1 SELECTING REHABILITATION STRATEGIES FOR FLEXIBLE PAVEMENTS IN TEXAS Andrew J. Wimsatt, Ph.D., P.E. Fort Worth District Pavement Engineer Texas Department of Transportation P.O. Box 6868 Fort Worth, TX Phone Fax Tom Scullion, P.E Texas Transportation Institute College Station, Texas 6,904 words (the word count includes one table and five figures at 250 words per table or figure) A Paper Prepared for Presentation and Publication at the Annual Meeting of the Transportation Research Board January 2003
2 Wimsatt and Scullion 2 ABSTRACT In September, 1996, the Texas Department of Transportation (TxDOT) funded a research project conducted at the Texas Transportation Institute (TTI) to develop a strategy selection procedure for rehabilitation of flexible pavements. The study concluded in August, Everyday, TxDOT District personnel (especially TxDOT District Pavement Engineers) are asked to make critical decisions on how best to rehabilitate a particular section of highway. The research project did generate an effective strategy selection procedure. This paper first discusses TTI Research Report , which summarizes the strategy selection procedure generated by the research project . The paper then describes the results of three case studies on roadways in TxDOT s Fort Worth District that need rehabilitation. The first two case studies include analysis of data from the Ground Penetrating Radar, Falling Weight Deflectometer, and Dynamic Cone Penetrometer. The resulting rehabilitation solutions are then generated with assistance from TxDOT s FPS19 flexible pavement design computer program. The last case study outlines a procedure for determining the optimum stabilizer content for an inplace cement stabilization project of an existing pavement. Keywords: Pavement Rehabilitation, Flexible Pavement, Falling Weight Deflectometer, Ground Penetrating Radar, Dynamic Cone Penetrometer.
3 Wimsatt and Scullion 3 SELECTING REHABILITATION STRATEGIES FOR FLEXIBLE PAVEMENTS IN TEXAS INTRODUCTION By Andrew J. Wimsatt, Ph.D., P.E., and Tom Scullion, P.E. The Texas Department of Transportation spends hundreds of millions of dollars each year rehabilitating the state s large highway network. In the next decade, it is anticipated that these sums of money will increase as the Department gradually transitions from the era of building new roads and adding capacity to the era of maintaining a network which is essentially in place. TxDOT District personnel, in general, have limited experience in the area of structural evaluation of existing facilities. In addition, up until recently, there were few guidelines available to assist TxDOT District personnel in defining feasible options when major rehabilitation work was required. As a result, in September, 1996, the Texas Department of Transportation (TxDOT) funded a research project conducted at the Texas Transportation Institute (TTI) to develop a strategy selection procedure for rehabilitation of flexible pavements. The study concluded in August, Everyday, TxDOT District personnel (especially TxDOT District Pavement Engineers) are asked to make critical decisions on how best to rehabilitate a particular section of highway. The research project did generate an effective strategy selection procedure; Mr. Scullion has been presenting the findings from the study as part of a four day training course on flexible pavement rehabilitation that is offered to TxDOT Personnel. The course also includes case studies of roadways that are candidates for rehabilitation. The course lecture and contents have also been incorporated into a personal computer CD-ROM based training course that is available through TxDOT s Research and Technology Implementation Office. This paper first discusses TTI Research Report , which summarizes the strategy selection procedure generated by the research project . The paper then describes the results of three case studies on roadways in TxDOT s Fort Worth District that need rehabilitation. The first two case studies include analysis of data from the Ground Penetrating Radar, Falling Weight Deflectometer, and Dynamic Cone Penetrometer. The resulting rehabilitation solutions are then generated with assistance from TxDOT s FPS19 flexible pavement design computer program. The last case study outlines a procedure for determining the optimum stabilizer content for an inplace cement stabilization project of an existing pavement.
