UTILIZATION OF RAP MATERIALS FOR THE DEVELOPMENT OF SUSTAINABLE CONCRETE PAVER BLOCK MIXES

Abstract

Gradual depletion of good quality natural aggregate sources and restrictions on quarrying activities to preserve the ecological balance has compelled researchers in the fields of construction materials to search for reliable alternative sources of supply. In this context, reclaimed asphalt pavement (RAP) can serve as one potential solution. RAP is a recycled material generated in large amounts when deteriorated asphalt pavements are milled or demolished for repair, rehabilitation, and reconstruction activities. Several research studies have considered the utilization of RAP for asphalt and cement concrete pavement applications like hot mix asphalt (HMA), Portland cement concrete (PCC), roller compacted concrete (RCC), and dry lean concrete (DLC). Successful utilization of RAP in the pavement industry can offer socio-economic and environmental benefits like safe disposal of an enormous amount of pavement solid waste, minimizing the use of natural aggregate sources and pollution related to quarrying activities, and incurring cost savings associated with the purchase and transportation of natural aggregates. However, although the recycling of RAP materials has been well established for asphalt pavement applications with published design specifications, no specific documentation or guidelines are available to date regarding their usage in concrete pavement applications. But, past literature has suggested utilizing RAP materials obtained through uncontrolled reclamation techniques from relatively older or highly aged asphalt pavements for concrete pavement applications, whereas the same retrieved through controlled reclamation process from newer or lesser aged asphalt pavements can be utilized back into them owing to higher asphalt binder concentration lessening the requirement of the fresh binder. Moreover, the suitability of RAP aggregates for special types of concrete pavement applications like concrete paver blocks (CPBs) has hardly been studied. In recent decades, CPBs have attained widespread popularity for various commercial, municipal, and industrial applications due to their superior strength, durability, and aesthetic features. The reduced energy requirements and carbon footprints associated with the production of these precast products have largely helped their integration into sustainable green infrastructure and low-impact development programs. Additionally, studies investigating the potential of RAP as an alternative aggregate source in concrete pavement applications like PCC, RCC, and DLC have mostly considered standard curing procedures like continuous water or moist curing for evaluating the hardened concrete properties. But, precast concrete products like CPBs are either cured in the air or are subjected to a combination of water and air (intermittent water) curing because of their mass production and lack of indoor storage facilities. Hence, this thesis aims to comprehensively investigate the various possible ways of maximizing RAP recycling for the sustainable development of CPB mixes. Furthermore, efforts were also made to optimize the RAP dosage for CPBs based on the effectiveness of different curing regimes to understand the possible utilization of RAP materials in the precast concrete industry from an economic viewpoint. It is envisaged that the observations and analyses from this research study would contribute to the limited literature on RAP concrete and could be a step forward toward the formulation of design specifications for the utilization of RAP materials in cement concrete pavement applications. Initially, the potential of RAP materials obtained through demolition process from a highly aged asphalt pavement in its as-received state (with stiff asphalt coating, dust impurities, and agglomerated particles) for the production of CPB mixes was investigated. Herein, the coarse and fine fractions of RAP materials were separately (individual effect) incorporated in volumetric proportions of 25%, 50%, 75%, and 100% as natural aggregate replacements, and the laboratory test results comparatively analyzed with reference to the recommended standard specifications of CPBs. Based on several mechanical, transport, durability, and microstructure properties, it was observed that the dosage of RAP as natural aggregate replacement as well as the durability of RAP concrete in CPBs could be increased using a staged mixing approach and a time-controlled dual-source compaction technique, which involves synchronized impact pressure and vibratory compaction energy. The adopted design methodology could also cater to the needs of stiff RAP based CPB mixes attempting to utilize a higher proportion of coarse aggregates in the aggregate matrix and produce different classes of CPBs with varying RAP proportions suitable for traffic applications. With standard water or