Reclaimed Asphalt Pavement Particles Research Articles

RAP chunks produced in cold milling operation of asphalt pavement: Evaluation, mechanism, and engineering investigation in China

Cold milling is a widely used method for rehabilitating asphalt pavement, generating reclaimed asphalt pavement (RAP) chunks. Within this process, aggregates within the asphalt pavement will be crushed, forming RAP agglomerates and aggregate breakdown. However, the mechanism of these phenomena has remained unclear, and a unified evaluation method has yet to be established. In this study, RAP agglomeration and aggregate fragmentation were characterized, five distinct methods were systematically assessed, and the mechanism of RAP agglomeration and breakdown was analyzed by discrete element method (DEM) simulation based on setting different particle contact parameters, then followed by a mechanical analysis, and demonstrated in engineering. The results revealed that both agglomeration and aggregate breakdown occur within RAP particles of various sizes, with the five methods showing similar trends in quantifying these effects. Through DEM simulations and mechanical analyses, the aggregate breakdown predominantly occurs at the cutter's motion trajectory of the cutter and during crack propagation, while agglomeration was mainly related to the sliding surface's area. The milling speed and depth positively impact RAP agglomeration, while negatively affecting aggregate breakdown, and milling drum speed exerts minimal influence on these phenomena. RAP agglomeration varies considerably in different engineering projects, and cold milling parameters should be determined based on the material composition of the asphalt pavement and design requirements to control agglomeration and breakdown rates of RAP.

Experimental and predictive study of self-compacting concrete containing reclaimed asphalt pavement

This work focuses on the reuse of reclaimed asphalt pavement (RAP) in the production of self-compacting concrete (SCC) for usage in the construction of rigid pavements. The article first describes an experimental study where five SCC mixtures incorporating 0%, 20%, 40%, 60% and 100% volumetric substitutions of natural aggregates by RAP were formulated. Several characterisation tests in the fresh and hardened states were then performed. The results of all performed tests showed that RAP has a slight negative effect on SCC performance in the fresh state. The most significant reduction observed does not exceed 18% for slump-flow and 10% for L-shaped box values when total substitution is applied. Furthermore, the mechanical properties noticeably decrease with an increase in RAP content, with reductions of up to 37% in 28-d compressive strength and 23% in 28-d indirect tensile (IDT) strength observed in the case of full replacement. However, it is possible to formulate SCC mixtures even with 100% RAP substitutions and meet specifications for rigid pavement construction. In addition, modelling of the various mechanical characteristics made it possible to find out the reason behind the reduction in these properties with RAP content. In fact, the thin bitumen film around RAP particles weakens their surface adhesion with the hydrated cement paste. With the intrinsic RAP properties found, the hardened-state characteristics of any other SCC mixture incorporating RAP could be predicted.

Effect of design parameters on degree of blending and performance of recycled hot-mix asphalt incorporating fine reclaimed asphalt pavement particles

Due to the rich binder content and high economic benefits of fine reclaimed asphalt pavement (RAP) particle, more and more attention has been paid to the effective design of recycled hot-mix asphalt (HMA) incorporating this material. This research explored the effect of design parameters on degree of blending (DoB) and performance of recycled HMAs incorporating fine RAP particles. Six groups of mixes were designed, including one no blending group, four experimental groups, and one full blending group. The different parameters of preheating temperature, compaction effort, asphalt content (AC), and blending duration, were involved in this research. It can be found that the recycled HMAs incorporating fine RAP particles under the traditional design proved to the risks in water stability and low-temperature performance. By rising the preheating temperature, decreasing compaction effort and increasing AC, or extending the blending duration, more RAP binder would be activated and blended with the virgin binder, and the poor rheological performance of recycled HMAs can be restored. The different design parameters have different effects on the performance. Besides, the actual contribution of DoB to performance was quantified. With an increase in DoB by 10%, the tensile strength ratio, shear strength, fracture energy, fracture strength, and abrasion loss percentage of recycled mixes can improve by around 15%∼28%, 0.15–0.34 MPa, 180–1969 J·m2, 0.25–0.34 MPa, and 7–18%, respectively. In addition, design incorporating fine RAP particles generally can bring significant advantages in energy consumption and CO2 emission.

