Laboratory-compacted Specimens Research Articles

The effect of specimen size on the unconfined compressive strength of cement-stabilized granular mixtures made of natural and recycled aggregates

The cement-stabilization technique is employed on natural and recycled granular materials to improve their mechanical properties. The strength of these materials is assessed by the unconfined compressive strength on laboratory compacted specimens, typically after 7 days of curing. Standards and technical specifications specify different values of specimen height and diameter and different loading modes of testing. This makes the comparison between different materials and with the acceptance limits of technical specifications difficult. The research investigates the effect of specimen size and loading mode on the unconfined compressive strength of both natural and recycled cement-stabilized granular materials. The results revealed significant differences in strength due to variations in specimen size and loading mode. As expected, an increase in specimen slenderness resulted in a decrease in compressive strength. A linear regression model was developed to quantify the effect of the experimental variables on the compressive strength of the two cement-stabilized materials.

Development of Macrotexture Test Method for Dense-Graded Asphalt Mixtures

The design and construction of pavements necessitate careful consideration of longevity, cost-effectiveness, sustainability, and safety, with a focus on enhancing the durability and lifespan of dense-graded asphalt (DGA) surface mixtures. This paper discusses a research project the Federal Highway Administration (FHWA) initiated to assess the macrotexture characteristics of DGA mixture pavement—particularly concerning safety at higher speeds. The project aims to develop an efficient testing procedure for measuring macrotexture during mixture design and field-testing processes, striking a balance between longevity, cost-effectiveness, sustainability, and safety in pavement design and construction. The study began with a comprehensive review of existing practices, leading to the selection and testing of a unique laser texture scanner capable of noncontact macrotexture measurement. The subsequent phase involved validating the suggested macrotexture measurement system through data collected from State-sponsored projects under the FHWA Mobile Asphalt Technology Center program. Macrotexture measurements were collected from field cores and laboratory-compacted specimens from various State departments of transportation projects and analyzed. Preliminary analysis of the limited dataset to date revealed a positive correlation between average macrotexture measurements from gyratory specimens’ top surface and field core macrotexture measurements. The FHWA developed a draft test method for measuring macrotexture on laboratory specimens and field cores, thereby aiming to reduce procedural variability. Interlaboratory macrotexture test results demonstrated good reproducibility, with an average coefficient of variation of approximately 20%. Overall, this research project seeks to advance the understanding and measurement of macrotexture in DGA mixture pavements and thereby contribute to the development of safe pavements.

Digital image colour analysis to evaluate the moisture sensitivity of hot-mix asphalt mixes

Moisture in hot-mix asphalt (HMA) can cause serious damage such as cracks and ruts if left untreated. This is due to the separation of binder and aggregates caused by moisture-induced stripping. Existing approaches for assessing moisture damage in asphalt are time consuming, destructive and expensive to implement. In contrast, digital image analysis provides a simple, inexpensive and non-destructive method for identifying moisture-sensitive mixes. Colour, as an essential characteristic of an image, effectively conveys information about its content. This study seeks to explore the potential of analysing digital image colour in the L*a*b* colour space to assess the moisture sensitivity of HMA after moisture-induced stress tester conditioning. Laboratory-compacted specimens of loose asphalt mixtures from the field were studied by image processing followed by statistical tests of significance. It is found that two parameters – namely, the ‘average colour value for the L* channel’ and the ‘standard deviation value for the b* channel’ – can be used to distinguish between moisture-sensitive and non-sensitive mixes. The findings of this study suggest that future research may find colour to be a helpful criterion for characterising asphalt materials using digital image processing.