4 Wimsatt and Scullion 4 RESEARCH PROJECT RESULTS TTI Research Report , titled Selecting Rehabilitation Options for Flexible Pavements: Guidelines for Field Investigations, summarizes the strategy selection procedure generated by the research project . Obviously, the first step in selecting the optimum rehabilitation strategy for a flexible pavement is to identify the cause of the existing pavement distress. The rehabilitation selection process is often straightforward once the cause has been identified. The report presents an updated summary of the techniques and interpretation guidelines that have been developed by the Texas Transportation Institute over the past two decades in order to conduct an effective investigation. The report first presents a summary of the different distress types frequently found on Texas flexible pavements rutting, block cracking, alligator cracking, failures, longitudinal cracking, transverse cracking, raveling, roughness, and flushing (or bleeding). The possible causes for each are described together with guidelines on how to conduct a failure investigation. For each distress type, a series of rehabilitation options are also presented. The report then presents guidelines on how TxDOT personnel can incorporate Ground Penetrating Radar (GPR), Falling Weight Deflectometer (FWD), and Dynamic Cone Penetrometer (DCP) information into the project evaluation process. In particular, the report indicates how the COLORMAP and Modulus Version 5.1 computer programs developed by TTI can be used to analyze GPR and FWD data, respectively. For example, GPR data is processed using the COLORMAP program to determine pavement layer thicknesses and the presence of excessive moisture or excessive air voids in pavement layers. GPR measures dielectric values for pavement layer materials . Figure 1 describes a typical GPR waveform. Normal dielectric values for various types of pavement layers are shown in Table 1. Dielectric values greater than those listed in Table 1 indicate the presence of moisture in the pavement layers. Dielectric values less than those listed in Table 1 indicate materials that contain excessive air voids (i.e., the layers are below optimum density). If the dielectric values are greater than 16, the pavement layers are most likely saturated with water. As a result, the GPR data can indicate where further investigation is warranted, especially in terms of coring or trenching operations. Use of GPR in the Fort Worth District was previously reported in a paper presented at the Transportation Research Board Annual Meeting in 1998 and published in the Proceedings of the Society of Photo-Optical Instrumentation Engineers . Also, FWD data is processed using the Modulus program to generate remaining life estimates and pavement and subgrade layer moduli values . As a result, the FWD data can indicate where the pavement structure may be excessively weak. The moduli values are used in the TxDOT Flexible Pavement System computer program (FPS19) to generate rehabilitation strategies . FPS19 uses a pavement performance equation that consists of the initial present serviceability index (PSI), the terminal PSI, the surface curvature index value generated by a 9,000 pound dual wheel load applied to a pavement structure, the number of 18 kip equivalent single axle loads that are forecast to be applied to the pavement structure, and a temperature constant assigned to each TxDOT District s geographical area . The surface curvature index is calculated in FPS19 using the average modulus values determined from the Modulus program
5 Wimsatt and Scullion 5 . The reliability level used for the case studies in this paper is set at 95 percent. The modulus assigned to a new asphalt concrete pavement (ACP) overlay layer is 500 ksi, which is the default value in the FPS19 program. Initial and terminal serviceability index values were set at 4.2 and 2.5, respectively. Finally, DCP data may be collected in order to verify the results of FWD data analysis, such as measuring base, subbase, and subgrade stiffnesses or determining the depth to a stiff layer. DCP information may also be collected in cases where the FWD cannot collect data, such as estimating subgrade moduli on a route where no roadway currently exists. Two equations described in TTI report are used to determine modulus values from the DCP : CBR = (292/PR)^1.12 E= 6.89*2500*(CBR)^0.64 Where, CBR = California Bearing Ratio, and PR = Penetration Rate, mm/blow E = Modulus, kpa In Texas, flexible pavement rehabilitation usually involve one of the following general strategies: (1) Base repair and asphalt concrete pavement (ACP) overlays, (2) removing defective ACP layers and replacing with new ACP layers, or (3) full depth pavement reclamation, where the existing pavement layers are first milled and then stabilized with lime, cement, fly ash, or a combination of such materials; and then compacted as a new base or subbase layer. ACP overlays are generally used over such stabilized layers. The TTI Research Report also includes recommendations to provide TxDOT personnel with new laboratory procedures for selecting the optimum stabilizer type and stabilizer content for reclamation projects.
