AASHO Road Test Tandem Axleload Data Adapted to Fundamental Concepts

The reptitions of tandem axleloads applied to the AASHO Road Test were analyzed by the same method previously reported in analyzing the corresponding single axleload data. The essential findings were: 1. One repetition of a 34-kip (151-kN) tandem axleload appears to cause the same damage as one repetition of an 18-kip (80-kN) single axleload. 2. The relationship of log repetitions versus axleload used by Kentucky appear to be equally valid for single and tandem axleloads. 3. The use of superposition principles, and equivalency of repetitions, in combination with terminal serviceability indiced permitted analyses of the tandem axleload data and comparison with equivalent Kentucky thickness designs converted to the AASHTO structural number. 4. The single axleload analyses were combined with the tandem axleload analyses and superimposed by Kentucky equivalent designes. The Kentucky designs, based upon elastic theory, encompassed 89 percent of the AASHO Road Test data

A Pavement Design Schema

Elastic theory and 40 years of empirical flexible pavement design in Kentucky have been joined into the design system presented herein. A brief discussion is presented of the coupling mechanisms relating experience to theoretical analyses. An annotated design procedure is presented as a guide for pavement designers. Design nomographs account for a wide range of input parameters and permit the designer a wide choice of alternative thickness designs

Adaptation of AASHO Interim Guide to Fundamental Concepts

Using 1971 vehicle classification counts and truck weights from nine Kentucky locations, equivalent axleloads (EAL\u27s) were calculated by several methods. Apparent discrepancies led to a review of axleload equivalency factors used to estimate either EWL\u27s (equivalent wheel loads) or EAL\u27s. Axleload equivalencies are determined as the ratio of the number of repetitions of a standard or reference load to the number of equivalent (damage-wise) repetitions of the load in question. The choice of equivalency factors can result in as much as a 40-percent difference in calculated EAL\u27s. Most of Kentucky\u27s traffic is made up of axleloads less than 80 kilonewtons (18 kips). The 1973 Kentucky design guide axleload factors are more severe than either the 1959 Kentucky or 1972 AASHO Interim Guide factors for axleloads less than 80 kilonewtons (18 kips). An extensive effort has been made herein to explain these differences. The AASHO Road Test has provided an independent source of data

Design Guide for Bituminous Concrete Pavement Structures

The proposed design guide includes procedures for estimating design EAL\u27s, CBR, modulus of the asphaltic concrete, and an array of tables and graphs to determine pavement and layer thicknesses

Development of a Thickness Design System for Bituminous Concrete Pavements

A pavement provides a functional surface for safe operation of a vehicle. The operator or passenger of a vehicle does not particularly care about the material from which the pavement structure is constructed. However, they are sensitive to such factors as speed, safety (skid resistance), and comfort (roughness). One aspect of pavement design is the selection of the thickness of the pavement and its various components sufficient to support vehicular loadings and to transfer those loadings through successive layers of the pavement - surface, base, and subgrade - to the soil on which the pavement rests. The structural design of a highway pavement involves a study of the soils, paving materials, and their behavior under load. The pavement must be adequate to support the wheel loads of motor vehicles. Each time a vehicle passes over a pavement, some stressing and straining of the surface and underlying layers occur. If the load is excessive or if the supporting layers are not sufficiently strong, repeated applications of the vehicular loadings will cause rutting and cracking that ultimately lead to a complete structural failure of the pavement. The pavement thickness design scheme suggested in this report provides a procedure by which the load-carrying capabilities of any individual layer or of the soil upon which the pavement rests are not exceeded

Rational Analysis of Kentucky Flexible Pavement Design Criterion

Rational criteria for the structural design of pavements are emerging from classical theories equated to the observed behavior of real pavements. Pavement behavior is known to be affected by traffic, variations in soil support, and variations of component thicknesses. Considerable attention has been devoted to the mechanistic response of pavements to static and dynamic loads and to the development of theoretical design procedures, which rely, in part, on the computation of certain critical stresses, strains, and(or) deflections in the structure. A computer program (1) for the elastic analysis of multilayered pavement systems has enabled an extensive investigation of the effects of soil support properties, the materials used in the pavement structure, and component thicknesses. In this study, this computer program was used to determine the patterns of stresses, strains, and deflections of the pavement system. In the second portion of the study, attempts have been made to show the relationships between these stresses, strains, and deflections and current and proposed design curves using the fatigue concept (equivalent axleloads – EAL\u27s, or equivalent wheel loads – EWL\u27s). From the mechanistic point of view, load deflection relationships outwardly portray the composite stiffness or rigidity of pavement systems. Contrary to general impressions, surface deflection is not a discrete, limiting parameter. Stresses and strains in the subgrade soil and in the extreme fibers of the bituminous concrete layers may (do) constitute overriding, fundamental limits. Therefore, thickness design criteria cannot be based directly upon deflection spectra. In other words, two different pavements having equal, 18-kip deflections are not necessarily equal designs unless all accompanying stresses and strains are also equal

