Structural Layer Coefficients Research Articles

Field investigation of material layer properties for emulsion-treated base layer application in low-volume roads

ABSTRACT The study presents the determination of pavement design input parameters-structural layer coefficients and back-calculated layer moduli for Emulsion-Treated Base (ETB) layer materials in a Low-Volume Road (LVR) application. Seven test sections were constructed on a stretch of LVR with varying emulsion contents and 50% Recycled Asphalt Pavement (RAP) aggregate incorporation in ETB mixes. Structural performance of the test sections was periodically observed using a Benkelman Beam Deflectometer (BBD) and the data was used for the estimation of the structural layer coefficient of ETB, based on the effective structural number of the pavement. The deflection bowl data of the test sections were also captured using a Falling Weight Deflectometer (FWD) and the modulus of ETB layer was back-calculated using a back-calculation software. The study proposes a structural layer coefficient of 0.23–0.29 and back-calculated moduli of 578–919 MPa for ETB mixes characterised by Indirect Tensile Strength (ITS) values ranging between 140 kPa and 245 kPa. Traffic level-based ETB mix specifications are also proposed in the study, which enables a mix designer to proportion the mix for a selected design traffic level. For a design traffic level of 1 million standard axles, the target dry ITS specification was found to be 188 MPa.

In-situ structural analysis of lightweight cellular concrete subbase flexible pavements

This study in Canada aimed to improve the performance of flexible pavements by comparing the in-place behavior of Lightweight Cellular Concrete (LCC) at three densities with traditional unbound granular material. The research investigated the effect of traffic load and testing temperature on pavement response and determined the structural layer coefficient for the 475 kg/m³ density. The structural layer coefficient for the LCC subbase at this density was 0.22. Results showed that pavements with LCC had over three times less stress at the top of the subgrade compared to the conventional granular material. The Weslea software, which uses a multi-layer elastic theory, appeared to overestimate stress responses for LCC pavements, and the research suggested a prediction modification for more accurate results. Overall, the study demonstrated that lightweight cellular concrete with different densities could improve pavement performance.

AASHTO 1993 Plus: an alternative procedure for the calculation of structural asphalt layer coefficients

ABSTRACT Many road agencies worldwide still use the empirical AASHTO 1993 pavement design method. Since the release of the AASHTO 1993 design guide, many aspects of pavement design have changed, including asphalt material properties, traffic, environmental factors, etc. However, many agencies use a single layer coefficient for asphalt mixtures and pavements are currently designed with inaccurate values, as proved by many. This paper presents a four-step procedure (called AASHTO 93 Plus method) to determine structural layer coefficients of asphalt mixtures. The AASHTO 93 Plus method considers viscoelasticity and temperature-dependency of asphalt mixtures to estimate the layer coefficient. Five-hundred-eighteen (518) flexible pavements from USA and Canada roads and 1599 HMAs from the Long-Term Pavement Performance (LTPP) database were analyzed. The results showed that the new material-specific layer coefficients were well above the typical value of 0.44. The total number of 3,726 IRI data records of these 518 pavements were converted to PSI and then compared to those obtained from the traditional AASHTO 93 and AASHTO 93 Plus methods for validation. Results confirmed that the conventional AASHTO 93 method predicts faster deterioration of pavement conditions over time, while the results of AASHTO 93 Plus method better aligned with the field data.

Use of coal ash cement stabilized material as pavement base material: Laboratory characterization and field evaluation

Coal fly ash (CFA) and coal bottom ash (CBA) are coal combustion's primary solid waste products. This solid waste management is a substantial challenge for power plants and local authorities. In order to apply coal ash as a pavement base material, the relationship of the mixture’s mechanical properties with structural design parameters for pavement design and analysis must be explored. No study has investigated the mechanical behavior of the 100% coal ash cement stabilized material (CACSM) as a pavement base layer. Therefore, this study aimed to investigate the structural layer coefficient of CACSM used in the American Association of State Highway and Transportation Officials (AASHTO) 1993 pavement design guide. The study consisted of laboratory material characterization and field evaluation. The mechanical property test revealed that CACSM has a self-healing ability; coal ash appeared to influence the secondary curing of the specimens significantly. Strength towards the ends of stage I and stage II showed the influence of high pozzolanic on the CACSM, which resulted in improved long-term performance. The layer coefficients of the locations in the field test ranged from 0.22 to 0.24. By contrast, for the granular base and cement-treated base, these values were 0.07 to 0.14 and 0.17–0.20, respectively. The study findings show that CACSM as a pavement base is a viable solution that can help develop transportation infrastructure with enhanced concrete durability and lower carbon footprint emissions.

