REQUIREMENTS ON CONCRETE FOR FUTURE RECYCLING


C Müller
Aachen University of Technology
Germany



ABSTRACT. Construction methods adopted within the structural engineering require to be sustainable and ecologically meaningful cycle for mineral building materials. Concrete, as a main construction material satisfies two essential properties required for sustainable building materials. lts production with the use of various secondary raw materials, such as material recovered from the wastes, is possible. As a results, it provides reduction in extraction of raw materials from nature resources and saving in quantity of waste going to disposal sites, and subsequently possible saving in energy and emissions connected with it. However, there are limits to how much material can be recycled and reused (for example, use of 1 m3 of "used" concrete in the production of 1 m3 of new concrete). The installation of a "passport for buildings", which contains information on the used building materials as well as on the Separation capability during demolition, their reuse and recycling could be a long term measure to realize sustainable cycles with mineral building materials.

Keywords: Attached mortar and cement pastew, flensity, Mineral building materials, Modulus of elasticity, Multiple recycling, Design for recycling, Environmental compatibility, Recycling, Reuse, Sustainable development, Water absorption.

Dipl.-Ing Christoph Müller, is a Research Engineer at the Institute for Building Materials Research (Institut für Bauforschung, ibac) of the Aachen University of Technology (RWTH Aachen), Germany, since 1994. His research interests include utilisation of industrial by-products in concrete, especially ecovery of recycled construction and demolition waste for use as concrete aggregates.

 

Introduction

The technical performance and ecological responsibility have come closer to each other in the consciousness of the society. Engineering performances are no longer analysed and evaluated on the basis of technical specifications only, but also on the background of their effect on the environment. With the background of the growing concern about the material and energy consumption since the beginning of the industrial age. The goals are set to conserve natural resources and minimise waste quantity, as weIl as the reduction of further environmental effects like the consumption of non renewable energy and emissions into soil, water and air.

Based on these aims, the "sustainable building material" can be defined as a material that

The properties ot concrete are analysed in this paper with the help of own investigations as well as information in literature with the background of this specification profile.



SECONDARY RAW MATERIALS IN CONCRETE PRODUCTION

Concrete satisfies an essential basic property requirements for sustainable building materials, mainly because its production is possible using various secondary raw materials (wastes for recovery). Besides the reduced extraction of raw materials (specially aggregates) and the conservation of disposal sites, the conservation of non-renewable energy and the emissions connected to it during the production of binding agents (cement) due to the use of secondary raw materials with binding agent properties (such as blast furnace slag, fly ash} can be mentioned. According to the German standard DIN 1045 [2] fly ash can be used as an addition in blended cements, as a concrete addition or as an aggregate in concrete. 67 M.-% (approx. 2.8 million tonnes) of the tly ash produced annually in Germany is utilized in the production of cement. concrete and concrete products [3]. On the other hand. three of the approx four million tonnes of blast-furnace slag produced annually in Germany is used in the production of blast furnace slag cements. A comparison of the energy consumption and the CO2 emissions in the produetion of 1 m3 of concrete of the strength class C25/30 when using a CEM lIl/A 32.5 with 60% blast furnace slag in comparison with the use of CEM I 32,5 R leads, for example, to a reduction in the energy consumption of around 43 % and in the CO2 emissions of around 50 %, taking account of the transportation of the aggregate over a distance of 40 km, of cement over 80 km and the fly ash over 100 km using a truck [4]. This corresponds to investigations in the U.S.A. [1].

 

Principles for the design for reuse and recycling

The multiple use of a material or product represents another way to conserve natural resources and avoid waste. This process is usually termed as "recycling". As regards to execution or design for reuse and recycling, one can differentiate between short-term measures to use building materials in order to design for reuse and recycling as far as it is possible today, and long-term strategies to optimize building materials, their combinations and building units as regards their ability to be recycled.

Basic strategies regarding the choice and the combination of building materials can already be determined and used today. Some basic rules were laid down in the guideline "Design methods for reuse and recycling of technical products" from the Association of German engineers [5], in order to guide engineers to design technical products "for reuse and recycling". Apart from the requirement of waste minimisation in the production process the following three basic rules are applicable for construction principles in structural engineering:

Reduction of the variety of materials,

Questions on the material reuse in concrete production on the background of these basics is discussed in the following sections.