Optimizing RAP sieving efficiency of linear vibrating sieve using DEM simulation

Producing recycled hot-mix asphalt mixtures (RHMA) requires an adequate stockpile of reclaimed asphalt pavement (RAP). To improve the content of RAP in RHMA and maintain the quality stability of RHMA, crushing and sieving technology was applied to pretreat RAP, which is mentioned in various technical standards across the world. However, the obstacle that affects the RHMA output during continuous production in any real-life project is the low efficiency of RAP crushing and sieving. A model of a three-dimensional, three-layer linear vibrating sieve was established in the numerical software of the Discrete Element Method (DEM) by considering the sieving combination commonly used in engineering. The Linear Model and Linear Parallel-Bond Model were defined for the contact properties of the RAP particles. Based on DEM simulation, the influence of sieving parameters such as vibration amplitude and frequency as well as inclination and length of the sieve mesh were evaluated by sieving efficiency. Furthermore, the optimal RAP sieving parameters were obtained and verified in the project. The results show that the sieving efficiency first increases and then decreases with an increase in amplitude, frequency, and inclination; however, the length of the sieve mesh has little effect on the sieving efficiency. The order of influence of the different parameters on the RAP sieving efficiency is as follows: amplitude > frequency > inclination > length. The optimal sieving parameters of RAP were 20 Hz, 3 mm, and 10°, respectively, for frequency, amplitude, inclination. The optimum sieve inclination of virgin aggregate was 20° higher than that of RAP. The deviation between the sieving efficiency of the DEM simulation test and the plant sieving test was within 15%, indicating that the DEM simulation had high accuracy. The study could be of interest to sieving plant manufacturers, pavement maintenance sector, and construction contractors.

The Impact of the RAP Cluster Dissociation on Gradation and Shape Properties of Aggregates from Recycled Asphalt Mixtures

Abstract The presence of clusters in reclaimed asphalt pavement (RAP) is a critical point that affects the RAP homogeneity. In this phenomenon, different RAP particles are working together as a single aggregate. The cluster dissociation occurs during hot asphalt mixture production and compaction in the field. This research is concerned with the impact of the cluster disintegration on the recycled asphalt mixture aggregate properties. Stages of binder extraction were performed in six RAP materials to partially activate the RAP binder and create different levels of cluster dissociation from 0 to 100 %. Then, the gradation curve and the aggregate shape properties were obtained for each level. Using a gradation curve (without RAP) as a reference, a hypothetical recycled asphalt mixture was created with different RAP percentages to investigate the impact of cluster dissociation on the aggregate properties of the recycled mixture. The results indicate that the RAP aggregate properties are significantly affected by the cluster dissociation. Besides, there was a significant impact on the partial cluster dissociation, because of RAP-binder activation, on the gradation curve and aggregate shape of the recycled mixture, mainly when the level of RAP-binder activated is between 40 and 70 %.

Application of Sieve Analysis to Estimate Recycled Binder Availability

One of the challenges of engineering asphalt mixtures containing reclaimed asphalt pavement (RAP) is uncertainty in the proportion of the total recycled asphalt binder that is available to interact and blend with the virgin asphalt, referred to as the recycled binder availability. The industry presently lacks a practical method to quantify RAP binder availability. Research has shown that the primary source of unavailable recycled binder is agglomerations of adhered RAP particles. The binder bound within the agglomerations is unavailable to contact and therefore blend with virgin asphalt. Building on this knowledge, this study establishes a practical method to quantify the extent of RAP agglomeration and, in turn, RAP binder availability by comparing the gradation of recovered RAP aggregates with that of the RAP itself. A size-exclusion method and corresponding predictive equation to estimate RAP binder availability from the high-temperature performance grade of recovered RAP binder and mixing temperature were also assessed. Four RAP sources were evaluated. Each RAP stockpile was paired with virgin aggregates from the same plant that the RAP was sourced at to produce eight mixtures. Tracer-based microscopy measurements within the eight mixtures were generally in good agreement with the estimations of recycled binder availability using sieve analysis. Implementing the size-exclusion method was challenging with local aggregate, and estimates using the predictive equation yielded in some cases good but overall poorer agreement with the measurements of recycled binder availability from tracer-based microscopy compared with the sieve analysis approach.