6 Wimsatt and Scullion 6 CASE STUDIES The following case studies illustrate the use of the above methods in rehabilitation strategy selection. SH 337, Palo Pinto County This roadway section, which is approximately 17.7 km (11 miles) long, was exhibiting substantial surface rutting up to 51 mm (two inches) deep when it was evaluated in May, The existing pavement structure consists of (on average) 203 mm (eight inches) of granular base and two seal coats placed in 1970, followed a seal coat placed in 1980, an open graded friction course placed in 1989, and another seal coat placed in The seal coats and open graded friction course together were approximately 51 mm (two inches) thick. However, before the last seal coat was placed, TxDOT maintenance personnel had applied many seal coat surface patches and ACP patches on this roadway due to the disintegration of the open graded friction course. GPR, FWD, DCP and coring operations were conducted on this roadway. GPR data was obtained at a 3.05 meter (ten foot) spacing along the roadway in the outside wheelpaths in both directions. FWD data was obtained at a 0.16 km (0.1 mile) spacing in each direction in the outside wheelpaths, resulting in 217 deflection basins. However, 18 of the deflection basins had to be omitted from analysis due to non-decreasing deflections. Also, two additional deflection basins were omitted because the deflections from the sensor located 1,829 mm (72 inches) from the load plate approached zero, which indicates that bedrock was immediately beneath the pavement structure; as a result, the Modulus program could not generate effective solutions for those basins. So, 197 deflection basins were used in the analysis. DCP data was obtained at four locations in the outside wheelpaths where cores were obtained and where rutting was present. Finally, cores were obtained approximately every 1.61 km (1 mile) along the roadway in the outside wheelpaths in both directions (a total of 24 cores). However, coring in the southbound direction were obtained 0.81 km (0.5 miles) from the cores obtained in the northbound direction in order to stagger the locations of the cores. The Modulus Version 5.1 computer program was used to backcalculate the pavement and subgrade layer moduli from the FWD data to use in the FPS19 design program. Since the surface layer was only 51 mm (2 inches) thick, the surface layer modulus was fixed at 2,067 kpa (300 psi), which was based on the overall surface visual condition. The Modulus program cannot reliably backcalculate reasonable moduli values for surface layers less than 76 mm (three inches) thick. This is a limitation with any FWD data backcalculation procedure, since such thin layers generate considerably smaller deflections than thicker layers, even if the moduli values for such thin layers varied widely. Average values obtained from the Modulus program were 296 kpa (43.0 ksi) for the 203 mm (eight inch) thick base, 134 kpa (19.5 ksi) for the upper 305 mm (twelve inches) of subgrade material, and 227 kpa (32.9 ksi) for the lower 7,620 mm (300 inches) of subgrade material. The corresponding coefficients of variation were 72 percent for the granular base layer, 99 percent for the upper 305 mm (twelve inches) of subgrade material, and 49 percent for the lower 7,620 mm (300 inches) of subgrade material. Bedrock is assumed to be underneath the subgrade. The
7 Wimsatt and Scullion 7 average absolute error per sensor value was 7.1%, with a coefficient of variation of 97 percent, which are considered acceptable values considering the condition of the roadway. The average absolute error per sensor value is an indication of how closely the deflection bowls computed with the backcalculated modulus values matches the corresponding measured deflection bowls. The FWD analysis indicated that, overall, the base material was in good shape from a structural standpoint. However, TxDOT personnel still had some concerns that the base material may be weak and contributing to the rutting observed on the roadway. So, to verify the results from the FWD data analysis, TxDOT personnel conducted DCP testing at four locations in the outside wheelpaths where cores were obtained and where rutting was present. Two of the DCP tests were conducted in the northbound lane and were located approximately 9.66 km (6 miles) and km (8 miles) from the southern end of the roadway. The other two DCP tests were conducted in the southbound lane and were located approximately 2.42 km (1.5 miles) and km (6.5 miles) from the southern end of the roadway. The 203 mm (eight inch) thick base material was still in good shape from a structural standpoint according to the DCP, with overall base modulus values of 413 kpa (60 ksi), 482 kpa (70 ksi), 531 kpa (77 ksi) and 317 kpa (46 ksi) obtained at each location. However, the subgrade just immediately below the base was quite weak, ranging in overall values from 69 kpa (10 ksi) to 103 kpa (15 ksi). However, this subgrade layer was quite shallow, since stiffer subgrade layers lie beneath it according to the FWD data analysis. So, the modulus values obtained from the DCP data compared favorably to the average values obtained from the FWD data