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chapter4_notes_ - Chapter 4 Pavement Design 1.1...

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Chapter 4 Pavement Design 1.1 INTRODUCTION Over 3 million miles (4.8 million kilometers) of highways in US (55% are paved). Several states (Pennsylvania, Texas, Illinois, and California) have pavement construction and rehabilitation budgets that exceed a billion dollars per year 1.2 PAVEMENT TYPES Why pavements? Typical soil-bearing capacities can be less than 50 lb/in 2 (345 kPa) and in some cases as low as 2 to 3 lb/in 2 (14 to 21 kPa). Typical automobile weighs approximately 2700 lb (12 kN), with tire pressures of 35 lb/in 2 (241 kPa). Typical tractor semi-trailer truck that can weigh up to 80,000 lb (355.8 kN), the legal limit in many states, on five axles with tire pressures of 100 lb/in 2 (690 kPa) or higher. 1.2.1 Flexible Pavements Figure 4.1 Typical flexible-pavement cross section. Components: Top layer made of asphaltic concrete, which is a mixture of asphalt cement and aggregates. The purpose of the wearing layer: Protect the base layer from wheel abrasion and to waterproof the entire pavement structure. Provides a skid-resistant surface for vehicle performance. Other layers (Fig. 4.1) – thicknesses vary with the type of axle loading, available materials, and expected pavement design life. 1.2.2 Rigid Pavements Rigid pavement is constructed with Portland cement concrete (PCC) and aggregates: Figure 4.2 Typical rigid-pavement cross section. 1.3 PAVEMENT SYSTEM DESIGN: PRINCIPLES FOR FLEXIBLE PAVEMENTS Assumed load distribution: Figure 4.3 Distribution of load on a flexible pavement.
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1.3.1 Calculation of Flexible Pavement Stresses and Deflections Boussinesq theory started by assuming that the pavement is one layer thick and the material is elastic, homogeneous, and isotropic. With point load: Figure 4.4 Point load on a one-layer pavement. The basic equation for the stress at a point in the system is 2 z P K = z σ (4.1) Where: z = stress at a point in lb/in 2 (kPa), P = wheel load in lb (N), z = depth of the point in question in inches (mm), and K = variable defined as () [] 2 / 5 2 / 1 1 2 3 z r π K + = (4.2) Where: r = radial distance in inches from the centerline of the point load to the point in question. More realistic approach to assume an elliptical area that represents a tire footprint instead of a point load. The tire footprint can be defined by an equivalent circular area with a radius calculated by: π p P = a (4.3) Where: a = equivalent load radius of the tire footprint in inches, P = tire load in lb, and p = tire pressure in lb/in 2 . Ahlvin and Ulery provided solutions for the evaluation of stresses, strains, and deflections at any point. The Ahlvin and Ulery equation for the calculation of vertical stress, is ( ) B A p = z + (4.4) Where: z = vertical stress in lb/in 2 , p = pressure due to the load in lb/in 2 ,
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A and B = function values, as presented in Table 4.1, that depend on z/a and r/a , the depth in radii and offset distance in radii, respectively, Where: z = depth of the point in question in inches (mm), r = radial distance in inches (mm) from the centerline of the point load to the point in question, and a = equivalent load radius of the tire footprint in inches (mm).
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This document was uploaded on 03/15/2012.

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chapter4_notes_ - Chapter 4 Pavement Design 1.1...

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