Microstructural transformation of a poly(styrene‐b‐butadiene‐b‐styrene), SBS, triblock copolymer blended with asphalt was studied as the asphalt composition was varied from 0 wt% to 96 wt%. Transmission electron microscopy (TEM), dynamic mechanical spectrometry (DMS), and differential scanning calorimetry (DSC) were used. The blends were made in batch mixers at 200°C, or by solution casting from a nonselective solvent (trichloroethane) at ∼28°C. Asphalt partially solubilizes the polybutadene (PB) midblock of the SBS producing saturated PB microdomains along with macrodomains of asphalt. When the asphalt concentration was varied from 10 to 90 wt%, the asphalt phase separated into a variable number of large domains, while the SBS‐rich regions formed a continuous matrix. Networks of SBS‐rich regions were observed at low magnification; these are referred to as macro‐networks. At higher magnification, networks that are stabilized by polystyrene (PS) microdomains (denoted micro‐networks) are also formed. The presence of a macro‐network is also confirmed by stress relaxation tests. The macro‐network broke down into microgel‐like structures when the asphalt composition exceeded 90 wt%. Examination of the interior of the SBS‐rich regions showed that the shape of the PS microdomains transformed from short cylinders to lamellae, hexagonally perforated lamellae (HPL), back to lamellae, short cylinders, and finally to spheres. DMS and DSC indicate a systematic increase in the PB glass transition temperature (Tg) and negligible change in the Tg of PS as the asphalt content increases. Triblock copolymers that can form a macro‐network at low concentration will be more desirable for highway pavement modification.
Chemical-specific emission rates for simple aromatic and polycyclic aromatic hydrocarbon compounds (PAHs) from bitumens during hot mix asphalt (HMA) production and placement activities were evaluated using a headspace gas chromatography method. Temperature-dependent headspace concentrations of the EPA listed aromatic and polycyclic aromatic compounds were measured in the laboratory using headspace gas chromatographs equipped with a variety of detectors. The methodology has previously been calibrated and verified by a program of simultaneous laboratory and field tests, and is accurate in identifying field inhalation exposure potential to individual compounds in asphalt fumes.The results indicated that chemical-specific emission rates of aromatic and PAHs are strongly linked to both the performance grade of the asphalt binder and the binder temperature. Individual chemical compounds were quantified for 22 paving grade bitumen sources from throughout the United States. Chemical-specific emission rates from each binder were measured at a series of temperatures spanning the range, typically used for application of HMA, and at temperatures well in excess of those used for HMA applications in the United States. Emissions of all detected compounds increased with elevating temperature. The amount and composition of PAHs were markedly influenced by changes in temperature. At binder temperatures at or below 190 • C, only very small amounts of mostly 2-and 3-ringed PAHs were emitted. Concentrations of individual two-ringed PAHs ranged from 0.5 to 11 μg/m 3 at 150 • C and ranged from 2.0 to 100 μg/m 3 at 190 • C. Concentrations of 3-ringed PAHs were below method detection level of 0.1 μg/m 3 and ranged from 0.1 to 120 μg/m 3 at 190 • C. Larger ring number PAHs were below detection at 150 • C and less than 10 μg/m 3 for 4-ringed PAHs at 190 • C. As binder temperature increased above the typical limit for HMA production and application, several PAHs with greater ring numbers (4-, 5-, and 6-ringed PAHs) and more potent toxicity equivalency factors (carcinogenic potential) were detected.
A 2.38-mm (No. 10) mesh crumb rubber from waste passenger tires was pretreated with a low-viscosity petroleum-based product and used as an aggregate replacement in asphalt concrete mixtures. The preconstruction testing consisted of the development of an appropriate aggregate gradation, mix design information, and fundamental mixture properties such as temperature susceptibility, moisture sensitivity, low temperature behavior, and permanent deformation characteristics. Five crumb rubber modified (CRM) asphalt concrete plus two control test sections were placed in Babbitt, Minnesota, in the fall of 1993. Variables in the CRM sections include pretreated and untreated crumb rubber and CRM mixtures in just the wear course or throughout the 150-mm (6-in.) pavement section. Laboratory results indicated that there was little difference in the temperature susceptibility between the mix design and behind-the-paver mixtures. Although the mix design samples indicated some increase in the moisture sensitivity of the CRM mixtures, no difference was seen in the behind-the-paver materials. There was some indication that the CRM mixtures would exhibit a greater ability to dissipate thermal stresses through a better ability to strain at cold temperatures. Some slight to moderate improvements were seen in both moisture sensitivity and low temperature behavior when the crumb rubber was pretreated. Falling weight deflectometer results were used to backcalculate the layer moduli for all of the test sections. Results indicate no difference in any of the layer moduli; these results agree with the laboratory resilient moduli results. After two winters, all test sections are performing equally well. Anticipated damage by snowplows to the coarser gradation used with the CRM mixtures did not occur.
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