fine-grained, clay soils. The GeoGauge deviated significantly from the laboratory values for the fine-grained soils. The results also show that both the GeoGauge and DCP over predicted or under predicted the laboratory measured values for the same material, with few exceptions. · HMA Layers. The PSPA average adjustment ratios were found to be relatively close to unity, with the exception of the I-35/SH-130 HMA base mixture. Conversely, the FWD adjustment ratios were significantly different from unity. The FWD over estimated the stone matrix asphalt (SMA) modulus for the overlay project and under estimated the HMA base modulus for the reconstruction projects--suggesting that the calculated values from the deflection basins are being influenced by the supporting materials. Accuracy and Precision of Different NDT Devices Tables 10 through 12 summarize the statistical analyses of the NDT devices included in the field evaluation projects for unbound fine-grained soils, unbound processed materials, and HMA mixtures, respectively. This information is grouped into two areas--those NDT devices with an acceptable to excellent success rate and those with poor success rates in identifying material/layer differences. Summary of Evaluations The steady-state vibratory (GeoGauge) and ultrasonic (DSPA) are the two technologies suggested for use in judging the quality of unbound layers, while the ultrasonic (PSPA) and
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13 Table 10. NDT device and technology variability analysis for the fine-grained clay soils. Statistical Value 95% Pooled Material Property NDT Devices Standard Precision Standard Error Tolerance Deviation NDT Devices with Good Success Rates Based on Modulus or Volumetric Properties GeoGauge 2.5 4.9 1.1 Structural Modulus, ksi DSPA 4.5 8.8 1.2 Properties Thickness, in. None NA NA NA Density, pcf None NA NA NA Volumetric Air Voids, % None NA NA NA Properties Fluids Content, % None NA NA NA NDT Devices with Poor (or Undefined) Success Rates Based on Modulus or Volumetric Properties DCP 3.8 7.4 1.9 Structural Modulus, ksi LWD/FWD 5.9 11.6 2.0 Properties Thickness, in. GPR, single antenna NA NA NA GPR, single antenna --- --- 4.2 Volumetric Density, pcf EDG 0.8 1.6 0.7 Properties Water Content, % EDG 0.2 0.4 0.5 Table 11. NDT device and technology variability analysis for the processed materials and aggregate base materials. Statistical Value 95% Pooled Material Property NDT Devices Standard Precision Standard Error Tolerance Deviation NDT Devices with Good Success Rates Based on Modulus or Volumetric Properties GeoGauge 2.5 4.9 1.8 Structural Modulus, ksi DSPA 4.5 8.8 1.5 Properties Thickness, in. None NA NA NA Density, pcf None NA NA NA Volumetric Air Voids, % None NA NA NA Properties Fluids Content, % None NA NA NA NDT Devices with Poor (or Undefined) Success Rates Based on Modulus or Volumetric Properties DCP 3.8 7.4 5.3 Structural Modulus, ksi LWD/FWD 5.9 11.6 2.0 Properties Thickness, in. GPR, single antenna 0.80 1.5 0.6 GPR, single antenna 3.4 6.7 3.0 Volumetric Density, pcf EDG 1.0 2.0 0.8 Properties Water Content, % EDG 0.2 0.4 0.6 Table 12. NDT device and technology variability analysis for the HMA mixtures. Statistical Value 95% Pooled Material Property NDT Devices Standard Precision Standard Error Tolerance Deviation NDT Devices with Good Success Rates Based on Modulus or Volumetric Properties Structural Modulus, ksi PSPA 76 150 56 Properties Density, pcf PQI & PT 1.7 3.4 2.5 Volumetric Air Voids, % None NA NA NA Properties Fluids Content, % None NA NA NA NDT Devices with Poor (or Undefined) Success Rates Based on Modulus or Volumetric Properties Modulus, ksi FWD 87 170.5 55 Structural GPR, single antenna 0.25 0.49 0.3 Properties Thickness, in. GPR, multiple antenna 0.27 0.55 --- Density, pcf GPR, multiple antenna 1.6 3.1 --- Volumetric Asphalt Content, % GPR, multiple antenna 0.18 0.36 --- Properties GPR, single antenna 0.40 0.8 2.1 Air Voids, % GPR, multiple antenna 0.22 0.4 ---
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14 non-nuclear density gauges (the PaveTracker was used in Part B) are the technologies suggested for use of HMA layers. The GPR is suggested for layer thickness acceptance, while the IC rollers are suggested for use on a control basis for compacting unbound and HMA layers. NDT Devices for Unbound Layers and Materials · The DSPA and GeoGauge devices had the highest success rates for identifying an area with anomalies, with rates of 86 and 79 percent, respectively. The DCP and LWD identified about two-thirds of the anomalies, while the GPR and EDG had unacceptable rates below 50 percent. · Three to five repeat measurements were made at each test point with the NDT devices, with the exception of the DCP. The LWD exhibited low standard deviations that were less dependent on material stiffness with a pooled standard deviation less than 0.5 ksi. One reason for the low values is that the moduli were less