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  • 1. Effective Approach to Improve Pavement Drainage Layers Imad L. Al-Qadi, F.ASCE1; Samer Lahouar2; Amara Loulizi3; Mostafa A. Elseifi4; and John A. Wilkes5Abstract: The objective of this study was twofold: ͑1͒ quantify the benefits of a specially designed geocomposite membrane ͓a lowmodulus polyvinyl chloride ͑PVC͒ layer sandwiched between two nonwoven geotextiles͔ to act as a moisture barrier in flexible pavementsystems; and ͑2͒ quantitatively measure the moisture content of unbound granular materials nondestructively. The geocomposite mem-brane was installed over half the length of a pavement test section at the Virginia Smart Road, while the other half of the test sectionconsisted of the same design without the interlayer system. An air-coupled ground penetrating radar ͑GPR͒ system with 1 GHz centerfrequency was used to monitor and detect the presence of moisture within the pavement system over different periods of time corre-sponding to different levels of water accumulation. Results of GPR data analysis indicated that the use of the geocomposite membranereduced water infiltration to the aggregate base layer by as much as 30% when measurements were performed after rain. It was also foundthat the moisture content underneath the interlayer was almost constant and therefore independent of the amount of rainwater, which is theprimary source of moisture in pavement systems that have a low water table. The impact of moisture in the granular layers wasinvestigated using the results of a deflection monitoring program. The results indicate that the area with the geocomposite membranealways showed less deflection than the area without the interlayer. The study recommends that any pavement drainage layer must bebacked by an impermeable interface, given that the water table is low.DOI: 10.1061/͑ASCE͒0733-947X͑2004͒130:5͑658͒CE Database subject headings: Surface drainage; Moisture content; Flexible pavements.Introduction formance. The use of geosynthetic material has also been recog- nized as a potentially reliable method to enhance the drainage ofSeveral problems associated with rapid deterioration and unsatis- aggregate and subgrade layers ͑Christopher et al. 1999; Elseififactory performance of pavement systems are directly related to et al. 2001͒.the accumulation of excessive moisture in subgrade and granular One of the simplest techniques with which to measure pave-layers. Examples of distress associated with the accumulation of ment drainage is to collect water at the pavement edges. But,water in pavement layers include stripping in hot-mix asphalt because some pavement layers are unbound in all directions,͑HMA͒ layers, loss of subgrade support, reduction in the stiffness water could drain vertically to the underlying layers or it could beof granular layers, and erosion of cement-treated base layers retained within the material. Such problems might be minimized͑Christopher et al. 1999͒. by adequate design of the lateral slopes, but the technique remains Different solutions have been suggested and implemented in unproven.flexible pavement design methods to improve drainage conditions Several techniques have been used to estimate the moistureand the ability to predict their impact on pavement performance content in pavement systems. Some techniques that detect the͑e.g., AASHTO 1993͒. Drainage layers and edge drains are ex- presence of moisture within the layers of a pavement system areamples of common additions to flexible pavement systems that based on electromagnetic ͑EM͒ energy, either by using groundmay prevent the detrimental effects of moisture on pavement per- penetrating radar ͑GPR͒ or time domain reflectometry ͑TDR͒ probes. In 1999, Al-Qadi and Loulizi demonstrated the use of a 1 Founder’s Professor of Civil and Environmental Engineering, 205 N. GPR system to detect the presence of moisture within pavementMathews, MC-250, University of Illinois at Urbana-Champaign, Urbana, systems based on variations of the reflection amplitude from theIL 61801. different layer interfaces ͑Al-Qadi and Loulizi 1999͒. In 2001, 2 Senior Research Associate, Virginia Tech Transportation Inst., 3500 Elseifi et al. presented the use of GPR and TDR probes toTransportation Plaza, Blacksburg, VA 24061-0536. 