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Oct/24/2018. Highway Engineering II CENG 4183. Lecture one.

Chapter One: Introduction 1.1. General 1.2. Overview of Pavement Structures 1.2.1. Flexible Pavements 1.2.2. Rigid Pavements 1.3. Basic Design Data.

1.1. General Flexible Pavement Layers Surface course Wearing course (Tack coat b/n binder and wearing course) Binder course Prime coat b/n base and binder course) Base course Sub base Capping layer (optional) Subgrade (existing soil).

1.2. Overview of Pavement Structures The basic idea in building a pavement for all-weather use by vehicles is to prepare a suitable sub grade, provide necessary drainage and construct a pavement that will: Have sufficient total thickness and internal strength to carry expected traffic loads, and distribute them over the sub grade soil without over stressing. Have adequate properties to prevent or minimize the penetration or internal accumulation of moisture, and Have a surface that is reasonably smooth and skid resistant at the same time, as well as reasonably resistant to wear, distortion and deterioration by vehicle loads and weather.

Cont.…………… The functional requirements of pavements are achieved through careful considerations of: Selection of pavement type, Selection of materials to be used for various pavement layers and treatment of subgrade soils, Structural thickness design for pavement layers, Subsurface drainage design for the pavement system, Surface drainage and geometric design, and Ridability of pavement surface..

Earth road Designed for a very low traffic Used when there is economic limitation The natural sub grade soil can be made to carry the traffic load after clearing and shaping Require reshaping after seasonal changes..

Cont.…………… Pavements are generally classified into two Categories, Flexible pavements Rigid pavements. The basis for classification is the way by which traffic loads are transmitted to the subgrade soil through the pavement structure. Flexible pavements Sufficient thickness for load distribution through a multilayer structure, It has low flexural strength and the load is largely transmitted to the sub grade soil through the lateral distribution of stresses with increasing depth. The stresses & strains in the sub-grade soil layers are within the required limits. The strength of subgrade soil would have a direct bearing on the total thickness of the flexible pavement..

Vertical stress. Foundation stress. Cont.…………… The pavement thickness is designed such that stresses on the subgrade soil are kept within its bearing capacity and the sub grade is prevented from excessive deformation. Its structural strength and smoothness depends to a large extent on the deformation of the subgrade soil Designed to take advantage of the decreasing magnitude of stresses with depth in a pavements.

Advantages to Flexible Pavement Adjusts to limited differential settlement Easily repaired (maintained) Additional thickness added any time Non-skid properties do not deteriorate Quieter and smoother Tolerates a greater range of temperatures Disadvantages to Flexible Pavement Loses some flexibility and cohesion with time Needs resurfacing sooner than PC concrete Not normally chosen where water is expected.

Generally, two types of construction have been used for flexible pavements, Conventional flexible pavement and Full-depth asphalt pavement..

Surface course: - The surface course is the top course of an asphalt pavement,(i.e. sometimes called wearing course. It is usually constructed by dense graded hot mix asphalt (HMA). It is a structural part of the pavement, which must be tough to resist distortion under traffic and provide a smooth and skid-resistant riding surface. The surface must be water proof to protect the entire pavement and subgrade from the weakening effect of water..

Cont.……………. Sub-base course: - It is the layer of material beneath the base course constructed using local and cheaper materials for economic reason on top of the subgrade. It provides additional help to the base-course and the upper in distributing the load. It facilitates drainage of free water that might get accumulated below the pavement. If the base course is open graded, the sub base course with more fines can serve as a filter between the sub grade and the base course. Sub-grade: - It is the foundation on which the vehicle load and the weight of the pavement layers finally rest. It is an in situ or a layer of selected material compacted to the properly drained desirable and compacted to receive the pavement layers..

[image] Transv Joint Longitudinal— Joint Surface Course Base Course (Optional) Subbase Course (Optional) Subgrade (Existing Soil).

Rigid Pavements Are constructed of cement concrete slabs A rigid pavement, by virtue of its rigidity, can be able to effect a slab action to spread the wheel load over the entire slab area. The structural capacity of the rigid pavement is largely provided by the slab itself. The effect of subgrade soil properties on the thickness of rigid pavement is less important than that of flexible pavement..

