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CHAPTER FOUR
Data Collection, Management, and Storage
This chapter covers FWD operations including data collec- Testing Procedures and Practices
tion, management, and storage.
The FHWA/LTPP program provides a manual for FWD
measurements (Schmalzer 2006). In addition, ASTM's
Data Collection Guidelines "Standard Test Method for Deflections with a Falling-Weight-
Type Impulse Load Device" (2005) covers deflection testing.
SHAs collect FWD data on a variety of public facilities. Many agencies reported using testing protocols that were
Several SHAs contribute FWD data to other state depart- developed in-house. On flexible pavements, 66% of SHAs
ments, especially in situations in which geotechnical data are reported using agency-developed FWD guidelines (Appen-
needed. Most frequently, state highways are tested; all survey dix B, Question 29). Similarly, 29% of SHAs reported using
respondents whose states have FWD programs reported test- their own FWD testing guidelines for rigid pavement testing
ing on SHA highways. Approximately 40% of respondents (Appendix B, Question 40). The following are a few exam-
reported testing city streets and 27% reported performing ples of how different agencies use their FWDs.
tests on airport runways (Appendix B, Question 23).
The Virginia DOT (VDOT) outlines its FWD testing
Data collection locations largely depend on the pave- practices in Test Method 68, "Non-Destructive Pavement
ment surface type and what sort of data the agency wishes Testing . . ." (2007). Tests are done "in accordance with
to obtain. On flexible pavements, 91% of survey respondents ASTM 4694-96" and VDOT's "Project Evaluation and Pave-
stated that the right-wheel path is tested. The left-wheel path ment Design--Appendix A." On flexible pavements, the
was tested only by 21% of survey respondents and lane cen- Test Method prescribes FWD testing "to assess the struc-
ters by 30%. Lane geometry is dominated by outer lanes in tural capacity of the pavement and estimate the strength of
locations where multiple lanes are present, as 63% of sur- subgrade soils. In addition to the structural capacity, the
vey respondents stated. Inner lanes were tested by 21% of elastic modulus for the surface, base, and subbase layers can
respondents. On rigid pavements, responses were similar be determined" ("Non-Destructive Pavement Testing . . ."
to those of flexible pavements. The right-wheel path (56%) 2007). Multilane roads are tested in the outside lane. Sam-
and outer lane (40%) were the most prevalent locations to pling is to be done based on 3.2 km (2 mi) intervals, and
perform FWD tests on rigid pavements. Additionally, slab conducted "in the wheel path closest to the nearest shoulder"
corners and edges were tested by 38% of respondents. ("Non-Destructive Pavement Testing . . ." 2007). For basin
testing on AC pavements, for example, the Test Method rec-
Preparation ommends 11 total drops:
Immediately before testing, the majority of SHAs reported · Two seating drops at 53 kN (12 kips)
some sort of preparation activity. Of the SHAs surveyed, · Three recorded drops at 27 kN (6 kips)
55% follow a written equipment inspection checklist before · Three recorded drops at 40 kN (9 kips)
departing for testing and the same percentage follow a writ- · Three recorded drops at 71 kN (16 kips)
ten warm-up procedure. Additionally, all testing guides
(e.g., LTPP, ASTM, and TxDOT) require a clean surface Temperature readings should be taken at the surface and
on which the load plate and sensors should be placed. The at the surface layer's mid-depth.
ASTM D4695-03 standard, for example, requires that the
test location "be free from all rocks and debris to ensure that For all pavement rehabilitation projects, the Idaho Trans-
the load plate . . . will be properly seated" ("Standard Guide portation Department considers FWD data or R-values.
for General Pavement Deflection Measurements" 2005). Either may prove that a candidate design has a design life of
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at least eight years. Additionally, if a candidate design has a · Section 2.4. The system shall measure deflections with
design life of more than eight years, and a modicum of addi- an absolute accuracy of better than 2% ±2 m, and
tional material and costs would yield a 20-year design life, with a typical relative accuracy of 0.5% ±1 m. The
the 20-year design life is put forth (Design Manual 2007). resolution of the equipment shall be 1 m.
· Section 6.1.4. All PC units shall have a multi-boot sys-
The Illinois DOT's (IDOT) Bureau of Materials and Physi- tem installed with boot options for MS-DOS 6.22 and
cal Research (BMPR) performs FWD tests given the fol- Windows XP.
lowing information ("Pavement Technology Advisory . . ." · Section 7.2.3. Data files shall be created for the FWD
2005): tests. The data files shall be composed of 80 charac-
ter records. A data file shall contain test results and
· Marked route descriptive information by roadway and roadway sec-
· Contract and section number (if available) tion. File names for deflection data files shall be in the
· Location map following specific format: DDPNNNNS.FWD
· Pavement type and thickness (cores may be needed to · The format shown is standard PC DOS format where:
verify thickness of pavement layers) - DD = District number ranging from 1 to 25,
· Contact information for requesting agency and traffic - P = Roadway prefix,
control provider - NNNN = Roadway number ranging from 0001 to
· Type of investigation desired 9999, and
- S = Roadway suffix.
If the investigation is for an overlay, agencies must also
provide traffic data, design load, design period, and deadline Further details include payment method, acceptance, and
date. warranty.
