Andrew Hanz

Measuring Effects of Warm-Mix Additives: Use of Newly Developed Asphalt Binder Lubricity Test for the Dynamic Shear Rheometer

An emphasis on environmental stewardship has prompted the use of warm-mix technologies aimed at allowing for production of conventional asphalt mixtures at reduced temperatures. Successful use of warm-mix asphalt (WMA) in field demonstrations has created a need for development of mix design procedures. A major impediment in development of these procedures is the evaluation of the effect of WMA technologies on asphalt binder and mixture workability. The objective of this study was to introduce a new test methodology for estimating asphalt binder workability by measuring the lubricity effects of a surfactant-based additive as well as binder foaming processes through novel use of the dynamic shear rheometer with a new testing fixture. The new test allows measuring the coefficient of friction of binders at various temperatures, loading rates, and normal force. Asphalt binder lubricity measurements were correlated mixture to workability tests defined by the compactive effort required to densify a mixture to 8% air voids. Mixture testing was conducted at temperatures ranging from 90°C to 135°C. Results of asphalt binder workability testing demonstrated a significant reduction in coefficient of friction due to the use of a surfactant-based WMA additive and identified a need for revised procedures for evaluation of foamed asphalts. Both warm-mix processes demonstrated enhanced mixture workability relative to the hot-mix asphalt; however, significant differences were not realized until compaction temperatures were below those normally used in production.

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.

Rheological Behavior of Emulsion Residues Produced by Evaporative Recovery Method

The increasing use of surface treatments has generated a need for the development of specifications and test methods for performance of construction and in-service properties of emulsions. A major impediment in characterizing emulsion is the establishment of a generally accepted residue recovery method. ASTM recently approved a procedure for a low-temperature evaporative recovery method. It involves consecutive 24-h curing periods at 25°C and 60°C in a forced-draft oven. Initial rheological evaluation of emulsion residues indicates that residue properties far exceed those of the unaged base binder and are closer to those of aged materials. Also, concerns have been expressed about the length of this procedure and the possibility for reducing the time required. This study focused on evaluating whether the full 48-h curing period is required and identifying the cause of different behaviors relative to base binders. Evaluation was conducted at high and intermediate temperatures on emulsion residues and base binders subjected to different aging conditions, with the dynamic shear rheometer. The study included neat and polymer-modified base asphalts, and latex, polymer-modified, and conventional cationic rapid set emulsions. Results indicate that oxidative aging contributes significantly to the change in rheology with time of curing, compared with unaged base binder. Especially for modified emulsions, the full 48-h curing period is needed for full development of rheological properties. However, the properties should be compared with short-term aged binders rather than unaged binders. Results support the ASTM method but suggest that residue be considered rolling thin-film oven-aged material rather than unaged material.

Asphalt Binder Contribution to Mixture Workability and Application of Asphalt Lubricity Test to Estimate Compactability Temperatures for Warm-Mix Asphalt

The disconnect between estimated compaction temperatures on the basis of viscosity and behavior observed in laboratory- and field-compacted mixes indicates that the sole use of viscosity is insufficient to characterize the role of the asphalt binder in mixture compaction. The asphalt lubricity test was presented as a complement or replacement to viscosity on the basis of the concept that it was necessary to measure the properties of binders in thin films to relate better to compaction of mixes and to detect the effects of warm-mix asphalt (WMA) additives. To assess the value of the asphalt lubricity test, the relative contributions of lubricity, viscosity, and aggregate gradation on mixture workability at different levels of compactive effort were studied with statistical analysis. The mixes included fine and coarse gradations prepared with various conventional and WMA-modified binders, compacted at a range of temperatures representative of those used for WMA. Results indicated that gradation and binder lubricity contributed significantly to mixture workability at conditions of lower temperature, lower density, or both. The effects of viscosity were found to be insignificant. Aggregate gradation was found to be the most influential factor, which indicated that, for some mixes, use of WMA additives was not always necessary to improve laboratory compaction. To support selection of compaction temperatures on the basis of binder properties, a method was presented to apply the asphalt lubricity test to estimate minimum WMA compaction temperatures.

