mechanical response of flexible pavements enhanced with geogrid-reinforced asphalt ... ·...

Mechanical response of flexible pavements enhanced withgeogrid-reinforced asphalt overlaysN. S. Correia1 and J. G. Zornberg2

1Postdoctoral Researcher, University of Sao Paulo at Sao Carlos, Department of GeotechnicalEngineering, 13566-536, Sao Carlos, SP, Brazil, Telephone: +55 16 3373 9505;Telefax: +55 16 3373 9509; E-mail: [email protected] (corresponding author)2Professor, The University of Texas at Austin, Civil Engineering Department, University Station C1792Austin, TX 78712-0280, USA, Telephone: +1 303 492 0492; Telefax: +1 512 471-6548;E-mail: [email protected]

Received 25 May 2015, revised 1 September 2015, accepted 19 September 2015,published 9 December 2015

ABSTRACT: The use of geosynthetic reinforcements placed underneath asphalt overlays, which hasbeen typically used to minimise problems associatedwith reflective cracking, is evaluated in this study asan approach to improve pavement structural capacity. Whereas geosynthetics have been used to increasethe pavement structural performancewhen used to reinforce the base aggregate, as reported in numerouslaboratory and evaluations experiments, such improvement has not been evaluated for the case ofgeosynthetics within asphalt layers. Accordingly, this paper presents the results of large model testsinvolving both reinforced and unreinforced hot mix asphalt overlays. Cyclic wheel loads were appliedusing an accelerated pavement test facility that was specifically developed for this research. A number ofpavement sections were constructed using a polyvinyl alcohol geogrid as reinforcement inclusions. Theresults show a considerable increase in pavements structural performance, as quantified by reduction ofstrains in the asphalt concrete layers, in the vertical stresses within the pavement layers and in theresilient displacements at the wearing surface. The use of geogrid reinforcements was also found to leadto reduce rutting and permanent lateral movements in the surface layer. Overall, the use of geogridswithin asphalt overlays was found to act as a reinforcement element that provide enhanced structuralcapacity to flexible pavements.

KEYWORDS: Geosynthetics, Asphalt overlay, Geogrid, Pavement, Reinforcement

REFERENCE: Correia, N. S. and Zornberg, J. G. (2016). Mechanical response of flexible pavementsenhanced with geogrid-reinforced asphalt overlays. Geosynthetics International, 23, No. 3, 183–193.[http://dx.doi.org/10.1680/jgein.15.00041]

1. INTRODUCTION

Geosynthetics have been used to improve the pavementstructural performance when used as reinforcementswithin the aggregate base layer, as reported in numerouslaboratory and field evaluations (Perkins 1999; Zornbergand Gupta 2010; Tang et al. 2015; Wu et al. 2015).However, geosynthetic reinforcements placed underneathasphalt overlays have been used almost exclusively toreduce the development of reflective cracking into asphaltoverlays, without accounting for their capability topotential increase the pavement structural capacity.This study evaluates the potential use of geosynthetic-reinforced asphalt overlays to enhance the overall pave-ment structural capacity. This approach is expected to leadto significant advances in pavement rehabilitation byextending the life of roadways, consequently reducingmaintenance costs.The use of geosynthetics to improve the service life of

pavements in asphalt overlays (either new asphalt layers or

localised repairs) has increased markedly in recent years.Field evidence and theoretical studies have indicated thatthe service life of flexible pavements can be extended byinstalling nonwoven geotextiles or geogrids between theexisting layer and the new asphalt overlays due to theability of the geosynthetic to minimise the development ofreflective cracks (Lytton 1989; Austin and Gilchrist 1996;Prieto et al. 2007; Khodaii et al. 2009; Virgili et al. 2009;Yu et al. 2013; Fallah and Khodaii 2015; Gonzalez-Torreet al. 2015). However, a more limited number of studieshas been initiated to assess the use of geosynthetics inasphalt overlays to improve the mechanical performanceof pavements, such as control of permanent displace-ments. While in many of these studies, the main focus hasbeen the control of reflective cracks, the reported evidenceof improved pavement capacity are summarised next.Brown et al. (1985) reported that polypropylene

geogrids placed underneath asphalt layers resulted in adecrease in rutting ranging from 20 to 58%. Austin andGilchrist (1996) reported that the use of geogrid in

