AASHTO. Guide for. Design of Pavement Structures. STNIE HIGHWAY. SSOCIATION OF O. Published by the. American. Table Flexible Pavement Design according AASHTO Guide for Design of Pavement. Structures Table Comparison of pavement structures. AASHTO Guide for Design of Pavement Structures David John. Loading Preview. Sorry, preview is currently unavailable. You can download the paper by .
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Offi cials (AASHTO) Guide for Design of Pavement Structures, WinPAS and the Supplement to the AASHTO Guide to Design. ImplementatIon of the aaShto deSIgn guIde for pavement StructureS In Kdot Flexible Pavement Design AASHTO versus Mechanistic-Empirical. The MDT Pavement Design Manual is available as a PDF file that includes all chapters and AASHTO Guide for Design of Pavement Structures ( Guide).
Obviously, if only swelling or only frost heaveis considered,there will be only one curve on the graph. A conceptuat example of the environmental serviceability loss versus time graph that m ay b e d e v e l o p e d to r a s p e c i fi c l ocati on. Design of PavementStructures The equation is applicable to flexible, rigid, and aggre gatvs urfaced road s.
The primary measureof serviceability is the PresentServiceabilityIndex PSI , which ranges from 0 impossible road to 5 perfect road. The basicdesignphilosophyof this Guide is the serviceability-performanceconcept, which provides a means of designing a pavement based on a specific total traffic volume and a minimum level of serviceability desired at the end of the performance period.
Selection of the lowest allowable PSI or terminal serviceability index pt is based on the lowest index that will be tolerated before rehabilitation, resurAn necessary. One criterion for identifying a minimum level of serviceability may be establishedon the basis of public acceptance. Terminal Serviceability Level Percent of People Stating Unacceptable In this design guide, rutting is consideredonly as a performance criterion for aggregate-surfacedroads.
Although rutting is a problem with asphalt concrete surface pavements, no design model suitable for incorporation into this Guide is available at this time.
It is important to note that the rut depth failure predicted by the aggregate-surfacedroad model does not refer to simple surface rutting which can be corrected by normal blading operations , but to serious rutting associated with deformation of the pavement structure and roadbed support.
The allowable rut depth for an aggregate-surfacedroad is dependent on the average daily traffic. Typically, allowable rut depths range from 1. When is the aggregate aggregatelossoccurs, the pavement structure becomes thinner and the load-carrying capacity is reduced. This reduction of the pavement structure thicknessincreases the rate of surface deterioration.
To treat aggregate loss in the procedure, it is to necessary estimate l the total thicknessof aggregate that will be lost during the design period, and 2 the minimum thickness of aggregate that is required to keep a maintainable working surfacefor the pavement structure.
Unfortunately, there is very little information available today to predict the rate of aggregateloss. Below is an exampleof a prediction equation developed with limited data on sectionsexperiencinggreaterthan 50 perccnt truck traffic ts, to: For relatively minor highwayswhereeconomicsdictate that the initial capital outlay be kept at a minimum, it is suggestedthat this be accomplishedby reducing the design period or the total traffic volume, rather than by designing for a terminal serviceabilitylessthan 2.
Since the time at which a given pavement structure reaches its terminal serviceability depends on traffic volume and the original or initial serviceability p , some consideration must also be given to the selection of po.
Once po and pt are established, the following equation should be applied to define the total change in serviceability index: A second equation, which was developed from a recent study in Brazil on typical rural sections,can be. I3 lossshould possible,localinformation about aggregate to the procedure.
For roadbed materials, laboratory resilient modulus tests AASHTO T should be performed on representative samples in stressand moisture conditions simulating those of the primary moisture seasons. Alternatively, the seasonal values may be determined by corresilient modulus relations with soil properties, i. The purpose of identifying seasonal moduli is to quantify the relative damage a pavement is subjectedto during each seasonof the year and treat it as part of the overall design.
An effective roadbed soil resilient modulus is then established which is equivalent to the combined effect of all the seasonal modulus values. The development of the procedure for generating an effective roadbed soil resilient modulus is presentedin Appendix HH of Volume 2 of this Guide. The seasonal moisture conditions for which the roadbed soil samplesshould be tested are those which result in significantly different resilient moduli.