Mitigation Strategy Selection for Asphalt Mixtures with High Reclaimed Asphalt Pavement Content

Economics and sustainability goals have driven higher use of recycled asphalt materials (RAMs), such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles, in the pavement industry. Owing to the brittle nature of aged binders found in RAMs, a mitigation strategy is usually needed to ensure adequate cracking performance. Determining a proper mitigation strategy is vital to designing long-lasting asphalt concrete (AC) pavements with balanced performance. This study explored the use of mitigation strategies on AC mixtures with intermediate and high RAP contents, using materials from four mixtures that were part of a mill and overlay project in Milford, Delaware. This project included one standard mixture and three experimental mixtures with 25%–40% RAP. This paper presents performance data for field cores, plant-mixed laboratory-compacted specimens, and laboratory-mixed laboratory-compacted specimens. The effects and benefits of using a softer binder versus a recycling agent and the effects of using both a softer binder and a recycling agent to mitigate the stiff RAP binder are explored. To understand the effects of each mitigation strategy with aging, data from one-year field-aged cores and from laboratory-aged specimens are shown. Adjusting only one variable to mitigate the increased stiffness and brittleness of recycled binders yielded the best results in this study. Additionally, some test parameters were found to be sensitive to aging, including the flexibility index from the semicircular bend test and the cracking test (CT) index from the IDEAL-CT, while others were not observed to be sensitive to aging, including the fracture energy measured by the low-temperature disk-shaped compact tension test.

Development and Verification of CTIndex Correction Methods to Normalize Air Void Content and Thickness of Field Cores

The IDEAL cracking test (IDEAL-CT) (ASTM D8225) provides a fracture mechanics-based parameter: cracking tolerance index or CTIndex which is sensitive to specimen thickness and air void (AV) content. The CTIndex of field cores with variable thicknesses and AV contents are not comparable to those of laboratory-compacted specimens prepared at specified values. This study analyzed the CTIndex sensitivity to these features by preparing reheated plant-mixed laboratory-compacted (RPMLC) specimens with variable thickness and AV content and developing thickness and AV content correction factors. The correction factors were obtained using trendline equations from the relationships between CTIndex and varying thickness and CTIndex and varying AV content. The correction factors were then applied to RPMLC specimens fabricated with variable thickness and AV content and compared with RPMLC specimens prepared at 62 ± 1 mm and 7 ± 0.5% AV. CTIndex results were comparable, and one correction method was proposed. This method was applied to replicate field cores of 22 independent mixtures and compared with RPMLC specimens fabricated at 62 ± 1 mm and 7 ± 0.5% AV. CTIndex results of field cores and the RPMLC specimens still showed differences after applying the correction method, suggesting the need for a different (larger) CTIndex threshold for field cores. Finally, more data are required to improve the proposed correction methods and more mixtures need to be evaluated to consider a wider range of thickness, AV, component materials, and aging conditions.

Comparison of asphalt mixtures containing polymeric compounds and polymer-modified bitumen based on the VECD theory

• Under the same volumetrics, the mixes have similar stiffness and fatigue resistance. • In the field, the mixtures modified with the dry method show a workability issue. • The mixtures modified with the dry method can be more prone to thermal damage. • An intermediate stiffness can ensure limited bottom-up cracking and thermal damage. • The polymer compounds improve the performance of a mixture with neat bitumen. The ‘dry’ method that can be used to produce modified asphalt mixtures is a less expensive, less energy-consuming, and faster process than the well-established ‘wet’ method. Moreover, the dry method allows the incorporation of hard plastics, even those plastics obtained from waste products. Although researchers agree that the dry method can improve the stiffness and rutting resistance (i.e., high-temperature performance) of asphalt mixtures, they have conflicting opinions regarding mixture fatigue and cracking resistance. In this regard, this paper aims to evaluate, through the application of viscoelastic continuum damage theory, the fatigue behavior of two compound asphalt mixtures that have been modified using the dry method. One of the studied compounds is composed of plastomeric polymer and the other is composed of waste plastic with the addition of graphene. A reference mixture containing polymer-modified bitumen (representing the wet modification method) was used for comparison. The experimental program involved dynamic modulus tests and uniaxial cyclic fatigue tests of laboratory-compacted specimens and cores extracted from full-scale field test sections. The test results from the laboratory-compacted specimens and field cores were input to FlexPAVE TM for pavement performance simulations. Under the same volumetric conditions, the three dense-graded mixtures broadly had comparable stiffness and fatigue resistance values at the material level. However, in the pavement-level simulations, the reference mixture exhibited much less damage after 30 years of service than the compound mixtures. Concerning the field test track, the air void contents of the mixtures varied due to workability issues related to the presence of the compounds. Optimum performance was obtained for asphalt layers that could be characterized by an intermediate stiffness level that ensured an adequate load distribution without negative consequences for the mixture’s fatigue resistance and thermal resistance.