Generalized Interface Shear Strength Equation for Recycled Materials Reinforced with Geogrids

In this research, large direct shear tests were conducted to evaluate the interface shear strength between reclaimed asphalt pavement (RAP) and kenaf geogrid (RAP–geogrid) and to also assess their viability as an environmentally friendly base course material. The influence of factors such as the gradation of RAP particles and aperture sizes of geogrid (D) on interface shear strength of the RAP–geogrid interface was evaluated under different normal stresses. A critical analysis was conducted on the present and previous test data on geogrids reinforced recycled materials. The D/FD, in which FD is the recycled materials’ particle content finer than the aperture of geogrid, was proposed as a prime parameter governing the interface shear strength. A generalized equation was proposed for predicting the interface shear strength of the form: α = a(D/FD) + b, where α is the interface shear strength coefficient, which is the ratio of the interface shear strength to the shear strength of recycled material, and a and b are constants. The constant values of a and b were found to be dependent upon types of recycled material, irrespective of types of geogrids. A stepwise procedure to determine variable a, which is required for analysis and design of geogrids reinforced recycled materials in roads with various gradations was also suggested.

Measurement of particle agglomeration and aggregate breakdown of reclaimed asphalt pavement

Cold milling of old asphalt pavement is widespread in pavement resurfacing. It produces reclaimed asphalt pavement (RAP) and changes the state of the old asphalt pavement; the pavement is cut into particles, known as RAP particles. RAP particles are composed of many smaller aggregates bonded by the aged binder (particle agglomeration). The aggregates contained in the old pavement can be crushed into smaller pieces in the milling operation (aggregate breakdown). Particle agglomeration increases the risk of insufficient moisture stability and fatigue resistance of the recycled asphalt mixture; aggregate breakdown often prevents the use of a higher RAP content in the new mixture. In this study, the effect of these two problems is studied through a pavement maintenance project. The effect of milling speed on the RAP particle agglomeration rate is evaluated. The size-susceptibility of the agglomerate d particles, gradation deviation of the recycled mixture, and morphology classification of RAP particles are discussed. The effect of milling speed on aggregate breakdown is also investigated. The results show that approximately 30%–50% of RAP particles are made of aggregates smaller than the corresponding particle size. The agglomeration rate increases with an increase in milling speed and RAP particle size. High milling speed produces a lower rate of aggregate breakdown, resulting in a higher coarse aggregate content and a lower filler and fine aggregate content than a low milling speed.

Laboratory Investigation of Compaction Characteristics of Plant Recycled Hot-Mix Asphalt Mixture

In this study, the compaction characteristics of recycled hot-mix asphalt (RHMA) were evaluated using the void content (VV), compaction energy index (CEI), slope of accumulated compaction energy (K), and lock point (LP). Then, the effects of the compaction parameters, including the gradation of the RHMA, reclaimed asphalt pavement (RAP) content, temperature of gyrations, and number of gyrations, on the compaction characteristics of RHMA were investigated. An orthogonal experiment was designed and the data collected were analyzed via range analysis; then, a regression model was generated relying on a quadratic polynomial. Furthermore, the regression model was used for the comparison and prediction of the mixture’s compactability during the material design. Finally, the compaction mechanism of RHMA was discussed from the perspective of the void content of RAP particles. The results showed that a finer aggregate gradation, a higher gyration temperature, a greater number of gyrations, and a higher RAP content were effective for increasing the compactability of RHMA. The range analysis results suggest that the gradation of RHMA has the greatest influence on compactability, followed by the RAP content. The RAP aggregate cannot diffuse to a new mixture completely, so the remained RAP particle reduces the void content of RHMA. Therefore, a higher RAP content up to 50% can help RHMA to achieve the designed void content with higher efficiency.

