The Use Of Polymer Modification In Slurry Surfacings.

Glynn Holleran, Vice President Valley Slurry Seal Company.

Introduction

The performance of road surfacings is an essential part of any road maintenance program. The purpose of road maintenance is to either protect or rehabilitate a road, making it effectively new again. However, in the last 20 years road budgets have shrunk for most state, county and city agencies. As well the road pavements themselves are growing older, the traffic levels are getting higher and politically aware tax payers are demanding that the roads be improved. What once were low traffic rural roads now carry commuters to new housing developments .In many residential areas the homeowners expect that the road will not only carry their cars but also be smooth places for their children to roller blade or throw a football.

Effective maintenance programs must therefore not only be low cost but cost effective too.

We need to do more with less and do it even better.

The key to doing more with less is in innovation and design, materials are an important part of this .This workshop is covering aspects of design and materials .

Road engineering has relied on naturally occurring materials for reasons of price and convenience. For the most part these materials have done an excellent job. However the extra demands placed on them has meant that enhancement of materials has often become necessary. In the era of " doing more with less" new pavements are expected to last longer and, old pavements to have extra life wrung out of them.

This implies and requires of the asphalt emulsion product provider, high performance binders that meet the requirements of the traffic, environment and climate.

Asphalt is made from naturally occurring crude oil. The performance of an asphalt is decided by the composition. This composition is decided by the crude oil source and the processing. Some crude oils can produce bitumen that is susceptible to water, high temperatures, and traffic loading ( thermal and shear susceptibility), or may age rapidly.

Such deficiencies may then be seen in hotmix that ravels ( disintegrates under traffic and heavy rain), prematurely wears, ruts or cracks. It may be seen in surface dressings as loss of stone under traffic and/ or rain, or as excessive softness in early life resulting in stone plucking at high pavement temperatures. It may be seen in slurry surfacings that bleed, ravel, crack prematurely, deforms or ages.

In fact all standard asphalts suffer from these problems under some circumstances.

The need is obviously for better asphalt, especially in conditions of high traffic, and low and high temperatures. The SHRP program designed " performance grades" to meet this aim. This is in essence to extend the performance of asphalts in the high and low temperature ranges. Polymers are clearly an excellent way of achieving the aim.

Response to Stress and Strain.

Viscoelasticity is about the reaction of a solid or semi solid to stress. Viscoelastic materials have a range of response based on the temperature and loading regimes imposed.

Polymers are viscoelastic. When stressed a polymer will respond in three ways, the relative levels of response will depend directly on the structure.

i) elastic deformation ( 100% recoverable instantaneously).

ii) delayed elastic deformation ( 100% recoverable but time dependent).

iii) Viscous or plastic flow ( not recoverable).

The response of any polymer will depend on the structure and the conditions of loading in terms of time and temperature. A single polymer that has quite different response when loaded at different temperatures and different rates.

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

Improvement of asphalt properties, ie rheology or how they respond to stress and strain at the conditions this paper concentrates on polymer modification, particularly by the use of latex and how it relates to slurry type applications.

2. POLYMER TYPES

"Polymer" is a derived word that means " of many parts". That is polymers consist of very large molecules made up of the same, repeating, molecular units. The molar masses of polymers can be from 10,000 to 10,000,000.

A copolymer has two different sorts of repeating units and a block copolymer has these repeating units in a regularly occurring block pattern. Depending on the size and positioning of the different molecular functions as well as the thermodynamic or energy considerations the polymer will form different structures and the polymer will have different properties.

The polymer structure determines the physical properties and the chemistry of the polymer will determine its reactivity, for example its aging and degradation properties.

Two basic types of polymer structure that are important for road applications are :

a) Elastomer- Rubber Like

b) Thermoplastic - plastic like.

The method of manufacture may vary, this may 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 dealing with a latex emulsion issues must be remembered. That is cationic latex is required for cationic asphalt emulsion, pH issues are important, you must in considering breaking of a modified emulsion 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:

a) elastomers :

SBR Latex.

Neoprene Latex

Natural Latex

Co- polymers (SBS)

Reclaimed rubber

b) Plastics : Polyolefin Polymers ( EVA, EMA, )

3. WHAT POLYMERS DO

We have seen that polymers have much bigger molecules than asphalt has. This allows them to have unique properties that may be imparted to the asphalt. The efficiency with which they can achieve this depends on the asphalt/ polymer compatibility, amount of polymer added and the polymer type. But in general we can say that a stiff plastic type polymer produces a stiff asphalt/polymer binder, a rubbery elastic polymer produces a more flexible asphalt polymer binder.

So polymers:

a) Raise Softening Point.- This assists in reducing bleeding.

b) Increase viscosity- This allows increases in the thickness of films on the aggregate and hence increases durability. It also assists in deformation resistance.

c) 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.

d) Increases elasticity- this is true of all the polymers , but most for the elastomers. A binder that can recover will resist traffic induced cracking better.

e) Increases cohesion or internal strength- This reduces deformation under traffic and increases stone retention.

f) Increases low temperature tensile strength and flexibility- this reduces the propensity to crack due to reflection or low temperatures..

4. RELATIONSHIP TO SLURRY SURFACINGS

The main types of slurry surfacing are slurry seals and microsurfacing. Each has its place and application areas. The factor that distinguishes a slurry seal from a microsurfacing is the thickness at which the material is laid. In general a slurry surfacing is laid at only one to one and a half stones thick. Microsurfacings are laid on the other hand in multiple layers .The issues therefore of performance are related to the thickness of the layers.

They are for slurry:

adhesion and cohesion

abrasion resistance

bleeding resistance

durability.

