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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-16-009 Date: March 2017 |
Publication Number: FHWA-HRT-16-009 Date: March 2017 |
As described in chapter 1, pavement deflection testing is a quick and easy way to assess the structural condition of an in-service pavement in a nondestructive manner. Over the years, a variety of deflection-testing equipment has been used for this purpose, from simple beam-like devices affixed with mechanical dial gauges to more sophisticated equipment using laser-based technology. Nevertheless, all pavement deflection-testing equipment basically operates in the same manner. A known load is applied to the pavement and the resulting maximum surface deflection (or an array of surface deflections located at fixed distances from the load, known as a deflection basin) are measured. Figure 1 shows a schematic of a deflection basin.
Figure 1. Diagram. Typical pavement deflection basin.
This chapter reviews the reasons for conducting deflection testing, provides a summary of commonly used deflection-measuring devices, describes common deflection-testing patterns, and discusses important factors influencing deflection measurements.
The primary purpose of deflection testing is to determine the structural adequacy of an existing pavement and to assess its capability of handling future traffic loadings. As observed in the work by Hveem, there is a strong correlation between pavement deflections (an indicator of the structural adequacy of the pavement) and the ability of the pavement to carry traffic loadings at a prescribed minimum level of service.(1) Early work attempted to identify maximum deflection limits below which pavements were expected to perform well, and these limits were based on experience and observations of performance of similar pavements. This concept quickly lent itself to overlay design, in that required overlay thicknesses could be determined based on trying to reduce maximum pavement deflections to tolerable levels.
When complete deflection basins are available, deflection testing can provide key properties for the existing pavement structure through backcalculation of the measured pavement responses. Specifically, for HMA pavements, the elastic modulus (E) of the individual paving layers can be determined, along with the resilient modulus (MR) of the subgrade. For PCC pavements, the elastic modulus (E) of the slab and the modulus of subgrade reaction (k or k-value) can be determined. In addition, deflection testing conducted on PCC pavements can be used to estimate the LTE across joints or cracks (see figure 2) as well as to identify loss of support at slab corners.
©National Highway Institute
δL = Deflection at loaded slab edge.
δU = Deflection at unloaded slab edge.
1 mm = 0.039 inches.
Figure 2. Diagram. Comparison of LTE.(9)
These properties of the pavement layers and of the subgrade are used in pavement design procedures or in performance prediction models to estimate the remaining life or load-carrying capacity of the pavement. They can also be used in elastic layer or finite element programs to compute stresses and strains in the pavement structure and are also direct inputs in many overlay design procedures to determine the required overlay thickness needed for the current pavement condition and the anticipated future traffic loadings.
Deflection data can also be used in a number of other ways to help characterize the condition of the existing pavement. For example, plots of deflection data along a pavement project can be examined for nonuniformity, which may suggest areas that require further investigation using destructive means. In addition, daily or seasonal deflection data can provide insight regarding a pavement’s response to environmental forces, including the effects of thermal curling, frozen support conditions, and asphalt stiffening. Some agencies also use deflection criteria to establish seasonal load restrictions for certain low-volume roads. Deflection testing has also seen some limited use as a means of monitoring the quality of a pavement during construction.(3) Finally, a few agencies conduct deflection testing at the network level to provide a general indication of the structural capacity of the pavement structure.
As alluded to in chapter 1, pavement deflection testing provides some distinct advantages over destructive testing, including the following:(10)
More rapid testing operation.
Relative ease of operation.
Lower operating cost.
Reduced manpower requirements.
Less intrusive procedure.
Increased number of test points.
At the time of this report, there were many different commercially available deflection-testing devices. These devices could be generally grouped based on the type of loading imparted on the pavement (static, steady-state vibratory, and impulse). More recently, a fourth type of deflection device was introduced, one in which dynamic deflections were continuously measured at highway speeds. This section describes each of these types of devices, including their principles of operation, advantages, and disadvantages.
