Why are there 3 modules for concrete

Recycled concrete - building material of the future?

The term “recycled concrete” has a certain negative image. As concrete experts, we should speak of “resource-saving concrete” or “R-concrete” in the future. Because when using recycled aggregates in concrete, we usually recycle high-quality demolition materials from the construction industry. The aim is to create an environmentally friendly, ecological and sustainable building.

The circular economy law has already prevailed in many areas of life. We write on “recycled paper”, we drink from “glass and plastic recycling bottles” and we build cars with “recycled steel”. So why not build concrete structures with recycled aggregates?

A wide range of analyzes and trends relating to construction waste and rubble are published every year in the monitoring report of the Construction Recycling Initiative. The latest figures from 2014 show around 68 million tons of building rubble and road debris from which the recycled aggregate for the concrete could be extracted. 80-90% of this material is currently used as a frost protection layer, trench filling and in the asphalt industry. The current Germany-wide use in concrete is less than 1%.

Table 1: Normative bases

The primary task of suitable demolition objects must be selective dismantling. This is the only way to achieve economic advantages in the manufacture of the RC aggregates through the targeted separation of the different building materials during dismantling. Subsequent separation at the recycling center before or after further processing is technically possible, but increases the cost of the RC material. After processing (crushing and sifting) the RC material is available as crushed aggregate in the usual grain sizes 2/8, 8/16 and 16/32. The fraction <2mm must not be used.

Normative regulations
The normative bases are regulated by a guideline of the DAfStb "Concrete according to DIN EN 206-1 and DIN 1045-2 with recycled aggregates according to DIN EN 12620" and the DIN 4226-101 and -102 newly published last year. The new release includes the classification of the RC aggregates into 4 types (type 1 and 2 for concrete according to DIN 1045-2 and types 3 and 4 for concrete outside of DIN 1045-2), the environmental compatibility with regard to hazardous substances and the in-house production control (WPK ) regulated (Fig. 2). This eliminates the cumbersome procedure of an abZ or approval in individual cases for environmental compatibility. A voluntary manufacturer's declaration is now sufficient. This regulation has led to a significant simplification in the use of RC material.

Table 2: Extract from DAfStb-Rili "Concrete with recycled aggregates"

The limit values ​​for the concrete composition are specified in the DAfStb guideline. Here, depending on the exposure classes, strength classes and RC material type 1 or 2, maximum RC material quantities are prescribed.

XC1 - XC4 max. 45% type 1 and max. 35% type 2 (internal components)
XF1 and XF3 max. 35% type 1 and max. 25% type 2 (external components)
XA1 max. 25% type 1 and max. 25% type 2

  • In principle, no recycled sand 0/2 may be used
  • Use only permitted up to strength class C30 / 37
  • Not used with prestressed concrete and lightweight concrete

From a technical point of view, the proportions are kept so low, because knowing that the aggregate also determines essential properties of the hardened concrete (e.g. E-module), no changes to the static calculations must be made. Thus every static design can be carried out with normal concrete or with R-concrete (according to the guideline) without adaptation.

The RC material manufacturer produces the 4 usual grain fractions with a lot of effort, from which the ready-mixed concrete manufacturer will purchase a maximum of 2 fractions (usually one fraction). After this consideration, the manufacturer of the RC material has to process approx. 5.5 tons of building rubble in order to ultimately be able to sell 1 ton of RC material 8/16. The material that has not been accepted must be integrated into other building materials. Fraction 0/2 is the most difficult to accommodate.

Depending on its classification as Type 1 or Type 2, the RC material may contain different proportions of concrete, mortar, unbound aggregates, masonry bricks and sand-lime bricks. Small and limited amounts of asphalt, plastics, plaster of paris, glass and floating wood and insulation materials are also permitted. That is why the WPK is very important here. Even if the limit values ​​are complied with, the same manufacturer may well have fluctuations in the petrographic composition of the RC grains.

When using RC material in concrete, a possible alkali silica reaction must also be taken into account. The guideline "Concrete with recycled aggregates" (09-2010) specifies that in a dry environment, i.e. moisture class WO, no further conditions have to be met. For components in a damp environment (WF), a distinction is made between:

The origin of the old concrete is known and no further measures are harmless
Origin of the old concrete not known E III-S
(unless the safety is determined by an expert opinion)

The statements on recycled aggregates in the Alkali Directive (10-2013) are partly contradicting the RC Directive. When classifying the aggregates in alkali sensitivity class E I, the recycled aggregates are strictly excluded. Section 4.4 of the Alkali Directive stipulates that the RC aggregates should be classified using a rapid test method and, if necessary, a concrete test in the cloud chamber. If this is not done, the RC aggregate must be assigned to alkali sensitivity class E III-S, regardless of whether the old concrete is known and harmless or not.

The German Institute for Building Technology (DIBt) has decided as follows:
"... as the alkali directive from 2013 reflects the more recent technical and scientific status compared to the RC directive from 2010, the specifications of the alkali directive are binding."

Conclusion: Recycled aggregates without further ASR tests are in principle to be classified in alkali sensitivity class E III-S.