Geosynthetics International, 2016, 23, No. 3

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an asphalt concrete layer contributed to reduce lateralflow and, hence, the development of permanent defor-mations. Comparisons between the results obtained inunreinforced and reinforced slabs resulted in ruttingreductions as great as 70%. An experimental studyconducted by Laurinavicius and Oginskas (2006) invol-ving the use of geogrids in asphalt overlays reportedreductions in rutting depth of over 50%. Bühler (2007)reported results of a field study in which geogrids werefound to reduce rutting depths by 40% in relation to theresults obtained in a control section. Using a MMLS3load simulator, Solaimanian (2013) reported a reductionof up to 85% in the permanent deformations of apavement model constructed using geogrids between aconcrete slab and an asphalt layer. Canestrari et al. (2015)conducted flexural beam tests in order to investigate theimpact of geogrid reinforcements placed at the interfacebetween two asphalt layers. The use of geogrids resulted indecreased permanent deformations in relation to unrein-forced specimens. More recently, Mounes et al. (2015)evaluated permanent deformations in geogrid-reinforcedasphalt concrete using dynamic creep tests, and concludedthat both the mechanical properties and the openingsize of the geogrid have a significant importance incontrolling permanent deformations. Pasquini and Bocci(2014) conducted interface shear tests on double-layeredreinforced samples focusing on interlayer shear resistanceand concluded that improved mechanical properties wereachieved, although they cautioned about reductions ininterlayer bonding (Zamora-Barraza et al. 2010; Ferrottiet al. 2012).A valuable approach to quantify the effectiveness of

geosynthetic reinforcements to improve the pavementstructural performance involves testing full-scale instru-mented pavement sections (Perkins 1999). However, mostof the current knowledge regarding the use of geosyn-thetics in a reinforcement function in a pavement haveinvolved the reinforcement of unbound aggregates(Perkins and Ismeik 1997; Holtz et al. 1998; USACOE2003). While these studies are relevant, the mechanismsidentified for geosynthetic-reinforced base layers cannotbe directly applied to understand the behaviour ofgeosynthetics in reinforced asphalt layers. Only a limitednumber of studies have been specifically conducted toinvestigate the improvement in the behaviour of reinforcedasphalt overlays in full-scale instrumented pavementsections, as detailed next.Siriwardane et al. (2010) reported the results of

instrumented pavement sections (with and without fibre-glass geogrids within the asphalt layer) as a part of alarge-scale pavement study. The inclusion of geogridswas found to lead to a decrease in vertical displacementsof approximately 38%, as well as a reduction in verticalstresses in the underlying layers. Graziani et al. (2014)reported the results of a studyon the structural response ofgeogrid-reinforced asphalt overlays, in which they focusedon monitoring the strains within the asphalt layer.The results showed an overall reduction of 65% peaktensile strains in the reinforced section with respect to theunreinforced section.

There are clear opportunities to capitalise on thepotential structural improvement that may be realisedwhen using geosynthetic-reinforced asphalt overlays.Accordingly, the objective of this research was to gainunderstanding on the reinforcement benefits of usinggeogrids within asphalt overlays, with a focus on theimprovements on the pavement performance. An acceler-ated pavement testing facility was designed and con-structed specifically for this study. The investigationinvolved large-scale laboratory instrumented pavementmodels, which were loaded using a rolling wheel simulat-ing a truck wheel load.

2. TEST FACILITY AND SCOPE OF THEEXPERIMENTAL PROGRAMME

2.1. Wheel tracking facility

The large-scale pavement models constructed as part ofthis study were loaded using a rolling wheel system tosimulate the load of a truck wheel. The wheel trackingapparatus was installed in a large steel testing box withinternal dimensions of 1.8 m (height), 1.6 m (width)and 1.8 m (length). The testing facility involved steel-reinforced walls that had been designed to withstand highlateral soil stresses with minimum lateral deformations.Accordingly, plane strain conditions were considered to berepresentative for the models. The stiffness of the structurewas enhanced by a reaction beam fixed to the walls ofthe box, which acted as part of the load transfer system.To properly represent the structural section of a flexiblepavement, the testing box was used to house threepavement structural layers: subgrade soil, aggregategranular base, and hot mix asphalt. Figure 1 illustratesthe wheel tracking facility.The wheel load is controlled by a hydraulic jack, which

has been designed to apply unidirectional loads of up to2 t. A hydraulic motor was used to control the movementof the entire system (wheel, jack, load cell). The test wheel(one-tyre configuration) travels at a speed of 3.6 km/h incontact with the tracking length of 1.0 m. The wheel