Comparison of Relative Structural Characterization Methods for Additive-Modified Asphalt Mixtures

The evaluation, adoption, and efficient deployment of asphalt additives in flexible pavements has historically been a relatively slow process, requiring years of testing and evaluation before a new product may be fully and readily used in pavement cross-sections. However, the fast-paced and innovative nature of the additive marketplace produces numerous products that have the potential to improve performance, reduce costs, or both, but have been hindered by slow deployment. To address the need for a more rapid set of evaluation tools, the National Center for Asphalt Technology Test Track initiated an Additive Group experiment in 2021 to build full-scale pavement test sections modified with several asphalt additives, including recycled tire rubber, recycled plastics, reactive polymers, and aramid fibers. Data will be collected from these test sections to develop a framework for evaluating asphalt additives in the future. Phase 1 of the investigation was to conduct limited laboratory testing and structural analysis from which a group of additives would be selected for full-scale construction and evaluation. Focused on fatigue cracking, this paper details the laboratory testing conducted for Phase 1 and two different structural pavement analysis methods using FlexPAVE™ and WESLEA. Even though the two structural pavement analysis methods are very different, the FlexPAVE and WESLEA analyses provided strongly correlated predicted cracking performance, remarkably similar predictions of equivalent pavement thicknesses, and nearly identical structural layer coefficients. It is recommended that these two approaches be further evaluated against laboratory and field performance data that will be collected from the Additive Group experiment.

Full-Scale Pavement Testing of a High Polymer-Modified Asphalt Concrete Mixture

Abstract This paper presents the validation process for the structural layer coefficient determined mechanistically for high polymer (HP) asphalt mixtures (i.e., 0.54) through full-scale laboratory testing of asphalt pavements. Two instrumented full-scale pavement experiments were conducted in the PaveBox setup at the University of Nevada, Reno. A flexible pavement structure with a conventional polymer-modified asphalt layer was evaluated in Experiment No. 1. Meanwhile, a flexible pavement structure with a reduced-thickness HP asphalt concrete (AC) layer was evaluated in Experiment No. 2. Similar vertical surface deflections (except under the center of the applied surface load), higher vertical stresses at the middle of the base layer, and similar vertical stresses below the subgrade surface were observed with the reduced thickness of the HP AC layer. Moreover, better overall fatigue and rutting performance in the AC layer, slightly higher rut depths in the unbound layers, and similar total rut depths were observed by means of mechanistic analysis with the use of HP AC pavement. In general, the results and findings of this study supported the selection of 0.54.

Structural Contribution of Cold In-Place Recycling Base Layer

Cold in-place recycling (CIR) of asphalt pavements is a process that has successfully been used for many years. The use of CIR for rehabilitation offers many advantages over traditional overlays due to its excellent resistance to reflective cracking and its environmentally friendly impacts. Despite the good performance and positive sustainability aspects of CIR, the structural contribution of the CIR base layer has not been well defined. In this research, CIR mixtures were designed with different asphalt emulsions. The mixtures were then subjected to dynamic modulus, repeated load triaxial, and flexural beam fatigue testing over a range of temperature and loading conditions. The performance test data generated were then used to develop CIR rutting and fatigue performance models used in the mechanistic analysis of flexible pavements. The technique used to develop the performance models leveraged the fact that the rutting and fatigue models for individual CIR mixtures were all within the 95 percent confidence interval of each other. A mechanistic analysis was conducted using the 3D-Move Mechanistic Analysis model. With the laboratory-developed performance models, the structural layer coefficient for the CIR base layer were developed for use in the 1993 AASHTO Guide for the Design of Pavement Structures. This analysis led to the determination of an average structural coefficient of the CIR base layer of 0.25.

Improvement of unbound granular pavement layers and subgrade with cement dust in Egypt

