Road Maintenance Solutions from One Source

by Glynn Holleran, VP of Technical and International Operations, VSS, West Sacramento, Calif, USA 

Blending tire rubber into an asphalt mix requires consideration of a variety of factors

The purpose of a road surfacing is, in essence, to transfer stress from moving wheel loads to the earth. To operate effectively, roads obviously must not crack, rut or wash away. The distresses of road surfaces generally take one of those three forms. However, in instances where the road surface or structure has failed, the rehabilitation method must address and correct the failure.

Asphalt (bitumen) is a naturally occurring material and can have limitations based on any of three things: its chemistry, the method by which it is refined and the materials with which it is blended. Adding polymers has been used to extend the application of asphalt and improve its properties, particularly in the areas of thermal susceptibility and flexibility. With the problem, in western countries, of the development of a vast used tire stockpile, researchers have turned to asphalt rubber as a polymer additive.

Tire rubber is a mixture of many materials, including some polymers. This rubber has been used extensively in asphalt rubber modified chip seals, gap-graded hot mix and dense-graded hot mix.

In order to make an emulsion, you must have a compatible asphalt rubber binder. The compatibility of asphalt/polymer systems is the structural arrangement of the polymer particles, chains or groups within the asphalt matrix.

In asphalt rubber, compatibility is an issue that has received little attention. Researchers believe a reaction occurs in which the asphalt and the rubber particle interact to form a gel-coated particle, like the process of swelling that occurs in polymer asphalt systems (See Figure 1).

Morphology, or structure, has a dramatic effect on rheology. This has been demonstrated by rheological studies of modified binders and the subsequent mix and product properties. Compatible systems perform better. In emulsification, the presence of large particles would make for a coarse, unstable emulsion.

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Rubber, emulsion must be blended with precision

Polymer emulsions have been used to a great extent in chip sealing and slurry/Micro-surfacing. The polymers generally are pre-blended with the asphalt; co-milled as latex, either by direct injection into the soap or pre mixed with the soap; or post-added.

Ground tire rubber usually cannot be added so simply. Pre-blended crumb must be very finely dispersed and be free of any metal (to avoid damage to the emulsion mill) to be able to be emulsified.

Tire rubber may also be incorporated in emulsion through post-addition approaches. Ground, solid crumb rubber has been added into slurry mixes as a dry ingredient, similar to the method mentioned above. In such a case, the rubber becomes a part of the aggregate phase and acts mainly as a filler. Such processes are in general use in the United States.

However, to create much of a change in elasticity, increase in cohesion, and other desirable properties, the rubber needs to be fully or partially digested so that it may coat particles. This is the basis of the process.

The increase in cohesion should improve properties such as deformation resistance (in rut filling), surface abrasion resistance and crack resistance, and allow increased binder films without flushing.

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Dispersion and post-addition work to improve adhesion

The process referred to as the wet process may be adapted for use with emulsions. If a suitable dispersing agent can be used that allows dispersion in the emulsion water phase, then a material, which is capable of being post-added, may be produced. The type is of obvious importance because it must not create an environmental hazard, nor degrade the asphalt or rubber properties. On the other hand, if the solvent is able to swell or soften the rubber, then it may improve wetting and adhesion.

A range of agents has been used to optimize the dispersion and other additives, such as wetting agents and carbon black. In general terms, what is preferred is an oil additive with a high aliphatic content and a boiling range that both meets emission requirements and allows swelling of the rubber. The material will be referred to by its designation of RG-1. RG-1 is a semi-swelled dispersion of crumb rubber (40 to 50 percent) in a petroleum solvent. It is supplied as a free-running, high-viscosity material that can be readily poured and pumped.

RG-1 is used by post-addition into the emulsion with simple mixing (See Figure 4). The emulsion is not greatly affected by adding RG-1 except that the sedimentation rate is high. This is not surprising because the RG-1 is a separate phase, and so the emulsion must be thoroughly mixed before use. There is no obvious breaking caused by the presence of the RG-1, and this is true to concentrations in excess of 20 percent.

Figure 4. Emulsion properties
Emulsion Type	% RG-1	Viscosity	Settlement (3 day)	Residue	Sieve
CRS-2	5	120	3	67	n/a
CRS-2	10	160	5	68	n/a
CRS-2	20	210	10	69	n/a
PMCRS-2	10	170	2	69	0.2
CQS-1h	10	39	1	60	0.1
LMCQS-1h	10	65	1	61	0.3
Micro	10	70	2	61	0.2
AR Fine	12% rubber	150	5	62	0.9
AR Mid	12	120	12	63	2
AR Coarse	12	85	15	62	3

Direct emulsification requires cautious approach

Direct emulsification of asphalt rubber has been reported in a number of patents. These patents are not very specific on the composition of the asphalt rubber, but do have in common that the asphalt and rubber are pre-blended and gelled. Although high levels of addition are claimed, they seem very difficult to achieve.

The work carried out here uses the compatibility approach and has been aimed at developing as high a concentration of asphalt rubber as possible. Analysis of the asphalt rubber used indicates that high styrene butadine rubber (SBR) content materials produce the best results.

As fine a dispersion of asphalt rubber as possible is attained through use of additives, asphalt composition control and high shear blending. Particle size of the rubber must be of a magnitude lower than the desired emulsion particle size.

Dispersion structure plays role in emulsification of asphalt rubber blends

In the examples shown here, blends of asphalt rubber were prepared at a set percentage of 15 percent. The rubber used was from a single source and represented standard car radial tire rubber - predominantly synthetic rubber. The asphalt was made compatible with the rubber by use of aromatic oil and added carboxylic acid.

Figures 5, 6 and 7, below, show the morphologies (structures) of three types, one where low shear was used, one where the system was compatible to a very fine distribution of 1 to 10 microns and an intermediate with some larger rubber particles.

Researchers emulsified this with a low zeta potential emulsifier, with a stabilizing additive, to optimize double layer thickness and create the most stable emulsion. This resulted in a modified asphalt with a balance of components with a high aromatic oil content. Figures 8, 9 and 10, below, show the resulting three emulsions. The coarser blends gave coarser emulsions.

It is clear that the emulsion can be formed and used. However, the morphology is a prime determinant of the physical properties, and this would seem to be a compromise from traditional asphalt rubber materials. In short, this is a different material.

Maintenance applications benefit from emulsified asphalt rubber

• Slurry and Micro-surfacing
  Research shows that slurry and Micro-surfacing applications have improved mix properties, such as cohesion, abrasion resistance and flexibility, when an emulsified asphalt rubber is used.
• Chip seal
  Research shows that chip seals with such emulsions have significantly improved resistance to stone loss and improved flexibility. z For Valley Slurry Seal, write in 867