Testing box

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Figure 1. View of the accelerated pavement testing facility

184 Correia and Zornberg


involves a rubber tyre characterised by a diameter of546 mm and awidth of 154 mm. The tyre pressure was setat 700 kPa. Additional details about the main character-istics of the accelerated pavement test facility are describedin Correia (2014).An in-house software program for data acquisition was

developed to control the load-time history of the wheel(cyclic moving load mode). The data acquisition was alsoused to collect data from the instrument devices installedwithin the pavement models. The software was pro-grammed to induce the load and wheel pressure pulseshown in Figure 2. The wheel load pulse involves a linearload that increases from 0 to 16 kN over 0.1 s, followed bya period of 1.0 s during which the load is held constant.The load is subsequently reduced to zero over a 0.1 speriod. The final period involves a 1.2 s period under zeroload before the subsequent load cycle is applied. Thevarious loading stages add up to a total load cyclecorresponding to 2.4 s. The output from the load cell for atypical load cycle application is also shown in Figure 2.The results in the figure show a very good match betweenthe target loads and actual applied loads. Each test wasconducted until reaching a total of 105 load cycles.

2.2. Pavement materials and layer properties

The geogrid-reinforced and unreinforced asphalt layerswere constructed over a dense uniform aggregate base,which was constructed over a soft subgrade. The subgradesoil was classified as MH and A-7-5 according to theUSCS (ASTM D 2487-11) and AASHTO (AASHTO M145 1999) classification systems, respectively. Themaximum dry density of subgrade, as determined byStandard Proctor tests was 14.9 kN/m² at an optimumwater content of 28.8%. The target dry density and watercontent were set as 14.7 kN/m² and 30.8% (2% aboveoptimum), resulting in a weak subgrade conditioncharacterised by a California bearing ratio of 4.5%. Thebulk density and moisture content of the soil weremeasured during placement of every 250 mm in elevation.The 1 m thick subgrade layer was compacted in thetesting box in 50 mm-thick lifts using manual proceduresinvolving a drop hammer.

The aggregate used as base material classifies as GPand A-1-a according to the USCS (ASTM D 2487-11)and AASHTO (AASHTO M 145) soil classificationsystems, respectively. The aggregate material was charac-terised by a maximum dry density of 24 kN/m3 and anoptimum moisture content of 6.5% based on StandardProctor tests. Compactions of the base layer wereconducted in 100 mm-thick lifts using a vibratory plate.The thickness of the base course layer ranged from 100 to200 mm, depending on the pavement model, and it wascompacted to a target relative compaction of 99% basedon Standard Proctor tests.The hot mixed asphalt (HMA) concrete was a 9.5 mm

(3/8″) dense-fine-graded mixture with a binder content of5.4% (Bitumen Penetration Grade 30/45), indicated for alltraffic conditions (FHWA and NAPA 2001). The HMAlayer was compacted in a single lift of 50 mm using avibratory plate. For the first stage of tests, the pavementmodels were composed of a soft subgrade soil, anunbound aggregate base and a HMA layer.The second stage of tests involved asphalt resurfaced

sections. A geogrid that had been specifically manufac-tured for asphalt application was selected as the reinforce-ment material (Hatelit XP 50). The geogrid wasmanufactured using high-modulus polyvinyl alchoholribs to form a biaxial product with quadrangularapertures. A lightweight polypropylene nonwoven geo-textile was attached on one side of the geogrid to facilitatethe installation and to allow a continuous bonding to theHMA layer during installation. Index tests were con-ducted to characterise this reinforcement product (Correia2014), with results presented in Table 1.For reinforced sections, the geogrid was cut into the

dimensions of the box area and subsequently placed overthe previously tested asphalt surface with the geotextilefabric facing down. The machine direction of the geogridwas installed parallel to the wheel tracking path. Previousto the geogrid installation, the asphalt surface had beenlevelled using HMA concrete. A tack coat rate of cationicrapid setting emulsion (0.6 l/m² residual) was sprayed inone application (DNER P00/043 2006). Figure 3 illus-trates the geogrid installation.After to the geogrid installation, a new HMA overlay

was compacted using the same asphalt concrete materialand conditions adopted for construction of the previousasphalt layer. The thickness of the asphalt overlay was

