APPLICATION OF GROUND PENETRATING RADAR FOR COLD IN-PLACE RECYCLED ROAD SYSTEMS

Transcription

1 APPLICATION OF GROUND PENETRATING RADAR FOR COLD IN-PLACE RECYCLED ROAD SYSTEMS Prepared By: Curtis Berthelot Ph.D., P.Eng. Tom Scullion P.E. Ron Gerbrandt P.Eng. Larry Safronetz Published in: Journal of Transportation Engineering American Society for Civil Engineers Jul/Aug 2001 Vol. 127 No 4

2 APPLICATION OF GROUND PENETRATING RADAR FOR COLD IN-PLACE RECYCLED ROAD SYSTEMS By: Curtis Berthelot Ph.D., P.Eng. 1 Tom Scullion P.E. 2 Ron Gerbrandt P.Eng. 3 Larry Safronetz 4 ABSTRACT: Grain transportation rationalization, economic diversification and value added initiatives within the Saskatchewan economy has, and will continue to, increase commercial truck traffic on many Saskatchewan roads. As a result, Saskatchewan Department of Highways and Transportation are investigating cold in-place recycling as a rehabilitation alternative for strengthening thin paved roads. However, different construction practices and years of maintenance and rehabilitation have led to many of these thin paved roads having variable structural composition. The effect of in situ variability on cold inplace recycle designs is further exacerbated by the inherent sensitivity of stabilizers such as asphalt emulsion, foamed asphalt, cementitious blends, and/or concentrated chemicals when integrated into different road materials. As a result, materials and structural design of cold in-place recycled thin paved road systems can be highly uncertain. Ground penetrating radar has been identified as an engineering diagnostic tool that can accurately quantify in situ structural composition and help reduce the uncertainty associated with material and structural design of cold in-place recycled road systems. This paper summarizes the principles of ground penetrating radar, discusses the use of ground penetrating radar as an engineering diagnostic tool for cold in-place recycling of thin paved roads, and presents two pilot case studies undertaken by Saskatchewan Department of Highways and Transportation that demonstrates the capability of ground penetrating radar to mitigate the uncertainty associated with cold in-place recycled road systems. 1 Assistant Professor of Civil Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5A9. 2 Research Engineer, Texas Transportation Institute, College Station, Texas, Preservation Engineer, Saskatchewan Highways and Transportation, Saskatoon, Saskatchewan, S7L 6M6. 4 Area Manager, Saskatchewan Highways and Transportation, Meadow Lake, Saskatchewan, S9X 1V8. Key words: ground penetrating radar, cold in-place recycling, road strengthening.

3 INTRODUCTION Grain transportation rationalization, economic diversification and value added initiatives within the Saskatchewan economy has, and will continue to, increase commercial truck traffic on many Saskatchewan roads. These increases in commercial truck traffic hold significant and often-immediate implications for Saskatchewan secondary roads because many were not designed to accommodate large numbers of heavily loaded commercial trucks. As a result, increasing commercial truck traffic has translated into the need to strengthen many Saskatchewan secondary roads. Of particular concern is the approximately 8600 kms of Saskatchewan non-structural thin membrane surfaced (TMS) and asphalt mat on subgrade (AMOS) roads of which some are experiencing significantly accelerated damage due to the increased truck traffic. TMS and AMOS roads are of particular concern because they are expensive to repair once surface breaks occur and these types of roads are particularly sensitive to heavy truck loading when subgrades are thaw-weakened during the spring period. Although strengthening the Saskatchewan TMS road network using conventional methods is tenable in some situations, conventional methods are not always the most technically or economically feasible solution in the time frame required to provide a sustainable level of service. This is especially true in areas with depleted aggregate sources. Therefore, in-place recycling is being investigated to provide a more pragmatic method for strengthening TMS roads using a phased build down" approach using in-place recycling equipment as shown in Figure 1.