3 Research Scientist, Virginia Tech Transportation Inst., 3500 Trans- qualitatively validate the efficacy of a specially designedportation Plaza, Blacksburg, VA 24061-0536. geocomposite membrane system to act as a moisture barrier 4 Senior Research Associate, Virginia Tech Transportation Inst., 3500 and to prevent the infiltration of water to the underlying layersTransportation Plaza, Blacksburg, VA 24061-0536. ͑Elseifi et al. 2001͒. Although the results of these studies demon- 5 President, CARPI USA, 3517 Brandon Ave., Suite 100, Roanoke, strated the ability to detect the accumulation of water in the pave-VA 24018. ment system, the quantification of such benefits has yet to be Note. Discussion open until February 1, 2005. Separate discussions achieved.must be submitted for individual papers. To extend the closing date by This paper presents two important issues: the effectiveness ofone month, a written request must be filed with the ASCE Managing the aforementioned geocomposite membrane as a moisture barrierEditor. The manuscript for this paper was submitted for review and pos-sible publication on January 13, 2003; approved on August 18, 2003. This and the utilization of GPR data to quantify the moisture infiltra-paper is part of the Journal of Transportation Engineering, Vol. 130, tion reduction to the granular base layers when using an imper-No. 5, September 1, 2004. ©ASCE, ISSN 0733-947X/2004/5- meable interlayer underneath an open-graded drainage layer658 – 664/$18.00. ͑OGDL͒. This study was conducted at the Virginia Smart Road658 / JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
  • 2. pavement test facility where the geocomposite membrane wasinstalled in two different sections to test its effectiveness as amoisture barrier.BackgroundVirginia Smart RoadThe Virginia Smart Road located in southwest Virginia is aunique, state-of-the-art, full-scale research facility for pavementresearch and evaluation of intelligent transportation system ͑ITS͒concepts, technologies, and products. The Virginia Smart Road isone of a few facilities to be built from the ground up with itsinfrastructure incorporated into the roadway. When completed,the Virginia Smart Road will be a 9.6-km connector highwaybetween Blacksburg and I-81 in southwest Virginia, with the first3.2 km designated as a controlled test facility. The flexible pave-ment part of the Virginia Smart Road test facility includes 12 Fig. 1. Ground penetrating radar survey van showing air-coupleddifferent heavily instrumented flexible pavement sections. antenna More than 500 instruments were embedded in the road duringconstruction to quantitatively measure the response of pavementsystems to vehicular and environmental loading. For a successful To precisely locate the data collected longitudinally on theinstrumentation strategy, at least two types of response ͑stress, road, a distance-measuring instrument ͑DMI͒ connected to thestrain, or deflection͒ should be compared simultaneously. There- survey vehicle wheel was used to trigger the pulses generated byfore, at the Virginia Smart Road facility, stress and strain are the GPR system. In this case, data were collected as a function ofcarefully monitored throughout the depth of the pavement system. the distance ͑i.e., n scans every meter͒ rather than as a function ofClimatic parameters, including temperature, base and subbase time ͑i.e., n scans every second͒.moisture, and frost depth, are monitored at different depths alongthe pavement. The calibration and installation of the instruments Moisture Content Estimation in Base Layers Usingat the Virginia Smart Road are presented elsewhere ͑Al-Qadi Ground Penetrating Radaret al. 2001a,b͒. It is possible that the moisture content of a pavement layer can be correlated to its dielectric constant. The presence of free moistureGround Penetrating Radar Principles in a pavement layer will increase its dielectric constant sinceThe principle of an impulse GPR system ͑the most common type water has a high dielectric constant ͑approximately 80͒ comparedof GPR systems commercially available͒ is based on sending an to the dielectric constant of the other pavement materials (ϭ1 forEM pulse through the GPR antenna to the ground and then re- air, and ranges between 3 and 6 for