than for the other devices. The coefficient of variation (COV), an estimate of the normalized dispersion, however, was higher. It is expected that the supporting layers had an effect on the results. The GeoGauge had a standard deviation for repeatability measurements varying from 0.3 to 3.5 ksi. This value was found to be material dependent. The DSPA had the lowest repeatability, with a standard deviation varying from 1.5 to 21.5 ksi. The reason for this higher variation in repeat readings is that the DSPA sensor bar was rotated relative to the direction of the roller, while the other devices were kept stationary or did not have the capability to detect anisotropic conditions. No significant difference was found relative to the direction of testing for fine-grained soils, but there was a slight bias for the stiffer coarse-grained materials. The EDG was highly repeatable with a standard deviation in density measurements less than 1 pcf, while the GPR had poor repeatability based on point measurements. Triplicate runs of the GPR were made over the same area or sublot. For comparison to the other NDT devices, the values measured at a specific point, as close as possible, were used. Use of point specific values from successive runs could be a reason for the lower repeatability, which are probably driver specific. One driver was used for all testing with the GPR. · The COV was used to compare the normalized dispersion measured with different NDT devices. The EDG consistently had the lowest COV with values less than 1 percent. The GeoGauge had a value of 15 percent, followed by the DSPA, LWD, DCP, and GPR. The GPR and EDG are dependent on the accuracy of other tests in estimating volumetric properties (density and moisture contents). Any error in the calibration of these devices for the specific material is directly reflected in the resulting values, which probably explains why the GPR and EDG devices did not consistently identify the areas with anomalies or physical differences. · Repeated load resilient modulus tests were performed in the laboratory for characterizing and determining the target resilient modulus for each material. Adjustment ratios were deter- mined based on uniform conditions. The overall average ratio for the GeoGauge for the stiffer coarse-grained materials was near unity (1.05). For the fine-grained, less stiff soils, the ratio was about 0.5. After adjusting for laboratory conditions, all NDT devices that estimate resilient modulus resulted in low residuals (laboratory resilient modulus minus the NDT elastic modulus). However, the GeoGauge and DCP resulted in the lowest standard error. The LWD had the highest residuals and standard error. · The DSPA and DCP measured responses represent the specific material being tested. The DCP, however, can be affected significantly by the varying amounts of aggregate particles in fine-grained soils and the size of the aggregate in coarse-grained soils. The GeoGauge measured responses are minimally affected by the supporting materials, while the LWD can be signifi- cantly affected by the supporting materials and thickness of the layer being tested. Thickness
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15 deviations and variable supporting layers are reasons for LWD's low success rate in identifying areas with anomalies or physical differences. · No good or reasonable correlation was found between the NDT devices that estimate modulus and those devices that estimate volumetric properties. · Instrumented rollers were used on too few projects for a detailed comparison to the other NDT devices. The rollers were used to monitor the increase in density and stiffness with increasing number of roller passes. One potential disadvantage with these rollers is that they may bridge localized soft areas. However, based on the results obtained, their ability of provide uniform compaction was verified and these rollers are believed to be worth future investment in monitoring the compaction of unbound materials. · The GPR resulted in reasonably accurate estimates to the thickness of aggregate base layers. None of the other NDT devices had the capability or same accuracy to determine the thickness of the unbound layer. NDT Devices for HMA Layers and Mixtures · The PSPA had the highest success rate for identifying an area with anomalies with a rate of 93 percent. The PQI identified about three-fourths of the anomalies, while the FWD and GPR identified about one-half of those areas. The seismic and non-nuclear gauges were the only technologies that consistently identified differences between the areas with and without seg- regation. These two technologies also consistently found differences between the longitudinal joint and interior of the mat. · The non-nuclear density gauge (PaveTracker) was able to identify and measure the detrimen- tal effect of rolling the HMA mat within the temperature sensitive zone. This technology was beneficial on some