Static deflection devices measure the pavement’s response under a static or slow-moving wheel load and include equipment such as the Benkelman Beam, plate bearing tests, and curvature meters.(11) Of these, the Benkelman Beam is the most commonly used, and in fact has a long history of use as a deflection-measuring device. The Benkelman Beam was developed by A.C. Benkelman while assigned at the Western Association of State Highway Officials Road Test in the 1950s.(12) It was also used at the American Association of State Highway Officials Road Test in the late 1950s and by the Asphalt Institute (AI) in the 1960s and 1970s for HMA overlay design.
The Benkelman Beam consists of a support beam, a probe arm, and a dial gauge (see figure 3). The device is used by placing the tip of the probe between the dual tires of a loaded truck, typically with an 8,172-kg (18,000-lb) axle load; as the loaded vehicle moves away from the beam, the upward movement of the pavement (termed the rebound deflection) is recorded by a dial gauge.(10)
1 inch = 25.4 mm.
Figure 3. Diagram. Schematic of Benkelman Beam device.(13)
The advantages of the Benkelman Beam include its ease of use, low equipment cost, and the existence of a large database from its use over many years.(9) Disadvantages of this device include the following:(9)
Difficulty ensuring that the front supports are not in the deflection basin (particularly on stiff pavement structures).
Difficulty or inability to determine the shape and size of the deflection basin (only maximum deflection is obtained).
Poor repeatability of measurements obtained by the device.
Labor-intensive and cumbersome nature of the device.
In the 1970s, automated deflection beams, such as the Lacroix deflectograph and the California Traveling Deflectometer, were developed to overcome the labor and speed disadvantages of the Benkelman Beam, but these still had the other limitations associated with the use of the Benkelman Beam.(10)
In this category of deflection devices, a relatively large static preload is applied to the pavement, with a sinusoidal force superimposed to simulate a dynamic loading condition. A typical loading series is shown in figure 4, in which the static load is constant and a dynamic force is produced at a fixed driving frequency.(9) The amplitude of the peak-to-peak dynamic force must be less than the static force to preclude the possibility of the device bouncing off of the pavement surface. However, the presence of the static preload may make it difficult to interpret the resultant deflection data because it could close voids beneath the surface or it could influence deflections on particularly stress-sensitive materials.(9)
©National Highway Institute
Figure 4. Graph. Typical output of vibrating steady-state force generator.(9)
The most common steady-state vibratory deflection devices are the Dynaflect and the Road Rater™. These are discussed separately in the following sections.
The Dynaflect device is second to the Benkelman Beam in terms of its longevity of use for deflection measurement. It is a trailer-mounted device that is quick and relatively simple to operate. Figure 5 shows the schematic of the Dynaflect device, including the loading wheels and geophones and the geophone configuration.
1 inch = 25.4 mm.
Figure 5. Diagram. Schematic of Dynaflect device.(10)
The Dynaflect device is a light-load, fixed-frequency device, with a static weight of 900 kg (2,000lb), that produces a 4.5-kN (1,000-lbf) peak-to-peak dynamic force at a fixed frequency of 8 cycles/s.(10) The load is applied through two counter-rotating eccentric masses, and the resulting deflections are recorded by five velocity transducers. The transducers are suspended from a placing bar and are normally positioned with one located between the two wheels and the remaining four placed at 300-mm (12-inch) intervals away from the wheels.
The Dynaflect device has the following technical limitations:(14)
However, the Dynaflect does provide a deflection basin, which allows more meaningful interpretation of the deflection data. In addition, a number of agencies have used the Dynaflect for many years, and it has demonstrated a high level of ruggedness and dependability.
The Road Rater™ is the other common type of steady-state vibratory deflection equipment. It is similar to the Dynaflect device in that a vibratory load is applied to the pavement, but it has the capability of applying greater loadings than the Dynaflect, depending on the model. For example, the static loads range from 10.7 to 25.8 kN (2,400 to 5,800 lbf), and the peak-to-peak dynamic loads range from 2.2 to 35.6 kN (500 to 8,000 lbf).(14) Moreover, the Road Rater™ applies the load to the pavement via a load plate, as opposed to the rigid wheels on the Dynaflect. Four velocity transducers are used to measure the deflections, one in the center of the plate and the other three placed at 300-mm (12-inch) intervals away from the load plate. The advantages and disadvantages of the Road Rater™ are similar to those of the Dynaflect.
