Comparison of Modified Asphalt Hot Mixes Using Devulcanized Crumb Rubber and SBS

 by Huang Wen Yuan

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1. Introduction to Rubberized Asphalt Concrete Technology

Rubber modified asphalt concrete hot mixes is accomplished by two methods – Dry Process and Wet Process.

In the Dry Process, crumb rubber modifier (CRM) is mixed together with heated aggregate and liquid bitumen in the pugmill.  In this process, CRM is considered more as filler or bitumen stabilizer.

In the Wet Process, CRM is thoroughly mixed in hot bitumen and the resulting blend is held at elevated temperature for a specific period of time for proper interaction of CRM and bitumen before incorporating the binder with the mineral aggregate. CRM is used in this process as a binder modifier.

This Wet Process Rubber Modified Asphalt or otherwise known as Asphalt Rubber (RAC) was introduced in the United States in the 1960’s and has been successfully adapted for commercial use by the Chinese paving industry since 2004.

RAC can perform better than conventional asphalt concrete, especially in extreme weather conditions and heavy-duty traffic, when the right materials and processing method were chosen.

Advantages of RAC over Conventional Asphalt Concrete:-

  • Improved durability when used as surface course or overlay due to better fatigue cracking resistance.  RAC is not only less brittle at low temperature, but also offer better heat resistance at higher temperature.
  • ŸReduction in tyre/pavement noise.
  • ŸIncreased flexibility when used as chip seals (SAM or SAMI) due to better reflective cracking resistance.

 Certain limitations have however prevented RAC from gaining wider acceptance, i.e.:-

  • Specialized blending equipment and storage facility are required and the higher capital outlay would be a deterrent for RAC to be used for smaller jobs.  Asphalt rubber has to be kept at high temperature and stirred continuously during transportation and storage to prevent segregation of particles in the binder.  Even so, it is required to be used as soon as possible because its viscosity keeps going down during high temperature storage.
  • This production method and delivery mode is totally different from conventional asphalt supply systems; asphalt suppliers are deterred from making the switch to produce asphalt rubber.
  • The binder content required for asphalt rubber mix design is higher than conventional hot mix asphalt concrete, thereby increasing total mixture cost.

 

2. Production of WDRA  

The terminal-produced Stabilized Rubber Modified Asphalt (WDRA) is produced in 2 steps:-

  • Tyre-derived crumb rubber is devulcanized and blended with other modifiers, stabilizers and additives, and finally pelletized to specific particle size.
  • As in the process for making SBS modified asphalt, WDRA modifier pellets are firstly mixed with hot bitumen in the tank, sheared into tiny particles and dispersed evenly into the binder while passing through a colloid mill, and held for a designated period of time for interaction. The end product, stabilized rubber modified asphalt (WDRA), is then delivered to a hot-mix plant’s asphalt tanks by conventional tank trucks and fed into the pugmill to produce hot mixture.

 

Material Selection and Bitumen Modification  

In the comparison study, raw materials were produced as follows:-

i. WDRA was processed as follows:-

  • PEN 70 bitumen / 18% WDRA modifier pellets.
  • Shearing for 30 minutes at the speed of 3500 rpm and temperature of 170 – 180°C.
  • Mixing for 30 minutes at the temperature range of 170 – 180°C.

Sheet 1 – Tests on WDRA
Sheet 1 Test on WDRA v3

 ii.  SBS modified asphalt was processed as follows:-

  • PEN 70 bitumen / 4% SBS modifier.
  • Shearing for 30 minutes at the speed of 3500 rpm and temperature of 170 – 180°C.
  • Mixing for 60 minutes at the temperature range of 170 – 180°C.

Sheet 2 – Tests on SBS Modified Binder
Sheet 2 Test on SBS Modified Binder

Observations

From the results of Sheets 1, 2 and 3, it was observed that:-

  • The similar production process for SBS bitumen binder can also be used for WDRA production.
  • WDRA binder has similar mechanical performance as asphalt rubber.
  • Both WDRA and SBS samples have a softening point difference of 0.5 °C after 48 hours storage at high temperature.  Thus, it can be deduced that segregation is similar for both modifiers.

Viscosity and softening point difference obtained from the 3 modifiers after storage @ 170°C.  

Sheet 3 – Storage Stability Test
 Sheet 3 Storage Stability Test v2

3.  Mineral Aggregate Gradation  

The mineral aggregates used are basalt coarse aggregate, limestone fine aggregate and mineral filler chosen from Shanghai Road and Bridge (Group) Co., Ltd.