Evaluation of internal structure of asphalt mixtures compacted using a virtual gyratory compactor

ABSTRACT Computational models are increasingly being used to simulate the behavior of asphalt mixtures on account of their ability to conduct a broad range of parametric, sensitivity and probabilistic analyses. However, computational models rely on mix geometry as a starting point for any of these analyses, and methods to create such a geometry are either onerous or inaccurate. In this regard, a computational model using an open source “Bullet” physics engine was developed in the previous studies to virtually compact asphalt mixtures in a gyratory compactor using real 3D aggregate shapes. This present study evaluates the internal aggregate structure of asphalt mixtures compacted using the aforementioned computational model and compares with the laboratory compacted specimens. Four asphalt mixtures were compacted both in laboratory using the Superpave gyratory compactor and virtually using the computational model. Two different set of aggregate shapes were used in virtual compaction of an asphalt mixture. Results indicate that the aggregates preferred to align themselves both in laboratory compaction and virtual compaction. The difference in orientation plane was attributed to the differences in gyration of the mold versus top plate in the laboratory versus virtual compaction, respectively, which can be addressed in future by improving the simulation model.

Development of swelling induced shear and slickensides in Vertisols

Vertisols shrink and crack as they dry, and swell as they wet. Shear surfaces, sometimes called slickensides, are often observed within the profile of such clay soils. A hypothesis for the appearance of these surfaces is in reaction to lateral swelling forces that induce shear within the soil mass. No direct proof supporting this hypothesis is found in the literature.Test data presented verify the development of lateral soil swelling pressure in response to wetting of a compacted clay soil. The data illustrate that the swell induced pressure can cause the soil to reach failure conditions and in essence, shear itself.During wetting, laboratory compacted specimens of clay were subjected to constant vertical pressure and allowed to swell in the vertical direction. Strain in the lateral direction was restrained and the horizontal soil pressures monitored as they approached a residual magnitude. From the series of tests it was found that the ratio between the residual horizontal swell pressure to the applied vertical pressure decreased monotonously with increase of applied vertical pressure.Mohr's circles representing the state of stress within the specimen at the residual state align along the Mohr-Coulomb failure envelope for tests performed at vertical pressures of 4 kPa through 27 kPa.These results provide credence to the hypothesis that slickensides are formed due to internal shear of the clay, and as such are expected to form at limited depths.

Evaluation of Asphalt Mixture Performance Using Cracking and Durability Tests at a Full-Scale Pavement Facility

In the asphalt materials community, the most critical research need is centered around a paradigm shift in mixture design from the volumetric process of the previous 20-plus years to an optimization procedure based on laboratory-measured mechanical properties that should lead to an increase in long-term pavement performance. This study is focused on advancing the state of understanding with respect to the value of intermediate temperature cracking tests, which may be included in a balanced mix design. The materials included are plant-mixed, laboratory-compacted specimens reheated from the 2013 Federal Highway Administration’s (FHWA’s) Accelerated Loading Facility (ALF) study on reclaimed asphalt pavement/reclaimed asphalt shingle (RAP/RAS) materials. Six commonly discussed intermediate temperature (cracking and durability) performance testing (i.e., Asphalt Mixture Performance Tester [AMPT] Cyclic Fatigue, Cantabro, Illinois Flexibility Index Test [I-FIT], Indirect Tensile Cracking [ITC, also known as IDEAL-CT], Indirect Tensile Nflex, and Texas Overlay Test) were selected for use in this study based on input from stakeholders. Test results were analyzed to compare differences between the cracking tests. In addition, statistical analyses were conducted to assess the separation among materials (lanes) for each performance test. Cyclic fatigue and IDEAL-CT tests showed the most promising results. The ranking from these two tests’ index parameters matched closely with ALF field performance. Furthermore, both showed reasonable variability of test data and they were successful in differentiating between different materials.