4 Figure 1. Cold In-Place Full Depth Recycling Although cold in-place recycling holds significant promise for cost effectively improving the performance of thin paved roads while at the same time optimizing the reuse of road materials already inplace and paid for by the taxpayers, different construction practices at the time of initial construction and years of maintenance and rehabilitation has resulted in highly variable in situ composition of many TMS roads. This variability often means that the evaluation of in situ materials for in-place recycling is highly uncertain in nature, and as a result, the uncertainty associated with the engineering the recycling rehabilitation of these types of road systems is increased compared to conventional road system designs. Ground penetrating radar (GPR) has been identified as an effective engineering diagnostic tool that can help reduce the uncertainty associated with the design and analysis of in-place recycled road systems (Maser and Scullion, 1992; Saarenketo and Scullion, 1994; Wimsatt, Scullion, Ragsdale, and Servos, 1998). PRINCIPLES OF PULSE GROUND PENETRATING RADAR Earthen materials are semi-transparent at electromagnetic wave frequencies from 10 to 5000 MHz. GPR employs electromagnetic waves to estimate the electrical properties of road materials including: dielectric permittivity, conductivity, and susceptibility (magnetic materials). Based on these material property measurements, GPR can be used as a non-destructive and non-intrusive road diagnostic tool that is analogous to the X-ray in the medical profession, and can be used to estimate layer thickness, voids, etc. The

5 principles of pulse GPR for use in road engineering applications are well documented (Scullion, T., Lau, C., and Chen Y., 1994; Maser, K., 1994). Pulse GPR operates on the principal of sending discrete pulses of electromagnetic waves (radar energy) into the road structure and capturing reflected radar waved from layer interfaces within the road structure if there is an electrical contrast between two adjacent layers. Conversely, if two adjacent layers have the same electrical properties and they are well bonded at their interface, there will be little radar waves reflected from that interface. As an example, it is often difficult to identify individual lifts within thick asphalt concrete pavements when it is constructed of multiple lifts with similar material properties. Radar Antenna A 0 A 1 A 2 Volts GPR Antenna Reflection A 0 HMA Surface Reflection A 1 Top Base Reflection A 2 Top Subgrade Reflection A 3 A 3 Asphalt Concrete Time Granular Base Subgrade t 1 t 2 t 1 = Travel Time in Asphalt t 2 = Travel Time in Base Layer Figure 2. Typical Air-Coupled GPR Reflection Profile Figure 2 illustrates a typical single reflected GPR waveform in terms of volts versus time expressed in nanoseconds. As seen in Figure 2, a typical granular base-asphalt concrete pavement structure generates a GPR reflection from the surface of the asphalt concrete A1, and reflections from the top of the granular base and subgrade are represented by peaks A2 and A3, respectively. The reflection amplitude of the reflected peaks can be used to estimate the dielectric permittivity, which is a measure of the extent to which the electric charge distribution in a material can be distorted or polarized by application of an electric field. The equations used to calculate dielectric permittivity of the asphaltic concrete and granular base layers may be summarized as:

6 .(1) ε a Am + A1 =.. Am A1 where: ε a = dielectric permittivity of surface, A1 = amplitude of reflection from surface in volts, and A m = amplitude of reflection from a metal plate, measured in volts..(2) ε b = 2 A1 A2 1 Am Am ε. a 2 A1 A2 1 Am Am where: ε b = dielectric permittivity of base layer, and A2 = amplitude of reflection from the top of the base layer, measured in volts. Because moisture content significantly influences dielectric permittivity of a material such as soils, dielectric permittivity can be used to estimate moisture content, density, and the presence of voids within earthen materials. In addition, the time delays between the reflected amplitudes make it possible to estimate layer thickness of each individual layer as well, which may be expressed mathematically as: h1 c t1 = ε a. (3) where: h 1 = thickness of surface, c = GPR speed of travel in free space as measured by the GPR, and t 1 = time delay between surface dielectric permittivity peak and base peak. Table 1. Dielectric Permittivity Values of Common Road Material Constituents Material Dielectric Permittivity Air 1 Water Dry Soil Solid Aggregate 10 to 81 depending on degree of saturation and mineralogy 5 to 15 depending on mineralogy and density 4 to 8 depending on rock mineralogy Asphalt Cement 2 to 5

