CE 445 Pavement Management
Notes developed spring 2000
Text: Pavement Management for Local Governments by Federal Highway Administration, 1985
Lecture 1 Introduction to Pavement Management
(Handout-Chap 1,2, and 3 of manual)
Pavement Management System
FHWA – Defines PMS as a set of tools or methods that can assist decision makers in funding cost effective strategies for providing, evaluating and maintaining pavements in a serviceable condition.
AASHTO – Guidelines for Pavement Management Systems.
Should include:
Inventory
Assessment of conditions
Needs determination
Prioritization
Budget development
Program
Feedback
Objective of PMS
Determine extent and usage of the road system
No miles
Volume
ESALS
% trucks
Accidents
Analyze current network conditions
By political division
By condition vs % system
Develop historical data
Determine condition vs time curve for the road segments. Useful in making future predictions
Analyze funding options
What is the affect of the decisions related to maintenance
Assist in marketing the road program
Approach
Network - includes all roads in a political entity. The type of approach considered in this course
Project - Determines the best solution for a given project determined from the network analysis
Chap I – Historical Methods
Roads and streets are major investments for most communities. They must be repaired, rehabilitated and reconstructed in a timely and cost effective manner.
Examples of management methods used by road agencies
Historical – Same as last year plus some increase for inflation
Last Year X (1+ adjustment%/100)
Time based with estimated life - determine how long between a maintenance activity and cost per sqyd and calculate the amount of and costs for each year of the specific activity. For example for a 100 mile system
Seal Coat every 3 years at a cost of $.50 per sqyd is
Resurface every 10 years at a cost of $2.00 per sqyd
Reconstruct every 50 years at a cost of $10.00 per sqyd.
For a total annual budget of
Emergency expenditures - sets aside a constant amount that the engineer estimates will be adequate to make emergency repairs to the road system. Some time this is linked with the deferred maintenance strategy
Political – Everyone gets a share based on their clout. May be partially based on past activities and distributing the available resource to all the citizens of the political unit.
Experience of the manager - A knowledgeable and experienced manager may have the figures developed in an informal manner and uses this number as the budget value. Not easily defended and is not very likely in the present engineering community.
A PMS approach would consider
When is the best time to do maintenance
What is the best maintenance strategy to use and when should it be used
What is the effect of no maintenance or deferred maintenance
From this curve it can be seen that if the road condition is fair it may be able to return the condition to good by doing a seal coat at a cost of $0.50 per sqyd compared to doing a reconstruction at a cost of $2.00 per sqyd.
Another example is shown below where the present worth of the strategies can be evaluated
No maintenance
Preventive maintenance
Obviously the present worth of the cost associated with the preventive maintenance is lower and is the preferred option.
The above graphs show the importance of timing and maintenance strategy. The only problem is to define the pavement life curves and that is one of the functions of the PMS program.
Lecture 2 Introduction Continued
Benefits of PMS
Documents existing and future conditions
Plans can be quantified
Alternatives can be compared on some constant basis
Citizens can be involved in the process and it is less mysterious
Show video related to PMS = 72,74,76 from LTAP
We have reviewed several elements related to PMS including the videos and several are starting to reoccur including
Maintenance activities
What and when
Budget – life cycle costing
Inventory and data base determination
Assessment techniques
One other consideration is in terms of
Project level PMS (explain)
Network level - the one we will work with
Lecture 3 Review of Roadway Cross Section Elements
Roadway X section elements (Chap 2) – related to network analysis.
Roadway elements
X-section
Pavement Type
Flexible
Rigid
Aggregate
Dimensions
Traveled way
Shoulders
Cross slopes
Sidewalks, etc
Drainage – key to good roads
Transverse
Cross slopes
Fore and back slopes
Catch basins
Longitudinal
Ditches
Side
Dimension
False
Curb and gutter
Under drains
Base and sub base
Edge drains
Traffic elements
Total count (AADT)
% trucks
ESAL
Directional split
Lecture 4 Pavement Design Related to Project PMS
(See Chap 3 of the handout)
Many options are available for the engineer designing pavements that are new or being rehabilitated. Generally the design process is slightly different for flexible and rigid pavements. Flexible pavements are designed using the AASHTO Structural Number concept while rigid pavements use the AASHTO depth determination nomographs or analysis from beam on elastic foundation methods.
Design is a function of
Traffic - usually in terms of ESALs
Materials - CBR, Mr, etc
Environment - seasonal frost, rainfall, drainage conditions etc.
Design Methods
AASHTO method of design -Guide for Design of Pavement Structures, 1993. Covers both flexible and rigid as well as aggregate surfaces. There are computer programs that do the solutions given the input.
Design standards as developed by SHAs such as MDOT.