For example, in a climate which is not subjected to extended sub-freezingtemperatures, it would be important to test for differencesbetween the wet rainy It and dry seasons. If operations make it difficult to test the roadbed soil for springthaw or winter-frozen conditions, then, for these extreme cases,practical values of resilient moduli of 20,to 50,psi may be used for frozen conditions, ind for spring-thaw conditions, the retained modulus may be 20 to 30 percent of the normal modulus during the summer and fall periods.
Two different procedures for determining the seasonal variation of the modulus are offered as guidelines. One method is to obtain a laboratory relationship betweenresilientmodulus and moisture content. Then, with an estimate of the in situ moisture content of the soil beneath the pavement, the resilient modulus for.
Another equation, developed through a British study done in Kenya, is more applicableto areaswhere there is very little truck activity and thus the facility is primarily used by cars. Since this equation below is for annual gravel loss,the total gravel loss GL would be estimated by multiplying by the number of years in the performance period:. It should be noted that there are seriousdrawbacks with all the equations shown here;therefore,whenever.
I4 each of the seasonsmay be estimated.
1993 AASHTO Flexible Pavement Structural Design
An alternate procedure is to back calculatethe resilientmodulus for using the proceduredescribedin Part different seasons pavements. III usingdeflectionsmeasuredon in-service factors to correct the These may be used as adjustment resilient modulus for a referencecondition. Besides defining the seasonal moduli, it is also necessaryto separate the year into the various component time intervals during which the different moduli are effective.
In making this breakdown, it is not necessaryto specify a time interval of less than one-half month for any given season. If it is not lengths,the possibleto adequatelyestimatethe season usermay refer to Section4. At this point, the length of the seasonsand the seasonal roadbed resilient moduli are all that is required in terms of roadbed support for the design of roads.
For the rigid pavements and aggregate-surfaced design of flexible pavements, however, the seasonal data must be translated into the effectiveroadbed soil resilient modulus described earlier. This is accomplished with the aid of the chart in Figure 2. The effective modulus is a weighted value that gives the equivalent annual damage obtained by treating each season independently in the performance equation and summing the damage.
It is important to note, however, that the effective roadbed soil resilient modulus determined from this chart applies only to flexible pavements designed using the serviceability criteria. It is not necessarily applicable to other resilient modulus-based design procedures.
Since a mean value of resilient modulus is used, design sections with coefficient of variations greater than 0. For example, if the mean value of resilient modulus is 10,psi, then approximately 99 percent of the data should be in a range of 5, to 14,psi.
The first step of this processis to enter the seasonal moduli in their respectivetime periods. If the smallest season is one-half month, then all seasonsmust be defined in terms of half months and each of the boxes is must be filled.
If the smallestseason one month, then all seasonsmust be defined in terms of whole months and only one box per month may be filled in. The next step is to estimatethe relative damage ur values corresponding to each seasonalmodulus. This is done using the vertical scale or the corresponding equation shown in Figure 2.
AASHTO Pavement Design 1986
For example, the. Design of PavementStructures relative damage corresponding to a roadbed soil resilient modulus of 4, psi is 0. Next, the - u, values should all be added together 12 increments and divided by the number of seasonal or 24 to determine the averagerelative damage. The effectiveroadbed soil resilientmodulus Mp , then, is the value correspondingto the averagerelative damage on the MR ur scale.
Since the k-value is directly proportional to roadbedsoil resilientmodulus, the season moduli developedin the previous lengthsand seasonal section will be used as input to the estimation of an effective design k-value.
But, becauseof the effectsof subbasecharacteristicson the effectivedesign k-value, its determination is included as a step in an iterative designprocedure seeChapter 3.
The developmentof the actual procedure for generating this effective modulus of subgradereaction is presentedin Appendix HH of Volume 2 of this Guide.
If, however, the user should have a better understanding of the "layer coefficients" seeSection 2. In general, layer coeflicients derived from test roads or satellite sectionsare preferred.
