® C O M P O ECOLOGIC ASPHALT (RARx )

C/ Narciso Monturiol s/n P.I. Rompecubas. 28340. Valdemoro (Madrid– España) TEF:+34 918 950 968

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ABSTRACT

A new technology which produces a “Reacted and Activated Rubber” (RAR),

which is an elastomeric asphalt extender, has been developed by hot blending and

activation of a rubber granulate with a selected asphalt binder and activated mi-

neral binder stabilizer (AMBS). RAR achieves similar results comparable to ot-

her types of polymer modified binders (PMB). However, a principle advantage

with RAR is that it can be easily added to any HMA manufacturing facility using

systems designed to feed particulate material into a batch plant (pug-mill) or

drum mix plant. This paper describes how RAR is produced from raw constituent

materials. Various tests on binder contrast the performance to typical paving gra-

des and PMBs that are used in the USA. Tests on mixtures in wheel tracking and

fatigue demonstrate how the binder performance tests translate into mixture

performance. In all cases evaluated, the RAR mixtures out-perform SBS modified

and even conventional rubber modified equivalent materials.

BACKGROUND

The components of RAR are asphalt, crumb rubber and an activated mineral bin-

der stabilizer. The asphalt cements or (bitumen as referred to in Europe and el-

sewhere) conceptually can be any straight run plain soft bitumen. The use of the

softer asphalt grades enables production of HMA at conventional mixing and la-

ying temperatures without losing the proper workability, despite the addition of

the crumb rubber.

The Crumb Rubber is usually consisting of scrap tires that are processed and fine-

ly ground by any proven industrial method. The scrap tires consist of combina-

tion of automobile tires and truck tires, and should be free of steel, fabric or fibers

before grinding. For the production of RAR, the crumb rubber particles should be

finer than 1.0 mm. A 30-40 mesh maximum particle size is preferred.

The Activated Mineral Binder Stabilizer (AMBS) is a new binder stabilizer that

was developed to prevent excessive drainage of the bitumen in SMA mixes du-

ring mix haulage, storage and lay down. This stabilizer (industrially known as

“iBind”) is an activated micro-ground raw silica mineral (40μm and finer), which

is a waste by-product of Phosphate Industries mining. The activation, made by

nano-monomolecular particle coating, was aimed at obtaining thixotropic and

shear-thinning properties for the asphalt, since the binder film/mastic in the mix

should possess high viscosity at rest (haulage, storage and after laying) - for redu-

cing drain-down, and low viscosity in motion (mixing and laying) - for maintai-

ning the proper workability.

During the research and development work, RAR was produced and tested at

different formulations, dictated by the type and relative proportions of its three

components. As an average, a typical RAR blend contains about 62% Crumb

Rubber, 22% soft bitumen, and 16% AMBS. After the material has reacted in the

blending equipment another 10% AMBS is added in a coating mixer unit to pre-

vent re-coagulation of the product.

RAR has been found to enhance the properties of the plain bitumen to higher le-

vels than polymer modified asphalt, and even higher than conventional asphalt

rubber blends. A hypothesized basic model for the mechanism of RAR as a bitu-

men enhancer is illustrated in FIGURE 1.

FIGURE 1 Suggested model for the mechanism of RAR as an asphalt extender

BINDER TESTING

Traditional binder has been conducted to define properties such as penetration, Brook-

field viscosity, softening point and resilience (ASTM D5329) on RAR modified binder.

In this example a RAR material was produced using a 35/50 Pen which also graded as a

PG70-22. The results from this evaluation are presented in FIGURE 2 which shows data

from two trial production runs (Trial 1 and 2). The changes in these properties are very

significant particularly as the RAR content increases above 15% of the binder.

