Performance Studies of Gap-Graded Mixes using Elastomer Modifier for Wearing Course

:- Road construction crossing the world created huge demand for construction materials. Natural aggregates is the prime material for the road construction use of aggregates leads to question about the preservation of natural resource. In other hand use of recycled pavement asphalt (RAP) as a alternative material in place natural aggregate which makes environmental friendly construction material. This research investigated the feasibility of using reclaimed asphalt pavement in stone mastic asphalt mix by modifying the binder with waste engine oil and elastomer modifier. RAP is replaced with aggregates at 10%, 20%, 30% & 40%. Further in order to increase the percentage of RAP in SMA mix waste engine oil and elastomer modifier is added i.e., RAP of 50%, 60% and 70%. The results indicated that 30% of RAP is found to be feasible through laboratory investigation for SMA mixes and 60% RAP is found to be feasible to use in SMA mix through laboratory investigation by modifying binder with waste engine oil and elastomer modifier.


Stone matrix asphalt
India which has an ideal vision for growth and development, of of infrastructure. Infrastructure development is a core aspect of connectivity which connects different places through different modes of transportation, wherein road connectivity is undoubtedly one of the most important mode. For a robust road connectivity network, the study of pavement structure and their durability and resistance will be crucial aspect. This makes us look at some of the key aspects like type of pavements and materials used for construction and the cost incurred to lay the pavement. The pavements undergo repetitive loads and the number of deflections and numerous loads are imposed once a wheel moves on the surface where the load is dissipated to the underlying layers of the pavement are subjected to diverse type of distress. The major distresses found in flexible pavements across the globe are rutting and fatigue. Stone matrix asphalt is found be and effective alternative on heavy traffic pavements due to its stone on stone contact it provides resistance to rutting and higher binder content increases fatigue life and reduces reflection cracking [1].

Reclaimed Asphalt Pavement
Reclaimed asphalt pavement are the materials derived from reprocessed pavement which contains asphalt and aggregates. The derived materials are subjected to crushing and screening to obtain RAP of well graded aggregates coated with bitumen. Recycling of existing asphalt pavements allows the industry to produce new pavements with effective utilization of material and energy making it cost effective [7]. For this study, RAP is procured from Chittoor district where some part of Bengaluru-Tirupati Highway (NH-206) was scarified for construction of cement concrete pavement.

Stabilizing Additive
SMA is a gap graded mix, contains high percentage of voids due to which the bitumen content is more. Hence, the stabilizing additive should be added to the mix to reduce the draindown characteristic of the mix [3]. In this study cellulose fiber is used as stabilizing additive.

Mineral Filler
Mineral filler is the material which passes through 0.075 mm sieve size. The major components of the filler consist of rock dust, portland cement, hydrated lime, ground limestone dust or fly ash. This makes the SMA mix stiffer and a rich binder. Breaching of the threshold level of the filler can lead to excessive stiffening of the mix causing unfavourable conditions for compaction. This might lead to the cracking of the susceptible mix. In general, aggregates passing through 0.075 mm sieve size should be 8-12% of the total amount of aggregate in SMA mix. The main function of filler is to fill the voids in aggregate skeleton and make the mix denser by improving the cohesion of the asphalt binder and the stability [2]. In this study, a waste material called baghouse dust is used as filler material in SMA mix.

Elastomer Modifier
The elastomer modifier consists of functional additive which is a smart solution to improve the properties of mix around the globe. It gives better durability, convincing anti-rutting characteristics, better aging resistance and enhanced performance. Apart from this, it is an alternative way to achieve a process of reliable modification of bituminous mix close to the same performance level with modified bitumen. In this study commercially available elastomer modifier pellets are used. This product is a pelletized blend of cellulose fiber and functional additive. The composition consists of 20% and 80% by weight respectively. The quantity of elastomer modifier is added at a rate of 12% or 15%. In this study dosage rate of 15% is considered for viscosity grade 30 which is recommended in the manual [20]. The mixing of elastomer modifier with the mix is carried out by two modification procedures i.e., dry process and wet process. In this research the modification of the bitumen is carried out by adopting dry process technique.

