Mugurel Turos

Determination of Asphalt Mixture Creep Compliance at Low Temperatures by Using Thin Beam Specimens

The thermal cracking (TC) module (TCMODEL) of the recently developed Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures (referred to as MEPDG) is used to predict thermally induced cracking in asphalt pavements over their service lives. The primary input to this model is the asphalt mixture creep compliance. The current standard test for determination of the creep compliance of asphalt mixtures is the indirect tensile test (IDT). This paper investigates the feasibility of using the bending beam rheometer (BBR) device to determine the low-temperature creep compliance of thin asphalt mixture beams (127 × 12.7 × 6.35 mm). The BBR device was used to evaluate 20 different asphalt mixtures, and the results were compared with the standard IDT results. Direct comparison of the BBR and the IDT results indicated that both methods produce slightly different creep compliance curves and that the relative ratio between the BBR and the IDT results varies with time and temperature. A simple phenomenological relation that gives good predictions of the IDT results on the basis of BBR creep compliance is proposed. The measured and predicted creep compliance curves were input into the MEPDG TC module, and the predicted depth of cracks and the amount of cracking were compared. The comparison showed that predicted creep compliance determined on the basis of the BBR results can be successfully used to estimate thermal cracking by use of the TCMODEL. It was concluded that the BBR device can be used for the practical and surrogate estimation of the creep compliance of mixtures. The proposed equation relating BBR and IDT creep compliances should be further validated with different types of mixtures.

Investigation of asphalt mixture creep compliance at low temperatures

ABSTRACT The creep compliance is one of the main material characteristics used to describe low temperature behavior of the asphalt mixtures. It also serves as a primary input to the current thermal cracking model in the US that is used to predict thermally induced cracking in asphalt pavements over their service life. The current standard method in the US to determine creep compliance of asphalt mixtures is the Indirect Tensile (IDT) test. This paper investigates the feasibility of using the Bending Beam Rheometer (BBR) device to determine the low-temperature creep compliance of thin asphalt mixture beams (127x12.7x6.35mm). The BBR was used to evaluate 20 different asphalt mixtures and the results were compared with the standard IDT results. Direct comparison of the BBR and the IDT results indicate that both methods produce slightly different creep compliance curves and the relative ratio between the BBR and the IDT results varies with time and temperature. A simple phenomenological relation was proposed that gives good predictions of the IDT results based on the BBR creep compliance. Furthermore, short-term aged asphalt binders used in the mixtures were also tested in the BBR. Modified Hirsch model was applied to the BBR results on both mixtures and binders and it was shown that this model is capable of producing quite accurate results in forward and inverse predictions using considered dataset. It was concluded that the BBR can be used in practical and surrogate estimation of the mixture creep compliance but proposed scheme requires validation on other mixture types.

Investigation on asphalt binder strength at low temperatures

A new strength test method for asphalt binders at low temperatures is investigated in this paper. A modified Bending Beam Rheometer (BBR), capable of applying loads at different rates, is used to obtain asphalt binder flexural strength. Strength values obtained from the BBR test are compared with strength values obtained from the Direct Tension Tester (DTT) test and significant differences are found between the two tests. Additional BBR strength tests are run on asphalt binder beams after replacing the ethanol used in the cooling bath with potassium acetate, similar to DTT testing conditions, and flexural strengths significantly larger than those obtained with ethanol are obtained. Weakest link theory is used to convert three-point bending strength to direct tension strength and to perform volume correction of DTT values. Based on the corrected values, the BBR and DTT strength measurements are similar, when the same cooling medium is used. The effect of the cooling medium on BBR creep stiffness is also evaluated. From the limited experimental tests, it appears that there are no significant differences in stiffness and m-value after 1 h of isothermal storage.

Comparison of Thermal Stresses Calculated from Asphalt Binder and Asphalt Mixture Creep Tests

Low-temperature cracking is a significant distress in asphalt pavements built in the northern United States and Canada. As temperature decreases, thermal stresses develop in the restrained asphalt surface layer; and when the temperature reaches a critical temperature, cracking occurs. The current guides use thermal stress as a critical input parameter in the low-temperature performance model. In this paper, statistical and graphical analyses are performed to compare thermal stresses that develop in an idealized asphalt pavement calculated from mixture creep data obtained using indirect tensile test (IDT) and bending beam rheometer (BBR) test. In addition, the idea of obtaining thermal stresses from binder BBR creep data is further investigated. Thermal stresses calculated using IDT and BBR mixture creep data, respectively, are similar. Thermal stresses calculated from binder creep data are significantly different than thermal stresses calculated from mixture creep data. The effect of physical hardening is investigated for a limited number of materials, and the effect on thermal stresses is significant.

