Asphalt Concrete Overlays Research Articles

Importance of Invasive Measures in Assessment of Existing Pavements

Countless miles of aged interstate highway pavement in the United States and significant expenditures associated with reconstruction highlight the importance of cost-effective rehabilitation measures. However, the prescription of ill-posed rehabilitation strategies due to improper assessment can be costly. This paper details a forensic investigation of a 22.5-km (14-mi) stretch of interstate highway in Oklahoma, consisting of multiple asphalt concrete (AC) overlays accumulated over a 40-year period. A thorough nondestructive investigation was carried out using falling weight deflectometer testing and ground penetrating radar. This was followed by a detailed invasive investigation involving coring, drilling and sampling, laboratory testing, and trenching. The pavement profile deduced from nondestructive test results alone failed to reveal a significantly weakened subsurface AC layer that was clearly revealed during invasive testing. Mechanistic analysis of the perceived pavement and actual pavement profiles reveal a significant difference in fatigue life. The reliance on nondestructive testing alone for pavement analysis and rehabilitation design would have been in significant error.

Evaluation of Accuracy of Surface Profilers

The Texas Department of Transportation (TxDOT) is implementing smoothness specifications based on profilograph testing as part of its construction quality control/quality assurance (QC/QA) program. Automated California-type profilographs are now used in most tests, in which the equipment is pushed over a prescribed wheelpath. It appears that smoothness specifications will continue to be based on the profilograph, at least for the short term. However, in view of advances in profiling technology, it becomes prudent to investigate other methods of measuring surface profiles and to develop smoothness specifications based on profilers that offer greater accuracy and higher production rates. In pursuit of its goal of providing smooth pavements, TxDOT initiated a research project with the Texas Transportation Institute to develop a smoothness specification for asphalt concrete overlays based on the new generation of pavement profilers, which offer greater accuracy in profile measurement relative to the profilographs now used in construction projects. Among other things, this research project evaluated a number of profile-measuring devices to establish the availability of equipment for implementing a new profile-based smoothness specification in Texas. This evaluation showed that lightweight profilers provide a basis for developing and implementing smoothness specifications that are based on surface profile. Since surface smoothness is commonly monitored using inertial profilers in pavement condition surveys conducted for pavement management, having the initial profile allows highway agencies to tie the as-built smoothness to the rest of the performance history and thus maintain a consistent historical record of surface smoothness throughout the pavement life cycle.

International Roughness Index of Asphalt Concrete Overlays: Analysis of Data from Long-Term Pavement Performance Program SPS-5 Projects

The Specific Pavement Studies 5 (SPS-5) experiment in the Long-Term Pavement Performance (LTPP) program was developed to investigate the performance of selected asphalt concrete rehabilitation treatment factors. Results of an analysis are presented that was conducted to compare the international roughness index (IRI) values of the test sections before and after overlay and to compare the IRI values that were obtained on the test sections subjected to different treatments. The data presented will be useful for highway agencies to gain an insight into the typical IRI values that can be expected when overlays are placed on asphalt concrete pavements and to obtain information on the reduction in roughness from an asphalt concrete overlay. The analysis indicated that the IRI of a pavement after overlay was not a function of the IRI before overlay. It was observed that a 50-mm-thick overlay (placed in one lift) was capable of achieving similar IRI values as a 125-mm-thick overlay (placed in two lifts). No differences in IRI values were obtained on overlays that were placed on pavement surfaces milled before the overlay when compared with surfaces not milled before the overlay. A frequency analysis of the IRI values after overlay indicated that in 85 percent of the cases, the IRI of the pavement was less than 1.2 m/km for both 50-mm- and 125-mm-thick asphalt concrete overlays.

Bayesian methodologies for evaluating rutting in Nova Scotia's Special B asphalt concrete overlays

In Nova Scotia, wheel path rutting (plastic deformation) is becoming one of the most significant manifestations of pavement distress on arterial 100 series highways. To address plastic deformation, the Nova Scotia Department of Transportation and Communications (NSDOTC) introduced, in 1989, a high friction asphalt concrete mix commonly referred to as Special B. In this study, an empirical model is developed to evaluate rutting in Special B using Bayesian regression software and methods. The Bayesian methodology allows for the inclusion of subjective data, referred to as prior data, from experienced engineers, as well as field data in its modelling approach. The model resulting from the data is referred to as the posterior. For this study a team consisting of members of NSDOTC and the Technical University of Nova Scotia and consultants from private industry was formed to transfer the technology of the Bayesian C-SHRP project. The field data were gathered from a data base of C-SHRP test sections in Nova Scotia. Five expert pavement engineers were solicited to provide the subjective data. Linear and linear-log functional forms were investigated for this project.

