NDT Technology for Quality Assurance of HMA Pavement Construction (2009)

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The approach taken for this project was to use fundamental properties needed for mixture and structural design for the control and acceptance of flexible pavements and HMA over- lays. The NDT technologies included in the field evaluation were evaluated for their ability to determine these properties accurately on a practical and effective QA program. The NDT technology or QA tests are used to confirm the design assump- tions for the materials placed. Chapter 2 identified those devices that were able to identify or discriminate areas with different material properties or con- ditions. Chapter 3 presents the evaluation of the NDT devices with the potential to determine the quality of the unbound and HMA mixtures placed on some of the projects. These devices include the GeoGauge for unbound materials and the PSPA for HMA mixtures. Other evaluated devices, such as the DCP, were not as successful in identifying anomalies. In addition, the intent of this chapter is to show the use of NDT devices that estimate modulus for defining construction quality. 3.1 Quality Control and Acceptance Application Of the many process control procedures that can be used in highway construction, process control charts, particularly statistical control charts, are most commonly used by contrac- tors and material producers for verifying that their process is under control. Although there are different approaches that can be taken in implementing NDT technologies to verify that the process is in control, statistical control charts were used within this project. As a result, the NDT test methods must produce results that can be adapted to existing AASHTO pro- cedures in pavement construction. The ASTM Manual on Presentation of Data and Control Chart Analysis was used for preparing practical procedures that contractors can use in deciding whether their process is in control (ASTM 1992). Similarly, there are different acceptance procedures that are used to judge whether the pavement material meets the required specifications. Two of the more common methods that have been used and adopted by most agencies are percent within limits (PWL) and average absolute deviation (AAD). PWL is the procedure used by over 75 percent of the agencies that have adopted statistical-based acceptance specifications. AASHTO R9, Acceptance Sampling Plans for Highway Con- struction, was used for preparing practical but effective pro- cedures that agencies can use in deciding whether the product meets their specifications (AASHTO 2003). Statistical control charts are the primary method for determining whether the construction is in-control or out- of-control, and PWL is the primary method for judging the acceptability of construction. To demonstrate the use of the NDT technology for use in a QA program, specific projects were selected to cover the range of conditions encountered during construction. The following table contains the steps needed to set up a QA program that uses NDT devices to judge the quality of construction of unbound materials and HMA mixtures using the material modulus. 3.2 Control Limits for Statistical Control Charts The upper and lower control or action limits are calculated from the NDT modulus tests in accordance with the following equations. Where: UCLX– = Upper control limit for the sample means. LCLX– = Lower control limit for the sample means. X –– = Target value for a project. s– = Pooled standard deviation that represents the pro- cess variance. LCL X A sX = −( )( )3 (17b) UCL X A sX = +( )( )3 (17a) C H A P T E R 3 Construction Quality Determination 80

81 The target value of the control chart for each material is the average value measured in the laboratory in accordance with AASHTO T 307 or the test protocol used by the agency. Both action and warning limits are normally included on the statis- tical control charts. The upper and lower action limits are set at three standard deviations from the target value, while the warn- ing limits are set at two standard deviations from the target. 3.2.1 Target Modulus or Critical Value The target value of the control chart for each material and project is the modulus measured in the laboratory. This aver- age laboratory value should be the same as the input to the MEPDG for structural design. Tables 35 and 36 list the target values for the unbound and HMA layers included in the field evaluation projects, respectively. 