Usually this is not a problem. According to the Alkali Directive, the necessary measures are linked to the moisture class and the cement content. Up to 350 kg / m³ cement and moisture class WF no additional measures are required. In addition, a cement with a low effective alkali content (na cement) would have to be used or an expert opinion would have to be obtained. The latest ASR investigations by the VDZ as part of the joint research project "R-Concrete" have shown that this regulation cannot reliably prevent ASR in northern Germany. The rule for E III-O or E III-OF would have to be applied here, which has no longer existed for concrete with recycled aggregates since 2007. For both alkali sensitivity classes, a cement with a low alkali content (na cement) with moisture class WF was prescribed for the production of recycled concrete. New rules for the ASR classification are currently being discussed.

For the production of R-concrete, the DAfStb guideline "Concrete with recycled aggregates" requires an extended initial test. Basically, the density and the LP content should be checked on the fresh concrete. In this context, the time-dependent course of consistency must also be determined after 10, 45 and 90 minutes. Both the core moisture and the surface moisture must be determined on the recycled aggregate. Dosing instructions for the subsequent addition of superplasticizer are also required. The solid concrete density and the concrete compressive strength are then again the usual tests as part of the initial test.

Every concrete manufacturer who uses RC material in concrete must make this clear for everyone in the delivery list and on the delivery note. During the concrete production, it must be ensured that the RC material must be subjected to a visual inspection with each delivery and, in addition to the FPC, the grain density and the water absorption must be determined after 10 minutes for each production week.

Laboratory tests

In order to be able to assess the performance of R-concrete a little better, comparative tests (normal GK and recycled GK type 1 and 2) were carried out in our concrete laboratory, the SCHWENK technology center. A C25 / 30 F4 was mixed and the course of consistency, need for superplasticizer, compressive strength, shrinkage behavior and E-modulus were determined at different times. The concrete composition was carried out as described below:

300 kg / m³ cement CEM II / A-M (V-LL) 42.5 N
40 kg / m³ limestone powder
w / c value 0.60
BASF Glenium Sky 681, concrete temperature 20 ° C, grading curve A / B 16
Grading curve A / B16
34% RC aggregate 2/16

The evaluation of the fresh concrete results showed that the concrete with RC material tends to need more superplasticizer in order to achieve the same consistency as the concrete with normal aggregate. As expected, the R-concretes stiffen more strongly depending on the increasing proportion of bricks. In the compressive strength test, the R-concretes have slightly lower compressive strengths (3-5 MPa strength loss) at each test point. The results of the shrinkage measurements were initially surprising because the concrete with type 2 RC material demonstrably shrinks less. The high proportion of brick chippings has a positive effect here. On the one hand, these usually porous parts store the water and thus lead to an "internal aftertreatment", which leads to higher degrees of hydration, a denser structure and thus to lower shrinkage rates. On the other hand, thermally treated aggregates tend to have less shrinkage deformation. With regard to other possible deformations of a component, the modulus of elasticity also plays a role in normal building construction from time to time. The zero concrete with its test value of 31500 MPa is slightly above the calculated value according to Eurocode EC2 for C25 / 30 with 31000 MPa. By using the softer RC material, especially type 2, the modulus of elasticity drops to 28200 MPa. This fact should be kept in mind and, if necessary, concrete technology intervened.

Only a few larger "lighthouse projects" have been built with R-concrete nationwide. Certainly also due to the fact that a general building inspectorate approval or approval in individual cases was still necessary until mid-2017. Due to the simplification, there is the idea of ​​using R-concrete more intensively, especially in the public sector. For the time being, implementation will only be possible in metropolitan areas where there is sufficient, easily usable construction waste available. The ecological advantage of R-concrete must not be diminished by the long transport routes of the RC grain. The recycling rate for mineral construction waste is already very high in Germany. An average of 95% of the recycled RC materials are used in road and landfill construction. It would be a wrong sign if the recycling in concrete were to be increased to such an extent that this RC grain would then have to be replaced by other, natural aggregates in road construction. A sensible balance must be created here and additional sources for RC material must be developed (e.g. separate excavated earth from coarse aggregate and then break it; the remaining excavated material can be used again as backfill material).

It could be shown that the production of R-concrete is technically possible. At every concrete mixing plant there would be the possibility of mixing and delivering such concrete. Whether the use of R-concrete makes ecological sense depends on many factors. The most important are:

  • Transport distance from the recycling point to the mixing plant
  • Difference in cost of the RC material compared to normal aggregates
  • Availability (quantity and continuity)
  • Are there additional storage and dosing options on the mixing plant?
  • Are there any economic incentives from the AG? (R-concrete is not cheaper than normal concrete !!!)
  • Are there regional manufacturers of RC material who meet all requirements?

The concrete and aggregate industry should advertise R-concrete positively in order to awaken and strengthen awareness of R-concrete in the minds of concrete users and builders. As self-evident as today is written on recycled paper or we come across other recycling materials more and more often in our daily lives, it should also be a matter of course to accept concrete with recycled aggregates without ifs and buts on our construction sites.

Dipl.-Ing. Roland Mellwitz - SCHWENK Zement KG - construction consulting