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Publication Number:  FHWA-HRT-16-011    Date:  December 2017
Publication Number: FHWA-HRT-16-011
Date: December 2017


Using Falling Weight Deflectometer Data With Mechanistic-Empirical Design and Analysis, Volume III: Guidelines for Deflection Testing, Analysis, and Interpretation


Aggregate: A collective term for the mineral materials such as sand, gravel, and crushed stone that are used with a binding medium (such as water, bitumen, portland cement, lime, etc.) to form compound materials (i.e., asphalt concrete, portland cement concrete, etc.).(70)

Alligator cracking: Interconnected or interlaced cracks forming a pattern that resembles an alligators hide.(71)

Analysis period: The period of time used in making economic comparisons between rehabilitation alternatives. The analysis period should not be confused with the pavement’s design life (performance period).(71)

Asphalt concrete (AC): A controlled mixture of asphalt cement and graded aggregate compacted to a dense mass. Also referred to as hot-mix asphalt (HMA).(72)

Axle load: Load exerted by a vehicle on the pavement surface via an axle.

Base: The layer or layers of specified or select material of designated thickness placed on a subbase or subgrade to support a surface course; layer directly beneath a PCC slab.(71)

Backcalculation: An iterative process by which pavement layer moduli, or other stiffness properties, are estimated from FWD deflection data. The process begins with a hypothesis of a given layer’s modulus, which is repeatedly compared with the FWD’s output using an iterative mathematical model. The iteration stops once a predetermined level of tolerance has been reached between subsequent calculated estimates.(4)

Chemically stabilized mixtures: Subgrade materials whose plasticity characteristics have been modified using materials such as lime, fly ash, or PCC.

Composite pavement: A pavement structure composed of an asphalt concrete wearing surface and PCC slab, or an asphalt concrete overlay of a PCC slab.(71)

Continuously reinforced concrete pavement (CRCP): PCC pavement containing longitudinal reinforcement at or above mid-depth designed to hold shrinkage cracks tightly closed. Transverse joints exist only for construction purposes and on-grade structures. Transverse reinforcement may or may not exist. Longitudinal joints exist similar to other types of concrete pavements.(72)

Crack: A break or disruption in the continuity of the pavement surface that may extend through the entire pavement thickness.

Crushed stone: A base (or subbase) course of designed thickness and constructed of graded and mechanically crushed mineral aggregate compacted above the subgrade.(72)

Curling: Deformation of a PCC slab caused by a temperature difference between the upper and lower surfaces.(73)

Deflection: Vertical deformation of a pavement under an applied load.(73)

Deflection basin: The bowl shape of the deformed pavement surface caused by a specialized load as depicted from the peak measurements of a series of deflection sensors placed at radial offsets from the center of the load plate.(18)

Deflection sensor: An electronic device(s) capable of measuring the relative vertical movement of a pavement surface and mounted to reduce angular rotation with respect to its measuring axis at the expected movement. Such devices may include seismometers, velocity transducers (geophones), or accelerometers.(18)

Deviator stress: In triaxial testing the difference between the axial stress applied by the testing apparatus and the confining stress (pressure).

Dowel: A load transfer device across a joint (usually a transverse joint) in a rigid slab, usually consisting of a plain cylindrical steel bar.(71)

Drop sequence: A sequence of load levels used during FWD testing.

Dynamic cone penetrometer (DCP): Testing apparatus for measuring the resistance of a granular material or soil against penetration of a cone driven into the soil by repetitive droppings of a mass on an anvil.

Elastic modulus: The relationship between stress and strain within a material’s elastic range. Thus, the flexibility of any object depends on its elastic modulus and geometric shape; however, it is important to note that strength (stress needed to break something) is not the same thing as stiffness (as measured by elastic modulus).(70)

Equivalent single axle load (ESAL): A numerical factor that expresses the relationship of a given axle load to another axle load in terms of the relative effects of the two loads on the serviceability of a pavement structure. ESALs are often expressed in terms of 80 kN (18,000-lb) single-axle loads.(71)

Falling weight deflectometer (FWD): Trailer- or truck-mounted equipment that applies an impact load to a pavement structure by means of a mass dropping on a set of buffers mounted on a loading plate resting on the pavement surface and measures the resulting deflections of the pavement.

