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Falling Weight Deflectometer Usage (2008)

Chapter: CHAPTER TWO Falling Weight Deflectometer Equipment

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Suggested Citation:"CHAPTER TWO Falling Weight Deflectometer Equipment." National Academies of Sciences, Engineering, and Medicine. 2008. Falling Weight Deflectometer Usage. Washington, DC: The National Academies Press. doi: 10.17226/13675.
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Suggested Citation:"CHAPTER TWO Falling Weight Deflectometer Equipment." National Academies of Sciences, Engineering, and Medicine. 2008. Falling Weight Deflectometer Usage. Washington, DC: The National Academies Press. doi: 10.17226/13675.
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Suggested Citation:"CHAPTER TWO Falling Weight Deflectometer Equipment." National Academies of Sciences, Engineering, and Medicine. 2008. Falling Weight Deflectometer Usage. Washington, DC: The National Academies Press. doi: 10.17226/13675.
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Suggested Citation:"CHAPTER TWO Falling Weight Deflectometer Equipment." National Academies of Sciences, Engineering, and Medicine. 2008. Falling Weight Deflectometer Usage. Washington, DC: The National Academies Press. doi: 10.17226/13675.
×
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Suggested Citation:"CHAPTER TWO Falling Weight Deflectometer Equipment." National Academies of Sciences, Engineering, and Medicine. 2008. Falling Weight Deflectometer Usage. Washington, DC: The National Academies Press. doi: 10.17226/13675.
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7 • One or more deflection sensors. (Note: Deflection basin tests require at least seven sensors.) • A system for collecting, processing, and storing deflec- tion data. Recorder systems, discussed in Section 8 of ASTM D4694-96, should display and store load measurements with a 200 N (45 lbf) resolution. Such systems should display and store deflection measurements with ±1 μm (0.039 mil) or less of resolution. Most of the FWDs used by SHAs are either towed by a vehicle or are built into a vehicle’s cargo area. Figures 1 and 2 depict one of each such FWD. FiguRe 1 trailer-towed FWD (Courtesy: Carl Bro). FiguRe 2 vehicle-mounted FWD (Courtesy: Foundation Mechanics). This chapter provides information on FWD ownership, types and configurations, manufacturers, and maintenance practices. FaLLinG WeiGhT deFLecToMeTer oWnership The 45 SHAs that responded to the survey reported own- ing 82 FWDs. Most were manufactured by Dynatest, but Carl Bro, JILS, and KUAB were also represented. Table 3 summarizes SHA FWD ownership by manufacturer (Appendix B, questions 2–6). TABLE 3 qUANTITIES OF FWDS OWNED By STATE HIGHWAy AGENCIES, By MANUFACTURER Manufacturer quantity of FWDs in Service (total) Ages of FWDs (years, average) Dynatest 61 14 JILS 15 6 KUAB 6 14 Carl Bro 0 Not applicable Other 0 Not applicable Total 82 11 FaLLinG WeiGhT deFLecToMeTer TYpes and conFiGuraTions Falling Weight deflectometer components The basic components of a FWD are defined by reference documents such as AASHTO and ASTM standards. For example, in ASTM D4694-96, the apparatus described is composed of the following (“Standard Guide for General Pavement Deflection Measurements” 2005, pp. 487–488): • An impulse-generating device with a guide system. This device allows a variable weight to be dropped from a variable height. • Loading plate, for uniform force distribution on the test layer. When the weight affects this plate, this loading plate ensures that the resulting force is applied perpen- dicularly to the test layer’s surface. • A load cell for measuring the actual applied impulse. CHAPTER TWO FaLLinG WeiGhT deFLecToMeTer equipMenT