HMA, between 4 and 9 forcording the pulses reflected from layer interfaces where there is aggregate, and between 6 and 15 for concrete͒ ͑Daniels 1996͒.contrast in the dielectric properties of the layers. Analysis of the Therefore, increasing moisture accumulation in a pavement layerreflected GPR signals allows the estimation of the pavement layer will cause a higher dielectric contrast at its interface with drierthicknesses and dielectric properties, which are related to the adjacent layers. Consequently, GPR signals reflected from inter-water content ͑Lahouar et al. 2002͒. These are the main applica- faces between two layers having increasingly different moisturetion of GPR for pavement assessment. contents will have greater amplitudes. Depending on the way the GPR antenna is deployed, GPR Quantitatively, the bulk dielectric constant of any material cansystems can be classified as air-coupled ͑or air-launched͒ or be computed from the individual dielectric constant of its compo-ground-coupled systems. In air-coupled systems, the antennas nents using a mixture law called the complex refractive index͑usually horn antennas͒ are typically 150–500 mm ͑6 –20 in.͒ model ͑Maser and Scullion 1992͒. This model expresses the di-above the surface. These systems give a clear radar signal and electric constant of a material, ␧ m , as follows:allow highway-speed surveys. However, since part of the EMenergy sent by the antenna is reflected back by the pavementsurface, they have a low depth of penetration into the pavement ␧ mϭ ͚ͩ ͱ ͪ i Vi ␧i 2 (1)structure. In contrast, a ground-coupled system antenna is in full where V i ϭvolume fraction of component i; and ␧ i ϭdielectriccontact with the ground, which gives a higher depth of penetra- constant of component i.tion ͑at the same frequency and EM energy output͒ but limits the A granular base layer is composed of aggregates, air voids,speed of the data collection survey. and water. Therefore, the dielectric constant of the base layer The GPR system used in this research was a SIR-10B con- could be expressed according to Eq. ͑1͒ as follows:nected to an air-coupled antenna, manufactured by Geophysical ␧ b ϭ ͑ V aggͱ␧ aggϩV airͱ␧ airϩV w ͱ␧ w ͒ 2 (2)Survey Systems, Inc. ͑GSSI͒. The air-coupled antenna was com-posed of a pair of separate horn antennas ͑one serves as a trans- where ␧ b ϭdielectric constant of the base layer; V agg and ␧ aggmitter and the other as a receiver͒ having a center frequency of 1 ϭfractional volume and dielectric constant of aggregate, respec-GHz, which corresponds to a pulse width of 1 ns. As depicted in tively; V air and ␧ airϭfractional volume and dielectric constant ofFig. 1, the antenna was mounted behind the survey van, with the air, respectively; and V w and ␧ w ϭfractional volume and dielectriccontrol unit set inside it. constant of water, respectively. JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 / 659 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
  • 3. Fig. 3. Section J pavement design Geocomposite Membrane Installation A geocomposite membrane was installed in two sections of the Virginia Smart Road ͑Al-Qadi et al. 2001a, b͒. This membrane consists of a 2-mm thick low modulus polyvinyl chloride ͑PVC͒ backed on both sides with nonwoven polyester geotextile. The primary function of the PVC geocomposite membrane is to con- trol water infiltration and fines migration. Typical values of per-Fig. 2. ͑a͒ Reflections from layer interfaces in typical pavement meability for the geocomposite membrane were reported to besystem and ͑b͒ radar scan obtained from typical pavement section between 1ϫ10Ϫ13 and 2ϫ10Ϫ13 cm/s, both essentially represen- tative of an impermeable material ͑Koerner 1994͒. This special design offers the potential to use this type of interlayer as a mois- ture barrier and as a strain energy absorber. The latter case was Eq. ͑2͒ could be transformed to yield the gravimetric moisture investigated and validated elsewhere ͑Al-Qadi and Elseifi 2002͒.content of the base layer according to the following equation In section J of the Virginia Smart Road, the geocomposite͑Maser and Scullion 1992͒: membrane was installed along half the section underneath an asphalt-treated OGDL to evaluate its effectiveness as a moisture ␥d barrier ͑a schematic of this