A Non-Destructive Pavement Evaluation Tool for Urban Roads

Transcription

1 A Non-Destructive Pavement Evaluation Tool for Urban Roads By K. O. Addo 1 Introduction Highway authorities and local municipalities periodically assess the condition of roads in their various jurisdictions and update their databases with new information. The key pieces of required information are remaining life and rehabilitation strategy. This information enables engineers to identify, prioritize and schedule roads that require rehabilitation as well as estimate costs. The process of determining prevailing road conditions is called pavement evaluation. The total length of road to be evaluated is often large and techniques that are fast, economical, repeatable and cause little delay to the motoring public are preferable. A requirement that is becoming increasingly popular with engineers is that the evaluation technique be objective enough to generate results that can be processed within a computerized framework. Like most roads, an urban road or street is a multi-layered flexible or rigid structure built on a subgrade. Typically, a road structure consists of an upper layer of asphalt or concrete overlying a gravel base/subbase on fill or native soil. Buried within this road structure are gas, water mains, sewers, cable TV and telephone conduits of different diameters at various depths and orientations. The presence of underground services makes it preferable to use non-destructive tests to evaluate urban roads. Four tools available for conducting non-destructive tests on roads are the 1. Benkelman beam, 2. Road Radar, 3. Falling Weight Deflectometer (FWD) or dynaflect, and 4. Spectral or Seismic Pavement Analyzer (SPA). The Benkelman beam test procedure involves the measurement of pavement surface rebound with a cantilevered beam when a truck loaded to 818 kg on its rear axle moves from rest. Measurements are made between the dual tires on the rear axle at specified intervals in the outer wheel path and are then corrected for temperature and seasonal variation. The corrected rebound values are used in a statistical manner to determine a most probable spring rebound (MPSR). The MPSR value, a specified design rebound and traffic number are used to enter a design chart (based on an accumulated experience 1. MTL (Metro Testing Laboratories) Engineering, Burnaby, BC 53

2 on similar roads) to determine the overlay required to extend pavement life to 2 years. This test is fast, simple and inexpensive. However, it does not provide thickness information and must be accompanied by other tests that provide this information. In general, Benkelman beam tests are performed if an overlay is the preferred rehabilitation strategy. The road radar is a sophisticated non-destructive tool for measuring pavement layer thickness. It uses a hybrid antenna system (comprising an air launched and surfacecoupled antennae) to emit and receive electromagnetic waves. By measuring radar signal velocity and travel times, the depths to interfaces of materials with unequal electrical properties are determined. The technique is fast since it is performed from a vehicle in motion at about 2 km/hr. The road radar can also be used for identifying delaminations and cracks perpendicular to the direction of travel. However, it does not provide strength or deformation properties. The dynaflect and Falling Weight Deflectometer are tools that measure surface deflection. In this technique, a number of geophones are used to determine the static deflection basin resulting from a vertical impact. A back-calculation procedure is then used to infer the thickness and resilient modulus of the constituent layers of the pavement structure. Due to the nature of the back-calculation algorithm, reliable layer thickness information is required to control the inversion process. Thus supplemental coring or road radar tests are required. The Seismic Pavement Analyzer (Nazarian et al, 1993) or the Spectral Pavement Analyzer SPA (Metro Testing Laboratories, 1994) uses a suite of wave propagation techniques to determine shear modulus, layer thickness, support conditions and to detect delamination. The acoustic contrast at layer interfaces is used to determine layer thickness. Shear modulus is calculated from wave propagation velocities. The ability of the SPA to simultaneously determine thickness and deformation modulus, without recourse to coring and drilling, makes it an attractive tool for evaluating urban roads. Coring and drilling may therefore be reserved for calibration and verification purposes only and do not have to be performed on a routine basis. This paper presents two case histories on the use of the SPA to evaluate urban roads. Layer thickness determined from the SPA is compared with core thickness. The SPA information is used to estimate remaining life on one road while it is compared with Benkelman beam rebound curves measured on the other. For the sake of completeness, an overview of the SPA and its basic concepts are also presented. Equipment Description The SPA equipment comprises a trailer towed by a van. The steel-framed single-axle trailer is approximately 3.2 m long and 1. m wide. Figure 1a shows a picture of the SPA at work on an urban road. An array of sensors, two impact hammers, a temperature sensing system, a hydraulic control system, an electrical unit and power source are mounted on the trailer. Table 1 shows the positions of the sensors relative to the impact hammers. 55