Development of Emulsion Residue Testing Framework for Improved Chip Seal Performance

To promote sustainability and optimize funding, state highway agencies are integrating pavement preservation strategies into the management of their roadway networks. Chip seals, a prominent pavement preservation alternative, are traditionally being used on low-volume roads with success in many states. However, as advancements in emulsion formulations continue and the use of chip seals on medium- and high-traffic facilities becomes more prevalent, there is a need to improve the methods to select and specify chip seal emulsions. The objective of this study was to develop an emulsion testing framework that captured properties related to critical distresses observed for in-service chip seals and considered the effects of traffic, environment, and aging. The proposed testing framework recommended the use of the dynamic shear rheometer and bitumen bond strength test to evaluate high-, intermediate-, and low-temperature performance. The testing protocols were applied to recovered and pressure aging vessel–aged emulsion residues from six emulsions widely used in Wisconsin, which included two emulsifier chemistries and two types of modification. The base binders before emulsification were also tested to assess the effects of the emulsification process. Results indicated that the proposed test methods could characterize material performance and differentiate between emulsion types and types of modification; however, additional research is needed to establish the relationship between laboratory-measured residue properties and performance of the full chip seal.

Challenges in using the Disc-Shaped Compact Tension (DCT) test to determine role of asphalt mix design variables in cracking resistance at low temperatures

ABSTRACT There have been many fracture tests – the Thermal Stress Restrained Specimen Test, the Single-Edge Notched Beam test, the Semi-Circular bend test, the Indirect Tensile Test and the Disc-Shaped Compact Tension (DCT) test – developed to understand thermal cracking in asphalt pavement. Among these tests, the DCT test is the most recent test method developed that has gained significant interest. This paper includes a laboratory study to measure the effect of different mixture design parameters on the DCT test results. The parameters include per cent binder replacement from recycled asphalt pavement, binder modification, low-temperature binder grade, oxidative ageing and mix design traffic level. To investigate the significance on the factors controlled, ANOVA and multi-linear regression analyses are used to show that only a few factors can be considered significant in terms of their effects on DCT parameters, and the significance of those factors could not explain the range in DCT response variables. Some of the trends in change in the DCT test responses with mixture ageing and some other factors are also found to be illogical. This paper does not offer solutions, but highlights some of the challenges experienced when applying the DCT test to performance specifications.

Impact of re-refined engine oil bottoms on binder properties and mix performance on two pavements in Minnesota

The use of vacuum distilled residues from the re-refining of used motor oils as blend stock in paving grade bitumen has occurred in isolated markets in North America for more than 30 years. Recently in the United States the increasing need for low stiffness bitumen for use in high binder replacement mixtures, coupled with economic considerations, has led to an expanding market for these products. Little in-depth investigation into the long-term impact of these additives on mixture performance has taken place until recently. Conflicting studies from researchers in Canada and private organizations have been published. Here, we review the eight-year performance of four virgin mixture test sections placed on a county highway Minnesota. Three test sections used PG 58-28 bitumens from different crude sources, one of which contained approximately 8% of the re-refined engine oil residuum. The fourth utilized PG 58-34 polymer-modified bitumen. Pavement distress surveys were conducted over the eight year period. Tests of the original bitumen following extended laboratory aging were conducted. Properties of the recovered bitumen from eight-year-old field cores were correlated to pavement distress. Double Edge Notched Tension (DENT) tests were conducted on 20and 40-hour PAV residues of the original bitumen. Iatroscan and extensive rheological testing were conducted on laboratory aged bitumen and on bitumen recovered from laboratory aged mixtures and field cores to determine the colloidal index, rheological index and the extent to which laboratory aged and recovered bitumen materials were m-controlled. Also evaluated were three test sections constructed at the Minnesota DOT MnROAD test site. A comparative study of PG 58-28, PG 58-34 (PMA) and PG 58-40 (PMA +re-refined engine oil bottoms) were evaluated over eight years. Bitumen used on the projects was tested as described above. The PG 58-40 exhibited the worst performance. The results of binder evaluations correlated to the fatigue cracking observed in the field with the test sections containing the re-refined engine oil residuum exhibiting the most extensive fatigue cracking. Based on this investigation we provide recommendations for test criteria to identify bitumens that could increase an asphalt mixture’s likelihood to exhibit poor durability and fatigue cracking performance.