Forward calculation: A noniterative process in which stresses, strains, and displacements are calculated from layer data and applied load.(4)

Fatigue cracking: Cracking of the pavement surface as a result of repetitive loading; may be manifested as longitudinal or alligator cracking in the wheelpaths for flexible pavement and transverse cracking (and sometime longitudinal cracking) for jointed concrete pavement.

Faulting: Difference in elevation across a joint or crack.

Flexible pavement: A pavement structure that maintains intimate contact with and distributes loads to the subgrade and depends on aggregate interlock, particle friction, and cohesion for stability.(71)

Gravel: Coarse aggregate resulting from natural disintegration and abrasion of rock or processing of weakly bound conglomerate.(72)

Ground-penetrating radar (GPR): Noninvasive tool that has been used to map subsurface conditions in a wide variety of applications. GPR is basically a subsurface anomaly detector; as such it will map changes in the underground profile due to contrasts in the electromagnetic conductivity across material interfaces. In a GPR system, short pulses of radio wave energy travel through the pavement structure and create echoes at boundaries of dissimilar materials, such as at an asphalt-base interface.(70)

Hot-mix asphalt (HMA): A controlled mixture of asphalt cement and graded aggregate compacted to a dense mass. Also referred to as asphalt concrete (AC).(72)

International Roughness Index (IRI): A measure of a pavement’s longitudinal surface profile as measured in the wheelpath by a vehicle traveling at typical operating speeds. It is calculated as the ratio of the accumulated suspension motion to the distance traveled obtained from a mathematical model of a standard quarter car traversing a measured profile at a speed of 80 km/h (50 mi/h). The IRI is expressed in units of meters per kilometer (inches per mile) and is a representation of pavement roughness.(1)

Joint: A pavement discontinuity, either longitudinal or transverse, made necessary by design or by interruption of a paving operation.

Jointed plain concrete pavement (JPCP): Jointed PCC pavement containing transverse joints spaced to accommodate temperature gradient and drying shrinkage stresses to avoid cracking. This pavement contains no distributed steel to control random cracking and may or may not contain joint load transfer devices.(72)

Lean concrete base: A base course constructed of mineral aggregates plant mixed with a sufficient quantity of portland cement to provide a strong platform for additional pavement layers and placed with a paver.(72)

Leveling course: A first lift applied to an existing pavement used to fill in ruts and make up elevation differences.(70)

Lime stabilized: A prepared and mechanically compacted mixture of hydrated lime, water, and soil supporting the pavement system that has been engineered to provide structural support.(72)

Linear elastic: A material property that allows an object or material to return to or be capable of returning to an initial form or state after deformation in a linear manner (e.g., a plot of a linear elastic material would show a straight line). Almost no material is completely linearly elastic, but many materials are linearly elastic over a certain range of stress/strain.(70)

Load cell: A cell capable of accurately measuring the load that is applied to the load plate and placed in a position to minimize the mass between itself and the pavement. The load cell should be positioned in such a way that it does not restrict the ability to obtain deflection measurements under the center of the load plate. The load cell should be water resistant and resistant to mechanical shocks from road impacts during testing or traveling.(18)

Load transfer efficiency (LTE): The ability of a joint or crack to transfer load from one side to another.

Loading plate: A plate capable of an even distribution of the load over the pavement surface for measurements on conventional roads and airfields or similar stiff pavements. The plate should be suitably constructed to allow pavement surface deflection measurements at the center of the plate.(18)

Longitudinal cracking: Pavement cracking predominately parallel to the direction of traffic.