7 The volumetric proportions of the individual constituents influence the dielectric permittivity measurement obtained from typical road materials. The dielectric permittivity of common road material constituents are summarized in Table 1. It is known that the proportions of individual constituents influence the moisture content and/or density properties of the composite road material, which is directly related to field performance. To illustrate the engineering significance of dielectric permittivity measurements, examples of how changes in road material and structural properties influence reflected GPR signals include: If the thickness of the surface layer increases, the time interval between peaks A 1 and A 2 illustrated in Figure 2 increases. Large changes in the surface reflection amplitude and shape of peak A 1 illustrated in Figure 2 would indicate changes in the density and/or moisture content of the surface hot mix asphalt concrete. If there is a defect within the surface layer, a reflection will be observed between peaks A 1 and A 2 illustrated in Figure 2. A positive reflection will be seen from trapped moisture in the defect whereas a negative reflection will be seen from decreased density such as that resulting from increased air voids found with severe stripping. Typical ranges of hot mix asphalt concrete dielectric permittivity are between 4 and 7, depending on the coarse aggregate type, mix type, and asphalt content. Measured dielectric permittivity significantly higher than this indicate the presence of excessive moisture or density within the mix. Measured dielectric permittivity significantly lower indicate low-density problems resulting from high air voids or the presence of unusual lightweight coarse aggregate such as shale or dried clay balls. If the moisture of the granular base and/or subgrade layer increases, the amplitude of reflection A 2 and/or A 3 illustrated in Figure 2 respectively would increase. Based on the discussion presented above, GPR information is a potentially valuable pre-engineering, tool for designing cold in-place recycled road systems. Of particular value are air-coupled GPR systems because they are not in contact with the road surface and therefore can be operated at highway speeds, thus requiring no traffic control while surveying roads. This study employed an air-coupled one GHz central frequency pulse GPR system as shown in Figure 3, to provide a continuous spatially referenced reflection profile along the outside wheel path of two roads proposed to be cold in-place recycled by SDHT on Highway and Highway

8 Figure 3. Air-Coupled Ground Penetrating Radar System SDHT HWY COLD IN-PLACE RECYCLED THIN MEMBRANE SURFACE CASE STUDY During the summer of 1999, SDHT and SaskPower International piloted a cold in-place recycling and lime flyash stabilization project to strengthen portions of Highway The as-built design of Highway was a 40 mm cold mix asphalt concrete thin membrane surface placed directly onto a prepared subgrade as illustrated in Figure 4.

9 Typical TMS System C L 50 mm Cold Mix Asphaltic Surface Prepared Subgrade Typical TMS System C L 150 mm Cold Mix Asphaltic Surface Prepared Subgrade C L mm Asphalt Concrete 150 mm Base mm Subbase Prepared Subgrade Typical Full Pavement System Figure 4. Typical TMS, AMOS and Full Pavement Structure Cross Sections Figure 5 illustrates the raw ground penetrating radar profiles retrieved from the survey of Highway and Figure 6 illustrates the surface layer thickness calculations obtained form the raw GPR information. As seen in Figure 5 and Figure 6, the ground penetrating radar evaluation of Highway identified the asphaltic wearing surface layer thickness ranged from 25mm to 200mm.

10 Wearing Surface Subgrade Figure 5. Highway GPR Reflection Profile North Bound Figure 6. Highway Cold Mix Wearing Course Thickness Profile

11 Because the proposed lime flyash stabilization has minimal effect on cold mix asphalt concrete, cold mix asphalt concrete thicker than 25 mm was rotomixed and windrowed off the side of the road as shown in Figure 7. Subgrade stabilization was performed with lime flyash to a depth of 150 mm as shown in Figure 8, and the cold mix asphalt concrete was bladed back onto the road surface and compacted as a black base wearing course as shown in Figure 9. Unconfined compressive strength and stiffness characterization was performed according to AASHTO T208 on rotomixed subgrade with and without lime flyash modification compacted at optimum moisture content and density after 14-day and 28-day moist cure periods. As shown in Figure 10, the addition of lime flyash significantly increased the unconfined compressive strength and stiffness of the Highway subgrade after a 14-day and 28-day moist cure period. The lime flyash stabilized section of is performing well after two years of service in the field. Figure 7. Highway Windrowed Cold Mix Asphalt Concrete

12 Figure 8. Highway 155 Rotomixing Lime Flyash Stabilizer into Subgrade Figure 9. Highway Replaced Cold Mix Asphalt Concrete Wearing Course

13 Axial Stress (kpa) Axial Strain (mm/mm) 0%LFA@7%W/C-28 Day Moist Cure 5%LFA@7%W/C-14 Day Moist Cure 5%LFA@7%W/C-28 Day Moist Cure Figure 10. Highway Rotomixed in situ Subgrade Unconfined Compressive Strength SDHT HWY COLD IN-PLACE RECYCLED ASPHALT MAT ON SUBGRADE CASE STUDY During the summer of 1999, SDHT piloted a cold in-place recycling project to strengthen portions of Highway 15-10, an existing AMOS structure. The as-built information available for Highway indicated a design hot mix asphalt concrete mat of 150mm placed directly on prepared clay till subgrade. Based on this information, the in-place recycling strategy was to add 50mm of virgin graded base aggregate to the surface of the existing road structure and rotomix the top 150mm of the composite aggregate-asphalt concrete mat and inject asphalt emulsion to provide a cohesive asphalt-aggregate wearing course, while at the same time reducing reflective cracking. A GPR survey was performed to quantify asphalt concrete mat thickness. As can be seen in Figure 11, the asphalt concrete mat thickness determined from the ground penetrating radar varied from slightly over 200mm to slightly less than 100mm. In addition, three areas where identified where a 25mm thick surfacing was placed over approximately 300 mm of select material which were later confirmed to be deep granular patches with a double seal surfacing constructed in locations to repair severe subgrade failures (Baker, Berthelot and Gerbrandt, 2000).

