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A falling weight deflectometer, towed by a truck
A falling weight deflectometer (FWD) is a testing device used by civil engineers to evaluate the physical properties of pavement. FWD data is primarily used to estimate pavement structural capacity for 1) overlay design and 2) to determine if a pavement is being overloaded. Use includes (but is not limited to) highways, local roads, airport pavements, harbor areas and railway tracks. The machine is usually contained within a trailer that can be towed to a location by another vehicle. It can also be built on a pickup truck or inside a mini van. There are also comprehensive units where a FWD device is mounted on a heavy truck together with a GPR cart and a TMA protection to have a complete road survey vehicle.
The FWD is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight, and transmitted to the pavement through a circular load plate - typically 300 mm diameter on roads and 450 mm on airports. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. The load plate can be solid or segmented. The advantage of a segmented load plate is that it adopts to the shape of the pavement, giving an even distribution of the load on uneven surfaces. Typically, the load for road testing is about 40 kN giving about 567 kPa pressure under the load plate (50 kN / 707 kPa according to European standard).
Load impact system[edit]
There are two different types of load impact systems; single-mass (e.g. Dynatest, Sweco - former Carl Bro, PaveTesting) and double-mass (KUAB).[1][2] In a single-mass system, a weight is dropped onto a single buffer connected to a load plate, which rests on the surface being tested. The load force is transferred through the plate, and the plate creates a deflection that simulates a wheel load. In the double-mass system, the weight drops onto a double-buffer system, which includes a first buffer, a second weight, and a second buffer. The principle is based on the law of conservation of momentum with an elastic collision of two unequal masses. The double-mass system essentially produces a longer loading duration that more precisely represents a wheel load. The double-mass system has higher reproducibility and gives a more accurate result on pavements built on soft soils.[3][4] The single-mass system may seriously overestimate the capacity of pavements built on soft soils due to the mass inertia of the pavement material.[5] The effect of the load pulse shape and rise time cannot be overlooked because it can affect the peak values of center deflection by as much as 10% to 20%.[6] However, single-mass FWDs are smaller, cheaper and faster. Low-cost FWD:s for the Indian market are currently (2015) being developed independently by Geotran, PaveTesting and KUAB.
Deflection sensors[edit]
Deflection sensors (geophones; force-balance seismometers) mounted radially from the center of the load plate measure the deformation of the pavement in response to the load. Some typical offsets are 0, 200, 300, 450, 600, 900, 1200 and 1500 mm. The deflections measured at these sensors are termed D0, D200, D300 etc. The advantages of seismometers compared to geophones are built-in calibration devices and higher range (5 mm vs 2 mm). Geophones are more sensitive to disturbance immediately before the impact since the initial error is integrated. Geophones however are much cheaper than seismometers. Dynatest, Sweco - former Carl Bro, Jils and PaveTesting use geophones while KUAB have seismometers in their standard FWD's and geophones in their low-cost models.
Analysis[edit]
FWD data is most often used to calculate stiffness-related parameters of a pavement structure. The process of calculating the elastic moduli of individual layers in a multi-layer system (e.g. asphalt concrete on top of a base course on top of the subgrade) based on surface deflections is known as 'backcalculation', as there is no closed-form solution. Instead, initial moduli are assumed, surface deflections calculated, and then the moduli are adjusted in an iterative fashion to converge on the measured deflections. This process is computationally intensive although quick on modern computers. It can give quite misleading results and requires an experienced analyst. Commonly used backcalculation software are:
- BAKFAA (Federal Aviation Administration)
- Clevercalc (University of Washington)
- ELMOD (Dynatest)
- Evercalc (WSDOT)
- KGPBACK (Geotran)
- MichBack (Michigan DOT)
- Modulus (TxDOT)
- PVD (KUAB)
- PRIMAX DESIGN / RoSy Design (Sweco, former Carl Bro)
![Free Backcalculation Software Free Backcalculation Software](/uploads/1/2/5/8/125850191/371353692.jpg)
Many analysts use simplified methods to calculate related parameters that are empirical in nature. The most common is maximum deflection under the centre of the load plate (D0) which is related to empirical measures such as the Benkelman Beam deflection (after minor adjustment for differences in the two devices). Historically some used the radius of curvature (D0-D200) but this is out of favour now because it is clear that the steel loading plate of 300mm diameter affects the shape of the deflection bowl between the centre (D0) and the D200 sensor at 200mm. However this means that a lot of useful information about the shape of the deflected bowl is wasted. Horak and Emery have published indices that use this information: BLI=D0-D300 and gives an indication of the basecourse performance, MLI = D300-D600 and gives an indication of the subbase performance, and LLI=D600-D900 and gives an indication of subgrade performance. These and other similar indices are known as shape factors. The FWD data can also be very useful in helping the engineer divide the length of the pavement into homogeneous sections.