Asphalt Mixture Quality Acceptance using the Hamburg Wheel-Tracking Test

This paper investigates the applicability of the Hamburg wheel-tracking test (HWTT) for asphalt mixture quality acceptance using laboratory-compacted specimens and field-compacted specimens. Density distribution functions for rut depths, stripping inflection points, and rutting resistance index (RRI) values used in the HWTT were obtained for mixtures with different nominal maximum aggregate size (NMAS) values and binder performance grades. Clear distinctions among the rut depth distributions for the high-temperature performance grade mixtures were observed in the laboratory-compacted specimens. The RRI values for both the laboratory and field-compacted specimens increased with an increase in the binder performance grade. In addition, the RRI values showed clear differences for different binder grades among the mixtures with the same NMAS. The range of the RRI distributions for the laboratory-compacted specimens was narrower than that of the field-compacted specimens. The stripping inflection points of the field-compacted specimens increased as the binder grade was increased, indicating better moisture damage resistance for stiffer mixtures. HWTT results were significantly influenced by the air voids content of specimens. The relationship between air voids content and RRI can be used for understanding the critical effect of in-place density in pavement performance. The laboratory-compacted and field-compacted specimens offer advantages and disadvantages. The laboratory-compacted specimens were much easier to fabricate to standard dimensions, and the field-compacted specimens present inherent variability in relation to air voids content, diameter, and thickness.

Microstructure characterisation of field and laboratory roller compacted asphalt mixtures

This study investigates the effect of field and laboratory roller compaction methods on the microstructure of asphalt mixtures. Loose material of varying hot mix asphalt (HMA) types was compacted in the laboratory using the steel-segmented roller compactor. Multiple compaction regimes were implemented by adjusting the compaction effort, temperature and mode. The internal structure of the laboratory roller compacted specimens was characterised in terms of the aggregate contact points, orientation and segregation by means of two-dimensional image analysis. The investigation in the laboratory demonstrated that the dominant parameters controlling the formation of the asphalt mixture microstructure are the compaction temperature and effort. In addition, the comparative evaluation of the field cores and laboratory compacted specimens revealed that the laboratory roller compaction method reproduces the microstructure induced by the field compaction. To conclude several laboratory compaction regimes may be established to closely duplicate the internal structure characteristics of field cores.

Total Recycled Asphalt Mixes: Characteristics and Field Performance

The use of recycled materials in asphalt concrete (AC) pavement has increased significantly because of their economic and environmental benefits. The use of recycled materials can pose risks to the performance of asphalt pavements, however. The Illinois Department of Transportation developed five total recycled asphalt (TRA) mixes in the pursuit of environmentally sustainable pavements. These mixes contain up to 60% asphalt binder replacement (ABR) obtained from reclaimed asphalt pavement (RAP) and recycled asphalt shingles. Virgin aggregates were replaced by 100% recycled aggregates including RAP, steel slag, and recycled concrete aggregate (RCA). Based on laboratory testing, all the mixes offered excellent rutting resistance because of their high ABR content. The TRA mixes were relatively less compliant and not very sensitive to field aging, whereas indirect tensile strength tests showed indistinguishable results. All mixes had comparable complex modulus |E*| and phase angle ([Formula: see text]) values at low temperatures. Laboratory-compacted specimens had relatively low flexibility index (FI) compared with field cores taken after construction. The FI values of the field cores decreased with aging, higher recycled materials content, or both. An exponential increase in transverse cracking was observed in the field cores because of their relatively high ABR, RCA/steel slag content, or both. The progression of field transverse cracking over time and FI values are well correlated. A three-dimensionally balanced mix design was introduced and used successfully to distinguish between AC mixes; it is proposed as a tool for better control mix designs and optimum field performance.