Cont.…………… Rigid Pavement Typical Applications High volume traffic lanes Freeway to freeway connections Exit ramps with heavy traffic Advantages of Rigid Pavement Good durability Long service life Withstand repeated flooding and subsurface water without Deterioration Disadvantages of Rigid Pavement May lose non-skid surface with time Needs even sub-grade with uniform settling May fault at transverse joints It may not be visible at night Noisy at night (when vehicles are passing) Rigid pavements The subgrade may provide a uniform support for the slab. However where the sub grade soil cannot provide a uniform support, there is always a necessity to build a base or Sub base course under cement concrete slab.

Main reasons for providing base-course under cement concrete slab are: Control of pumping Control of frost action (it is not common in Ethiopia) Improvement of drainage Control of shrinkage and swell As working platform for construction Concrete pavements can be classified in to four types: Jointed Plain Concrete Pavement (JPCP), Jointed Reinforced Concrete Pavement (JRCP), Continuous Reinforced Concrete Pavement (CRCP), and Pre-stressed Concrete Pavement (PCP). Comparison of Rigid and Flexible pavements, The following main differences between rigid and flexible pavements can be cited. The manner in which vehicle loads are transmitted to subgrade Design life and precision (The quality of being reproducible) Maintenance requirements Initial cost Suitability for stage construction Surface characteristics Permeability and Traffic dislocation during construction..

Highway and Airport Pavement Airport pavements are generally Thicker than highway pavements and require better surfacing materials due to: The gross-weight of an airplane is usually greater than that of a heavy truck, but the number of load repetitions on airport pavements is usually smaller than that on highway pavements. The arrangement and spacing of wheel loads on airport and highway pavements are different. A typical tire pressure on highway pavements is much lesser than that of airport pavements Vehicle loads are applied near to the edge of highway pavements but far away from the outside edge of airport pavements. Unlike highway pavements, airfield pavements are subjected to an impact loading. The design load of airport pavements is the wheel load of the largest aircraft During takeoff time due to heavy fuel weight. Although wheel loads can be used as design loads, number of repetitions of standard axles is the commonly used design parameter for highway pavements..

1.3 Basic Design Factors Design factors can be divided into four broad categories: Traffic loading, Environment, Materials and Failure criteria. Traffic loading The loading applied by traffic is one of the major factors affecting the design and performance of pavements. The most important aspects of traffic loading that are considered in the analysis and design of pavements are: The configuration, Magnitude and Repetitions of axle loads.

Environment The environmental factors that influence pavement design include Temperature and Precipitation. Different standards of pavement design consider the effects of these factors in various ways. Softens AC in cold weather and hardens in hot weather Materials Pavement materials include soils, aggregates, bituminous binders and cement. The properties of these materials under traffic loading in a given environmental conditions is fundamental for the proper design of pavement structures. Moreover, if economically constructed facilities are to be obtained, locally available materials are to be used efficiently. Materials Properties Stiffness, Poisson Ratio, Strength, Durability Permeability Thermal-Volumetric Properties.

Types of Material Behavior Elastic -recoverable, Plastic -non-recoverable, Viscous -non-recoverable, Combination Visco-elastic –recoverable Performance and Failure Criteria Pavements are normally designed and constructed to provide, during the design life, a riding quality acceptable for both private and commercial vehicles with acceptable maintenance. Distress Types in Flexible Pavements * Longitudinal * Transversal crack.

* Fatigue cracking *Block cracking * De-bonding (adjacent layers of HMA lose adhesion to one another and can become separated).

[image] original profile weak asphalt layer shear plane blSUG.

* Stripping * Bleeding * Corrugation & Shoving. [image].

* Ravelling (The progressive disintegration of an HMA layer from the surface downward ) * Potholes (potholes are the end result of fatigue cracking. As alligator cracking becomes severe……………..).

Thickness Design Material Design Construction Practices & Quality Control.

Thank you.