Road Maintenance Solutions from One Source

by Glynn Holleran

Though no quick fix, polymers can play a positive role in mix performance

Glynn Holleran is Vice President of Valley Slurry Seal Co. (VSS) and Director of VSS Asphalt Technologies, a division of Valley Slurry Seal Co. The companies are in West Sacramento, Calif.

With road pavements growing older and traffic levels growing higher, the need for low-cost, effective road maintenance programs is imperative. The challenge for pavement maintenance contractors becomes doing more with less and doing it better. Certainly, the demand is that new pavements last longer and old pavements have every last bit of life wrung out of them. The answers often lie in innovative emulsion technology, the use of high performance binders, and improved pavement maintenance methods. Some of today's most cost-effective pavement maintenance solutions involve the use of polymer modification in slurry surfacings. 

Asphalt is usually made from naturally occurring crude oil and its performance is decided by composition. This composition is affected by the crude oil source and its processing. Some crude oils can produce bitumen that is susceptible to water, high temperatures, high traffic and rapid aging. In hot mix or slurry surfacings, such deficiencies may cause raveling, premature wear, ruts or cracks, or even disintegration under heavy traffic or the extremes of weather. 

Obviously there's an ongoing need for better asphalt, especially in conditions of high traffic and low and high temperatures. The SHRP program, in which polymer modification plays an integral part, has designed performance grades to meet this aim.

The method of polymer manufacture may vary and result in a solid polymer that must be blended with the asphalt before emulsification or the process may produce a latex. A latex, sometimes called a dispersion, is actually an emulsion. So when you're dealing with a latex, you must keep emulsion issues in mind. That is, cationic latex is required for cationic asphalt emulsion, and pH issues are important. In considering breaking of a modified emulsion you must look at the emulsifier used in the latex and allow for it.

Many different polymer types have been used to modify asphalt for slurry. The most important of these are: 1) Elastomers such as SBR Latex, Neoprene Latex, Natural Latex, Co-polymers (SBS), and reclaimed rubber; 2) Plastics such as Polyolefin Polymers (EVA, EMA).

Polymers affect binder properties

Combined with asphalt, polymers create large changes in the physical properties of the final binder. Polymers are viscoelastic. This means that they will recover when stressed and will flow when heated. Also, they can be elastic at lower (operational) temperatures and still flow easily when heated (handling).

In fact, polymers will respond in three ways: 1) elastically; 2) elastomerically (time-dependent elasticity) and; 3) viscously (plastically). The response of any polymer will depend upon the structure and the conditions of loading in terms of time and temperature. A single polymer has quite a different response when loaded at different temperatures and rates.

When mixed with an asphalt, the polymer will impart this elasticity and flow resistance if the asphalt and the polymer are compatible.

So polymers:

• Raise the softening point - this assists in reducing bleeding.
• Increase viscosity - this allows increases in the thickness of films on the aggregate and hence, increases durability. It also assists in deformation resistance.
• Decrease thermal susceptibility - this gives the effect of a softer binder at low temperatures and a stiffer binder at higher temperatures - that is a better balance of properties.
• Increase elasticity - this is true of all polymers, particularly for the elastomers. A binder that can recover will resist traffic-induced cracking better.
• Increase cohesion or internal strength - this reduces deformation under traffic and increases stone retention.
• Increase low temperature tensile strength and flexibility - this reduces the propensity to crack due to reflection or low temperatures.

How polymers enhance slurry mixes

Polymers can extend the performance of slurry and Micro-surfacing in: adhesion and cohesion; abrasion resistance; bleeding resistance and durability. For Micro-surfacing, polymers provide the added benefit of deformation resistance, especially in ruts. While slurry surfacing is laid at only one stone thick, Micro-surfacings are laid in multiple layers, often on high-volume, high-traffic roads where toughness and resistance to tearing become more important.

To achieve this using polymers, 1 to 5 percent latex is required, depending on the required property build. Special rut filling mixes may also be made using polymers. Increasing the stiffness of the binder in the Micro-surfacing can increase the stability of the mix beyond that of the hot mix it is repairing. In these applications, SBS or EVA at 5 percent work best; however, 3 to 5 percent latex will deliver significant improvements (See Figure 1, page 16).

