Falling Weight Deflectometer Usage (2008)

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21 Forward calculation Forward calculation is a process of using the equations of elastic layer theory to calculate stresses, strains, and deflec- tions caused by surface loads at any point in a pavement system. Computer programs such as BISAR, CHEVLAy2, and ELSyM5 are used for forward calculation. The pro- cess is “forward” in the sense that it is closed form. By con- trast, back-calculation uses forward calculation iteratively, together with numerical methods to assist with convergence, to “back out” the pavement layer moduli from measured sur- face deflections. hogg and deLMaT Methods Hogg (Stubstad et al. 2006) and DELMAT (Hossain 2006) methods have been used as checks and balances for backcal- culated data. As an example, Florida uses the Hogg model and has shown reasonable agreement with backcalculated moduli for asphalt and subgrade layers. Load Transfer efficiency Adjacent JPC slabs should move together when a load is applied to one of them; faulting can result if they do not. The degree to which adjacent slabs move together is defined as LTE. LTE is calculated from FWD deflections by placing the load cell on one PCC slab and then placing a sensor on an adjacent unloaded slab. When the weight is dropped, the measured deflections are used to calculate LTE with Eq. (1), %100×= loaded unloaded D DLTE (1) where Dunloaded represents the deflection of the unloaded PCC slab, and Dloaded represents the deflection of the loaded PCC slab (Pierce et al. 2003). The work done by Gawedzinski (2005) for IDOT pro- vides a comprehensive example of load transfer analysis and contains a discussion of the state of the practice for LTE. Once FWD data have been collected from the field, multiple analysis tools are available to the SHA. These software pack- ages typically calculate pavement layer modulus, a mate- rial parameter that is essential for pavement layer design. According to survey data, 90% of SHAs use FWD data for pavement layer modulus estimation (Appendix B, question 58). On JPC pavements, FWD data can be used to determine LTE by placing one sensor on one slab and a second sensor on a neighboring slab and determining how each slab moves when the weight is dropped. These data are analyzed with the help of computer software. FWD data analysis software may be provided by FWD vendors, academic institutions, or government bodies. This chapter briefly explores FWD data analysis methods and the software developed around them. Data analysis tools are not necessarily compatible with data from FWD tests. Sources of incompatibility may be job type (e.g., parking lots vs. highways vs. airports), lack of compatible file formats between FWD models, or differing analysis software configurations. A 2001 study suggests fix- ing these issues by standardizing file formats among manu- facturers, allowing data analysis software to vary test site stationing, and allowing quality controls such as the SLIC transform, nondecreasing deflections, and overflow checks (Schmalzer 2001). daTa anaLYsis MeThods Back-calculation The most common back-calculation method is an iterative mathematical process. The method assumes that a unique set of layer moduli result in the deflections measured by the FWD. The data analyst, based on experience and judgment, selects seed moduli to calculate deflections. These calcu- lated deflections are compared with the output from the Bossinesq equations, or a two- or three-dimensional finite- element model. After the first calculation, the seed moduli are adjusted and the calculation is repeated. The iteration stops once a predetermined level of tolerance has been reached between the measured and calculated deflections. From this iteration, layer moduli are estimated. CHAPTER FIVE daTa anaLYsis

22 aashTo darWin Used by 21% of survey respondents (Appendix B, question 59), DARWin automates the processes in the 1993 AASHTO Guide for the Design of Pavement Structures. Version 3.1, the most recent version, is designed for 32-bit versions of Windows operating systems. Back-calculation is based on a mathematical iteration. As inputs, the program accepts Dynatest version 20, KUAB, and PDDX files, but it cannot use Dynatest version 25 files (“AASHTOWare DARWin Product Features” 2005). carl Bro rosy desiGn for roads Based on a fast Fourier transformation algorithm, Carl Bro’s RoSy DESIGN software calculates pavement layer stresses and strains in a 32-bit Windows environment. Additionally, RoSy DESIGN is able to link backcalculated data to a geo- spatial database. The program is compatible with “data from any FWD equipment” (“Carl Bro RoSy DESIGN Product Data Sheet” 2006). Despite its availability with all Carl Bro FWDs, no SHAs reported using RoSy DESIGN (Appendix B, question 59). dynatest eLMod A plurality of SHAs surveyed reported that Dynatest’s ELMOD (an acronym for Evaluation of Layer Moduli and Overlay Design) is their FWD analysis package of choice. Used by 21% of survey respondents, ELMOD is currently at version five. Back-calculation is based on a mathematical iteration, and the user may choose between the finite-ele- ment, linear elastic theory or a nonlinear subgrade algo- rithm by means of add-ons. The program operates in a 32-bit Windows environment. ELMOD 5 is able to read Dynatest versions 9, 10, 20, and 25 files, as well as Microsoft Access database files generated by Dynatest WinFWD. KuaB erida For FWD data analysis, KUAB provides the ERIDA soft- ware package. KUAB’s pavement analysis software is com- patible with Dynatest FWD equipment (“Pavement Analysis Software” 2003). The program supports KUAB’s FWD for- mat, Unicode text files, and PDDX files. Despite its compat- ibility and possible bundling with KUAB FWDs, no SHAs reported using ERIDA for data analysis (Appendix B, ques- tion 59). ModcoMp Currently in version 6, MODCOMP was reported as the data analysis package of choice by two SHAs. The program sup- ports linear and nonlinear modeling criteria (Stubstad et al. 2000). MODCOMP is freely available and was developed by quality control/quality assurance of analyzed data The Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements (Schmalzer 2006) lists the software data checks required. These data checks are discussed in greater detail in chapter four: • Roll-off • Nondecreasing deflections • Overflow • Load variation—set to ±18 kN + 0.02X (±4,000 lbf + 0.02X), where X represents successive drop loads • Deflection variation—set to ±2μm + 0.01X (±0.0787 mils + 0.01X), where X represents successive mea- sured deflections FWD data should be processed onsite using FWDCon- vert and FWDScan (Schmalzer 2006). Once at the office for processing, the data are subjected to a variety of software and correction algorithms. other Findings In the early 1960s, California began measuring pavement deflections using a device of its own design. Known as the California Traveling Deflectometer, the device exerted a simulated 80 kN (18,000 lbf) single-axle force, while a Benkelman Beam measured the resulting deflections. This unique device is a fundamental standard by which Califor- nia compares all other nondestructive pavement deflection testing devices. While using the Caltrans flexible pavement rehabilitation design method, deflections are correlated to the California Deflectometer. Additionally, FWD data are correlated to a standardized reference FWD load, FWDref, which transmits a 40 kN (9,000 lbf) load to a loading plate 305 mm (12 in.) in diameter. The correlation function is illustrated in Eq. (2), (2) where D(CD) is the California Deflectometer equivalent deflection and D(FWDref) represents deflections measured by the reference FWD. Additionally, any FWDs must pos- sess a valid calibration certificate issued by an FWD calibra- tion center (“California Test 356 . . .” 2004). daTa anaLYsis soFTWare FWD manufacturers bundle their own software with the FWDs they sell, but each software package is available for separate purchase. The following sections describe the vari- ous software packages available.