3.2.2 Combined or Pooled Standard Deviation The pooled standard deviation was calculated in accordance with the AASHTO R9-03, Standard Recommended Practice for Acceptance Sampling Plans for Highway Construction. The pooled standard deviation was determined for each project and unbound material using the NDT results for the areas without anomalies or physical differences. The pooled stan- dard deviations for each project and material are listed in Tables 35 and 36 for the unbound and HMA layers, respec- tively. These values were used to determine whether the proj- ects were in-control or out-of-control, using the action limits: upper control limits (UCL) and lower control limits (LCL) provided in Tables 35 and 36. 3.3 Parameters for Determining PWL 3.3.1 Determining Quality Indices The upper and lower quality indices are calculated in accordance with Equations 18 and 19, respectively. The upper and lower specification limits were determined using data from all projects with similar materials. Q X LSL s L = − (18) Unbound Materials HMA Mixtures 1. Develop M-D relationships in the laboratory prior to construction for the unbound material to determine the maximum dry unit weight. Select the target density and water content for compacting the unbound layer. 1. Conduct an HMA mixture design to determine the target gradation and asphalt content. Select the target density and job mix formula for the project mixture or lift being tested. The target job mix formula will likely be revised based on plant produced and placed material. 2. Prepare and compact test specimens at the average water content and dry density expected during construction; based on the project specifications. 2. Prepare and compact test specimens at the target asphalt content and the average density expected during construction; based on the project specifications. 3. Measure the repeated load resilient modulus in accordance with the agency’s procedure (AASHTO T307 or NCHRP 1-28A, as required by the MEPDG). Determine the resilient modulus at a selected stress state. The resilient modulus should equal or exceed the value used during design. If the agency does not have a resilient modulus testing capability, the FHWA-LTPP regression equations can be used to estimate the target value, until the laboratory resilient modulus test has been completed (see Equations 1 through 15). 3. Measure the dynamic modulus in accordance with the agency’s procedure or the test protocol in accordance with the MEPDG. Determine the dynamic modulus for the test temperature expected during acceptance testing. Two values should be extracted from the test results or master curve; one for the day of paving (an elevated temperature expected after compaction) and the other for one or multiple days following placement. This target value for one or more days following placement will need to be adjusted back to a standard temperature depending on the actual pavement temperature. 4. Define the adjustment factor or ratio for the unbound material to laboratory conditions. Low stress states were used in establishing the ratios for this project. 4. Define the adjustment factor for the HMA mixtures to laboratory conditions. A load frequency of 5 Hz was used in establishing the adjustment ratios for this project. 5. Determine the combined or pooled standard deviation of the modulus for setting up the control limits of the unbound layer for the contractor (see Section 3.3). Establish the action, as well as warning, limits for the statistical control charts; upper and lower control limits (see Section 3.2). 5. Determine the combined or pooled standard deviation of the seismic modulus for setting up the control limits of the HMA mixture for the contractor (see Section 3.3). Establish the action, as well as warning limits for the statistical control charts; upper and lower control limits (see Section 3.2). 6. Determine the upper and lower specification limits (see Section 3.3) for the resilient modulus of the unbound material. This includes the upper and lower specification limits for the resilient modulus of the unbound layer. 6. Determine the upper and lower specification limits (see Section 3.3) for the dynamic modulus of the HMA mixture. This includes the upper and lower specification limits for the dynamic modulus of the HMA mixture. 7. Prepare the statistical control charts. 7. Prepare the statistical control charts. 8. Determine the PWL criteria for different conditions. 8. Determine the PWL criteria for different conditions.