Under the category of impulse load deflection devices is the FWD, which is the most common deflection-measuring device in use today and is, therefore, the emphasis of this project. As shown in figure 6, the FWD releases a known weight from a given height onto a load plate resting on the pavement structure, producing a load on the pavement that is similar in magnitude and duration to that of a moving wheel load. A series of sensors are located at fixed distances from the load plate so that a deflection basin can be measured. Variations in the force applied to the pavement are obtained by varying the weights and the drop heights; force levels from 13 to more than 222 kN (3,000 to more than 50,000 lbf) can be applied, depending on the equipment type.
1 inch = 25.4 mm.
Figure 6. Diagram. FWD testing schematic.
Developed in the 1970s, the FWD emerged in the 1980s as the worldwide standard for pavement deflection testing. The equipment of two FWD manufacturers―Dynatest® and KUAB―are described in the following sections as illustrative examples, but there are also several other manufacturers of FWD equipment.
The Dynatest® FWD is a trailer-mounted system (see figure 7) with an operations control computer located in the tow vehicle. The computer controls the complete operation of the FWD, including the lowering and raising of the load plate and deflection sensor bar as well as the sequencing of drop heights. Many FWDs are fitted with external cameras to help operators precisely align on selected testing locations.
Figure 7. Photo. Dynatest® heavyweight FWD.
Dynatest® currently offers two FWD trailer-mounted models, the 8000 and the 8081. The 8000 model applies peak impact loads in the range of 7 to 120 kN (1,500 to 27,000 lbf), whereas the 8081 model (termed the “heavy weight deflectometer”) applies peak impact loads in the range of 30 to 320kN (6,500 to 71,800 lbf).(15) Typical testing production rates range from about 200 to 300points per day, depending on traffic control requirements and specific testing locations (e.g., basin testing versus joint/crack load transfer testing).
Two different plate sizes can be used with the Dynatest® FWD: a 300-mm- (11.8-inch-) diameter plate or a 450-mm- (17.7-inch-) diameter plate. The smaller plate is typically used for street and highway pavements, whereas the larger plate is commonly used on airfield pavements (and generally on the heavyweight FWD model).
The Dynatest® FWD is used in the Federal Highway Administration’s (FHWA) LTPP Program, for which pavement deflection measurements have been routinely collected on more than 900pavement sections since the late 1980s. FHWA has established four regional FWD calibration centers across the United States to provide annual calibrations on the FWD equipment to ensure it is operating within allowable tolerances. Dynatest® also performs calibrations at its facilities in Florida and California, and at least one other State has its own calibrating facility.
Like the Dynatest® FWD, the KUAB FWD is a trailer-mounted device with a loading system and series of deflection sensors. However, it has its own defining characteristics, including a metal housing completely enclosing the loading system (see figure 8). Other characteristics include the following:
A two-mass system in which the initial load mass is dropped onto an intermediate buffering system. That buffering system transmits the force to another buffering system, which in turn transmits the load to the load plate. The use of the two-mass system creates a smoother load pulse than can be delivered by a single mass system.(14)
A segmented load plate in which each quarter circle of the plate is capable of conforming to the shape of the pavement surface being tested.
©Engineering and Research International, Inc. (ERI)
Figure 8. Photo. KUAB 2m-FWD.(16)
Several KUAB models are available, with the primary difference being the magnitude of the load that can be applied. The heaviest KUAB device can impart a load of 294 kN (66,000 lbf), making it suitable for use in airfield applications.
As with the Dynatest® FWD, the KUAB has two loading plates available: 300 or 450 mm (11.8or 17.7 inches) diameter. Also, the testing operation is completely automatic, so productivity levels of 200 to 300 points/h can be achieved.
In the preceding discussions on impact load deflection devices, a number of advantages were cited for the equipment, including the following:(14)
Realistic simulation of actual wheel loading.
High productivity.
Ability to measure deflection basin.
Ability to measure joint/crack load transfer.
