Effects of Newly Developed Cellulose Oil Palm Fiber in the Fatigue Failure of Stone Mastic Asphalt

Fatigue or tensile cracking along wheel paths of vehicles are predominant on Malaysian roads as compared with other forms of distress. This is primarily due to accelerated loading from trucks, which is causing the authorities millions of ringgit on road maintenance alone. This situation is further aggravated with the traditional use of soft 80-100 penetration binders, which are poor in shear strength. At the same time, the use of additives such as Ethylene Vinyl Acetate (EVA) has proved costly. With the rising cost of asphalt in Malaysia, construction and rehabilitation of asphalt road pavements are expected to constrain the road agencies’ budget in the coming years. The objectives of this study were to research the rheological properties of newly developed cellulose oil palm fibers and their potential in resisting fatigue failure of Stone Mastic Asphalt (SMA). The research was undertaken in two parts. The first part of the study was carried out at UPM on SMA with granite aggregates. The selection of aggregate and asphalt for the study were done based on typical SMA mix requirements. Utmost importance was given to the use of the newly developed cellulose oil palm fiber in SMA. Out of the six types of cellulose fibers obtained through various types of pulping procedures, the Chemical Refined(Chem-R) Cellulose Fiber gave the best performance in terms of drain-down and rheological properties such as complex shear modulus. As such Chem-R cellulose fiber was selected and used throughout the study in proportions of 0.0%, 0.2%, 0.4%, 0.6%,0.8%,1.0% in 100mm cylindrical SMA14 mix design and fatigue and IDT tests. SMA specimens, prepared with the above cellulose fiber proportions were tested to simulated loading and temperature conditions in accordance with the American Standard for Testing and Materials (ASTM) and Association of American State Highway and Transportation Officials (AASHTO) Standards. The various proportions of cellulose oil palm fiber tested in 100 mm cylindrical specimens showed remarkable improvement in terms of fatigue life, stiffness and modulus. All of the SMA14 specimen properties increased as the fatigue life increased to a maximum value that corresponds to about 0.6% fiber. Remarkably, at 0.6% optimum fiber content, the initial strain decreased while the stiffness modulus increased, as compared with SMA14 specimens without fibers. The diameteral fatigue, and beam flexure tests have become popular in the Super Pave and AASHTO Tests. Along with that, new approaches in the fatigue analysis such as Dissipated Energy Ratio(DER), and Stiffness have also become very useful in the analysis of asphalt beams. Tests carried out on SMA9.5 beam specimens with the same cellulose fiber proportions as in SMA14, displayed similar trends in the fatigue performance of cellulose fibers regardless of the aggregate and gradation types. Maximum performance curves for fatigue life, stiffness and DER for the SMA9.5 beams were established. The fatigue life of beam specimens showed a maximum value between 0.6 and 0.8% of fiber contents, and the trend was similar for other parameters such as stiffness and DER. The results indicated that use of cellulose oil palm fibers greatly reduced the stiffness of the SMA9.5 and increased the number of load cycles to failure. These special characteristics of the fibers are expected to extend the life span of SMA pavement in the field. In addition, DER value was found to be the lowest for 0.6% cellulose oil palm fibers indicating a decreased loss of energy through dissipation. The more energy is retained and stored the longer life of the SMA pavements. Another important aspect observed in this study was the resistance of cellulose oil palm fibers to fatigue failure of SMA mixes. Several 150mm IDT samples were tested to determine the maximum indirect tensile stress, crack initiation, and propagation. The specimens tested in accordance with AASHTO TP-9 standard showed an increase in indirect tensile stress at 0.6% fiber proportions before taking a down turn. This seems to be promising for more new research in the area since previous research by others showed that gap graded mixes such as SMA displayed poor tensile strength. Two new approaches were undertaken to study the resistance of cellulose fibers against fatigue life of SMA. The first crack tensile stress and the maximum tensile stress values were used to quantify the fatigue resistance of the newly developed cellulose oil palm fibers. The fiber fatigue resistance quantifying approach is termed as Sustenance Ratio (SR). SR in this newly developed approach is defined as the ability of cellulose fibers to carry the maximum applied load to the first crack load divided by the time taken or total number cycles to failure. The unit of measure can be kN/sec or kN/cycles. Using this newfound analogy, the SR of various fiber percentages in SMA9.5 cylindrical specimens were determined. It was observed that the SR decreased to the lowest point at 0.6% fiber content, indicating a higher fatigue resistance. It was observed that the lower the SR the higher the fatigue resistance of fibers. In summary, it has been shown that the addition of Cellulose Oil Palm Fiber (COPF) up to 0.6% provides the maximum fatigue resistance to SMA which can be measured in terms of SR for various temperatures and load configuration. Another concept that was developed in this study was the Crack Meander (ξ) concept analogous to that of a river meander. Theoretically the lower the resistance encountered along the path of crack propagation, the more linear the line of crack becomes. The crack initiation and propagation within the 40mm gauged stress zone, was captured using a SLR camera, and the crack pattern was digitized. It was observed that the crack started to meander as the fiber proportions in the mix increased. A maximum meander was observed at a fiber content of 0.6%. The crack propagation within the stress zone appeared to be pinned by fiber reinforcements thus causing the line of crack to meander and propagate through weaker matrix.

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