The Influence of Reclaimed Asphalt Pavement on the Mechanical Performance of Bituminous Mixtures. An Analysis at the Mortar Scale

The main characteristics of bituminous mixtures manufactured with a considerable amount of reclaimed asphalt pavement (RAP), compared to conventional mixtures, are a reduction in workability, an increase in stiffness, and a loss of ductility, due to the presence of the aged bitumen contained in the RAP particles. To minimize these impacts, softer binders or rejuvenators are commonly used in the design of these mixtures in order to restore part of the ductility lost and to reduce the stiffness. In spite of previous investigations demonstrating that the mortar plays an essential role in the workability, long-term performance, and durability of bituminous mixtures (where cracking, cohesion, and adhesion problems all start at this scale), not many studies have assessed the impacts caused by the presence of RAP. In response to this, the present paper analyzes the workability, fatigue performance, and water sensitivity of bituminous mortars containing different amounts of RAP (from 0% to 100%) and rejuvenators. Mortar specimens were compacted using a gyratory compactor and studied via dynamic mechanical analysis under three point bending configuration. The results demonstrated that the presence of RAP reduces the workability and ductility of asphalt mortars. However, it also causes an increase in their stiffness, which induces a more elastic response and causes an increase in their resistance to fatigue, which could compensate for the loss of ductility. This aspect, together with the low water sensitivity shown, when using Portland cement as an active filler, would make it possible to produce asphalt materials with high RAP contents with a similar long-term mechanical performance as traditional ones. In addition, the use of rejuvenators was demonstrated to effectively correct the negative workability and ductility impacts caused by using RAP, without affecting the fatigue resistance and material adhesion/cohesion.

Characterization of agglomeration of reclaimed asphalt pavement for cold recycling

Agglomeration of reclaimed asphalt pavement (RAP) is one of the most important factors affecting performance of cold recycled asphalt mixture. This research is conducted to evaluate the agglomeration of RAP collected from various sources including both raw milled-off and plant-crushed RAP. Three different test methods were used to examine the breaking of agglomerates of RAP, including asphalt extraction test, modified LA abrasion test and mixing test. LA abrasion revolution of RAP samples was performed at various rotations of 50r, 100r, 200r and 300r, and mixing test was performed at different mixing time of 0.5 min, 1 min, 2 min and 3 min. Effects of the testing conditions and plant-crushing on RAP de-agglomeration were evaluated. The obtained results showed that all three tests could effectively reflect the agglomeration property of RAP materials. Good correlations were found among results of the three tests, while abrasion test and mixer test typically have lower loss% than asphalt extraction test. A general indicator of agglomeration degree was proposed for the tests to describe the agglomeration property of RAP and a classification method of RAP particles was proposed based on the results and findings in the tests. Weak structure of RAP has significantly higher agglomeration degree, while old aggregate has very low agglomeration degree. It is suggested to reduce content of weak RAP in the cold recycling process due to its negative impact on the performance of regenerated mixture.

A review of thermal processes in the production and their influences on performance of asphalt mixtures with reclaimed asphalt pavement (RAP)

Abstract The use of reclaimed asphalt pavement (RAP) in asphalt mixtures is considered as one of the best sustainable practices in the construction of transportation infrastructures. Extensive efforts have been made to design, produce, and evaluate RAP mixtures produced by the hot recycling technology in asphalt plants. However, the inconsistencies or contradictive conclusions regarding the performance of RAP mixtures reported in the literature impede the use of high percentage of RAP in asphalt mixtures. One of the knowledge gaps for the discrepant behaviors of RAP mixtures is the negligence of the influence of production thermal processes, e.g. mixing time, production temperature, silo storage temperature and time, etc., on the mechanical performance of RAP mixtures. In this context, this paper synthetically reviewed the fundamental thermodynamics and kinematics involved in the plant production of RAP mixtures and the corresponding numerical methodologies to quantify the thermal processes. Three prevailing numerical methods, including overall energy balance method, 1-D thermal-granular model, and coupled computational fluid dynamics and discrete element method, are used to quantify the temperatures of hot gases, virgin aggregates, and RAP particles in the drum dryer. Then, the key production parameters that could influence the performance of RAP mixtures were identified based on the experimental studies. The review results from the experimental studies highlighted the importance of mixing time, mixture discharge temperature, and silo storage time on the mechanical performances of laboratory-produced and plant-produced RAP mixtures. The review findings from this study can be used to guide the best practice of the production of RAP mixtures and further contribute to the energy saving and sustainable construction of using RAP in asphalt mixtures.