At the same time, however, the FWD does have some disadvantages, such as the following:(14)
High initial cost.
Need for traffic control.
Relatively complex electromechanical system.
When interpreting FWD deflection data, the loading time is important to consider when evaluating differences in backcalculated moduli of viscoelastic materials because shorter loading times generally result in higher backcalculated modulus values for HMA.(14) The Dynatest® FWD produces a loading time of about 28 to 30 ms, whereas the KUAB produces a loading time of about 80 ms.(14)
In the last decade, considerable work has been conducted on the development of deflection-measuring equipment capable of collecting continuous deflection data along the length of a project. Continuous deflection profiles are noted to provide the following advantages over discrete deflection measurements:(17)
The entire length of the pavement project can be investigated. Thus, there is no danger of missing critical sections and no uncertainty about a test section being representative of the entire pavement system.
The spatial variability in deflections owing to pavement features such as joints, cracks, patches, and changing constructed or subgrade conditions are identified.
Testing and measurement operations are more efficient because there is no time lost stopping and starting.
At the time of this report, two such devices were under development, the rolling dynamic deflectometer (RDD) and the rolling wheel deflectometer (RWD). Although both of these devices were still in the prototype stage with no production models available, the following sections describe some of the characteristics of each device.
The RDD, developed at the University of Texas in the mid-1990s, is a truck-mounted deflectometer that applies large cyclic loads to the pavement and measures the induced cyclic deflections as it moves along the roadway.(18) Several deflection sensors are used on the RDD to measure deflections at different distances from the loaded areas. Often, however, only the maximum deflection is collected and studied to provide an indication of the overall stiffness of the pavement so the pavement can be divided into areas of similar response.(17) Deflection testing can be performed while the RDD vehicle travels at speeds of up to 2.4 km/h (1.5 mi/h).
Figure 9 shows a schematic drawing of the RDD. The truck has a gross weight of about 20,000kg (44,000 lb). A large diesel engine on the rear of the truck powers a hydraulic pump. This hydraulic system powers the loading system, which applies a combined static and dynamic sinusoidal force to the pavement through two loading rollers. The hydraulic system is capable of generating dynamic forces up to 154 kN (34,700 lbf) at frequencies from 5 to 100 Hz. The dynamic forces are transferred down the stilt structure, to the loading frame, and then through the loading rollers to the pavement (see figure 10). The force applied to the pavement is measured with load cells located between the loading frame and the bearings of the loading rollers. The displacements induced by the applied dynamic force are measured with multiple rolling sensors that are pulled along with the truck.
Figure 9. Diagram. Schematic of an RDD.(18)
Figure 10. Diagram. RDD loading and deflection measurement systems.(18)
Several applications are especially well suited for RDD testing. One is quality assurance and quality control (QA/QC) because continuous profiling can help identify all sections of the pavement system not conforming to specifications.(18) Another application is the development of load ratings for pavements because RDD testing can be performed along the entire pavement to identify critical sections.(18) Continuous profiling can also eliminate or limit the need for traffic control and associated costs.
The RWD is a dual-wheel, single-axle semitrailer equipped with four spot lasers mounted on an aluminum beam beneath the trailer to measure deflections (see figure 11). The trailer is 16 m (53ft) long and can vary the single axle load from 8,160 to 10,890 kg (18,000 to 24,000 lb) through the use of water tanks permanently installed over the rear axle.(19) The long trailer was selected to minimize differential bouncing from the front to the rear of the trailer and to allow for the long beam length so that the forward lasers are sufficiently away from the rear tractor axle.(19) The aluminum beam is 7.8 m (25.5 ft) long and is outfitted with four spot lasers mounted 2.6 m (8.5 ft) apart with the rearmost laser placed 152 mm (6 inches) behind the axle centerline.(20)
Figure 11. Photo. RWD collecting deflection data (aluminum beam beneath trailer contains laser sensors).(20)
The RWD configuration allows collection of deflection data at speeds up to 88 km/h (55 mi/h) at intervals of 12.2 mm (0.5 inches). In a field trial, such a high productivity allowed the collection of deflection data from more than 483 km (300 mi) of pavement in a single day.(20) Evaluation studies of those data found that the deflection data collected by the RWD compared favorably with that collected by an FWD; moreover, multiple RWD passes on several days for the same section produced results that were reasonable in magnitude and showed fair repeatability.(19) Other items of note from the field trials include the following:(19,20)
The RWD prototype was physically limited from being able to measure deflections directly at the axle centerline between the dual tires. The laser used to calculate deflection was located 277 mm (10.9 inches) forward of the axle centerline, which meant that, when combined with the delay in the peak deflection (which occurred approximately 150 mm (6 inches) behind the axle centerline), the RWD effectively measured deflections about 406 to 457 mm (16 to 18 inches) forward of the actual peak deflection. This resulted in smaller deflections and provided less contrast between pavements of varying stiffnesses.
