Feasible Use Of Recycled Concrete Aggregates As Unbound Road Base Material In South Africa

Abstract

Sustainability is one of the biggest challenges facing the world today. Construction and demolition waste (C&DW) has become a financial burden globally. Landfills are reaching maximum capacity earlier than expected and one of the biggest contributors to this phenomenon is C&DW. This is generated by the need for new infrastructure, refurbishment and demolition of existing infrastructure as a result of urbanisation that is sweeping through the world. New avenues have to be explored to reduce the financial costs and environmental effects instead of the old-fashioned way of hauling C&DW to landfills. There is uncertainty with regards to defining CD&W in the world: every country has its own definition, and there is no exact amount available for the total of CD&W generated worldwide. The composition of C&DW varies based on site activity and is, usually, mainly made up of concrete. The United States alone generates about 100 million tons annually of C&DW, and roughly more than 50% of the landfill volume is used for C&DW. About 5 to 8 million tons of C&DW is generated annually in South Africa, where the community is not keen to recycle and few organisations are involved in recycling. C&DW is produced by crushing and screening of material from old infrastructure and may be produced by processing the material on site or off-site. There are mobile processing machineries available in the market which gives flexibility to production of C&DW. The material is crushed and screened for production and there is technology available to assist with the removal of detrimental substance and steel. The recycling of C&DW is still in its early stages. Although the European Union and several American States have high expectancy in C&DW material as a commodity, it took ISO 9000 until 1996 to come up with standards for recycled material. In South Africa there are currently no local standards for C&DW and there is consequently a need to develop standards to promote the use of C&DW. The introduction of standards will give environmental value to the construction industry and eradicate financial losses, and will also assist to overcome barriers. The South African experience regarding C&DW is not totally different to the world. The South African construction industry is not keen to recycle and uses mainly virgin aggregates from natural reserves. There are some similarities such as differences among local authorities with regards to the definition of C&DW. Most local authorities define C&DW as a waste generated through demolition, excavation and building activities. The Environmental Conservation Act, Act 73 of 1989, and the National Water Act, Act 36 of 1998 define waste as unwanted or surplus material, with potential to create pollution. The South African construction industry views the restrictive definition of waste as one of the barriers for implementation of recycling of C&DW. The C&DW in South Africa is mainly made of concrete. The concrete industry comprises cement manufacturers, aggregates producers, admixture suppliers, cement extender suppliers, precast and readymix concrete producers, structural engineers, building contractors and small-scale concrete consumers such as home builders. This use of virgin aggregates has adverse effects on the environment. It also leads to more extraction and less usage as a result of availability of resources, towering energy consumption, pollution of the environment by C02 emissions, lack of rehabilitation, ecosystem disturbance and generation of waste and neglectful disposal. The sieve analysis and densities of C&DW obtained from returned readymix concrete aggregates for the purpose of this study indicated the material was well graded and dense, which indicated the material could easily be compacted on site. The plasticity index test indicated that the material was non-plastic, which indicated in turn that the material would not be susceptible to big volumetric changes as a result of changes in moisture content. It was found that the optimum moisture content was on the higher side and the maximum dry density was on the lower side when compared to virgin aggregates usually used as base course material in South Africa. The high optimum moisture content was due to absorption of water by the cementitious paste from previous concrete. The lower maximum dry density was due to loss of aggregates stiffness since C&DW was subjected to secondary crushing. The California bearing ratio (CBR) for C&DW was on the lower side resulting in loss of aggregates stiffness due to secondary crushing; the results were also inconsistent as a result of different classes of concrete blended to obtain C&DW. It was found that the UCS met COLTO requirements for C3 and C4 material and it increased as the cement content increased. Contrary to unconfined compressive strength (UCS), the indirect tensile strength (ITS) was less than that of C3 and C4 material. The ITS increased as the cement content was increased which was a positive indication that higher ITS could be achieved. The maximum dry density of stabilised C&DW increased as well and the increase may be attributed to addition of cement and cementitious properties from crushed concrete aggregates which also contributed in the hydration process. Physical and chemical test results obtained in this study indicated that C&DW could be successfully recycled, which correlates with the literature review. European countries and Japan have successfully recycled C&DW; ISO 9000 was established in 1996 for C&DW. South Africa has to change its perception towards C&DW and develop a local standard to be used by the industry to promote the use of C&DW.