Elastic modulus is a fundamental engineering property of any paving or roadbed material. For those material types which are subject to significant permanent deformation under load, this property may not reflect the material's behavior under load. Thus, resilient modulus refers to the material's stress-strain behavior under normal pavement loading conditions.
The strength of the material is important in addition to. Chart for estimating effective roadbod soil resilient modulus for flexible pavements designed using the serviceability criteria.
Chart for estimating effective roadbed soilresilient modulus for flexible pavements designed using the serviceabilitycriteria. Generally,the layer thickness is rounded to the nearestinch, bui the use of controlled grade slip form pavers may permity--nch increments.
In addition to the design i-value, other inputs required by this rigid pavement design nomograph include: Th design example in Appendix I provides an illustratio: The approach to considering the effectsof swellinl and frost heave in rigid pavement design is almos identical to that for flexibre pavements Section 3.
Roadbed swelling and frost heaveare both important environmental considerationsbecause their potential of effect on the rate of serviceability loss. Swelling refers to the localized volume changesthat occur in exfansive roadbed soils as they absorb moisture. A diainage system can be effective in minimizing roadbed swelling if it reducesthe availability of moistureforabsorption. Frost heave, as it is treated here, refers to the localized volume changesthat occur in the roadbed as moisture collects, freezesinto ice lenses,and produces distortions on the pavementsurface.
Like swelling, the effects of frost heave can be decreased by providing some type of drainage system. This not only protects against frost heave, but also significantly reduces or even eliminates the thaw-weakening that may occur in the roadbed soil during early spring.
If either swelling or frost heave is tobe considered in terms of their effects on serviceability loss and the need for future overlays, then the following procedure should be applied.
Lectures 4 to 8 - Design of Pavement Structures AASHTO summerized.pdf
It requires the plot of slrviceability loss versus time developed in Section 2. The procedure for considering environmental serviceability loss is similar to the treatment of stage construction strategies becauseof the planned future need for rehabilitation. In the stage construction approach, an initial PCC slab thicknessis selected and the corresponding performance period sendce life determined. An overlay or series of overlays which will extend the combined performance periods past the desired analysis period is then identified.
The difference in the stage construction approach when swelling andlor frost heave are considered is that an iterative process is required to determine the length of. To consider analysis periods which are longer than this maximum expected performance period o. It is also important to recognize the need to compound the reliability for each individual stageof the strategy.
Conversely, 9: To evaluate secondary stagesof such stageconstruction alternatives, the user should refer topart III of this Guide which addressesthe design for pavement rehabilitation. That part not only provides a irocedure. Design Requirements stiffness, and future mechanistic-based procedures may reflect strength as well as stiffness in the materials characterization procedures.
In addition, stabilized base materials may be subject to cracking under certain conditions and the stiffness may not be an indicator for this distresstype.
Different notations are used to expressthe moduli for subbase Err , base Egs , asphalt concrete Enc , and portland cement concrete Ec. The procedure for estimating the resilient modulus of a particular pavement material dependson its type.
Relatively low stiffnessmaterials, such as natural soils, unbound granular layers, and even stabilized layers and asphalt concrete, should be tested using the resilient modulus test methods AASHTO T Although the testing apparatus for each of thesetypes of materials is basically the same, there are some differences, such as the need for triaxial confinement for unbound materials.
Alternatively, the bound or higher stiffnessmaterials, such as stabilized basesand asphalt concrete, may be tested using the repeated-load indirect tensile test ASTM D This test still relies on the use of electronic gaugesto measuresmall movements of the sample under load, but is less complex and easier to run than the triaxial resilient modulus test.
Because of the small displacements and brittle nature of the stiffest pavementmaterials, i. Thus, it is recommended that the elastic modulus of such high-stiffness materials be determined according to the procedure described in ASTM C The elastic modulus for any type of material may also be estimated using correlations developed by the state'sdepartment of transportation or by some other reputable agency. The following is a correlation recommended by the American Concrete Institute l for normal weight portland cement concrete:.