section is shown in Fig. 3; all desig- ͱ␧ b Ϫ1Ϫ ͑ ͱ␧ aggϪ1 ͒ ␥ agg nations are in accordance with the Virginia Department of Trans- mcϭ (3) ␥ portation specifications͒. Section J is located in a cut with a sub- ͱ␧ b Ϫ1Ϫ d ͑ ͱ␧ aggϪ22.2͒ grade material classified as A-1-a based on the AASHTO ␥ agg classification ͑corresponding to GP-GM in the United Classifica-where mcϭgravimetric moisture content in percent; ␥ d ϭdry tion System͒. This describes a material consisting predominantlydensity of the base layer; and ␥ aggϭdensity of aggregate. of stone fragments or gravel. During construction of this project In Eq. ͑3͒, the dry density, aggregate density, and aggregate ͑1999͒ and based on regular deflection testing, the subgradedielectric constant could be estimated from direct measurements modulus was estimated to be 335 MPa with the presence of a stiffof samples extracted from the aggregate base layer. In contrast, layer at a depth of 4.6 m. In this project, the ground water table isthe aggregate base dielectric constant can be determined from low and was not detected in any of the test sections. Therefore,GPR data using the following equation ͑Al-Qadi and Lahouar the main source of water in the granular layers is precipitation ͩ ͪ2004͒: that infiltrates through the layered system. The section has a 5% ͩ ͪ longitudinal slope and 2% lateral slope, which is considered ideal A0 2 A1 2 1Ϫ ϩ for positive drainage within the pavement structure. While this AP AP ͩ ͪ ␧ b ϭ␧ HMA (4) geocomposite membrane has been used on two bridge decks in 2 A0 A1 Italy and is widely utilized in dam applications in Europe, it has 1Ϫ Ϫ never been used on any roads or bridges in the United States prior AP AP to its installation at the Virginia Smart Road. Since the installationwhere ͑as presented in Fig. 2͒ A 0 and A 1 ϭthe amplitudes of the at the Virginia Smart Road in 1999, it was installed on a bridgesurface and HMA/base interface reflections, respectively; A P deck underneath HMA overlay in Delaware in 2000.ϭthe reflection amplitude collected over a calibration metal plate Prior to installation, the area to be covered with the geocom-placed on the pavement surface ͑thus, A P represents the negative posite membrane was cleared of any loose aggregates. The instal-of the incident signal͒, and ␧ HMAϭthe HMA layer dielectric con- lation of the geocomposite membrane in section J ͑moisture bar-stant, which is determined separately from the GPR data accord- rier over a granular material͒ did not necessitate the use of aing to the following equation ͑Al-Qadi and Lahouar 2004͒: ͩ ͪ prime coat between the geotextile and the underlying layer ͑21-B A0 2 aggregate base layer͒. A prime coat is not effective when applied 1ϩ AP to a granular material ͑e.g., 21-B͒ due to the nature of the surface, ␧ HMAϭ (5) which accumulates a large amount of loose aggregates, and due to A0 the fact that greater friction exists between the geocomposite 1Ϫ AP membrane and the aggregate layer when the prime coat is absent.660 / JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
  • 4. Fig. 6. Average daily rainfall during survey period and base layer moisture content measured by time domain reflectometry under mois- ture barrier brane. The only precaution during installation that was outside normal operations was to avoid sudden application of truck brakes on the geocomposite membrane layer to prevent it from wrinkling. Temperature and moisture sensors were placed on both sides of the geocomposite membrane, while three pressure cells were installed under the geocomposite membrane. More details about the installation are presented elsewhere ͑Al-Qadi et al. 2001a,b͒. Fig. 4. Welding process at longitudinal joints Data Collection Since the geocomposite membrane was installed over half of sec- Five rolls, each 37-m long and 2.05-m wide, were installed tion J at the Virginia Smart Road, its effectiveness as a moistureover the complete width of the road and extended 2.15 m into the barrier was quantified by comparing the moisture content of theshoulder. Additional rolls were then installed to achieve a 50-m two section halves. For that, GPR data were collected over differ-long installation. Transverse joints between PVC rolls were stag- ent periods of the year from the same locations. The