Unbonded concrete overlays (UBOL) consist of a new Portland Cement Concrete (PCC) layer placed on an existing PCC pavement. The new concrete layer is separated from the existing pavement by an interlayer system, allowing these overlays to be placed on distressed PCC pavements. The interlayer system usually consists of a thin open graded or dense graded hot mix asphalt (HMA) layer or a non-woven geotextile fabric. An effort is currently being undertaken to develop a mechanistic-empirical design procedure for UBOLs. To develop this procedure it is necessary to identify the distresses that commonly develop in these pavement structures. Performance data from in-service pavements at the Minnesota Road Research Facility (MnROAD), in Michigan and Missouri, as well as data in the Long Term Pavement Performance (LTPP) database was reviewed. It was found that primary cracking mechanisms include longitudinal cracking in the wheelpath, at random locations and at mid-lane, and transverse cracking due to reflective cracking, reflective distress and erosion along the transverse joint. Many of these distresses appear to be at least partially caused by breakdown of the interlayer.

Bonded concrete overlays of asphalt pavements (BCOA), also known as whitetopping, consist of a thin concrete overlay on distressed asphalt or composite pavements. They typically have smaller panel sizes than traditional jointed plain concrete pavements (JPCP) in order to reduce stress levels. Faulting at the transverse joints is a distress in BCOA that is not currently addressed in current design procedures. The pumping mechanism that leads to this distress is the focus of this paper. Pumping in BCOA can develop at either the bottom of the overlay slab within the asphalt layer or at the bottom of the asphalt in the granular layer. It is essential to identify where the faulting initiates for each pavement structure, if accurate faulting models are to be developed for these pavement structures. An examination was conducted of 21 BCOAs located at the Minnesota Road Research Facility (MnROAD) to investigate how faulting initiates in BCOA and the rate at which it develops. To do this, the relationship between deflection load transfer efficiency (LTE) from falling weight deflectometer (FWD) testing and transverse joint faulting was examined. It was found that these parameters can be used in successfully defining when faulting initiates at the top of the asphalt and when it initiates at the top of the granular layer below the asphalt. Finally, the rate of the development of faulting in BCOAs resulting from pumping at the concrete/asphalt interface as well as pumping below the asphalt layer was compared to faulting predictions using current models. A good agreement between the observed and predicted faulting was not achieved.

The performance of concrete pavements and overlays is highly dependent on the uniformity, durability, and stability of the underlying support layers. Erosion of support layers can lead to pavement distresses and a reduction in pavement life. A review of existing erodibility performance tests that assessed stabilized support layers was first conducted to identify and evaluate their suitability for adaptation. The Hamburg wheel tracking device (HWTD) test was selected to assess the erosion potential of asphalt and cement stabilized support layers. Field testing with distress surveys, falling weight deflectometer, and coring was completed to obtain HWTD specimens and link laboratory results to pavement performance. The HWTD test was performed on cores obtained from in-service cement and asphalt stabilized support layers, a cold in-place recycling (CIR) mixture, and cement stabilized laboratory mixtures. As expected, an increase in cement content within cement stabilized mixtures decreases the likelihood of erosion with the HWTD. Additionally, conventional asphalt stabilized base layers were highly erosion resistant. Erosion resistant cement stabilized bases (including full-depth reclamation) should target an average HWTD erosion depth ≤2 to 4 mm (0.08 to 0.16 in.) after 10,000 load cycles based on the functional classification and expected traffic volume of the pavement section. Likewise, asphalt stabilized bases (including CIR and support layers for concrete overlays) should target an average HWTD erosion depth ≤12.5 mm (0.5 in.) after 7,500 load cycles with performance grade 64 binder.