If standard agency practice dictates the useof center-point loading, then a correlation should be made betweenthe two tests. Becauseof the treatment of reliability in this Guide, it is strongly recommended that the normal construction specification for modulus of rupture flexural strength not be used as input, since it representsa value below which only a small percent of the distribution may lie.
If it is desirableto use the construction specification, then some adjustment should be applied, based on the standard deviation of modulus of rupture and the percent PS of the strength distribution that normally falls below the specification: Permissible number of specimens, expressed as a percentage, that may have strengths lessthan the specification value.
A value for this coefficient is assignedto each layer material in the pavement structure in order to convert actual layer thicknesses into structural number SN. This layer coefficient expressesthe empirical relationship between SN and thickness and is a measure of the relative ability of the material to function as a structural component of the pavement. The following generalequation for structural number reflects the relative impact of the layer coefficients a, and thickness D,: Design of Pavement Structures lime, lime flyash, and cement flyash are acceptable materials, and each agency should develop charts.
Asphalt Concrete Surlace Course. Caution is recommendedfor modulus values above , psi. Although higher modulus asphalt concretesare stiffer and more resistant to bending, they are also more suiceptible to thermal and fatigue cracking.
Gronular Base Loyers.
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Although the elastic resilient modulus has been adopted as the standard material quality measure,it is still necessary to identify corresponding layer coefficients because of their treatment in the structural numberdesign approach. Research and field studies indicate many factors influence the layer coefficients, thus the agency's experience must be included in implementing the results from the procedures presented. For example, the layer coefficient may vary with thickness, underlying support, position in the pavement structure, etc.
It should be noted that laboratory resilient modulus values can be obtained that are significantly different from what may exist for an in situ condition. For example, the presenceof a vcly stiff unbound layer over a low stiffness layer may result in decompaction and a corresponding reduction of stiffness. As a guideline for successive layers of uubuuud rnatcrials, the ratio of resilient modulus of the upper layer to that of the lower layer should not exceed values that result in tensile stressesin unbound granular layers.
The discussionof how thesecoefficientsare estimated isseparated into five categories, depending on the type and function of the layer material. These are asphalt concrete, granular base, granular subbase, cementtreated, and bituminous base. Other materials such as.
The following relationship may be used in lieu of Figure 2. Variation in granular base layer coefficient a 2 with various base strength parameters 3l. Note, E", is a function of not only moisture but also the stress state 6. Values for the stressstate within the. Typical values for use in design are:.
Each agency is encouraged to develop relationships for their specific base materials e. Granulor Subbose Layers. Err - 0. Typical values for k, range from to , while k, varies from 0. As with the base layers, each agency is encouraged to develop relationships for their specific materials; however, in lieu of this data, the valuesin Table 2. Stress states 0 which can be used as a guide to select the rnodulus value for subbase thicknesses between 6 and 12 inches are as follows:. Guidance not providedhere is for any detailed drainage designsor construction methods.
Furthermore, criteria on the ability of various drainagemethodsto removemoisture from the pavement not provided. It is up to the design are to engineer identify what level or quality of drainage is achieved under a specificsetof drainage conditions.
Below are the generaldefinitions correspondingto different drainagelevelsfrom the pavement structure: Quality of Drainage Water Removed Within. Bituminous-Treated Bases.
Variation in granular subbase layer coefficient a3l with various subbase strength parameters Scale derived from correlations obtained from Texas. Variation in granular eubbase layer coefficiont. V a ri a ti o n i n a , fo r b i tu mi n o us-treated bases w i th base strength parameter 3. Flexibte Povements. The treatment for the expected level of drainage for a flexible pavement is through the use of modified layer coefficients e.
The factor for modifying the layer coefficient is referred to as an m, value and has been integrated into the structural number SN equation along with layer coefficient a, and thickness Di ; thus: The conversion of the structural number into actual pavement layer thicknessesis discussedin more detail in Chapter 3.
Obviously, the latter is dependent on the average yearly rainfall and the prevailing drainage conditions.
As a basisfor comparison, the m, value for. A discussion of how these recommended m, values were derived is presentedin Appendix DD of Volume 2. Finally, it is also important to note that thesevalues apply only to the effects of drainage on untreated base and subbase layers. Although improved drainage is certainly beneficial to stabihzed or treated materials, the effects on performance of flexible pavements are not as profound as those quantified in Table 2.