Objectives of the work
• To assess the performance of stone matrix asphalt mix with partial replacement of reclaimed asphalt pavement. • To determine the effect of elastomer modifier in stone matrix asphalt mix with reclaimed asphalt pavement.

Scope of the work
Recycling asphalt pavement initiates a cycle of 4R principles, where the materials can be recycled and reuse which leads to a sustainable use of natural resources. Reclaimed asphalt pavement is one such alternative to unexploited materials which can reduce the over use of limited aggregates. By using reclaimed asphalt pavement sustainability can be achieved without compromising its structural performance. As RAP is an aged material, to regain the fresh properties different commercial rejuvenators are used. There are instances wherein commercial rejuvenators are not available. Therefore, this study analyses the potential of modifying the asphalt with elastomer modifier as a substitute for commercial rejuvenators to increase the percentage of the reclaimed asphalt pavement materials which can meet the intensified needs more legitimately.

Aggregates
The aggregates were procured from different crushers. The basic tests were carried out on the aggregates and it was found that aggregates procured from KMS crusher-Bagalur, Karanataka were found to be within the required limits as per MoRTH [16] for SMA mix. The basic test results are depicted in Table 1.  Table 2.

Bitumen
In the present study Viscosity Grade (VG) -30 is used, which is obtained from HINCOL Pvt Ltd, Mangalore. The tests on the bitumen is carried out as per IRC SP: 73:2013 -Guidelines for Paving Bitumen Specifications [19]. The results are tabulated in the following Table 3.

Elastomer modifier
In this research, a commercially available elastomer modifier is used which is the combination of cellulose fibre and functional additive. It is a patent product obtained from SMART technologies, Peenya, Bengaluru. The characteristics of the modifier are shown in Table 4.

Gradation of aggregates
After the basic tests, the aggregates are further used in the determination of the proper blend to give a good mix consisting of different aggregate sizes. The different sizes of aggregates used to obtain the proper blend are 20mm downsize, 12mm downsize, and 6mm downsize and stone dust. 4000gm of aggregates were taken for sieve analysis. The aggregate gradation is carried out using the trial and error method, to determine the individual percentages of the different size aggregates to be used in the mix and should be within the limits as per MoRTH Table 500-37 [16] .The sieve analysis results and obtained gradation of wearing course for conventional SMA mix is as shown in Table 5.

Marshall properties
The SMA mix design basically relies on volumetric properties such as air voids, voids in mineral aggregate, voids in the coarse aggregate and binder content. Another important consideration in designing SMA mix is the filler/additive. In this study cellulose fibre was used as stabilizing additive and baghouse dust was used as filler material. The marshall specimen are casted and compacted with 50 blows on either side as per marshall procedure. The SMA mix design shall meet0the requirements0given in Table 6. The optimum binder content and optimum RAP content with and without elastomer modifier is obtained by casting marshall specimens for different bitumen contents. Three specimens are casted for each bitumen content. The marshall test is carried out for three SMA mixes i.e., conventional SMA mix, SMA mix with RAP without elastomer modifier and SMA mix with RAP and elastomer modifier. For Conventional SMA mix the marshall specimens were casted for 5.75%, 6%, 6.25% and 6.5% bitumen content and were tested. The optimum binder content was found to be 6%. The optimum RAP was determined by replacing RAP in different percentages i.e., 10%, 20%, 30%, 40% and 50%. The bitumen was also varied in same range as in conventional SMA mix. The marshall stability at 30% RAP replacement with 6% bitumen content was found to be highest when compared to other proportions. Hence, the optimum RAP was considered as 30%. An attempt is made to enhance the optimum RAP by adding elastomer modifier at a quantity of 15% of the weight of bitumen. This elastomer modified is added to the mix by dry process method. The bitumen content of 6% obtained for 30% RAP replacement was considered for further RAP replacement of 30%, 40%, 50%, 60%, 70% and 80%. The marshall properties at 60% RAP showed better results compared to other proportions. The marshall results of all the mixes are tabulated in the Table 7.