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Table 1. Properties of geogrid reinforcement used in this research

Characteristic Geogrid

Polymer Polyvinyl alcoholAperture size (mm×mm) 40×40Tensile strength (kN/m)Machine direction (mm) 50Cross machine direction (mm) 50Secant stiffness at 2% strain (kN/m) 890Elongation at break (%) 5.6

Results obtained using ASTM D6637-11 test method.

Mechanical response of flexible pavements enhanced with geogrid-reinforced asphalt overlays 185


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60 mm. The same procedure was used for construction ofboth reinforced and unreinforced sections.

2.3. Instrumentation and scope of the testing programme

An instrumentation programme was designed to monitorthe pavement sections during traffic loading in order toquantify the mechanical response of the pavementmaterials. The instrumentation involved sensors suitableto measure surface vertical displacements, stresses andstrains in the asphalt layers, as wells as stresses in the base

course and subgrade layers. The data acquisition systemused in this study was configured to record information onthe entire time history in order to capture the full responseof the models under application of the load cycles(Figure 2). All pavement sections were constructed usinga similar instrumentation layout. Figure 4 presents thecross-section view of pavement layers and location of theinstrumentation devices.Pressure transducers were used to monitor vertical

stresses within the pavement layers. The sensors wereinstalled: (1) at the interface between the old and newasphalt layers, (2) at the interface between the base courseand asphalt layer (1 MPa range), (3) in the middle ofthe base course (500 kPa range) and (4) at the top ofthe subgrade (200 kPa range). H-type asphalt straingauges (ASG) were used to measure the asphalt concretestrains at the interface between reinforced and unrein-forced asphalt layers. The ASG has a physical range of±2000 με. The procedures described by Timm et al. (2004)and Graziani et al. (2014) were adopted to minimisepossible damage to these sensors induced by aggregateparticles.A total of eight linear position transducers (LPT) were

used to monitor surface deformation in the transversedirection to the wheel path. The sensors were placed onboth sides at four different locations, as follows (distancesmeasured from the wheel load centreline): 240, 390,540 and 690 mm. Figure 5 illustrates a plan view of

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Figure 3. View of the geogrid installation within the testing box

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Figure 4. Cross-section view of the pavement layers showing location of the instrumentation devices. Note: dimensions are in mm



the test section showing the location of the LPTs,ASGs and pressure transducers. Additional measure-ments of permanent displacements in the wheel loadarea were conducted using a transverse surface profile.Measurements were taken after 1, 10, 100, 500, 1000, 2500and 5000 passes, and at intervals of 10 000 wheel passesthereafter. Additional details about the characteristics ofthe instrumentation used in this study are provided atCorreia (2014).Table 2 provides an overview of the scope of the testing

programme. It includes a total of three pavement models(tests 1, 2 and 3), conducted using two stages of tests ineach model (stages A and B). In the first stage of tests 1and 2, identical control sections were loaded until reach-ing a total of 105 cycles. The control sections tested aspart of this stage (sections 1A and 2A) involved flexiblepavement models composed of a 50 mm-thick HMAlayer, a 200 mm-thick aggregate base layer and a1 m-thick subgrade layer. In the second stage of thesetests, the HMA overlay was constructed over the pre-viously loaded asphalt surface in order to simulate theloading conditions in an asphalt rehabilitation project.

