Recycling demolished concrete waste
Worldwide consumption of concrete is estimated at 20 billion tonnes. To produce such a huge volume of concrete, we need 14 billion tonnes of natural aggregate. To supply aggregate, it is necessary to cut a huge volume of mountains or dig aggregate quarries. On the other hand, we are demolishing concrete structures due to deterioration as well as replacement of low-rise buildings by high-rise buildings. It is estimated that every year the world has to reckon with the problem of disposing of 12 billion tonnes of demolished concrete waste generated from the demolition of buildings and other reinforced concrete infrastructures. To make construction materials sustainable and to keep our world habitable for the future generations, we need to strike a balance between the demand for aggregate for construction works and the volume of demolished concrete waste. Through recycling of demolished concrete waste, this balance can be established. By recycling demolished concrete waste in new construction works, it is possible to (i) solve the environmental problems related to the production of natural aggregate and (ii) solve the disposal problem associated with the demolished concrete waste.
Bangladesh is a fertile and densely populated delta country. Due to the scarcity of natural sources of stone aggregates, bricks are widely used as coarse aggregate in concrete and as blocks for making walls. Conventionally, bricks are produced by burning clay at a temperature of more than 1100°C which causes enormous environmental pollution. Around 17.2 billion clay burnt bricks are produced every year through around 5,000 brick kilns across the country. During production of these bricks, about 10 million tonnes of carbon dioxide are emitted to the atmosphere. The brick industry is also responsible for about 40 percent of fine particles in the air of Dhaka city. Moreover, in order to supply raw material (clay) to the brick kilns, about one metre depth of fertile soil is removed from an area of 40km2 every year. As a result, agricultural production is hindered significantly. Also, agricultural production is severely affected around the brick kilns due to the deposition of dusts on the plants. Moreover, environmental pollution by brick kilns causes severe problems to human health. Switching to alternative sources of coarse aggregate can help to overcome these concerns.
It should be noted that concrete is the second most consumed material after water. Water is recycled through the God-gifted natural water cycle. Recycling of concrete is also indispensable to keep our environment healthy and sustainable.
In Bangladesh, most of the demolished buildings are made of brick chips. To understand the possibility of recycling demolished concrete as coarse and fine aggregate for new construction works, we conducted a research study where demolished concrete blocks were collected from 33 demolished building sites. Then the blocks were crushed into coarse and fine aggregates. The properties of aggregates were investigated and compared with the virgin brick aggregate and natural sand. By using the recycled aggregate, more than 1,000 concrete cylindrical specimens were made for evaluation of mechanical properties of concrete. The results revealed that recycled brick aggregates satisfy international standards, such as ASTM C33, and it is possible to produce concrete of strength 2,500 to 5,000 psi by using recycled brick aggregate. In addition to the cylindrical concrete specimens, large-sized RC beam specimens were also made and investigated under loads. The results were compared with different internationally renowned design code provisions, such as ACI, AASHTO, BS, CSA, FIB, Euro Code, and JSCE. The provisions of different design codes can be used safely for evaluation of load-carrying capacity of RC beams made with recycled aggregate. In general, to improve strength of concrete made with recycled brick aggregate, it is necessary to use lesser amounts of water during mixing concrete, and water reducing admixture can be used to improve flow ability of concrete. The presence of different sizes of particles is also necessary as per standard. Our research results related to these investigations were published in internationally renowned journals, such as ASCE Materials Journal, Journal of Construction and Building Materials, and Journal of Engineering Structures.
For general awareness of recycling demolished building waste, public projects were made with recycled aggregate in many countries like Germany, USA, Japan, etc. The same policy can be adopted in Bangladesh as well. Recycling of demolished concrete waste will help in achieving several Sustainable Development Goals (SDGs) set by the United Nations (UN), such as SDG 9 (Industry, Innovation, and Infrastructure); SDG 11 (Sustainable Cities and Communities); SDG 12 (Responsible Consumption and Production); and SDG 13 (Climate Action). Considering sustainability of construction materials as well as sustainability of our environment, every particle of construction waste is expected to be recycled properly thorough the innovative ideas of our engineers keeping in mind that demolished concrete is not waste but a resource.
Dr Md Tarek Uddin, PEng., is Professor, Department of Civil and Environmental Engineering, Islamic University of Technology (IUT).