8 FDOT follows LTPP guidelines for sensor spacing, but load levels differ. Additionally, rigid pavements are not tested; “the procedure used by FDOT to predict the embankment Mr is applicable only to flexible pavements . . . [if a request involves composite or rigid pavements] Limerock Bearing Ratio tests will be used in lieu of FWD tests” (Holzschuher and Lee 2006). Figure 3 gives the percentages of SHAs who developed their own spacing and load guidelines. Addition- ally, histograms of sensor spacings at the project level, at the network level, during research, and during other projects are provided in Appendix B. FaLLinG WeiGhT deFLecToMeTer ManuFacTurers Information from four FWD equipment manufacturers was gathered for this synthesis. Those manufacturers were Carl Bro; Dynatest; Foundation Mechanics, who offers FWD equipment through its JILS division; and KUAB. The fol- lowing paragraphs describe each of those manufacturers’ equipment and features based on the information gathered. Because of the ever-evolving technology, other equipment and additional features may be offered by FWD manufacturers. carl Bro The Carl Bro Group, acquired by the Dutch consultancy Grontmij in August 2006, offers three types of FWD: trailer- mounted, vehicle-mounted, and portable FWDs. The Carl Bro trailer-mounted FWD is the PRI2100. The FWD is mounted to the tow vehicle by a double-axle trailer. According to correspondence with Carl Bro, the mass sensor spacing and Target Loads The newest version of the LTPP FWD manual details physi- cal setup, loads, test plans, error checks, software, and cali- bration protocols. Sensor spacings depend on the pavement surface being tested and the number of sensors on the FWD. For basin testing, the LTPP FWD manual (Schmalzer 2006) requires: • 0, 203, 305, 457, 610, 914, 1,219, 1,524, and −305 mm (0, 8, 12, 18, 24, 36, 48, 60, and −12 in.) for nine-sensor FWDs. • 0, 203, 305, 457, 610, 914, and 1,524 mm (0, 8, 12, 18, 24, 36, and 60 in.) for seven-sensor FWDs on flexible pavements. • −305, 0, 305, 457, 610, 914, and 1,524 mm (−12, 0, 12, 18, 24, 36, and 60 in.) for seven-sensor FWDs on rigid pavements. Additionally, target loads of 26.7, 40.0, 53.4, and 71.2 kN (6,000, 9,000, 12,000, and 16,000 lbf) ±10% are defined for LTPP pavement tests. Test locations are specified for PCC testing at joint approach, joint leave, and corners. The Florida DOT (FDOT) publishes its own guidelines for FWD use. In addition to intradepartmental report require- ments, the document gives FWD apparatus parameters, data analysis techniques, and crew requirements. Other jurisdictions shared their sensor spacing and load methods in the survey. As the LTPP guidelines suggest, sen- sor spacing varies depending on test type and pavement sur- face. Most SHAs, however, follow FWD guidelines of their own creation rather than the LTPP guidelines. For example, FiguRe 3 survey response to Question 29, “What kind of flexible pavement field testing manual does your agency use?”