Asphalt Institute design method for flexible pavements.
PCA method for concrete pavements
COE for government projects
Overlay design is based on the additional thickness required for future traffic. AASHTO has a procedure
Pavement selection - based on life cycle analysis and should include fixed cost, annual maintenance, rehabilitation costs, etc. Life cycle costs are also a function of interest rate, inflation and other costs associated with money.
Lecture 5 Maintenance Techniques - Network PMS
Generally maintenance is considered in terms of a set of general activities with different specific activities related to each surface type.
Asphalt
Routine-pothole repair, crack and joint sealing and leveling
Preventive-surface treatments, fog coats, sand seals and seal coats
Rehabilitation-resurfacing and some improvement to drainage or improvement of strength. Usually includes a structural improvement.
Reconstruction-reworking the grade including earthwork and resurfacing.
Aggregate
Routine- blading, pot hole repair
Preventive- ditch cleaning, spot aggregate, shaping
Rehabilitation- scarify, and compact, add aggregate.
Reconstruct-geometric changes, safety improvements, surface improvements
Concrete
Routine- crack sealing, repair of blow outs
Preventive- clean and seal joints
Rehabilitation- slab replacement (50%), structural overlay
Reconstruction- geometric and safety improvements, replace pavements, drainage improvements
Lecture 6 Inventory (Chap 6)
The inventory is the data collection phase of the PMS. It may be the most important as all other parts of the PMS are related to the data collected in this phase. This should include
Dimensions of surface, shoulders and sidewalks
Materials
X-section components
History
Drainage
Section number and identification
Traffic count, etc.
One of the most important elements of the inventory phase is the determination of the section boundaries. Or how the roads are divided into manageable sections. There are several ways this is done. The basic requirement is that each section has consistent properties in the section. Section boundaries may be determined by
Change in number of lanes
Change in pavement type
Change in pavement structure
Change in drainage
Traffic volume
Political subdivision
Street intersections
Topographic features
Constructions sections
At a maximum the section is less then 1 mile in length and the resulting data base will be a function of the number of sections. A problem is how to treat the intersection of two streets. Possible solutions are:
Treat intersection as a separate section
Include intersection in a preferred section such as N-S.
Collected Data
Section description- include name and to and from. May be by latitude and longitude by state wide coordinates or by mile post.
Functional classification
AASHTO basis
County designation
State trunk line designation
NHS
STP
Act 51 designation
state trunk line
county primary
county local
city major
city minor
Pavement structure- include type and depth if available.
History and record of maintenance
Cost data related to maintenance activities
Traffic
Parking
Volume
Geometric
Curvature
Vertical alignment
Drainage
Ditch and internal if used
Comments are usually included on the inventory
Several examples of data collection sheets are given in Chap 6 Figs 6-8,-9 and -10. Data is collected in order of importance from the arterial to the local access.
Lecture 7 Condition Assessment Chap 7
Before any decisions can be made related to maintenance activities it is necessary to determine the condition of the existing facilities. The condition can be assessed several ways but usually consider:
Structural capacity- load carrying capability of the pavement section.
Ride quality - the most sensitive to public opinion
Skid resistance - related to safety of the road section
Distress surveys - relates condition to observable conditions of the pavement.
What ever method is selected to monitor it is necessary to have a program of regular measurements in order to get a detailed impression of the progress of the condition
The curve developed by periodic determination of the conditions can be used to develop a model that can be used to predict the behavior of similar pavement sections.
the coefficients a, b. c and d are determined by statistical analysis and the independent variables S, RC, SR and DISS are measurable conditions related to the pavement condition PC. In the above form it suggests linear analysis other methods are available.
The pavement condition Vs time curve can also be used to show the effects of maintenance
Determine results of change in maintenance activities
Track performance of strategies
The time of observations related to condition determines the expense related to the program as the measurement of the various conditions is expensive and labor intensive. Typical frequencies of assessment are
Every year
Every two years ( many states use this time period)
Every five years
Several types of assessment are available
Direct
Roughness testing
Structural testing
Skid resistance
Indirect
Distress
Some of the methods are high tech and some are rather primitive.
Lecture 8 Data Collection Methods and Equipment
There are many methods used to collect data for condition assessment. Some are specific to a particular condition and others are integrated systems. In the following each condition is treated separately.
Friction Measurement
One of the important elements of a road related to safety if the ability to develop friction between the tires and the roadway. The lower the friction the more likely that accidents will occur. The measurement of friction is basic and only requires some equipment that will apply a normal force and measure the shear force required to overcome the friction. The equation is
Fr is the measured shear force, N is the normal force and f is the friction factor. Generally, the higher the friction factor the better the safety characteristics of the road. Typical methods of measuring the shear force are
Locked wheel trailer device - ASTM E274-85
This device takes a lot of time to collect the data and is fairly expensive although it is well recognized and used.
