Falling Weight Deflectometer Usage (2008)

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.

15 Testing procedures and practices The FHWA/LTPP program provides a manual for FWD measurements (Schmalzer 2006). In addition, ASTM’s “Standard Test Method for Deflections with a Falling-Weight- Type Impulse Load Device” (2005) covers deflection testing. Many agencies reported using testing protocols that were developed in-house. On flexible pavements, 66% of SHAs reported using agency-developed FWD guidelines (Appen- dix B, question 29). Similarly, 29% of SHAs reported using their own FWD testing guidelines for rigid pavement testing (Appendix B, question 40). The following are a few exam- ples of how different agencies use their FWDs. The Virginia DOT (VDOT) outlines its FWD testing practices in Test Method 68, “Non-Destructive Pavement Testing . . .” (2007). Tests are done “in accordance with ASTM 4694-96” and VDOT’s “Project Evaluation and Pave- ment Design—Appendix A.” On flexible pavements, the Test Method prescribes FWD testing “to assess the struc- tural capacity of the pavement and estimate the strength of subgrade soils. In addition to the structural capacity, the elastic modulus for the surface, base, and subbase layers can be determined” (“Non-Destructive Pavement Testing . . .” 2007). Multilane roads are tested in the outside lane. Sam- pling is to be done based on 3.2 km (2 mi) intervals, and conducted “in the wheel path closest to the nearest shoulder” (“Non-Destructive Pavement Testing . . .” 2007). For basin testing on AC pavements, for example, the Test Method rec- ommends 11 total drops: • Two seating drops at 53 kN (12 kips) • Three recorded drops at 27 kN (6 kips) • Three recorded drops at 40 kN (9 kips) • Three recorded drops at 71 kN (16 kips) Temperature readings should be taken at the surface and at the surface layer’s mid-depth. For all pavement rehabilitation projects, the Idaho Trans- portation Department considers FWD data or R-values. Either may prove that a candidate design has a design life of This chapter covers FWD operations including data collec- tion, management, and storage. daTa coLLecTion GuideLines SHAs collect FWD data on a variety of public facilities. Several SHAs contribute FWD data to other state depart- ments, especially in situations in which geotechnical data are needed. Most frequently, state highways are tested; all survey respondents whose states have FWD programs reported test- ing on SHA highways. Approximately 40% of respondents reported testing city streets and 27% reported performing tests on airport runways (Appendix B, question 23). Data collection locations largely depend on the pave- ment surface type and what sort of data the agency wishes to obtain. On flexible pavements, 91% of survey respondents stated that the right-wheel path is tested. The left-wheel path was tested only by 21% of survey respondents and lane cen- ters by 30%. Lane geometry is dominated by outer lanes in locations where multiple lanes are present, as 63% of sur- vey respondents stated. Inner lanes were tested by 21% of respondents. On rigid pavements, responses were similar to those of flexible pavements. The right-wheel path (56%) and outer lane (40%) were the most prevalent locations to perform FWD tests on rigid pavements. Additionally, slab corners and edges were tested by 38% of respondents. preparation Immediately before testing, the majority of SHAs reported some sort of preparation activity. Of the SHAs surveyed, 55% follow a written equipment inspection checklist before departing for testing and the same percentage follow a writ- ten warm-up procedure. Additionally, all testing guides (e.g., LTPP, ASTM, and TxDOT) require a clean surface on which the load plate and sensors should be placed. The ASTM D4695-03 standard, for example, requires that the test location “be free from all rocks and debris to ensure that the load plate . . . will be properly seated” (“Standard Guide for General Pavement Deflection Measurements” 2005). CHAPTER FOUR daTa coLLecTion, ManaGeMenT, and sToraGe