Bending beam rheometer testing of asphalt mixtures

This paper provides a test protocol for performing creep tests on asphalt mixture beam specimens using the bending beam rheometer, and addresses the issues related to performing this test. First, a detailed sample preparation procedure is presented and the experimental data are provided to assess the consistency of this method. Then, three loading methods are investigated, and results are analysed and compared using statistical tools. Finally, a preliminary investigation on the representative volume element of asphalt mixtures at low temperatures is performed by testing beams of different sizes.

The flexural strength of asphalt mixtures using the bending beam rheometer

University of Minnesota M.S. thesis. December 2010. Major: Civil engineering. Advisor: Dr. Mihai O. Marasteanu. 1 computer file (PDF); iv, 66 pages.

Pressure Aging Vessel and Low-Temperature Properties of Asphalt Binders

The oxidative aging during the service life in asphalt binders used in construction of asphalt pavements significantly affects the performance of these pavements. A study investigating the effect of the pressure aging vessel (PAV) laboratory aging procedure on low-temperature properties of asphalt binders is presented. Bending beam rheometer creep tests and direct tension fracture tests were performed on laboratory-aged binders as well as extracted binders. Significant differences existed between extracted binder and PAV binder behavior at low temperatures. PAV aging was not always detrimental for asphalt binder fracture properties compared with the aging from rolling thin-film oven testing.

Testing protocol to obtain failure properties of asphalt binders at low temperature using creep compliance and stress-controlled strength test

Good fracture properties are an essential requirement for asphalt materials used in the construction of pavements in cold regions. For asphalt binder, two instruments were developed during SHRP research effort to investigate the low-temperature behaviour of these materials: bending beam rheometer (BBR) and direct tension tester (DTT). These two devices are used to obtain the performance grade (PG) of asphalt binders in the US. The DTT is expensive and the results are less repeatable due to a complex sample preparation and difficult-to-achieve strain-controlled loading. For these reasons, many agencies do not use DTT when selecting asphalt binders, and rely entirely on the creep properties obtained with the BBR testing method. In this paper, a modified BBR is used to perform three-point bending strength tests on asphalt binder beams at low temperature, with the final goal of developing a specification for binder selection, similar to the current PG specification. BBR strength and DTT tests are first performed on a common set of binders, and the results are compared using size effect theory. Then, additional tests are performed and a simple and practical strength test procedure based on BBR creep data and on BBR strength data obtained under constant loading rate is proposed.