The key to any good mix design is to first determine the desired outcome - will it mix, set and perform? In truth, you need to know in what way it will perform. This will tell you, if the spec does not, what to use - a Micro-surfacing, a slurry seal or a cape seal.

Considerations throughout the design process include job site - whether it's highway or residential - traffic type, volume and constraints, surface feature bleeding, ruts, type of cracks, grades, corners, and speed limits.

You should always talk to your emulsion manufacturer about local specs and requirements. Most states will have a spec for a latex or polymer-modified emulsion. The state of California, for example, controls on the basis of standard emulsion parameters such as viscosity, settlement, storage stability, sieve test, demulsability, residue by evaporation and ash content. 

With stringent specs, testing comes into play. Binders must be carefully measured (See Figure 2, page 16). Traditionally, this has been done by either instrumental or mechanical tests or a mix of both. The residue is first recovered from the emulsion by distillation or evaporation. 

Currently a matter of debate, the effect of air and heat on some polymers is critical. Oven evaporation at moderate temperatures - 140 to 160° F (60 to 70° C) - is the favored method. Certainly a hot plate should not be used.

The residue is subjected to an infrared analysis to determine polymer content. In California, the content is 2.5 percent minimum for slurry and 3 percent for Micro-surfacing. The residue is also tested for physical properties, penetration and ductility. In future years, the Strategic Highway Research Program (SHRP) dynamic shear rheometer may be used as it can measure stiffness, consistency and elasticity directly.

In addition to the infrared test, the residue may be measured for torsional recovery. In this test, a spider or Y-shaped jig is placed in a pen cup while the sample is poured and solidified around it. A torsional shear is applied to turn the jig 180° F (82° C) and the recovery is measured- 77° F (25° C). This can be correlated to the concentration of the polymer present. It is also dependent upon the polymer type. For example, the SBS will give higher numbers than SBR or neoprene. For 3 percent latex, 18 to 25 percent is normal.

Manufacturing methods may affect final emulsion properties. Latex may be added in a number of ways:

• Pre-blended in the soap solution - This is the normal route for neoprene and natural latex, but may also be used for SBR. This method limits latex level as pH and other compatibility issues become important. The limit is usually 3 percent.
• Co Mill - The latex is metered into the soap line just before the mill. This method usually leads to higher viscosity emulsions as packing is altered by the application of shear to the latex emulsion. Levels of up to 12 percent have been added this way.
• Post addition - High levels of latex, up to 20 percent, may be added; however, this method generally reduces viscosity. Also, the emulsion is not very storage stable and will require mixing before use.

As to mix times, keep in mind that latex contains its own emulsifier system. If not compensated for in the emulsion design, this can lead to increased mix times and slow setting. During the entire design and manufacturing process, you will notice effects in the tests:

• Wheel track abrasion loss - SBR at 3 percent can reduce loss by 50 percent in one-day soaks and 67 percent in six-day soaks. This indicates that the surface is not only tougher but more abrasion resistant. Adhesion and water resistance are also improved. Neoprene and SBS also improve this by 40 to 50 percent (See Figure 3, this page).
• Cohesion - SBS can actually give reductions in cohesion, due mostly to the softness of the polymer. SBR will increase cohesion by 25 percent.

Polymers improve deformation resistance

Depending upon how it is measured, either by wheel tracking of thick sections or by the lateral displacement test, it has been found that varying levels of improvement are achieved by SBR, EVA and SBS. SBR at 3 percent will decrease lateral displacement by close to 90 percent; SBS by 30 to 40 percent; and EVA by 70 to 80 percent.

The Schulze-Bruer-Ruck rating is significantly improved by SBR, SBS and natural latex (See Figure 4, this page). Not all tests address all performance issues. For example, there is no real test for cracking resistance. Abrasion loss is also an area that requires some scrutiny. In future years, it is expected that some of the mix tests now developed for SHRP and other design methods will be used. In Australia, work has been carried out using compacted specimens and subjecting them to creep and resilient modulus testing. In such tests the value of polymers can be predicted.

Quite simply, polymers are means of extending the performance of asphalt. These additives improve resistance to bleeding, traffic, high and low temperatures, cracking and deformation. Latex is a convenient method by which to modify both Micro-surfacing and slurry seal emulsions. Most importantly, though, these additives are not a magic pill. Good design and pavement maintenance methods are still essential.