RWD results were sensitive to factors that did not affect FWD-measured deflections, such as driver habits (uniform speed and minimizing sudden steering corrections), pavement texture, and roughness.
The RWD experienced a warming up effect prior to stabilization of readings in that the first one or two runs of the repeated measurements showed systematically higher deflections than the others.
Although the RWD was still in the phase of prototype testing and improvements, the potential benefit of the RWD would be that it would help highway officials prioritize and target funding and projects to those segments of the highway network that needed structural improvement and rehabilitation.(20)
Because the pavement deflections measured with the different devices reflect different loading conditions (static versus dynamic) and load duration, the pavement deflections obtained from the various deflection devices cannot be universally substituted for one another. However, sometimes there is a need to convert deflections from one device into deflections obtained from another; for example, the AI overlay design procedure is based on Benkelman Beam deflections, but many agencies have moved to the FWD device while still using the AI design procedure.(21) To address this need, some very rough correlations have been developed, but these should be used cautiously because they are often based on limited data and are valid only for the specific set of conditions under which the procedure was developed.(14) Some of these general relationships are provided in figure 12 through figure 17.
Figure 12. Equation. Conversion of FWD deflection to Benkelman Beam deflection—method 1.(14)
Where:
BB = Benkelman Beam deflection, 0.001 inches.
FWD = FWD maximum deflection, 0.001 inches (normalized to a 40-kN (9,000-lbf) load applied on a 300-mm (11.8-inch) diameter plate).
Figure 13. Equation. Conversion of FWD deflection to Benkelman Beam deflection—method 2.(21)
Where:
BB = Benkelman Beam deflection, 0.001 inches.
FWD = FWD maximum deflection, 0.001 inches (normalized to a 40-kN (9,000-lbf) load applied on a 300-mm (11.8-inch) diameter plate).
Figure 14. Equation. Conversion of Dynaflect deflection to Benkelman Beam deflection—method 1.(14)
Where:
BB = Benkelman Beam deflection, 0.001 inches.
D =Dynaflect maximum deflection, 0.001 inches.
Figure 15. Equation. Conversion of Dynaflect deflection to Benkelman Beam deflection—method 2.(21)
Where:
BB = Benkelman Beam deflection, 0.001 inches.
D = Dynaflect maximum deflection, 0.001 inches.
Figure 16. Equation. Conversion of Road Rater™ deflection to Benkelman Beam deflection—method 1.(14)
Where:
BB = Benkelman Beam deflection, 0.001 inches.
RR = Road Rater™ maximum deflection, 0.001 inches (at 36 kN (8,000 lbf)).
Figure 17. Equation. Conversion of Road Rater™ deflection to FWD deflection.(14)
Where:
FWD = Maximum FWD deflection, 0.001 inches (under 36 kN (8,000 lbf) on a 300-mm (11.8‑inch) diameter plate).
RR = Road Rater™ maximum deflection, 0.001 inches (at 36 kN (8,000 lbf) on a 300-mm (11.8‑inch) load plate).
Typical testing patterns for FWD testing vary, depending on the purpose of the testing and on the type and condition of the pavement. For network-level testing, deflection testing is conducted at greater intervals, commonly 150 to 450 m (500 to 1,500 ft) in a single traffic lane.(14) This level of testing is normally sufficient to provide a general indicator of structural adequacy of the pavement network.