The main goal of this study is to assess the technical viability of using cement dust as part of the fines percentage in the unbound granular base/subbase pavement layers’ and subgrade soil as a viable sustainable solution. This study investigates the effect of adding cement dust to five types of pavement road materials which are collected from different ongoing roadway construction projects (1) Reclaimed Asphalt Pavement (RAP), (2) crushed stone base aggregates, (3) blend of crushed stone base aggregates with RAP, (4) crushed stone subbase aggregates, and (5) sandy subgrade soil. The resilient modulus (Mr) is selected as the main characteristic for evaluating the investigated materials’ stiffness. The regression parameters (k1, k2, and k3) of the universal Mr prediction model are found by fitting the experimental results of Mr testing for two replicates per each material type. The results show that using 3–5% of cement dust improves the estimated Mr of the investigated base/subbase materials and subgrades at the anticipated field stresses by 8–21%., As a result the structural layer coefficients are enhanced by 11–17%. KENLAYER nonlinear damage analysis confirms that using 3–5% of cement dust improves the predicted rutting life by up to 12% and the fatigue life up to 27%. Based on a typical pavement structure, enhancing pavement base/subbase layers and subgrade soils with 3–5% of cement dust reduces asphalt layer thickness from 11.25 to 12.50% and increases base/subbase layer thickness from 8.75 to 12.50% to maintain the same predicted rutting and fatigue lives of the typical structure. This positively affects the total construction cost in addition to the ecological benefits.

Mechanistic-based verification of a structural layer coefficient for high polymer-modified asphalt mixtures

Extensive laboratory performance testing and advanced statistical analyses led to the determination of a fatigue-based structural layer coefficient of 0.54 for high polymer-modified (HP) asphalt concrete (AC) mixtures in new pavements. The objective of this paper was to verify, using mechanistic-empirical (ME) analyses, the structural capacity of HP AC pavement sections designed using the 0.54 layer coefficient. The ME analysis for rutting, shoving, and top-down cracking in the AC layer resulted in most of the cases in a better performance for the HP AC mixes when compared with their respective PMA AC mixes. Moreover, the rut depths determined in the unbound layers were observed to be lower than the maximum allowable rut depth indicating an acceptable performance for the HP pavement sections which had a thinner AC layer thickness. In summary, the verification efforts supported the use of a structural coefficient of 0.54 for HP AC mixes in new pavements.

Methodology for Determination of the Structural Layer Coefficient of Unbound Base Materials in Florida

The main goal of this study was to assess methodologies for the determination of base structural layer coefficients (SLCs) in Florida base materials. The aim was to streamline the traditional empirical and observationally based technique for determining layer coefficients either by an accelerated loading process or by the computer simulation of pavement performance. The paper describes a methodology in which the structural numbers (SN) of two paired test sections are used to backcalculate the unknown SLC. The SNs are determined from the AASHTO performance equation using a prediction of pavement performance from the University of Florida top-down cracking (TDC) model. The paper presents a positive application of the methodology for two test sections in Louisiana and two sections in Florida.

Fatigue-Based Structural Layer Coefficient of High Polymer-Modified Asphalt Mixtures

Florida Department of Transportation uses the 1993 AASHTO guide to conduct new and rehabilitation designs for all the state’s flexible pavements. Based on previous experience, a structural layer coefficient of 0.44 was found to be well representative of the department’s conventional polymer-modified (PMA) asphalt concrete (AC) mixtures. If the positive impact of the polymer on the layer is assumed to be maintained at higher contents, then the use of high polymer-modified (HP) asphalt binder may lead to a higher AC structural layer coefficient and a reduced AC layer thickness for the same design traffic and serviceability design loss. The objective of this paper was to determine a fatigue-based structural layer coefficient for asphalt mixtures that contain HP binder using comprehensive mechanistic analyses. This approach relied on combining measured engineering properties and performance characteristics of AC mixtures with advanced flexible pavement modeling (3D-Move). A total of eight PMA and eight HP AC mixtures were designed and evaluated in the laboratory. Overall, the HP AC mixtures showed similar or lower dynamic modulus and better fatigue performance models when compared with those of their respective PMA AC mixtures. However, the fatigue-based structural layer coefficients, determined via mechanistic analysis using the service life approach, ranged between 0.33 (lower than 0.44) and 1.32 (greater than 0.44). Using advanced statistical analyses, a fatigue-based structural layer coefficient of 0.54 was determined for HP AC mixtures. This coefficient should still be verified for other modes of distress.

Layer coefficients of cold in-place recycled layers from performance data

ABSTRACTWith cold in-place recycled (CIR) pavements becoming an increasingly popular treatment for pavement life extension, it is important for local agencies to have data that justifies the design parameters that influence the effectiveness of such treatments. Studies have shown that the effectiveness of CIR depends on its milling depth, and in order for agencies to better decide the pavement design, depth of CIR to be milled and placed, the authors have developed a procedure to back-calculate the structural layer coefficient of CIR layers based on performance data, in order to evaluate their structural capacity. This paper attempts to establish a relation between the structural capacity of the CIR layer and the international roughness index (IRI) of the pavement over time, using the results of previous research determining a definite differential between the IRI of pavements with different CIR thicknesses.