9 Foundation Mechanics, inc. Based in California, Foundation Mechanics, Inc., sells FWDs under its JILS nameplate. Fifteen of the 81 FWDs used by survey respondents were manufactured by JILS. The com- pany provides FHWA-compliant calibration services at its El Segundo, California, facility. JILS offers trailer-mounted and vehicle-mounted FWDs. JILS’ trailer-mounted FWD is the JILS-20. This FWD includes a 305 mm (12 in.) loading plate, distance measurer, video monitoring system, and temperature measurement hardware. JILS provides a separate gasoline engine for the FWD hydraulic system, allowing for independent vehicle and FWD operation. Up to ten deflection sensors are sup- ported. The company provides a laptop, which includes their JTEST FWD monitoring software. FWD data are output in raw data format, but they can be converted to PDDX format (“JILS, Falling Weight Deflectometers: JILS 20” 2007). For heavier loads, the company offers the JILS-20HF. While supporting heavier drop loads, the features and speci- fications are otherwise similar to the JILS-20 (“JILS, Falling Weight Deflectometers: JILS 20HF” 2007). The company’s vehicle-mounted FWD, the JILS-20T, is otherwise identical to the trailer-mounted JILS-20. JILS provides a Ford F350 pickup with dual rear wheels as the carrying vehicle (“JILS, Falling Weight Deflectometers: JILS 20T” 2007). KuaB Engineering and Research International, Inc., based in Savoy, Illinois, sells trailer-mounted and vehicle-mounted FWDs under its KUAB nameplate. The company offers FWD repair and calibration services. According to information provided by KUAB, four mod- els are sold under the KUAB name. All four models support up to seven deflection sensors. The company supplies a 300 mm (12 in.) load plate, which is available either segmented or solid. Additionally, an aluminum cover, automatic ambient temperature sensors, surface temperature sensor, distance measurers, and a laptop are all included. The company pro- vides three days of training to operators. The models are dif- ferentiated by their loading capacities and installation types. The KUAB 50, for example, offers a load range from 12 to 50 kN (2,698 to 11,240 lbf). This model is only available as a trailer-mounted FWD. The KUAB 120, conversely, adds a 450 mm (18 in.) solid or segmented load plate and has a load range from 7 to 120 kN (1,574 to 26,977 lbf). The KUAB 150 brings possible loads from 12 to 150 kN (2,698 to 33,721 lbf), and the KUAB 240 supports loads from 20 to 240 kN (4,496 to 53,954 lbf). Furthermore, the KUAB 120, KUAB 150, mechanism generates force magnitudes up to 250 kN (56,200 lbf). Carl Bro provides a laptop personal computer (PC) with software, which controls FWD operation and records distance measurement. The company supplies their “RoSy DESIGN” back-calculation software, but the FWD output may be used with other back-calculation packages. Three temperature sensors, nine deflection sensors, a four-split loading plate, a time history module, and warning lights are also supplied (Carl Bro 2006). The vehicle-mounted Carl Bro PRI2100 is integrated into a van. Otherwise, it is identical to the trailer-mounted PRI2100. The company recommends the vehicle-mounted FWD to reduce “mobilization time” and safety risks associ- ated with trailers. The PRIMA 100 by Carl Bro is a portable FWD. This device is designed to be carried by one person, with no need for a tow vehicle. Included with the PRIMA 100 are 100 and 300 mm (4 and 12 in.) loading plates and a 10 kg (22 lb) mass. Data are collected onto a personal digital assistant or laptop through a direct cable or Bluetooth wireless con- nection. A single load cell and up to two additional deflec- tion sensors are supported. The device is powered by four AA-size batteries. dynatest The Dynatest Group of Denmark, the United States, and the United Kingdom manufactured 59 of the 81 FWDs used by survey respondents. In addition to FWD equipment, Dyna- test provides FHWA-compliant FWD calibrations at its Starke, Florida, facility. Dynatest’s FWDs are either trailer- or vehicle-mounted. Based on information provided by Dynatest, the Model 8000E FWD supports drop masses from 50 to 350 kg (110 to 770 lb). The resulting applied force thereby ranges from 7 to 120 kN (1,500 to 27,000 lbf). The company supplies loading plates of diameters 305 mm (12 in.) and 450 mm (18 in.), and a segmented loading plate 305 mm (12 in.) in diameter is available for separate purchase. The system supports from 7 to 15 deflection sensors. Additionally, Dynatest supplies a laptop PC with FWD monitoring software. The system’s Pavement Deflection Data Exchange (PDDX)-formatted FWD output is compatible with Dynatest’s Elmod back- calculation software package. The Dynatest Model 8081 applies heavier loads than the Model 8000E. Capable of 30 to 240 kN (6,744 to 53,954 lbf) impact loads, the Model 8081 supports load masses between 200 and 700 kg (441 and 1,543 lb). Features and specifi- cations are otherwise similar to the lighter-weight Model 8000E. Similar to Model 8000E, model 8081 outputs to PDDX format.