For project-level testing, the spatial location of the deflection points should be adequate to capture the variability in structural capacity of the pavement; pavements with greater variability in structural condition should be subjected to a greater number of deflection measurements.(11) Typical project-level testing intervals for both HMA and PCC pavements are between 30 and 150 m (100 and 500 ft), with the shorter testing interval warranted for pavements in poorer condition and the larger testing interval appropriate for pavements in better condition. Even shorter testing intervals are sometimes used for research projects. If needed, a testing pattern can be set up to stagger the tests across traffic lanes, although often, traffic control constraints may prevent that. Recommended testing locations for specific pavement types include the following:
HMA pavements: Outer wheelpath of the outer traffic lane.
PCC pavements: Basin testing in the center slab of the outer traffic lane (to minimize edge effects) and joint load transfer testing in the outer wheelpath of the outer traffic lane. In addition, testing at selected slab corners locations may also be conducted to detect the presence of voids.
Routine FWD calibration is a vital component to ensure accurate loading and deflection measurements. As outlined in AASHTO R32-09, Calibrating the Load Cell and Deflection Sensors for a Falling Weight Deflectometer, FWD calibration should include the following:(22)
Annual calibration of the load cell and deflection sensors using an independently calibrated reference device (referred to as “reference calibration”). Deflection sensors are also compared with each other (referred to as “relative calibration)”. Annual calibration should also be conducted as soon as possible after load cell or deflection sensor replacement. Annual calibration is performed by a certified technician.
Monthly relative calibration of the deflection sensors. Monthly deflection sensor calibration is conducted using a relative calibration stand supplied by the FWD manufacturer and is different than the relative calibration conducted during annual calibration. Relative calibration should also be conducted immediately after replacement of a deflection sensor. Relative calibration does not require a certified technician.
A number of factors affect the magnitude of measured pavement deflections, which makes the interpretation of deflection results difficult. To the extent possible, direct consideration of these factors should be an integral part of the deflection-testing process so that the resultant deflection data are meaningful and representative of actual conditions. For example, conducting load transfer testing on PCC slabs in the afternoon of a warm day (when the slabs have expanded and the joints are tight) produces very high load transfer results, which likely are not representative of the load transfer capabilities during cooler temperatures (when the slabs have contracted and the joints are open). Recognizing and accounting for these factors before the establishment of a field testing program helps ensure the collection of useful deflection data that can be used in subsequent backcalculation analyses.
The major factors that affect pavement deflections can be grouped into categories of pavement structure (type and thickness), pavement loading (load magnitude and type of loading), and climate (temperature and seasonal effects). Each of these is discussed in the sections that follow.
In essence, the deflection of a pavement represents an overall system response of the surface, base, and subbase layers, as well as the subgrade itself. Thus, the properties of the surface layer (thickness and stiffness) and of the supporting layers (thickness and stiffness) all affect the magnitude of the measured deflections. Generally speaking, “weaker” systems deflect more than “stronger” systems under the same load, with the exact shape of the deflection basin related to the stiffness of the individual paving layers.(9) As a general rule, pavements of similar materials (flexible or rigid) exhibiting greater deflections typically have shorter service lives. Figure 18 illustrates typical flexible and rigid pavement deflection responses to loading.
Figure 18. Diagram. Comparison of typical flexible and rigid pavement deflection responses.
Many other pavement-related factors can affect deflections, including the following:
Testing near joints, edges, cracks, or in areas containing structural distress (such as alligator cracking), can produce higher deflections than testing at interior portions of the pavement.
Random variations in pavement layer thickness can create variabilities in deflection.
Variations in subgrade properties and the presence of underlying rigid layers (such as bedrock or a high water table level) can provide significant variability in deflections.