Incorporating the Strength Provided by Subgrade Stabilization in the Flexible Pavement Design Procedures

Abstract Chemical stabilization has been successfully used to treat weak subgrade soil by permanently changing its physical and chemical properties, such as reducing plasticity and increasing stiffness and load bearing capacity, so it can serve as a construction platform during constructions and provide long-term structural stability. The Ohio Department of Transportation (ODOT) has adopted a policy to globally stabilize the subgrade for new construction. However, the strength provided by subgrade stabilization has not been incorporated into the design procedures for flexible pavement. Therefore, in this study, the impact of subgrade stabilization on the stiffness and performance of pavement layers was investigated. In situ testing, including the falling weight deflectometer (FWD) and the portable seismic pavement analyzer (PSPA), was performed on 20 pavement sections within the state of Ohio to evaluate pavement sections constructed with stabilized subgrade. Backcalculation of pavement layers’ moduli was carried out on the FWD data collected. Finite element analysis was performed using ABAQUS to validate the pavement layers’ stiffness obtained from backcalculation. The validated layer moduli were used to determine structural numbers and layer coefficients of pavement layers. The result was used to investigate the effect of subgrade stabilization on pavement design procedures. Significant reduction in the pavement thickness and distress was obtained as a result of incorporating subgrade stabilization into pavement design procedures.

Initial Performance of Virginia's Interstate 81 In-Place Pavement Recycling Project

Rehabilitating or reconstructing existing asphalt pavements by applying in-place pavement recycling techniques has been shown to be more cost-effective and more environmentally friendly than with traditional techniques. However, the scanty information in the literature documenting the structural and functional performance of these processes for high-volume roadways, leads to a perception by pavement engineers that the techniques are suitable only for lower-volume roads. This paper presents performance data for the 3.66-mi section of Interstate 81 in Virginia that was rehabilitated through the use of full-depth reclamation, cold central-plant recycling, and cold in-place recycling during the 2011 construction season. The performance data used to document the pavement condition included the structural capacity obtained by a falling-weight deflectometer and rut depth and ride quality measurements collected by an inertial profiler. The structural layer coefficients for the recycled materials were at the upper range of values reported in the literature. The pavement sections continued to perform well in regard to their functional and structural condition after the application of approximately 6 million equivalent single-axle loads and nearly 3 years of service. Two sections with different overlay thicknesses in the right lane also performed well after nearly 3 years of service.

Mechanistic structural properties of foamed asphalt stabilised base materials

Foamed asphalt stabilised base (FASB) material is a partially bound material consisting of four phases of aggregate skeleton, air voids, foamed binder and water. Its mechanical properties and behaviour are greatly influenced by field placement and curing conditions. Given appropriate design and application, the structural capacity of FASB lies somewhere between granular aggregate base (GAB) and hot mix asphalt. Therefore, when FASB is used in the place of GAB, the required thickness of the pavement section can be reduced, resulting in cost savings in addition to recycling benefits. The objective of this study was to investigate and explain the distinct mechanical behaviour of this material and address the most important performance-related parameters of FASB – stiffness and permanent deformation resistance. Triaxial dynamic modulus (|E*|) and repeated load permanent deformation (RLPD) tests were performed on field-cured FASB cores. The influence of confining stress on dynamic modulus was found minor as compared with that of loading rate and temperature. At a 25°C temperature and a 10 Hz loading rate typical for base layer conditions in high-volume highway pavements, the mean value of dynamic modulus of a new FASB was found to be 4310 MPa, much greater than what would be expected from conventional GAB. RLPD test results demonstrated satisfactory resistance to rutting. The in situ long-term stiffness of FASB measured using a Dynatest falling weight deflectometer was of an order of magnitude greater than the stiffness of GAB. A structural layer coefficient of 0.142 cm− 1 (0.36 in.− 1) was recommended for this material for use in the empirical 1993 American Association of State Highway and Transportation Officials design procedure.

Bearing capacity evaluation of rubblized concrete pavements

The paper presents the findings of a research work performed on a real scale concrete pavement project where Rubblizing technology was used for its structural rehabilitation. Rubblizing may be defined as a fracture technique in which a concrete pavement slab is transformed in a granular base with a very high Modulus. This technique, fractures the concrete slab in angular pieces by using a concentrated dynamic load of low amplitude and high frequency. The research work was based on field study on the rehabilitation of 5 km motorway. The structural evaluations where made, before, during and after one year construction. Measurements and site evaluation where made by using DCP, Light Weight Deflectometer and FWD (on top of asphalt layer) and excavating inside pits. The structural capacity of the Rubblized layer was evaluated through theoretical analysis. Because of the anisotropic properties of the Rubblized layer the results are presented using AASHTO structural layer coefficient. The structural layer coefficients recommended are between the range of 0.25 and 0.30 for concrete slabs with thickness grater than 220 mm.