Test 1 involved a section constructed without reinforce-ment (section 1B). Test 2 was constructed using layers ofthe same characteristics of those in layers of Test 1, butincluded a geogrid-reinforcement in the HMA overlay(section 2B). In both tests, the newly placed HMA overlywas loaded until reaching 105 cycles. Test 3 was conductedusing the same materials and stages as those in test 2, butinvolved a base course of reduced thickness (100 mm).This configuration was adopted in order to simulate acomparatively less rigid reinforced pavement structure.Table 3 presents the most relevant properties of the

asphalt concrete layers used in each pavement section.These characteristics include the layers thickness (t),maximum theoretical specific gravity (Gmax), air voidcontents (AV), indirect tensile strength (IDT) and resilientmodulus (Mr). The asphalt concrete layers in all sectionsand stages presented a comparatively high air voidscontent, which was attributed to the relatively lowcompaction effort used in this study.

3. RESULTS

3.1. Evaluation of rutting depth

Accumulation of permanent vertical deformations in oneor more layers within a pavement system leads to thedevelopment of rutting depth. Rutting severely impactsthe pavement serviceability, having a significant effect onthe cost and schedules of pavement maintenance. In thisstudy, the development of permanent displacements onthe pavement surface was measured during the first andsecond stages of tests. Rutting depth measurements werealso used to define the termination of the tests (failurewas set as 25 mm rutting). Permanent (plastic) displace-ments were obtained by subtracting resilient (elastic)displacements from total vertical displacements. Figure 6shows the development of cumulative plastic displace-ments obtained during the first stage of tests 1A, 2A and3A (before placement of HMA overlay). The results onrutting depths obtained for the two identical controlsections 1A and 2A show very good repeatability,including essentially the same rut depths at the wheelpath (Figure 6a), as well as similar movements alongsidethe wheel path (Figure 6b). The comparison between theresults in the first stage of the tests provided evidence ofthe consistency of the construction of the pavementmodels, as well as confidence of the quality controlmeasures adopted in this investigation. The results provide

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Table 2. Scope of the testing programme

Test Stage Profilename

Geogrid installation Base coursethickness (mm)

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Unreinforced HMA overlay 1B – 105

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Geogrid-reinforced HMA overlay 2B Between old and new asphalt layers 105

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Geogrid-reinforced HMA overlay 3B Between old and new asphalt layers – 105



a good basis for the subsequent comparison of theperformance of unreinforced and geogrid-reinforced sec-tions. Figure 6 also shows the results obtained insection 3A, constructed with a reduced base coursethickness. As expected, the pavement with reducedoverall stiffness (3A) shows a significant difference inthe cumulative permanent deformation in comparisonwith the response observed in sections 1A and 2A.In particular, the lateral movement alongside the wheelpath on the surface in Section 3Awas approximately twiceof the values observed in sections 1A and 2A. However,it should be noted that the development of plasticdeformations had not stabilised in any of the threesections by the time that the maximum number of cycleswas reached. No significant fatigue cracks were noted incontrol sections.

Figure 7 presents the cumulative permanent displace-ments obtained during the second stage of the tests. Theresults indicate that the presence of the HMA overlay ledto an increased pavement stiffness and to comparativelysmaller rutting in all tests. However, the results also showthat the presence of the geogrid lead to a substantialdecrease in rutting. The benefits of reinforcementinclusion were observed to occur early in the test(Figure 7a), after applying the initial load cycles. Inparticular, a comparison of the rutting response ofsections 1B and 2B reveals that the use of geogridreinforcement led to 40% decrease in rutting depthsafter 105 cycles. Furthermore, a comparison of theresponse of tests 3B and 1B reveals that the use ofasphalt reinforcement led to accumulated rutting that is

Table 3. As-constructed asphalt concrete layers properties

Property 1A 1B 2A 2B 3A 3B

t (mm) 53.0 61.08 52.1 62.0 52.5 60.0Gmax (ASTM D2041M-11) 2.676 2.676 2.677 2.677 2.672 2.672AV (ASTM D3203M-11) 9.84 10.2 10.4 11.4 9.8 9.0IDT at 25°C (ASTM D6931-12) 1.09 0.97 1.04 0.88 1.12 0.98Mr (ASTM D7369-11) 3730 4977 3595 4600 3581 4660

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Figure 6. Results of rutting depth obtained during the first stageof tests: (a) cumulative plastic displacements along the wheel path;(b) rutting profiles after 105 cycles