Applicability of New Flexible Pavement Smoothness Specification for Asphalt Overlays

The Texas Department of Transportation plans to implement smoothness specifications based on profilograph testing for all asphalt concrete paving projects as part of its construction quality control/quality assurance program. Smoothness specifications have been developed for newly constructed asphalt and portland cement concrete pavements. In a move to develop a similar specification for asphalt concrete overlays, the Texas Transportation Institute was asked to evaluate the applicability of the existing flexible pavement smoothness specifications for the quality control and quality assurance of pavement rideability on overlay construction projects. This study was intended to provide the state with the information necessary to evaluate the improvement in ride quality that may be expected from placement of asphalt overlays, particularly for thin overlays [38 to 51 mm (1.5 to 2 in.)], which are generally constructed in Texas. To collect data to determine whether the existing specification can be implemented for overlay construction work, TTI monitored a number of overlay projects during the 1994 calendar year. Profilograph measurements were made during these projects before any surface preparation, after surface preparation, and after placement of the asphalt concrete overlay. All of the projects monitored involved thin overlays. The data collected indicate that the existing smoothness specifications can be implemented on overlay projects, provided that appropriate surface preparations (e.g., milling, level-ups) are conducted to correct existing surface distresses before placement of the overlay.

Bayesian methodologies for evaluating rutting in Nova Scotia's Special B asphalt concrete overlays

The increase of truck traffic on Nova Scotia's 100 series highways has resulted in an increase of wheel path rutting. To remedy this problem, in 1989 the Nova Scotia Department of Transportation and Communications (NSDOT&C) introduced a high friction Type B asphalt concrete (AC) mix, more commonly referred to as Special B. In this paper, an empirical model is developed to evaluate the performance of Special B as a rut resistant AC mix. The model is developed using the Bayesian regression software and methods developed by the Canadian Strategic Highway Research Program (C-SHRP).

Effect of Initial Pavement Smoothness on Future Smoothness and Pavement Life

Initial pavement smoothness has an effect on the future smoothness and life of asphalt concrete (AC) and portland cement concrete (PCC), as well as AC overlays of existing AC and PCC pavements. Initial pavement smoothness has a significant effect in 80 percent of new construction (both AC and PCC pavements) and in 70 percent of AC-overlay construction. Furthermore, using two different analysis techniques, added pavement life can be obtained by achieving higher levels of initial smoothness. Combined results of roughness-model and pavement-failure analyses indicate at least a 9 percent increase in life corresponding to a 25 percent increase in smoothness from target profile index (PI) values of 7 and 5 in./mi (0.11 and 0.08 m/km) for concrete and asphalt pavements, respectively.

Use of Pavement Management System Data For Pavement Engineering Applications

Agencies have collected data for pavement management for many years now. These data have been used primarily to develop short- and long-range rehabilitation programs. Another valuable use of these data is improvements to engineering procedures. The Illinois Pavement Feedback System (IPFS) is a comprehensive inventory and monitoring system that was designed in 1984 to provide data for both programming and engineering uses. Over the past 10 years many engineering issues have been addressed through the use of these data. IPFS data were used to provide feedback to designers and administrators on the design of new (or original) pavements and on overlay designs. Results from a survival analysis of thousands of kilometers of original jointed reinforced concrete pavement (JRCP), continuously reinforced concrete pavement and hot-mix asphalt concrete pavements and asphalt concrete (AC) overlays of these pavements were used. Key results relate to performance variability (a wide range of performance was found for identical designs) and thus the need for a design reliability greater than 50 percent, the expected pavement in-service life in terms of both age and traffic [equivalent single axle loads (ESALs)] of each pavement type and overlays (for example, the mean life of JRCP was 24.1 years and 16.6 million ESALs and the mean life of the first thick AC overlay was 14.5 years and 23 million ESALs), the effect of overlay design thickness, and the effect of concrete material durability (D cracking reduced ESALs carried by 50 percent).