Study of In Situ Pavement Material Properties Determined from FWD Testing

The Vermont Agency of Transportation (Agency) developed pavement design procedures patterned after the American Association of State Highway and Transportation Officials (AASHTO) pavement design model described in the AASHTO 1993 pavement design guide (guide). A consequence of using the structural number approach to pavement design is the need for structural layer coefficients ( ai ) to characterize the component materials of the pavement structure. The agency has developed and tested a method for determining layer coefficients using a falling weight deflectometer (FWD), and the resulting layer coefficients are representative of the in situ behavior of the pavement materials. The agency found layer coefficients determined for unbound subbases to be reasonable, while layer coefficients estimated for asphalt cement concrete materials were generally 25–35% higher than AASHTO’s implied maximum of 0.44. However, a statistical analysis indicates considerable support for the predictive qualities of FWD derived l...

Practical Approach to Criteria for the Use of Lime–Fly Ash Stabilization in Base Courses

In October 2000 the Mississippi Department of Transportation (MDOT) initiated a study to evaluate the long-term performance of lime–fly ash (LFA) stabilized soil as a base course material. This study entailed performing falling weight deflectometer (FWD) tests on both newer and older pavements and coring pavement at each FWD location to observe the condition of the layers, to obtain pavement thicknesses, and to perform unconfined compressive strength (UCS) testing. Visual observation, backcalculated modulus, and in situ structural layer coefficient values showed that MDOT LFA-stabilized soil base courses have highly variable material properties and thicknesses. Recommendations were made to increase the average LFA material property values and to reduce the spread in these values by increasing the required compaction of the LFA-stabilized soil base layer to 100% standard Proctor effort, setting the required in situ Proctor UCS at 400 psi, and reducing variability by either improving the current method of field-mixed-in-place stabilization or requiring plant-mixed material with placement of the blended material via a paver. It is further recommended to increase the typical LFA-stabilized soil base layer design thickness from 6 to 8 in. and to use a 6-in. chemically stabilized subgrade layer to provide additional support to the pavement structure.

Practical Approach to Criteria for the Use of Lime-Fly Ash Stabilization in Base Courses

In October 2000 the Mississippi Department of Transportation (MDOT) initiated a study to evaluate the long-term performance of lime-fly ash (LFA) stabilized soil as a base course material. This study entailed performing falling weight deflectometer (FWD) tests on both newer and older pavements and coring pavement at each FWD location to observe the condition of the layers, to obtain pavement thicknesses, and to perform unconfined compressive strength (UCS) testing. Visual observation, backcalculated modulus, and in situ structural layer coefficient values showed that MDOT LFA-stabilized soil base courses have highly variable material properties and thicknesses. Recommendations were made to increase the average LFA material property values and to reduce the spread in these values by increasing the required compaction of the LFA-stabilized soil base layer to 100% standard Proctor effort, setting the required in situ Proctor UCS at 400 psi, and reducing variability by either improving the current method of f...

Accelerated Pavement Testing Evaluation of the Structural Contribution of Full-Depth Reclamation Material Stabilized with Foamed Asphalt

Research was conducted to determine the effectiveness of the use of foamed asphalt-stabilized reclaimed asphalt pavement from full-depth reclamation (FAS-FDR) as base material for flexible pavements. The experiment, conducted at the Civil Engineering Infrastructure Systems Laboratory of Kansas State University, consisted of constructing four pavements—one with a 9-in. conventional Kansas AB-3 granular base and one each with 6, 9, and 12 in. of FAS-FDR—and subjecting them to a full-scale accelerated pavement test. All four pavement sections were loaded with 500,000 axle load repetitions, at room temperature and under moderate moisture levels in the subgrade soil. The measured stresses and strains as well as the permanent deformation (rutting) observed on the pavement sections indicated that FAS-FDR can be used successfully as a base material. The measured rut depths and compres-sive vertical stresses at the top of the subgrade suggest that a 1-in. FAS-FDR base shows performance equivalent to that of a 1-in. conventional Kansas AB-3 granular base. The effective structural number computed from the falling weight deflectometer tests on the as-constructed pavements showed that average structural layer coefficient for the FAS-FDR base material was 0.18.