 

RECYCLED AGGREGATES FOR CONCRETE IN BUILDING CONSTRUCTION


T
he evaluation of numerous experiments [6,7] has showed that, in most cases, pure demolished concrete as well as demolished masonry can basically be used as aggregates in the production of new concrete. A proposal for the characterization of concrete with recycled aggregates, on the basis of the results in [6,8]. A guideline "concrete with recycled aggregate" [9] was drawn out by the German Committee on reinforced concrete on the basis of research results, and this is initially applicable tor concrete aggregates made from demolished concrete, later to be extended to aggregates made from mineral building materials (rubble). The guideline applies to the production and processing of concrete according to the German standard DIN 1045 [2] with the restrictions mentioned in [10]. These restrictions refer to the applications of the concrete and the degree of substitution of natural aggregates (primary aggregates) with recycled aggregates (secondary aggregates). The restrictions of the degree of substitution result from the requirement that concretes that are produced under the conditions of the guideline should, without any change or restriction, fulfill the requirements for concretes according to DIN 1045 for the named areas of use in the same way as concretes with primary aggregates. Thus, the static modulus of elasticity of concrete is largely dependent on the properties of the aggregate. Recycling aggregates made from demolished concrete must have a mean density p > 2000 kg/m3 [9], as per the rules of use, this leads to a minimum mean density of the aggregate mixture made of natural sand-gravel (from the rhine area) and recycled aggregate of around 2350 kg/m3. After this, a reduction in the modulus of elasticity of max. 25 % as compared to concretes with natural aggregates with identical compositions is to be expected (Figure 1). Since the density and thus the modulus of elasticity for natural normal aggregates is also capable of large fluctuations, a reduction in the modulus of elasticity up to 30 % is to be expected for other identical (in composition) concretes with sand-gravel from the rhine area in comparison to the sand-gravel from the main area.

Working out the guideline led to basic restrictions in the cycle capability as regards questions in connection with the alkali-silica reaction (ASR) and the frost resistance of concrete. lf this problem is analysed on the background of the basics previously described for recyclable building materials, it can be ascertained that the regulations for concrete production [0/2, 14] for various applications, contradict the basic rule for the reduction and/or minimization of the material variety [5], or at least restrict the recycling capability of the concrete. In both cases (ASR and frost resistance), the problem as regards the recycling capability lies in the fact that "critical" aggregates were used in the initial concrete under harmless conditions, but could be used as RCA in potentially reactive (or possibly harmful) environmental conditions. Aggregates with a reduced frost resistance from concretes from interior building parts could be used in concrete for exterior building parts with freeze/thaw attack. AlkaIi-reactive aggregates that were originally used in a constantly dry environment aggregates and cements with high alkali contents might be exposed to humid conditions when reused as RCA [11].



INVESTIGATIONS



Materials and Concrete Mixes

Aggregate mixtures with defined compositions were produced to investigate the influence of fluctuations in aggregate properties on the properties of fresh and hardened concrete. Table 1 shows the properties of the initial materials (aggregates). Two normal Portland cements which comply with cement standard ENV 197 [12] and conform with strength classes 32.5 R (PC1) and 42.5 R (PC2), were chosen. A sand-gravel respectively sand-recycled-aggregate mixture with a maximum grain size of 16 mm was used in the concrete tests.

Table 1
Properties of crushed mineral huilding materials (Reeyeled aggregates)


The grading curve of all concretes corresponded to A16/B16 according to German Standard DIN 1045 [2]. The aggregates were first dried in a furnace and then wetted by mixing them for 10 minutes with water Wa10min. Cement and the remaining amount of water were then added. The additional amount of absorbed water have been considered in the mixture calculation by weight, not volume. lt is assumed that the aggregates take in this amount of water totally.

Table 3 gives the mixture compositions and fresh concrete properties of the concretes.

Experimental

The consistency was measured as spread on the flow table according to German standard DIN 1048 [13] 10 minutes after mixing. Test specimens for hardened concrete studies were stored 1 day in the formwork, 6 day under water. thereafter in air at a temperature of 200 C and a relative humidity of 65 % up to testing. Various strength and deformation parameters were determined in order to characterize the mechanical properties of the concretes. Cube compressive strength and splitting-tensile strength (150-mm cubes) were tested after 28 days. Stress-strain curves in compression were determined at an age of 28 day on cylinders with a diameter of 150 mm and a height of 300 mm. The strains were measured by the means of strain gauges fitted along the longitudinal and circumferential axis of the specimens. The stress-strain tests on concrete specimens were performed; and cylinder compressive strength, static modulus of elasticity (compression), poissons ratio, and the ultimate compressive strains, were determined.