One of the most obvious factors that affects pavement deflections is the magnitude of the applied load. Most modern deflection equipment can impose load levels from as little as 13 kN (3,000lbf) to more than 245 kN (55,000 lbf), and it is important that appropriate load levels be targeted for each application. For example, for most highway pavement testing, a nominal load level of 40 kN (9,000 lbf) is often used because this is representative of a standard 80-kN (18,000-lbf) axle load. On the other hand, load levels of 156 to 200 kN (35,000 to 45,000 lbf), selected to match the wheel loads of commercial aircraft, may be needed on heavy-duty airfield pavements.
An important reason for selecting test loads as close as possible to the design loads is the nonlinear deflection behavior exhibited by many pavements. This is generically shown in
figure 19, in which a pavement structure exhibits a deflection of 0.028 mm (0.001 inches) under a 4.4‑kN (1,000-lbf) loading, and a deflection of 0.35 mm (0.014 inches) under a 40-kN (9,000lbf) load. Had the 40-kN (9,000 lbf) deflection been projected based on the 4.4-kN (1,000lbf) load, a deflection of 0.25 mm (0.01 inches) would have been erroneously projected. Nonlinear pavement response can result from a number of factors, including viscoelastic behavior, stress sensitive materials, and nonuniform support conditions.
©National Highway Institute
Figure 19. Graph. Nonlinear pavement deflection response.(9)
Pavement deflection response can also be affected by the type of loading; a slow, static loading condition produces a different response than a rapid, dynamic loading condition. In general, the more rapid the loading (i.e., the shorter the load pulse), the smaller the deflections; this is why the static load devices (such as the Benkelman Beam) tend to produce deflections larger than those produced by dynamic loading devices (such as the FWD).
Temperature is a very important factor that must be considered as part of any pavement deflection-testing program. For HMA pavements, the stiffness of the asphalt layer decreases as the temperature increases, resulting in larger deflections (see figure 20). Therefore, correction of the measured deflections to a standard temperature is required to perform meaningful interpretations of the data. Thickness design procedures also typically assume a standard HMA temperature. Correction factor charts are available to assist in converting deflections to a standard temperature, generally 20 °C (68 °F). When correcting to a standard temperature, FWD testing should ideally occur within a reasonable range of the standard temperature.
°C = (°F -32)/1.8.
1 MPa = 145 lbf/inch2.
Figure 20. Graph. HMA elastic modulus as a function of middepth pavement temperature.(23)
PCC pavement deflections are also affected by temperature, in both basin testing and in joint and corner testing. Differences in temperature between the top and bottom of the slab cause the slab to curl either upward (slab surface is cooler than the slab bottom) or downward (slab surface is warmer than the slab bottom). If basin testing is conducted when the slab is curled down, or if corner testing is conducted when the slab is curled up, the slab could be unsupported and greater deflections may result. Figure 21 shows the effect of daily temperature variations on backcalculated modulus of subgrade reaction (k-value).
°C = (°F -32)/1.8.
1 MPa = 145 lbf/inch2.
Figure 21. Graph. Variation in backcalculated k-value due to variation in temperature gradient.(24)
Temperature also affects joint and crack behavior in PCC pavements. Warmer temperatures cause the slabs to expand and, coupled with slab curling effects, may “lock up” the joints. Deflection testing conducted at joints when they are locked up results in lower joint deflections and higher LTEs that are misleading regarding the overall load transfer capabilities of the joint. Figure 22 shows the variation in computed LTEs throughout the day, with the higher values computed from data collected in mid-afternoon.(24) Because of these effects, it is normally recommended to conduct FWD testing early in the morning or during cold periods of the year on PCC pavements.
°C = (°F – 32)/1.8.
Figure 22. Graph. Daily variation in the calculated LTEs (leave side of joint).(24)
Further insight on the effects of both the weighted average slab temperature and temperature gradients on the LTEs for doweled (restrained) and undoweled (unrestrained) joints can be obtained from figure 23 and figure 24. Figure 23 provides LTEs for two 240-mm (9.5-inch) slabs (one slab is doweled and the other is undoweled) with a 4.6-m (15-ft) joint spacing. The graph shows the equivalent linear gradient and average slab temperature present at the time of testing significantly influenced the LTE of an undoweled joint. The LTE for the doweled joint was not influenced by either the equivalent linear gradient or average slab temperature.
