Concrete recycling
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.
Circular economy
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 01, 01, and 17.01.06*, and 17.01.07. 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.
In the context of a circular economy the most efficient way to use concrete after fulfilling its initial purpose may not be clear. Factors to be considered include the quality of the recovered material as well as technical or other regulatory requirements.
Currently, there is not an End-of-Waste criteria for concrete materials in the European Union. 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
Reuse of blocks in original form, or by cutting into smaller blocks, has even less environmental impact; however, a limited market now 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.Other cases of re-use are possible with pre-cast concrete pieces: through selective demolition, such pieces can be disassembled and collected for further use in other building sites. Studies show that back-building and remounting plans for building units is an alternative for a kind of construction which protects resources and saves energy. Especially long-living, durable, energy-intensive building materials, such as concrete, can be kept in the life-cycle longer through recycling. Prefabricated constructions are the prerequisites for constructions necessarily capable of being taken apart. In the case of optimal application in the building carcass, savings in costs are estimated in 26%, a lucrative complement to new building methods. However, this depends on several courses to be set. 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.
Recycling
Concrete debris is routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, changing regulation/laws and economic benefits. Concrete can be recovered – crushed and reused as aggregate in new projects.Recovering concrete reduces resource exploitation and associated transport costs, and reduces landfill. However, it has little impact on reducing greenhouse gas emissions as most emissions occur when cement is made. At present, most recovered concrete is used for road sub-base and civil engineering projects.
By far the most common method for recycling dry and hardened concrete involves crushing. The input material can be returned concrete which is still fresh, from ready-mix trucks, production waste at a pre-cast production facility, or waste from demolition. The most significant source is demolition waste, preferably pre-sorted post-demolition. Specific processing sites are typically able to produce higher quality aggregate. Screens are used to achieve desired particle size, and remove dirt, foreign particles and fine material from the coarse aggregate.
The final product, Recycled Concrete Aggregate, has an angular shape, rougher surface, lower specific gravity, higher water absorption, and pH greater than 11 – this elevated pH increases the risk of alkali reactions. RCA's lower density usually increases project efficiency and lowers job cost – RCA yields more volume by weight. The physical properties make it the preferred material for applications such as road base and sub-base. This is because recycled aggregates often have better compaction properties and require less cement for sub-base uses. Furthermore, it is generally cheaper to obtain than virgin material.
Cement
Pulverized concrete can replace flux material in electric arc furnaces. The process produces “reactivated cement” as a byproduct. Furnaces need flux, to purify the steel. If the leftover slag is cooled quickly in air, it becomes Portland cement. The technique also significantly reduces CO2 emissions compared to conventional methods.Applications
The main commercial applications are:- Aggregate base course, or the untreated aggregates used as foundation for roadway pavement, is the underlying layer which provides a structural foundation for paving.
- Aggregate for ready-mix concrete, by replacing from 10 to 45% of the virgin aggregates with a blend of cement, sand and water. Because the RCA contains cement, the ratios of the mix have to be adjusted to achieve desired structural requirements such as workability, strength and water absorption.
- Soil stabilization, with the incorporation of recycled aggregate, lime, or fly ash into marginal quality subgrade material used to enhance the load bearing capacity of that subgrade.
- Pipe bedding: serving as a stable bed or firm foundation in which to lay underground utilities. Some countries' regulations prohibit the use of RCA and other construction and demolition wastes in filtration and drainage beds due to potential contamination with chromium and due to pH impacts.
- Landscape materials: Includes boulder/stacked rock walls, underpass abutment structures, erosion structures, water features, retaining walls.
Cradle-to-cradle challenges
The path towards circularity goes beyond concrete technology itself, depending on multilateral advances in the cement industry, research and development of alternative materials, building design and management, and demolition as well as conscious use of spaces in urban areas to reduce consumption.
Process
Re-purposing urbanite involves selecting and transporting the pieces, and using them as slabs or bricks. The pieces can be shaped, for example using a chisel; this can be labor-intensive.Crushing involves removing trash, wood and paper; removing metals such as rebar, using magnets and other devices, to be recycled separately; sorting the aggregate by size; crushing it using a crushing machine; and removing other particulates by methods such as hand-picking and water flotation.
Crushing at the construction site using portable crushers is cheaper and causes less pollution than transporting material to and from a quarry. Large road-portable plants can crush concrete and asphalt rubble at 600 tons per hour. These systems normally include a side discharge conveyor, a screening plant, and a return conveyor from the screen back to the crusher for re-crushing large chunks. Compact, self-contained crushers can crush up to 150 tons per hour and fit into tighter areas. Crusher attachments to construction equipment such as excavators can crush up to 100 tons per hour and make crushing of smaller volumes economical.
To produce clean aggregates from crushed concrete waste, very careful dismantling and demolishing is needed to keep the concrete stream away from other materials that would diminish its quality. Once separated, the broken concrete is then sent to a wet recycling process, where the coarse fraction of broken concrete is washed to produce clean aggregate, whereas the residue generated from the washing process is sent to landfill in the form of sludge.
Uses
Large pieces of concrete rubble can be used in walls as building stones, as slabs in walkways, or as riprap revetments to reduce stream bank erosion. Ecology 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 to make new concrete.
Well-graded and aesthetically pleasing materials can be used as landscaping stone and mulch.
Wire gabions, can be filled with crushed concrete and stacked as retaining walls or privacy walls.