Influence of Crushing Size of the Aggregates on Dense Bituminous Mix and Experimental Study on Mechanical Properties

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1 Influence of Crushing Size of the Aggregates on Dense Bituminous Mix and Experimental Study on Mechanical Properties Assistant Professor, Gogte Institute of Technology. Belagavi ABSTRACT The aggregate source, physical properties and the production technology by crushers has great influence on performance of the bituminous mixes. Aggregates are produced from crushers which may be at single stage jaw crusher alone and may be associated with secondary stage cone crushers. This crushing pattern has very much influence on the shape indices of aggregates, which is one of the vital properties. Shape indices largely depend on the feeder size of the aggregate boulders which is varied from 350mm to 600mm. Firstly fed into the Primary stage jaw crushers. Further they get reduced to the half of the size in some make or to one third size in another type of the crushers before they are fed into secondary cone crushers. This has been observed practically by selecting METSO. The shape indices (Combined Flakiness and Elongation index)as per the specification by MORT&H is to be less than 30 % for all mix designs which is difficult to achieve even with advanced crushers like METSO. This is due to uncontrolled feeder size of boulders into primary jaw crushers. Aggregates from METSO crusher have been selected and used for design of Dense graded bituminous macadam (DBM) to understand the importance of the feeder size and its influence on the Mechanical properties of Bituminous mixes. Aggregates are separated into different shapes such as flaky, elongated and angular using conventional thickness and elongation gauge and are grouped into three type on the basis of Shape indices value such as < 30 %, > 30 % and Normal (as obtained from crusher). Marshall Method of Asphalt mix Design as per the procedure MS-II is being carried out. Key words: Crusher, shape indices 1.0 INTRODUCTION Road is one of land transportation infrastructure is very important in supporting the economic for both regional development. The quality of material for road construction will also influence the road performance. The aggregate constitute major component of Flexible pavement layers. As such the resulting properties of the pavement layers largely depends upon the properties of its constituent aggregates which in turn depends on its composition, size, source etc. the variety of aggregates widely exhibit the variable results when tested for compliance with respect to the specifications laid down by agencies such as the MoRT&H. Besides all these the aggregates shape also plays a significant role. The aggregate shape, apart from rounded, angular is also expressed in terms of its shape factors such as flakiness and elongation. The sum of flakiness and elongation indices of an aggregate mix is termed as its combined index. The longevity, structural stability and the orientation of the aggregates under the wheel loads are very much dependent upon the Particle shape of the aggregates. It affects the gradation requirement, mix design properties both of unbound and bituminous courses of the flexible pavement. This one particular property of the aggregates has influence on the overall performance of the pavement. The MoRT&H has specified an upper limit of combined index as 30% for all types of bituminous mixes. The process of obtaining the aggregates with desired shape and combined index has become a very tedious task to achieve in the field. If one goes strictly by the MoRT&H specifications, the aggregates with combined index higher than 30% have to be discarded which is very difficult as the aggregate resources are briskly exhausting. It is necessary to find means of using even the marginal aggregates. The most commonly used crushers in India are Jaw crushers which are single stage crushers. It is difficult to control flakiness of aggregates within specified limits using these crushers alone. Some of the case studies in NHDP (National Highway Development Programme) have proved that with use of Cone crushers as primary crushers and Vertical Shaft Impact (VSI) as secondary crushers, the combined index can be controlled to even less than 25%. The cost of VSI crusher is 3 to 4 times higher than Jaw crushers. But the problem is schedule of rates where there is no additional cost specified for aggregates supplied from VSI crushers 1. 5