GRAPHENE NANO-PLATELET (GNP) REINFORCED ASPHALT BINDERS AND MIXTURES

Researchers at the University of Minnesota have developed a graphene nano-platelets reinforced asphalt binder that has superior mechanical properties over pavement service temperatures, compared to existing binder formulations. The costs of these materials are comparable to polymer modified binders that are considered as best performers in pavement construction. Examples will be presented to show that for some formulations, asphalt binder strength at low temperatures doubled compared to the original binder, and fracture energy increases significantly for asphalt mixtures. It was also found that the addition of graphene nano-platelets significantly reduces the compaction effort required to prepare asphalt mixtures. For some formulations, the reduction in compaction effort is more than half of the original mixtures. Results will be presented for mixture test samples prepared using traditional mixture preparation, as well as mixture test samples prepared from loose mix. The effect on rutting behavior is also discussed Keywords:Asphalt, Compaction, Complex Modulus, Fracture-toughness, Low-Temperature E&E Congress 2016 | 6th Eurasphalt & Eurobitume Congress | 1-3 June 2016 | Prague, Czech Republic GRAPHENE NANO-PLATELET (GNP) REINFORCED ASPHALT BINDERS AND MIXTURES Mihai Marasteanu Professor Email: maras002@umn.edu Jia-Liang Le Assistant Professor Email: jle@umn.edu Mugurel Turos Scientist Email turos001@umn.edu Department of Civil, Environmental, and GeoEngineering University of Minnesota Minneapolis, MN 55455 USA ABSTRACT Researchers at the University of Minnesota have developed a graphene nano-platelets reinforced asphalt binder that has superior mechanical properties over pavement service temperatures, compared to existing binder formulations. The costs of these materials are comparable to polymer modified binders that are considered as best performers in pavement construction. Examples will be presented to show that for some formulations, asphalt binder strength at low temperatures doubled compared to the original binder, and fracture energy increases significantly for asphalt mixtures. It was also found that the addition of graphene nano-platelets significantly reduces the compaction effort required to prepare asphalt mixtures. For some formulations, the reduction in compaction effort is more than half of the original mixtures. Results will be presented for mixture test samples prepared using traditional mixture preparation, as well as mixture test samples prepared from loose mix. The effect on rutting behavior is also discussedResearchers at the University of Minnesota have developed a graphene nano-platelets reinforced asphalt binder that has superior mechanical properties over pavement service temperatures, compared to existing binder formulations. The costs of these materials are comparable to polymer modified binders that are considered as best performers in pavement construction. Examples will be presented to show that for some formulations, asphalt binder strength at low temperatures doubled compared to the original binder, and fracture energy increases significantly for asphalt mixtures. It was also found that the addition of graphene nano-platelets significantly reduces the compaction effort required to prepare asphalt mixtures. For some formulations, the reduction in compaction effort is more than half of the original mixtures. Results will be presented for mixture test samples prepared using traditional mixture preparation, as well as mixture test samples prepared from loose mix. The effect on rutting behavior is also discussed KEY WORDS: Asphalt, Compaction, Complex Modulus, Fracture Toughness, Low Temperature 1. INTRODUCTION In the past decade, significant efforts have been devoted to improving the performance of asphalt pavements. A major part of these efforts was focused on the development of new asphalt-based pavement materials with better performance and increased durability. Recently, there has been an emerging interest in applying nanotechnology to asphalt pavement materials [1, 2] and a number of attempts were made to incorporate carbon nanotubes (CNTs) into the asphalt binders and mixtures. It was found that the dispersion of CNTs in asphalt binders represents a major challenge [2], and the very high material cost make the application of CNTs in asphalt pavements prohibitive. In this study, the application of Graphene Nanoplatelets (GNPs) to asphalt binders and mixtures is investigated. The GNP is made from exfoliated graphene, which has been shown to possess superior mechanical and
electron transport properties [3-6]. The aspect ratio of GNP is significantly lower than that of CNT, which makes dispersion easier. In addition, the cost of GNP is comparable to some common polymer modifiers such as styrene butadiene styrene (SBS), which makes it a very attractive candidate for asphalt paving applications. 2. EXPERIMENTAL WORK The experimental work consisted of testing performed on asphalt binders over a wide range of temperatures and on testing performed on asphalt mixtures at low temperatures. Different types of GNP materials were used in the experiments: MICRO750 (M750): a graphene nano-flake powder with minimum 96.22% carbon and an average surface area of 12 m/g; MICRO850 (M850): a graphene nano-flake material with 99.54% carbon and an average surface area of 12m/g; and 4827: a surface enhanced synthetic graphite material with 99.66% carbon and 0.34% ash and an average surface area of 250 m/g. 2.1. Asphalt binder testing Binder testing was done following the Superpave Performance Grade (PG) Specifications that are detailed in a number of AASHTO specifications and are based on rheological measurements using a Dynamic Shear Rheometer (DSR) used to obtain complex modulus and phase angle of viscoelastic materials, a Bending Beam Rheometer (BBR) used to obtain the creep stiffness and the slope of the creep stiffness curve called m-value, and strength tests performed at low temperatures. In this investigation, a modified BBR instrument called BBR Pro, which has a proportional valve that E&E Congress 2016 | 6th Eurasphalt & Eurobitume Congress | 1-3 June 2016 | Prague, Czech Republic offers a complex control of the pressure in the air bearing system and a 44 N load cell, was used to perform strength tests under constant loading rate until failure. The procedure is explained in detail elsewhere [7]. Preliminary experiments were performed on a plain PG 58-28 asphalt binder in unaged condition and focused on low temperature properties. The GNP samples were carefully added to the hot asphalt binder and then mixed with a glass rod until a homogeneous mix was observed. An initial proportion of 6% by weight was used. No problems related to potential clustering of the graphene platelets were detected. BBR creep stiffness results, obtained at -24°C on two replicates, are shown in the Figure 1. Figure 1. Preliminary BBR results for unaged binder PG58-28 It can be observed that the addition of the graphene type 850 significantly increases the creep stiffness of the asphalt binder; the stiffness more than doubles compared to the plain asphalt binder. Smaller changes are observed for the other types of graphene. At the same time, the slopes of the curves do not change, which indicates that the “m-values” do not change and that the relaxation properties are not affected by the significant increase in stiffness. Bending Beam Rheometer strength tests were also performed on the binder beams at the same temperature of -24°C. The stress strain curves and the strength results for the two replicates tested are presented in Figures 2 and 3. Figure 2. Preliminary BBR stress-strain results for unaged binder PG58-28 10 100 1,000 10,000 1 10 10