Figure 24 shows the relationship between load transfer and equivalent linear temperature gradient at the time of testing for two 190-mm (7.5-inch) slabs with a 6.1-m (21-ft) joint spacing. This figure shows that even when the LTE of a doweled joint was low (approximately 60percent), it was still not significantly influenced by the weighted average temperature or the equivalent linear gradient.(25)
©National Academy of Sciences. Reproduced with permission of the Transportation Research
Board (TRB)
°C = (°F – 32)/1.8.
1 cm = 0.39 inches.
Figure 23. Graph. Variation in the calculated LTEs for two slabs tested at different temperature gradients and weighted average slab temperatures.(25)
©National Academy of Sciences. Reproduced with permission of the TRB
°C = (°F − 32)/1.8.
1.81 cm = 0.39 inches.
Figure 24. Graph. Relationship between LTEs and equivalent linear temperature gradients for two joints with low LTEs.(25)
Seasonal variations in temperature and moisture conditions also affect pavement deflection response. Generally speaking, deflections are greatest in the spring because of saturated conditions and reduced pavement support and lowest in the winter when the underlying layers and subgrade are frozen (see figure 25). This is the reason that many agencies located in seasonal frost areas place spring load restrictions on their secondary flexible pavements; otherwise, a significant amount of damage could be inflicted on the pavements when the pavement layers are in a weakened and saturated state. PCC pavements are less affected by seasonal variations in support conditions.
Figure 25. Graph. Seasonal effects on pavement deflection.(26)
Backcalculated modulus values can also vary seasonally. For example, figure 26 illustrates the variation of the computed elastic modulus values for the different layers of a pavement test section (276251) located in Minnesota.(23) The modulus values were provided over a 3-year period, and several abrupt spikes were observed during winter periods in which the HMA layer became very stiff and the prevailing frozen conditions resulted in higher modulus values for the base, subbase, and subgrade. (Note that the time scale on the x-axis in figure 26 is categorical not continuous, based on when the FWD testing was performed.)
Seasonal variations are also apparent on PCC pavements. For example, figure 27 shows seasonal variations in the backcalculated support properties of the subgrade.(27)Both the backcalculated k‑value and the backcalculated elastic modulus of the subgrade are shown in that figure, with a noticeable decrease in the support conditions observed during the springtime. As another example of seasonal effects on PCC pavements, figure 28 shows the average LTEs over a 2-year period.(24) As a general trend, the LTE parallels the surface temperature, generally decreasing with the decreases in the surface temperature and increasing with increases in the surface temperature.
1 MPa = 145 lbf/inch2.
Figure 26. Graphs. Comparison of monthly variation in elastic modulus (in MPa) for pavement layers and subgrade.(23)
ES = Elastic modulus of the subgrade.
1 MPa = 145 lbf/inch2.
Figure 27. Graph. Seasonal variation in backcalculated subgrade modulus.(27)
Figure 28. Graph. Seasonal variation in LTE and PCC surface temperature.(24)
Pavement deflection testing is recognized as a reliable, quick, and cost-effective method for determining the structural condition of existing pavements. Specifically, deflection measurements can be used for backcalculating the elastic moduli of the pavement structural layers and for estimating the load-carrying capacity for both HMA and PCC pavements. In addition, in PCC pavements, identification of loss of support at slab corners and the evaluation of the joint or crack load transfer can be performed using deflection testing.
A number of different types of equipment are available for the collection of pavement deflection data, including static devices (e.g., Benkelman Beam), steady-state vibratory devices (e.g., Dynaflect and Road Rater™), and impulse devices (e.g., FWD). The features and operating characteristics of these devices are described, and it is noted that the FWD has become the worldwide standard for pavement deflection testing, largely because of its ability to closely simulate the loading characteristics of a moving wheel load. Deflection devices capable of providing continuous deflection measurements are currently being developed.
Pavement deflections represent an overall system response of the pavement structure and subgrade soil to an applied load. The major factors that affect pavement deflections can be grouped into categories of pavement structure (type and thickness), pavement loading (load magnitude and type of loading), and climate (temperature and seasonal effects).