Establishing a resilient modulus test protocol for miniature cylindrical asphalt mix specimens

The resilient modulus is a very important parameter to be identified and used in pavement design. The indirect tensile test is the most common repeated load test to measure the resilient modulus of asphalt mixtures and the easiest method to test laboratory compacted specimens and field core specimens. The resilient moduli of asphalt mixtures are measured using the indirect tensile procedure (ASTM D4123). A metal loading strip with a concave surface with the radius of the test specimen is required to apply load to the specimen. Typically, specimens are either a nominal 100mm or 150mm in diameter with a minimum thickness over diameter ratio of 0.4. However, typical 100mm diameter core samples with 40mm to 50mm thickness taken from the site most often do not fulfil the minimum ratio of 0.4 after the samples are trimmed for testing. As such, a new procedure was developed to test specimens smaller than 100mm in diameter. This expected to be very useful and minimize the volume requirement from the field. This novel protocol for resilient modulus test using miniature specimens of 37.5mm and 56.3mm in diameter has great potential for practical relevance for the industry.

Modeling rutting susceptibility of asphalt pavement using principal component pseudo inputs in regression and neural networks

Permanent deformation is a major load-associated distress occurring in flexible pavement systems and increases with load repetitions affecting road roughness, serviceability, and the international roughness index (IRI). Early detection of rutting is necessary for maintenance and rehabilitation activities, but due to the complex behavior of asphalt mixtures, accurately predicting the permanent deformation of asphalt pavement is difficult. Historically, multivariate regression modeling and recently, artificial neural networks (ANNs) are used widely for material properties prediction. The ability to model accurately the response variable is adversely affected when inputs have pairwise correlations. To overcome this barrier, principal component analysis (PCA), as a dimensionality reduction technique, can be used to produce uncorrelated linear combinations of the original inputs as illustrated in this work using 83 (i.e., samples) laboratory compacted specimens from the State of Wisconsin. Asphalt binder, aggregate, and mix properties are obtained and used as the model inputs. The response parameter is the accumulated strain at the corresponding flow number. Using the developed pseudo inputs from PCA, a multivariate regression and an ANN model are generated and were able to fit the test cases with rfit of 0.8 and 0.97 respectively. The developed machine learning-based framework is shown to be a capable tool in estimating the rutting behavior of asphalt mixture.

In-situ assessment of the stress-dependent stiffness of unbound aggregate bases: application in inverted base pavements

Unbound aggregate bases are the primary structural components in many flexible pavements. The response of the unbound aggregate base is critical to the overall performance of the pavement, particularly in inverted base pavements given the proximity of the base to the traffic loads. The behaviour of granular materials such as unbound aggregate bases is inherently nonlinear and anisotropic. An experimental methodology is developed to assess the in-situ stress-dependent small-strain stiffness of unbound aggregate bases under controlled load using wave propagation techniques. CODA wave analysis is used to detect small changes in travel time. The methodology is applied in two distinct case histories of inverted base pavements. Results show that field-compacted granular bases exhibit higher stiffness, lower stress sensitivity and more pronounced anisotropy than laboratory-compacted specimens. The discrepancy in stiffness observed among the two field case histories is primarily attributed to traffic preconditioning sustained by the older pavement. Additional results show that the effect of suction on the stiffness of coarse-grained granular bases is insignificant.

Use of the Resilient Modulus Test to Characterize Asphalt Mixtures with Recycled Materials and Recycling Agents

Asphalt mixture stiffness is an important parameter used during pavement design and the evaluation of pavement performance. This study used the resilient modulus ( MR) test (ASTM D7369) to measure the stiffness of several types of asphalt mixtures and various specimen types. Five types of asphalt mixtures were studied: virgin mixture without recycled materials; control mixture with recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), and warm-mix asphalt additive; and three other mixtures with RAP, RAS, and different types of recycling agents. The specimen types used in the study were field cores, on-site plant mix laboratory-compacted specimens, and reheated plant mix laboratory-compacted specimens. The objectives of the study were to assess the effect on MR values of ( a) smoothness of field core surfaces and ( b) measurement angle in field cores and laboratory-compacted specimens. In addition, the study compared the MR results for the various mixture and specimen types. The results indicated that smoothness and measurement angle had no statistically significant effect on MR at the 95% confidence level. However, the test was able to identify statistically significant differences between mixture and specimen types. Finally, a comparison between MR and dynamic modulus (DM) test (AASHTO TP79) results showed equivalent ranking of the mixtures, confirming that MR is a practical alternative to DM testing.