Analysis of Tandem Axleloads by Elastic Theory

AASHO Road Test tandem axleloads were analyzed to determine the magnitude of the tandem axleload that causes the same damage as the 18-kip (80-kN) single axleload. The procedure is outlined and is the same used to analyze single axleloads. The essential findings were as follows: 1. One repetition of a 34-kip ( 151-kN) tandem axleload appears to cause the same damage as one repetition of an 18-kip (80-kN) single axleload. 2. The relationships of log repetitions versus axleload used by Kentucky appear to be equally valid for single and tandem axleloads. 3. The use of superposition principles, and equivalency of repetitions, in combination with terminal serviceability indices permitted analyses of the tandem axleload data and comparison with equivalent Kentucky thickness designs converted to the AASHTO structural number. 4. Single axleload analyses were combined with the tandem axleload analyses and superimposed by Kentucky equivalent designs. The Kentucky designs, based upon elastic theory, enveloped 91 percent of the AASHO Road Test data. 5. For pavement design, the estimated fatigue for a 20-year design using the AASHTO damage factors will be reached in 16.2 years using the Kentucky damage factors. The Kentucky damage factors more closely approximate observed behavior than the AASHTO factors

Design Guide for Bituminous Concrete Pavement Structures

To determine pavement thicknesses from design charts and tables, it is necessary to know only the EAL\u27s (equivalent axle loads), the CBR of the subgrade soil, and the modulus of elasticity of the bituminous concrete. Charts permit selection of pavement structures employing alternative proportions of bituminous concrete and crushed stone base. Total thickness varies according to the proportions chosen. It is implicitly intended that the selection of alternative structures be based on engineering considerations, such as 1. estimates of comparative construction costs, 2. compatibility with cross section template and shoulder designs, 3. uniformity or standardization of design practices, 4. highway system classification, 5. engineering precedence, and 6. utilization of indigenous resources. Designs based on 33- and 67-percent proportions (thickness of pavement structure) of bituminous concrete and crushed rock base, respectively, conform with the current design chart (for high-type pavements) of the Kentucky Department of Transportation, representing conventional or precedential designs. The charts otherwise represent theoretical extensions of conventional designs and, from a theoretical standpoint, provide equally competent structures. Heretofore, the Kentucky design system was based on EWL\u27s (equivalent wheel loads). The proposed system is based on EAL\u27s. This transformation was made for the sake of unifying design practices and standardizing design terms. EAL\u27s are defined here as the cumulative number of equivalent 18-kip axleloads in the design lane. An approximate conversion is made by dividing EWL\u27s by 32 - that is, divide by 2 to reduce two-directional EWL\u27s to one direction and divide by 16 to convert from a 10-kip axleload (or 5-kip wheel load) to an 18-kip axleload. Normally, traffic volumes are estimated in connection with needs studies and in the planning stages for all new routes and for major improvements of existing routes. Whereas the anticipated volume of traffic is an important consideration in the geometric design of a roadway, composition of the traffic in terms of axle weights and lane distributions is essential to the structural design of pavements. Traffic volumes used for EAL computations should therefore be reconciled with other planning forecasts of traffic. Historically, actual growths, particularly in EAL\u27s, have exceeded forecasts in the majority of cases. Even though predictions of traffic volumes may be reasonable, estimates of EAL\u27s are also dependent upon predictions of vehicle types and loadings over the design life. Again, previous experience shows an underestimation of EAL\u27s due to inadequate predictions (or even the disregard of known overloads) of vehicle loadings. Thus, the design lives of the pavements may differ from the geometric design period. Computation of EAL\u27s involves an estimate of the total number of vehicles during the design life and multiplying factors for various vehicle types and loading configurations and magnitudes to convert traffic volumes to EAL\u27s. Ideally, yearly increments of EAL\u27s could be calculated and summed; this approach would permit consideration to be given to anticipated changes in legal weight limits, changes in styles of cargo haulers, and changes in routing

Fatigue Damage of Flexible Pavements Under Heavy Loads

A modified Chevron N-Layer computer program has the capability of calculating the work done by the total load on a given load group. Earlier analyses of AASHO Road Test sections and test vehicles had permitted the development of damage factor relationships. This paper presents seven, namely two-tire and four-tire single axles, tandems, triaxles, and four-axle, five-axle, and six-axle groups. The two-tire axle (front or steering axle) has the most severe damage relationship. The 80-kN (18-kip) four-tire single axle was used as the reference axle and was assigned a damage factor of 1.0 for a specific amount of work . Other axle arrangements and total loads producing that amount of Work were 63.6 kN (14.3 kips) for the two-tire axle, 166.4 kN (37.4 kips) for eight-tire tandems, 250.2 kN (56.25 kips) for twelve-tired triaxles, 333.6 kN (75.0 kips) for a sixteen-tired four-axle group, 415.0 kN (93.3 kips) for a twenty-tired five-axle group, and 496.4 kN (111.6 kips) for a twenty-four-tired six-axle group. Using the damage factors for the various axle groupings, one trip of a vehicle having a gross weight of 534 kN (120 kips) can produce up to approximately 17 times the damage of an 80-kN (18-kip) axleload, depending on the particular axle groupings involved. Equally as significant effects can be attributed to the distribution of loads on a given type of vehicle. For example, a 355.9-kN (80-kip) vehicle having 53.4 kN (12 kips) on the steering axle and 151.3 kN (34 kips) on each of two sets of tandem axles has an equivalent damage factor of 1.80 per trip. If the load distribution is changed to 40.0 kN (9 kips) on the steering axle and 157.9 kN (35.5 kips) on each of two sets of tandem axles, the total damage factor per trip is reduced to 1.76. Other configurations and various ranges of loads are presented, evaluated in terms of damage per trip, and discussed