1、Designation: D 4748 06Standard Test Method forDetermining the Thickness of Bound Pavement LayersUsing Short-Pulse Radar1This standard is issued under the fixed designation D 4748; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the nondestructive determina-tion of thickness of bound pavement layers using short-
3、pulseradar. Bound pavement layers are defined as the upper layers ofa pavement, consisting of materials such as bituminous,concrete, portland-cement concrete, roller-compacted concrete,and stabilized bases. Bound pavement layers does not includegranular base and subbase materials.1.1.1 As the electr
4、omagnetic wave generated by radarpropagates through the bound pavement layers, the wave isattenuated, dispersed and reflected at layer interfaces. At somedepth, due to the wave attenuation and dispersion, the reflec-tions at the layer interfaces cannot be detected by the radar.This maximum penetrati
5、on depth is a complex function ofradar system parameters such as transmitted power, receiversensitivity, center frequency and bandwidth of the radar systemand signal processing, as well as the electromagnetic propertiesof the pavement materials and environmental factors such asmoisture content.1.1.2
6、 Radar system resolution is determined mainly by thetransmitted pulse length and bandwidth of the radar. A typicalsystem for this application usually has a resolution sufficient todetermine a minimum layer thickness of 40 mm (1.5 in.) to anaccuracy of 65mm(60.2 in.). Improvements in systemresolution
7、 may be possible with additional signal processing.1.2 This test method may not be suitable for application topavements which exhibit increased conductivity due to theincreased attenuation of the electromagnetic signal. Examplesof senarios which may cause this are: extremely moist or wet(saturated)
8、pavements if free electrolytes are present and slagaggregate with high iron content.1.3 The values stated in mm-kilogram units are to beregarded as the standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the us
9、er of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Specific hazardstatements are given in Section 6.NOTE 1Bound pavement layers are defined as the upper layers of apavement, consisting of materials such a
10、s bituminous, concrete, portland-cement concrete, roller-compacted concrete, and stabilized bases. Boundpavement layers do not include granular base and subbase materials.2. Summary of Test Method2.1 Since this test method is based upon measurementsperformed by a short-pulse radar system, a brief de
11、scription ofthe operating principles of such a system are included herein.2.2 The detection of an interface between two differentmaterials by radar depends upon the partial reflection ofincident energy at that interface. The amplitude of the reflectedenergy at that interface, with respect to the inc
12、ident energy, isrelated to the relative dielectric constants of the two materialsaccording to the formula:AA05=e12 =e2= e11 = e2(1)where:A = the reflected energy, amplitudeA0= the incident energy, amplitudee1= the dielectric constant material 1, ande2= the dielectric constant material 2.The ability
13、to detect the thickness of a layer depends on thecontrast between the dielectric constant of that layer and thelayer beneath. A sufficient contrast for thickness determinationusually exists between asphaltic layers and soil or aggregatebase materials. Such a contrast may not always be sufficientbetw
14、een concrete and agggregate base materials, or betweenindividual layers of asphalt.2.3 Layer thickness can be determined using radar technol-ogy if the dielectric constant of that material and the two-waytravel time for the radar wave to pass through the layer areknown. The relationship is defined b
15、y the following equation:T 5Dt 3 c2=er(2)where:1This test method is under the jurisdiction of ASTM Committee E17 on Vehicle- Pavement Systems and is the direct responsibility of Subcommittee E17.41 onPavement Testing, Evaluation, and Management Methods.Current edition approved Dec. 1, 2006. Publishe
16、d December 2006. Originallyapproved in 1987. Last previous edition approved in 1998 as D 4748 98.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.T = layer thickness,c = speed of light in air, 300 mm/nsec for T in mm (11.8in/nsec for
17、T in inches),er= relative dielectric constant of layer, andDt= two-way pulse travel time through layer (in nanosec-onds).2.4 The dielectric constant of a pavement material can varysomewhat depending on aggregate types, asphalt cementsources, density, cracking, and moisture content. Using anair-coupl
18、ed horn antenna radar this variation may be calculateddirectly from the radar data by using the known dielectricconstant of air and the equation in Section 2.22.5 Determining the dielectric constant when using groundcoupled dipole antenna radar requires an independent means,such as coring or a radar