PurposeThe time-varying equivalent linear temperature gradient (ELTG) significantly affects the development of faulting and must therefore be accounted for in pavement design. The same is true for faulting of bonded concrete overlays of asphalt (BCOA) with slabs larger than 3 x 3 m. However, the evaluation of ELTG in Mechanistic-Empirical (ME) BCOA design is highly time-consuming. The use of an effective ELTG (EELTG) is an efficient alternative to calculating ELTG. In this study, a model to quickly evaluate EELTG was developed for faulting in BCOA for panels 3 m or longer in size, whose faulting is sensitive to ELTG.Design/methodology/approachA database of EELTG responses was generated for 144 BCOAs at 169 locations throughout the continental United States, which was used to develop a series of prediction models. Three methods were evaluated: multiple linear regression (MLR), artificial neural networks (ANNs), and multi-gene genetic programming (MGGP). The performance of each method was compared, considering both accuracy and model complexity.FindingsIt was shown that ANNs display the highest accuracy, with an R2 of 0.90 on the validation dataset. MLR and MGGP models achieved R2 of 0.73 and 0.71, respectively. However, these models consisted of far fewer free parameters as compared to the ANNs. The model comparison performed in this study highlights the need for researchers to consider the complexity of models so that their direct implementation is feasible.Originality/valueThis research produced a rapid EELTG prediction model for BCOAs that can be incorporated into the existing faulting model framework.

Standard roller-compacted concrete (RCC) test methods such as the modified proctor, Vebe, and vibratory hammer are used to determine final mixture proportions, evaluate consistency, and prepare cylindrical specimens for strength. However, these methods do not replicate the method and levels of compaction observed in the field and require multiple iterations to find a constructable and sustainable RCC mixture for pavements. Better linking RCC compaction properties with mixture volumetrics can enable more efficient RCC constituent selection and proportions. For this research, a typical RCC pavement mixture was selected from which the aggregate voids filled by paste (VFP) varied six levels to achieve mixtures ranging from underfilled (VFP=63.4 %) to overfilled (VFP=126.9 %). A gyratory compactor with a vertical and torque load cell was employed to continuously measure the compaction energy evolution in each specimen and for all mixtures. The volumetric compaction energy required to reach 90 % of the absolute density (E0-90 initial) and incremental energy to compact 90-95 % absolute density (E90-95 final ) was determined and used to evaluate RCC paveability and compactability, respectively, and relate it to mixture volumetrics. All RCC mixtures were able to achieve the initial density but at least equifilling was required to achieve final density under reasonable energy levels (<= 2.70 MJ/m3), with compaction energy decreasing exponentially as the VFP increased. The gyratory compactor more efficiently compacted the reference mixture (VFP=117.9 %) to the same density as the modified proctor test (1.88 vs 2.70 MJ/m3) or cylinders with the vibratory hammer (3.10 vs 10.0 MJ/m3). The gyratory compactor with a perforated mold also enabled quantification of paste mobility within the aggregate skeleton. The time needed for initial and final seepage through the mold perforations was better and more sensitive than the Vebe and vibratory hammer compaction times, with seepage times ranging from 5 to 166 seconds for RCC mixtures from overfilled to underfilled. The perforated mold and a wet sieve analysis also allowed for experimental determination of intergranular voids (IGV) and total paste volume (TPV) of the RCC mixtures, respectively, which had an average relative error of 3.4 % and 4.0 %, respectively.

Transverse joint faulting is a distress that develops in unbonded concrete overlays (UBOL). Historically, faulting models used for predicting the performance of a UBOL have not accounted for the effects of the interlayer between the overlay and the existing pavement on the development of faulting. This is a significant limitation since characteristics of the interlayer play a primary role in the rate at which faulting develops in UBOLs. To develop a more robust faulting prediction model for UBOLs, enhancements were made to the current process to address this limitation. This includes the use of a structural response model that can account for the effects of the interlayer properties on the response of the UBOL. Additional enhancements include the use of a deflection basin of the overlay (in lieu of corner deflections of an equivalent slab system for accumulating differential energy [DE]), the incorporation of an erosion model that can account for the erodibility of the interlayer material, the adjustment of the incremental faulting equations to accommodate small slab sizes that are common in UBOLs, and a national calibration using faulting data from in-service UBOLs. This enhanced faulting model has been implemented in the mechanistic-empirical design tool Pitt UBOL-ME.