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Figure 7. Results of rutting depth obtained during the secondstage of tests (after HMA overlay application): (a) cumulativeplastic displacements along the wheel path; (b) rutting profilesafter 105 cycles



similar to the observed in the unreinforced section, whichwas constructed with a base course layer that is twice asthick as that in the reinforced model.The results at the end of the tests in Figure 7b indicate

that the unreinforced section (1B) shows higher perma-nent lateral movements next to the wheel path than thoseobtained when compared with the geogrid-reinforcedsections (2B and 3B). It should be noted that all sectionswere constructed with asphalt surface courses of similarproperties and air void contents (Table 3). Overall, it isclear that the presence of the geogrid was effective inreducing permanent deformations of the asphalt layersduring cyclic tests (approximately a 40% reduction). Therutting depth results in geogrid-reinforced asphalt over-lays obtained in this study were consistent with the trendsreported by Brown et al. (1985), Laurinavicius andOginskas (2006), Bühler (2007) and Siriwardane et al.(2010).The rate of vertical displacements has also been utilised

to evaluate rutting reduction (Wu et al. 2015), which isdefined as the changing rate (velocity) of the verticaldeformations. Figure 8 shows the rate of permanentdisplacements, as obtained for tests 1B, 2B and 3B.As shown in the figure, the permanent displacementrate of the section without geogrid (1B) was greaterthan that of the geogrid-reinforced section (2B). Thepermanent displacement rate became smaller withincreased number of cycles for all tests. However, forsection 2B, it became non-significant after 30 000 loadcycles, whereas for sections 1B and 3B, the permanentdisplacement rate remained six times higher and com-paratively similar between these sections. The resultspresented in Figures 7 and 8 suggest that the geogridreinforcement placed at the bottom of the HMA overlaywas effective in reducing rutting depths and permanentlateral movements of the wearing surface, as well as therate of permanent displacements.

3.2. Evaluation of the deflection basins

The area of pavement deflection under and near the loadapplication is known as the ‘deflection basin’. Theanalysis of the deflection basin (resilient deformation) in

the asphalt surface provides relevant information for thestructural evaluation of new or in-service flexible and rigidpavements (ASTM D4695-03). Figure 9 shows represen-tative results of the deflection basins acquired in the studyat the end of 105 load cycles. Figure 9a shows thedeflection basin results for sections 1A, 2A and 3A. Asshown in the figure, the deflection basins for the identicalcontrol sections 1A and 2Awere found to be very similar.The shape of the curves in the vicinity of the wheel loadarea depends primarily on the stiffness of the asphaltconcrete layer, not on the stiffness of subgrade. In the caseof section 3A, the characteristics of the deflection basinswas primarily affected by the quality of the underlyinglayers, as shown by the outer edges of the deflection basin.Figure 9b presents the results for the deflection basinsobtained at the end of the second stage of the tests 1B, 2Band 3B, showing the general asphalt overlay contributionin reducing resilient deformations. The maximum deflec-tion in section 1B was as high as 0.36 mm. On the otherhand, the basin shapes and magnitude of the maximumdeflections obtained for the geogrid-reinforced section 2B(max 0.16 mm) were similar to those obtained insection 3B (max 0.22 mm). The basin shapes obtainedfor these tests represents the significant geogrid con-tribution to improve pavement structural stiffness.Comparisons of sections 1B and 2B reveals that the use

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of the geogrid was responsible for a reduction of 55%in the maximum resilient deformations of the asphaltoverlay. Overall, the inclusion of geogrid in the asphalticlayer was found to lead to an enhanced overall stiffness inthe pavement.