16 • Section 2.4. The system shall measure deflections with an absolute accuracy of better than 2% ±2 μm, and with a typical relative accuracy of 0.5% ±1 μm. The resolution of the equipment shall be 1 μm. • Section 6.1.4. All PC units shall have a multi-boot sys- tem installed with boot options for MS-DOS 6.22 and Windows XP. • Section 7.2.3. Data files shall be created for the FWD tests. The data files shall be composed of 80 charac- ter records. A data file shall contain test results and descriptive information by roadway and roadway sec- tion. File names for deflection data files shall be in the following specific format: DDPNNNNS.FWD • The format shown is standard PC DOS format where: − DD = District number ranging from 1 to 25, − P = Roadway prefix, − NNNN = Roadway number ranging from 0001 to 9999, and − S = Roadway suffix. Further details include payment method, acceptance, and warranty. The Vermont Agency of Transportation (VTrans) uses data from FWD tests for pavement designs. For pavement rehabilitation projects, the VTrans guide lists FWD data along with traffic, climate, materials and soils properties, existing pavement condition, drainage, and safety data as use- ful inputs. FWD data are used to calculate SNeff for flexible pavements and to calculate effective slab depth Deff for rigid pavements. Data are collected at the following increments: [H]alf-mile [805 m] increments in the right wheel path. The opposing lane should be tested at alter- nating locations so that information is obtained at quarter-mile [0.4 km] increments. Multiple lane highways should be tested across the section to obtain representative information Pavement Design Guide . . . 2003, p. 7). The Georgia DOT (GDOT) uses sensor spacings identical to the FHWA/LTPP manual for flexible pavements; however, they use a unique set of spacings on rigid pavements. The GDOT spacings are as follows (Pavement Design Manual 2005): • Flexible: 0, 203, 304, 457, 610, 914, and 1,524 mm (0, 8, 12, 18, 24, 36, and 60 in.) • Rigid and Composite: −304, 0, 304, 610, 914, 1,219, and 1,524 mm (−12, 0, 12, 24, 36, 48, and 60 in.) Test locations vary by job type. GDOT’s mainline AC pavement testing is done in “the right lane, right wheel path. If there is extensive wheel path cracking then offsetting to the mid-lane path would be acceptable but should be noted.” Twenty locations per 1.6 km (1 mi) should be tested in all at least eight years. Additionally, if a candidate design has a design life of more than eight years, and a modicum of addi- tional material and costs would yield a 20-year design life, the 20-year design life is put forth (Design Manual 2007). The Illinois DOT’s (IDOT) Bureau of Materials and Physi- cal Research (BMPR) performs FWD tests given the fol- lowing information (“Pavement Technology Advisory . . .” 2005): • Marked route • Contract and section number (if available) • Location map • Pavement type and thickness (cores may be needed to verify thickness of pavement layers) • Contact information for requesting agency and traffic control provider • Type of investigation desired If the investigation is for an overlay, agencies must also provide traffic data, design load, design period, and deadline date. When the California DOT (Caltrans) evaluates a PCC pavement for rehabilitation, it considers replacing individ- ual slabs. To determine whether slab replacement is a viable strategy, Caltrans suggests spacing FWD sensors at 300 mm (12 in.) increments from the load plate. In addition, the load transfer efficiency at joints and cracks, as well as the presence of voids at corners, can be evaluated quickly . . . NDT [nondestructive testing] alone cannot, however, completely identify which pavement component is responsible for weak- nesses, or whether moisture-related problems exist. A pavement drainage survey and limited coring may also be required (“Slab Replacement Guidelines” 2004, p. 13). Appendix D of the New Mexico DOT’s “Infrastructure Design Directive” (Harris 2006, p. 28) provides the sensor spacings, location requirements, and testing procedures for FWD testing by the agency. The LTPP one year calibration requirement is met, but sensor spacing is unique to the state. Seven sensors should be placed at 0, 203, 305, 457, 610, 762, and 1,219 mm (0, 8, 12, 18, 24, 30, and 48 in.) from the cen- ter of the load plate. The load plate should have a 300 mm (12 in.) diameter. Successive measurements should be taken every 76 m (250 ft), using a 40 kN (9,000 lbf) load. TxDOT explicitly specifies their FWD and FWD data recording system in their specification. Additional equip- ment, such as a distance measurer, PC, and flat-panel display are described. Elements of the TxDOT specification include the following (Imler 2002):