19、 based dielectric constant measurementusing two or more antennas.3. Significance and Use3.1 This test method permits accurate and nondestructivethickness determination of bound pavement layers. As such,this test method is widely applicable as a pavement system-assessment technique.3.2 Although this
20、test method, under the right conditions,can be highly accurate as a layer-thickness indicator, consis-tently reliable interpretation of the received radar signal todetermine layer thicknesses can be performed only by anexperienced data analyst. Such experience can be gainedthrough use of the system
21、and through training coursessupplied by various equipment manufacturers or consultingcompanies. Alternatively, the operator may wish to use com-puter software to automatically track the layer boundaries andlayer thickness, where applicable4. Interferences4.1 Determinations made with radar are advers
22、ely affectedby surface and subsurface water. In the case of horn antenna,standing water on the surface of the pavement decreases theamount of energy that penetrates the pavement. This effect isdifficult to measure and may vary dramatically over a shorttime interval due to variations in the thickness
23、 of the waterlayer caused by run-off or evaporation. However, in general,testing with horn antennas shall not be conducted in thepresence of standing water.4.2 The apparatus is subject to interference from othersources of electromagnetic radiation. Interference from nearbyhigh-power transmitters man
24、ifests itself as large, high-frequency variations in the radar return across the entiremeasurement depth. Other sources of intermittent interferencemay include mobile phones and CB radios. Testing shall not beconducted in the presence of observed interference.4.3 Large objects such as vehicles have
25、the potential tointerfere with the radar return. A conservative, equipmentindependent approach to minimize the effects of large objectsis to maintain these objects at a distance outside the zone ofinfluence as calculated by the following expression:d 5t2 3 k(3)where:d = the zone of influence,k = mul
26、tiplication constant, 3.28 for d in meters (1 for d infeet), andt = time in nanoseconds of the measurement time window.For horn antennas, this equation does not take into account thespacing between the horn and the pavement. The closer thespacing between the horn and the pavement, the less influence
27、from large objects and therefore a more liberal approach to thedistance calculation above is permissible.5. Apparatus5.1 The apparatus consists of an antenna, radar transducer,and suitable display devices. The radar transducer consists of atransmitter, receiver, and timing and control electronics. T
28、hedisplay device may be an oscilloscope, grey-level chart re-corder, or a CRT monitor, or a personal computer with a dataacquisition board.5.2 The schematic drawing in Fig. 1 illustrates the equip-ment and configuration. The antenna and transducer are typi-cally in close proximity to one another, li
29、ghtweight, andmaneuverable, either by hand or a suitable support or mountingsystem. Mobility is desirable for positioning over the test area.Display devices are generally heavier and mounted on amovable platform such as a cart or a vehicle.5.2.1 The transducer is connected to the antenna and display
30、devices. The transducer generates, transmits, and receiveslow-power broad band radio frequency (rf) signals through theantenna. The rf signals are then converted into an audio-frequency signal suitable for display and resulting interpreta-tion.5.3 Two basic types of antenna systems are in use (1)air
31、-coupled horn antennas; and (2) ground-coupled dipoleFIG. 1 Equipment ConfigurationD4748062antennas. The horn antennas are specifically designed toradiate into the air, and thus to be used at some distance abovethe pavement surface, usually 8 to 20 inches (20 to 50 cm). Theground-coupled antennas ar
32、e specifically designed to operate incontact with the pavement surface.5.3.1 The general specifications for the radar system appa-ratus components are:5.3.1.1 AntennaA bandwidth shall be appropriate tohandle selected transmitted pulse frequency components.5.3.1.2 Transmitter, short-pulse (0.5 to 2.0
33、 ns) low-power (1to 20 W maximum).5.3.1.3 Receiver ProcessingA wide bandwidth capable ofprocessing time signals corresponding to a depth of severalfeet.5.3.2 The general specifications for the display devices are:5.3.2.1 Grey-Level Chart Recorder, with analog signal inputcapability that has external
34、ly-synchronized expandable datadisplay and 16 grey-levels.5.3.2.2 Oscilloscope A single-channel analog input oscil-loscope that is externally synchronized.5.3.2.3 Video/Computer Monitor that has analog input andis externally synchronized. It shall have 16 grey-levels and 16colors and continuous real
