Asphalt mixtures, sulfur

The optimum or minimum allowable substitution ratio is then established by means of a series of justification tests at different binder contents. Figure 15 shows a comparison between the Marshall design properties of a conventional mixture using an asphalt binder and a 30 70 SEA binder. As indicated the optimum substitution ratio based on the maximum stability and equivalent air voids is about 1.7 1. Since minimizing the substitution ratio has a direct impact on the economic benefits to be realized by replacing the asphalt with sulfur these justification tests are to be recommended in all mix designs. 

Gallaway, . M. Saylak, D. "Sulfur Extended Asphalt (SEA) Sulphur/Asphalt Mixture Design and Construction Details - Lufkin Field Trials" Report FHWA-TS-78-203, January 1976 (reprint 1977). 

The current sulfur-asphalt technologies are the result of research and development carried out since the late 1960 s, but the sulfur-asphalt concept is far from new. The foundation was laid in the 1930 s by Bencowitz 9) and his co-workers who produced and patented sulfur-asphalt mixtures which had several advantages over regular asphalt. 

Figure 11. Modification of temperature susceptibility of asphalt mixtures with sulfur...

Mix and material properties tests on a wide variety of S-A—S mixtures were performed using the aggregate and asphalt types discussed above. The specific mixture ratios evaluated ranged from 2 1 to 5 1 wt % sulfur to asphalt. The maximum amount of sulfur used in any mixture was 20 wt %. For comparison purposes, sand-asphalt (0% sulfur) and sand-sulfur (0% asphalt) mixes were also evaluated. 

As discussed above, Figures 6 and 7 reaffirm that air voids are reduced as both asphalt and sulfur contents are increased. The Thermopave mixture ratio (S A S = 80.5 6 13.5) satisfies the 15% maximum allowable value prescribed by Shell (2). Figure 8 shows that the unit weight of the mix increases to some maximum value with asphalt as well as sulfur content. This, again, is caused by the physical properties of the... 

Thermal Expansion. Experimental results obtained from S—A—S mixtures and a conventional asphaltic concrete are also given in Table VII. Published data on asphalt cement, asphaltic concrete sulfur, sand, and limestone are also provided. The overall thermal expansion coefficient of the composite is derived from the combined effects of the individual ingredients in the mixture and the air voids present in the final material. Any combination which tends to decrease the air voids content... 

The experimental data correlated quite well with those computed using the Rule of Mixtures. The computed values are given in parentheses in Table VII. The Thermopave S-A-S mixture (80.5 6 13.5) had a thermal expansion coefficient of 29.3 X 10"6 in./in.-°C which is about 30% higher than that of the A/C material used for comparison. This difference could have a significant effect on the stresses developed at the interfaces between adjacent layers of A/C and sulfur-asphalt mixtures. At this writing a more in-depth evaluation of the effects of the missmatch in thermal properties is in progress at the Texas Transportation Institute. 

A series of standard pavement evaluation tests have been performed on a large number of mixtures comprised of different percent-by-weight ratios of sand, asphalt, and sulfur. The testing program performed in this task was designed to evaluate qualitatively the influence of both material and process variables on engineering properties. The following represent some of the conclusion reached ... 

The chemical modifiers, such as sulfur, lignin and certain organo-metallic compounds, do not really modify the bitumen but, rather, the properties of the asphalt mixture. The sulfur or the lignin extend the properties of the bitumen and modify the properties of the asphalt. Hence, the sulfur and the lignin are called bitumen extenders. The organo-metallic compounds modify the asphalt mixture by their catalytic action. 

The bitumen extenders replace the bitumen needed in an asphalt mixture while the organo-metallic compounds are added to the bitumen. The typical range of bitumen replacement seems to be 30% to 40%, when sulfur is used, and 10% to 12% when lignin is used. When organo-metallic compounds are used, approximately 2% to 3%, by mass of bitumen, is added. 

A variety of materials has been proposed to modify the properties of asphaltic binders to enhance the properties of the mix (112), including fillers and fibers to reinforce the asphalt—aggregate mixture (114), sulfur to strengthen or harden the binder (115,116), polymers (98,117—121), mbber (122), epoxy—resin composites (123), antistripping agents (124), metal complexes (125,126), and lime (127,128). AH of these additives serve to improve the properties of the binder and, ultimately, the properties of the asphalt—aggregate mix. 

SAS mixes with S A ratios of 1.0 to 2.5 1.0 are recommended for use in flexible pavement mixture designs, while S A ratios greater than 5 1 can be used in situations requiring rigid pavement designs. A typical SAS formulation is 82 parts sand, 6 parts asphalt and 12 parts sulfur by weight. 

The effect of sulfur and asphalt contents in SAS mixtures on Marshall Stability is shown in Figure 3 [15]. The stability values tend to increase with sulfur content but decrease with asphalt addition. It is interesting to note that without the sulfur and asphalt, sand mixes would have little or no stability. The data also indicate a wide variety of mix designs are possible whose stabilities are consistant with Asphalt Institute suggested values for conventional asphaltic mixes. 

A characteristic of single sized sands is their comparatively high air void contents which usually exceed 30 percent. Since sulfur s role in SAS mixtures is to fill these air voids without the aid of mechanical densification, both economic and performance considerations would require analysis of the maximum permissible air void content the mixture may possess and still be relatively impermeable to water without sacrificing structural integrity. Figure 4 [15] shows the relationship between air voids content and permeability for both SAS and asphaltic concretes as determined... 

Figure 6. Heated dump body truck developed by Shell Canada, Ltd. used for hauling sand-asphalt-sulfur paving mixtures. [Photographed at the Kenedy County Texas field trials in 1977 (28)].

The effect of simulated brine and gasoline spills on sulfur pavement were studied. Whereas salt based deicers would have minimal effect, organic solvents or naphtha mixtures can solubilize free sulfur in addition to attacking the asphalt. 

Both cellulosic wastes and lignins were found to have only limited applicability to the problem at hand. Sulfur, however, has been shown to be a very useful material for this purpose. Elemental sulfur may be used to extend asphalt, as in sulfur extended asphalt (SEA) binders, or chemically modified sulfurs may completely replace asphalt in paving mixtures. 

Solvents used in commercial operations at the present time are furfural, phenol, cresylic acid, Chlorex, nitrobenzene, and sulfur dioxide. The Duosol process utilizes a solvent called Selecto which is a mixture of phenol and cresylic acid. The propane used in the Duosol process precipitates the asphalt (18). 

Solvent extraction has become the most widely used method of refining lubricating oils. Selective solvents which extract the less desirable constituents include phenol, furfural, dichloroethyl ether, mixtures of cresylic acid and propane, and liquid sulfur dioxide. Liquid propane precipitates asphaltic constituents and wax and retains the more desirable oil components in solution. Dewaxing may also be accomplished by other solvents such as mixtures of benzene and methyl ethyl ketone. 

Ci uuc oil—complex, naturally occurring fluid mixture of petroleum hydrocarbons, yellow to black in color, and also containing small amounts of oxygen, nitrogen, and sulfur derivatives and other impurities. Crude oil was formed by the action of bacteria, heat, and pressure on ancient plant and animal remains, and is usually found in layers of porous rock such as limestone or sandstone, capped by an impervious layer of shale or clay that traps the oil (see reservoir). Crude oil varies in appearance and hydrocarbon composition depending on the locality where it occurs, some crudes being predominately naphthenic, some paraffinic, and others asphaltic. Crude is refined to yield petroleum products. See distillation, hydrocarbon, sour crude, sweet crude, asphalt, naphthene, paraffin. 

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