17 Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements (Schmalzer 2006): • Roll-off: occurs when a single deflection sensor fails to return to 0 within 60 ms of the weight being dropped. • Nondecreasing deflections: occurs when deflections measured do not decrease as distance from the load cell increases. • Overflow: occurs when a deflection sensor measures a deflection beyond its range. Also referred to as an “out- of-range” error. • Load variation: occurs when the drop load varies by more than 0.18 kN (40.5 lbf) plus 2% of the average load. • Deflection variation: occurs when the measured deflec- tions from the same drop height vary by more than 2 μm (0.08 mils) plus 1% of the average deflection. The survey of SHA representatives revealed how fre- quently these data checks are used. The results are shown in Figures 4 and 5. travel directions. AC shoulders are tested at locations no more than 76 m (250 ft) apart “to help determine if the shoulders are structurally sufficient to carry travel lane traffic during construction.” On rigid pavements, FWD tests are done to “determine overall stiffness, material properties, load trans- fer at the joints, and for void detection.” On continuously reinforced concrete pavement (CRCP), tests are conducted in the centers of lanes. Distances between tests are left to engineering and statistical judgment. Testing at cracks on CRCP are done “at cracks that are spalled or have faulted” (Pavement Design Manual 2005). On jointed plain concrete (JPC), at least 12 tests should be done per directional kilo- meter (20 tests per directional mile). Additionally, tests on PCC slabs should be done only when the PCC surface tem- perature is between 10° and 27°C (50° and 81° F). Composite pavements (i.e., AC over PCC) are treated as rigid pavements where reflection cracks are present. Field data quality control and quality assurance FWD data gathered in the field are subject to quality checks before being sent to the office for further process- ing. Five specific error-checking methods are defined by the FiguRe 4 survey responses to question 55: “Which of the following data checks are performed by FWD operators?” (Check all that apply.)

18 FiguRe 6 survey responses to question 39: “under which of these conditions is flexible pavement testing not allowed?” (Check all that apply.) FiguRe 5 survey response to question 56: “What software is used to perform quality checks in the field?” Responses as reported by shas. Worker safety With the goal of protecting workers and the safety of the motoring public, SHAs institute restrictions on FWD testing depending on environmental conditions. The survey results noting when FWD testing is prohibited are presented in Figure 6 for flexible pavement testing and in Figure 7 for rigid pavement testing.

19 FiguRe 7 survey responses to question 50: “under which of these conditions is rigid pavement testing not allowed?” (Check all that apply.) Despite accident prevention measures such as traffic con- trols, 29% of survey respondents reported accidents occurring within the past five years. Of the 19 accidents reported, very few of these accidents involved injuries or fatalities. Details for the accidents are summarized in Figure 8. FiguRe 8 survey responses to question 80: “Please describe the type(s) and severity* of FWD related accidents within the past 5 years.”

20 FiguRe 9 survey response to question 53: “in which format does your FWD equipment give its output?” (Check all that apply.) Recommendations were given for traffic control and flag- ger placement at the 2001 FWDUG meeting (“FWD: Past Meetings” 2001). Assuming a 3.2 km (2 mi) work zone on a two-lane road, two flaggers should be employed; each flag- ger should stand 61 m (200 ft) from the traffic-facing fenders of test vehicles (Heath 2001c). For FWD-specific operations, flaggers need only be 15 m (50 ft) away from an FWD vehi- cle (Heath 2001a). On bridge decks, tapered cones are placed 132 m (435 ft) from the FWD vehicle (Heath 2001b). daTa ManaGeMenT Falling Weight deflectometer Field data File Types Although AASHTO recommends the PDDX data format for FWD output files, the survey revealed 12 distinct file for- mats in use. These formats are shown in Figure 9. Because so many file formats are in use, analysis software may not be compatible with FWD output from all agencies. PDDX file conversion software, such as PDDX Convert, can be utilized to convert many file formats to PDDX (Orr et al. 2007). Backups The Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements (Schmalzer 2006) requires that users back up the test data in PDDX for- mat on removable media. These backups should be made at the test site immediately after collection. These data should stay with the tow vehicle until received at the office, where they are uploaded and archived (Schmalzer 2006, p. 41). In practice, SHAs are backing up data. Seventy-eight percent of SHAs surveyed reported that FWD data files are backed up to removable storage media, such as floppy discs, compact discs, or Universal Serial Bus (USB) flash drives (Appendix B, question 62). daTa sToraGe FWD field data are potentially useful for future applica- tions, especially as analysis technologies evolve. Notably, the survey indicated that 89% of survey respondents keep raw FWD field data for more than five years and 84% keep these data indefinitely (Appendix B, question 63).