PERFORMANCE EVALUATION OF RAP FOR SUSTAINABLE FLEXIBLE PAVEMENTS

Abstract

Low initial construction cost along with a smooth riding surface makes flexible pavement as the first choice for the construction of roads in the global domain. In the past decade, more than 95% of the Indian road network was paved using bituminous mixtures, however, the shifting of choice in road construction to other alternatives has seen a booming trend in the recent years owing to the significant rise in asphalt binder cost and shortage of natural aggregates. In countries like India, millions of tons of flexible pavement waste commonly known as Reclaimed Asphalt Pavement (RAP) materials are being generated annually at an alarming rate. Generation of such an enormous amount of waste annually often leads to being dumped in open landfills or nearby the vicinity of roads which contributes towards environmental impacts. RAP is the material obtained when an existing aged distressed flexible pavement is milled, removed, or demolished for various maintenance activities such as overlay, repairing and rehabilitation etc. Proper processing of RAP material could lead to the accumulation of recycled bitumen as well as recycled aggregates which holds great potential for utilization in flexible pavements construction. Moreover, recycling of RAP material would not only address the issue pertaining the waste management but would also enable to bring down the cost of the project owing to the reduction in consumption of virgin aggregate and binder demand. However, the main hurdle associated with the recycling of RAP material is the presence of stiffened bitumen coating due to oxidation of maltenes during the service life and stockpiling duration of RAP material. Hence, these valuable aggregates have been oftenly used in non-structural application or lower layers of the pavement rather than the upper layers inspite of knowing the fact that this RAP materials contains substantial amount of good quality aggregtaes except the aged hardened binder film which otherwise could be used in the upper layer construction. Considering this grey area, efforts have been made to venture out a novel approach to characterize the properties of RAP materials collected from different sources and age of the pavements and thereby judiciously evaluate its suitability to implement it in various layers of flexible pavement. Hence, to arrive at a better understanding about the utilizations of the RAP materials, in the present study, is an attempt has been made to evaluate the efficacy of RAP materials in the laboratory for construction of flexible pavements as a whole. The studies were conducted examine the suitability of RAP materials for the construction of pavement layers such as Granular Sub-Base (GSB), Wet Mix Macadam (WMM) and bound base course Dense Bituminous Macadam (DBM).To study the effect of exposure life and types of RAP materials; RAP materials from two different sources were considered viz. RAP1 and RAP2. Both RAP1 and RAP2 were reclaimed using controlled milling technique. While, RAP1 was the result of reclaimation (in the form of chunks) of 3 to 5 years old pavement section, RAP2 was obtained from a highly distressed pavement section of about 15 years old. Both the fractions of RAP i.e. coarse RAP and fine RAP (individually and combinedly) were used for replacement of coarse natural aggregates (CNA) and fine natural aggregates (FNA), respectively. The replacement ratios considered in the present study were 50% and 100%, for preparations of unbound and bound base courses. The considered mixtures were studied for various properties such as Optimum Moisture Content (OMC), Maximum Dry Density (MDD), California Bearing Ratio (CBR), Permeability, Marshall Stability, Indirect Tensile Strength (ITS), Indirect Tensile Ratio (ITR), Rutting Resistance, Fatigue Life, Abrasion Resistance and advanced performance parameters such as Cracking Resistance, Resilient Modulus, Compaction Energy Parameters, Seismic Modulus, and Design Modulus. The highlight of the present study is that the performance of bituminous mixes containing RAP is primarily influenced by the amount of added virgin bitumen and secondarily by the source. Laboratory studies reveled that RAP materials containing both factions of RAP if included individually offer comparable results as that of conventional mixture, thereby, opening a scope for utilization of any fraction of RAP for pavement application. Initially, effort was made to part replace natural aggregtates by RAP materials (RAP1) formulating unbound sub base (GSB) and base courses (WMM) mixtures to evaluate the performace related parameters. Reductions in overall density and moisture content was observed for both GSB and WMM mixes and this is primarily due to low density bitumen coating around the RAP aggregates and secondly owing to the hydrophobic nature of bitume. Furthermore, the incorporations of RAP1 was also noted to affect the maximum dry density of the GSB and WMM mixes at a considerable rate. It was also noted that there has been significant alterations in the CBR value for both the layers especially when RAP1 is incorporated in higher proportions. This is mainly due to the presence of agglomerated particles in coarse RAP1 and gap-graded fine aggregate skeleton. Nevertheless, the latter condition could be addressed by filling the gaps of fine RAP1 using natural fines. Contrarily, the presence of bitumen coating around RAP1 was able to facilitate with better drainage properties owing to its hydrophobic nature which further indicates suitability of RAP1 for GSB mixes.To increase the percentage of RAP to be used in the bound base (DBM) course of flexible pavements, the approach of controlled replacements i.e. sieve by sieve was followed. Coarse and fine natural aggregates were considered for partial replacement by RAP aggregates, individually and combinedly. The optimum RAP binder demand for the above-mentioned mixes was evaluated using the Asphalt Institute MS-2 guidelines and was confirmed with the Marshall method of mix design. Based on the test results and analysis, it was observed that the Marshall stability of the DBM mixes could be improved considerably by the incorporations of RAP. The enhancement in the Marshall stability upon incorporation of RAP may primarily be due to the presence of aged-hardened binder in the RAP which may have facilitated with better stiffness in the RAP mixes. Similarly, the ITS of the considered mixes was also noted to be enhanced by about 35% when RAP was incorporated. However, the moisture susceptibility was noticed to