of the Part B projects to optimize the rolling pattern initially used by the contractor. · Three to four repeat measurements were made at each test point with the NDT devices. The PSPA had a repeatability value, a median or pooled standard deviation, of about 30 ksi for most mixtures, with the exception of the US-280 supplemental mixture that was much higher. The FWD resulted in a comparable value for the SMA mixture (55 ksi), but a higher value for the US-280 mixture (275 ksi). The non-nuclear density gauges had repeatability values similar to nuclear density gauges with a value less than 1.5 pcf. The repeatability for the GPR device was found to be good and repeatable, with a value of 0.5 percent for air voids and 0.05 inches for thickness. · The PSPA moduli were comparable to the dynamic moduli measured in the laboratory on test specimens compacted to the in-place density at a loading frequency of 5 Hz and the in-place mixture temperature, with the exception of one mixture--the US-280 supplemental mixture. In fact, the overall average ratio or adjustment factor for the PSPA was close to unity (1.1). This was not the case for the FWD. Without making any corrections for volumetric differences to the laboratory dynamic modulus values, the standard error for the PSPA was 76 ksi (laboratory values assumed to be the target values). The PSPA was used on HMA surfaces after com- paction and the day following placement. The PSPA modulus values measured immediately following compaction were found to be similar to the values one or two days after placement-- when making proper temperature corrections in accordance with the master curves measured in the laboratory. · A measure of the mixture density or air voids is required in judging the acceptability of the modulus value from a durability standpoint. The non-nuclear gauges were found to be acceptable, assuming that the gauges have been properly calibrated to the specific mixture-- as for the PSPA.
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16 · Use of the GPR single antenna method, even with mixture calibration, requires assumptions that specific volumetric properties do vary along a project. As the mixture properties change, the dielectric values may or may not be affected. Use of the proprietary GPR analysis method on other projects was found to be acceptable for the air void or relative compaction method. This proprietary and multiple antenna system, however, was not used within Part A of the field evaluation to determine its success rate in identifying localized anomalies and physical differ- ences between different areas. Both GPR systems were found to be very good for measuring layer thickness along the roadway. · Water can have a definite effect on the HMA density measured with the non-nuclear density gauges (PQI). The manufacturer's recommendation is to measure the density immediately after compaction, prior to allowing any traffic on the HMA surface. Within this project, the effect of water was observed on the PQI readings, as compared to dry surfaces. The measured density of wet surfaces did increase compared to dry surfaces. From the limited testing completed with wet and dry surfaces, the PaveTracker was less affected by surface condition. However, wet versus dry surfaces was not included in the field evaluation plan for different devices. Based on the data collected within the field evaluation, wet surfaces did result in a bias of the density measurements with this technology. · Another important condition is the effect of time and varying water content on the properties of the HMA mixture during construction. There have been various studies completed using the PSPA to detect stripping and moisture damage in HMA mixtures. For example, Hammons et al. (2005) recently used the PSPA (in combination with GPR) to successfully locate areas with stripping along selected interstate highways in Georgia. The testing completed within this study also supports the use of ultrasonic-based technology to identify such anomalies. · The instrumented rollers used to establish the increase in stiffness with number of passes was correlated to the increases in density, as measured by different devices. These rollers were used on limited projects to develop or confirm any correlation between the NDT response and the instrumented roller's response. One issue that will need to be addressed is the effect of decreasing temperature on the stiffness of the mixture and how the IC roller perceives that increase in stiffness related to increases in density of the mat and a decrease in mat temperature as it cools. A potential disadvantage with these rollers is that they will bridge segregated areas and may not accurately identify cold spots in the HMA mat. However, based on the results obtained, the ability to provide uniform compaction was verified and the rollers are believed to be worth future investments in monitoring the compaction of HMA