FIGURE 2 Traditional binder properties, a) Viscosity, b) Ring and Ball Softening Point,

c) Penetration and d) Resilience

PG GRADE

This RAR was then combined with a typical PG64-22 used in the USA obtained from

Ergon. This testing was conducted by Dongre Testing Laboratories and evaluated how

the RAR affects the PG grade using laboratory blended samples of the resulting binder

and the methods contained within AASHTO M320. The results from the DSR and BBR

testing are summarized in FIGURE 3.

FIGURE 3 PG Passing temperatures for high and low condition for trial 1 and 2 data sets

RECOVERY

In addition, testing has also been performed in the newer Multiple Stress Creep and

Recovery (MSCR) test, as specified in ASTM D7405 or AASHTO TP70, which has

more recently been introduced to better define the binder performance in relationship to

permanent deformation with the evaluation of the Jnr parameter. The concept of this

parameter is very important for ensuring that the material adopted tends to maintain

good behavior characteristics as the magnitude of stress increases.

It should be noted that the adoption of a very heavy grade would be considered equiva-

lent to 2-grade bumps using the system as specified in AASHTO M320. It is interesting

to note the percent recoveries obtained from the two tests (ASTM D7405 and ASTM

D5329) are in very good agreement (see FIGURE 5) considering these are conducted at

very different test temperatures, rates of loading and test conditions

FATIGUE

Extensive flexural fatigue tests, using the four point bending beam test device (ASSHTO

T321), were performed on variety HMA mixes. The conditions selected for the tests were to

load at a frequency of 10 Hz at a temperature of 20oC and a constant displacement to obtain

a level of between a nominal 250 and 700x10-6 strain. Generally 4 data points (2 at a high

strain level and 2 at a low strain level) were evaluated to obtain a log-log fatigue relations-

hip. From this expression the mean life at a strain of 500x10-6 was estimated and the asso-

ciated standard error.

Tests were made on polymer modified SMA mixes, AR-GAP conventional mixes and dense

graded mixes. These are compared to RAR gap graded mixes which were formulated having

the same binder content and add mix content as the reference AR-GAP “A” mix. The refe-

rence AR GAP “A” Graded mix had 8.23% AR Binder and 1% Portland cement or hydrated

lime, giving a total additive content of 9.23%.

PERMANENT DEFORMATION

Permanent deformation of asphalt mixtures can be accessed via a range of different labora-

tory techniques. The concept used was to compare the RAR modified materials to some

well-known good performing products. A polymer modified SMA material with fiber addi-

tion (referenced as Fiber/PMB) is considered the initial control. The same mix type but with

a blend of polymer and AMBS (referenced as AMBS/PMB) provides a second control of a

good performing product with respect to permanent deformation. These mixtures have good

performance history when used in a previous road project. The RAR mixtures have all been

used with a gradation typical of AR-GAP mixtures since this allows more space within the

aggregate structure to accommodate binder as used for the fatigue testing. The same blend

percentages were used for this testing as described earlier for the fatigue testing – that is a

total asphalt plus RAR content of 9.23% in the materials. All the data obtained were avera-

ged from two replicates with the exception of the B6.19RAR3.04 material which had a total

of 4 replicates. The detailed data obtained from this testing are presented TABLE 1 and

summarized in FIGURE 7 which shows that the high binder content materials with the RAR

additive have lower deformation than the well performing SMA mixtures.

FIGURE 4 Non-recoverable creep compliance (Jnr) versus % recovery

FIGURE 5 % Recovery (ASTM D7405) vs. Recovery %/Resilience (ASTM D5329)

MIXTURE TESTING

To investigate the effect of the binder properties on mixture performance the RAR was

added to several different types of mixtures including SMA and those used in normal

asphalt rubber gap graded mixes (ARGAP).

Mixes were chosen so that good contrast in performance could be obtained.

