Structural Design
The goal of structural design is to determine the number, material composition and thickness of the different layers within a pavement structure required to accommodate a given loading regime. This includes the surface course as well as any underlying base or subbase layers. This module is focused on the structural design of new pavement.
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Concepts
Pavement Structure
HMA pavements are flexible pavements. Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.
Methods
Empirical Pavement Design
An empirical approach is one which is based on the results of experiments or experience. Generally, it requires a number of observations to be made in order to ascertain the relationships between input variables and outcomes.
Mechanistic-Empirical Pavement Design
Mechanics is the science of motion and the action of forces on bodies. Thus, a mechanistic approach seeks to explain phenomena only by reference to physical causes.
AASHO Road Test
The AASHO Road Test, a $27 million (1960 dollars) investment and the largest road experiment of its time, was conceived and sponsored by the American Association of State Highway Officials (AASHO) as a study of the performance of highway…
The AASHTO Reliability Concept
"The reliability of the pavement design-performance process is the probability that a pavement section designed using the process will perform satisfactorily over the traffic and environmental conditions for the design period." (AASHTO,…
Poisson's Ratio
An important material property used in elastic analysis of pavement systems is Poisson's ratio. Poisson’s ratio is defined as the ratio of transverse to longitudinal strains of a loaded specimen. This concept is illustrated in Figure 1.
FHWA Bridge Formula
Since an individual set of bridge design computations cannot be done for every type of truck that may use a particular highway, bridge designers use a standard vehicle, the HS20-44, for estimating critical stresses, strains, or…
Flexible (asphalt) pavements
1993 AASHTO Flexible Pavement Structural Design
Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance). This article presents the 1993 AASHTO Guide basic design equation for flexible pavements.
Structural Number
The Structural Number (SN) represents the overall structural requirement needed to sustain the design's traffic loadings.
Flexible Pavement Empirical Design Example
Design a new flexible pavement for a major interstate highway using the following conditions (four lanes each direction):
Flexible Pavement Response
How a pavement responds to applied stresses determines how it will behave structurally. Stresses and the resultant pavement response are the combined result of loading, environment, subgrade and pavement material characteristics.
Flexible Pavement Mechanistic Models
Mechanistic models are used to mathematically model pavement physics.
A Mechanistic Computer Program
PAVEXpress also covers Mechanistic Pavement Design! https://www.pavexpress.com
Flexible M-E Failure Criteria
The main empirical portions of the mechanistic-empirical design process are the equations used to compute the number of loading cycles to failure.
Failure Criteria Example
Results from a mechanistic pavement analysis are compared to empirically derived failure models to determine pavement life (in terms of loads to failure) and the controlling failure mechanism.
Perpetual Pavements
"Perpetual Pavement" is a term used to describe a long-lasting structural design, construction and maintenance concept for HMA pavements.
Rigid (concrete) pavements
1993 AASHTO Rigid Pavement Structural Design
Empirical equations are used to relate observed or measurable phenomena with outcomes.
Rigid Pavement ESAL Equation
At first glance, this equation looks quite complex - it is.
Rigid Pavement Empirical Design Example
Design a new rigid pavement for a major interstate highway using the following conditions (four lanes each direction):
Rigid Pavement Response
Rigid pavements respond to loading in a variety of ways that affect performance (both initial and long-term). The three principal responses are:
Rigid Pavement Response Calculations
The following examples illustrate how rigid pavement responses to different stresses are calculated.
Rigid Mechanistic Models
Mechanistic models are used to mathematically model pavement physics.
Rigid M-E Failure Criteria
The main empirical portions of the mechanistic-empirical design process are the equations used to compute the number of loading cycles to failure.
Rigid Pavement Truck Types
There are two basic truck types used for mix transport:
Joint Design
Joints, which are integral to JPCP and JRCP, and also necessary in CRCP, must be designed to minimize slab cracking, joint deflection, joint stresses and roughness as well as accommodate the intended joint sealant.
Reinforcing Steel Design
In CRCP and JRCP, reinforcing steel is used to hold tightly together any cracks that may form. Cracks formation depends upon temperature, moisture and base material friction. As the slab cools and loses moisture, it will contract.
PPRC
The Partnered Pavement Research Center (PPRC) is a multiyear contract with the California Department of Transportation (Caltrans). It involves the University of California Pavement Research Center (UCPRC) together with Dynatest and CSIR.
Evaluating existing pavements
Backcalculation
"Backcalculation" is a mechanistic evaluation of pavement surface deflection basins generated by various pavement deflection devices.