3.3. Evaluation of vertical stresses in pavement layers

Instrumentation results also allowed evaluation of thevertical stresses induced by the loading wheel passes(dynamic stress response of the test sections). Specifically,stresses were measured using pressure transducers locatedbeneath the wheel path at the bottom of the asphalt layer,in the middle of the base course and at the top of thesubgrade layer, as in Figure 4. The pressure transducerinstalled at the bottom of the HMA overlay did notproduce reliable results in response to traffic loading. Thisis because transducers were partially damaged duringtests, probably due to the proximity (60 mm) of the wheelload contact area.A typical response of the pressure transducers, as

obtained during the second stage of tests, is provided inFigure 10. The stress results show a clear differencebetween the response of the unreinforced section (1B) andthe reinforced section (2B). An average of 32% reductionin vertical stresses at the bottom of the asphalt layerwas observed in the geogrid-reinforced section 2B inrelation to that in the unreinforced section 1B(Figure 10a). In the middle of the base course, a 30%reduction in vertical stresses was observed in thereinforced section (Figure 10b). Finally, vertical stressesinduced at the top of the subgrade resulted in an average36% reduction due to the presence of reinforcement inthe asphalt layer (Figure 10c). The results for reinforcedsection 3B, in comparison with the results obtained insection 1B, show slightly higher peak values at the bottomof the asphalt layer (Figure 10a). At the middle of thebase course, the pressure transducer stopped respondingduring traffic load, so the results will be further estimated.At the top of the subgrade, the difference in inducedstresses between sections 1B and 3B was more pronounced(Figure 10c). Nonetheless, this difference can be attrib-uted to the shorter distance for stress dissipation betweenthe pavement surface and the base layer in Section 3B(100 mm-thick base), when compared with the configur-ation in section 1B (200 mm-thick base).Figure 11 presents the vertical stresses measured in the

three pavement sections. The results in Figure 11a clearlyillustrate the contribution of the polymeric geogrid inreducing the vertical stresses across the base course layersof the pavement sections. These results are consistent withthe reductions in vertical stresses on the top of thesubgrade, as reported by Siriwardane et al. (2010) forthe case of fibre-glass geogrids placed within the asphaltlayer. Figure 11b illustrates the stress distributionmeasured on pavement layers of reinforced section 3B.The estimated vertical stress level in the middle of the basecourse is also shown in the figure. Furthermore, inFigure 11, the stresses in unreinforced section 1B werefound to be similar to those in reinforced section 3B(reduced base thickness). The vertical stress results

provide additional evidence that the presence of geogridsin the asphalt layer provides structural benefits to thepavement, leading to reductions in the vertical stresses inthe underlying pavement layers.

3.4. Evaluation of strains within the asphalt layer

ASGs were installed in the longitudinal and transversedirections of the wheel path (Figure 5) in order to measureload-induced strains in the asphalt layer during trafficloading. Figure 12 illustrates typical elastic strainsrecorded by the ASG devices in the longitudinal andtransverse directions during the tests. Figure 13 shows

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ss (k

Pa)

Ver

tical

stre

ss (k

Pa)

Bottom of asphalt layer

Time (s)

1B

2B3B

0

10

20

30

40

50

60

70

80

90

100Middle of base course

Top of the subgrade

1B

2B

0

10

20

30

40

50

60

70

80

90

100

500.0 500.4 500.8 501.2 501.6 502.0 502.4

Time (s)500.0 500.4 500.8 501.2 501.6 502.0 502.4

Time (s)500.0 500.4 500.8 501.2 501.6 502.0 502.4

1B

2B3B

Figure 10. Typical vertical stress response to cyclic wheel load:(a) bottom of asphalt layer; (b) middle of base course; (c) top ofthe subgrade



the strains measurement, as obtained by the sensorsin the transverse direction (ASG-T), for tests 1B, 2Band 3B. The results indicate that the peak transversetensile strains recorded for section 1B was approximately200 με, and the peak strains in section 2B was compara-tively smaller (115 με). Table 4 summarises the averagevalues of peak tensile strains in the asphalt layer, asmeasured in both longitudinal (ASG-L) and transverse(ASG-T) directions. Comparison of the tensile strain

values reported in Table 4 for sections 1B and 2Bhighlights the effectiveness of the geogrid to reduceasphalt concrete strain levels (over 40%). Whereas thegeogrid-reinforced section 3B exhibited higher transversestrains than the unreinforced section 1B, which wasresponsible for potential fatigue cracking, the results inthe longitudinal direction were found to be comparativelysimilar. These results illustrate the potential of geogridreinforcement to reduce asphalt fatigue cracking duringthe service life of the pavement.