analysis. Figure 2 shows a graph from one of the DCP tests. Observation of the cores by TxDOT personnel indicated that the overlying seal coat AC binder did not completely penetrate the open graded friction course, so voids were still present in this layer that could trap water. GPR data also indicated that this layer was holding water. This was causing the open graded friction course to become unstable due to freeze/thaw cycles during the winter and due to the asphalt stripping from the aggregate in this layer. So, as a result, the upper 51 mm (two inches) needed to be removed. Figure 3 shows a graph of the dielectric value measured along the roadway by the GPR at the interface between the open graded friction course and the underlying seal coat. The twenty-year traffic forecast from TxDOT s Transportation Planning and Programming Division indicated that the average daily traffic (ADT) in 2002 is 2,000 and is forecast to be 2,700 in The truck traffic on this roadway section is 16.4% of the ADT. The forecasted accumulated twenty year 18 kip equivalent single axle load value is estimated to be 1,012,000. Using the above average modulus values from the FWD analysis and the traffic forecast data, and taking into account that the upper two inches were to be removed, the FPS19 flexible pavement design computer program indicated that a 76 mm (three inch) ACP overlay would last for approximately nine years before another overlay is needed. TxDOT s pavement design manual indicates that such an overlay should perform a minimum of eight years according to the pavement design procedure . So, the proposed strategy exceeded the minimum requirements. The Modulus program analysis of the FWD data also indicated approximately twenty areas where base repair would possibly be needed. Based on the resulting visual observations of those areas, TxDOT personnel estimated that 836 square meters (1,000 square yards) of base repair would
8 Wimsatt and Scullion 8 possibly be needed after the two inches of seal coats and open graded friction course was removed. 277,951 square meters (332,477 square yards) of surface removal was estimated for this roadway segment. So, to summarize, the rehabilitation strategy for this roadway was to remove the top 51 mm (two inches) of the seal coat and open graded friction course, repair failed areas of base material, and then place 76 mm (three inches) of ACP. This rehabilitation project was let to contract in April, 2002 and completed in September, The contractor had to repair only 542 square meters (648 square yards) of base material. As a side note, TxDOT s Fort Worth District regularly places seal coats on top of open graded friction courses. The District placed approximately 1,494 centerline km (928 centerline miles) of open graded friction courses from 13 mm (0.5 inches) to 38 mm (1.5 inches) thick on roadways during the 1980 s and early 1990 s. As of July, 2002, approximately 961 centerline km (597 centerline miles) have seal coats on top of existing open graded friction courses in the District, with the earliest seal coat placed in The SH 337 roadway section has been to date the only section observed in the District where such substantial rutting distress occurred where a seal coat had been placed on top of an existing open graded friction course. US 377 Southbound Lanes, Hood County This roadway section, which is approximately 16.9 km (10.5 miles) long, was exhibiting substantial alligator cracking and potholes in the southbound lane when the roadway was evaluated in TxDOT maintenance forces had also placed several seal coat patches, ACP patches, and ACP overlays. The southbound lanes were originally constructed in 1979 with 152 mm (six inches) of lime stabilized subgrade where clay subgrade was present, 254 mm (ten inches) of granular base and a 54 mm (two inch) ACP surface. ACP level up courses and two open graded friction courses were then placed in 1988 and 1992, respectively. The total surfacing was approximately 178 mm (seven inches) thick. As a side note, the northbound lanes were performing satisfactorily mainly because they were built using a different and thicker pavement structure. Several areas of the roadway were placed over subgrade consisting of fractured limestone bedrock, while the rest of the roadway was placed over clay subgrade. FWD data and GPR data were first obtained. GPR data was obtained at a 3.05 meter (ten foot) spacing along the roadway in the outside wheelpath. FWD data was obtained at a 0.16 km (0.1 mile) spacing in the outside wheelpath. Cores were taken at select locations in the outside wheelpath based on the GPR data analysis. The authors determined that DCP data was not needed for this particular project, since the GPR data analysis and the cores verified the source of the distress as discussed below. GPR data indicated that the open graded friction courses were holding water where maintenance forces had placed ACP overlays. Figure 4 shows one GPR trace where a dielectric value of 33.9 was measured 43 mm (1.7 inches) from the pavement surface. As mentioned earlier, dielectric values greater than 16 indicate that the pavement layers are most likely saturated with water.