Results And Discussion

All mixtures were proportioned to reach a spread on the flow table between 450 - 490 mm. The amounts of superplasticiser Iay between 0.23 and 0.81 M.-% (of cement) for w/c = 0.55 and 1.14 to 1.84 M.-% (of cement) for w/c = 0.33. The mechanical properties of the concretes are summarized in Table 2.

The density of the recycled aggregates investigated and thus the dry density of the concrete influence the modulus of elasticity to a much larger extent than the compressive strength (Table 2). Concretes with recycled aggregates are, as regards their deformation behaviour (static modulus of elasticity), to be categorized in the region between concretes with normal and light-weight aggregates. The concretes tend to light-weight concrete with a decrease in the density of the aggregates (Figure 1).

 

 

LIMITS OF REUSE

Limits are set for the "direct" material utilization of crushed concrete as concrete aggregate (i.e. use of 1 m3 of "used" concrete in the production of 1 m3 of new concrete) in its own cycle (i.e. concrete of a specific quality: e.g. C25/30; X0, XC1), even for pure concrete (here: concretes of the same technical quality) as weIl as when considering the existing preparation techniques (mechanical crushing) as weIl as possible crushing techniques (e.g. electro-mechanical [14] or electro-dynamic [15] crushing) in the future. Around 30 M.-% of the crushed concrete results in crushed sand <2 mm during a normal mechanical crushing process for concrete (e.g. in an impact crusher).

Table 2 Mechanical properties of the concretes, tested after 28 days

Table 3 Mixture compositions and fresh concrete properties of the concretes


Figure 1 Influence of recycled aggregates on the modulus of elasticity (own investigations and [16])


One m3 of crushed concrete corresponds to roughly 700 kg crushed sand < 2 mm. lf the aggregate is to be used for concrete in (very) dry (X0, XC1) environment with a strength class C25/30, then 35 Vol.-% of crushed concrete > 2 mm and 7 Vol.-% crushed sand < 2 mm relative to the total aggregate can be used according to [9]. Combined with sand-gravel, this corresponds to a recyclable amount of approx. 515 kg crushed concrete > 2 mm and a recyclable amount of crushed sand < 2 mm of approx. 105 kg per m3 of concrete, i.e. approx. 585 kg crushed sand < 2 mm and approx. 1095 kg crushed concrete > 2 mm cannot be kept in the cycle directly, and a "direct" recovery quota of 27 M.-% relative to the initial concrete results (Figure 2). If one assumes a theoretical digestion of 100 % in the fraction > 2 mm, then the fraction < 2 mm that cannot be kept in the cycle can increase to around 910 kg, while the aggregate > 2 mm could be completely reused. The "direct" recovery quota would, in this case, be around 60 M.-% relative to the initial concrete, i.e. around 1400 kg of concrete would remain in the cycle.

 

MULTIPLE RECYCLING

The possibility of multiple recycling of a building material at the highest possible quality level is part of the realization of genuine material cycles. The question of the multiple material recycling of concrete and the changes in the properties connected with this depends, as has been mentioned in the previous section, heavily on the type of crushing employed crushing. The digestion of the concrete is decisive in this connection. One can differentiate between two basic considerations:

Figure 2 "Waste balance sheet" of 1 m3 concrete


Whereas in the first case, the aggregate recovered (> 2 mm) could normally be reused up to 100 % without a negative influence on the concrete properties, the content of crushed concrete with attached cement paste has to he limited in the new concrete. At the moment digestion rates of around 60 % can be attained with electro-mechanical crushing methods (sonic impulse). This method is nevertheless not competitive compared to common mechanical crushing methods (impact crusher: roughly 1.5 kWh/t concrete) because of its high energy consumption (around 12 kWh/t). The digestion for customary impact crushing is around 6 % [18]. Theoretical as weIl as practical investigations on multiple recycling are thus always connected to an assumption as regards the crushing method employed. At the moment, tests on multiple recycling for common mechanical crushing are being carried out at the Institute for Building Materials Research of the Aachen University of Technology (ibac). An assessment of the test results up till now using corresponding model conceptions indicates, that a reduction in the static modulus of elasticity of max. 35 % is to be expected for a triple processing of concrete. The final screening based on the test data is yet to he carried out.


