KLK583 Evaluation of Skid Resistance of Idaho Pavements at Different Speeds
ITD Research Project 266; Task Order Number UI-17-03
Principal Investigators
- Emad Kassem
- Richard Nielsen
ITD Project Manager
- James Poorbaugh
FHWA Project Advisor
- Kyle Holman
Student Researchers
- —
Research Problem Statement
ITD manages about 12,000 lane miles annually. Maintaining this road network at an adequate level of skid resistance is a key component of road safety. Skid resistance decreases significantly in wet conditions where water acts as a lubricant between pavement surface and tires. ITD pavement engineers use the measured skid numbers to determine if a treatment should be applied to improve surface friction. The current practice at ITD is to measure the skid number using a Dynatest 1295 Locked Wheel Friction Testing Trailer with a smooth tire. The left wheel of the friction testing trailer is locked to measure the skid number at 40 mph. In many cases the friction number is measured at lower speeds due to state speed limits, geometry of roads and size of skid truck. It is well established that the skid number decreases with speed. In addition, several interstates in Idaho have a speed limit up to 80 mph while the skid number is still measured at 40 mph; potentially unsafe for the operators and for the motoring public. Therefore there is a need to investigate and develop a correlation between skid numbers at lower speeds and higher speeds. This study will investigate and develop a correlation between skid numbers collected at speeds from 20 mph to 60 mph and the relative number at 40 mph. Such correlations will be developed based on comprehensive field testing locations in many districts and cover pavements with different surfaces (e.g., concrete, asphalt, and other road design mixes) and traffic levels. Pavement engineers can use such correlations to determine the skid number at the desired speed and the need for surface treatment.
Project Objectives
ITD measures the skid number at lower speeds from 20-39 mph when speed limits, geometry of roads and size of skid truck does not permit collecting friction measurements at 40 mph. Collecting friction data at lower speeds does yield higher skid numbers than actual values. Although the data is collected at lower speeds necessary for safe collection, the data cannot be used in confidence for roadways with higher speed limits. ITD engineers need accurate friction measurements at all collection speeds in order to determine the need for surface treatment. A correlation between skid numbers at lower and higher speeds will be very useful for ITD engineers to determine if a given pavement has an adequate level of skid resistance and if such pavement needs a corrective action to improve surface friction.
The main objective of this study is to study and establish correlations between skid numbers measured at different speeds (lower and higher than 40 mph) and the reference skid number measured at 40 mph. Such correlations shall be developed based on comprehensive evaluation of surface characteristics of various pavement test sections. These correlations shall enable ITD to determine accurate friction numbers and the need for surface treatment to improve the skid resistance.
Project Tasks
The goal of this project will be achieved by executing a number of tasks. Firstly, the research team will conduct a literature search to review and document the current performance tests used by various states to evaluate the resistance of asphalt mixtures to rutting, cracking, and moisture damage. The research will identify the most promising tests to evaluate the above mentioned distresses based on the reported performance in the literature. Secondly, the research team will identify test sections in Idaho with different resistance to rutting, cracking, and moisture damage. Thirdly, the team, with the help of ITD, will recover field cores as well as laboratory-prepared samples. Fifthly, the team will compare the laboratory results to field performance and identify the tests that provide close performance to field observations. Sixthly, the team will develop performance specifications for each test selected to evaluate various distresses (rutting, cracking, moisture damage). Finally, the team will prepare a final report with the proposed performance tests and their specifications in ITD formats. The project tasks are outlined below.
Task 1: Literature Review
Under this task, the team will conduct a thorough review to document the current tests used by various transportation agencies to review cracking, rutting, and moisture damage of asphalt mixtures. In addition, the team will review the specification limits set by different states for these tests. The literature review will include, but not be limited to:
- Cracking tests: several tests are currently used to evaluate the resistance to asphalt mixtures to cracking including semi-circular bending test (SCB), Texas overlay, bending beam fatigue test, direct tension cyclic fatigue test, indirect tension (IDT), and asphalt pavement analyzer (APA) Junior (Jr) fatigue test.
- Rutting tests: various tests are used to asses the rutting of asphalt mixtures. These tests include Hamburg wheel tracking, APA and APA Jr, and dynamic repeated load tests.
- Moisture damage tests: one of the early methods used to evaluate the resistance of asphalt mixtures to moisture damage was developed by Dr. Robert Lottman at the university of Idaho. This test is used by pavement community worldwide with some modifications. Other tests used to evaluate moisture susceptibility of asphalt mixture include Hamburg wet test and APA Jr.