Sieve analysis (passing %) was performed on the aggregates and the results as follows:-  

Sheet 4 – Mineral Aggregate Gradation
Sheet 4 Mineral Aggregate Gradation v2 

4. Mixture Designs and Performance Test

3 HMA gradations according to China’s Department of Transportation specifications were chosen for this comparison – 3 were modified with WDRA and 2 with SBS.

i. WDRA Modified Asphalt :-  

  • ARHM13 (Gap graded hot mix)
  • AC13 (Continuous graded asphalt concrete)
  • SMA13 (Stone Matrix Asphalt)

ii. SBS Modified Asphalt :-

  • AC13 (Continuous graded asphalt concrete)
  • SMA13 (Stone Matrix Asphalt)

Sheet 5 – Mineral Gradations
Sheet 5 Mineral Gradation

Same mixing process was used for incorporating the SBS modified and WDRA binders with the mineral aggregate:-

  • Heating temperature for asphalt @ 170°C
  • Heating temperature for mineral aggregate @ 180°C
  • Mixing temperature @ 170 – 180°C

The viscosity of WDRA is much higher than that of the SBS modified asphalt, so there is no requirement for fibres in SMA13 to improve asphalt stability.

During the mixing process, it was observed that segregation, coating, color and luster of WDRA is identical with SBS modified asphalt.

Sheet 6 – Marshall Stability and Flow Test
Sheet 6 Marshall Stability and Flow Test

From the results of the Marshall Stability Test in Sheet 6, the WDRA binder is suitable for all the 3 types of mixture design.

No fibres were used in WDRA SMA13.

The binder content for both the WDRA and SBS modified asphalt are about the same for each of the SMA13 and AC13 mixes.

Stability for both SBS modified asphalt mixes is higher than WDRA, which however does not mean the WDRA is inferior to the SBS in its resistance to high temperature (refer to Sheet 7).

Sheet 7 – Freeze-Thaw and Stability of the Mix Designs
Sheet 7 Freeze-Thaw and Stability of Mix Designs

Observations

The results obtained from tests in Sheets 6 and 7 indicated that when WDRA (without addition of fibres) was used to modify ARHM13, optimum binder content was achieved at 5.5% (compared to 6% for the SBS-modified SMA13).  Binder content was about the same as SBS modified asphalt when WDRA was used to modify SMA13.

This observation shows WDRA could be used for a wider selection of mix designs while maintaining the same or even better performance and cost characteristics as SBS.

It is evident that the lower binder content of WDRA compared to conventional Wet Process Asphalt Rubber (RAC) does not affect its moisture stability and high temperature resistance.

Performance of WDRA is comparable to the SBS modified asphalt.

This could be explained by the better dispersion of rubber in WDRA (attributed to its production process) has improved cohesion of the asphalt binder with aggregate.

 

5. Rutting

Picture 1  Rutting Plates Comparing Field Blend RAC and WDRA Mixes Rutting

Left: Asphalt rubber           Right: WDRA modified asphalt

The above picture was taken of the rutting test for AC13 mixture designs.

Observations

It can be observed that permanent deformation for both samples is identical, but the color and luster of the WDRA sample is more intense.

 

6. WDRA used as Chip Seal

Picture 2 – WDRA Used in Stress Absorbing Membrane Interlayer (SAMI-R)WDRA

Photographs from Picture 2 were taken after a truck drove over the interlayer before the wearing course was laid.  The enlarged photographs on the right showed the WDRA interlayer exhibited very strong cohesion to the old pavement.  When a heavy truck passed directly over the newly sprayed membrane, the membrane itself was intact but pavement material and road mark was torn off.  

Observations  

WDRA also exhibits advantages for the stress absorbing membrane interlayer (SAMI-R) as follows:-

  • Easier to spray and more uniform.
  • Better reflective cracking resistance.
  • Lower cost.

 

7. Summary

Using the same production mode as SBS modified asphalt, WDRA has reached the same performance results as SBS modified asphalt.

Due to the compatibility of materials used in the mix design, WDRA demonstrated similar or superior stability and durability – registering better performance in low temperature cracking resistance, anti-stripping, flexibility, fluidity and workability than SBS modified asphalt.

WDRA when used in all the commonly used mixture types performed at least as well as the SBS modified asphalt.

The potential to expand scale and applications of WDRA appears promising.