COMBINATION OF CONCRETE WITH OTHER BUILDING MATERIALS


Further restrictions in the material recycling of concrete can result due to a combination of concrete with other building materials. In recent times an increase in the use of floor screeds based on calcium sulphate can be investigated. While being used as floor screeds on separating Iayer, in the frame of the demolition work, the screed can be easily separated ("if one does it"), in the case of a bond screed, the screed and the concrete slab and/or floor can only be separated whith a lot of effort and have to be crushed together, while a separation would create problems because ot their very similar densities. Assuming customary thicknesses tor screed and concrete constructions, the result would be calculated sulphate contents in the mineral material mixture of up to 5M.-%, which would lie distinctly above the technicalIy admissible values (1 M.-% according to [8]). In this case constructive separatibility ot the building materials is important.

The density of the aggregates is an essential factor as regards the deformation behaviour of concretes with recycled aggregates (Figure 1). lf a relatively large fluctuation of the density is allowed, then the possible degrees of substitution of natural aggregate with recycled aggregate are reduced. lf, tor example, the density of recycled aggregates were restricted to a minimum value of 2200 kg/dm3 (customary values for crushed concrete >2 mm: 2200 to 2300 kg/m3), recycled aggregate could be used when maintaining the restrictions for the crushed sand according to [9] in the fraction > 2 mm combined with sand-gravel of 85 Vol.-% (= 58 Vol.-% relative to the total aggregate). The "direct" recovery quota would then rise from 30% to 46%. The recovery quota would be 55 % by limiting the density ot the recycled aggregates to a minimum value ot 2300 kg/m3.

Concrete is used in combination with masonry in the structure of numerous houses, commercial and industrial buildings. lf one assumes what has always been customary, that during demolition the building materials for the interior as weIl as doors, windows etc. are first removed separately and then the structure work is completely demolished then it can be deduced in which combinations these building materials should be employed, in order to be used as recycling aggregates with the lowest possible effort in the preparation process. An example here is the common use of concrete (density = 2300 kg/m3) and clay brick masonry (brick density = 800 kg/m3, net dry density = 1600 kg/m3). lf the recycled aggregate is to be used in concrete in (very) dry environment according to the present regulations [9] then the construction excluding concrete should consist of a maximum of 10 Vol.-% from vertically perforated clay brick masonry. lf one assumes a lower limit ot 2350 kg/m3 for die density of the aggregate mixture of natural and recycled aggregates, then on the basis of existing investigations in the fraction > 2 mm 90 Vol.-% recycled aggregates can be used if the construction consists 70 Vol.-% of concrete and 30 Vol.-% of clay brick masonry. lf the construction is made up of only 30 Vol.-% and 70 Vol.-% of clay brick masonry, 58 Vol.-% recycled aggregates > 2 mm can be used. The use of crushed sand (< 2 mm) was not considered here. lf the constructions and the aggregates recovered from them contain less concrete, this reduces either the usable aggregate contents, or corresponding changed concrete properties have to be considered.



COMPARISON OF DIFFERENT CONSTRUCTION ALTERNATI VES (EXAMPLES)


lf design engineers should be able to design for reuse and recycling, they need tools to compare different alternatives for construction elements. Such a tool can be a catalogue with different construction alternatives on the same level of technical requirement. These schedules have to be based on the actual state of knowledge, should be renewed after appropriate time intervals and can contain guidelines for the future reuse and recycling. An analysis of various construction alternatives, as in the example shown (Figures 3), can be differentiated at will if aspects of reuse are included in die decision. This assumes that reuse is "basically possible", but also can include the determination (as accurately as possible) of substance parameters of a construction as regards their reuse in comparison to the specifications of the reuse (field of application). In the example shown in Figure 3, the technical specifications for an outer wall were defined based on the coefficient of heat transfer k of approx. 0.44 W/(m2K). The different load-bearing capacity and economic feasability has to be noted at this point. The level of use to be attained is decisive for such evaluations. lf the material life cycle for composite substances and/or for walls with more than one layer is defined according to the building material with the highest mass fraction present, and if one makes it a top priority to use these main components in their own life cycles (this classification is based on the demand, that every material should be hold in it's own cycle as long as possible to avoid wastes), then the evaluation would be different compared to when the utilization on any kind of level is accepted, and if one only attempts to exclude those substances that are either hard to or impossible to reuse when the composition disintegrates. What all the four examples of walls (WA) have in common, is that they consist solely of mineral building materials and thus satisfy a basic requirement as regards the minimization of the material variety.