FWD data can also be used to calculate the degree of load transfer between adjacent concrete slabs, and to detect voids under slabs.
Other models[edit]
A Light Weight Deflectometer (LWD) is a portable falling weight deflectometer used primarily to test insitu base and subgrade moduli during construction. LWD measurement is quicker than the isotope measuring method and requires no reference measurements. The equipment has no radioactive sources requiring safety courses and can be operated by one operator, allowing for the analysis of collected data and printing out of data files on site.[7] The most simple LWD's have no load cell but use only a nominal load value while Dynatest and KUAB LWD's use a load cell for measurement. The most common LWD's have only one geophone in the centre, while Dynatest LWD's can also have sensors at 300 and 600 mm positions. KUAB LWD's come with 300 and 600 mm sensors as standard.
A Heavy Weight Deflectometer (HWD) is a falling weight deflectometer that uses higher loads, used primarily for testing airport pavements. Maximum load for HWD:s are typically around 300 kN (Dynatest, Sweco - former Carl Bro and PaveTesting) and 600 kN (KUAB). A common misunderstanding is that a higher load is needed to test an airport's capability to handle heavy aircraft. But in fact, the testing methods are not designed to test the strength of the construction but to find the material properties of the construction.
A Rolling Weight Deflectometer (RWD) is a deflectometer that can gather data at a much higher speed (as high as 55 mph) than the FWD, which allows the data to be collected without traffic control and lane closure[8]. It is a specially designed tractor-trailer with laser measuring devices mounted on a beam under the trailer. Another advantage of the RWD over the FWD is that it can gather continuous deflection data as opposed to discrete deflection data collected by the FWD. RWD development has been carried out independently by Applied Research Associates (ARA) since 2005 and KUAB Sweden since 1991.[9]
![Free Free](/uploads/1/2/5/8/125850191/499132771.jpg)
The test materials are described in ASTM D 4694, and the test method is defined in ASTM D 4695.140
References[edit]
- ^'KUAB two-mass FWD'. Iowa State University. Retrieved 12 March 2014.
- ^'KUAB two-mass FWD patent'. Google patents. Retrieved 6 July 2015.
- ^Meier, Roger W. 'Backcalculation of Flexible Pavement Moduli from Falling Weight Deflectometer Data Using Artificial Neural Networks'. US Army Corps of Engineers. Retrieved 24 January 2017.
- ^Shahin, M.Y. (2007). Pavement Management for Airports, Roads, and Parking Lots (2 ed.). US: Springer. ISBN0387234659.
- ^Crovetti, JA; Shahin, MY; Touma, BE. 'Comparison of Two Falling Weight Deflectometer Devices, Dynatest 8000 and KUAB 2M-FWD'. ASTM International. doi:10.1520/STP19799S.Cite journal requires
|journal=
(help) - ^'Testing Different FWD Loading Times'. Bulletin. Stockholm, Sweden: Department of Highway Engineering, Royal Institute of Technology. 8. 1980.
- ^'Light Weight Deflectometer YouTube Video'. Cooper Technology. Retrieved 3 March 2014.
- ^Elbagalati, Omar; Elseifi, Mostafa A.; Gaspard, Kevin; Zhang, Zhongjie (16 June 2017). 'DEVELOPMENT OF AN ARTIFICIAL NEURAL NETWORK MODEL TO PREDICT SUBGRADE RESILIENT MODULUS FROM CONTINUOUS DEFLECTION TESTING'. Canadian Journal of Civil Engineering. doi:10.1139/cjce-2017-0132.
- ^'Megascale RWD patent'. Google patents. Retrieved 29 October 2015.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Falling_weight_deflectometer&oldid=933882160'
AASHTOWare Pavement ME Design is available for license to agencies with membership or associate membership in AASHTO and to other entities located in the United States. Each licensee shall designate a primary contact person through whom all support and maintenance will be coordinated with the contractor support team.
