This Paper is a Contribution to the International Symposium

"Sustainable Construction:
Use of Recycled Concrete Aggregate"

11-12 November 1998

University of Dundee, Concrete Technology Unit, London (UK)

This Paper is a Contribution to the International Symposium

"Sustainable Construction:
Use of Recycled Concrete Aggregate"

11-12 November 1998

University of Dundee, Concrete Technology Unit, London (UK)

German Committee for Reinforced Concrete (DAfStb) - Code:
Concrete with Recycled Aggregates

ABSTRACT. Until now, there are no guidelines in Germany regulating the use of recycled demolition material as aggregate for concrete according to German Code DIN 1045 (Concrete and Reinforced Concrete; Design and Execution). At the moment, building supervisory authorities and the German Institute of Technology (DIBt) respectively, have to give permission for using concrete made of recycled demolition material for concrete constructions. To change this status, the German Ministry of Research promotes a research program called „Recycling of Mineral Building Materials" (Baustoffkreislauf im Massivbau, BiM), which shall create a code additional to the German Code DIN 1045 to regulate the use of recycled concrete as aggregate. Concrete made with this aggregate is uniform to concrete made with natural dense aggregate.

Keywords: Guideline, Demolition Rubble, Recycled Aggregate, Requirements, Concrete with Recycled Aggregate, Water Absorption Capacity, Superplasticiser.

Univ.-Prof. Dr.-Ing. Peter Grübl is Director of the Institut für Massivbau (Institute of Concrete Construction and Technology) at the Darmstadt University of Technology in Darmstadt, Germany. His main research activities are the sustainable use of building materials in structures as well as the durability and restoration of concrete structures. Professor Grübl has published and lectured widely and gained the Otto-Graf-Prize for his research achievements on the durability of concrete. He is a member in national and international committees, is involved in developing European Codes and is Cluster Co-ordinator for Recycling in Construction in the European Thematic Network.

Dipl.-Ing. Marcus Rühl is a research assistant at the Institut für Massivbau (Institut of Concrete Construction and Technology) at the Darmstadt University of Technology in Darmstadt, Germany. He specialises on the effects of aggregate made of recycled demolition rubbish regarding properties of freshly mixed and hardened concrete and is also involved in teaching concrete technology.

INTRODUCTION

Construction, demolition, reconstruction and restauration of buildings result in high quantities of constructions and demolition waste. Relevant amounts of mineral rubble are used as engeneering material for roads, earth constructions and dikes. In the next years the volume of construction and demolition waste material is expected to rise considerably. Therefore, new fields of application have to be initiated and developed. By postulating a hierarchy in managing, meaning avoidance of waste, prior to recycling, prior to deposition, a German law (Kreislaufwirtschafts- und Abfallgesetz [1], law to avoid waste in the producing industry) forces industry and science to strengthen their efforts finding new solutions.

Recycled aggregate is the result of processing appropriate construction and demolition waste. Thereby it is to distinguish between concrete rubble and mineral building material rubble. The processing leads to crushed sand, crushed stone and crushed gravel, derived from concrete rubble and mineral building material rubble respectively. Figure 1 shows the denomination of the different types of recycled aggregate.

On the basis of the results of a big research project "Recycling of Mineral Building Materials" (Bausstoffkreislauf im Massivbau, BiM), financed by the German governement and the building industry, a German guideline is now elaborated to regulate the use of recycled aggregate derived from concrete rubble. At the end of the R&D project, the guideline will be extended on recycled aggregate derived from mineral building material.

In this paper the content of the guideline "Concrete with Recycled Aggregate derived from Concrete Rubble", released by the German Committee of Reinforced Concrete (Deutscher Ausschuß für Stahlbeton, DAfStb[2]) is presented.

The guideline consists of two parts. Part one, named "Concrete Technology", deals with particularities referring to the concrete production and handling, which have to be complied with the manufacturing process. In part two, named "Aggregate such as Crushed Stone and Crushed Sand derived from Concrete Rubble" the requirements are listed, which the recycled aggregate has to fulfil in addition to the requirements for concrete aggregate according to German Code DIN 4226 (Aggregate for Concrete: Items, Marking and Requirements) for natural dense aggregate.

PART 1 CONCRETE TECHNOLOGY

The first part of the guideline is subdivided into the following chapters:

• Scope
• Requirements for Concrete Properties
• Requirements for Concrete Composition and Production
• Initial Test
• Design Values
• Monitoring

Scope

This paragraph provides which construction components are allowed to make with concrete containing recycled aggregate. Table 1 gives an overview of these components.