be higher with RAP (RAP1 and RAP2) incorporation. It was also observed that the inclusions of RAP could result in enhancing the abrasion resistance of the RAP incorporated DBM mixes indicating its suitability in high-intensity traffic conditions. In contrast, the overall performance of RAP2 mixes was noted to be better than the RAP1 mixes which is in contradiction to the general perception that less stiffened bitumen coated RAP would perform better than highly stiffened RAP aggregates. This finding indicates that the performance of DBM mixes would mainly be governed by the amount of extra virgin binder added rather than the binder already available in the RAP. Nevertheless, the total cost of production of RAP incorporated DBM mixes would still be considerably dependant on the amount of RAP added. Since the combined RAP fractions exhibited inferior properties than their counterpart mixes, Warm Mix Asphalt (WMA) technique was adopted. Besides improving the moisture susceptibility and preventing further-ageing (during the hot mix asphalt production) in the RAP mixes, this approach would also contribute in reducing the carbon-footprints during the construction stage. As expected, it was observed that the inclusions of WMA in RAP inclusive DBM mixes reduced the mixing temperature as well as improved the Marshall Stability and Indirect Tensile Strength by about 10% and 15%, with respect to the control mix. Additionally, it was also observed that the WMA incorporation in the considered mixes did not much affect the production cost (a marginal increase of 3 to 7% only). AASHTO R30 which is the prevailing guideline pertaining to the assessment of long-term performance of asphalt mixes does not consider the effect of moisture on the long-term ageing of asphalt mixes, and hence, the effect of temperature and moisture on the long-term ageing was considered in the present study. Comparison between the ageing conditions (AASHTO R30 and Developed Method) demonstrated that there is a significant impact of moisture on the long-term ageing of RAP incorporated asphalt mixes. Moreover, the incorporations of WMA additives would result in an added advantage in terms of better long-term ageing resistance. In the present study, efforts were made to examine the potential of bituminous mixes containing RAP up to 70% employing for both organic and chemical warm mix additive. To further increase the percentage of RAP beyond 70%, Foam Mix Asphalt (FMA) Technology was employed. FMA is produced by combining the hot binder with cold water because of which the water turns into steam and expands to about 10 to 12 times its initial volume. This results in reducing the viscosity of the binder and thus ensuring proper coating of aggregates at a lower temperature. Additionally, the conventionally used filler i.e. portland cement was replaced by fly ash and bagasse ash in order to further induce sustainability in the considered mixes. It was observed that the application of FMA technology was able to impart better ITS considerably by about 140%. Similarly, moisture resistance was noted to be improved by about 22 to 30% while the rutting resistance was enhanced by about 11 to 18% when RAP was incorporated. However, the resistance to abrasion was observed to be lowered down indicating the inability of the spot weld bond to resist impact loading, thereby resulting in higher abrasion loss. On the other hand, the incorporations of RAP resulted in decreasing the Marshall stability of the FMA mixes owing to the presence of asphalt coating around the RAP aggregates which may have hindered the spot-welding process due to lower mixing temperature. Meanwhile, the utilization of bagasse ash and fly ash also provided favorable results which clearly indicates its potential to be used in RAP incorporated FMA mixes. Fly ash and bagasse ash incorporations were noted to improve the rutting resistance by about 12 to 17%. In fact, these mixes were noted to be even better than the conventional mix. In comparison, the baggage ash incorporated FMA mixes were noted to possess better Marshall stability and moisture resistance as compared to the fly ash incorporated FMA mixes, irrespective of the foaming temperature. To induce further sustainability in FMA mixes, coarser fractions of RCA and RAP was incorporated in proportion of 25% to 100% (10 to 40% by weight of the total aggregates), separately and combinedly. The considered mixes were cast with the help of a Superpave Gyratory compactor with an aim to simulate the real time field compaction. It was observed that the porous adhered mortar content around the RCA could absorb the foamed binder which resulted in reducing the effective binder content, thereby, decreasing the density of RCA incorporated FMA mixes. But when 10% RCA was used in combination with 30% RAP, the density of FMA mixes was minimally affected. On the other hand, incorporation of 30% RCA alone was observed to show an adequate ITR value indicating its suitability to be used in DBM mixes. Contrary to the available literature, higher proportions of RAP had been found to contribute towards the strength properties despite the brittle nature of the RAP binder. The results of resilient modulus depicted that inclusions of RCA would ensure lower thermal conductivity in the foam mixes. Moreover, when the temperature is above 40°C, the delayed hydration of residual cement grains could impart higher stiffness in the FMA mixes, ensuring lesser stresses in the pavements owing to which reduction in the pavement thickness crust could be achieved. In terms of sustainability, blending of 10% RAP and 30% RCA using FMA technology could result in a more stiffer mix as well as rendered cost-effective in comparison to the mix containing natural aggregates only. Moreover, the incorporations of this blended (10% RAP and 30% RCA) mix in FMA adopted DBM mixes would also increase the resistance to abrasion. However, when RCA was incorporated in 40% without any proportion of RAP, the rutting resistance was noted to be lowered by 88%, with respect to the control mix. Moreover, the findings from the present study demonstrated the unsuitability of RCA if added in excess of 30% for the production of DBM mixes using Foam Mix Asphalt Technology, whereas, in the case of blended mix, this proportion is restricted to 10% RAP and 30% RCA only. Nevertheless, this could be eradicated by the addition of synthetic polypropylene fibers. In fact, fibers if added (in the blended mixes) in an optimum dosage could even yield better performance properties than the conventional mixes containing natural aggregates.