When the California DOT (Caltrans) evaluates a PCC The Vermont Agency of Transportation (VTrans) uses
pavement for rehabilitation, it considers replacing individ- data from FWD tests for pavement designs. For pavement
ual slabs. To determine whether slab replacement is a viable rehabilitation projects, the VTrans guide lists FWD data
strategy, Caltrans suggests spacing FWD sensors at 300 mm along with traffic, climate, materials and soils properties,
(12 in.) increments from the load plate. existing pavement condition, drainage, and safety data as use-
ful inputs. FWD data are used to calculate SNeff for flexible
In addition, the load transfer efficiency at joints and pavements and to calculate effective slab depth Deff for rigid
cracks, as well as the presence of voids at corners, pavements. Data are collected at the following increments:
can be evaluated quickly . . . NDT [nondestructive
testing] alone cannot, however, completely identify [H]alf-mile [805 m] increments in the right wheel
which pavement component is responsible for weak- path. The opposing lane should be tested at alter-
nesses, or whether moisture-related problems exist. nating locations so that information is obtained at
A pavement drainage survey and limited coring may quarter-mile [0.4 km] increments. Multiple lane
also be required ("Slab Replacement Guidelines" highways should be tested across the section to
2004, p. 13). obtain representative information Pavement Design
Guide . . . 2003, p. 7).
Appendix D of the New Mexico DOT's "Infrastructure
Design Directive" (Harris 2006, p. 28) provides the sensor The Georgia DOT (GDOT) uses sensor spacings identical
spacings, location requirements, and testing procedures for to the FHWA/LTPP manual for flexible pavements; however,
FWD testing by the agency. The LTPP one year calibration they use a unique set of spacings on rigid pavements. The
requirement is met, but sensor spacing is unique to the state. GDOT spacings are as follows (Pavement Design Manual
Seven sensors should be placed at 0, 203, 305, 457, 610, 762, 2005):
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 · Flexible: 0, 203, 304, 457, 610, 914, and 1,524 mm (0, 8,
(12 in.) diameter. Successive measurements should be taken 12, 18, 24, 36, and 60 in.)
every 76 m (250 ft), using a 40 kN (9,000 lbf) load. · Rigid and Composite: -304, 0, 304, 610, 914, 1,219, and
1,524 mm (-12, 0, 12, 24, 36, 48, and 60 in.)
TxDOT explicitly specifies their FWD and FWD data
recording system in their specification. Additional equip- Test locations vary by job type. GDOT's mainline AC
ment, such as a distance measurer, PC, and flat-panel display pavement testing is done in "the right lane, right wheel path.
are described. Elements of the TxDOT specification include If there is extensive wheel path cracking then offsetting to
the following (Imler 2002): the mid-lane path would be acceptable but should be noted."
Twenty locations per 1.6 km (1 mi) should be tested in all
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travel directions. AC shoulders are tested at locations no more Long-Term Pavement Performance Program Manual for
than 76 m (250 ft) apart "to help determine if the shoulders Falling Weight Deflectometer Measurements (Schmalzer
are structurally sufficient to carry travel lane traffic during 2006):
construction." On rigid pavements, FWD tests are done to
"determine overall stiffness, material properties, load trans- · Roll-off: occurs when a single deflection sensor fails to
fer at the joints, and for void detection." On continuously return to 0 within 60 ms of the weight being dropped.
reinforced concrete pavement (CRCP), tests are conducted · Nondecreasing deflections: occurs when deflections
in the centers of lanes. Distances between tests are left to measured do not decrease as distance from the load
engineering and statistical judgment. Testing at cracks on cell increases.
CRCP are done "at cracks that are spalled or have faulted" · Overflow: occurs when a deflection sensor measures a
(Pavement Design Manual 2005). On jointed plain concrete deflection beyond its range. Also referred to as an "out-
(JPC), at least 12 tests should be done per directional kilo- of-range" error.
meter (20 tests per directional mile). Additionally, tests on · Load variation: occurs when the drop load varies by
PCC slabs should be done only when the PCC surface tem- more than 0.18 kN (40.5 lbf) plus 2% of the average
perature is between 10° and 27°C (50° and 81° F). Composite load.
pavements (i.e., AC over PCC) are treated as rigid pavements · Deflection variation: occurs when the measured deflec-
where reflection cracks are present. tions from the same drop height vary by more than
2 m (0.08 mils) plus 1% of the average deflection.
Field Data Quality Control and Quality Assurance
The survey of SHA representatives revealed how fre-
FWD data gathered in the field are subject to quality quently these data checks are used. The results are shown in
checks before being sent to the office for further process- Figures 4 and 5.
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.)
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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 results noting when FWD testing is prohibited are presented
the motoring public, SHAs institute restrictions on FWD in Figure 6 for flexible pavement testing and in Figure 7 for
testing depending on environmental conditions. The survey rigid pavement testing.
FIGURE 6 Survey responses to question 39: "Under which of these conditions is flexible pavement testing
not allowed?" (Check all that apply.)
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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- few of these accidents involved injuries or fatalities. Details
trols, 29% of survey respondents reported accidents occurring for the accidents are summarized in Figure 8.
within the past five years. Of the 19 accidents reported, very
FIGURE 8 Survey responses to question 80: "Please describe the type(s)
and severity* of FWD related accidents within the past 5 years."
