Fig. 3-1: Spread of wheel-load through pavement structure
Pavements constructed of hot-mix asphalt are typically designed based on traffic projections over a 20 year period. Yet despite our best efforts, it is not uncommon to see severe rutting and cracking in asphalt pavements well before then as environmental conditions and heavy traffic loadings take their toll. The result: rougher rides, higher user costs, higher pavement maintenance and rehabilitation costs, and more work zones for motorists to negotiate. In order to provide a pavement which will serve its intended purpose for a reasonable time at a reasonable costs, Engineers must utilize proper design procedures based on projected traffic over the design period and consider the environmental conditions, subgrade strength, material properties and other factors that will allow construction of a pavement that will perform satisfactorily. When designing and building a road for all-weather use by vehicles, the basic objectives are to:
(a) Have sufficient total thickness and internal strength to carry
expected traffic loads.
(b) Prevent the penetration and/or internal accumulation of moisture,
and
(c) Have a top surface that is smooth, skid resistant, and resistant
to wear, distortion and deterioration by traffic, weather and deicing chemicals.
The subgrade ultimately carries all traffic loads; therefore, the structural function of a pavement is to support a wheel load on the pavement surface and transfer and spread that load to the subgrade, without over-taxing either the strength of the subgrade or the internal strength of the pavement itself. Figure 3-1 shows the wheel load being transmitted to the pavement surface through the tire. The pavement then spreads the wheel load to the subgrade which reduces the maximum pressure on the subgrade. Figure 3-2 shows how a wheel load, W, slightly deflects the pavement structure, causing both tensile and compressive stresses within the pavement. By proper selection of pavement materials and with adequate pavement thickness and strength, the pressure at the bottom of the pavement will be small enough to be easily supported by the subgrade and the pavement will be able to resist the internal stresses caused by the loading.
Fig. 3-1: Spread of wheel-load through pavement structure
In determining pavement thickness, the following factors are considered:
(a) Traffic - The amount of traffic predicted to use the facility. The number of trucks predicted to use the highway is particularly important, as one pass of a tractor trailer truck equals approximately 4,000 passenger vehicles,
(b) Soil Support Value - The soil subgrade strength, i.e., the type of soil of which the subgrade is composed - sand, clay, silt, etc,
(c) Regional Factor- Accounts for the effect of various climatic conditions. For instance, the effect and number of freeze-thaw cycles in the mountain region will require a thicker pavement structure than the milder climate in the eastern part of the state, and
(d) Strength and other influencing characteristics of the materials available or chosen for the layers or courses in the total asphalt pavement structure.
(e) Economics
From these factors, a structural number is calculated. The structural number is an index number derived from an analysis of traffic, soil conditions, and regional factors. This number is used to determine the thickness of the total pavement and the thickness of the various layers. The following is a relative comparison of strength of various asphalt mix layers to each other and to other base types:
Fig. 3-2: Pavement deflection results in tensile and compressive
stresses
in pavement structure
An asphalt pavement structure consists of all courses or layers above the prepared subgrade or foundation. The upper or top layer(s) is the asphalt surface course. The surface course(s) may range from less than an inch (25mm) to several inches (mm) in thickness. The surface course is a high density layer designed to prevent penetration or internal accumulation of moisture. It is also designed to be skid resistant, resistant to wear, distortion, and deterioration by traffic, weather and deicing chemicals and is made using a relatively small maximum size aggregate. The layer placed immediately below the surface course is the intermediate course. The intermediate course is a high density material and is made using a slightly larger maximum size aggregate. When the expected traffic is very high or other conditions dictate, an asphalt base course may be utilized. The base is also a high density material and is made using an even larger maximum size aggregate. The base is the structural strength element of a pavement. Its main purpose is to distribute traffic wheel loads over the subgrade and, therefore, is almost entirely designed for that purpose. Base course mixes can be constructed in relatively thick layers at a reasonable cost due to the large aggregate size and therefore, require a lower asphalt binder content. The thickness of the base course is usually dependent upon the overall strength requirements for a particular pavement based upon the anticipated traffic loading.
Listed in the following table are the Superpave mix types. The first
letter of the mix type designation indicates the type of mix (Surface,
Intermediate
and Base), the number indicates the nominal aggregate size in millimeters,
and the letter at the end indicates the level of traffic loading (measured
in ESALs) which the mix is design to carry with satisfactory performance.
As an example, an S 12.5 C mix is a surface mix with a nominal maximum
aggregate size of 12.5 mm and a design loading of 3 to 30 million
ESALs and will be produced using a PG 70-22 asphalt binder.
3.3 PAVEMENT LAYER DEPTH GUIDELINES (FOR PAVEMENT DESIGN PURPOSES)*
A. Single Lift Depths (Minimum – Maximum)**
B. Maximum Layer Total Depths**
3.4 SUPERPAVE APPLICATION RATES OF SPREAD PER INCH (MM) DEPTH
Note: Always refer to the contract and/or typical sections for the specified rate and approximate depth to be placed.
3.5 TYPICAL SUPERPAVE SINGLE LAYER DEPTHS / RATES
*Approximate Minimum Thickness
(See Typical Section Pg. 3.7 in Printed version of 2007 QMS Manual)
3.6 TYPICAL ASPHALT BINDER CONTENTS*
* By Weight of Total Mix
The following are layer coefficients used in designing and building a pavement structure to the required structural number. These coefficients are per inch (mm) depth for the appropriate type mix or base.