data weregered to prevent the creation of a weak joint across the pavement collected from two identical 20-m segments from section J. Thelane. At the longitudinal joints, the joints were welded by apply- only difference between the two segments was the presence of theing hot air to melt the uncovered PVC end; see Fig. 4͑a͒ for geocomposite membrane on top of the aggregate base layer in oneillustration. The welding was then carefully checked; see Fig. of the segments. The surveys were performed on eight different4͑b͒. The upper surface of the geocomposite membrane was dates: December 7, 2001; January 30, April 24; May 22, June 19,primed using PG 64-22 asphalt binder at an application rate of July 16, August 22, and September 12, 2002. As shown in Fig. 6,1.45 kg/m2 . Fig. 5 illustrates the application of the prime coat on the surveys were carried out after different weather conditionstop of the geocomposite membrane. A 75-mm thick asphalt varying from dry ͑i.e., it did not rain or snow for more than atreated OGDL was then placed on top of the geocomposite mem- week͒ to wet ͑rainfall was recorded in the week preceding the survey but it did not rain at least 2 days before data collection͒. Another survey was performed on the 21-B aggregate base layer on August 16, 1999, during road construction and after the 21-B layer was completed. This survey was completed the same day as nuclear gauge measurements were taken on the aggregate base layer to facilitate density and moisture content estimation for quality control purposes. Data Analysis and Results As shown by Eq. ͑3͒, the moisture content of the aggregate base layer is related to four parameters: the aggregate base layer di- electric constant, the dry density of the layer, and the density and dielectric constant of the aggregate particles. While the dielectric constant of the aggregate base layer was determined by Eq. ͑4͒, the remaining three parameters were estimated and assumed con- stant over the whole section. One technique to determine these parameters is to correlate the dielectric constant ͑or its square root͒ to the moisture content measured from the same locationsFig. 5. Application of prime coat on top of geocomposite membrane and then deduce the values of the unknown parameters from the JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 / 661 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
  • 5. Table 1. Moisture Content Measured by Nuclear Gauge Table 2. Average Moisture Content for Sections with and without Bulk density Dry density Geocomposite Membrane on Different Survey Dates Gravimetric, mcStation (kg/m )3 Pcf ͑%͒ (kg/m )3 Pcf ␧b Gravimetric moisture content ␧b ͑%͒111ϩ60 1,893 118 4.5 1,812 113 6.9 Survey Moisture Moisture111ϩ90 2,066 129 5.4 1,961 122 7.6 date barrier Normal barrier Normal112ϩ15 1,961 122 5.7 1,855 116 7.9112ϩ35 2,020 126 5.3 1,918 120 7.3 12/7/2001 5.85 6.22 3.31 3.79Average 1,985 124 5 1,886 118 7.4 1/30/2002 5.96 6.62 3.80 4.29Std dev 65 4 0.4 57 3.5 0.4 4/24/2002 5.95 7.12 3.45 4.89 5/22/2002 5.57 6.00 2.96 3.52 6/19/2002 5.09 5.80 2.31 3.25regression equation. To apply this method, the moisture content 7/16/2002 5.32 5.61 2.61 3.01measured using a nuclear gauge over the 21-B layer during con- 8/22/2002 5.84 6.23 3.31 3.80struction of the road can be used. The aggregate base dielectric 9/12/2002 5.37 5.46 2.69 2.81constant over the locations where the nuclear gauge measure-ments were taken was determined from GPR data collected thesame day. The aggregate base dielectric constant was computed in outlined procedure. The average dielectric constant and moisturethis case using Eq. ͑5͒ rather than Eq. ͑4͒ since the base layer was content found for each survey date and for each segment arethe top layer at that time. The results found from these measure- presented in Table 2. The base dielectric constant was found usingments are summarized in Table 1. Eq. ͑4͒ because the surveys were performed on the wearing sur- Fig. 7 shows the base layer gravimetric moisture content ͑mea- face layer. Fig. 8 shows the average reduction in moisture contentsured by nuclear gauge͒ as a function of the square root of its in the segment with the geocomposite membrane compared to thedielectric constant. According to this graph, a linear relationship segment without the geocomposite membrane.