9 Wimsatt and Scullion 9 The cores obtained indicated that the open graded friction course layer placed in 1988 was disintegrating, which resulted in the surface distress. However, the ACP layers underneath the open graded friction courses were still in good shape. Therefore, TxDOT personnel concluded that the open graded friction course layers had to be removed, along with any ACP overlays on top of those layers, which translated to 102 mm (four inches) of surface removal depth. The Modulus Version 5.1 computer program was used to backcalculate the pavement and subgrade layer moduli from the FWD data to use in the FPS19 design program. A total of 107 FWD deflection basins were obtained. Two methods were used to obtain design modulus values. For the first method, 54 of those basins were not used in the design modulus calculations because the Modulus program indicated that the depth to bedrock for those basins was less than 1524 mm (60 inches). This indicates that limestone bedrock was present and thus lime treatment of the subgrade was not used. As a result, the average values obtained from the Modulus program for the 53 basins analyzed were 3,004 kpa (436 ksi) for the 178 mm (seven inch) thick composite ACP layer; 309 kpa (44.8 ksi) for the 254 mm (ten inch) thick granular base layer; 5,161 kpa (749 ksi) for the 152 mm (six inch) thick lime stabilized subgrade layer, and 74 kpa (10.8 ksi) for the subgrade layer. The corresponding coefficients of variation were 61 percent for the composite ACP layer, 57 percent for the granular base layer, 39 percent for the lime stabilized subgrade layer, and 28 percent for the subgrade material. The Modulus program indicated that the average depth of the subgrade layer is 2,078 mm (81.8 inches). Bedrock is assumed to be underneath the subgrade. The average error per sensor was 4.2%, with a coefficient of variation of 65 percent, which are considered acceptable values considering the condition of the roadway. As a side note, the high modulus value (5,161 kpa or 749 ksi) for the 152 mm (six inch) thick lime stabilized subgrade layer is common for such layers in the Fort Worth District. Generally, five percent lime by weight is used for stabilized existing clay subgrade layers in the District. For the second method, all 107 deflection basins were used, but the lime treated subgrade layer was analyzed as part of the overall subgrade layer, and the depth of the subgrade layer was set at 7,520 mm (300 inches). As a result, the average values obtained from the Modulus program for the 107 basins analyzed were 2,528 kpa (367 ksi) for the 178 mm (seven inch) thick composite ACP layer; 501 kpa (72.7 ksi) for the 254 mm (ten inch) thick granular base layer and 365 kpa (53 ksi) for the 7,520 mm (300 inch) thick subgrade layer. The corresponding coefficients of variation were 63 percent for the composite ACP layer, 103 percent for the granular base layer, and 78 percent for the subgrade material. Bedrock is assumed to be underneath the subgrade. The average error per sensor was 3.8%, with a coefficient of variation of 79 percent, which are considered acceptable values considering the condition of the roadway. For both methods, the FWD data analysis indicated that the base material was in good shape from a structural standpoint and that no base repair was needed. The FPS19 computer program uses a modulus value for ACP layers obtained at a standardized temperature of 25 degrees Celsius (77 degrees Farenheit). The modulus of the 178 mm (seven
10 Wimsatt and Scullion 10 inch) thick composite ACP surface layers was backcalculated from FWD data at an average ACP pavement temperature of 13 degrees Celsius (56 degrees Farenheit). So, the 3,044 kpa (436 ksi) and 2,528 kpa (367 ksi) values are multiplied by 0.41 to generate design moduli values of 1,233 kpa (179 ksi) and 1,040 kpa (151 ksi). These design moduli values are used for the 76 mm (three inches) of ACP that remain after the upper 102 mm (four inches) have been removed. The temperature correction factor is based on a standardized curve developed by Mr. Scullion based on unpublished TTI lab data for use on Texas ACP layers. The equation for the curve is: CF = [(1.8*T+32)^2.81]/200,004 Where: CF= Correction Factor T= Temperature, degrees Celsius The twenty-year traffic forecast from TxDOT s Transportation Planning and Programming Division indicated that the average daily traffic (ADT) in 2002 is 22,600 and is forecast to be 37,100 in The truck traffic on this roadway section is 7.7% of the ADT. The forecasted accumulated twenty year 18 kip equivalent single axle load value is estimated to be 5,289,000. Using the above average moduli values from the FWD analysis and the traffic forecast data, and taking into account that the upper 102 mm (four inches) were to be removed, the FPS19 flexible pavement design computer program indicated that a 127 mm (five inch) ACP overlay would last for approximately ten years before another overlay is needed using the moduli values obtained from the first method; and twelve years using the moduli values obtained from the second method. The TxDOT pavement design manual indicates that such an overlay should perform a minimum of eight years according to the pavement design procedure . So, the proposed strategy exceeded the minimum requirements. So, to summarize, the rehabilitation strategy for this roadway was to remove the upper 102 mm (four inches) of open graded friction courses and ACP level up courses, and then place 127 mm (five inches) of ACP. This rehabilitation project was let to contract in May, 2001 and was completed in September, No base repairs were necessary for this project. The project is performing well. FM 2331, Johnson County, Full Depth Pavement Reclamation Candidate Texas has many miles of pavements consisting of granular base with a thin surface treatment. These pavement structures are structurally inadequate for the current level of truck traffic loading in many locations. One popular rehabilitation strategy for such pavement structures is full depth pavement reclamation, where the existing pavement layers are milled; stabilized with lime, cement, fly ash, or a combination of such materials; and then compacted as a new base or
