Resource-saving concrete – The next-generation material

Fine recycled aggregates, so-called crushed sands, are not defined as a main constituent in accordance with DIN EN 197-1, Section 5.2, and are therefore not allowed to be used in cement. Cements employing crushed sand as main constituent are subject to approval. With regard to environmental compatibility it would then probably also be necessary to comply with the maximum values stipulated in DIN 4226-101:2017-08 for the parameters relating to eluates and solids. Verification of both the cement properties demanded in DIN EN 196-1 and DIN EN 197-1 and the durability-related concrete properties would be required.

The Portland cements CEM I 42,5 R and 52,5 R formed the basis for the R cements (laboratory cements) produced in this project. The crushed sands differed in terms of their composition (origin), particle size and treatment method. Use was made of crushed sands obtained from the processing of sleepers (CTG), track ballast (UTG), crushed concrete (BTG) and masonry rubble (MTG). After delivery, the crushed sands were dried and ground to a fineness of approximately 4 000 cm²/g acc. to Blaine in an intermittently operating laboratory ball mill. The Portland cements were then mixed with the flour-fine crushed sands in the laboratory to form R cements. Proportions of 10 and 30 mass % crushed sand were set in the cement for production of the laboratory cements. In addition to this, large-scale operational trials were performed to produce R cement by way of joint grinding at a cement works. Proportions of 8 and 15 mass % crushed sand were set in the works cements.

The crushed sands used in the VDZ laboratory experiments and full-scale operational trials complied with the environmental requirements of DIN 4226-101. The maximum values for the parameters relating to eluates and solids were satisfied. Depending on their material/granulometric composition, the R cements corresponded to strength classes 42,5 N to 52,5 R in accordance with DIN EN 197-1. Concretes were produced on the basis of selected R cements and tested. The compositions of the concretes were as follows: B1: Cement content (c) = 300 kg/m³, water-cement ratio (w/c) = 0.60; B2: c = 320 kg/m², w/c = 0.50. The cube test method was employed to determine the freeze-thaw resistance of the B1 concretes. These tests correspond to the standards applied to date by the German Institute for Building Technology (DIBt) for the approval of cements. The concretes were subjected to up to 100 freeze-thaw cycles (FTC) with one cycle per day. The scaling of the concretes differed only slightly depending on the type of crushed sands. There was only a low level of scaling (max. 4.7 mass %) when using laboratory cements with up to 30 mass % crushed sand in combination with CEM I 42,5 R. The 10 mass % limit value for scaling after 100 freeze-thaw cycles employed in the DIBt approval tests was satisfied by a considerable margin. R cements with 30 mass % crushed sand in combination with CEM I 52,5 R and R cements with 10 mass % crushed sand in combination with CEM I 42,5 R were used to produce B2 concretes for examination of the internal microstructural damage (relative dynamic elastic modulus) in the CIF test (Capillary suction, Internal damage and Freeze-thaw test). The test specimens were made and analysed in accordance with the Technical Report CEN/TR 15177. Testing was conducted over the course of 56 freeze-thaw cycles.

Taking the R cements with 30 mass % crushed sand in combination with CEM I 52,5 R
as an example, the relative dynamic elastic modulus of the concretes as a function of the number of freeze-thaw cycles developed as follows: Three out of the four concretes satisfied the assessment criterion for the CIF test in accordance with the code of practice ‘Freeze-thaw testing of concrete ̓ issued by the Federal Waterways Engineering and Research Institute (BAW) and attained a relative dynamic elastic modulus of > 75 % after 28 freeze-thaw cycles. The concrete produced with the R cement Z 30 UTG (track ballast) did not conform to the above-mentioned assessment criterion. An overall view of the results obtained for R cements in the project reveals that these cements, with up to 30 % crushed sand, could at least be used in interior component concretes. A continuous, uniform material flow of corresponding quality between the processing plant and the cement works would however be essential.

The European standard EN 15804 sets down regulations for the life cycle assessment (LCA) of construction products. In 2017, VDZ published the LCAdata for the production of an average cement made in Germany in a revised environmental product declaration in accordance with EN 15804. This LCA was based on environment-related production data from German cement works.

The first topic to be dealt with in the ‘Life cycle assessment ̓ work package of the R concrete project was the influence of the use of crushed sand on the LCA of cement production. In particular, this involved giving consideration to the environmental impact associated with the processing of old concrete (crushing, sieving, etc.) and the use of these crushed sands in place of the raw materials currently used for cement production. A comparison between the data published by the association and the newly determined LCA parameters was intended to show the extent to which the use of RC fine material can help to reduce the environmental impact of cement production.

In a similar manner as for cement, consideration was also to be given to the possible effect of the use of RC aggregate on the LCA for the production of concrete. In 2018, VDZ will be producing updated LCA studies for concretes of six different strength classes, based on typical concrete formulae determined by the German Ready-Mixed Concrete Association (BTB) and the German Association for Precast Concrete Construction (FDB). These LCA studies, published in environmental product declarations, are intended to be used for comparison purposes.

The alkali reactivity class has to be stated for aggregates for concrete in accordance with EN 206-1 and DIN 1045-2. If necessary, concrete manufacturers can take preventive action to avoid damage as a result of alkali-silica reaction (ASR). Prior to issuing of the 2010 DAfStb Specification Concrete with recycled aggregates, the measures for E III-O aggregates had to be taken for any recycled aggregates that could not be clearly assigned to a non-critical alkali reactivity class. Since 2007 (in accordance with the alkali guideline) and 2010 (in accordance with the guideline of the German Committee for Structural Concrete (DAfStb) on concrete with recycled aggregates), the recycled aggregates have had to be assigned to alkali reactivity class E III-S in such cases. This brought about a change in the preventive measures. In the project ‘R concrete – Resource-saving concrete – The next-generation material ̓ (sub-project 5), this change was checked by conducting ASR performance tests on concretes.

The results show that the current (E III-S) measures are not always adequate for recycled aggregates. The expansion of the two concretes in the 60 °C concrete test is well in excess of the limit value of 0.2 mm/m after 140 days employed in France and Switzerland and specified in AFNOR FD P 18-456.

DAfStb recommendation

On the basis of the results obtained, the DAfStb recommends that recycled aggregates should be treated, as was the case before 2007 and 2010, as aggregates of alkali reactivity class E III-O if these are used in the ice-age deposition region of Northern Germany, as specified in the alkali guideline, and verification of non-criticality is not possible or is not performed for the aggregate. In the rest of Germany, recycled aggregates for which verification of non-criticality is not possible or not performed can continue to be assigned to alkali reactivity class E III-S.