3 Table 1 Sensor Hammer Configuration Sensor Number Distance from Hammer to Sensor (mm) Hammer HF Hammer LF A A A A A G G G G4 Adjustable The trailer is particularly designed and built for rapidly collecting wave propagation data on roads and runways. The trailer is fitted with a special lighting system that redirects vehicular traffic away from the testing lane. The sensor array consists of five accelerometers and four geophones that are acoustically shielded from its support through enclosure in PVC holders and special vibration isolators. These are fitted with rubber feet for better energy transfer. Two instrumented hammer sources (high and low frequency) with adjustable but limited strokes are also housed on the trailer. Sensors and hammers are independently raised or lowered by a hydraulic system via mechanical springs. The SPA equipment comprises a compressor-charged air tank that supplies power for raising, lowering and firing the hammers. A pressure-sensing switch automatically controls the compressor. Pressure for raising, lowering and holding down the receivers as well as firing the hammers are individually controlled and monitored. Activation of the hammers and sensors may be enabled from hardware or software. Coupled with leads from the hammers and sensors, the hardware control is a lightweight box that permits the use of the trailer with third party data acquisition systems and interpretation software. The tow vehicle is equipped with an optional Global Positioning System (GPS) and a distance-measuring device. This arrangement permits the operator to reliably identify test stations on the road and eliminates the need to manually mark locations for testing. Recently, a video camera system has been installed to take still photographs of test locations. Such photographs are valuable tools in the interpretation of complex waveforms measured on urban roads. A twin temperature-sensing device is also mounted beside the hammer assembly for measuring air and pavement temperatures. The latter is used for adjusting measured modulus to the standard 2 C. 56

4 The field microcomputer is equipped with analogue-to-digital and signal conditioning boards. A software program is used to collect and interpret data in real time on the road. The software also saves all raw and processed data to disk for further review and reanalysis if necessary. Figure 1b shows the control computer displaying intermediate spectral analysis of surface waves (SASW) results in real time. For off-road and other locations that are inaccessible to the SPA, a portable onboard geophone and hammer system may be used for testing. This off-road system comprises a laptop equipped with an A/D card in an expansion chassis, geophones, an interface unit that links the laptop and geophones. Basic SPA Concepts Compared with other techniques of evaluating pavements, the analysis of SPA data is complex. The entire analysis is therefore done with a computer program. For the purposes of completeness, a brief overview of the methods used in this computer program is outlined. These are 1. Impact echo 2. Impulse response and 3. Spectral-analysis-of-surface-waves (or SASW). Impact Echo This test is used for determining the thickness of the paving interface or depth to defect in the paving layer. The defect could be a void, crack or a deteriorated zone. An impact echo measurement is made with a geophone placed close to the wave generation point. The trace recorded by the geophone is converted to displacement and Fourier transformed to determine the relevant resonant frequency. The thickness or depth to reflector is determined from the following relationship. v p z = 2 f [1] where z = depth to reflector f = resonant frequency = compression wave velocity v p The principle underlying this equation is that the stress wave undergoes multiple reflection between the surface and the interface. On each arrival at the surface, a characteristic displacement is produced, thus setting up a periodic waveform. The period, (T), of this wave equals the ratio of the length of the total travel path (2 z ) to the compression wave velocity ( v p ) or z T = 2 [2] v p 57