Evaluating Moisture Susceptibility of Cold-Mix Asphalt

Moisture damage–related distress is a primary concern limiting the application of cold-mix asphalt (CMA) as an alternative to hot-mix asphalt in the field. Thus, a simple and effective method is needed in the materials selection process to identify moisture susceptible materials. This study presents a modification to the boiling test procedure specified in ASTM D3625 based on the concept that a mix designer can limit moisture damage by controlling coating quality. The test was modified to present CMA-specific sample preparation guidelines and to remove subjectivity from the test by quantifying coating according to image analysis. Coating quality was evaluated with the coating loss ratio, defined as an index to compare coating before and after the boiling of cured loose mixtures. The relationship between coating quality and moisture resistance was verified with two separate tests: the AASHTO TP-91 binder bond strength test and the AASHTO T-283 tensile strength ratio test. The binder bond strength test provided a direct measure of the bond between binder and aggregate and how it was influenced by moisture. The effect of coating quality on bulk mixture performance was evaluated through comparison with the tensile strength ratio test. All compacted mixtures were produced with a newly developed CMA mix design method to maintain air void levels between samples. Results show that image analysis can be effectively applied to quantify the results of the boiling test, and rankings from all three tests are consistent; this efficacy suggests that the evaluation of coating quality with the modified boiling test can be used as an effective initial evaluation of the potential for CMA moisture susceptibility.

Investigation of Sterol Chemistry to Retard the Aging of Asphalt Binders

The addition of petroleum-derived oils (aromatic or paraffinic) and bioderived oils (typically low-viscosity, unsaturated hydrocarbons modified to remove acid functionality) to bitumen has become a burgeoning business to facilitate the use of increasing amounts of reclaimed asphalt pavement, reclaimed asphalt shingles (RAS), and combinations. These additives have been marketed as softening agents, to reduce high- and low-temperature performance grades, and as rejuvenators. Given the aging mechanism of bitumen and chemical changes that occur during aging, it is unlikely that any true form of rejuvenation is possible. In this paper data are provided on investigations into the use of a chemical class known as sterols, which can be obtained from seed oils and tall oil to retard the aging of bitumen. Investigations were carried out with RAS, with re-refined engine oil bottoms, and with the accelerated aging of bitumen and bituminous mixtures that contained sterol chemistry. The results demonstrated that, relative to controls without sterol, increases in the bitumen rheological index were reduced, and decreases in the colloidal index were reduced, as were decreases in the difference between bitumen low-temperature stiffness critical temperature and bitumen m-value critical temperature (ΔTc). Investigations with atomic force microscopy of bitumen samples with sterol additive, compared with those without, were used to understand the sterol additive function as the bitumen was aged.

Development of Loaded Wheel Test for Evaluating Bleeding in Chip Seals

Bleeding is one of the most common distresses that leads to premature failure of chip seals. Bleeding reduces the surface texture of the pavement and thus compromises the safety of the traveling public, particularly during wet seasons and at intersections. Although factors that lead to premature bleeding are known, currently no laboratory test methods for evaluating bleeding in chip seals have been specified. The objective of this study was to develop a laboratory testing procedure and analysis framework for the evaluation of bleeding in chip seals by means of the loaded wheel test (LWT), specified in ASTM D6372. A new sample preparation, testing procedure, and analysis framework were proposed in this study to account better for the effects of existing pavement substrate, traffic load, traffic volume, and temperature. After samples were tested, bleeding was assessed on the basis of the percentage of asphalt at the surface of the seal; this percentage was determined with image processing analysis software (IPAS2) and surface texture measurements based on the sand patch method specified in the ASTM E965 standard procedure. Results indicated that the modified LWT method could quantify bleeding potential and bleeding development of laboratory prepared chip seal samples. Experimental factors including temperature, number of loading cycles, and contact stress were found to influence bleeding. Results showed that a moderate relationship existed between texture loss and the percentage of bleeding as measured by the IPAS2. Limited field validation results showed that texture loss could be an indicator of bleeding resistance.