Framework for Selecting Effective Preventive Maintenance Treatments for Flexible Pavements

Preventive maintenance techniques are considered useful in extending the life of a pavement if applied at the right time. Discussed here is a framework for a process that can be used to select the proper maintenance strategies for different distress types in asphalt pavements, depending on traffic level and environment. Maintenance treatments addressed include only crack seals, fog seals, slurry seals, microsurfacings, chip seals, thin asphalt concrete overlays, and other thin surface treatments. Types of distress considered include roughness, rutting, fatigue cracking, longitudinal cracking, raveling, weathering, and bleeding. Decision trees, based on the authors' experiences, are presented to illustrate the process in selecting appropriate maintenance treatments. A framework for evaluating the cost-effectiveness of the various maintenance treatments is presented. Existing methods for evaluating cost-effectiveness of maintenance are discussed together with their strengths and weaknesses. An example of cost-effectiveness for different maintenance treatments is also briefly presented.

Impact-Echo Studies of Interfacial Bond Quality in Concrete: Part l-Effects of Unbonded Fraction of Area

This paper is the first of two papers that summarize results from a research project on the feasibility of using the impact-echo method to determine interfacial bond quality in layered structures, such as overlaid concrete slabs or repaired concrete structures. For purposes of this study, interfacial bond quality is based on unbonded fraction and bond tensile strength. This paper focuses on demonstrating how the unbonded fraction at an interface affects impact-echo response. Results obtained from numerical, experimental, and field studies for concrete structures with concrete overlays or repairs and concrete slabs with asphaltic concrete overlays are presented. Results show that very small unbonded fractions at an interface do not significantly affect impact-echo response. As unbonded fraction increases, a shift in the solid structure's thickness frequency occurs and a small amplitude peak corresponding to reflections from the interface becomes discernible. As the unbonded fraction becomes large, the shift in the thickness frequency becomes more pronounced, and a distinct, larger amplitude peak corresponding to reflections from the interface becomes apparent. Eventually, the response resembles that of a crack at the interface.

Examination of Washington State Department of Transportation Transfer Functions for Mechanistic-Empirical Asphalt Concrete Overlay Design

A review of transfer functions for mechanistic-empirical design procedures is addressed. Specific emphasis is placed on those transfer functions currently used by the Washington State Department of Transportation (WSDOT) and shift factors that relate estimates of laboratory to field fatigue cracking. To achieve this goal, brief discussions about how the WSDOT transfer functions were developed or chosen are presented. A comparison of WSDOT with South African transfer functions is presented. This comparison is of special interest because the South African transfer functions have been updated recently and are in part based on extensive accelerated pavement testing. Finally, mechanistic-empirical overlay designs have been performed by WSDOT for more than 10 years, and a selection of prior overlay projects is reviewed to examine fatigue cracking shift factors. Only projects exhibiting fatigue cracking or its early manifestation are used. The annual visual distress surveys contained in the WSDOT Pavement Management System make this review a bit easier because all pavement sections on the WSDOT route system have been systematically monitored for the preceding 26 years. The conclusion is that the laboratory-based tensile strain relationship currently used by WSDOT must be shifted to predict field fatigue cracking. Such shift factors appear to fall most commonly into a range between 4 and 10.

Development of Bayesian Regression Model to Predict Hot-Mix Asphalt Concrete Overlay Roughness

Predictive equations are central to the many tools highway professionals use to design, maintain, and manage our highway infrastructure. However, adequate data bases to support the development and updating of these models are often lacking. These data bases are often inadequate in sample size, noisy, or incomplete. Conventional statistical modeling tools, such as classical regression analysis, meet with limited success in these applications. A promising solution lies in the use of Bayesian regression, which explicitly allows expert judgment, collected from in-house or external experts, to be used to supplement poor-quality data. An overview is presented of the development of a Bayesian predictive model to forecast the progression of roughness in hot-mix asphalt concrete overlays using data and expert judgment provided by Alberta Transportation and Utilities.