82 Where: QL = Lower quality index. QU = Upper quality index. USL = Upper specification limit. LSL = Lower specification limit. Q USL X s L = − (19) s = Sample standard deviation of the lot. X – = Sample mean of a lot. The upper and lower quality indices are used to determine the total PWL for each lot of material using Equation 20. The upper and lower PWL values are then determined from the Q-tables provided in the AASHTO QC/QA Guide Specification. Action Warning Limits, ksi Project Identification Material Target Modulus, ksi Pooled Standard Deviation, ksi UCL LCL I-85, AL Low Plasticity Clay 4.0 0.8 5.6 2.4 NCAT, OK High Plasticity Clay 6.9 2.0 10.8 3.00 SH-21, TX High Plasticity Clay 26.8 2.5 30.4 23.2 TH-23, MN Soil-AggregateEmbankment 16.4 1.0 17.8 15.0 US-2, ND Soil-AggregateEmbankment 19.0 2.6 22.7 15.3 SH-130, TX Improved Soil Embankment 35.3 2.8 39.3 31.3 NCAT, SC Crushed Granite Base 36.1 2.7 41.4 30.8 NCAT, MO Crushed Limestone Base 19.2 2.7 24.5 13.9 TH-23, MN Crushed Stone Base 24.0 2.6 27.7 20.3 US-53, OH Crushed Stone Base 27.5 1.6 30.6 24.4 NCAT, FL Limerock Base 28.6 3.5 35.4 25.5 US-2, ND Crushed Aggregate Base 32.4 4.5 38.8 26.0 US-280, AL Crushed Limestone Base 48.4 10.0 62.7 33.7 NOTE: The target modulus for the South Carolina crushed granite base was determined using the FHWA-LTPP regression equation, because the densities were significantly below the maximum dry unit weight of the material during NDT testing. The pooled standard deviation for this project was assumed to be equal to the Missouri limestone base because the same contractor placed both materials. Table 35. Parameters used to prepare statistical control charts for the unbound layers included in the field evaluation projects. Action Warning Limits, ksiProject Identification Material Target Modulus, ksi Pooled Standard Deviation, ksi UCL LCL I-85, AL SMA 250 14 270 230 TH-23, MN HMA Base 810 35 860 760 US-280, AL HMA Base 650 45 715 585 I-35, TX HMA Base 800 57 910 690 I-75, MI Type 3-C 400 86 520 280 I-75, MI Type E-10 590 86 715 465 US-47, MO Surface Mix 530 60 615 445 US-47, MO Base Mix 420 36 470 370 I-20, TX CMHB Base 340 40 420 260 US-53, OH HMA Base 850 44 915 785 US-2, ND HMA Base 510 33 555 465 NCAT, SC HMA Base 410 58 525 295 NCAT, FL HMA Base 390 40 470 310 NCAT, FL PMA Base 590 45 675 505 NCAT, AL PG76-Sasobit 610 40 690 530 NCAT, AL PG76-SBS 640 45 725 555 NCAT, AL HMA Base 450 50 550 350 NOTE: The Texas SH-130 target modulus was determined from Witczak’s regression equation because changes were made to the mixture just before NDT testing. Table 36. Parameters used to prepare statistical control charts for the HMA layers included in the field evaluation projects.

Where: PWL = Percent within limits. PWLL = Percent within limits from the lower specification limit. PWLU = Percent within limits from the upper specification limit. PWL PWL PWLL U= + −100 (20) 3.3.2 Determining Specification Limits Tables 37 and 38 list the target values for the unbound and HMA layers included in the field evaluation projects, respec- tively. These values were used to determine the PWL for the different materials used in the field evaluation projects and were compared to the control limits determined for each project. Project Identification Material Median Standard Deviation, ksi Specification Tolerance, (-) ksi I-85, AL Low Plasticity Clay NCAT, OK High Plasticity Clay SH-21, TX High Plasticity Clay 2.0 3.3 TH-23, MN Soil-AggregateEmbankment US-2, ND Soil-AggregateEmbankment SH-130, TX Improved Soil Embankment 2.1 3.5 NCAT, SC Crushed Granite Base NCAT, MO Crushed Limestone Base TH-23, MN Crushed Stone Base US-53, OH Crushed Stone Base NCAT, FL Limerock Base US-2, ND Crushed Aggregate Base US-280, AL Crushed Limestone Base 3.0 5.0 Table 37. Upper and lower specification limits for the unbound layers and materials included in the field evaluation projects. Project Identification Material Median Standard Deviation, ksi Specification Tolerance, + ksi I-85, AL SMA 15 30 TH-23, MN HMA Base US-280, AL HMA Base I-35, TX HMA Base I-75, MI Type 3-C 50 100 I-75, MI Type E-10 US-47, MO Surface Mix 70 140 US-47, MO Base Mix I-20, TX CMHB Base US-53, OH HMA Base US-2, ND HMA Base NCAT, SC HMA Base NCAT, FL HMA Base NCAT, FL PMA Base 50 100 NCAT, AL PG76-Sasobit NCAT, AL PG76-SBS 45 90 NCAT, AL HMA Base 50 100 Table 38. Upper and lower specification limits for the HMA layers and mixtures included in the field evaluation projects. 83