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Re use of rubble from demolished concrete structures

FORCETOC

Concrete recycling is the use of rubble from demolished concrete structures. Recycling is cheaper and more ecological than trucking rubble to a landfill. Crushed rubble can be used for road gravel, revetments, retaining walls, landscaping gravel, or raw material for new concrete. Large pieces can be used as bricks or slabs, or incorporated with new concrete into structures, a material called urbanite.

Concrete is an excellent material with which to make long lasting and energy efficient buildings. However, even with good design, human needs change and potential waste will be generated. Concrete may be considered waste according to the European Commission decision of 2014/955/EU for the List of Waste under the codes: 17 (construction and demolition wastes, including excavated soil from contaminated sites) 01 (concrete, bricks, tiles and ceramics), 01 (concrete), and 17.01.06* (mixtures of, separate fractions of concrete, bricks, tiles and ceramics containing hazardous substances), and 17.01.07 (mixtures of, separate fractions of concrete, bricks, tiles and ceramics other than those mentioned in 17.01.06). It is estimated that in 2018 the European Union generated 371,910 thousand tons of mineral waste from construction and demolition, and close to 4% of this quantity is considered hazardous. Germany, France and the United Kingdom were the top three polluters with 86,412 thousand tons, 68,976 and 68,732 thousand tons of construction waste generation, respectively. Currently, there is not an End of Waste criteria for concrete materials in the EU. However, different sectors have been proposing alternatives for concrete waste and re purposing it as a secondary raw material in various applications, including concrete manufacturing itself.

Reuse of blocks in original form, or by cutting into smaller blocks, has even less environmental impact; however, only a limited market currently exists. Improved building designs that allow for slab reuse and building transformation without demolition could increase this use. Hollow core concrete slabs are easy to dismantle and the span is normally constant, making them good for reuse. The viability of this alternative has to be studied as the logistics associated with transporting heavy pieces of concrete can impact the operation financially and also increase the carbon footprint of the project. Also, ever changing regulations on new buildings worldwide may require higher quality standards for construction elements and inhibit the use of old elements which may be classified as obsolete.

Large pieces of concrete rubble (urbanite) can be used in walls as building stones, to reduce stream bank erosion. Ecology blocks (eco blocks) are made from recycled concrete and used for retaining walls and other temporary structures, and have also been used for hostile architecture. Small pieces are used as gravel for new construction projects. Sub base gravel is laid as the lowest layer in a road, with fresh concrete or asphalt poured over it. The US Federal Highway Administration may use such techniques to build new highways from the materials of old highways. Concrete pavements can be broken in place and used as a base layer for an asphalt pavement through a process called rubblization. Crushed concrete free of contaminants can be used as raw material (sometimes mixed with natural aggregate) to make new concrete. Well graded and aesthetically pleasing materials can be used as landscaping stone and mulch.

Improper disposal and treatment of concrete waste negatively affect soil, but proper treatment and recycling processes can be used to amend and stabilize soil. In general, alkali activated mixtures improve and stabilize soil through cation exchange, hydration reactions, and enhanced pozzolanic reactions. Ca2+ ions in an alkali activated mixture exchanges with other metal ions, decreasing electric double layers and increasing flocculation, making soil more granular and friable. Alkali activated mixtures improve soil by sorbing the water in the soil through hydration reactions, which decreases the water content in the soil and improves soft soil with a high moisture content. Finally, the dissociation of calcium oxide in water in the soil increases electrolyte concentrations and pH, and hence SiO2 and Al2O3 dissolve more readily and promotes pozzolanic reactions. Materials such as Portland cement, fly ash, and lime are already used extensively to amend and stabilize soil, so the same concept can be extended to concrete waste, which is itself an alkali activated mixture. In general, studies have shown that the cementitious material of concrete waste that is added to weak soil causes hydration reactions that increase the soil pH, amount of Ca2+, and amount of free Ca(OH)2 that could react with SiO2 and Al2O3 through pozzolanic reactions that improve soil.

Concrete waste contains abundant silicon and some aluminum, so they can be used to synthesize geopolymers. Geopolymeric binder combined with metakaolin can yield material with desired silicon, aluminum, and calcium contents. Geopolymer concrete from waste concrete has been analyzed, and it has been suggested that it could be used in applications that require moderately strong concrete, thermally insulating concrete, lightweight concrete, and bricks or blocks.

Concrete waste that is rich in alkaline calcium compounds can be used to remove and recover various elements from an aqueous solution. Waste concrete has been used as a sorbent to remove phosphorus from wastewater after the removal of excess sludge in sewage treatment plants. Concrete waste may also be used as an inexpensive gas treatment agent. This would offer advantages over using conventional gas treatment agents because concrete waste is cheap and produced in large amounts. Research has shown that waste concrete can contribute to the sorption of NO2, SO2, and Fluorine gas.

There have been concerns about the recycling of painted concrete due to possible lead content. The Army Corps of Engineers' Construction Engineering Research Laboratory (CERL) and others have studied the risks, and concluded that concrete with lead based paint should be safely used as fill without an impervious cover as long as it is covered by soil. Some experiments showed that recycled concrete is less strong and durable than concrete from natural aggregate. This can be remedied by mixing in materials such as fly ash.