Elie Y. Hajj

Laboratory Evaluation of Mixes Containing Recycled Asphalt Pavement (RAP)

ABSTRACT This paper presents the findings of a laboratory-based research project that evaluated the impact of three RAP sources at three levels of RAP content (0, 15, and 30%) on the mechanical properties of the final mix. Two asphalt binder grades were targeted for the final mix. The binder grade for the new asphalt binder was selected using appropriate blending charts for high and low temperatures. Overall the addition of RAP to a mixture resulted in an acceptable moisture resistance, however a reduction in the unconditioned and conditioned tensile strength was observed. In most of the cases, the addition of RAP to a mixture resulted in an equivalent or better rutting resistance than the virgin mix (0% RAP). Depending on the RAP source and content, the addition of RAP to a mixture with an unmodified target asphalt binder resulted in either a better or worse fatigue resistance. On the other hand the addition of RAP to a mixture with a polymer modified target asphalt binder resulted in a worse fatigue resistance regardless of the RAP source and content. Finally, the addition of RAP to a mix resulted in a similar or better resistance to thermal cracking than the virgin mix and reasons for that were proposed.

Oxidative Aging of Asphalt Binders in Hot-Mix Asphalt Mixtures

This study evaluated the effect of different aggregate sources along with their Corresponding change in mixture characteristics to determine the influence of both on binder oxidation rates and changes in mixture stiffness when compacted mixtures were exposed to laboratory aging conditions. The two aggregate sources, Colorado and Nevada, had different gradations and different water absorption rates, which led to differences in the calculated asphalt binder apparent film thicknesses (AFT) for each mixture. Two asphalt binders, an unmodified PG 64-22 and a styrene–butadiene–styrene–modified PG 64-28, were used. The overall findings of the study indicated that both the aggregate and mixture characteristics influenced the oxidation rates of the binder, with the two binders oxidizing by similar amounts when aged in mixtures with the same characteristics (AFT and mixture air voids). The oxidation changes in the binder had differing effects on the stiffness of the mixture as a function of age. Not only were the aggregate and mixture characteristics important to the mixture stiffness and aging relationship, but the binder characteristics themselves, in particular polymer modification, influenced the aging and stiffness relationships of mixtures with age.

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.

Impact of Lime and Liquid Antistrip Agents on Properties of Idaho Hot-Mix Asphalt Mixture

In 2005, the Idaho Transportation Department constructed a field project on State Highway 67 (SH-67) to evaluate the impact of hydrated lime and a liquid antistrip agent on the mechanical properties of an Idaho hot-mix asphalt (HMA) mixture. This paper documents the results of a laboratory evaluation on field-produced HMA mixtures sampled from lime and liquid antistrip test sections on SH-67. As the two mixtures were subjected to multiple freeze-thaw (F-T) cycles, the mixture treated with hydrated lime (lime mixture) maintained good resilient-modulus properties over the entire 21 F-T cycles. The mixture treated with the liquid antistrip agent (liquid mixture) fully disintegrated after 22 F-T cycles. The results of a mechanistic analysis showed that, as a result of multiple F-T cycling, the liquid mixture will incur a 220% increase in potential rutting compared with the lime mixture, which will incur only a 65% potential increase in rutting. The dynamic moduli in compression demonstrated that both dry and moisture-conditioned lime mixtures can be considered less susceptible to rutting compared with liquid mixtures. Furthermore, the lime mixtures demonstrated better resistance to moisture damage than the liquid mixtures on the basis of the ratio of dynamic modulus in tension of moisture-conditioned to dry mixtures. The additional stiffness of the lime mixtures did not contribute to premature fatigue cracking when mixtures were subjected to dynamic creep testing in the tensile mode of loading.

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.

Long-Term Performance of Reflective Cracking Mitigation Techniques in Nevada

Hot-mix asphalt (HMA) overlay is one of the commonly used methods for rehabilitating deteriorated pavements. The Nevada Department of Transportation (DOT) uses HMA overlays as a rehabilitation technique for the majority of the states flexible pavements. One major type of distress influencing the life of an overlay is reflective cracking. In the past, Nevada DOT has experimented with a number of techniques— such as cold in-place recycling, reinforced fabrics, stress relief courses, and mill and overlay—to reduce the impact of reflective cracking on HMA overlays. Several projects were constructed under each category. The long-term field performance of various Nevada DOT reflective cracking mitigation techniques was evaluated; the techniques were used on flexible pavements at 33 field projects. Performances of the various projects were analyzed by fatigue, transverse, and block cracking measurements from Nevada DOTs pavement management system data. In addition, the statistical approach called principal component analysis was used to asses the effectiveness of each of the reflective cracking techniques. The study indicated that cold in-place recycling and mill and overlay were the most effective treatments for reflective cracking of HMA overlays over HMA pavements under Nevadas conditions, except when the existing pavement experiences severe alligator cracking. In such situations, it is recommended that HMA pavement be subjected to reconstruction or full-depth reclamation.

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.

Equivalent Loading Frequencies for Dynamic Analysis of Asphalt Pavements

AbstractAn investigation of the existence of predominant frequency, fp (or frequencies), and the ability to predict critical pavement responses in asphalt layers using those predominant frequencies has been presented. The study used an extensive database of computed pavement response histories of four different asphalt pavement structures (thin and thick) at two temperatures (21 and 40°C) subjected to a tandem axle load at three speeds (16, 65, and 97  km/h) using the pavement response analysis program 3D-Move Analysis. The conversion of load duration to frequency and the validity of using one response component (σzz) alone in the estimation of the pulse time (tp) were explored. Instead of focusing on only σzz, attention is given to all pavement response components (stresses and strains). In addition, verification of whether the use of a consistent set of fp value(s) can in fact adequately capture all components of pavement response has also been undertaken. The viscoelastic properties that are representa...

Evaluation of selected warm mix asphalt technologies

The overall objective of this study was to assess the performance of warm mix asphalt (WMA) mixtures in South Dakota based on laboratory and field evaluations. The study included three WMA technologies (Advera, Evotherm, and foaming) and three aggregate types (limestone, quartzite, and natural gravel). A comprehensive laboratory programme was conducted to assess the impact of WMA in terms of stiffness and mixtures’ resistance to moisture damage, rutting, fatigue cracking, and thermal cracking. Overall, the WMA mixtures performed comparatively similar to the hot mix asphalt (HMA) mixtures. The impact of the WMA technology was found to be aggregate dependent. WMA test sections were constructed on four Highways in South Dakota to validate the laboratory findings. During construction, percent compaction, roughness measurements were collected. Field observations indicated that WMA mixtures can be successfully produced and constructed using South Dakotas materials.

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.