Individual Workstation License
The individual workstation license allows only one user to use the software at a time. With this option, the computer operates in standalone mode. An agency can purchase multiple workstation licenses.
Description | Annual License Fee | |
---|---|---|
Individual Workstation | $6,400 |
Site License
A site license is based on the maximum number of concurrent users accessing the program through a single license server in the network. A site license requires a connection to a license server when installing the software.
A site license permits cities/counties, and contractors/consultants employed by the licensing agency access to the product on the Member Department’s network.
Note: AASHTO member agencies wishing to exercise this option will be required to register the names and contact information for all contractors/consultants using AASHTOWare Pavement ME Design via their Site License and will be responsible for protecting AASHTO’s intellectual property rights to the AASHTOWare Pavement ME Design product by having each contractor execute the Contractor Agreement in the form specified in Appendix A of the Supplemental License Agreement, and providing a copy of the executed Contractor Agreement to AASHTO.
Description | Annual License Fee | |
---|---|---|
Site License – up to 9 concurrent users | $25,400 | |
Site License – up to 14 concurrent users | $38,200 | |
Site License – up to 20 concurrent users | $50,800 |
Note: Purchasers of the Site License for up to 20 Concurrent Users will have an option to increase the number of concurrent users for $2500 each.
Backcalculation Tool
The Pavement ME Deflection Data Analysis and Backcalculation Tool is a standalone software program that can be used to generate backcalculation inputs from Falling Weight Deflectometer (FWD) files to the AASHTO Pavement ME Design software for rehabilitation design. The tool is capable of analyzing raw deflection data files from three FWD testing devices: Dynatest®, JILS and KUAB. The tool provides three major functions: pre-processing deflection data (including project segmentation), backcalculation, and post-processing of results to generate inputs for Pavement ME rehabilitation design. This tool uses the EVERCALC® algorithm for the iterative backcalculation process.
Although the tool is included with the Pavement ME Design software, it can also be licensed separately and used as a standalone single user application. A training presentation is available at http://me-design.com/MEDesign/Webinars.html.
Description | Annual License Fee | |
---|---|---|
Standalone Single User Backcalculation Tool | $1,250 |
There is no evaluation or educational version for the backcalculation tool.
International License
AASHTOWare Pavement ME Design is available for licensing to entities located outside the United States and that do not have membership in AASHTO. These international entities must license through Applied Research Associates, Inc. (ARA), ARA can be reached at:
Applied Research Associates, Inc.
100 Trade Centre Drive, Suite 200
Champaign, IL 61820
(217) 356-4500
Fax: (217) 356-3088
Shobhit Mundra, email: [email protected]
100 Trade Centre Drive, Suite 200
Champaign, IL 61820
(217) 356-4500
Fax: (217) 356-3088
Shobhit Mundra, email: [email protected]
Educational Option
This is a no cost workstation version of the AASHTOWare Pavement ME Design software that is modified for use by educational institutions within the jurisdictions of our Member Departments, and/or Associate Members for teaching purposes in the classroom only. This option is not available to individuals and it is not to be used for research, graduate work or any other purpose. (An SI version is available for those institutions located within the jurisdictions of international AASHTO Member Departments.)
The no cost educational version differs from the full version as follows:
- Limited design types–new asphalt and concrete (JPCP and CRCP), AC/AC overlays, AC/JPCP overlays, unbonded PCC overlays.
- Limited analysis period: 30 years.
- Limited climate stations: 8 to 10 stations from around the country representing each climate zone.
- No batch mode and sensitivity analysis.
- Output reports (PDF and Excel) include the watermark text “Educational Version.”
- No access to intermediate output files.
Note: A licensing agreement executed by the institution assuring compliance with the education and training limitation is necessary to exercise the educational license option.
The license agreement is between the institution and AASHTO. Therefore, the educational version of the software is limited to one copy per institution and should be used in a computer lab environment, installed on computers owned by the institution. It should not be installed on individual personal student machines.
If the software is installed on multiple machines, the machines should be ghosted.
Only one copy of the educational software is licensed to each institution and the maximum number of seats per institution is limited to 25.