The Importance of Concrete Recycling in the Construction Industry
Resources are becoming scarcer. Finite raw materials in particular require highly efficient management. This topic is ubiquitous in the construction industry as well. After all, enormous quantities of materials are needed and used here every day. Resource efficiency therefore plays a major role in the construction industry. On one hand for cost reasons, on the other hand due to the constantly increasing demand for energy-efficient and sustainable buildings. The reuse of raw materials is at the top of the agenda. For example, structural steel has been remelted for reuse for quite some time. Now concrete recycling is also gaining increasing importance.
Two Different Types of Concrete Recycling
Provided that the concrete is sorted properly by type, concrete can be recycled completely. A distinction must be made between fresh concrete recycling and hardened concrete recycling. The term fresh concrete recycling refers to internal material loops within the respective concrete plant. In this process, excess concrete or unplaced concrete is deliberately separated and fed back into production. These surplus materials consist of material obtained when cleaning pumps and mixers at the plant.
Fresh Concrete Recycling: Focus on Grain Groups in Residual Concrete and Residual Water
These surplus materials account for approximately 2.5 percent of total production at a concrete plant. In fresh concrete recycling, fresh water and primary aggregates are replaced by certain grain groups contained in residual concrete and by residual water. How residual concrete, residual mortar, and residual water may be used is governed by a DafStb guideline (DafStb: Deutscher Ausschuss für Stahlbeton). Fresh concrete recycling is now actually carried out by virtually all German concrete manufacturers.
Recovery of Residual Water and Aggregates
In terms of processing, the not yet hardened mortar or concrete is carefully washed out. Through this process, the residual water and aggregates can be recovered in the form of an aqueous suspension of fine particles. The aggregates have – due to equipment requirements – grain diameters of over 0.125 millimeters or 0.250 millimeters. These consist of cement particles and similar fine materials. According to DIN EN 1008, residual water containing these particles may be added to fresh water or mixing water in concrete construction. This is referred to in the industry as residual water recycling. The legally permissible ratio of fresh water to residual water always depends on the required concrete quality and the level of fine content in the residual water. It should be noted that high-strength concrete, air-entrained concrete, and other concretes with particularly high quality requirements are generally only permitted to be mixed with fresh water.
Concrete Recycling: Study on the Use of Residual Water
However, there are now also various studies and scientific papers that advocate for the use of residual water in the recycling of high-quality concrete. In particular, a study as part of the Industrial Joint Research Promotion Program (IGF) has addressed this topic comprehensively and in detail. According to this, the use of residual water leads to different processing results, depending on how dense the residual water used is. This means it always depends on the water content and fine particle content.
Study Results
- increased early strength
- leads to higher 28-day strengths in concrete with lower target strength
- with regard to the normatively regulated range for residual water use, slightly lower carbonation resistance is observed
- outside the normatively regulated range, carbonation resistance is significantly lower
- in the frost test, concrete without artificially created air pores experiences increased weathering
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Hardened Concrete Recycling: Proper Sorting as an Important Prerequisite
By contrast, in so-called hardened concrete recycling, concrete from demolition is reprocessed. A prerequisite for this is the most precise possible sorting by type during building demolition. In the subsequent recycling process, the concrete is crushed and sieved using a crusher. This produces recycled concrete sand and recycled concrete aggregate. In the case of reinforced concrete, reinforcing steel is also sorted out separately. This scrap steel can subsequently be remelted and then formed into new steel products.
The crushed sand and recycled concrete aggregate can generally be used for various purposes. For example, recycled concrete is made from recycled concrete aggregate. The corresponding requirements are explicitly specified in the standard DIN 4226-100. For example, when producing recycled concrete, a modified water demand must be taken into account in the material distribution.
At a Glance – the Advantages of Recycled Concrete
Basically, the use of recycled concrete offers various advantages, which in some cases only become apparent to a construction company in the long term:
- initiates a circular economy, which is counted among environmental protection measures
- demolition material and construction waste are not deposited in landfills and disposal sites, but are purposefully reprocessed
- no transportation costs and no landfill costs
- no costs for new concrete
- reduces the emissions and energy-intensive production of cement
- brings independence from suppliers and price increases