(R 2 ϭ0.88) can be established between the moisture content and It should be noted that the moisture content values found bythe dielectric constant. In addition, Eq. ͑3͒ can be linearized using this technique may be underestimated because the base dielectricTaylor series expansion about point ␧ b0 in the middle of the in- constant ͑from which the base layer moisture content is com-terval of the expected values of the base dielectric constant, ac- puted͒ is underestimated. In fact, estimating the base dielectriccording to the following: constant using Eq. ͑4͒ does not account for any EM energy loss occurring in the HMA layer ͑i.e., the HMA layer is considered 2 ͱ␧ b0 ϩc 1 ϩc 2 2c 1 ͱ␧ b0 ϩc 1 c 2 Ϫ␧ b0 mcϷ ͱ␧ b Ϫ ϭa ͱ␧ b ϩb lossless͒. However, the effect of this error is eliminated when ͑ ͱ␧ b0 ϩc 2 ͒ 2 ͑ ͱ␧ b0 ϩc 2 ͒ 2 comparing the moisture content of the different sections ͑such as (6) in Fig. 8͒ because the comparison is based on the ratio of mois-where c 1 and c 2 ϭtwo constants given by ture contents, which are linear functions of the dielectric constant as shown in Eq. ͑6͒. Therefore, using GPR for estimating the base ␥d c 1 ϭ1ϩ ͑ ͱ␧ aggϪ1 ͒ (7) layer moisture content might not be very accurate unless calibra- ␥ agg tion cores were taken to estimate the different parameters needed for the analysis along with the EM energy loss occurring in HMA. ␥d c 2 ϭϪ1Ϫ ͑ ͱ␧ aggϪ22.2͒ (8) However, the technique could be used to monitor moisture ␥ agg changes in pavements over time by performing surveys during Using the constants aϭ0.0628 and bϭϪ0.1187 ͑obtained different periods and comparing the moisture content ratios. Thefrom Fig. 7͒, Eq. ͑6͒ yields the values of the aggregate dielectric results of such surveys would be more accurate and reliable.constant and the ratio of dry density to aggregate density as fol- According to Table 2, the moisture content in the segment withlows: ␧ aggϭ5.65 and ␥ d /␥ aggϭ0.68. the geocomposite membrane is always lower than the segment The GPR data collected after completion of the road were without the geocomposite membrane. Moreover, the moistureanalyzed for aggregate base moisture content using the previously content difference is particularly high for the April 2002 and June 2002 surveys because of the rain recorded during these periods. It is also noted that the moisture content underneath the geocompos- ite membrane is almost constant and, therefore, is independent of rainfall. This result is confirmed in Fig. 6 where the volumetric moisture constant underneath the membrane, measured by TDR probes, was found to be constant and independent of rainfall. A comparison between the two segments with and without the geocomposite membrane ͑Fig. 8͒ shows that, for dry conditions, the moisture content in the segment with the geocomposite mem- brane is comparable to that in the segment without the geocom- posite membrane ͑approximately a 4 –13% difference͒. However, after rain, the difference between the two segments reached 30%. This indicates that the 21-B layer will have high moisture content in the event of rain if the geocomposite membrane does not exist. This may not be desirable as it may reduce the resilient modulusFig. 7. Correlation between moisture content and dielectric constant of that layer and, hence, the structural capacity of the pavementof base layer system. It is clear from this analysis that the geocomposite mem-662 / JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
  • 6. Fig. 8. Moisture content reduction between sections with andwithout geocomposite membrane on different survey dates Fig. 9. Measured deflection by the last Sensor (Dϭ72 mm) with and without geocomposite membranebrane is effective in decreasing moisture that infiltrates the granu-lar base layers. It is also equally important that the presence of an 1989͒. It appears that preventing water from reaching the un-OGDL with significant slopes in both directions may not prevent bound materials will reduce the detrimental effect of spring thawor significantly reduce the moisture infiltration to underlying un- cycle on the granular layers.bound layers. This observation may well explain the ‘‘less thanexpected’’ performance of pavement systems having OGDLs.Considering the