Pavement Design - Civil Engineering

Pavement Design

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What will you learn? 

You will learn about types of pavement structure, asphalt pavement design method, rigid pavement design method

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What competency do you want to expect? 

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Knowledge competency: You will recognize types of pavement structure, asphalt pavement design method, rigid pavement design method. Skill competency: you are able to bridge communication between community and engineering service provider (road planner/designer/contractor) in regards with suitable pavement structure to support development of community and region according to cost availability and site condition

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Contents   

Types of pavement structures Asphalt pavement design method Rigid pavement design method

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Cross section of roman road

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Types of pavement: roman road pavement

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Types of pavement: early modern road pavement (Tresaguet, Telford, McAdam)

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Types of pavement: early modern road pavement (Tresaguet, Telford, McAdam)

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Types of pavement: modern road pavement

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Details of rigid pavement joints

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Types of rigid pavement (max. length of 6 m per slab)

(b) Jointed reinforced concrete pavement (JRCP) (max. length of 30 m per slab)

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Road construction on swamp and wetland area

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Haul road

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Asphalt pavement design method     

Load distribution in asphalt pavement Properties of flexible pavements Design objectives and constraints AASHTO, 1993 Design Method Pavement damage

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Load distribution in asphalt pavement

Asphalt pav. is designed to provide sufficient thickness to distribute the applied load with depth MANAGEMENT OF INFRASTRUCTURE AND COMMUNITY DEVELOPMENT

Properties of flexible pavements  

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Fast Deterioration with Time Repeated Loads Variable Load Configuration Variable Load Magnitude Variable Tyre Pressure Traffic Growth Change of Material Properties with Environmental Conditions Change of Subgrade Properties with Distance Channelized Traffic Load Multi-Layer System Unconventional Failure Definition

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Design objectives The objectives of pavement design can be listed as follow  Maximum economy, safety, and serviceability over the design period  Maximum or adequate load-carrying capacity in terms of load magnitude and repetitions  Minimum or limited deteriorations over the design period  Minimum or limited noise or air pollution during construction  Minimum or limited disruption of adjoining land use  Maximum or good aesthetics

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The constraints The pavement designer typically faces several economic, physical, and technical design constraints such as,  Availability of time and fund for design and construction  Minimum allowable level of serviceability before rehabilitation  Availability of materials  Minimum and maximum layer thickness  Capabilities of construction and maintenance personnel and equipment  Testing capabilities  Capabilities of structural and economic models available  Quality and extent of the design data available

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AASHTO, 1993 Design Method  

Equivalent Single Axle Load (ESAL) Design Procedure

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Equivalent Single Axle Load (ESAL) 

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Traffic loads applied on the pavement surface range from light passenger cars to heavy trucks. To design a pavement section the damage caused by all axle loads that will be applied on the pavement during its designed life has to be considered. Different magnitudes and different numbers of repetitions are converted to an equivalent number of repetitions of a standard axle load that causes the same damage to the pavement. A standard axle load was selected as 18000 Lb (80 kN) applied on a single axle with a dual wheel at each end. The ESAL is the equivalent number of repetitions of the 18-kip (80 kN) standard axle load that causes the same damage to the pavement caused by one-pass of the axle load in question. Equivalent Axle Load Factors (EALF) to relate the damage caused by different load magnitudes and axle configurations to the standard axle load as shown in Equation below

where Wt18 is the number of 18-kip (80-kN) single-axle load applications to time t (failure) and Wtx is the number of x-axle load applications to time t (failure).