5 Substituting frequency for the reciprocal of the period ( f equation [1]. = 1 ) in equation [2] yields T To obtain the compression wave velocity, the arrival times of P-waves at two known sensor stations are used in the calculation process. Usually, these arrivals are readily picked off at stations further from the impact point. Distance from the source is required for the wave energy to fractionally separate into the component wave types. The compression wave velocity is calculated from the following equation v p = d t t 2 1 where v p = compression wave velocity t 2 t 1 d = arrival time at far receiver = arrival time at near receiver and = distance between sensors. The compression wave velocity computed from equation [3] may be used directly in equation [1] to calculate depth to the reflector or to calculate Young s modulus as shown in equation [4] E = ρv 2 p [4] where E = Young s modulus ρ = mass density v p = compression wave velocity Figure 2 shows typical power spectra and coherence plots used in data reduction. Sansalone and Carino (1988) described the impact echo test in detail. Impulse Response This test is performed to determine the shear modulus of the subgrade and the damping ratio of the pavement system. The magnitudes of these parameters are measures of subgrade competence and condition of support respectively. The test is similar to the impact echo test except that the force-time function f ()of t the impact and the geophone response, x(), t are recorded. The impulse response spectrum, or mobility spectrum in this case, is calculated as follows. where * X( f ) F ( f ) H( f ) = F( f ) F( f ) [5] H( f ) = mobility spectrum X( f ) = Fourier Transform of x( t) F( f ) = Fourier Transform of f (), t and F ( f ) = complex conjugate of F( f ). [3] 58

6 The flexibility spectrum, from which conditions of support and the existence of voids may be determined, is then computed from equation [5] using numerical integration. Figure 3 shows typical complex stiffness (the reciprocal of flexibility) and coherence plots. Due to background and other noise, the flexibility spectrum is curve-fitted before deriving the modal parameters. For analytical simplicity, the road structure is assumed to have a single-degree of freedom defined by a natural frequency (f), a gain factor and a damping ratio. Higgs (1979) presented details of the impulse response test. The shear modulus (G) of the subgrade may then be calculated equations given by Dobry & Gazetas (1986) and Richardson and Formenti (1982). Spectral Analysis of Surface Waves (SASW) This is the main method for determining resilient modulus and thickness profiles in layered systems such as pavements. The key principle is the dispersion of surface waves - which means that surface waves of different frequencies propagate at different depths. Thus by measuring the propagation velocity of waves of different frequencies, the variation of velocity (or stiffness) with depth is obtained. An impact with certain desired characteristics is used to generate waves that are monitored at two or more receiver stations. The computations involved in carrying out an SASW analysis may be outlined as follows. For simplicity, only two sensors are used but the procedure is readily applicable to multiple pairs of receivers at different spacing. 1. Transform the pairs of signals ( x t and y t ) into the frequency domain ( X f and Y f ). 2. Compute the mean of the cross-spectrum in the frequency domain and extract the phase (φ( f )) information as a function of frequency S ( f ) = xy X ( f )* Y ( f ) [6] where S xy f = cross-spectrum of x and y t t X( f ) = Fourier Transform of x t Y ( f ) = complex conjugate of the Fourier Transform of y t The function φ( f ) is bounded between +π and -π and may be unwrapped using the following equation φ u( f )= φ +2 kπ [7] where k is an integer. If the given pair of sensors is a distance d apart, then the phase velocity is given by d v = ω r φ [8] where v r = phase velocity, ω = angular frequency, 2πf. 59

