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Enhancement of Performance of Hot Mix Asphalt Concrete by Polymer Modified Bitumen

INTRODUCTION

The rehabilitation and improvement of our road network is of strategic importance to any government committed to providing better infrastructure facilities to enable the rapid development of our nation.

This goal of providing better roads, in Sri Lanka, has over the last two decades resulted in the development of new roads as well as the improvement of existing roads for the convenience of commuters. Though, the government and the owner of these road networks, the Road Development Authority (RDA) expect a significant life span of these new and rehabilitated roads. However, unsatisfactory conditions and pre-mature failures have been observed in many of the roads all over the Sri Lanka. The pre-mature failures include; surface cracking, raveling, stripping the top layer of bitumen, corrugations, base failures and so on.

Bitumen is one of the oldest known engineering materials, having been used for nearly thousands of years in as adhesive, sealant, preservative, waterproofing agent and pavement binder. By selecting good crude oil and proper refinery processes, good bitumen properties can be obtained. However the limited resources for producing good-quality bitumen and the lack of effective control actions during its refinery process, as well as other economic influences have made industries pay more attention on bitumen modification. Additionally, the pavement industry has developed rapidly all over the world during the last few decades, especially in developing countries, significantly increasing its demand. Following the rapid development in Sri Lanka, increased traffic load, higher traffic volume, and insufficient maintenance have led to the occurrence of severe distresses (e.g. rutting and cracking) on the surface of our roads.

POLYMER MODIFIED BITUMEN

Bitumen plays a very important role in hot mix asphalt (HMA), and improvement in its composition can improve the properties of HMA significantly. Polymer modification of bitumen is the incorporation of polymers in bitumen by chemical reaction or mechanical mixing. During the last few decades, significant research has been done on the polymer modification of bitumen with both lab and field tests completed. Access Engineering PLC has done extensive lab works in this area with the assistance of the Research and Development Division of RDA along with field trials Joint inspections at regular intervals are carried out to obtain the necessary test data in order to establish the road’s long term performance. The Kadawatha Nittabuwa Road project (KNRP) in 2016 is one of the key projects which was done through this procedure

Popular types of polymer used in modified bitumen

PLASTOMER TYPE

  • Polyethylene (PE)
  • Polypropylene (PP)
  • Ethylene-vinyl acetate (EVA)
  • Ethylene-butyl acrylate (EBA)

ELASTOMER TYPE

  • Styrene-butadiene-styrene (SBS)
  • Styrene-isoprene-styrene (SIS)
  • Styrene-ethylene/butylene-styrene (SEBS)

These polymers are documented to improve the properties of bitumen including its stiffness at high temperatures and cracking resistance at low temperatures, as well as giving it better moisture resistance or longer fatigue life. Considering the climate characteristics in Sri Lanka, it is highly recommended to use only elastomer types of modifiers. For the above mentioned project a branded elastomer type (SBS) modifier, by the name “Elvaloy” polymer, by DuPont, was used.

Key Features and Benefits of (Polymer Modified Bitumen) PMB Asphalt Production and Laying

  • A special mixing plant is required to mix the polymers with the bitumen
  • The polymer has to mix with general bitumen type 60/70 under special conditions
  • The PMB has to mix with other aggregates materials at a temperature 20 0C more than what would be required using normal bitumen
  • Strict quality control is necessary to maintain a quality product
  • The laying and compaction temperatures are also 15 0 C high than with conventional asphalt
  • Traffic can be opened within 4 hr of laying where as in conventional asphalting a minimum 12 hr time period is needed.
  • Less compaction effort would be sufficient.
  • Initial cost can be 15 % higher than conventional asphalt but as a whole it has significant advantages including a reduced lifecycle cost.
  • The lifetime of PMB asphalt roads can be more than twice that of conventional asphalt roads.                           

The Elvaloy elastomer polymer once mixed into conventional bitumen (60/70), enhances the performance of the Marshall Properties of final asphalt product. The most important Marshall properties of asphalt are Flow (mm) and Stability (kN). The following table shows the basic and improved values of these properties.

Stability parameter shows significant increment.
Flow parameters also exhibits clear enhancement.
Parameter Neat 60/70 Bitumen PMB by Elvaloy Enhancement (%)
Stability (kN) 20.8 27.6 33
Flow (0.55 mm) 10.3 12.0 17

PERIODIC VISITS AND TESTS BY AEPLC & RDA

In order to establish long term performance, the site is jointly visited with RDA personnel to test the laid road stretches at the Kadawatha Nittabuwa Road project (KNRP).

The overall positive aspects of the work have yet to be realised. Test results yield a lower aging of the asphalt and maintaining of the flow and stability at satisfied levels compared to the same age asphalt with neat bitumen. These initial results point to a guarantee of long term performance and further tests will validate these expectations.

Content Submitted by Dr. Praneeth Wickramarachchi

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