AASHTO Method for Flexible Pavement Design

1 GRID-DE-6 1999 TENAX Corporation4800 East Monument StreetBaltimore, Maryland 21205tel: (410) 522 - 7000fax: (410) 522 - 7015 TENAXT echnical Reference GRID-DE-6 GEOGRID REINFORCEMENT OF Flexible PAVEMENTS:A PRACTICAL PERSPECTIVEGRID-DE-6 Geogrid Reinforcement of Flexible Pavements: A Practical PerspectiveBy Aigen Zhao and Paul T. FoxworthyRecent efforts by the AASHTO Subcommittee on Materials, Technical Section 4E, to develop ageogrid/geotextile specification for Pavement reinforcement have initiated very positivediscussions. The Geosynthetic Materials Association has participated in the discussion and maderecommendations to AASHTO with the presentation of a draft White Paper addressinginstallation survivability and specifications. The overwhelming comments back from thereviewers of the White Paper clearly show the need to 1) demonstrate the performance andcost benefits of geogrid reinforcement, and 2) develop a Design procedure incorporating geogridwith value-added benefits, in addition to the installation survivability aspects already reinforcement has been used in the Design and construction of pavements for over adecade, yet there exists no Design Method incorporating geogrid mechanical properties as directdesign parameters.

2 Due to the complexity of layered Pavement systems and loading conditions,there may never be a simple Design Method identifying the properties of a geogrid as directdesign parameters for reinforced Pavement systems. Rather, a series of performance based testsshould be conducted to evaluate the structural contribution of geogrid reinforcement to pavementsystems, from which Design parameters could be derived and incorporated into a paper presents a practical perspective to address: 1) a modified AASHTO Design Method forreinforced pavements, 2) performance tests to support and verify the Design parameters, and 3)cost benefit and constructability analyses. Performance data and analyses presented here arelimited to multilayered polypropylene biaxial AASHTO Design Method for Geogrid Reinforced Flexible PavementsExisting Design methods for Flexible pavements include: empirical methods, limiting shearfailure methods, limiting deflection methods, regression methods, and mechanistic-empiricalmethods.

3 The current AASHTO Method is a regression Method based on the results of road AASHTO Method utilizes an index termed the structural number (SN) to indicate therequired combined structural capacity of all Pavement layers overlying the subgrade. Therequired SN is a function of reliability, serviceability, subgrade resilient modulus, and expectedtraffic intensities. The actual SN must be greater than the required SN to ensure long termpavement actual SN value for a unreinforced Pavement section is calculated as follows:22211mdadaSN + = Eq. (1)where a1 a2 are the layer coefficients characterizing the structural quality of the asphalticconcrete (AC) layer and the aggregate base course (BC) in a Pavement system. A subbase layerGRID-DE-6can be included in Eq. (1) if desired. d1, d2 are their thicknesses; and m2 is the drainagecoefficient for the granular modification to equation (1) is introduced to account for the structural contribution of ageogrid reinforcement to Flexible pavements.

4 Eq. (2)where LCR is the layer coefficient ratio. Equation (2) can be used to calculate the base coursethickness for geogrid reinforced pavements by rearranging its terms: Eq. (3)When the layer coefficient ratio, LCR, is greater than 1, the thickness of the geogrid reinforcedbase course is reduced compared to unreinforced sections; similarly, if the base course thickness isheld constant, the structural number of the reinforced section increases. An increased structuralnumber implies an extended service life of the Pavement for the same traffic concept of layer coefficient ratio was introduced over a decade ago (Carroll, Walls and Haas1987, Montanelli, Zhao, and Rimoldi, 1997) to quantify the structural contribution of a geogridin a Flexible Pavement . This concept was established based on the reinforcing mechanism thatgeogrid provides lateral confinement to the base course material and improves the layer coefficientof the reinforced base.

5 The next section addresses the controlled laboratory Pavement testsperformed to develop this Design parameter for multilayered polypropylene biaxial geogrids. Thefollowing sections provide field verification through nondestructive tests and full-scale Laboratory Pavement TestingLaboratory tests were performed to study Flexible Pavement systems under cyclic loadingconditions, and to quantify the structural contribution of a geogrid reinforcement. The test setupis shown in Figure 1. Cyclic loading was applied through a rigid circular plate with a diameter of300 mm. The peak load was 40 kN with an equivalent maximum stress of 570 kPa. Asphalticconcrete, aggregate base course and subgrade soil layers were included in the Pavement asphalt thickness was 75 mm, and the base thickness was 300 mm. A multilayeredpolypropylene geogrid manufactured by continuous extrusion and orientation processing wasused in the test, its properties are listed in Table 1.