For example in the study conducted for permanent deformation some good performing

materials were selected as controls and the performance of the RAR modified

materials were compared to these. One of the major issues when comparing results

from different modifiers is how that modifier affects items such as the aggregate struc-

ture (for example VMA and air voids) and the effect that these parameters might have

on the property being evaluated. In order to reflect this concern we have often compa-

red against several mixture types with very different structures. While these compari-

sons are not perfect we consider this a valid approach as this alternate RAR modifica-

tion developed.

FIGURE 6 Flexural Fatigue results on different HMA mixes comparing to RAR ones, tested in

accordance with AASHTO M321 at 10Hz, 20oC and 500x10-6 strain

TABLE 1 Wheel tracking data on mixtures modified with RAR and two SMA controls

FIGURE 7 Wheel tracking data on GAP graded mixtures with RAR and PMB/SMA mixes

SUMMARY

This development combines an existing technology of rubber in asphalt mixes but produ-

ces a product that has been reacted with an activated mineral binder stabilizer in a unique

manner. The product has been subjected to extensive laboratory investigation and tests

performed during the R&D stage, has shown that HMA, produced with RAR, outperforms

conventional HMA and even modified and conventional asphalt rubber mixes. In general,

RAR is an elastomeric asphalt extender that modifies asphalt, increasing its PG grading,

resilience, and recovery properties. Different types of HMA produced with RAR showed

much better stability, rutting and fatigue resistance under attractive cost/benefit conditions.

THE MAIN ADVANTAGES OF RAR AS AN ASPHALT MODIFIER IN HMA

ARE AS FOLLOWS:

Easy and fast production. No need for AR or modifier blenders.

No need for re-heat cycles associated with conventional AR in the asphalt mixing

plant or job site.

Since the RAR product is a dry granulated material it is easy to handle, store and

transport.

RAR can be fed to any asphalt mixing plant directly to the pugmill or the dryer

drum.

When blended with the asphalt binder in the mixing plant, a unique asphalt rubber

binder is formed to provide better resilience and recovery and higher viscosity and

softening point.

With increasing RAR content in the combined binder (RAR plus asphalt) any PG

Grade binder can be formed (both positive and negative temperature gains to the PG

grade).

With the correct RAR content, any type of hot AR mix can be produced (Dense Gra-

ded, SMA, Open Graded, Gap Graded, etc.).

Can make new improved hot AR mixes (with even more crumb rubber) that are

stronger more resilient, and exhibit better recovery, rutting and fatigue resistance.

RAR eliminates the need for cellulose fibers in SMA/porous mixes.

The crumb rubber particles contain a large amount of inorganic materials that are elec-

tro-statically surface charged (fillers, vulcanization materials, and various additives).

The activator of the silica mineral particles of the AMBS is composed of organic mole-

cules that are partly electro-statically surface charged (ammonium head) and contains

organic hydrophobic chains. When the activator particles are present in a liquid me-

dium, such as asphalt binder, they can be attracted and connected to other particles with

opposite charge. Charged organic chains of the activator in the AMBS are able to create

a connected network of particles. When the fine RAR particles (elastomeric material)

are blended in the liquid medium with the activated silica particles, the charged

molecules of the AMBS particles are connected to the rubber particles at charged sites

on the inorganic materials. In this way, when all the above materials are blended toget-

her with the hot liquid bitumen, an inner network of the elastomeric material and the

AMBS particles is formed in the bitumen.

This network, together with the unique elastic and networking capabilities of the elasto-

meric material derived from the reaction and activation of rubber at high temperatures

structurally enhances the bitumen resulting in improved mechanical properties. These

are evident in the elastic behavior and better performance in tests such as the bending

beam fatigue test. RAR is also coated with a special formulation of AMBS that once

dispersed into the bitumen also attaches itself to the aggregate. This connection impro-

ves binder aggregate interactions improving moisture sensitivity responses. As such the

new networks bound aggregate, bitumen, elastomeric material and AMBS particles.

Such a network cannot be formed when just rubber and bitumen are blended together

(without AMBS), as in any conventional asphalt rubber technology.

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