moist curing for 28 days, CPB mixes containing any fractions of coarse RAP as partial/total replacement of natural coarse aggregates were found suitable for medium-traffic/medium-duty applications, whereas replacement of natural fines up to a proportion of 50% by fine RAP aggregates could be recommended for very heavy-traffic/heavy-duty applications. Assessing the economic and environmental benefits highlighted the potentially sustainable aspects of recycling RAP fractions into these precast units. Thereafter, the effect of combined RAP fractions (incorporating both coarse and fine RAP at the same time) in proportions of 50% and 100% as natural aggregate replacement and different curing regimes were investigated by implementing the same design methodology for the development of CPB mixes. The laboratory results were comparatively analyzed with those of companion mixes containing individual proportions of coarse and fine RAP to understand the effect of individual and combined RAP fraction utilization in cement concrete mixes. Considering the hardened concrete properties, an initial 7 days of water curing or water spraying followed by air curing could be suggested as an alternative curing method to standard water curing for RAP inclusive CPB mixes. When subjected to continuous water curing, combination of water and air curing, or prolonged air curing, lower proportions of individual and combined RAP fractions could be used in CPB mixes for very heavy-traffic applications, whereas higher replacement levels should be restricted to medium-traffic areas. Despite the deterioration in transport and durability properties of CPB mixes with incorporation of RAP aggregate fractions, the water absorption and abrasion resistance characteristics fulfilled the acceptance criteria of heavy-traffic applications as per ASTM, IS, and IRC standards. However, the suitability of CPBs for different traffic applications needs to be validated through strength parameters. The microstructure of the CPB mixes characterized through scanning electron microscopy revealed the presence of large voids or pore spaces and micro-cracks for the heat-cured counterparts making heat curing unsuitable for RAP based CPB mixes. The presence of carbonation was detected in all the RAP inclusive CPB mixes when exposed to air curing for a prolonged period. It was also observed that dense aggregate gradations could be achieved through the blending of natural and RAP aggregates which limits the negativities associated with RAP utilization to some extent. Hence, an attempt was further made to improve the performance of CPB mixes containing 50% natural aggregates and 50% RAP aggregates using fillers like wollastonite (naturally occurring calcium metasilicate mineral) and jarosite (hazardous zinc industry waste). But before attempting their use in the RAP CPB mix, the utilization potential of jarosite as Portland cement substitute in conventional concrete block and concrete pavement mixes was investigated, wherein an optimum replacement level of 15% for this high specific surface area filler was conservatively recommended based on the criterion of strength enhancement or concreting application in an aggressive environment. Further, it was recommended to utilize such filler material only after an amount of processing (i.e., oven drying and milling/crushing/grinding) and to use a staged mixing approach or increase the mixing time with other ingredients of concrete. For the RAP CPB mix, the studied filler materials, namely wollastonite and jarosite, were used to part replace 5 – 15% and 10 – 20% of Portland cement by volume, respectively. Apart from their individual effects, the efficacy of wollastonite-jarosite blends was also investigated under continuous water curing and water spray curing regimes. The recommendations pertaining to the utilization of filler materials in conventional concrete mixes (discussed above) were followed. Statistically significant improvement in the flexural strength, tensile splitting strength, and abrasion resistance properties of the RAP CPB mix was found with inclusion of these fillers in correct proportions. However, the compressive strength (in most cases), permeable voids, water absorption, and water permeability properties showed an insignificant improvement. The pozzolanic reactivity exhibited by these fillers was substantiated with thermogravimetric analysis, while the micro-reinforcing ability of fine wollastonite particles and the ultrafine nature of processed jarosite bringing about microstructure densification was qualitatively validated using microstructural analysis. The leachate concentrations of heavy metals in the RAP CPB mixes containing jarosite were found to be within permissible limits. It is anticipated that higher economic and ecological benefits could be reaped with the utilization of combined RAP fractions and these fillers as Portland cement substitutes in the CPB mixes.