The avoidance of gypsum plaster (interior plaster) and/or heat insulating plaster with non-mineral aggregates is a further contribution as regards the optimization of the recycling capability. While WA 1 has a carrying function as well as largely being responsible for the total heat conservation, WA 2-4 fulfills these functions with two wall layers and/or two wall Iayers and additional

Figure 3 Examples for walls with a coefficient of heat transfer k 0.44 W/(m2K)


insulation either totally separated or through the "division of labour". ln the case of WA 3 and 4, the mineral fibre is a hinderance during the material preparation; a separation of the mineral fibre boards from the rest of the construction is basically possible, nevertheless with additional effort. With the aim of a complete material utilization independent of the level of re-utilisation, the variations WA 1 and 2 are the most suitable since they can be used without having to be separated when used in road construction. On the other hand, the re-utilisation at the same technical level is either only possible to a small extent or not possihle at all. In the case of WA 3, the mineral components can be used to produce new bricks (i.e. in their own cycle), when separated from the insulation during demolition. Additionally, a theoretical possibility exists of the use as a concrete aggregate and in road construction.

WA 4 offers the possibility of using a part of the facade (facing concrete) in the case of element recycling (i.e. without further effort spent on material preparation). The carrying concrete wall can be and reused in concrete production when separating the insulation during demolition. Separating the concrete from the steel reinforcement does not present any problems and reusing the steel reinforcement is possible. If the insulation is not already separated in the demolition phase, the effort necessary for the material preparation increases, i.e. apart from crushing and sieving, a further wet or dry preparation is necessary. The interior plaster will he enriched mainly in the sand fraction of the concrete demolition. The four examples displayed here show that the question ahout the recycling capability of building parts, even when they consist solely of mineral building materials, cannot be answered with a definite "yes" or "no" ,and the assessment of the recycling capability of mineral building materials and building material combinations is hard to quantify. Even when non mineral components are avoided as far as possible, the assessment might not be uniform depending on the level of reuse and recycling that is to be attained. Furthermore, it is to be considered that for a general ecological point of view, the additional energy needed for further use (compared to the use of a corresponding primary raw material) and the emissions that go with it can certainly be a criterium for excluding a specific Option of utilization.

 

ENVIRONMENTAL COMPATIBILITY

The emission of environmental relevant compounds from cement-based building materials can occur due to the washing-off and/or leaching of essentially inorganic substances (contact with a leachant e.g. in the form of rain or groundwater), due to the emission of fugitive compounds (mainly of organic origin) as weIl as the emission of radioactivity (increase in the natural radioactive exposure). According to the present state of investigations, one can assume that concrete with a dense structure, which contains only standardized and approved concrete raw materials does not represent a danger to soil, water or air [18]. In the future, a scheme should be set up classifying concretes with certain compositions (classification of materials) for certain applications (classification of environments or applications). New materials and/or new material compositions (e.g. substituting natural aggregates by waste material) can then be classified by using the established test procedures and by comparing these results with those of already classified materials.

 

CONCLUSIONS

The concrete with desirable properties can be produced using various secondary raw materials (wastes for recovery), and their use can reduce consumption of natural resources as weIl as the conservation of disposal sites. This can result in the saving of non-renewable energy and the emissions associated with the process. Even if the energy consumption during the production phase, when considering the building materials, products and buildings during their total life span (production, use, demolition and reuse) normally only is a fraction of 15 % of the energy consumption for the total life span [5]. lt will be a significant ecological contribution, when the use of secondary raw materials leads to environmentally compatible and technically equivalent to building products.

An important step in maintaining and encouraging the recyclability of concrete is the ability to separate the concrete construction by-products from other building materials that would either be incompatible in a common preparation process, or would at least restrict the recycling. The recovery of mineral building material mixtures is influenced by the material composition used in the construction. Even the "direct" material utilizability of concrete is restricted in its pure type recovery. At present, the maximum direct recovery quota of concrete during the material processing and reuse as an aggregate is around 60 %, with consideration to possible future development in the preparation techniques.

Another important condition for a technical and ecological successful recycling of mineral building materials is the declaration of the building materials/substances used in the original concrete. One could then examine the critical substance and substance combination. This can be regarded the best method of recovery during demolition and controls the harmful substances. This also true for natural aggregate concrete. For example, use of alkali-reactive aggregates. Concretes with various compositions requires proper sorting during demolition. Therefore, the installation of a "passport for buildings", which contains information on the used building materials as weIl as on the separation capability during demolition, their reuse and recycling could be a long term measure to realize sustainable cycles with mineral building materials.

 

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