2 1.1 SHAPE FACTOR OF AGGREGATES The particle shape of aggregate is determined by the percentages of flaky and elongated particles contained in it. For bituminous macadam type, the presence of flaky and elongated particles is considered undesirable as they may cause inherent weakness with possibilities of braking down under heavy loads. In general, it is also found that, in Jaw crushers flakiness index increases with decrease in size of aggregate produced 1 (maximum between 12.5 to 6.3 mm sieve size). Sridhar Raju et al., (2006) from their studies on aggregate shape concluded that with increase in combined flakiness and elongation indices there is reduction in bitumen content and improvement in mix properties. They also stated that based on laboratory studies it can be concluded that the mixes with combined flakiness and elongation indices of 35 percent can be adopted foe design of mixes without compromise in performance. D. Sakthibalan(2009) studied on influence of aggregate flakiness on dense bituminous macadam and semi dense bituminous concrete mixes and concluded that flakiness index up to 32 percent for DBM and 33 percent for SDBC can be permitted without compromising specification requirements. Meor Othman Hamzah et al., (2010) studied on effects of asphalt mixtures incorporating geometrically cubical aggregate to optimize the design of HMA mix. The partial substitution of normal aggregate in hot mix asphalt with geometrically cubical aggregate was investigated. They concluded that the asphalt content for all geometrically cubical aggregate percentage is found to have lower B.C than the normal aggregate mixture. This is due to the fact that more asphalt binder is required to coat the flat, elongated aggregate in the mixture. Ganapati Naidu and S. Adiseshu studied on Influence of coarse aggregate shape factors on bituminous mixtures and concluded that the mixes prepared by replacing 20 percent aggregates shown higher stability values. Cubical particles exhibit interlock and internal friction, and hence results in greater mechanical stability than the blade, rod, and disk shape aggregates. Particle shape parameter values obtained were higher for cubical shape aggregates and lower for blade shape aggregates. Paul K. Oduroh et al., (2000) studied on flat and elongated aggregates in superpave regime and concluded that there is no detrimental effect in mixes containing up to 40% of 3:1 Flaky and Elongated aggregates. OBJECTIVES AND SCOPE OF STUDY To study the effect of flaky and elongation indices on volumetric properties of DBM mixes To study the effect of flaky and elongation indices on laboratory performance of DBM mixes 2.0 LABORATORY STUDIES Aggregates: For the Laboratory study, the aggregate of sizes 40mm, 20mm, 12.5mm and stone dust were obtained from Maidala near Tumkur District. The Physical properties of aggregates are given in Table 1. Table 1 Physical properties if aggregates Test Method Results Requirements Impact (%) IS:2386 Part IV % Max 27% Crushing (%) IS:2386 Part IV Specific Gravity IS:2386 Part IV Water Absorption (%) IS:2386 Part IV Max 2% Abrasion (%) IS:2386 Part IV % Max 35% Bitumen: 80/100 penetration grade bitumen obtained from Mangalore refinery was used in the study. The physical properties of the bitumen are given in Table 2. 6

3 Table 2 Properties of 80/100 grade bitumen Test Description Limits Requirements Results Penetration (mm) at 25 0 C (IS: ) IS: Ductility (cm) at 25 0 C (IS: ) IS: Min 75 >75 Softening Point ( 0 C) (IS: ) IS: Min Specific Gravity IS: Min Flash Point( 0 C) (IS: ) IS: Min Aggregate Gradation: The gradation obtained using 40mm, 20mm, 10mm, stone dust along with MoRT&H specified gradation are shown in Figure 1. Marshall Mix Design: The Marshall test method was used to select the optimum binder content (OBC). Dense Bituminous Macadam Mix samples were prepared using 75 blows of Marshall Hammer on each face at binder percentages increasing in 0.5 percent increments, with three replicates at each binder contents. Bulk density, stability, flow, air voids, VFB & VMA were calculated for each specimen and average for the replicates at the same binder content were also calculated as given in Table 3. Marshall samples were prepared at OBC and tested for bulk density, stability, flow, air voids, VFB & VMA. The values along with MoRT&H Specification for verification are given in Table 4. 7

4 Properties Bulk Density (gm/cc) Stability (kg) Table 3 Physical properties of DBM mixes using various binders CI>30% Normal CI<30% Flow(mm) Air Voids (%) VMA (%) VFB (%) Table 4 Mix Properties for varying combined flakiness and elongation indices Property CI<30% Normal CI>30% MoRT&H specifications OBC % (by weight of agg) Min Bulk density (g/cc) Stability (kn) Min 900 Flow Value mm Air Voids (%) VFB (%) to 75 VMA (%) Min 11 From Table 4, it can be observed that the binder content increased by 0.2 percent for CI>30%, and also there was reduction in density, stability and air voids. It was also observed that the decrease in CI resulted in increased density, stability and sufficient air voids. There is no much difference in the VMA to accommodate sufficient bitumen for durability of the mix. Moisture Sensitivity Characteristics: To find the moisture sensitivity of mixes with varying combined indices, indirect tensile strength (ITS)tests were conducted on dry and wet samples as per AASHTO-T-283. The indirect tensile strength ratio values are given in Table 5. Table 5 ITS ratio for varying FI and EI Sl.No. CI ITS (kg/cm 2 Retained ITS ) (kg/cm 2 ITS Ratio ) 1 CI>30% Normal CI<30% ANALYSIS AND DISCUSSION OF RESULTS 4.1 Stability and Flow From Marshall Mix design, specimens were prepared at optimum binder content of 4.7%, 4.5% and 4.35% for CI>30%, Normal (unasserted), CI<30% respectively. Then the specimens were tested for Marshall Stability and flow value. The results are shown in Table 6. 8

