Development of ‘green’ rigid pavements incorporating recyclable rubber and paper mill products.
Nhlabatsi, Senzo Brian.
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Natural aggregate resources used as conventional fillers in concrete production are constantly diminishing. Several studies have suggested that there will be a lot of pollution in the degradation of waste rubber products. This research explores the potential use of waste tyre rubber crumbs in a concrete mixture for rigid (concrete) roads. The research also focuses on the possibility of using paper mill ash, a by-product of the paper mill industry, as a new innovative binder in a concrete mixture with rubber aggregate. Specifically, concrete mixtures with river sand partially replaced by crumbed rubber and Portland cement partially replaced by paper mill ash were produced for a new concrete. The objective was to optimise proportion and size of tyre rubber and paper ash particles to improve material performance. The sustainable mixes incorporated a constant 5% cement substitution with paper mill ash and river sand substitution with crumb rubber proportions of 5%, 10% and 15% by mass of sand. A class 40/19 control mixture comprised of crumb rubber particles of sizes ranging from 1mm-5 mm considered for a mix with 0.49 water/cement ratio was designed as per the C&CI design method developed by the Concrete Institute, South Africa. Laboratory tests were conducted, and the effects of the paper mill ash and crumb rubber inclusions were ascertained by comparing the results for mixtures containing the two waste stream materials to a control mixture only with the conventional materials (river sand, stone and Portland cement). The concrete was tested for fresh and hardened properties including density, compressive strength, splitting strength and flexural strength. Hardened concrete samples were then extracted to carry out a durability analysis by performing a chloride conductivity, water sorptivity and oxygen permeability test. This research has uncovered that replacing conventional aggregate with waste paper mill ash and rubber lead to, on average, improvements in the durability of concrete, and a degradation of the mechanical strength. The loss of mechanical strength can be attributed to the sense that the resulting lower density and nature of waste tyre rubber weakens the interfacial bonding and thus resulting in a heterogeneous particle distribution within the concrete. However, the research reveals that the use of recycled rubber and paper mill ash-modified concrete in road pavements is feasible when low proportions of paper mill ash and crumb rubber are added. The marginal loss of strength with the offsetting improvements in durability implies that there exist an optimum combination of proportion and size of the rubber crumbs and paper mill ash that will improve the material performance under cyclical vehicle loads.