Institute for Transportation

About the research

This project outlines the development and implementation phase of chip seal performance specification for Oregon. Two chip seal workshops were held to involve chip seal stakeholders including a variety of Oregon Department of Transportation (ODOT) personnel and contractors. A voluntary survey about the potential specification changes was developed and results are presented. A series of specification meetings were held with ODOT and industry to re–evaluate the specification and potential changes line–by–line. The specifications developed as a result of the specification meetings are presented and specification guidance is provided. Flow charts were developed aid in specification processes and action items. This report also contains follow–up work from research in SPR 777 (https://www.oregon.gov/ODOT/Programs/ResearchDocuments/SPR777_ChipSeal.pdf). Chip seal design and a design spreadsheet are discussed. Finally, the Oregon DOT conducted a demonstration project under the new specification. The details and results of the demonstration project are presented.

About the research

This project collected and analyzed event based vehicle detection data from multiple technologies at four different sites across Oregon to provide guidance for deployment of non-invasive detection for use in adaptive control, as well as develop a true life cycle cost comparison of various detection sources. Data during fair weather from non-invasive detection zones co-located with inductive loops were compared under based upon typical metrics used for driving adaptive algorithms, such as activations, occupancy, and time to gap out. From this analysis, it was recommended that non-invasive detection sources should be used with caution for developing data for adaptive control, as the inherent nature of their operation differs from inductive loops. From a life cycle cost standpoint, non-invasive units were shown to be much more expensive than inductive detection over the typical lifespan, but modest efficiency and safety improvements possible due to expanded features of the non-invasive units may overcome this cost deficit. Lastly, due to performance issues encountered with over 50% of the inductive loop detectors within this study, it is highly recommended to implement some type of continual detector health monitoring program regardless of technology selected for deployment.

Subcontractor to Northern Arizona University on this project.

About the research

Recently, the Oregon Department of Transportation (ODOT) has identified hot mix asphalt concrete (HMAC) pavements that have displayed top-down cracking within three years of construction. The objective of the study was to evaluate the top-down cracked pavement sections and compare the results with the non-cracked pavement sections. Research involved evaluating six surface cracked pavements and four non-cracked pavement sections. The research included extensive field and laboratory investigations of the 10 pavement sections by conducting distress surveys, falling weight deflectometer (FWD) testing, dynamic cone penetrometer (DCP) testing, and coring from the cracked and non-cracked pavement sections. Cores were then subjected to a full laboratory-testing program to evaluate the HMAC mixtures and binder rheology. The laboratory investigation included dynamic modulus, indirect tensile (IDT) strength, and specific gravity testing on the HMAC cores, binder rheological tests on asphalt binder and aggregate gradation analysis. The FWD and DCP tests indicated that top-down cracked pavement sections were structurally sound, even some of the sections with top-down cracking showed better structural capacity compared to non-cracked sections. The study also found that top-down cracking initiation and propagation were independent of pavement cross-section or the HMAC thickness. The dynamic modulus testing indicated that cores from all the t op-down cracked pavement sections except one section (OR 140) possessed stiffer mixtures than that of non-cracked pavement sections. All four non-cracked pavement areas were found to be exhibiting fairly high IDT strength, and low variability in IDT strength and HMAC density when compared to top-down cracked sections as indicated by the IDT strength tests and air void analysis. Asphalt binder rheological test result indicated that asphalt binders from all the top-down cracked sections except OR140 showed higher complex shear modulus (stiffer binder) compared to non-cracked pavement sections. The study concluded that top-down cracking could be caused by a number of contributors such as stiffer HMAC mixtures, mixture segregation, binder aging, low HMAC tensile strength, and high variability in tensile strength or by combination of any.

About the research

The Oregon Department of Transportation (ODOT) is in the process of implementing the recently introduced AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) for new pavement sections. The majority of pavement work conducted by ODOT involves rehabilitation of existing pavements. Hot mix asphalt (HMA) overlays are preferred for both flexible and rigid pavements. However, HMA overlays are susceptible to fatigue cracking (alligator and longitudinal cracking), rutting, and thermal cracking. This study conducted work to calibrate the design process for rehabilitation of existing pavement structures. Forty-four pavement sections throughout Oregon were included. A detailed comparison of predictive and measured distresses was made using MEPDG software Darwin M-E (Version 1.1). It was found that Darwin M-E predictive distresses did not accurately reflect measured distresses, calling for a local calibration of performance prediction models. Darwin M-E over predicted total rutting compared to the measured total rutting and most of the rutting predicted by Darwin M-E occurs in the subgrade. For alligator (bottom-up) and thermal cracking, Darwin M-E underestimated the amount of cracking considerably as compared to in-field measurements. A high amount of variability between predicted and measured values was observed for longitudinal (top-down) cracking. The performance (punch-out) model was also assessed for continuously reinforced concrete pavement (CRCP) using Darwin M-E’s default (nationally calibrated) coefficients.

Four distress prediction models (rutting, alligator, longitudinal, and thermal cracking) of the HMA overlays were calibrated for Oregon conditions. It was found that the locally calibrated models for rutting, alligator, and longitudinal cracking provided better predictions with lower bias and standard error than the nationally (default) calibrated models. However, there was a high degree of variability between the predicted and measured distresses, especially for longitudinal and transverse cracking, even after the calibration. It is believed that there is a significant lack-of-fit modeling error for the occurrence of longitudinal cracks. The Darwin M-E calibrated models of rutting and alligator cracking can be implemented, however, it is recommended that additional sites be established and included in the future calibration efforts to improve the accuracy of the prediction models.

About the research

Chip seals or seal coats, are a pavement preservation method constructed using a layer of asphalt binder that is covered by a uniformly graded aggregate. The benefits of chip seal include: sealing surface cracks, keeping water from penetrating the surface, provides an anti-glare surface, minimizes the effect of aging as it seals the pavement surface, provides a highly skid-resistant surface, and is cost effective.

This study summarizes performance and the methodology used for developing specifications and a rational chip seal design in Oregon. Test sections included both emulsified asphalt and hot-applied chip seal applications. The pre- and post-construction pavement performance information is presented and analyzed. Post-construction analysis of the chip seals includes macrotexture analysis, dynamic friction testing to measure microtexture, and pavement performance surveys. The underlying pavement conditions were classified from being very good to very poor performance. In this study, a comparison of field performance on test section is developed to recommend best practices and develop a rational design methodology. A comparative study between the application of McLeod method and New Zealand method is performed to evaluate the best chip seal design methodology for adoption into the State chip seal specifications. The results will also determine if the macro-texture based New Zealand chip seal performance specification is applicable for Oregon chip seals.