Micro-mechanical interaction of activated fly ash mortar and reclaimed asphalt pavement materials

Reclaimed asphalt pavement (RAP) is a milled material obtained from the distressed pavement sections, containing a thin coating of aged bitumen on the aggregate surface. In spite of their inferior properties, RAP has been promoted to use in new pavement layers after blending with virgin aggregates (VA) and/or chemical stabilizers such as fly ash. Due to thin amorphous asphalt coating present on RAP aggregates, expected level of improvement is not witnessed. Hence, the non-reactive type fly ashes can be activated in an alkali environment, such as lime, sodium hydroxide, potassium hydroxide or a combination of sodium hydroxide and sodium silicate to further improve the reactivity to participate in the pozzolanic reactions. The present study investigates the degree of chemical and micro-mechanical interactions between the alkali (sodium hydroxide) activated fly ash mortar and RAP:VA base course mixes on strength development. To accomplish this task, specimens were prepared at various proportions of RAP and VA materials; stabilized with fly ash at 20% and 30% by dry weight with and without alkali activation at 2% and 4% sodium hydroxide (NaOH) solution. These mixes were cured for 1, 7 and 28days and then tested for their unconfined compressive strength (UCS). Mineralogical studies such as X-ray fluorescence (XRF), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) were then conducted on the powdered samples to identify and quantify the formation of possible hydration products. Since the RAP aggregates are coated with an amorphous asphalt coating, the amount of exposed aggregate surface of RAP particles plays a major role in the formation of pozzolanic compounds with fly ash. To verify the amount of exposed surface of RAP particles, surface area analysis was performed through high definition image processing. The interaction mechanism of RAP with other constituents of the mix is also studied. It is observed that a random RAP sample has exposed aggregate surface area between 15 and 70%, which is responsible for high pozzolanic reactions between the alkali activated fly ash mortar and RAP aggregates. The data are presented in terms of the exposed surface area and the strength characteristics of the mixes.

Back-calculation method for determining the maximum RAP content in Stone Matrix Asphalt mixtures with good fatigue performance based on asphalt mortar tests

This paper presents a simple method to determine the amount of Reclaimed Asphalt Pavement (RAP) that can be added to Stone Matrix Asphalt (SMA) mixtures without compromising the fatigue resistance. Dynamic Shear Rheometer tests (DSR) on binder and mortars composed with fine RAP particles, called Selected Recycled Asphalt Pavement (SRAP) are used together with the Nielsen model to back-calculate the norm of the complex modulus of the bituminous blend of fresh and RAP binder. Fatigue properties are derived from parameter G*sinδ and Linear Amplitude Sweep tests. The analysis indicates that a limiting SRAP binder content of 23% can be included in SMA mixtures with satisfactory fatigue performance.

A new laboratory methodology for optimization of mixture design of asphalt concrete containing reclaimed asphalt pavement material

The reduction of virgin bitumen added to asphalt mixtures containing Reclaimed Asphalt Pavement (RAP) is based on the typical assumption that all the aged binder function in the same way as the virgin binder. However, recent studies conducted by the authors for a specific case show that a blend or mobilization of RAP binder are negligible. The aged bitumen becomes softer acting as glue facilitating cluster formation between small-size RAP particles. The reduction of small-size particles causes changes in the target grading curve and in the voids-fill, affecting the compactability of RAP mixtures. Therefore the target grading curve of RAP mixtures needs to be readjusted, using different proportions of virgin aggregates and taking into account the cluster phenomenon. The objective of this paper is to develop a new mix design approach for RAP mixtures, taking into account the cluster phenomenon and the contribution of the aged bitumen in the compactability. The virgin aggregates, filler and RAP are investigated and individually included in the calculation. 3D images of the virgin aggregates allowed the determination of new surface area factors; the concept of critical filler concentration led to the definition of the minimum bitumen quantity required to maintain the mastic in a diluted state and fill the voids. A RAP clustering model was introduced to predict the agglomeration of small-size RAP particles. The readjustment of the target grading curve was analytically calculated, allowing the correct estimation of the amount of virgin bitumen to be added to asphalt mixtures. Finally, a first verification of the entire process was carried out performing laboratory tests. These promising results enable the challenge of a new mix design optimization for HMA with high RAP content to be addressed.