The outcome of the literature review will be identifying the most promising laboratory tests that will be conducted in Task 4 to evaluate the resistance of asphalt mixes to the above mentioned distresses. Researchers will work closely with ITD's project Technical Advisory Committee to select performance tests to be reviewed. many factors shall be considered when selecting the performance tests including their applicability to simulate field performance, simplicity of the test, applicability to Idaho conditions, availability of the equipment at ITD laboratories, and being cost- and time-effective.
Task 2: Identify and select pavement sites for evaluation
The research team will work with ITD district material engineers to identify and select candidate pavement sites across Idaho for evaluation. Several criteria will be considered when selecting the test sections. These criteria include:
- Distress type and severity: pavement test sections that manifest various distresses (cracking {fatigue and thermal], rutting, moisture damage) and other sections with good performance and without major distresses will be identified.
- Mixture design: asphalt pavements with different mixture design shall be included. The number of test sections at various mix design shall be decided by the research team and ITD Technical Advisory Committee and based on the frequent use of a given mix design on highways.
- Construction materials: asphalt pavements made of different binder grades and sources as well as aggregates shall be considered.
- Pavement structure and support: asphalt pavements with different structural design (layer thickness) as well as base and subgrade strength shall be considered. Pavement distresses are not only related to poor asphalt courses but in many cases they are related to poor (insufficient) structural support.
- Environmental conditions: the test sections shall be distributed across the state and sites from all six districts will be included in this study. The environmental conditions have a great impact on the performance of asphalt mixtures and the type and degree of severity of various distresses.
- Traffic level: interstate highways, US highways, and state highways subjected to different traffic levels shall be considered.
- Service life: asphalt pavements sites with different ages shall be included. Pavement sites with more than five years in service are recommended so that pavements had enough time to develop distresses.
Task 3: Conduct field performance evaluation and collect cores and virgin materials
Under this task, the research team with help from ITD will obtain field cores and collect virgin materials (binders and aggregates) from the selected sites under Task 2. Field Cores will be extracted from the wheel path and shoulder. In case there is no shoulder, field cores between wheel path will be obtained. It is expected that some field cores collected from the wheel path will be distressed compared to cores from the shoulder or between wheel path while no or less traffic applied, respectively. In addition, the research team will obtain the mix design of the selected sites, The team will document the distress type and its severity as well as obtain the mix design of the selected sites. The team will document the distress type and its severity as well as possible causes for such distress. The results of field performance evaluation will be compared to laboratory performance evaluation under task 5. The following information will be obtained for selected sites:
- Project Location
- Date of construction
- Pavement structure (layer thickness) and material properties
- Traffic volume
- Historical performance data such as rutting and cracking
- Maintenance and rehabilitation history
- Climate information
- Ground water table level
Task 4: Conduct laboratory performance tests
Laboratory asphalt mixture test samples will be prepared using the virgin materials in accordance with the mixture design collected in Task 3. Part of laboratory test samples will be subjected to long-term aging to simulate the conditions of asphalt mixtures in the pavement sites. Studies at Texas A&M showed that placing asphalt mixture test samples in an environmental chamber at 60°C for a few months (about three months) simulate several years of asphalt pavement aging tin the field. Previous studies showed that fatigue cracking and moisture damage may increase as the asphalt mixture ages in the field. A comprehensive laboratory testing program will be conducted on both laboratory test samples and recovered field cores. The selected tests under Task 1 will be conducted to evaluate the resistance of asphalt mixtures to cracking (fatigue and thermal), permanent deformation or rutting, and moisture damage. At least three different tests will be evaluated to study the resistance of asphalt mixtures to various distress modes (fatigue and thermal cracking, rutting, and moisture damage). At this time and based on the limited literature review performed by the research team during the course of the preparation of this task order, the following tests are thought:
- Cracking tests: direct tension cyclic fatigue test, semi-circular bend (SCB), indirect tension (IDT) may be conducted to evaluate the resistance of asphalt mixture to fatigue and thermal cracking.
- Rutting tests: Hamburg wheel tracking or AOA Jr, dynamic repeated load tests, and flow number and flow time test may be conducted to evaluate the resistance of asphalt mixtures to rutting.
- Moisture damage tests: modified Lottman test, IDT, and direct tension cyclic, and Hamburg wheel tracking or APA Jr in wet conditions may be used to study the moisture susceptibility test samples.
Furthermore, under Task 4, the research team is planning to conduct inter-laboratory experiments to compare the repeatability and variability of different performance tests conducted at the university of Idaho and other ITD laboratories. It is envisioned that the inter-laboratory experiments will include performance tests (rutting, cracking, and moisture damage) for three different mixtures. At least three replicates will be evaluated in each test.
