Design Parameters
The inputs every pavement design starts from: the subgrade, the traffic loads and the environment, with the tests and equations used to put a number on each.
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Traffic and loads
Loads
One of the primary functions of a pavement is load distribution. Therefore, in order to adequately design a pavement something must be known about the expected loads it will encounter.
Equivalent Single Axle Load
Although it is not too difficult to determine a wheel or an axle load for an individual vehicle, it becomes quite complicated to determine the number and types of wheel/axle loads that a particular pavement will be subject to over its…
Flexible Pavement ESAL Equation
At first glance, this equation looks quite complex - it is.
AASHTO Vehicle Definitions
Trucks and Buses
Based on typical ESAL equivalency factors and the fourth power law it is clear that heavy vehicles cause a majority of pavement structure damage (with the notable exception of studded tire wear).
Weigh-in-Motion
Weigh-in-motion (WIM) devices are designed to capture and record truck axle weights and gross vehicle weights as they drive over a sensor.
Additional Tire Information
Figure 1 shows the following tire terminology:
Studded Tire Information
Tire studs are small metal protrusions inserted into winter tires to improve tire-road friction in snow or ice conditions.
Design Life
Pavements are typically designed for a specified "design life". Design life (or "design period") is the time from original construction to a terminal condition for a pavement structure.
Subgrade and materials
Subgrade
The "subgrade" is the in situ material upon which the pavement structure is placed.
Stiffness and Strength Tests
Subgrade materials are typically characterized by their resistance to deformation under load, which can be either a measure of their strength (the stress needed to break or rupture a material) or stiffness (the relationship between stress…
Elastic Modulus
Elastic modulus is sometimes called Young's modulus after Thomas Young who published the concept back in 1807.
Resilient Modulus
The Resilient Modulus (M_{R}) is a measure of subgrade material stiffness. A material's resilient modulus is actually an estimate of its modulus of elasticity (E).
California Bearing Ratio
The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%).
Resistance Value
The Resistance Value (R-value) test is a material stiffness test. The test procedure expresses a material's resistance to deformation as a function of the ratio of transmitted lateral pressure to applied vertical pressure.
Modulus of Subgrade Reaction
The modulus of subgrade reaction (k) is used as a primary input for rigid pavement design. It estimates the support of the layers below a rigid pavement surface course (the PCC slab).
Plate Load Test
The plate load test (see Figure 1 and 2) can be used to measure the modulus of subgrade reaction for in situ material. The test involves pressing a steel bearing plate into the surface to be measured with a hydraulic jack.
Shrinking and Swelling Soils
Some soils will change volume significantly depending upon their moisture content.
Triaxial Test
In a triaxial resilient modulus test a repeated axial cyclic stress of fixed magnitude, load duration and cyclic duration is applied to a cylindrical test specimen.
Environment, frost and drainage
Environment
A pavement must be able to function within the environment in which it is built. The environment can vary greatly across the globe at any one time and it can also vary greatly across time at any one place.
Temperature Variations
Extreme temperature variations can causes severe pavement damage due to expansion, contraction and (in the case of rigid pavements) slab curling. Additionally, asphalt binder rheology varies with temperature.
Frost Action
Frost action can be quite detrimental to pavements and refers to two separate but related processes:
Calculation of Frost Depth
Some equations and variables in this section are expressed in English units only due to their source.
Stefan Formula
Some of the first studies of freeze/thaw depth were made by Josef Stefan in 1889, in connection with ice formation and melting in the Polar oceans (Paynter, not date given).
Modified Berggren Formula
The modified Berggren formula was developed in the early 1950s to address the shortcomings of the Stefan formula.
Frost Action Mitigation
The following sections describe the more common methods for dealing with frost action:
Capillary Rise
Capillary rise is the rise in a liquid above the level of zero pressure due to a net upward force produced by the attraction of the water molecules to a solid surface (e.g., soil or glass).
Drainage
Proper drainage is important to ensure a high quality long lived pavement; moisture accumulation in any pavement structural layer can cause problems.
Surface Drainage
Surface drainage is concerned with removing all water that is present on the pavement surface, shoulder surface or any other surface from which it may flow onto the pavement.
Subsurface Drainage
Subsurface drainage is concerned with removing water that percolates through or is contained in the underlying subgrade.