Maintenance: The preservation of the entire roadway, including surface, shoulders, roadsides, structures, and traffic control devices.(71)

Mechanistic-empirical: A design philosophy or approach in which fundamental material responses are used in conjunction with empirically derived relationships to accomplish the design objectives.

Milling: Mechanical process in which a portion of a pavement surface is removed.

Modulus of elasticity (E): The stiffness of a material as defined in terms of the ratio of stress to strain in the elastic portion of a stress-strain curve.

Modulus of rupture (MR): The flexural bending strength of concrete.

Modulus of subgrade reaction (k): Westergaard’s modulus of subgrade reaction for use in rigid pavement design (the load in pounds per square inch on a loaded area of the roadbed soil or subbase divided by the deflection in inches of the roadbed soil or subbase, MPa/mm (lbf/inches2/inch)). The value used in design is the dynamic modulus of subgrade reaction as directly backcalculated from FWD deflections or backcalculated from deflections obtained from the elastic layered program where resilient moduli values are assigned to each layer. The traditional modulus of subgrade reaction is the static value which is approximately one-half that of the dynamic value.(71)

Nonlinear material: A pavement material having properties such that the relationship between stress and strain is nonlinear.

Pavement performance: Measure of accumulated service provided by a pavement (i.e., the adequacy with which it fulfills its purpose). Often referred to as the record of pavement condition or serviceability over time or with accumulated traffic.(71)

Pavement structure: A combination of subbase, base course, and surface course placed on a subgrade to support the traffic load and distribute it to the roadbed.(71)

Portland cement concrete (PCC): A composite material consisting of portland or hydraulic cement, water, and coarse and fine aggregate.

Poisson’s ratio: Ratio of the transverse strain (perpendicular to the applied load) and longitudinal strain (elongation) of a material specimen in one-direction loading conditions.

Raveling: A pavement distress characterized by the loss of surface material and degradation of the binder material.

Reflective cracking: Cracks in asphalt or concrete surfaces of pavements occurring over joints or cracks in underlying layers.

Resilient modulus (MR): A standardized measurement of the modulus of elasticity of roadbed soil or other pavement material.(71)

Rigid pavement: A pavement structure that distributes loads to the subgrade, having as one course a PCC slab of relatively high-bending resistance.(71)

Roadbed: The graded portion of a highway between top and side slopes, prepared as a foundation for the pavement structure and shoulder.(71)

Rutting: Longitudinal surface depressions in the wheelpath of an HMA pavement caused by plastic movement of the HMA mix, inadequate compaction, or abrasion from studded tires (such abrasion can also be observed on PCC pavements).(74)

Soil aggregate: Natural or prepared mixtures consisting predominantly of stone, gravel, or sand that contain a significant amount of −75-μm (No. 200) silt-clay material.(72)

Soil cement: A mechanically compacted mixture of soil, portland cement, and water, used as a layer in a pavement system to reinforce and protect the subgrade or subbase.(75)

Spalling: The cracking, breaking, or chipping of pavement edges in the vicinity of a joint or crack.(74)

Stabilized base: A base course constructed with a stabilizing material, usually AC or portland cement.(72)

Subbase: The layer or layers of specified or selected materials of designated thickness placed on a subgrade to support a base course.(71)

Subgrade: The top surface of a roadbed upon which the pavement structure and shoulders are constructed.(71)

Surface Curvature Index (SCI): Difference between the deflection recorded at the center of the dynamic load and the deflection recorded at a nearby offset (usually up to 900 mm (35 inches) maximum).(70)

Transverse cracking: A discontinuity in a pavement surface that runs generally perpendicular to the pavement centerline. In HMA pavements, transverse cracks often form as a result of thermal movements of the pavement or reflection from underlying layers. In PCC pavements, transverse cracks may be caused by fatigue, loss of support, or thermal movements.(74)

Warping: Deformation of a PCC slab caused by a moisture differential between the upper and lower surfaces.(74)

Wheel load: The portion of a loaded axle that is transmitted to the pavement on a wheel.

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