5 Table 6 Stability and flow values for mixes Case Stability (kg) Flow (mm) CI<30% Normal (Unsorted) CI>30% The results show that the mixes with CI<30% have relatively higher stability. These mixes had 5 percent higher stability compared to Normal (unsorted). But mixes prepared with CI>30% resulted in 7 percent in stability when compared to mixes with Normal (unsorted). The results showed that the stability is not much affected for mixes with CI>30% when compared to MoRT&H specification of CI of 30 percent fulfilling the criteria of minimum 900kg stability. 4.2 Moisture Sensitivity The moisture sensitivity tests were conducted on DBM mixes with combined flakiness and elongation indices of CI>30%, Normal (unsorted Aggregates) and CI<30% as per AASHTO-T-283 at air voids of approximately 6 percent. The average indirect tensile strength ratio were 84.61, and percent respectively, fulfilling the minimum criteria of 80% (ASTM standards). The results showed that the even the mixes prepared with combined flakiness and elongation indices of 35 percent satisfied the MORT&H Specification. 4.3 Density and Voids ratio relationship The density values at OBC are 4,and 2.22 gm/cc for combined indices greater than 30 percent, for normal (unsorted aggregates)and combined indices less than 30%. Density from these results decreases from and percent. Air voids obtained from mix changes decrease from combined indices less than 30 percent to combined indices greater than 30 percent. 4.4 VMA and VFB relationship At optimum binder content (OBC) VMA values are 16.96, and percent for combined indices greater than 30 percent, for normal (unsorted aggregates) and combined indices less than 30 percent. VMA values increases by 7.13 and 8.25 percent if CI increased from less than 30 percent to greater than 30 percent. 5.0 CONCLUSIONS From laboratory studies it can be concluded that increase in binder content from 4.35, 4.5 and 4.7 for combined indices of greater than 30 percent, for normal (unsorted aggregates) and for combined indices less than 30 percent respectively for DBM mixes. Air voids increased with increase in combined indices in the mix, from which it can be inferred that both flaky and elongated aggregates gives rise to more voids during reorientation of aggregates as proved by researchers earlier. From laboratory study it can be concluded that the stability decreases with increase in combined indices. The values of stability satisfy the minimum value of 900 kg as specified in MoRT&H. It has been found very difficult to obtain aggregates with combined flakiness and elongation indices less than 30 percent which ie specified in MoRT&H, this work has given way for utilizing aggregates of up to 50% combined indices for state highway projects and other small projects. REFERENCES 1) Oduroh PK, Mahboub KC and Anderson RM, Flat and elongated aggregates in superpave regime. J Mater Civil Eng 2000:12(2): ) Chen J, Chang MK, Lin KY, Influence of coarse aggregate shape on the strength of asphalt concrete mixtures, J Eastern Asia Society of Transportation Studies 2005:6: ) Sridhar Raju, Sunil Bose, B.M. Sharma and K. Venkat Narayan. Fatigue and permanent defrormation characteristics on flakiness and elongation Indices of dense mixes. 4) Ganpathi Naidu. P and S. Adiseshu, Inflence of coarse aggregate shape factors on bituminous mixtures, International Journal of engineering Research and Applications. 5) Meor Othman Hamazah, Mailiana Azura Ahmad Puzi and Khairun Azizi Mohd. Properties of geometrically cubical aggregates and its mixture design ) D. Shaktibalan. Influence of Aggregate Flakiness on Dense Bituminous Macadam & Semi Dense Bituminous Concrete Mixes Indian Geotechnical Society Chennai chapter. 9