35、-time display capability.5.3.3 A power source, typically 110 V ac or 12 V dc, shallbe available for the apparatus.6. Hazards6.1 WarningThe radar apparatus used in this test methodis potentially a microwave radiation hazard.All personnel shallstand clear of the region directly under the antenna when
36、thesystem is energized.6.2 Electromagnetic emissions from the radar apparatus, ifthe system is improperly operated, could potentially interferewith commercial communications, especially if the antenna isnot properly oriented toward the ground. Take care to ensurethat all such emissions from the syst
37、em comply with Part 15 ofthe Federal Communications Commission (FCC) Regulations.6.3 Take care to ensure that appropriate traffic controlmeasures are employed when operating the radar apparatus onhighways, roads, and airports. Such measures are essential forthe safety of system operators as well as
38、that of the generaltraveling public.7. Calibration and Standardization7.1 A calibration time constant, CT, is established for theradar system by measuring the time interval between reflec-tions from two precisely spaced metal plates as follows:7.1.1 Separate two metal plates by a distance, dc, of ap
39、proxi-mately 300 65mm(1260.2 in.) using four small nonconductive spacers, one attached to each corner of the metalplates (see Fig. 2). The upper plate is approximately 125 by125mm (5 by 5 in.) and the lower plate is approximately 300by 300 mm (12 by 12 in.).7.1.2 Place the calibration fixture below
40、the antenna andenergize the radar system.7.1.3 Measure the time delay between the two reflectionsfrom the pair of plates by observing the received signal on thedisplay device. This time delay is the calibration time constant,CT, in nanoseconds. (See Fig. 3)7.1.4 The calibration time constant does no
41、t have to bemeasured before each attempt to determine pavement layerthickness. The radar system itself should remain in calibrationfor long periods of time (1 day). If adjustments or changesthat affect the radar system timing are made, a calibrationmeasurement shall be made or the system shall be ca
42、pable ofself calibration on a continuous basis.7.2 The relative dielectric constant, er, for the pavementlayer of interest must be estimated or calculated. This dielectricconstant is used to calculate thicknesses of pavement layers.7.2.1 Position the antenna over an area of known layerthickness. Typ
43、ically, a precise layer thickness for calibrationpurposes is determined by taking a core sample and measuringthe appropriate layer thickness from the sample or alternativelyrefraction techniques can be used to measure in situ materialFIG. 2 Calibration MeasurementFIG. 3 CTMeasurementD4748063velociti
44、es. If it is impossible to core or obtain in situ velocitymeasurements and obtain an accurate calibration of the dielec-tric constant, then “best-guess” estimates shall be made by theoperator based on known dielectric constants for given mate-rials.7.2.2 Using the radar display device, measure the t
45、imeinterval in nanoseconds, Dt, between the surface reflection ortop of the layer reflection and the corresponding reflection fromthe bottom of the layer of interest.7.2.3 Calculate the relative dielectric constant, er, for thatlayer using the equation:er5FDt 3 dc2 3 T 3 CTG2(4)where:er= relative di
46、electric constant of layer,Dt = two-way pulse travel time through layer (in nanosec-onds),dc= distance between metal plates,T = measured layer thickness, andCT= calibration time constant (in nanoseconds).7.2.4 The relative dielectric constant thus calculated can beused for all measurements made wher
47、e the pavement layermaterials are identical to those in the locale of the calibrationcore. If the materials change, a new calibration core may benecessary.7.2.5 Typical relative dielectric constants are given in Table1.8. Procedure8.1 Continuously traverse the radar antenna across thepavement to be
48、tested. The speed of the traverse will determinethe number of data points per unit distance. For horn antennas,the traverse speed is constrained only by the desired spacing ofradar scans. For ground coupled antennas, the traverse speed islimited to approximately 8 kph (5 mph) in order to maintainste
49、ady ground contact.8.2 Identify the signal reflection associated with the surfaceof the pavement or upper interface of the layer of interest usingthe display devices.8.3 Identify the coherent signal reflection associated withthe lower surface interface of the layer of interest.8.4 Determine the time interval, Dt, between these tworeflections.9. Calculation9.1 Calculate the thickness of the layer of interest using theconstants CTand erobtained in Section 7 and the time intervalDt, (in nanoseconds) determined in 8.4, using the followingequation:T 5Dt 3 dc2=er3 CT(5)where:T = measure