23 other software packages Fourteen percent of survey respondents reported using in- house developed software packages for their FWD data analysis. calBack Caltrans, the University of California at Berkeley, and the University of California at Davis are developing a Java- based back-calculation software package. CalBack, cur- rently at version 0.9, provides a GUI to linear elastic model calculations. The program accepts raw FWD data from JILS and Dynatest FWDs and operates in a 32-bit Windows envi- ronment (CalBack Manual 2006). Falling Weight deflectometer area WSDOT developed FWD AREA, a software program that takes Dynatest FWD Field Program output files, sensor offsets, pavement material type, temperature correction factor, plate radius, and geographic information as inputs. Using these inputs, FWD AREA calculates load deflections, deflections normalized to the 40 kN (9,000 lbf) reference FWD, loading area, and deflections normalized for tempera- ture. FWD AREA can display the deflection data in plots of deflection versus sensor spacing (“Falling Weight Deflecto- meter” 2007). Microsoft excel The FDOT FWD handbook describes data analysis in the office using Microsoft Excel. It provides proprietary mac- ros for forward estimation and “Greenbar” forms, as well as milepost versus deflection tables (Holzschuher and Lee 2006). anaLYsis ouTpuT FiLe TYpes Data analysis output files are a function of the analysis software that creates them. From the literature review and survey, little relevant information was found. CalBack and ModTag explicitly show a button labeled “Export to Excel” (CalBack Manual 2006). Cornell University. It is designed to work in a command line environment under DOS (Schmalzer et al. 2007). Virginia department of Transportation ModTag VDOT added a GUI to MODCOMP. The resulting program, ModTag, is currently in its third major revision. The program operates in a 32-bit Windows environment, but the calcula- tion methods are identical to MODCOMP. ModTag incorpo- rates several data quality assurance measures, including the SLIC transform. Texas Transportation institute ModuLus The Texas Transportation Institute, a collaborative facility operated by Texas A&M University and the TxDOT, devel- oped the graphical FWD data analysis package MODULUS. Among survey respondents, 14% reported MODULUS as their data analysis package of choice. MODULUS is cur- rently at version six (Liu and Scullion 2001). The program operates in a 32-bit Windows environment. Back-calcula- tion techniques are based on the linear elastic model. The program reads Dynatest FWD files and is freely available (Schmalzer et al. 2007). eVercaLc Developed by the Washington State DOT (WSDOT), EVER- CALC was reported by 15% of survey respondents as their primary FWD data analysis package. Version 5.0, the most recent version, operates in a 32-bit Windows environment. Back-calculation techniques are based on the linear elastic model. As inputs, EVERCALC accepts Dynatest versions 20 and 25 FWD output files. The program is freely available. sLic Transform SLIC is an algorithm for finding errors in FWD data. The algorithm compensates for misplaced sensors and other data issues by applying a logarithmic normalization. The SLIC algorithm can easily be added to FWD field software such as VDOT’s ModTag and WSDOT’s FWD AREA (Stubstad 2006). During the development of the Asphalt Layer Condition Assessment Program pavement layer condition software, the program’s outputs were compared with MODULUS 5.1 at two locations. Both programs accept raw FWD data, includ- ing sensor spacing, as inputs. Unlike MODULUS, however, this program applies the SLIC method to screen out invalid data (Xu et al. 2003).