mixtures. Limitations and Boundary Conditions · All NDT devices suggested for QA application, with the exception of the GPR and IC rollers, are point specific tests. Point specific tests are considered a limitation because of the number of samples that would be required to identify localized anomalies that deviate from the population. Ultrasonic scanners are currently under development so that relatively continuous mea- surements can be made with this technology. These scanners are still considered in the research and development stage and are not ready for immediate and practical use in a QA program. GPR technology to estimate the volumetric properties of HMA mixtures is available for use on a commercial basis, but the proprietary system has only had limited verification of its potential use in QA applications and validation of all volumetric properties determined with the system. Similarly, the IC rollers take continuous measurements of density or stiffness of the material being compacted. During the field evaluation, some of these rollers had both hardware and
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17 software problems. Thus, these devices were not considered immediately ready for use in a day-to-day QA program. The equipment, however, has been improved and its reliability has increased. The technology is suggested for use on a control basis but not for acceptance. · Ultrasonic technology (PSPA) for HMA layers and materials; suggested for use in control and acceptance plans. Test temperature is the main boundary condition for the use of the PSPA. Elevated tem- peratures during mix placement can result in erratic response measurements. Thus, the gauge may not provide reliable responses to monitor the compaction of HMA layers or define when the rollers are operating within the temperature sensitive zone for the specific mixture. These gauges need to be calibrated to the specific mixture being tested. However, this tech- nology can be used in the laboratory to measure the seismic modulus on test specimens during mixture design or verification prior to measuring the dynamic modulus in the laboratory. A limitation of this technology is that the results (material moduli) do not provide an indication on the durability of the HMA mixture. Density or air void measurements are needed to define durability estimates. The DSPA for testing unbound layers is influenced by the condition of the surface. High modulus values near the surface of the layer will increase the modulus estimated with the DSPA. Thus, the DSPA also needs to be calibrated to the specific material being evaluated. · Steady-state vibratory technology (GeoGauge) for unbound layers and materials; suggested for use in control and acceptance plans. This technology or device should be used with caution when testing fine-grained soils at high water contents. In addition, it should not be used to test well-graded, non-cohesive sands that are dry (i.e., well below the optimum water content). The condition of the surface of the layer is important and should be free of loose particles. A layer of moist sand should also be placed underneath the gauge to fill the surface voids and ensure that the gauge's ring is in contact with about 75 percent of the material's surface. Placement of this thin, moist layer of sand takes time and does increase the time needed for testing. These gauges need to be calibrated to the specific material being evaluated and are influenced by the underlying layer when testing layers that are less than 8 in. thick. These gauges are not applicable for use in the laboratory during the development of moisture- density (M-D) relationships that are used for monitoring compaction. The DSPA technology is applicable for laboratory use to test the samples used to determine the M-D relationship. A relative calibration process is available for use on a day-to-day basis. However, if the gauge does go out of calibration, then it must be returned to the manufacturer for internal adjustments and calibration. These gauges do not determine the density and water content of the material. Alternate devices are necessary to measure the water content and density of the unbound layer. · Non-nuclear density gauges (electric technology) for HMA layers and materials; suggested for use in control and acceptance plans. Results from these gauges can depend on the condition of the layer's surface--wet versus dry. It is recommended that the gauges be used on relatively dry surfaces until additional data become available pertaining to this limitation. Free water should be removed from the surface to minimize any effect on the density readings. However, water penetrating the surface voids in segregated areas will probably affect the readings (i.e., incorrect or high density compared to actual density from a core). The PSPA was able to identify areas with segregation. These gauges need to be calibrated to the specific material under evaluation.