Draindown test
The draindown characteristics for the SMA mix were conducted for the conventional SMA mix as well as for the SMA mix with optimized RAP with and without elastomer modifier, the results are tabulated in Table 8. The draindown test results yielded good results, the draindown of bitumen of the specimens are found out to be well within the limit of 0.3% maximum as prescribed in MoRTH [16].

Moisture Susceptibility
The main factor leading to premature failure of the bituminous pavements is due to the presence of moisture in the pavement surface and the inability of aggregates to retain the coating in the presence of moisture. Hence, it is important to conduct tests regarding moisture susceptibility. Tensile strength ratio is carried out on the basis of the indirect tensile strength procedure this test is done for the conventional SMA mix as well as for the SMA mix with optimized RAP with and without elastomer modifier. The number of blows for ITS is found out at 32 blows on both sides for conventional SMA mix and for RAP replacement with and without elastomer modifier is found to be 28 and 25 blows each side respectively. The results for ITS and TSR for conventional SMA mix and SMA mix with optimized RAP with and without elastomer modifier are tabulated in Table  9. The results show that the SMA mix with optimized RAP with elastomer modifier is found to more moisture susceptibility than conventional SMA mix and optimized RAP without elastomer modifier.

Rutting characteristics
The rutting tests are conducted on the conventional SMA mix as well as for the SMA mix with optimized RAP with and without elastomer modifier. The test is carried out to find out the resistance to deformation due to repeated wheel load for wearing course. For this test, the maximum and minimum layer thickness is considered which is 40mm and 50mm thick respectively, with tyre pressure of 549 kPa (5.6 kg/cm 2 ) for each specimen at a frequency of 25 passes/min and it is carried out at room temperature until 10000 passes or 12mm rut depth. The graphs of 40mm and 50mm thick specimens are shown in Figure 1 and Figure 2 respectively. The rutting test is conducted for conventional mix and optimized RAP with and without elastomer modifier. The thickness of test specimens is 40mm and 50mm. The result shows that for 40mm thick beam the rut depth is 5.97, 4.59 and 3.73mm respectively and for 50mm thick beam 4.48, 4.04 and 3.15mm respectively for different mix. Hence by adding elastomer modifier it increases the RAP content to 60% and decreases rut depth.

Repeated load fatigue test
The fatigue test is conducted on the conventional SMA mix and optimized RAP with and without the elastomer modifier. In this test marshall specimens of 32, 28 and 25 blows respectively and tested in four-point loading apparatus. The stress level and deformation are kept constant which is 10% and 5 mm respectively. The test was carried out to find out the resistance to deformation due to repeated wheel load. The test results are shown in Figure 3. From the results obtained it is observed that the failure criteria for 5 mm deformation at 10% stress level shows that 60% RAP with the addition of elastomer modifier in SMA mix increases the fatigue life cycles compared to conventional mix and optimum RAP without elastomer modifier.

CONCLUSIONS
The physical tests on virgin aggregates, reclaimed asphalt pavement, baghouse dust and bitumen (VG-30) were carried out. Laboratory investigations on marshall mix specimens of varying percentages of RAP replacement with and without modifier was done. Performance tests like draindrown, moisture susceptibility, rutting and fatigue tests were conducted and the following conclusions were made: 1. The results obtained from basic laboratory tests of virgin aggregates, reclaimed asphalt pavement aggregates, baghouse dust and bitumen (VG-30) are within the specified limit as prescribed by MoRTH 500-35. 2. To determine the amount of RAP in the mix, marshall properties are determined. At 30% RAP replacement, the stability value of the mix is found to be increased by 1.55 times than the conventional SMA mix. The obtained marshall properties are within the specified limits as per MoRTH. 3. By adding elastomer modifier, the optimum RAP is found to be 60% and marshall properties are within the specified values as prescribed in MoRTH. 4. The draindown test results yielded good results, the draindown of bitumen of the specimens was found out to be well within the limits i.e., maximum of 0.3% as prescribed in MoRTH. 5. The moisture susceptibility test results showed that SMA mix with60% RAP with addition of elastomer modifier is found to be greater than the conventional SMA mix and SMA mix with 30% RAP without elastomer modifier.