Task 5: Comprehensive evaluation of laboratory and field performance data
The results of the laboratory testing program conducted under Task 4 will be analyzed and compared to the performance evaluation in the field. Statistical analysis methods will be used to analyze the test results and evaluate the significance of the effect of different variables (e.g. traffic level, aggregate gradation, aggregate type, binder grade, etc.) on the performance test results. The expected outcome of this task shall enable us to:
- Evaluate the applicability of various performance tests to asses the resistance of asphalt mixtures to rutting, cracking, and moisture damage, in addition to compare the results to field observations. The evolution of various test methods shall cover the simplicity of the test, availability of test devices in a typical asphalt laboratory, need for skilled staff, and required time and cost of the test.
- Identify the most promising and applicable test(s) that provide close performance prediction to field observation for each pavement distresses (rutting, cracking, and moisture damage).
Task 6: Develop performance test specifications
Based on the findings of Task 5, the research team will develop performance specifications for each test selected to evaluate the resistance of asphalt mixtures to cracking, rutting, and moisture damage. These limits or specifications shall be augmented to the mix design used by ITD. The proposed test method(s) will be prepared in ITD standard test formats. In addition, the researchers will organize a workshop to demonstrate and train ITD personnel on the proposed test methods and seek feedback from ITD engineers.
Task 7: Prepare the final report
The research team will prepare a comprehensive final report at the end of the study to document the research methodology, present the results and analysis, summarize the findings, and provide recommendations and propose plans for future studies and implementation. In addition, the final report shall include the proposed test methods to evaluate the performance of asphalt mixtures in ITD formats. The report will be edited by a professional editor and reviewed by an external expert.
Project Communication Schedule
The research team will prepare the typical monthly progress report (ITD Form 771). There will be frequent — at least quarterly- meetings with the project technical advisory committee from ITD to discuss progress. Depending on time and TAC availability, these meetings could be held in person, face-to-face or via video conferencing. As a minimum, the research team will have three face-to-face meetings; kick-off meeting, progress meeting (after 1 year), close-out meeting (toward the end of the project).
Needs and Requirements
Like other projects with ITD, this is a cooperative effort between ITD and U of I. The research team will seek help and support of ITD to execute the project tasks. Specific needs are:
- Identification and selection of field test sections as per Task 2.
- Measuring the skid number at different speeds for the selected test sections as per Task 3 and Task 6.
- Providing skid performance data from TAMS database and/or other record that enable the team to study the skid history of the selected test sections.
Required Outputs/Deliverables
Required project output will be mainly:
- An Excel-based application that can utilize ITD friction data, once collected, to produce interpolated numbers at 40 mph from within the collection speeds prior to TAMS uploading.
- Statistical correlations between skid numbers at low and high speeds to the relative speed of 40 mph for different pavement types (flexible and rigid).
- Final report.
There are six deliverables as outlined below:
Deliverable No. 1: Literature review
A technical memorandum that summarizes the literature review conducted under Task 1.
Deliverable No. 2: Test section selection
A technical memorandum will summarize the candidate test sections selected by the research team and ITD under Task 2.
Deliverable No. 3: Preliminary results of skid and friction characteristics of the test sections
A technical memorandum shall report the preliminary skid and friction characteristics of the test sections.
Deliverable No. 4: Final results of skid and friction characteristics of the test sections
A technical memorandum shall report the final results of skid and friction characteristics of the test sections. This memorandum will also include preliminary correlations of skid resistance at different speeds.
Deliverable No. 5: Draft report
The draft report shall document the research methodology, present the results and analysis, summarize the findings, and provide recommendations and propose plans for future studies and implementation.
Deliverable No. 6: Final report
The final report shall address the ITD comments on the draft report as well as comments from the external reviewer.
Implementation
ITD engineers can use the developed correlations to determine the skid number at the desired speed and if a surface treatment is required to improve the skid resistance. ITD may change their practice of collecting friction measurements based on the findings of this study (i.e. specify different speeds). The Project Sponsor, Project Manager, and TAC members will meet with the researchers to discuss study recommendations as part of the report review process. The researchers will assist the ITD Project Sponsor, Project Manager, and Technical Advisory Committee in developing a plan to implement study recommendations as appropriate. The plan will identify the steps needed to implement recommendations requiring action, a schedule for implementation activities, and the individuals responsible for specific implementation steps.
Expected Outcomes/Savings to ITD
The outcomes of this project will improve safety and thus reduce the number of accidents and save lives. Operators of the friction testing unit will be able to collect relevant data at speeds that would not impede traffic flow. In regards to process efficiencies, there is a sizable amount of data that is currently collected, but is not valid at speeds under 40 mph, the agency stands to gain much more applicable data for the systems model if researched.
Monthly Progress Reports (ITD 771) and Task Progress Charts
Project status
Active
Draft report
Due April 2019
Final report
Due June 2019