RAP MATERIALS FOR SUSTAINABLE PAVEMENT QUALITY CONCRETE

Abstract

Utilisation of Reclaimed Asphalt Pavement (RAP) aggregates in rigid pavement mix has been gaining momentum but confined to laboratory level of research with few field applications. Past research suggests that RAP aggregates has the potential to be an alternative to natural aggregates which could suppress the pressure of dumping of the same on landfills and resolve the issue of depletion of virgin aggregates. Recycling of RAP aggregates back in bituminous mixes has been well established with published specifications for the design such as IRC:120 (2015). But it is recommended to utilise RAP aggregates which are retrieved from flexible pavements of younger age (age of 5 to 10 years) with higher bitumen content than older pavements with lower bitumen content and subjected to oxidation due to stockpiling, since the former would reduce the amount of fresh bitumen to be added. The RAP aggregates that are retrieved from older pavements is beneficial in recycling to rigid pavement mix. Therefore, the recycling of RAP aggregates can go hand - in - hand between flexible and rigid pavements. In case of utilisation of RAP aggregates in cement concrete mixes, the focus has been given to the coarse fraction of RAP aggregates with limited literature on fine fraction. Hence the scope of the study lies on analysing the feasibility of utilising fine fraction of RAP aggregates as partial or complete alternative to natural fine aggregates in pavement quality concrete mixtures, based on their performance with respect to strength, durability and permeation characteristics. By doing so, the observation and analysis can be added to the limited literature and could be a step forward in preparing standard specifications for recycling of RAP in rigid pavements. To achieve the objectives step – by- step procedure is adopted. Initially, the fine fraction of RAP aggregates and natural fine aggregates were assessed for comparative study where it was observed that even though the parent rock is the same between them the specific gravities are different, the former being lower than the latter. The fine RAP was coarser in particle size distribution and darker in colour with layer of dust upon the asphalt film on the aggregate which could not be removed even after washing. In order to analyse the sole effect of fine fraction of RAP aggregates without the blinding effect of natural coarse aggregates, cement mortar samples were prepared with 25,50, 75 and 100% replacement of natural fine aggregates. By analysing the effect of gradation and mode of replacement (weight and volume replacement) upon strength parameters it was observed that gradation does have statistically significant effect on strength parameters. It is recommended to adopt volume replacement methodology rather than weight replacement technique since the latter fails to address the variation in specific gravities between the types of aggregates, producing samples with unequal volumetric content and resulting in unstandardized comparison. Even though with the incorporation of RAP aggregates in mortar mixes resulted in reduction strength parameters, mixes with 25% and 50% of RAP aggregates were able to achieve 28-day characteristic compressive strength needed for Ordinary Portland Cement (OPC) Grade 43. It was also observed that flexural strength of RAP mortars had undergone lower reduction than compressive strength which was noteworthy. Addition of mineral admixtures namely silica fume and activated sugarcane bagasse ash as partial replacement of cement improved the strength parameters and moreover had a significant impact on reduction in drying shrinkage, even being at par with conventional mixes. With the addition of mineral admixtures, even 75% replacement of fine aggregates was possible while meeting the stipulated strength requirements. Interesting behaviour was observed for resistance against sulphate attack for RAP inclusive cement mortars where gain of strength was obtained for 25% and 50% replacement mixes even after 90 days of sulphate attack. The behaviour was replicated by mortar mixes with mineral admixtures at the same RAP replacement level. The gain as well as loss in strength due to sulphate attack was substantiated using pore structure characteristics obtained by Mercury Intrusion Porosimetry (MIP) and the delay in formation and filling of salt – crystallization products in pores of the samples. Porosity determined through MIP increases with RAP content and existing strength – porosity models holds good. A pragmatic approach was adopted when concrete mixes were prepared with RAP aggregates. The requirement of slump of 25±10mm was given priority than maintaining a constant water – to – cement ratio. Concrete mixes included the mixes with 25,50, 75 and 100% RAP content as well as the optimal mineral admixture mixes corresponding to each replacement level. Flexural strength is a primary factor for the design of rigid pavements. RAP inclusive concrete mixes with 25,50 and 75% RAP content and their corresponding mineral admixture mixes were able to attain the target flexural strength of 4.85MPa with a maximum reduction of 9.7% when compared to conventional mix. RAP inclusive concrete mixes possess the character of increased rate of development of strength than conventional mixes. The RAP concrete mixes did not facilitate to corrosion due to absence of carbonation, maintenance of alkaline pH of 12-13 and the chloride content was within the stipulated limits. At the same time from presence of entrained air bubbles observed from MIP analyses showed the qualitative ability of RAP mortars and concrete to protect the cement paste from frost damage and salt decay. Fractal dimensional analysis was used to describe the pore – solid structure of RAP inclusive cement mortars and concrete. Multifractal dimensions were displayed by the mixes and pore – mass fractal dimension was able to describe the pore – solid structure. These characteristics shows the potential of RAP aggregates to act as an alternative to natural aggregates for rigid pavements. Performance evaluation of optimal mixes of RAP concrete was assessed using pavement prototype analysis where the variation of temperature with respect to depth and corresponding stresses developed for pavement quality concrete section were assessed. From the analysis that was conducted for a duration of 365 days, it was observed that the temperature profile was non – linear in nature and was truly linear only for less than 1%. The RAP inclusive prototype exhibited lowest value of maximum positive (78.57% reduction) and negative (2.04% reduction) temperature differential and also exhibited reduced total temperature stress in comparison to conventional mix. The assumption of linear temperature profile failed to identify the nature of stresses (compressive or tensile) at most 50% of cases. Hence it is recommended to at least consider the non – linearity nature of temperature profile and amend the design codes pertaining to temperature profile and stresses where blind assumption of linearity shall not be taken.