Creating load-bearing precast parts through the use of recycled concrete (by Herbert Kahmer, Beton-Betz)

Creating load-bearing precast parts through the use of recycled concrete by Herbert Kahmer (Beton-Betz)

In Germany, recycled concrete (R-concrete) is still being produced only to a small extent for load-bearing components, although normative foundations exist, and ecological incentives are given. Concrete precast plants have not yet adapted their product range to this portfolio. Accordingly, this article provides an overview of the challenges faced by precast plants in the implementation of R-concrete and describes how recycled concrete can make use of loadbearing prefabricated concrete elements.

Market development

Already in the year 2000, the artist Friedensreich Hundertwasser created the "Waldspirale" in Darmstadt - a promising building with R-concrete despite it contained only a relatively low proportion of recycled aggregates. Further projects in the following years had, with a few exceptions, a volume of less than 1,000 m3, which did not noticeably increase the share of R-concrete in Germany.

Major projects that have now become public:

- 2016: Humboldt University Research and laboratory building in Berlin

- 2017: District building in Ludwigsburg (concrete and reinforced concrete)

- 2018: Leonardo-da-Vinci-Gymnasium (grammar school) in Berlin-Buckow

- 2018: Technical Town Hall in Tübingen

This reticence of the parties involved is quite different in our neighbouring countries. There, the awareness of the material and the trust in it are much more clearly pronounced. In Switzerland, for example, the use of recycled concrete is no longer labelled separately. A share of at least 15% in public works measures is intended to contribute to the use of recycled concrete in building construction, even with content of broken masonry.

Current funding measures, for example for the EU funded project "Secondary Raw Materials for Concrete Precast Products" ( SeRaMCo ), represent the starting point here and offer a knowledge platform of international exchange with the aim of carrying out further pilot projects. Within the SeRaMCo project, SySpro member Beton-Betz is one of the main partners who focuses on obtaining the necessary building permits for load-bearing components.

Crushing technology

In the SeRaMCo project, best practice is an essential point. In a first step, Beton-Betz decided to focus on its own concrete residues, which are usually either generated in the production process or result from misalignments from changes in the planning stage. Collected over years, more than 2,000 tons were produced here, resulting in the return of over 2,000 m3 of R-concrete or 100 concrete basements after recycling. A challenge!

For the practical application, Beton-Betz gathered the appropriate know-how via SeRaMCo. The first focus was on crushing technology. SeRaMCo adapted specifications for gentle processes and economic procedures, and the latter have been used as a basis when awarding the contract for crushing work. Concrete remains from our own production can therefore be reused with a mobile jaw crusher and a treatment plant with three decks for the 0/2, 3/16 and 17/50 mm fractions. The sand and grain fractions thus obtained are stored on the company premises and can be integrated into the production process.

The main hurdles in breaking/crushing are dust and noise; therefore, early coordination with the monitoring authority (for industrial plants in accordance with the Federal Immission Control Act) is mandatory. After all, the process takes about two weeks including the pre-crushing and extraction of steel parts with approx. 30 tons/hour, which is significantly slower than the crushing process with over 300 tons/hour. This shows that, as is well known, the effort is not uneconomical.

The real result was not surprising either: the approximately 2.000 tons of material resulted in about 50% of the total of crushed sand, which later turned out fine and required an increased

water demand and influenced other properties.

Figure 1 shows the mobile crushing system and Figure 2 gives an overview of the grain fractions obtained after processing.

Normative requirements for R-concrete

The Directive of the German Committee for Reinforced Concrete (DAfStb) "Concrete according to DIN EN 206-1 and DIN 1045-2 with recycled rock grains in accordance with DIN EN 12620" applies to: the pure use of recycled coarse aggregate grains > 2 mm of types 1 and 2 for the production and processing of concrete in accordance with DIN EN 206-1 and DIN 1045-2 up to a pressure-proof class of C30/37.

The resulting aggregate may be classified in type 1 according to The DafStb Directive: Type 1 must have at least 90% concrete, mortar, concrete bricks and a maximum of 10 % masonry bricks, limestone bricks, porous concrete.

Within the scope of the class WF according to alkali directive concretes up to exposure class

XC4 and XF1 as well as concretes with high water penetration resistance in accordance with

DIN 1045-2, section 5.5.3, may be used. This makes it available for the usual floor and wall components of residential and commercial construction, provided that the proportion of the surcharge is limited to 35 vol.%.

It is assumed that the origin of the recycled aggregate is known. On the basis of their origin, the aggregate contained in the old concrete can clearly be assigned to a harmless alkali sensitivity class. The manufacturer must prove to the user the safe origin with regard to the alkali sensitivity class and document it in writing. This shall be verified by the relevant recognised monitoring bodies.

