Engineering and environmental evaluation of spent coffee grounds stabilized with industrial by-products as a road subgrade material

Current construction technology increasingly seeks the sustainable usage of waste by-products as a resource material. This paper evaluates the viability of utilizing spent coffee grounds (CG), a highly organic beverage waste, to be stabilized as a road subgrade material. The additives used in this research incorporates industrial by-products such as fly ash (FA), ground granulated blast-furnace slag (S) as well as traditional binders such as portland cement (PC) and hydrated lime (L). CG collected from a coffee roaster were mixed with controlled additive content ratios by mass to assess the effects of these common engineering stabilizers towards the load-bearing capacity of CG. The additive contents of FA and S were 10, 20, 30, 40, and 50 % whereas the PC and L additive contents were 3 and 5 % by dry unit weight. Modified proctor compaction tests, 7-days unconfined compressive strength (UCS) tests, and California bearing ratio (CBR) tests were carried out to determine the optimum moisture content and bearing strength of the different mixes produced. It found that as the proportion of additives in the specimen increased, the optimum moisture content of the additive-stabilized CG specimens subsequently decreased. Regardless of the type of stabilizers used, the UCS strength increases were found to be nominal. FA and S mixes above the 20 % additive contents satisfied the requirements for subgrade materials; however, the low PC and L contents were insufficient to meet subgrade requirements. The research findings indicate that instead of being disposed of into landfills, stabilized CG has the potential to be used as a subgrade material. Such a sustainability driven approach for reuse of CG will have the potential to divert CG from landfills and at the same time utilize CG as a viable construction material.