11 Wimsatt and Scullion 11 subbase layer. For the past five years, the Texas Transportation Institute has been evaluating a new test procedure, the tube suction test (TST), to identify poorly performing unstabilized base materials. Figure 5 shows the testing apparatus . However, this test has recently been extended to assist with selecting the optimal stabilizer type and stabilizer content. The proposed testing procedure is discussed in more detail in TTI Research Report To summarize, for each stabilizer content tested, the corresponding samples are compacted using standard TxDOT specifications at the optimum moisture content. The samples are placed in a sealed bag and then are put into a 100 percent humidity curing room for seven days. After seven days, the unconfined compressive strength if one of the samples is measured. The other sample is then subjected to the Tube Suction Test, which includes a four day drying period and then a ten day capillary rise period. The surface dielectric of the sample is measured periodically using a portable dielectric probe for the ten day period. Bases with dielectric values less than 10 are expected to perform very well in the presence of water. The unconfined compressive strength of the sample subjected to the Tube Suction test is then measured at the completion of the Tube Suction Test. The pavement material from a section of FM 2331 in Johnson County was selected as a candidate for in place cement stabilization. This pavement section consists approximately of three seal coats over 203 mm (eight inches) of granular base material. The pavement material was subjected to the test using the following criteria for determining an acceptable cement stabilizer content: (1) Minimum seven day unconfined compressive strength of the first sample (UCS1)= 1,723 kpa (250 psi) (2) Final Measured Dielectric Value less than 10, and (3) Minimum seven day unconfined compressive strength of the second sample (UCS2) that is subjected to the TST is at least 80 percent of the strength of the first sample. Cement contents of zero, two, three, and four percent were used in the testing. The results are as follows: (1) For zero percent cement, UCS1 = 503 kpa (73 psi), UCS2= 400 kpa (58 psi), final measured dielectric = 5.8 (2) For two percent cement, UCS1= 1,550 kpa (225 psi), UCS2=1,860 kpa (270 psi), final measured dielectric=4.8 (3) For three percent cement, UCS1=2,336 kpa (339 psi), UCS2=1,950 kpa (283 psi), final measured dielectric=4.9 (4) For four percent cement, UCS1=2,356 kpa (342 psi), UCS2=2,315 kpa (336 psi), final measured dielectric= 4.5
12 Wimsatt and Scullion 12 Based on the results above, the three percent cement content was selected to be used. All of the materials passed the tube suction test dielectric criteria, even with the untreated raw material. The tube suction test results indicates that the existing base material is of good quality. For pavement thickness design purposes, a modulus value of 689 kpa (100 ksi) is currently used in the FPS19 flexible pavement design computer program for such a stabilized base material. This value is considered to be conservative. The Fort Worth District has previously used in place cement stabilization of existing granular bases on four roadway sections: FM 917 in Johnson County, FM 51 in Parker County, FM 1938 in Tarrant County, and FM 4 in Johnson County. The FM 917 and FM 51 projects were completed in 1997; the FM 1938 and FM 4 projects were completed in The cement content for these projects ranged from 2 to 3 percent and was determined using on the 1,723 kpa (250 psi) minimum seven day unconfined compressive strength criteria. All four projects have been performing well, with minor transverse reflective cracking from the stabilized bases being the only significant visual distress present. The FM 917 project was discussed in a paper presented at the Transportation Research Board Annual Meeting in 1998 and published in the Proceedings of the Society of Photo-Optical Instrumentation Engineers .
13 Wimsatt and Scullion 13 SUMMARY TTI Research Report has provided effective practical guidelines for determining the optimum rehabilitation strategies for flexible pavement structures. The methodologies in the research report helped TxDOT personnel determine the cause of existing pavement distress and generate appropriate rehabilitation strategies for the projects described in this paper and for other candidate rehabilitation projects in Texas. As mentioned earlier, the research project results and several case studies have been incorporated into a four day training course on flexible pavement rehabilitation that is offered to TxDOT personnel. The course lecture and contents have also been incorporated into a personal computer CD-ROM based training course that is available through TxDOT s Research and Technology Implementation Office.