Concrete technology

In addition to crushing technology, the development of concrete formulation plays an essential role. Here, too, the experts of SeRaMCo-Partners provided crucial clues. The laboratory studies showed optimal results up to a maximum of 10% crushing sand to the targeted 35 vol.% crushed gravel lying on the safe side on 30 vol. % have been limited.

Recycled aggregates which originate from concrete from the concrete producer's production and has been reprocessed without prior use, may be used without restriction up to a share of 5 Vol .% , based on the total mass of the aggregate.

The recipe now found therefore differs from the possibilities in German standards, especially since aggregate fines are not regulated or permitted in the DAfStb Directive. Accordingly, extensive material testing was required.

1. a) Aggregated

For the natural aggregates the use of aggregates in accordance with DIN EN 12620, is fundamentally required. The conformity of system 2+ was demonstrated in compliance with the rule requirements set out in Table U.1 of DIN 1045-2: 2008-08 for all aggregate groups used.

The material composition of the recycled aggregates > 2 mm must comply with the requirements of Table 1 of DIN EN 12620:2008-07, Section 5.8. According to Table 2 in Section 5.8, there are essentially about 20 properties to check:

- Material Composition

- Grain shape

- Shell content

- Fine parts of coarse aggregate

- Tolerance of bulk density

- Frost resistance according to DIN 1367-1

As to 2/16 mm the tests on bulk density yielded 2.36 kg/dm 3 found what is well below the natural material at 2.56 kg/dm3 for 2/8 and 2.64 kg/dm3 for 8/16.

1. b) Fines

The testing of the properties of the fines is carried out analogously to DIN EN 12620 for fine rock grains.

The bulk grain density was set at 2.30 kg/dm 3 which is well below the natural material at 2.63 kg/dm3.

It must also be documented that the fines come from the same production as the above-case crushed gravel and is to be classified as type 1 of known origin.

For the use of fines, it must also be demonstrated that the design according to Eurocode 2 is given. For this purpose, bond tests must be carried out comparatively between normal concrete and R-concrete as a pull-out test according to RILEM, see next graphic.

The results of the tests show that the anchoring or transfer-lengths of the concrete rods are greater than they would be under EC2. At present, the building authority determines the increase factor, which according to the expert should be in the order of 10%.

1. c) Concrete

The tests provided for in the DafStb Directive for concrete, in addition to those referred to in DIN EN 206-1 and DIN 1045-2 researched the following:

- A presentation of the concrete production concept, in particular the determination of the required added water depending on the moisture of the aggregate, taking into account the water absorption in relation to the desired water cement value

- Extended initial tests with changes in consistency, increase of the consistency value according to the dosing chart, moisture content of the coarse aggregate (core moisture and surface moisture).

- Production control per production week with regard to bulk density, water content, water absorption, air content and compression.

The following further tests are recommended by the expert as part of the extended initial research:

- Gap tensile splitting strength according to DIN EN 12390-6

- Modulus of elasticity according to DIN EN 12390-13

- Concrete to detect the frost resistance of the recycled coarse

- Water penetration depth according to DIN EN 12390-8

The important properties of the R-concrete according to the DAfStb directive, correspond essentially to those of normal concrete. If crushed sand is additionally used, then the processability requires an increased cement content. Even with a small proportion of fines, the concrete properties deviate from the normative numerical values, in particular the modulus of elasticity as well as creeping and shrinking effects.

The following trend has been observed for early age strength (3 days) (important for precast members) for C25/30:

- Normal concrete (initial mix) (w/z=0.58, 280 kg cement): ßc,cube,3d = 34.5 Mpa. This value was also obtained for R-concrete.

- R-concrete with additional 10% R-sand (w/z=0.60, 310 kg cement): ßc,cube,3d = 21.2 MPa

Note:

Due to the increased water demand for the fines (30% for R-gravel without fines) the cement quantity was increased by 30 kilograms to maintain the water-cement ratio. This may be bypassed if an innovative concept is adopted, e.g. additional cement replacement can be bypassed by Coal fly ash and limestone flour as well as use of modern fluids. This avoids a negative impact on the life cycle assessment.

Reducing the crushing sand to 5% resulted in a 3-day strength that was only 10% below the strength of the normal concrete (initial mix).The 28-day strength was then 40.1 MPa and was thus just under 10% below the normal concrete (initial mix) with 44.5 MPa.

For the water penetration depth, the R-concrete had a depth of penetration about 10% lower due to the increased fine parts.

The E-module is reduced from 32,000 MPa for normal concrete (initial mix) to 29,100 MPa. According to EC2, the design value is Ecm = 31,000 MPa; the zero-concrete thus barely reaches the strength class C30/37 with Ecm = 33,000 MPa, wherein the R-concrete as C16/20 is assigned.