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Equivalent Axle Load Factors, Single axles

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Equivalent Axle Load Factors, tandem axles

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Equivalent Axle Load Factors, triple axles

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Equations to calculate pavement thickness

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Lx is the load in kips on one single axle, one set of tandem axles, or one set of triple axles; L2 is the axle code (1 for single, 2 for tandem axles, and 3 for triple axles); pt is the terminal serviceability index b18 is the value of bx when Lx is equal to 18 and L2 is equal to one. SN is the structural numbers, which is an index that combines the effect of material properties, layer thicknesses and drainage quality

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ESAL at first day and cumulatif ESAL during design life 

ESAL at first day may be calculated as follow

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Cumulatif ESAL during design life can be calculated as follow

where  Ni is the number of repetitions of axle group i,  EALFi is the equivalency factor for axle group  m is number of axle groups  n is the designed life of the pavement in years  i is the expected annual traffic growth rate.

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Since the EALFs are not very sensitive to SN, a SN value of 5 may be assumed in most cases. Unless the design thickness is significantly different, no iterations will be needed

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Design Procedure  

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Step 1 — Reliability Step 2 — Overall Standard Deviation Step 3 — Cumulative Equivalent Single Axle Load Step 4 — Effective Roadbed Soil Resilient Modulus Step 5 — Resilient Moduli of Pavement Layers Step 6 — Serviceability Loss Step 7 — Structural Numbers Step 8 — Structural Layer Coefficients Step 9 — Drainage Coefficients Step 10 — Layer Thicknesses Step 11 — Freeze or Thaw and Swelling (additional) Step 12 — Life-Cycle Cost (additional)

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Step 1 — Reliability 

A reliability level (R) is selected depending on the functional classification of the road and whether the road is in urban or rural area. The reliability is the chance that pavement will last for the design period without failure. A larger reliability value will ensure better performance, but it will require larger layer thicknesses.

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Step 2 — Overall Standard Deviation 

The overall standard deviation ðSoÞ takes into consideration the variability of all input data. The 1993 design guide recommends an approximate range of 0.4 to 0.5 for flexile pavements

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Step 3 — Cumulative Equivalent Single Axle Load 

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In this step, the designer assumes a designed life, typically in the range of 10 to 20 years. The cumulative expected 18-kip (80-kN) ESAL (W18) during the designed life in the design lane is then determined as discussed earlier. If the cumulative two-directional 18-kip ESAL is known, the designer must factor the design traffic by directions by multiplying by the directional distribution factor (D) to get the ESAL in the predominate direction. For example, if the traffic split during the peak hour is 70 – 30%, D is taken as 0.7. To get the ESAL in the design (right) lane, the design traffic in the predominant direction is multiplied by the lane distribution factor (L)

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Step 4 — Effective Roadbed Soil Resilient Modulus

Worksheet for estimating effective roadbed soil resilient modulus

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Step 5 — Resilient Moduli of Pavement Layers 

The resilient moduli ðMRÞ of the surface, base, and subbase layers are either determined using laboratory testing or estimated using previously developed correlations

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Step 6 — Serviceability Loss 

The serviceability loss is the difference between the initial serviceability index (po) and the terminal serviceability index (pt). The typical Po value for a new pavement is 4.6 or 4.5. The recommended values of pt are 3.0, 2.5 or 2.0 for major roads, intermediate roads and secondary roads, respectively

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Step 7 — Structural Numbers 

The required structural number above the subgrade (SN3) is determined using following equation (that also can be described using the figure).

Figure 8.22 MANAGEMENT OF INFRASTRUCTURE AND COMMUNITY DEVELOPMENT

Step 8 — Structural Layer Coefficients 

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The structural layer coefficient is a measure of the relative ability of a unit thickness of a given material to function as a structural component of the pavement. Three structural layer coefficients (a1, a2 and a3) are required for the surface, base and subbase, respectively.