14 Wimsatt and Scullion 14 REFERENCES  Scullion, Tom, Selecting Rehabilitation Options for Flexible Pavements: Guidelines for Field Investigations, Research Report , Texas Transportation Institute, The Texas A & M University System, September, 2001, 86 pp.  Wimsatt, Andrew, Tom Scullion, John Ragsdale, And Stacia Servos, The Use of Ground Penetrating Radar Data in Pavement Rehabilitation Strategy Selection and Pavement Condition Assessment, Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE), Volume Society of Photo-Optical Instrumentation Engineers, Bellingham, Washington, 1998, pages  Scullion, Tom, Yiqing Chen, and Chun Lok Lau, Colormap Version 2 User s Guide with Help Menus, Research Report , Texas Transportation Institute, The Texas A & M University System, November, 1999, 116 pp.  Scullion, Tom, and Chester H. Michalak. Modulus 5.0: User s Manual. Research Report , Texas Transportation Institute, the Texas A & M University System, November, 1995, 104 pp.  Scullion, Tom, and Chester H. Michalak, Flexible Pavement Design System (FPS) 19: User s Manual, Research Report , Texas Transportation Institute, The Texas A & M University System, September, 1998, 74 pp.  Scrivner, Frank H., W.M. Moore, W.F. McFarland, and G.R. Carey, A Systems Approach to the Flexible Pavement Design Problem, Research Report 32-11, Texas Transportation Institute, Texas A&M University, College Station, Texas, 1968, 106 pp.  Pavement Design Manual, Texas Department of Transportation, Austin, Texas, March 2001, 48 pp. Available at under the Design Section.  Saarenketo, T. and Tom Scullion, Using Suction and Dielectric Measurements as Performance Indicators for Aggregate Base Materials, Transportation Research Record 1577, Transportation Research Board, Washington, D.C., 1997, pages
15 Wimsatt and Scullion 15 LIST OF TABLES TABLE 1. NORMAL DIELECTRIC CONSTANT VALUES FOR VARIOUS PAVEMENT LAYERS
16 Wimsatt and Scullion 16 LIST OF FIGURES FIGURE 1. PRINCIPLES OF GROUND PENETRATING RADAR  FIGURE 2. DYNAMIC CONE PENETROMETER TEST RESULTS, SH 337 FIGURE 3. FIGURE 4. DIELECTRIC VALUE MEASURED AT THE INTERFACE BETWEEN THE OPEN GRADED FRICTION COURSE AND THE UNDERLYING SEAL COAT, SH 337 NORTHBOUND. GPR TRACE, US 377 SOUTHBOUND. FIGURE 5. TUBE SUCTION TEST APPARATUS 
17 Wimsatt and Scullion 17 TABLE 1 - NORMAL DIELECTRIC CONSTANT VALUES FOR VARIOUS PAVEMENT LAYERS Pavement Type Dielectric Value Range Asphalt Concrete Pavement (Normal Aggregate) Asphalt Concrete Pavement (Lightweight Aggregate) Flexible Base (Granular Base) Cement Treated Base Concrete Pavement Dielectric constants greater than those listed indicate the presence of moisture in the system. Dielectric constants greater than 16 indicate layers saturated with water. Dielectric constants less than those listed indicate the presence of excessive air voids. Water has a dielectric constant of 81.
18 Wimsatt and Scullion 18 Figure 1- Principles of Ground Penetrating Radar 
19 Wimsatt and Scullion 19 0 SH 337 Core 13 DCP Test- 5/17/01 Cumulative Number of Blows mm(8in.)BaseMaterial, Overall Modulus = 413 kpa (60 ksi) Subgrade Material, Overall Modulus = 103 kpa (15 ksi) Figure 2 Dynamic Cone Penetrometer Test Results, SH 337
20 Wimsatt and Scullion 20 Dielectric at the Interface between the OGFC and Lower Seal Coat, SH 337 NB Significant Moisture Present Normal Range Distance, km Figure 3 Dielectric Value Measured at the Interface between the Open Graded Friction Course and the Underlying Seal Coat, SH 337 Northbound.
21 Wimsatt and Scullion 21 Figure 4 GPR Trace, US 377 Southbound.
22 Wimsatt and Scullion 22 Figure 5 Tube Suction Test Apparatus 
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