Machine vision based characterization of particle shape and asphalt coating in Reclaimed Asphalt Pavement

Reclaimed Asphalt Pavement (RAP) particles are created from the impact removal and/or reprocessing of existing asphalt layers. RAP particles contain a combination of asphalt and aggregates with varying degrees of coating and morphology. Particle size and shape properties, amount of asphalt coating the RAP particles, and the binder content of the RAP are among the important engineering properties that control the performance of this material. This paper introduces an innovative machine vision-based inspection system to quantify the percentage of asphalt coating in different RAP aggregate sources. The Enhanced-University of Illinois Aggregate Image Analyzer (E-UIAIA) is used to acquire the color RGB images of RAP particles from six different sources with sizes between 1/4 in. (6.35 mm) and 1/2 in. (12.5 mm). The influence of asphalt coating percentage on the RAP particle size and shape properties are quantified in this paper. Then, using the advanced color image thresholding scheme incorporated in the E-UIAIA, the corresponding segmented binary images of RAP particles are generated. A newly defined image mean property is used as an automatic variable threshold limit to segment the bright areas in the associated grayscale version of RAP images to detect the uncoated areas on each particle. A relationship was found between the results of the proposed image processing technique in terms of asphalt coating percentages and the asphalt content of the RAP. Furthermore, the asphalt surface coating percentages could be successfully correlated to the fracture energies of concrete specimens containing these RAP particles blended with other virgin aggregates.

Reclaimed asphalt binder aging and its implications in the management of RAP stockpiles

Reclaimed asphalt pavement (RAP) is a granular composite geomaterial that is highly reactive to the environment under normal atmospheric conditions. The binder aging processes that contribute to pavement deterioration do not stop upon reclamation. Here we report on the results of conventional and a novel RAP characterization test. RAP binder content has been observed to increase as RAP-particle size decreases. A simple analytical model is proposed to explain the results. Binder properties also change as a function of RAP-particle size. A simple thermomechanical test developed as part of this study shows differences in the temperature-dependent deformation of RAP particles of different sizes. These results evidence qualitative differences in binder viscosity, which can be attributed to RAP-binder aging.

Thermal conductivity of reclaimed asphalt pavement (RAP) and its constituents

Up to the present, most work on the use of reclaimed asphalt pavement (RAP) has been empirical in nature. Very recent advances have demonstrated that finite-element techniques can be effectively used for modelling asphalt mixing drums in order to optimise the relative proportions of new and recycled materials and to determine the amount of time required to achieve full melting inside of the drum. A necessary prerequisite for the modelling is a definitive knowledge of the thermal conductivities of RAP and its components. This need motivated the present experimental work which encompassed RAP particles, RAP particles with the asphalt binder removed, and pure asphalt binder of different degrees of ageing. Also evaluated were taconite tailings, residual rock from the processing of iron-containing ore, and sand. The tailings have been mentioned as a candidate aggregate. The conductivity results for the solid media were related to three metrics: (a) the size ranges of the solids, (b) the density of the sample as a whole and (c) the porosity of the sample. All of the conductivity results for the investigated solid media fell in the range from 17 to 30 W/m °C. The measured conductivities of the binder ranged from 0.17 to 0.19 W/m °C.

Laboratory Investigation on Residual Strength of Reclaimed Asphalt Mixture for Cold Mix Recycling

The purpose of this study is to evaluate the residual strength of reclaimed asphalt pavement (RAP) to understand the strength forming mechanism of RAP in cold recycled asphalt mixture. The mechanical properties of the RAP were investigated as compared to the properties of pure aggregates. Triaxial shear tests were conducted to analyze the residual cohesion of asphalt binder in the RAP. The gradation and specific surface area of RAP were analyzed to understand the agglomeration structure of coarse RAP particles. Splitting strength of cement-RAP mixtures was tested as compared to cement-aggregate mixtures. It can be concluded that the RAP in the cold recycled asphalt mixture does not act exactly like ＂black color＂ aggregates. The aged asphalt in the RAP has active cohesion although its cohesion strength is much lower than the binder in the hot-mix asphalt mixture. The coarse RAP particles formed by agglomeration of fine particles and the aged binder have important influences on the strength of cold recycled asphalt mixtures. The cement-RAP mixture has much lower splitting strength than the cement-aggregate mixture with the same gradation and cement content. It is important to add new aggregate into the recycled mixture to increase the effective aggregate surface area that can be coated with the new binder.