4. CONCLUSIONS

Whereas most previous research on the use of geosyn-thetics to reinforce asphalt layers has focused on mini-mising reflective cracking, this study focuses on the useof geosynthetic reinforcements to increase the overallpavement structural capacity. Specifically, this researchfocused on quantifying the improved permanent andresilient deformations, asphalt concrete strains andvertical stresses expected in pavement layers when usinggeogrids to reinforce asphalt overlays. An APT facilitywas developed for this study. The investigation involvedlarge-scale laboratory instrumented pavement modelsconstructed using polyvinyl alcohol geogrids as reinforce-ment inclusions. Based on the results obtained from thisresearch, the following conclusions can be drawn.

• Analysis of the results obtained in pavement controlsections constructed using the same structural layers

(a)

(b)

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200

Pav

emen

t dep

th (m

m)

Average vertical stress (kPa)

1B2B

Asphalt layer

Middle base course

Top of subgrade

Basecourse

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200

Pav

emen

t dep

th (m

m)

Average vertical stress (kPa)

3B

Asphalt layer

Top of subgradePredicted verticalstress level

Base course

Figure 11. Measured vertical stresses in pavement layers:(a) sections 1B and 2B; (b) section 3B

–200

–100

0

100

200

300

400

520 522 524 526 528 530

Mic

rost

rain

(µε)

Time (s)

Interface of asphalt layers Test 3B

ASG-T

ASG-L

Figure 12. Typical response of ASGs in longitudinal andtransverse direction, as obtained in test 3B

–200

–100

0

100

200

300

400

520 522 524 526 528 530

Mic

rost

rain

(µε)

Time (s)

Interface of asphalt layers1B2B3B

ASG-T

Figure 13. Typical strains at the interface of asphalt layersmeasured by ASG-T, as obtained in tests 1B, 2B and 3B

Table 4. Average of the peak tensile strains in asphalt concretelayers

Section Longitudinal strains (με)ASG-L

Transverse strains (με)ASG-T

1B 280 2002B 125 1153B 300 330



and materials were found to provide essentially overallrutting and deflection basin behaviour after wheelcyclic loading. This provides confidence on the qualitycontrol measures adopted in the experimental com-ponents of this study.

• The improvement in pavement performance thatresults from the use of geogrids in asphalt overlayscould be quantified by comparing the mechanicalperformance of unreinforced and reinforced sections.In particular, the use of geogrid reinforcement resultedin a 40% decrease in rutting depth and significantlyless permanent lateral movements, as well asreductions in the rate of permanent displacements.The use of geogrid reinforcement also resulted inreductions in the asphalt concrete strain levels thatwere as high as 40%. In addition, resilient displace-ment levels were reduced up to 55%. Finally, over 30%reduction in vertical stress levels in pavement layerswas also attributed to the geogrid inclusion within theasphalt overlays.

• The performance of a pavement section including ageogrid-reinforced asphalt overlay and a reduced basecourse thickness was found to be comparable to that ofa pavement section including an unreinforced asphaltoverlay and a thicker base course (twice as thick in thisparticular study). Specifically, similar behavior wasobserved in terms of permanent and resilient displace-ments. Vertical stresses registered in pavement layersand asphalt concrete strain levels in the longitudinaldirection were also found to be similar.

Overall, the experimental results generated as part of thisstudy illustrate the improved mechanical performance thatresulted from the use of geogrids in asphalt overlays interms of rutting depths, deflection basins, asphalt con-crete strains and vertical stresses. In addition to the morecommon use of geosynthetics to minimise reflectivecracking, geogrid reinforcements within asphalt overlayswere found to be effective in enhancing the overallstiffness of the pavement. Consequently, the inclusion ofgeogrid in the asphalt overlay was recognised as acting asa reinforcement element, providing enhanced structuralcapacity to flexible pavement structures.

ACKNOWLEDGEMENTS

The authors are grateful to the late B. Bueno for hiscontributions to this study. Support provided by theNational Council for Scientific and TechnologicalDevelopment (CNPq) and by Huesker Geosynthetics isalso gratefully acknowledged.

NOTATION

Basic SI units are given in parentheses.

AV air void contents (dimensionless)Gmax asphalt concrete maximum theoretical specific

gravity (dimensionless)

Mr resilient modulus (Pa)t thickness (m)

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