6 The details of the laboratory tests arepresented by Cancelli et al. (1996).SNa dLCR adm= + 112 2 2**22112*maLCRdaSNd =GRID-DE-6 Figure 1. Controlled laboratory Pavement testsTable 1. Properties of the Multilayered Geogrid Used in the TestsMachine DirectionCross Machine DirectionUnit weightg/m2 240 Open Area % 75 Peak tensile modulus @2% strainkN/m220325 Tensile modulus @5% strainkN/m180260 Junction 2 shows Pavement surface rutting for both control and geogrid reinforced sections. Thenumber of loading cycles versus subgrade CBR is presented in Figure 3 for rut depths of 25 mm , [-]050100150 VERTICAL SETTLEMENT, [mm]300 mm GRAVEL Unreinforced CBR 1% Reinforced CBR 1%Unreinforced CBR 3%Reinforced CBR 3%Unreinforced CBR 8%Reinforced CBR 8%Unreinforced CBR 18%Reinforced CBR 18%Figure 2. Pavement surface ruts for control and reinforced , [%]CYCLE, [-]Unreinforced 25mm RUTR einforced, 25mm RUTU nreinforced RUTR einforced, RUTF igure 3.

7 Loading cycle number for control and reinforced at two rut 4 depicts the relationship between the calculated layer coefficient ratio and subgrade CBRbased on Pavement testing data from both control and reinforced sections. The layer coefficientGRID-DE-6ratio was calculated for each subgrade CBR based on procedures contained in the AASHTOG uide for the Design of Pavement Structures (1993). First, the structural number of the controlsection was calculated using Eq. 1. Second, the total number of 18-kip equivalent single axleloads (ESAL) that the control section could be expected to sustain before failure wasbackcalculated from the AASHTO Flexible Pavement Design curve, assuming reliability = 95%,standard deviation = , Design serviceability loss = 2, layer coefficient of asphalt = , layercoefficient of aggregate base course = , drainage coefficient = 1, and subgrade resilientmodulus = 1500 * CBR value.

8 Third, the load correction ratio was calculated by dividing thetotal expected ESAL by the actual number of rigid circular plate load applications required toreach the predetermined rut depth failure criteria (25mm, or ). The failure criterion of mm rut depth was used since under a CBR of 18 the Pavement never reached a 25mm rutdepth. Fourth, this load correction ratio was used to calculate the expected total number of ESALto failure for each reinforced section with the same subgrade CBR. Fifth, the structural numberof each reinforced section was determined from the AASHTO Flexible Pavement designnomograph. Finally, the layer coefficient ratio for each subgrade CBR was then calculated bysolving Eqs. (1) and (2). The layer coefficient ratio is presented as a function of subgrade CBRvalues in Figure 4, and as shown, the lower the subgrade CBR, the greater the layer CBR 02468101214161820 Layer Coefficient Ration, 4.

9 Layer coefficient ratio vs. subgrade CBRN ondestructive FWD TestsNondestructive tests were conducted in Wichita, Kansas, to evaluate the effectiveness of geogridmaterials in improving the structural capacity of Pavement sections using AASHTO nondestructive testing and analysis procedures. To accomplish this objective, several residential,collector, and arterial street segments, previously constructed using geogrid materials, wereGRID-DE-6identified for nondestructive testing. ground penetrating radar (GPR) and falling weightdeflectometer (FWD) tests were conducted on these existing geogrid reinforced paved roads. TheFWD load plate used for this project was 285 mm in diameter, and two tests targeted to producenominal loads of approximately 9,000 pounds were performed at each test location. FWD testswere conducted at 100 foot spacing in the outside wheel path of each travel lane and staggered toprovide 50 foot coverage along the street GPR tests were first conducted to identify individual uniform Pavement sections alongeach street segment.

10 A core sample of the asphaltic concrete surface and hand auger sample ofthe aggregate base course were obtained on Dallas Street to provide ground truth for calibrationof the GPR data. The GPR data was then analyzed at each FWD test point to produce layerprofiles for each street segment and to further delineate the uniform sections shown in Table 2. Summary of Uniform SectionsStreetFromToAverageACAverageBase Segment Section Station(ft)Station(ft)Thickness(in)CoV*( %)Thickness(in)CoV*(%)GeogridSterling10+ 005+ +009+ +0015+ +0024+ +9852+ +005+ +5028+ +9240+ * CoV = Coefficient of Variation = Standard Deviation Divided by the MeanField roadbed soil resilient modulus Mr values for each FWD test location were backcalculatedfrom deflection data. The structural capacity of each uniform Pavement section was thenevaluated in terms of an effective structural number (SNeff) using the nondestructive deflectiontesting approach outlined in the 1993 AASHTO Guide.