Ultra low noise rubber pavements with
very high crumb rubber content
By Nils-Ake Nilsson, Sweden and Jorge Sousa, PhD., Portugal

   Acoustic Control AB of Sweden, Consulpav, and Recipav of Portugal teamed up in placing a new ultra low noise asphalt rubber mix on a surface course in Braganca, Portugal.
   Acoustic Control has been contracted by Traffic & Public Transport Authority, City of Gothenburg, Swedish Tire Recycling Administration (SDAB) and SKANSKA (one of the major road contractors in Sweden) to develop this new mix for the purpose of reducing noise.
   Nils-Ake Nilsson, Acoustic Control AB, worked with Paulo Fonseca, Recipav, and Dr. Jorge Sousa, Consulpav to place a demonstration project in Braganca, Portugal. The Portuguese Road Administration has agreed to assist in the evaluation process by allowing the placement of this surface on one of the most travelled arteries of the city of Braganca.
   A gap graded Asphalt Rubber Hot Mix section was placed in an adjacent lane to this ultra-low noise mix to compare the noise reducing properties of both materials. Noise measurements were performed comparing the noise reductions obtained by the two surfaces to that of the original pavement of conventional mix in a nearby lane.
   The evaluation of the measurements are still to be performed so no information concerning the reduction in dBA’s is available at this time.
   Earlier experiments in Sweden, using the dry process introduction of the rubber and a higher rubber content compared to the one now studied in Braganca indicated that 11-14 dB(A)-units of noise reduction is possible. However, due to the dry manufacturing method this mix did though not resist the traffic load for very long.
   One of the prime targets of the ultra low noise mix now tested in Braganca was to find out whether the “wet” production method would give us the necessary durability for this type of surface. We expect the version of the surface now tested in Braganca to have somewhat less noise reduction because of the high priority we gave to strength and durability for this test. 

New Asphalt-Rubber Road in Braganca, Portugal - Jorse Sousa, left, and Nils-Ake-Nilsson stand beside the new ultra quiet road surface. Note the special air flow resistance measuring device that Nilsson is carrying. It is used for fine tuning the flow resistance of the surface during compaction.

   The ultra low noise mix now studied on Braganca contained about 18% Asphalt-Rubber binder. Yes, it is no misunderstanding, 18%! The crumb rubber content of the 18% binder was about 45%. Yes, it is no misunderstanding, 45%!
   The new Asphalt-Rubber blending equipment developed by Hotmix Industries San Antonio, Texas, just acquired by Recipav, was used in the production of this asphalt rubber binder. The mix was produced on a batch plant and placed with conventional paving equipment.
   Flexural fatigue tests indicated that this very high rubber content mix has about 40 to 50 better resistance to fatigue compared to open graded asphalt rubber mixes at high strain levels!
   These values indicate that possible higher thickness reductions can be achieved or that with more crumb rubber much higher pavement lives can be attained. Due to a pending joint patent by Acoustic Control, Traffic and Public Transport Authority and the City of Gothenburg, Sweden, details of the mix cannot be disclosed at this time.
   It is clear from this experiment that we are just in the infancy of what can be achieved with Asphalt-Rubber. So far developments on Asphalt-Rubber have mostly been done based on raw experience from the use by the States and other highway agencies. However, with new equipment on the market and with new performance evaluating techniques, the road is now open to the development of exciting new materials with much higher binder and crumb rubber contents resulting in cost benefit ratios never dreamed possible.
   Current trends have led to a mechanism of producing binders with rubber that tend to
minimize its use. This experiment demonstrates that we may have been looking at the wrong end of the issue. Far more interesting mixes can seemingly be developed with high crumb rubber contents. For more information, contact the companies at www.acoustic.se, www.consulpav.com, or www.recipav.pt.


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