Development of Bayesian Regression Model to Predict Hot-Mix Asphalt Concrete Overlay Roughness

Predictive equations are central to the many tools highway professionals use to design, maintain, and manage our highway infrastructure. However, adequate data bases to support the development and updating of these models are often lacking. These data bases are often inadequate in sample size, noisy, or incomplete. Conventional statistical modeling tools, such as classical regression analysis, meet with limited success in these applications. A promising solution lies in the use of Bayesian regression, which explicitly allows expert judgment, collected from in-house or external experts, to be used to supplement poor-quality data. An overview is presented of the development of a Bayesian predictive model to forecast the progression of roughness in hot-mix asphalt concrete overlays using data and expert judgment provided by Alberta Transportation and Utilities.

Examination of Washington State Department of Transportation Transfer Functions for Mechanistic-Empirical Asphalt Concrete Overlay Design

A review of transfer functions for mechanistic-empirical design procedures is addressed. Specific emphasis is placed on those transfer functions currently used by the Washington State Department of Transportation (WSDOT) and shift factors that relate estimates of laboratory to field fatigue cracking. To achieve this goal, brief discussions about how the WSDOT transfer functions were developed or chosen are presented. A comparison of WSDOT with South African transfer functions is presented. This comparison is of special interest because the South African transfer functions have been updated recently and are in part based on extensive accelerated pavement testing. Finally, mechanistic-empirical overlay designs have been performed by WSDOT for more than 10 years, and a selection of prior overlay projects is reviewed to examine fatigue cracking shift factors. Only projects exhibiting fatigue cracking or its early manifestation are used. The annual visual distress surveys contained in the WSDOT Pavement Management System make this review a bit easier because all pavement sections on the WSDOT route system have been systematically monitored for the preceding 26 years. The conclusion is that the laboratory-based tensile strain relationship currently used by WSDOT must be shifted to predict field fatigue cracking. Such shift factors appear to fall most commonly into a range between 4 and 10.

Asphaltic concrete overlays on surface-treated pavements with lateritic gravel bases

The design and performance of asphaltic concrete overlays for surface-treated pavements with lateritic gravel bases and subbases using the results of Benkelman beam deflection measurements are discussed. It is found that failure to consider pavement distress factors, local materials and environmental conditions in design are inadequate and early signs of distress are exhibited. It is concluded that the high percentage of unpredictable generated traffic is an important contributing factor to performance. An assessment of stress interaction between the existing surface-treated pavement with granular bases and subbases and the asphaltic concrete overlay needs to be considered in the design process. An appropriate approach would be to consider the overlay design as a new asphaltic concrete-surfaced pavement with granular base and subbase.

Detecting Delaminations in Concrete Slabs With and Without Overlays Using the Impact-Echo Method

The paper demonstrates the feasibility of detecting delaminations in reinforced concrete slabs using the impact-echo method, a nondestructive testing technique based on transient stress wave propagation. The results of two laboratory studies are discussed. One study involved detecting artificial delaminations embedded at unknown locations in a reinforced concrete slab. All the artificial delaminations in the slab were located. The second study was aimed at showing the feasibility of detecting delaminations in reinforced concrete slabs with asphalt concrete overlays. Two reinforced concrete slab specimens with corrosion-induced delaminations were tested. Prior to overlaying the slabs with asphalt concrete, the depths of delaminations as determined by impact-echo testing were verified by drilling at selected points. After the asphalt concrete overlays were applied, the slabs were retested. It was found that the impact-echo method could successfully locate the delaminations in the slabs through the asphalt concrete overlays.

Use of geotextiles for reinforcement and strain relief in asphalt concrete

This paper describes the three uses of geotextiles in asphaltic concrete overlays: (a) reinforcing (b) strain relief and (c) undersealing. Reflection cracking through overlays is caused by both loads and thermal contraction, and cracks may be retarded or arrested by geotextiles if the proper material properties are selected and good construction techniques are used. Prediction of crack growth is made with fracture mechanics and this is the basis of an overlay design computer program which is described herein. Fracture properties of asphaltic concrete under fatigue loading and thermal contraction conditions are presented and the way they are altered by the addition of geotextiles is illustrated. The design equations for the thickness of an overlay with a reinforcing grid are also given. Some cautionary rules of thumb, concerning the appropriate pavement conditions when geotextiles may be expected to prolong the life of an overlay and when they may not, are also given.