7 A plot of phase velocity versus frequency is called a dispersion curve and contains stiffness and layering information. However, not all the dispersion points thus calculated are valid. A parameter called root mean square coherence is used to define dispersion points that may be invalid. 3. Calculate the root mean square coherence γ xy ( f ) xy = [9] xx S ( f ) S ( f ) S ( f ) where γ xy ( f ) = root mean square coherence, S xx f = Auto power spectrum of x and t S yy f = Auto-power spectrum of y. t yy The root mean square coherence is a measure of signal quality and values of.95 or higher may be used to filter out suspect dispersion points. Figures 4a, 4b, 5a and 5b show typical spectral functions used in the analysis of SASW data. An iterative numerical process called inversion is then used to determine the variation of shear wave velocity ( v s ) with depth. The references at the end of this paper may be consulted for further information on this numerical procedure. The shear modulus ( G ) and the dynamic Young s modulus ( E ) (which is equivalent to the resilient modulus) may then be determined from the following equations G = ρv 2 s [1] E = 2G( 1+ ν ) [11] where ρ = mass density, v s = shear wave velocity and ν = Poisson s ratio. The modulus of asphalt layers is strongly dependent on temperature. This temperature dependency has been well documented by many researchers (Witczak, 1972) and is incorporated into post-processing software. The above is a simplified explanation of the concepts SASW. The actual numerical routines deployed in the computer programs used in analyzing the data reported in this paper are more sophisticated. In addition, special add-on programs are used to read the large volume of numerical data into spreadsheet software for further processing and automated plotting. Test Sites The two pavement sites tested with the SPA are both located in the City of Surrey in British Columbia. Surrey is reported to be the fastest growing city in Canada and its population has increased substantially in the last five years. Consequently, traffic loads on its arterial, collector and local roads have exceeded anticipated growth and pavement 6

8 life is expected to be much shorter than initially designed for. One such road is 72 Avenue, a two-lane wide collector. The section on this road from 145 th Street to 152 nd Street was tested with the SPA in the spring of The length of the test section was approximately 13 meters. 144A Street is the second test section. This two-lane 4 meter local road links 144 Street and Highway 99A. It also provides a more direct access to City Hall from the highway. A number of distress characteristics such as surface depressions, longitudinal and alligator cracks were observed on the road. According to the city s pavement management system, these two roads were due for evaluation and the SPA was used in conjunction with the Benkelman beam. A sketch of the relative locations of the two test sections is shown in Figure 6. Field Procedure A trained technician typically conducts the field tests and is responsible for ensuring that the appropriate traffic control measures are in place. Once positioned at a test location, a GPS (global positioning system) reading may be taken. On activation from the keyboard, the array of seismic receivers and hammer source are lowered onto the pavement surface. Three sets of up to eight hammer impacts are generated. For each set, the output of the hammer load cell and designated sensors are recorded and saved during the last three impacts. The designated group of sensors is activated as required by a multiplexer. The first few hammer hits in each set are used for setting the gains of the amplifiers. When the impacts are completed, the sensors and hammers sources are automatically raised. While the sensor array is in contact with the pavement surface, the ground and air temperatures are recorded. A photograph of the test location may be then be taken. The operator then examines the recorded waveforms and either moves on to the next location, repeats the test or takes a core sample. A distance-measuring device is used for positioning at the next location. It takes less than a minute to test a given location. Results The layer and stiffness information determined from the SPA tests are presented in Figures 7, 8, 9 and 1. Layer thickness information available from coring and drilling has also been indicated on the figures. Figure 11 shows Benkelman Beam results on 144 A Street. Table 2 shows a summary of the thickness information presented in Figures

9 Table 2 Comparison of Asphalt Thickness from SPA and Coring Site Lane Station (1 m) Asphalt Thickness (mm) Difference (%) Core Condition SPA Coring 72 Ave. North Coarse (+35) overlays South Coarse 144A St. Northbound (+85) <.1 2 overlays Degraded Southbound Degraded Degraded Degraded During field testing, it was noted that the paving layer on 72 Avenue was in a much better condition than 144A Street. There were less pavement distress characteristics (cracks, surface depressions, etc.). The results in Table 2 indicate that asphalt core thickness was in better agreement with the SPA results on 72 Avenue. The presence of cracks and other distress features cause complexity in waveform shapes that ultimately affect data interpretation and the significance of the measured parameters. For example, on pavements with multiple overlays or delaminated paving layers, the SPA gives the thickness of the uppermost overlay or the depth to the shallowest delamination and not the total asphalt thickness. Station 1+6 on 144A Street and 9+8 on 72 Avenue are examples. On resurfaced older roads, the lower half of the asphalt layers is degraded. This deterioration sets up an acoustic contrast that causes changes in wave propagation. This is equivalent to material boundary resulting in an underestimation of the asphalt thickness. The underestimation of asphalt thickness on 144A Street was partly due to poor asphalt quality in the older overlays. The shear modulus determined from SASW tests have been compared in the past with modulus determined from other methods (such as the down-hole, cross-hole seismic cone penetration tests) and found to be reliable (Addo and Robertson, 1992). Since most pavement engineers are familiar with the Benkelman beam, results of this test on 144 A Street is presented in Figure 11. The decline in base and sub-base modulus towards Station 4+ in the subsurface layers supports the high rebound values similarly recorded by the beam. Ongoing studies appear to indicate that the Benkelman beam deflections 62