The structural planner has to take this influence into account. In this case, there may be larger values for the form work jacking. However, extension of time period for removing forms adjusted posttreatment are usually not required.

Accordingly, Beton-Betz had had to perform various tests on structural members to verify creep and shrinkage which is necessary to perform the  usability design.

In autumn 2017, floor slabs components were made of recycled concrete (30% crushed gravel and 10% crushed sand) and subjected to climatic loading on the Beton-Betz storage site to show the shrinkage influences. After the second winter, there were no significant differences to normal concrete at the bottom of the slab panel.

The upper picture shows the floor panels made with R-concrete. The lower picture shows the sight of this floor slab from beneath. Cracks are not visible.

However, an important task for future series production is the control with regard to the residues from small rebar or wires, which are similar to the shredded e-pipes and spacers and other built-in parts that should float upwards when compacting the fresh concrete. The left image shows the tip of a so-called plastic nail.

The creeping behaviour was also investigated on the basis of floor panel components, as a comparison of 18 cm thick hollow slab (double walled with 7 cm shells without core concrete): normal concrete C25/30 and R-concrete 30 % coarse plus 10% crushing sand.

Various preloads were carried out with steel coils and after that a continuous stand with 5 kN load – as a long term test - started in April of 2019. As a result of this permanent load, a steel tension stress of 250 MPa was generated, i.e. about the full load level was reached. The computational concrete compressive stress was 8 MPa.

Due to the lower E-module and the lower tensile strength, the initial deflection at 22 mm was significantly above normal concrete with 12 mm. After 15 weeks, the deformations for both concretes had doubled. There is therefore no discerning effect on creep. However, the structural planner must determine the proper deflection according to EC2, what means in the cracked stage, by using the actual material properties.

The upper picture shows the test setup as a single span (5,40 m ) system with loads on squared timber. The lower image shows the preloads with coils up to the yield limit of the reinforcing steel. The pictures show the deflection measured in the middle of the slab after 7 weeks of permanent load of 5 kN.

Construction approval

The building authority in Tübingen is responsible for the supply area of Beton-Betz, and has already approved some projects with R-concrete. In addition to the usual documents such as execution plans and structural calculations, expert approvals were required about the concrete technology and the EC2 design, although the first project concerns a simple residential building. In that case, exterior and interior walls with 20 cm thickness and 18 cm thick slabs are to be used.

Concrete components produced using recycled aggregates in accordance with the DafStb Directive may be dimensioned in accordance with EC2, I.e. DIN EN 1992-1. The proof of the limit state of load-bearing capacity, the design rules for the anchoring lengths and the limitation of deformations are essential.

The limited state of the load-bearing capacity according to section 6.1 of EC2 is not relevant as the strength values do not differ from normal concrete. The enlarged anchorage lengths according to 9.2.1.4 and 9.2.15 of EC2 affect the floor slabs, since the walls are considered unreinforced or low and dimensioned according to section 12.6.5.2.

The graphic shows a section of the construction of the house with ceiling elements and double walls.

The limitation of deformations relates to the floor slabs (deflections with direct calculation according to section 7.4.3 of the standard) . The recalculation of the component test shows an agreement with this calculation method. The coefficient of crack distribution according to equation 7.19, which describes the torn and uncracked parts of the component, is therefore essential for the deflection. The tested panel body with R-concrete (10% crushing sand) shows  cracked range about 40% larger due to the lower tensile strength. The E-module is also 40% lower, the double deflection can be represented by the theoretical approach according to EC2. Accordingly, the deflection for the reference project reaches 20% higher values with the 5% addition of crushing sand. However, the precast floor slabs have a 30% lower deflection than in-situ concrete slabs, so that an upgrade of slab thickness, concrete grade or reinforcements are not required.

However, the design of the building and assembly stage requires special attention in special cases. In the assembly stage, the deflection also plays a role in larger (from 12 mm) lattice girder top bar diameter (above 10 mm). The deflection is limited to 1 cm after building approval, whereby the E-module of the young concrete is used. Lying on the safe side, until exact approval tests are available, the support spans are limited to 80% of normal concrete.

Conclusion

In the preparation of a reference project, it was shown that the concrete prefabricated factory has only little efforts on application of structural members designed with recycled aggregates.

After further reference projects and statements about the service stage design for use,  a building permit is advantageous to include a comprised testing procedure which will reduce the additional costs. Environmental aspects of innovative concrete formulations for cement replacement can also be integrated, as R-concrete production is already associated with special aspects.

In compliance with the above-mentioned boundary conditions, the decision on R-concrete does not have to be made early during the planning phase. There is sufficient time to draw the attention of the parties to the conditions. At present, the costs are not likely to be significantly higher than those of normal concrete. Even with increased costs, the environmental benefits are obvious. In addition to the protection of gravel storage and landfill c