Mohammad Zia Alavi

Influence of Hydrogreen Bioasphalt on Viscoelastic Properties of Reclaimed Asphalt Mixtures

The incorporation of reclaimed asphalt pavement (RAP) into asphalt mixtures exposes some challenges from the design perspective because of the aged asphalt binder in RAP. Steps are being taken to offset the addition of stiff materials, often with the use of rejuvenating additives. This paper summarizes the laboratory evaluation of one of the available bio-rejuvenating agents called BituTech RAP. High RAP content mixtures used in Manitoba, Canada, were evaluated to study the impact of BituTech RAP on the viscoelastic properties of asphalt mixtures to overcome any possible moisture damage or thermal cracking problems that might arise in such a wet–freeze environment. The laboratory experiment consisted of the production and test of mixtures that contained 15% and 50% RAP, with and without BituTech RAP. The 2S2P1D analogical model was used to generate the complex modulus (E*) of the various evaluated mixtures and to assess the influence of BituTech RAP on the storage and loss moduli. The addition of BituTech RAP improved the moisture resistance of the mixtures that contained RAP, as observed after three freeze–thaw cycles. The addition of BituTech RAP restored the thermal cracking properties of the mixtures revealed by the thermal stress restrained specimen test. The use of BituTech RAP could result in cost savings without the need to use a softer binder, as long as the high-temperature properties of the mixtures were not jeopardized.

Evaluating Adhesion Properties and Moisture Damage Susceptibility of Warm-Mix Asphalts: Bitumen Bond Strength and Dynamic Modulus Ratio Tests

Through development and evaluation of the warm-mix asphalt (WMA) mixture design process, increased moisture susceptibility has been cited as one of the potential critical failure modes for WMA. Reduced production temperatures can affect the drying of the aggregate before mixing, the development of adhesion at the asphalt–aggregate interface, and binder stiffness. The objective of this research is to identify the significance of these factors and to define their relative contribution to mixture resistance to moisture damage. To evaluate the contribution of asphalt binder–aggregate adhesion, the bitumen bond strength (BBS) test was implemented on dry and moisture-conditioned samples. The effect of production temperature was simulated by heating aggregate substrates to hot-mix asphalt (HMA) and WMA temperatures before applying the asphalt binder. Furthermore, the effect of reduced binder stiffness resulting from lower production temperatures was considered through establishing two controls for mixture performance testing: a conventional HMA and a mixture prepared at WMA temperatures without WMA additives. The relevance of these factors was established through comparison with mixture performance as measured by the reduction in dynamic modulus, as a function of conditioning cycles. Mixture sample preparation allowed for consideration of residual moisture in aggregate that might have been associated with WMA and provided a control HMA sample prepared under standard conditions to establish a performance benchmark. Recommendations were made for incorporation of these new test methods into current WMA mixture design specifications. In summary, both BBS and dynamic modulus testing indicated that specific WMA additives could improve the mixtures moisture resistance and could offset any negative effects from the reduced production temperatures on moisture susceptibility. Therefore, selecting appropriate warmmix additives during the mix design process can help mitigate potential moisture damage associated with WMA.

Low-temperature properties of plant-produced RAP mixtures in the Northeast

The impact of reclaimed asphalt pavement (RAP) materials on pavement performance is an important topic of study in the industry due to environmental and cost benefits. The primary concern for increasing allowable RAP percentages in hot mix asphalt relates to the presence of aged materials, which may embrittle the mixture and decrease cracking resistance. Low-temperature cracking is a major distress in cold temperature climates, such as the Northeastern United States. Currently, there are several procedures to analyse low-temperature performance of asphalt binders and mixtures. However, these methods use different starting (initial) temperatures and cooling rates that may not represent actual field temperatures and cooling rates. This paper presents the results of a study on low-temperature performance of plant-produced RAP mixtures. Eighteen mixtures from three states were tested with varying RAP contents (0–40%) and different virgin binder grades. The objectives of the study were to: (1) evaluate the impact of cooling rate and starting temperature on the critical cracking temperature of RAP materials; (2) evaluate the impact of RAP content on the low-temperature properties of mixtures; (3) evaluate the benefit of using softer virgin binder grades to mitigate the impact of the aged RAP binder in the mixture; and (4) to compare the low-temperature cracking properties determined from different mixture and binder tests. Based on the results, warmer starting temperatures and faster cooling rates result in warmer critical cracking temperatures for all mixtures. Through use of the uniaxial thermal stress strain test, it was found that the addition of RAP alters fracture behaviour from ductile failure towards a brittle failure. Based on results from the indirect tensile test, tensile strength increases with RAP content. However, due to a faster-building thermal stress, warmer critical cracking temperatures result. It was also determined that degree of blending may impact the effectiveness of using softer binder grades at higher RAP percentages to improve low temperature cracking resistance. The data also show that analysis procedure and test protocols can have a profound effect on critical cracking temperature. The conclusions presented reinforce the need for more accurate representation of RAP materials, and careful selection of analysis parameters.