There are many urban jointed plain concrete pavements (JPCPs) constructed annually in the U.S. by counties, cities, and municipalities, which typically use state department of transportation standards but are not part of a systematic check to determine if design, construction, and performance are meeting expectations. A field evaluation survey with nondestructive testing was initiated on urban JPCPs throughout Illinois to assess performance and identify any deficiencies in design and construction. A total of 67 JPCPs were selected based on their design features and age. A significant number of sections (57%) were selected because they exhibited some level of premature cracking. Based on the field investigation, multiple mechanisms were proposed to explain the premature cracks. Many transverse and longitudinal cracks observed had excessive slab lengths or widths that were inconsistent with best slab geometry practices. There was no link established between observed cracking and the slab-base frictional restraint or the concrete mixture. For some sections, ultrasonic tests over the contraction joints determined certain joints had not activated and likely contributed to transverse cracks in adjacent slabs. Lack of dowel bar lubrication at the transverse contraction joints was also a plausible mechanism that restrained joint movements and produced excessive tensile stresses in the slab, which led to premature transverse cracks. The mechanisms leading to almost all observed premature cracks for these urban JPCPs were attributed to not adhering to existing state standards (slab geometry limits) and specifications (saw-cut timing/depth and dowel lubrication) during the pavement design and construction phases.

A research study investigated longitudinal cracking developing along an experimental unbonded concrete overlay (UBOL) on I-72 near Riverton, Illinois. The project evaluated existing literature on UBOL (design, construction, and performance), UBOL case studies, and mechanistic-empirical design procedures for defining the mechanisms that are contributing to the observed distresses. Detailed distress surveys and coring were conducted to assess the extent of the longitudinal cracking and faulting along the longitudinal lane-shoulder joint. Coring over the transverse contraction joints in the driving lane showed stripping and erosion of the dense-graded hot-mix asphalt (HMA) interlayer was the primary mechanism initiating the longitudinal cracks. Cores from the lane-shoulder joint confirmed stripping and erosion was also occurring there and leading to the elevation difference between the driving lane and shoulder. Field sections by surrounding state departments of transportation (DOTs), such as Iowa, Michigan, Minnesota, Missouri, and Pennsylvania, with similar UBOL design features to the I-72 section were examined. Site visits were performed in Illinois, Michigan, Minnesota, and Pennsylvania, while other sections were reviewed via state DOT contacts as well as Google Earth and Maps. Evidence from other DOTs suggested that HMA interlayers, whether dense graded or drainable, could experience stripping, erosion, and instability under certain conditions. An existing performance test for interlayers, i.e., Hamburg wheel-tracking device, and current models reviewed were not able to predict the distresses on I-72 eastbound. Adapting a dynamic cylinder test is a next step to screen HMA interlayers (or other stabilized layers) for stripping and erosion potential. To slow down the cracking and faulting on I-72 eastbound, sealing of the longitudinal lane-shoulder joint and driving lane transverse joints is suggested. To maximize UBOL service life, an HMA overlay will minimize water infiltration into the interlayer system and significantly slow down the HMA stripping and erosion mechanism that has led to longitudinal cracking and lane-shoulder faulting.

A research investigation was conducted on the erosion potential of stabilized subbases under concrete pavements and asphalt layers supporting concrete overlays. Through field surveys and testing in Illinois, this project evaluated if existing concrete pavements with stabilized subbases and concrete overlays were exhibiting potential erosion of the underlying support layer. The field evaluation testing included falling weight deflectometer testing, distress surveys, coring, and ultrasonic tomography scanning. A laboratory performance test was also established using the Hamburg wheel-tracking device to assess the erodibility of the various stabilized subbase layers for new construction and existing asphalt layers available for a concrete overlay. The analyzed field test results were coupled together with the laboratory performance testing to provide recommendations for updating the Illinois Department of Transportation’s “Bureau of Design and Environment Manual” guidance. No changes were recommended for hot-mix asphalt stabilized subbases, but testing using the Hamburg wheel-tracking device should be considered for Portland cement concrete stabilized support layers (e.g., CAM II) under concrete pavements. For testing of asphalt support layers for concrete pavement overlays, the Hamburg wheel-tracking device is recommended with performance criteria similar to flexible pavements for appropriate functional classes.