10 may be predicted by softening the shear modulus from SPA tests to about 2% of its initial value. The shear modulus of the subgrade on the entire test section (determined from the impulse response test) varies only slightly but the rebound values are very different from one end to the other. While there was other evidence to support high rebound values, the low rebound values may partly be attributable to thicker asphalt layers. Remaining Life The key question in most pavement evaluation work is How bad is it? If the answer can be given in terms of a numerical rating, then the task of prioritizing roads for rehabilitation becomes easier. As a preliminary procedure, the design traffic was used in conjunction with the average thickness and modulus of the worst 2% of the test locations. Based on previous experience, the 2 th percentile value (i.e. 8% of the data is better) was picked to be more representative of the weaker locations and at the same time avoids outliers. Using damage models, elastic layer theory and traffic data provided by the City of Surrey, the 2 th percentile values of shear modulus and thickness were used in a computer program to estimate remaining life based on fatigue and rutting. This computer program, similar to existing programs such as the Chevron N-Layer, BISAR or DAMA from the Asphalt Institute, uses a mechanistic-empirical approach to predict remaining life by computing the strains at layer interfaces. The use of shear modulus instead of resilient or Young s modulus in these computations is equivalent to a softening factor of about 2-5. This softening factor accounts for the low strain level at which seismic wave propagation tests are conducted (1-4 %) and the loading frequency. On 72 Avenue, the remaining life was calculated to be 3.3 years. This value agreed very well with the prediction of the Pavement Management System. It must be emphasized that this is only preliminary. Further verification is required before any generalizations can be made. Conclusion The case histories presented indicate that asphalt thickness can be estimated from SPA tests to an accuracy of less than 6 % if the asphalt is in good condition. Further research is required to improve the estimation process and to reliably determine the thickness of the remaining pavement layers. Based on previous correlation, the shear modulus of the pavement layers determined from the SPA was adjusted for strain level and used to determine an acceptable remaining life that was in close agreement with an independent pavement management system. Further work is required to statistically account for the wide variations in pavement properties, a factor that seems to have significant effect on the estimated remaining life. 63

11 In the 1993 AASHTO Guide for Design of Pavement Structures, the inability to determine layer thickness and material type from NDT was one reason cited in support of destructive testing (page III-49). While destructive testing is good practice, the use of NDT to simultaneously determine thickness and modulus will ultimately reduce the frequency of coring and drilling. Acknowledgements The research and development of the SPA equipment was undertaken by Metro Testing Laboratories and was financially sponsored in part by the Industrial Research Assistantship Program of the National Research Council of Canada. I thank my colleagues Drs. Soheil Nazarian, Mark Baker and Kevin Crain for their assistance. The field support of Curtis Syrnyk and Paul Hii are greatly appreciated. Finally, I thank Brian Snow of Web Engineering, John Paley of R.F. Binnie & Associates and the City of Surrey for sponsoring the road evaluation projects. References Addo, K. O. and Robertson, P. K. (1992), Shear Wave velocity Measurement of Soils using Raleigh waves, Canadian Geotechnical Journal, Vol. 29, No. 4, pp Dobry, R. and Gazetas, G. (1986), Dynamic Response of arbitrary shaped foundations. ASCE Journal of Geotechnical Engineering, Vol. 112, No. 2, pp Higgs, J., (1979), Integrity testing of piles by the shock method. Concrete October, 1979, pp. 31. Metro Testing Laboratories (1994), Development of an automated SASW Test Equipment. Research Report Submitted to the Industrial Research Assistantship Program, National Research Council, 35 pp. Nazarian, S., Baker, M. and Crain, K. (1993), Development and Testing of a Seismic Pavement Analyzer. National Research Council, Report No. SHRP-H-375, 165 pp. Richardson, M.H. and Formenti, D.L. (1982), Parameter Estimation from Frequency Response Measurements Using Rational fraction polynomials. Proceedings of the 1 st International Modal Analysis Conference (Society for Experimental Mechanics, Orlando, FL, pp Sansalone, M. and Carino, N. J. (1986), Impact Echo A method for flaw detection in concrete using transient stress waves. Report NBSIR , National Bureau of Standards, Gaithersburg, MD. Witczak, M. W. (1972), Design of full-depth asphalt airfield pavements, Proceedings of the International Conference on the Structural Design of Asphalt Pavement, London, England, Vol. 3, pp