Rigid Pavemenls. The treatment for the expected level of drainage for a rigid pavement is through the use of a drainage coefficient, Co, in the performance equation. It has an effect similar to that of the load transfer coefficient, J. As before, the latter is dependent on the average yearly rainfall and the prevailing drainage conditions. A discussion of how these recommended Co values were derived is also presented in Appendix DD of Volume 2. Recommended m, values for modifying structural layer coefficients of untreated baseand subbase materials in flexible pavements.
Design Requiements Table 2.
Load transfer devices, ag9regate interlock, and the presenceof tied concrete shoulders all have an effect on this value. Generally, the J-value for a given set of conditions e.
As a general guide for the range of J-values, higher J's should be used with low k-values, high thermal coefficients, and large variations of temperature. Each agencyshould, however. As a generalguideline,the dowel diameter should be equal to the slab thicknessmultiplied by Vt inch e.
The dowel spacingand length are normally l2 inches and l8 inches, respectively. Jointed Povemenfs. The value of J recommended for a plain jointed pavement JCP orjointed reinforced concrete pavement JRCP with some type of load transfer device such as dowel bars at the joints is 3. This value is indicative of the load transfer of jointed pavementswithout tied concreteshoulders. Forjointed pavementswithout load transferdevices at the joints, a J-value of 3. This basicallyaccountsfor the higher bending stresses that develop in undowelled pavements, but also includes some consideration of the increasedpotential for faulting.
If the concrete has a high thermal On coefficient, then the value of J should be increased. Part I of this Guide provides some other generalcriteria for the consideration andlor design of expansionjoints, contraction joints, longitudinal joints, load transfer devices,and tie bars in jointed pavements.
Continuously Reintorced Psvemenfs. The value of J recommended for continuously reinforctcdc: Tied Shoulders or lTidened Outside Lanes. One of the major advantagesof using tied PCC shoulders or widened outside lanes is the reduction of slab stress and increasedservicelife they provide.
To account for this, significantly lower J-values may be used for the design of both jointed and continuous pavements. For continuously reinforced concrete pavements with tied concrete shoulders the minimum bar size and maximum tie bar spacing should be the same as that for tie bars between lanes , the range of J is between 2.
This value is considerably lower than that for the design of concrete pavementswithout tied shoulders becauseof the significantly increased load distribution capability of concrete pavementswith tied shoulders.
For jointed concrete pavements with dowels and v, tied shoulders, the value of J should be between 2. The lower J-value for tied shoulders assumes traffic is not permitted to run on the shoulder. Experience has shown that a concreteshoulder of 3 feet or greater may be considered a tied shoulder.
Pavements with monolrthic or tied curb and gutter that provides additional stiffness and keeps traffic away from the edge may be treated as a tied shoulder. Obviously, if various types of base or subbase are to be considered for design, then the corresponding values of LS should be determined for each type. A discussionof how the loss of support factor was derived is present in Appendix LL of Volume 2 of this Guide. The LS factor should also be considered in terms of differential vertical soil movements that may result in voids beneath the pavement.
Thus, even though a nonerosive subbaseis used, a void may still develop, thus reducing pavement life. Generally, for active swelling clays or excessive frost heave,LS valuesof 2. Each agency'sexperiencein this area should, however, be the key element in the sclection of an appropriate LS value. Write a review Rate this item: Preview this item Preview this item.
Washington, D. English View all editions and formats Rating: Subjects Pavements -- Design and construction. Pavements, Concrete -- Design and construction. Pavements -- Maintenance and repair.
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This can lead to invalid results at the least and incorrect results at the worst.
The rest of this section will discuss the specific assumptions, inputs and outputs associated with the AASHTO Guide flexible pavement empirical design equation. Variation in granular eubbase layer coefficiont.
Although improved drainage is certainly beneficial to stabihzed or treated materials, the effects on performance of flexible pavements are not as profound as those quantified in Table 2. The selection of longer time periods than can be achieved in the field will result in unrealistic designs.
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