IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Issue: 07 | Jul-2014, Available @ http://www.ijret.org 83 INFLUENCE OF ADDITIVES ON THE DRAIN DOWN CHARACTERISTICS OF STONE MATRIX ASPHALT MIXTURES Bindu C.S 1 , Beena K.S 2 1 Associate Professor, Division of Civil Engineering, School of Engineering, Cochin University of Science& Technology, Kerala, India 2 Professor, Division of Civil Engineering, School of Engineering, Cochin University of Science& Technology, Kerala, India Abstract In Kerala highways, where traditional dense graded mixtures are used for the surface courses, major distress is due to moisture induced damages. Development of stabilized Stone Matrix Asphalt (SMA) mixtures for improved pavement performance has been the focus of research all over the world for the past few decades. Many successful attempts are made to stabilize SMA mixtures with synthetic fibres and polymers. India, being an agricultural economy produces fairly huge quantity of natural fibres such as coconut, sisal, banana, sugar cane, jute etc. . Now- a -days the disposal of waste plastics is a major concern for an eco- friendly sustainable environment. This paper focuses on the influence of additives like coir, sisal, banana fibres (natural fibres), waste plastics (waste material) and polypropylene (polymer) on the drain down characteristics of SMA mixtures. A preliminary investigation is conducted to characterize the materials used in this study. Drain down sensitivity tests are conducted to study the bleeding phenomena and drain down of SMA mixtures. Based on the drain down characteristics of the various stabilized mixtures it is inferred that the optimum fibre content is 0.3% by weight of mixture for all fibre mixtures irrespective of the type of fibre. For waste plastics and polypropylene stabilized SMA mixtures, the optimum additive contents are respectively 7% and 5% by weight of mixture. Due to the absorptive nature of fibres, fibre stabilizers are found to be more effective in reducing the drain down of the SMA mixture. The drain values for the waste plastics mix is within the required specification range. The coir fibre additive is the best among the fibres investigated. Sisal and banana fibre mixtures showed almost the same characteristics on stabilization. Keywords: Stone Matrix Asphalt, stabilizing additives, fibres, waste plastics, drain down, stabilizing capacity. --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION Stone Matrix Asphalt (SMA) has been first introduced in Europe for resisting damage from the studded tires better than other type of HMA [1]. In recognition of its excellent performance, a national standard was set up in Germany in the year 1984. The Indian Roads Congress has adopted a tentative SMA specification [2]. One test road was constructed in Delhi in October 2006, using SMA as a surfacing course [3]. By considering the advantage of the proven field performance of this test track and in other regions of developed countries with climatic conditions reasonably close to that of India, SMA can be considered as the right choice for long lasting pavements. SMA is a gap graded bituminous mixture containing a high proportion of coarse aggregates and filler with relatively less medium sized aggregates [4]. It has rich mastic comprised of bitumen, fines, and mineral filler. Additives such as fibres or polymers are used as a stabilizer to secure the mastic within the overall structure and prevent the draining off during storage, transportation and placing of SMA. Drain down is considered to be that portion of the mixture (fines and bitumen) that separates itself from the sample as a whole and flows downward through the mixture [5]. Drain down test is more significant for SMA mixtures than for conventional dense-graded mixtures. It can be used to determine whether the amount of drain down measured for a given bituminous mixture is within the specified levels (should not exceed 0.3% by weight of the mixture [6]). SMA mixtures exhibited a very high bitumen binder film thickness (6-7% by weight of mix). This high binder content and the filler content as compared to that of dense-graded HMA lead to higher susceptibility for the bitumen binder to drain off the aggregate skeleton in SMA mixtures. Irregular distribution of bitumen binder due to its drain down can lead to raveling of zones with low bitumen binder content and reduction of permeability in zones with accumulation of bitumen binder [7, 8]. Potential problems with SMA mixtures are drainage and bleeding. Storage and placement temperatures cannot be lowered to control these problems due to the difficulty in obtaining the required compaction. Therefore, stabilizing additives has been added to stiffen the mastic and thereby reducing the drainage of the mixture at high temperatures and to obtain even higher binder contents for increased durability [9]. Fibres or polymers can be used as the additives. In this study the influence of natural fibres (coir, sisal and banana fibre), a polymer (polypropylene) and a waste material (used plastics in shredded form) on the drain down characteristics of SMA mixtures are studied.