12 a. Hardware b. Software Figure 1. The SPA equipment 65

13 1.2.8 Logarithm of Power Spectrum Impact Coherence Frequency (khz) a. Coherence and Power Spectrum of Impact Logarithm of Power Spectrum Sensor Frequency (khz) b. Power Spectrum of Sensor Signal Figure 2. Typical power spectra from Impact Echo test. 66

14 15 Measured Curve Fit Coherence Stiffness (MPa) Frequency (khz) a. Measured and fitted real stiffness 15 Stiffness (MPa) 1 5 Measured Curve Fit Frequency (khz) b. Measured and fitted imaginary stiffness Figure 3. Typical stiffness and coherence plots from Impulse Response test 67

15 1 Root Mean Square Coherence Frequency (Hz) a. Coherence Function Phase (radians) Frequency (Hz) b. Phase of the Cross-Spectrum Function Figure 4. Typical spectral functions used in SASW 68

16 -5 Unwrapped Phase (radians) Frequency (Hz) a. Unwrapped Phase.5 Wavelength (m) Raleigh Wave Phase Velocity (m/s) b. Raleigh Wave Dispersion Curve Figure 5. Unwrapped phase and dispersion curves 69

17 N 72 Avenue 152 St. 144 St. City Hall Hwy A St. Hwy 99 A Figure 6. Location of test sites (not to scale) 7

18 5 Asphalt Base 1 Depth (mm) Asphalt Core Station (1 m) a. Layer Thickness 1 4 Shear Modulus (MPa) Asphalt Base Subbase Station (1 m) b. Stiffness Profile Figure 7. SPA test results in the North Lane of 72 Avenue 71

19 5 Asphalt Base 1 Depth (mm) Asphalt Core Station (1 m) a. Layer Thickness 1 4 Shear Modulus (MPa) Asphalt Base Subbase Station (1 m) b. Stiffness Profile Figure 8. SPA test results in the South Lane on 72 Avenue 72

20 .1 Depth (m) Asphalt Asphalt Base Base/Subbase Station (1 m) a. Thickness from SASW and Coring / Drilling Shear Modulus (Pa) Asphalt Base Subbase Subgrade Station (1 m) b. Stiffness Profile Figure 9. Thickness and stiffness profiles on 144 A Street (Northbound). 73

21 .1 Depth (m) Cores Asphalt Base Subbase Asphalt Base Station (1 m) a. Thickness Profile Shear Modulus (Pa) Asphalt Base Subbase Subgrade Station (1 m) b. Stiffness Profile Figure 1. Layer thickness from SPA and Coring/Drilling on 144 A Street (Southbound) 74

22 2.5 2 Uncorrected Rebound (mm) Northbound Southbound Station (1 m) a. Rebound Curve. Most Probable Spring Rebound (mm) Pavement Temperature = 25 deg. C Seasonal Correction Factor = 1.2 Design Rebound = 1.4 Northbound Lane Southbound Lane Station (1 m) b. Most Probable Spring Rebound (MPSR) Figure 11. Benkelman Beam test results on 144A Street 75