Transverse joint faulting is a common distress in unbonded concrete overlays (UBOLs). However, the current faulting model in Pavement mechanistic-empirical (ME) is not suitable for accurately predicting the response of UBOLs. Therefore, to develop a more accurate faulting prediction model for UBOLs, the first step was to develop a predictive model that would be able to predict the response (deflections) of these structures. To account for the conditions unique to UBOLs, a computational model was developed using the pavement-specific finite element program ISLAB, to predict the response of these structures. The model was validated using falling weight deflectometer (FWD) data from existing field sections at the Minnesota Road Research Facility (MnROAD) as well as sections in Michigan. A factorial design was performed using ISLAB to efficiently populate a database of fictitious UBOLs and their responses. The database was then used to develop predictive models, based on artificial neural networks (ANNs), to rapidly estimate the structural response of UBOLs to environmental and traffic loads. The structural response can be related to damage through the differential energy concept. Future work will include implementation of the ANNs developed in this study into a faulting prediction model for designing UBOLs.

This study investigated the causes for premature, transverse cracking on urban jointed plain concrete pavements in Illinois. A field survey of 67 sections throughout Illinois coupled with ultrasonic evaluation was completed to synthesize the extent of premature cracking on urban JPCP. The visual survey showed some transverse and longitudinal cracks were a result of improper slab geometry (excessive slab length and width). Ultrasonic tests over the contraction joints determined some notched joints had not activated and adjacent transverse cracks were likely formed as a result. Three-dimensional finite-element analyses confirmed that cracking would not develop as a result of normal environmental factors and slab-base frictional restraint. The concrete mixture also did not appear to be a contributing factor to the premature cracks. Finally, the lack of lubrication on dowel bars was determined to potentially be a primary mechanism that could restrain the transverse contraction joints, produce excessive tensile stresses in the slab, and cause premature transverse cracks to develop.

Unbonded concrete overlays (UBOL) consist of a new Portland cement concrete (PCC) layer placed on an existing PCC pavement. The new concrete layer is separated from the existing pavement by an interlayer system, allowing these overlays to be placed on distressed PCC pavements. The interlayer system usually consists of a thin open graded or dense graded hot mix asphalt (HMA) layer or a non-woven geotextile fabric. In an effort to develop a mechanistic-empirical design procedure for UBOLs, the effects of the interlayer properties on the performance of the overlay must first be established. There are many variables to consider when selecting an interlayer for UBOLs such as, drainability, frictional restraint, prevention of reflective cracking, and stability of a mixture to prevent erosion or consolidation/ rutting. Performance data from in-service pavements at the Minnesota Road Research Facility (MnROAD), Michigan, Missouri, and Pennsylvania, as well as data in the Long-Term Pavement Performance (LTPP) database is examined to establish the influence of interlayer characteristics on overlay performance. . Many of the distresses are at least partially caused by breakdown of the interlayer. Longitudinal cracking develops in the wheelpath due to erosion or consolidation of an asphalt interlayer, while transverse cracking can occur due to reflective cracking, reflective distress, or erosion along the transverse joint. The characteristics of the interlayer also play a role in the rate of the development of faulting. Based on this evaluation, factors that should be considered for optimizing the performance of the interlayer are defined.

The faulting model currently adopted in the Pavement ME design procedure accounts for the pavement response, climatic conditions, traffic, and erodibility of the base. The same faulting model is applied to all jointed concrete pavements regardless of the pavement structure (conventional concrete pavement, unbonded concrete overlay, bonded concrete overlay, etc.). This implies the pumping mechanism is the same for all pavement structures. It also assumes that the rate of the development of faulting and the maximum faulting that will occur is the same regardless of the pavement structure. The focus of this study is to investigate the mechanisms contributing to the development of voids beneath the slabs and faulting for a range of pavement structures. The results are used to determine the suitability of applying one faulting model to all pavement structures. The focus is directed toward undoweled structures since dowelled joints significantly reduce the potential for the development of faulting. It was found that the development of faulting is delayed, develops at a slower rate, and the magnitude of the faulting tends to be higher in overlays as compared to that in jointed plain concrete pavements. In conclusion, a single faulting model is not suitable for application for all pavement structures.