10 • Giving operators an ownership stake in the FWD equipment they operate • Overhauling the equipment when needed A few SHAs reported that their FWDs have not yet needed “significant maintenance.” Manufacturers’ recommendations FWD equipment manufacturers generally follow LTPP guidelines for equipment maintenance and offer mainte- nance services on the equipment they sell. For example, Carl Bro supplies a comprehensive maintenance list to its clients, which breaks down maintenance activities by individual FWD component. In their preventative maintenance check- list, Dynatest recommends that brake operation, hand brake pump, load plate lubrication, tires, and belts be inspected on a daily basis; their FWD equipment checklist is summarized in Table 4. In addition to providing JILS-FWD maintenance ser- vices, Foundation Mechanics, Inc.’s maintenance personnel are able to log into their clients’ JILS-FWD computers over the Internet to review files and perform diagnostic tests. KUAB offers a preventative maintenance program as an option with their FWDs, which includes cleaning, inspec- tion, and calibration. The procedures typically take between three and four weeks to complete and are done by KUAB personnel at their Savoy, Illinois, facility. Merits of Falling Weight deflectometer Versus other nondestructive Testing devices The Missouri DOT (MoDOT) detailed the merits of FWD usage in an undersealing study. Before employing FWDs to detect voids, MoDOT used to test load transfer efficiency (LTE) by the “proof-rolling method.” A dump truck was filled to give a rear-axle load of 80 kN (18,000 lbf), and its and KUAB 240 are available as single-axle trailer-mounted FWDs or as vehicle-mounted FWDs. MainTenance pracTices Most SHAs perform regular maintenance on their FWD equipment and their tow vehicles. These maintenance activi- ties are separate from calibration and can include mechani- cal lubrication, replacement of consumable parts, leak repair, cleaning, and other activities that keep the mechani- cal devices in working order. state highway agency Falling Weight deflectometer Maintenance Although most SHAs do not have a written FWD mainte- nance plan (Appendix B, question 7), maintenance activi- ties are performed by SHA personnel. FWD equipment and tow vehicle maintenance is performed by SHA employees among 87% of survey respondents (Appendix B, questions 9–10). Seventeen percent of SHAs reported that they fol- low the manufacturer’s guidelines for regular maintenance. Those who did not follow the manufacturer’s guidelines usu- ally stated that maintenance activities are done when needed. Twenty-five agencies (listed in Appendix B, question 11) provided other suggestions on keeping their FWD equip- ment in working order, including the following (Appendix B, questions 8 and 11): • Cleaning the sensors and holders with an emery cloth • Storing FWD equipment and vehicles in a heated garage when not in use • Bleeding hydraulic lines annually • Following a brief maintenance checklist before depart- ing for a job • Including maintenance activities when FWDs are calibrated TABLE 4 DyNATEST RECOMMENDED MAINTENANCE ACTIVITIES AND FREqUENCIES Frequency Activities Daily, or as required Check brake operation, check hand brake pump for free movement, check load plate for lubrication, clean clamping magnets/disks/springs. Weekly Tire pressure (approx. 32 psi), lug nuts tight (75–90 ft-lb), check breakaway feature (actuator), check brake lock operation, inner catch parts lubricated with Teflon, external weight guides lubricated with Teflon, lubricate SD foam guides with silicone spray, check/refill battery level, clean infrared/air sensors. Monthly Check brake fluid level (DOT type 3 only), check rubber stabilizers for tightness, nuts and bolts tight, R/L cable checked for bends/breaks, check deflector holders––tips tight, clean sensor cables with soapy water only, check charging system connections, coat terminals with corrosion inhibitor, check hydraulic fluid level, clean/inspect electronic connections, perform relative calibration, wash equipment. Annually Change hydraulic fluid, change hydraulic fluid filter, perform reference calibration. Source: DyNATEST.

11 • Less manpower is required. • Lanes are closed for less time. • “No influence of shoulder movement to apparatus.” • Dynamic FWD impulse loads provide a more realistic simulation of truck movements. • Multiple load levels are possible. rear tire was placed 1 ft past a transverse joint between two slabs. Gauges then gave the deflections generated by the load on each slab. When measured, LTE is less than 65% and loaded side deflections were greater than 0.44 mm (17.5 mils), the slab was undersealed. FWDs are preferred over the proof-rolling method, for the following reasons (Donahue 2004):

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 381: Falling Weight Deflectometer Usage examines usage by state departments of transport of the falling weight deflectometer (FWD) to measure pavement deflections in response to a stationary dynamic load, similar to a passing wheel load.

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