Chart for estimating structural layer coefficient of dense-graded asphalt concrete based on the elastic (resilient) modulus (a1)

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Chart for estimating structural layer coefficient of base course (a2)

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Chart for estimating structural layer coefficient of sub base (a3)

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Step 9 — Drainage Coefficients 

Drainage coefficients are measures of the quality of drainage and the availability of moistures in the granular base and subbase

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Step 10 — Layer Thicknesses 

Minimum Thickness (in.) (AASHTO, 1993)

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Sample Problem 1

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Solution

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Solution (continued)

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Solution (continued)

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Rigid pavement design method 

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Characteristics and Load Transmission of Rigid Pavements Considerations for Structural Design of Rigid Pavement Computation of Design Traffic Loading Material Properties for Design of Rigid AASHTO Procedure for Thickness Design of Concrete Pavement Reinforcement Design of Rigid Pavement Joints and Load Transfer Design

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Characteristics and Load Transmission of Rigid Pavements The rigid pavement relies on rigid slab action and designed to spread the load over a large area

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Considerations for Structural Design of Rigid Pavement (a) Determination of soil properties, design traffic loadings and environmental parameters (b) Selection of materials for various pavement layers (c) Structural thickness design of pavement layers (d) Drainage design for the pavement system (e) Safety and geometric design

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Computation of Design Traffic Loading The computation of design traffic loading involves the following steps: (a) Estimation of the initial year traffic volume and composition (b) Estimation of the annual traffic growth rate by vehicle type (c) Estimation of directional split of design traffic (d) Estimation of design lane traffic (e) Estimation of the magnitudes of wheel loads by vehicle type (f) Computation of the number of applications of wheel loads in the design lane 

Information concerning (a) and (b) can be obtained from traffic survey and forecast based on historical trends or prediction using transportation models.

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Computation of Design Traffic Loading Directional Split and Design Lane Traffic Loading

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Computation of Design Traffic Loading Traffic Loading Computation:  Structural analysis and design of pavement requires the knowledge of (a) the magnitudes of axle loads in the design traffic, and (b) the number of times each of these loads will be applied on the design lane during the design life of the pavement.  Two forms of field survey are required to obtain the required information from similar highway type within the same region. First, traffic count surveys must be conducted to determine the number of vehicle types in the design traffic. For pavement design, it is necessary to classify vehicles by size and axle configuration, such as cars, buses, single-unit trucks, and different types of multiple-unit trucks. Second, a survey to measure the axle or wheel loads of each vehicle type. Such axle or wheel load survey can be performed at weighing stations or using weigh-in-motion devices. Data collected from the two forms of survey enable one to compute the number of repetitions by axle type

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Vehicle Classification for Pavement Design

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Example of Axle Load Data for Pavement Design

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ESAL factor in rigid pavement design

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AASHTO Load Equivalency Factors for Rigid Pavements Based on Terminal Serviceability Index of 2.5 for Tandem Axles and pt of 2.5

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AASHTO Load Equivalency Factors for Rigid Pavements Based on Terminal Serviceability Index of 2.5 for Tandem Axles and pt of 2.5

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AASHTO Load Equivalency Factors for Rigid Pavements Based on Terminal Serviceability Index of 2.5Triple Axles (i.e., Tridem Axles) and pt of 2.5

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AASHTO Load Equivalency Factors for Rigid Pavements Based on Terminal Serviceability Index of 2.5Triple Axles (i.e., Tridem Axles) and pt of 2.5

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Sample problem 2

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Sample problem 3

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Formula for Computing Total Design Loading

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Sample problem 4

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Material Properties for Design of Rigid Pavement

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AASHTO Procedure for Thickness Design of Concrete Pavement: Procedure     

Reliability Pavement Material Properties Load Transfer Coefficient Drainage Coefficient Slab Thickness Requirement

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Reliability

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Chart for estimating composite k

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Chart for k as a function of bedrock depth

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Chart for estimating relative damage to rigid pavements

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Correction of effective modulus of subgrade reaction for potential loss of subbase support

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Values for Loss of Support Factor LS

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Sample problem 5 and 6 

Sample problem 5

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Sample problem 6

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Load Transfer Coefficient

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Drainage Coefficient

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Slab Thickness Requirement and sample problem 7

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Sample problem 7

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Rigid pavement thickness design chart

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Reinforcement Design of Rigid Pavement

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Reinforcement Design for JRCP

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Sample problem 8

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