Unbonded concrete overlay of concrete pavement (UBOL) is an effective rehabilitation method involving the construction of a new concrete pavement over a deteriorated concrete pavement, separated by an interlayer. While UBOL is used in practice to improve the structural capacity of existing concrete pavements, the performance of the interlayer is not currently accounted for in the pavement mechanistic–empirical design process. Therefore, the objective of this research is to improve prediction of UBOL performance by accounting for the effects of asphalt interlayer consolidation on the development of longitudinal cracks in the wheelpath. First, a laboratory investigation was performed using beams cut from in-service pavements in Michigan, Minnesota, and Pennsylvania to assess the susceptibility of permanent deformation of asphalt interlayers. This data was utilized in conjunction with a finite element analysis to develop/calibrate a permanent deformation prediction model for dense graded asphalt interlayers. The framework of the model follows that of the permanent deformation prediction model for asphalt surface pavements incorporated into the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic–Empirical Pavement Design Guide. In addition, a field analysis was conducted, using the Long-Term Pavement Performance (LTPP) database, to assess longitudinal cracking in the wheelpath caused by permanent deformation in asphalt interlayers. The laboratory-calibrated permanent deformation model was then validated using the performance data for UBOLs in the LTPP database and deformation thresholds for asphalt interlayers were established. This research resulted in the development of a framework for the prediction of longitudinal crack development in UBOLs because of permanent deformation in the asphalt interlayer.

Bonded concrete overlays of asphalt pavements (BCOA) consist of a concrete overlay placed on an existing asphalt or composite pavement. This technique is intended as a cost-effective rehabilitation solution for marginally distressed in-service asphalt or composite pavements. BCOA with panel sizes between 4.5 ft and 8.5 ft have become popular as they reduce curling stresses while keeping the longitudinal joints out of the wheelpath. The BCOA-ME (mechanistic empirical) design procedure and Pavement ME short jointed plain concrete pavement (SJPCP) module can both be used to design BCOA with mid-size panels. However, these design procedures differ in the assumptions used to develop the mechanistic computational model, fatigue models used to predict failure, treatment of environmental conditions, estimate of asphalt stiffness, consideration of structural fibers, the application of traffic loading, and the calibration process. This results in the procedures producing different overlay thicknesses and predicted distresses. The strengths and limitations of each procedure are evaluated and comparisons are made between the design thicknesses obtained from them.

Reflective cracking can be a concern in unbonded concrete overlays of existing distressed concrete pavements. In these structures, an inter layer, commonly hot-mix asphalt (HMA) or a nonwoven geotextile fabric, is placed to isolate the overlay concrete from the existing concrete pavement. The interlayer minimizes interaction between the overlay and the existing concrete pavement, which helps prevent distress in the existing pavement from propagating into the overlay. In this study, a laboratory investigation was used to examine the influence of HMA and geotextile fabric interlayer systems on the potential for development of reflective cracking. A laboratory test was conducted by using stacked beam specimens separated by an interlayer to evaluate the potential for a discrete crack in the lower beam reflecting up through the interlayer and into the overlay beam. The study revealed that reflective cracking was more likely when there was loss of support beneath the existing pavement. Reflective cracking did not occur with any of the interlayer systems when the beam was fully supported. The nonwoven geotextile inter layers that were tested were more effective than the HMA interlayers at preventing reflective cracking. Even though the fabric interlayer had a higher load ratio than the HMA interlayers, a relatively large load was required to generate reflective cracking regardless of the interlayer type (HMA or fabric) and, therefore, both may be suitable interlayer alternatives for the prevention of reflective cracking.