14 Granular Patches Figure 11. GPR Asphalt Mat Layer Thickness Profile of Highway Post construction evaluation of Highway revealed variable performance along the entire section. Localized performance problems included soft spots as shown in Figure 12 which developed quickly after placement. Forensic analysis of the rotomixed material retrieved from the soft spots determined that a high proportion of clay fines and clay balls from the subgrade had been incorporated into the recycled mix where the asphalt mat thickness was less than anticipated as shown in Figure 13. The incorporated clay fraction into the mix prematurely broke the asphalt emulsion and resulted in the mix to be dry and tender.

15 Soft Spots Figure 12. Localized Soft Spots of Highway Figure 13. Rotomixed Clay Balls from Subgrade of Highway 15-10

16 Conversely, granular patches identified in the GPR survey were found to produce rotomixed material that exhibited a stable mix as shown in Figure 14. In areas where the asphalt concrete mat thickness was greater than the specified 150mm, a significant portion of the old asphalt concrete mat was left intact. Although the stability of the recycled mix in these areas was found to be excellent, because the rotomixer did not penetrate full depth into the mat there is potential for reoccurrence of reflective cracking. Therefore, based on the GPR information, the rotomixer depth should have been set to match the variable asphaltic mat layer thickness along the entire section. Granular Patch Figure 14. Granular Patch of Highway SUMMARY AND CONCLUSIONS Saskatchewan Department of Highways and Transportation are investigating cold in-place recycling and full depth stabilization as a potentially cost effective rehabilitation alternative for strengthening thin paved roads. Pilot studies undertaken in 1999 on Highways and revealed considerable in situ structural variability due to different construction practices at the time of initial construction and years of maintenance and rehabilitation treatments. As a result, the design and quality control/quality assurance of inplace recycled road systems can be highly variable in terms of materials and structural composition. The effect of in situ variability is further exacerbated if stabilization additives such as asphalt emulsion, foamed

17 asphalt, cements, flyash, and/or chemical stabilizers are used for adding strength to the in situ materials given the sensitive nature of many additives when exposed to different materials. Based on the cold in-place recycling performed during the Highway and pilot case studies, GPR has been found to be an effective pre-engineering diagnostic tool that can be used to help reduce the uncertainty associated with in-place recycling existing road structures. Future work should be undertaken to evaluate the use of GPR as an ongoing performance-monitoring tool to quantify changes in layer thickness, moisture content, and/or density of recycled road systems over time. APPENDIX I. REFERENCES Baker, D., Berthelot, C., and Gerbrandt, R. (2000). Full-Depth Cold In-Place Recycling/Stabilization for Low Volume Road Strengthening: A Case Study on Highway Transportation Association of Canada. Edmonton. CDROM Proceedings Paper 9-1. Maser, K., and Scullion, T. (1992). "Automated Pavement Subsurface Profiling Using Radar Case Studies of Four Experimental Field Sites. Transportation Research Record 1344, Transportation Research Board, Washington, D.C. Maser, K., (1994). "Ground Penetrating Radar Surveys to Characterize Pavement Layer Thickness Variations at GPS Sites. Strategic Highway Research Program Research Report SHRP-P-397. National Research Council, Washington D.C. Saarenketo, T., and Scullion, T., (1994). Ground Penetrating Radar Applications on Roads and Highways. Texas Transportation Institute Research Report F. Texas A&M University, College Station Texas, USA. Scullion, T., Lau, L., and Chen, Y., (1994). "Implementation of the Texas Ground Penetrating Radar System. Texas Transportation Institute Research Report Texas A&M University, College Station Texas, USA. Wimsatt, A., Scullion, T., Ragsdale, J., and Servos, S., (1998). "The Use of Ground Penetrating Radar Data in Pavement Rehabilitation Strategy Selection and Pavement Condition Assessment. Paper No , Transportation Research Board 77 th Annual Meeting, Washington, D.C.