A method to estimate the thermal stress build-up in an asphalt mixture from a single-cooling event

This paper suggests a method to estimate the thermal stress build-up in asphalt from a single-cooling event based primarily on the measured bitumen rheology. In order to check the reasonableness of the calculation, first the thermal stress-restrained specimen test (TSRST) was performed on a laboratory-compacted, cylindrical asphalt specimen. Concurrent to the TSRST test, the thermal strain was measured from an unrestrained asphalt specimen. As a result, the thermal stress build-up and coefficient of thermal expansion were determined. The bitumen from the TSRST specimen was recovered and the bitumen low and intermediate temperature rheological properties were determined using a dynamic shear rheometer (DSR) technique (commonly referred to as 4-mm DSR) that allows testing to−40°C by way of a correction for instrument compliance. The estimated and measured TSRST thermal stress build-up were compared and found to be remarkably similar. Also, the TSRST thermal stress build-up was compared with the estimated thermal stress build-up using the methodology in ASTM D6816-11, which includes an empirical pavement constant (PC), and they were found to be significantly dissimilar suggesting that simply multiplying the binder thermal stress by a PC (18 in this case) does not provide a particularly good estimate of the mixture thermal stress build-up.

Low Temperature Characterization of Asphalt Mixtures by Measuring Visco-Elastic Properties under Thermal Loading

Low temperature thermal cracking is a common type of failure in asphalt pavement that occurs particularly in cold regions or locations with significant daily temperature fluctuations. The resistance of asphalt mixtures to low temperature cracking is generally influenced by the thermal contraction/expansion, visco-elastic, and fracture properties of the asphalt mixture. The accurate characterization of these properties is essential to the meaningful modeling of thermal cracking in asphalt pavements and, thus, the design of thermal cracking resistant mixtures. This paper describes a fundamental approach to determine the visco-elastic properties of asphalt mixtures from direct measurements of thermal stress and strain using a uniaxial test device recently developed at the University of Nevada, Reno. The relaxation modulus was computed in the temperature domain using linear visco-elastic constitutive equation, known as Boltzmanns superposition principle, from the measured, thermally-induced stress and strain during the test. Five distinct stages were identified from the relaxation modulus change with temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. The proposed approach was used to assess four hot mixed asphalt mixtures made from three aggregate sources with different mineralogy (Quartzite, Limestone and Rhyolite) and two different binder grades (PG64-22 and PG64-28) from different sources. It was found that the evolutions of thermal stress and strain are significantly influenced by the grade of asphalt binder and aggregate source. The relaxation modulus and the derived thermal visco-elastic properties were highly affected by the grade of asphalt binder.

Approach for Quantifying the Effect of Binder Oxidative Aging on the Viscoelastic Properties of Asphalt Mixtures

Because of the noted influence of oxidative aging on mixture properties and pavement performance, it is becoming imperative to have a more complete understanding of the influence of asphalt binder aging on the viscoelastic behavior of asphalt mixtures. This study proposes a new approach to correlating the oxidative aging of asphalt binder for carbonyl functional groups with the viscoelastic behavior of asphalt mixtures for a continuous relaxation spectrum. The asphalt mixture complex modulus and the carbonyl area for the recovered asphalt binder were measured for mixtures subjected to varying durations of long-term aging in the laboratory. The continuous relaxation spectrum was obtained analytically from the 2S2P1D model of complex modulus of asphalt mixture through the inverse Fourier–Laplace transform approach. A consistent horizontal shift in the continuous relaxation spectrum was observed for all mixtures with the increase in aging duration. However, the shape and the amount of shifting of the spectra were mixture dependent. In particular, mixtures with higher asphalt binder absorption exhibited the greatest shift in the continuous spectra for both unmodified and polymer-modified asphalt mixtures. Good correlations were observed between the carbonyl in asphalt binder and the continuous relaxation spectrum parameters of the asphalt mixture. Such relationships should permit the incorporation of long-term oxidative aging directly into the constitutive equation used in pavement response analyses.