Mu-Meter ASTM E 670-87
A trailer that uses the friction relation ship to determine the friction factor. Wheels on the trailer are oriented at an angle of 71/2 o to the direction of travel. The friction force is measured by a transducer between the trailer and truck.
Laboratory method - British portable tester ASTM E 303
A pendulum device that measures friction between the surface and the pad.
We have this device. The problem is the value determined is good for relative measurements between surfaces but it does not relate directly to the friction factor observed on pavements.
Roughness Measurement
Roughness is the variations in the surface along and transverse to the roadway. It is the primary criteria by which the traveling public judges the quality of a road. In addition roughness can lead to accelerated wear because wheel loads that are displaced can lead to harmonic forces that lead to more distortions.
Measuring the distortions can be done in several ways.
Transverse
Rod and level - a traditional survey method. Takes a long time to do but accuracy is good.
Dip stick - a device that is "walked " across the road and measures the incremental changes in vertical elevation. This method is also slow but accurate
Road roughness meter - records dynamic response of vehicle as it is driven along the roadway. Requires electronic equipment
Longitudinal
Slope variance method - AASHTO method that measures the slope change as a standard beam ( 25.5 ft) is towed along the profile of the road. Measurements are made at a regular basis and the slope variance is calculated.
XI is the measured variance at 1 ft intervals. The speed of the forward movement is 5 mph = 7.33 fps resulting in a measurement every 0.14 sec
Note that this value is the slope variance used in the AASHTO design equation for flexible and concrete roads.
SV is the slope variance, C and P are cracks, and RD is the rut depth value
This method is slow and with the advent of better electronics other method are used that are related to the response of the vehicle to inertia forces and measured with accelerometers. The relationship develops as follows
u(x) is the height of the vehicle above the pavement, u(x) is the vertical position at any point in time and is developed from the basic definitions
This requires some electronic instrument that will collect the data and perform the integration. For example the
South Dakota Profiler
Law Profiler
Others being developed and in use by various agencies
Transverse measurements
Rut depth based on horizontal measurements
Where the rut depth is calculated using the equation
This is a measure of the rate of change in the slope of the road with higher values indicating deeper rut depth. Measurement may be made at small increments of distance in the x direction and the equation then is a summation
Other electronic devices are available and are becoming more useable and inexpensive. Also emerging is the area of laser technology and it is being used to measure rut depth very accurately.
An example of an integrated vehicle that does most of the measurements listed above is the ARAN
ARAN® is a multi-functional data collection vehicle which gathers highway information while travelling at highway speeds. Videotape of the highway, ditches, and abutting properties is collected and maintained by the Pavement Management Section. In addition to the video, physical properties of the pavement surface are also collected. The data gathered is analyzed to assign a Pavement Condition Rating (PCR), predict future deterioration, and make recommendations on where
pavement expenditures should be made.
Structural Evaluation
Evaluation of pavement strength is difficult but there are many methods that are being used including destructive and nondestructive.
Nondestructive Methods usually related to the defection basin developed during a loading or impulse.
Static - Plate bearing AASHTO T 222-81 is used to determine the modulus of subgrade reaction. This value is determined as
y is the deflection under the 10 psi loading
The test is time consuming and disruptive to traffic.
Benkleman Beam - ASTM
Uses a swinging beam suspended under a vehicle where the deflections are measured as the vehicle slowly moves forward. A tradition method but hard to relate results to a soil property
Vibratory
Dynaflect Road Rater- uses a set of geophones set between the wheels of a test trailer. A dropped weight produces a response in the soils that is measured by the geophones and from the response it is possible to calculate soil properties that are related to the strength.
FWD - the falling weight defectometer is a very popular method of determining the soil properties from accelerometers placed on the roadway
Spectral Analysis of Surface Waves - used by some researchers but not too popular at present
Destructive Methods
Samples of the roadway materials are taken and the strength of the materials are evaluated in the laboratory using conventional triaxial tests or resilient modulus tests
Whatever method is used to evaluate the structural adequacy of the materials they should produce a result that can be used in some statistical evaluation of the condition of the road and can be combined with other values to produce an overall rating of the roadway
pavement condition is a function of friction(FR), rut depth(RD), structure strength(ST) and profile(P). For this linear model it is possible to add other independent variables as they develop.
Lecture 9 Distress Measurement
Many of the PMS procedures that are used at the local level need an assessment technique that is cheap and easy to do. The usual process to satisfy this requirement is a distress survey. The survey is conducted by making visual observations of the distresses and relating the magnitude, severity and extent to a maintenance strategy.