For microsurfacing the same list applies but added are considerations of deformation resistance, especially in ruts

Microsurfaces are often laid on high volume, high traffic roads and so the toughness and resistance to tearing by traffic become more important.

Polymers can extend the performance of slurry and microsurfaces in these areas. To achieve this 1-5% latex is required. The level used will depend on the required property build.

Cape seal is a combined slurry/chipseal technique in which a chip seal is applied and a slurry used to fill the majority of the void structure to give a smooth , low noise surface of high skid resistance. In such applications the function of the slurry mix is mostly to protect the chip seal and thus increase durability. The presence of polymer in the chipseal can be an effective crack treatment and the polymer in the slurry will improve the stone retention and increase the allowable film thickness in the mix without bleeding. 3-5% latex is required in the chip seal and 1-3% in the slurry.

Special rut filling mixes may be made using polymers. Increasing the stiffness of the binder in the microsurfacing can increase the stability of the mix beyond that of the hotmix it is repairing.

In these applications SBS or EVA at 5% work best but 3-5% latex will give significant improvements.

Design Using Polymers.

a) Choice Of Emulsion.

Most states will have a specification for a latex or polymer modified emulsion. The state of California for example controls on the basis of standard emulsion parameter such as viscosity,settlement,storage stability, sieve test, demulsability, residue by evaporation and ash content.

However, as the properties of the binder are important it must be measured as well. Traditionally this has been done by either instrumental or mechanical tests or a mixture of both . The residue is first recovered from the emulsion by distillation or evaporation.

The effect of air and heat on some polymers is critical and this is currently a matter of debate. Oven evaporation at moderate ( 60-70C) temperatures is the favored method.

Certainly a hot plate should not be used.

The residue is subjected to an infrared analysis to determine polymer content ( 2.5% minimum in California for slurry and 3% for microsurfacing).

The residue is also tested for physical properties. Penetration and ductility. In future years the SHRP dynamic shear rheometer is likely to be used, this can measure stiffness or consistency and elasticity directly.

Alternatively 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 and the sample poured and solidified around it. A torsional shear is applied to turn the jig through 180 degrees and the recovery measured ( 77F).

This can be correlated to the concentration of the polymer present. It is also dependent on the polymer type and SBS will give higher numbers than SBR or neoprene.18-25% is normal for 3% latex.

b) Manufacturing Method.

The manufacturing method may effect the final emulsion properties. Latex may be added a number of different ways .

i) Preblended in the soap solution:

 

This is the normal route for neoprene and natural latex but may be used also for SBR.

This method limits latex level as pH and other compatibility issues become important. Limit is usually 3%.

 

ii) 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% have been added in this way.

iii) Post Addition.

 

High levels of latex , up to 20% may be added in this way, however this method generally reduces viscosity and the emulsion is not very storage stable and will require mixing before use.

Talk to your emulsion manufacturer on the local specifications and the requirements for your mixes.

c) Mix Design.

The good new on mix design is that no real changes have to be made. The key to any good mix design is to first determine the desired outcome.

The basic questions that need to be asked ( Benedict):

i) Will it mix?

ii) Will it set?

iii) Will it perform?

Actually you need to know " perform in what way", this will tell you, if the specification does not , what to use, a microsufacing, a slurry seal, a cape seal.

The determinants of this are job site, traffic type,volume and constraints, surface features- bleeding, ruts, cracks ( types), highway, residential, grades, corners, speed limits etc.

Polymers will be the performance enhancer to meet these needs.

The design then follows as normal , and as has been discussed in this workshop.

In the process you will notice the following effects in the tests:

i) Wet track Abrasion Loss.

SBR at 3% can reduce loss by 50 in one day soaks and 67% in 6 days. This indicates that the surface is not only tougher and more abrasion resistant but the adhesion and water resistance is also improved. Neoprene and SBS also improve this by 40-50%.

 

ii) Cohesion.

SBS can actually give reductions in cohesion, due mostly to the softness of the polymer. SBR will increase cohesion by 25%.

 

iii) Deformation Resistance.

Depending on how it is measured , either by wheel tracking of thick sections or by the lateral displacement test , different levels of improvement are achieved by SBR, EVA and SBS. SBR at 3% will decrease lateral displacement by close to 90%. SBS 30-40% and EVA by 70-80%.

iv) Compatibility.

Schulze- Bruer, Ruck Rating is significantly improved by SBR, SBS and natural latex.

 

v) Mix Times.

Latex contains its own emulsifier system. If not compensated for in the emulsion design, this can lead to increased mix times and slow setting.

 

vi ) Other

The tests here do not address all the performance issues, there is not real test for cracking resistance for example. Abrasion loss is also an areas that requires some scrutiny. In future years you could expect that some of the mixture tests now developed for SHRP and in other rational design methods will be used. For example work has been carried out using compacted specimens in Australia and subjecting them to creep and resilient modulus testing. In such tests the value of the polymer can be predicted. However they are currently only suitable for rut filling mixes or in overlays where the microsurfacing is laid relatively thickly.

 

 

d) Application Using Latex modified Emulsion Systems.

There are few extra requirements for such systems. The emulsion should be stable and easy to handle and give no real differences in application. Appearance may be lighter.

If very high levels of latex have been used a tackiness may appear in the matt. This will disappear with traffic, if there is pick up a light sand coat may be required.

5. CONCLUSIONS

1. Polymers are means of extending the performance of asphalts.

2. Polymers improve resistance to bleeding, traffic, high and low temperatures, cracking, and deformation.

3. Latex is a convenient method by which to modify both microsurfacing and slurry seal emulsions.

4. They are not a magic bullet good design and practice are still essential.

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