Mechanical Responses and Viscoelastic Properties of Asphalt Mixtures under Heavy Static and Dynamic Aircraft Loads

The introduction of larger aircraft on flexible airfield pavements has led to a need for asphalt mixtures capable of sustaining such heavy loads. This laboratory and analytical study investigated the mechanical responses of a number of modified asphalt mixtures to identify their potential for use in airfield aprons and taxiways that were subjected to heavy, static, or slow-moving aircraft loads. The airfield flexible pavement section constructed at the FAA's National Airport Pavement Test Facility Construction Cycle 1 was modeled by using the three-dimensional finite element analysis software ABAQUS. Laboratory-compacted specimens of each modified asphalt mixture were tested by using AASHTO standards to determine volumetric properties and mechanical responses. The effects of static and dynamic aircraft loading were evaluated in ABAQUS with the material properties of the mixtures determined in the laboratory. On the basis of the findings of this study, it appears that several mixtures more commonly used in highway pavements, including modified mixtures, warm-mix asphalt, and reclaimed asphalt pavement, perform similarly to or even outperformed the FAA standard asphalt mixture. The results of this initial study support the idea that an opportunity exists for airports to implement emerging asphalt paving materials without compromising the pavement design life.

Influence of asphalt compaction procedure on 3D deformation properties

In this study, the effect of various laboratory procedures for compacting hot-mix asphalt to form test specimens is investigated. Based on the triaxial cyclic compression test (TCCT) according to the European specification EN 12697-25, the resistance to permanent deformation of the specimen obtained from different types of laboratory compaction is studied. The deformation behaviour of the laboratory-compacted specimens is always compared with the deformation properties stated in TCCT for the asphalt mix specimen taken from on-site roller-compacted road pavement. As a result, the effect of the choice of laboratory compaction procedure on the resulting deformation behaviour in TCCT is assessed.

Performance-Related Characterization of Bituminous Binders and Mixtures Containing Natural Asphalt

Natural asphalt as an additive in bituminous mixtures was subjected to evaluation by means of performance-related laboratory tests carried out on materials sampled from a production plant and a test section. Compaction issues were discussed by referring to binder viscosity and by considering the volumetrics of laboratory-compacted specimens. Mixture stiffness was assessed by means of repeated load indirect tensile tests carried out on field cores and by constructing pseudo master curves. Models were employed for the quantification of effects associated to enhanced compaction and binder ageing. Fatigue was also investigated by means of indirect tensile tests.

Comparison between fatigue performance of horizontal cores from different asphalt pavement depths and laboratory specimens

This study presents a comparison between the fatigue performance of field cores obtained from different layers (top and bottom) and laboratory produced–laboratory compacted specimens. The field cores were obtained from the Accelerated Pavement Testing (APT) facility at Turner-Fairbank Highway Research Center (TFHRC), located in McLean, Virginia, USA. Uniaxial push-pull (compression-tension) fatigue tests were conducted on samples obtained from the top 50 mm and the bottom 50 mm layers of 100 mm thick asphalt pavements. The samples were obtained by horizontally coring slabs extracted from six APT lanes. The damage characteristic curves (i.e., C versus S curve of the viscoelastic continuum damage (VECD) theory) and the fatigue lives of the field cores were compared with those of the laboratory specimens. The experimental test results and the subsequent analysis findings revealed comparable damage characteristic curves and fatigue lives for the top and bottom field specimens; meanwhile, the field and laboratory specimens exhibited different damage characteristic curves and fatigue lives.