constructability problems with OGDL, a triplanar Conclusionsgeosynthetic drainage layer may provide an alternative pavementdrainage system ͑Christopher et al. 1999͒. Ground penetrating radar was used to estimate the moisture con- tent of an unbound aggregate subbase layer in two pavement sec- tions, one with and one without geocomposite membrane, placed underneath an OGDL. Moisture accumulation in the aggregateImpact on Pavement Serviceability base layer increased its dielectric constant, which was estimated from GPR data. The moisture content underneath the geocompos-High moisture contents may significantly reduce the bearing ca- ite membrane was always lower than that in the section withoutpacity of unbound subbases and subgrades, causing progressive the geocomposite membrane. Moreover, the geocomposite mem-failure of the pavement system. Cedergren predicted a reduction brane allowed as much as a 30% reduction in the estimated mois-of 50% in the pavement service life if a pavement base is satu- ture content of the base layer when surveyed a few days after rain.rated as little as 10% of the time ͑Cedergren 1974͒. To evaluate Based on this research, two main conclusions are made:the effect of moisture on pavement structural integrity, use was 1. A method to quantify the moisture content in granular mate-made of a deflection monitoring program using a falling weight rial nondestructively was presented and validated.deflectometer ͑FWD͒. From the period between March 2000 and 2. The benefits of using an impermeable interface as a moistureDecember 2001, FWD measurements were regularly performed barrier underneath OGDL are validated using GPR surveyson a selected set of points in section J. Two points were located in and FWD deflection measurements.the area with the geocomposite membrane, and three other points The study recommends the use of an impermeable interfacewere located in the area without the membrane. Within the con- underneath drainage layers, given that the water table is low, totext of this study, in which the major interest is to detect the ensure effective water drainage of pavements.variation in structural capacity of the subbase and the subgrade,use was made of the deflection measured away from the center.The last sensor of the FWD, which was located a distance of 72 Acknowledgmentsmm from the center, is regularly used to diagnose the integrity ofthe subgrade by assuming that the higher the deflection, the This research was sponsored by the Virginia Center for Innova-weaker the subgrade ͑Chester and Scullion 1995͒. tive Technology, Virginia Transportation Research Council, Vir- Fig. 9 illustrates the measured deflections with and without ginia Department of Transportation, Carpi USA, and Atlanticgeocomposite membrane from the period between March 2000 Construction Fabrics, Inc. The writers would like to acknowledgeand December 2001. As can be noticed in Fig. 9, the area with the the assistance of B. Diefenderfer, G. Flintsch, W. Nassar, A.geocomposite membrane always exhibited less deflection than the Appea, K. Light, and K. Taylor.area without the geocomposite membrane. Given the uniformityof the subgrade throughout this section, it may be concluded thatthe accumulation of moisture in the area without the geocompos- Referencesite membrane resulted in a decrease of the bearing capacity of thesubbase and the subgrade material. It may also be noticed in Fig. AASHTO. ͑1993͒.9 that a sharp increase in deflection occurred during the spring Al-Qadi, I. L. and Elseifi, M. A. ͑2002͒. ‘‘Analytical modeling and fieldthaw season. This cycle is repeated every year and may be noticed performance testing of geocomposite membrane in flexible pavementtwice during the monitoring period. During the AASHO road test, systems.’’ Proc., 7th Int. Conf. Geosynthetics, 907– was reported that the spring thaw cycle resulted in a sharp Al-Qadi, I. L., and Loulizi, A. ͑1999͒. ‘‘Using GPR to evaluate the effec-decrease in pavement serviceability ͑Ullidtz and Ertman Larsen tiveness of moisture barriers in pavements.’’ Structural Faults and JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2004 / 663 Downloaded 06 Mar 2011 to Redistribution subject to ASCE license or copyright. Visit ttp:// h
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