The Importance of Concrete Recycling in the Construction Industry
Resources are becoming increasingly scarce. Finite raw materials in particular require highly efficient management. This is an omnipresent issue in the construction industry as well. After all, huge quantities of materials are needed and built in every single day. Resource efficiency therefore plays a major role in the construction sector. On the one hand for cost reasons, on the other hand due to rising demand for energy-efficient and sustainable buildings. The reuse of raw materials is at the top of the list. For example, construction steel has been melted down for reuse for quite some time now. Concrete recycling is now gaining increasing importance as well.
Two Different Types of Concrete Recycling
Assuming proper material separation, concrete can be completely recycled. A distinction must be made between fresh concrete recycling and hardened concrete recycling. The term fresh concrete recycling refers to internal material cycling within a concrete plant. Here, excess concrete or unused concrete is deliberately separated and fed back into production. These excess materials are materials obtained when cleaning pumps and mixers at the plant.
Fresh Concrete Recycling: Grain Groups in Residual Concrete and Residual Water in Focus
These excess materials account for around 2.5 percent of total production at a concrete plant. In fresh concrete recycling, fresh water and primary stone aggregates are replaced by specific grain groups contained in residual concrete as well as by residual water. A DafStb guideline (DafStb: German Committee for Reinforced Concrete) regulates exactly how residual concrete, residual mortar, and residual water may be used. Fresh concrete recycling is nowadays actually carried out by virtually all German concrete manufacturers.
Recovery of Residual Water and Stone Aggregates
In terms of processing method, the not yet hardened mortar or concrete is carefully washed out. Through this process, residual water and stone aggregates can be recovered in the form of an aqueous suspension of fine particles. The stone aggregates have – depending on the equipment – grain diameters larger than 0.125 millimeters or 0.250 millimeters. These are cement particles and similar fine materials. According to DIN EN 1008, residual water with 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 material content in the residual water. It should be noted that high-strength concrete, air-entrained concrete, and other concretes with particularly high quality requirements may generally only 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 for recycling high-quality concrete. In particular, a study within the Industrial Collective Research (IGF) program has addressed this topic comprehensively and in detail. According to the findings, the use of residual water leads to a changed processing result, the higher the density of the residual water used. This means it always depends on the water content and fine material 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, a slightly lower carbonation resistance is observed
- outside the normatively regulated range, carbonation resistance is significantly lower
- in the frost test, concrete without artificially generated air pores shows increased erosion
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Hardened Concrete Recycling: Proper Material Sorting as Important Prerequisite
In contrast, in so-called hardened concrete recycling, concrete demolition waste is reprocessed. A requirement for this is the most accurate possible material-specific pre-sorting when demolishing buildings. As part of the subsequent recycling process, the concrete is crushed and screened using a crusher. This produces recycled concrete sand and concrete chips. If it is reinforced concrete, reinforcing steel is also sorted out separately. This steel scrap can subsequently be melted down and reformed into new steel products.
The crushed sand and concrete chips can generally be used for various purposes. For example, recycled concrete is made from concrete chips. The corresponding requirements are explicitly specified in the DIN 4226-100 standard. For instance, in recycled concrete production, a modified water requirement must be taken into account in the material distribution.
At a Glance – The Advantages of Recycled Concrete
In principle, the use of recycled concrete offers various advantages, some of which may only be noticeable to a construction company in the long term:
- initiates a circular economy, which counts as an environmental protection measure
- demolition material and construction waste are not deposited on landfills and extraction sites, but deliberately reprocessed
- no transport 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