Prediction of Asphalt Pavement Temperature Profile with Finite Control Volume Method

The accurate prediction of the pavement temperature profile and history is critical for the calculation of the viscoelastic response of asphalt concrete pavements under traffic and thermal loadings. An accurate prediction is also essential in the estimation of the asphalt binder oxi-dative aging. A sound alternative approach is proposed to predict the pavement temperature profile with the finite control volume method in a fully implicit scheme. The model for the finite control volume method provides a fundamental and clear understanding of the heat energy balance, including the incoming and outgoing thermal energies, in addition to the dissipated heat in the system. Also, the variability in the materials’ thermal properties in a multilayered pavement structure can be remedied by assigning appropriate thermal properties to each control volume or cell. The fully implicit scheme improved the time efficiency of the calculation significantly. Moreover, enhancements were made by defining the accurate heat balance equations for the pavement surface and the bottom boundary conditions. With reliable meteorological data, solar radiation, and monthly variable pavement surface radiation properties, accurate prediction of the pavement temperature profile was made possible as validated by comparison with field measurements. Consequently, Windows-compatible software has been developed on the basis of the proposed model, which is capable of effectively predicting the pavement temperature profile and history.

Effect of select warm-mix additives on thermo-viscoelastic properties of asphalt mixtures

In this paper, the thermo-viscoelastic properties of hot and warm asphalt mixtures from South Dakota, USA, were determined from direct measurements of thermally induced stress and strain. The approach to determine the relaxation modulus, viscous flow, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages was described and used to assess the influence of select warm-mix additives on low-temperature properties. The evaluated warm-mix additives as well as the aggregate source were found to have inconsistent effects on the thermo-viscoelastic properties of the mixtures. It was observed that mixtures with similar thermal stress build-up can have significantly different relaxation modulus values depending on the coefficient of thermal contraction of the respective mixture. In summary, the proposed methodology was able to discern the slight differences caused by the warm-mix asphalt treatments on the same binder and two aggregates.

Evolution of Thermoviscoelastic Properties of Asphalt Mixtures with Oxidative Aging

The uniaxial thermal stress and strain test (UTSST) provides a fundamental approach to characterize the thermoviscoelastic properties of asphalt mixtures and permits the pragmatic evaluation of changes in the stiffness and overall behavior of mixtures as a function of oxidative aging. The UTSST modulus was computed in the temperature domain with a linear viscoelastic constitutive equation from the measured thermally induced stress and strain. Five distinct stages, here named thermoviscoelastic properties, are identified from the modulus as a function of temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. Through consideration of the thermoviscoelastic properties, marked differences in the aging process were noted in the evaluation of two binders and two aggregate sources over a range of air void levels. Typically, decreases in the viscous response of the mixtures as well as corresponding increases in both the stiffness and brittle behavior are presented as a function of aging. The evaluated behavior of the mixtures also provides a clearer understanding of the significant influence the air void level, or mixture density, has on the binder oxidation and overall mixture performance. The evaluation method provides definitive measures to monitor the progression of multiple aspects of the response of asphalt mixtures to thermally induced loading.

Influence of Reclaimed Asphalt Pavement on Performance-Related Properties of Gap-Graded Rubberized Hot-Mix Asphalt

Rubberized hot-mix asphalt (RHMA) has been widely used in construction projects by the California Department of Transportation (Caltrans) for the environmental benefits of its recycled waste tires and for its improved fatigue and reflective cracking resistance. Currently, Caltrans does not permit the use of reclaimed asphalt pavement (RAP) in any gap- or open-graded rubberized asphalt mixes. However, given the cost and environmental benefits of RAP to replace portions of required virgin binder and aggregates in conventional mixes, interest is growing in the addition of some RAP to RHMA mixes as well. This study investigated concerns about this proposed practice. Three phases of laboratory testing (i.e., asphalt binder testing, fine aggregate matrix mix testing, and full-graded mix testing) were conducted to evaluate the effects of the addition of RAP into new RHMA mixes. The results indicated that the gap-graded aggregate structure of RHMA might limit the amount of RAP that could be used in the mix. Only 10% RAP by binder replacement could be achieved for the mix tested in this study, but the other specified volumetric requirements were still met. Replacement of a portion of asphalt rubber binder with age-hardened RAP binder increased the binder stiffness at low and high temperatures, which indicated enhanced rutting performance but diminished low-temperature cracking performance. Test results from full-graded mixes indicated similar trends, with improved rutting performance with the addition of RAP but also with significantly poorer fatigue and reflective cracking resistance.