The above mentioned restrictions for outdoor use make sure, that no material with potential for alkali silica reaction is used even though special cement has been used or will be applied. The guideline, however, allows in general for reinforced concrete the addition of recycled aggregate up to 5 % by mass., referring to the total aggregate mass.

Requirements for Concrete Properties

Concrete containing recycled aggregate has to comply with the same requirements as concrete made with natural aggregate. This principle makes it easy to use concrete made with recycled aggregate, because during the planning stage of a construction in general it is not clear, whether there will be in use a concrete containing recycled aggregate or a concrete made with natural aggregate.

Requirements for Concrete Composition and Production

The values of maximum amount of recycled aggregate in concrete as shown in Table 2, reflect the cognitions made so far and are chosen in such a way, that there is no loss in the margin of safety of the complete construction, when they are applied.

When producing concrete with recycled aggregate, the same measures have to taken into account when natural dense aggregate are used. However, the water absorption capacity of this material [3] cannot be neglected. Figure 2 shows the relation between the dry volume density of the recycled aggregate and the water absorption, given for 10 minutes and 24 hours of water absorption time. Because of this effect, the core moisture of the aggregate has to be known for the dosage of aggregate and water. Therefore an "effective water-cement-ratio" is defined. For calculating, the ten minute water absorption is appropriated [4], because during this time the water absorption value reaches up to 90 % of the 24 hour water absorption value.

If the core water content has approximately the magnitude of the 24 hours value of water absorption, no withdrawal of mixing water by the aggregate will take place during the handling of the concrete.

Initial Test

Due to water absorption and the core moisture content of recycled aggregate, a strong consistency change between the time of mixing and fitting into form boards can accure. Adjusting the consistency by adding water on site is not allowed. That is why the guideline regulates within the initial test to create an instruction regarding the amount of superplasticiser necessary to compensate the change in consistency. Figure 3 shows the relation between the amount of superplasticiser dosage and the change of the flow table test value for a certain type of superplasticiser

This instruction assures, that the consistency laid down in the contract, will be attained reliably on site. After addition of superplasticiser, the workability of concrete made of recycled aggregate does not differ from the workability of concrete made of natural aggregate.

Design Values

Building components made of concrete with recycled aggregate, can be designed with the same characteristic values as components of concrete made with natural aggregate. This is due to the fact, that the recycled aggregate can only be used mixed with natural aggregate. The portions were chosen very carefully as to comply with this demand. Even in constructions in which deformations have to be considered, the hardened concrete’s properties, resulting from the use of recycled aggregate, have to be evaluated by tests.

Monitoring

To make sure, that there are no impurities or foreign substances in the recycled aggregate, each delivery has to be checked organoleptic. To detect material deviations of the recycled aggregate, the dry volume weight in each production week and the water absorption capacity (10 min and 24 hours) have to be determined. To avoid the not allowed use of concrete made from recycled aggregate in prestressed concrete elements, the concrete has correspondingly be marked in the producers type description.

PART 2 AGGREGATE SUCH AS CRUSHED GRAVEL AND CRUSHED SAND DERIVED FROM CONCRETE RUBBLE

Part two of the guideline provides, with which the recycled aggregate derived from concrete rubble has to comply. It is subdivided into:

• Scope
• Preparation Technology and Preparation Process
• Requirements
• Conformity Certificate
• Production, Packing, Transport, Storage and Identification

Scope

For the time being the requirements stated in this part, cover recycled aggregate derived from concrete rubble. Later on, when the research results are available, they will be enlarged on aggregate derived from mineral building material.

Preparation Technology and Preparation Process

The properties of the recycled aggregate depend on the preparation process. Therefore special care has to be taken into this process. It influences mainly the grain distribution and the grain form. A combination of a jawcrusher in the first hackling phase and a rotating crusher in the second hackling phase, leads to the best results regarding size distribution and shape. By application of wet processing harmful substances can be eliminated nearly complete. Furthermore, the grains will be stripped of crushing dust which is advantageous regarding concrete technology.

Basic requirement for producing high quality recycled aggregate is the selection of the material entering the preparation process. This presumes a well organised acceptance and storage of the incoming material as well as an effective material management.

Requirements

The recycled aggregate must comply with the same requirements as natural aggregate laied down in the German Code DIN 4226 and in additional demands depending on following attributes:

• Compound
• Dry Volume Density
• Water Absorption
• Environmental Constraints

 

Requirements regarding the compressive strength of the concrete of the demolished construction are not fixed. Instead of this, the processing procedure is specified. For the production of crushed gravel and crushed sand only precrushed concrete with grain size larger than 32 mm is allowed to use. This preceding crushing of the concrete represents a selection process concerning the grain strength. Experimental data show, that recycled aggregate derived from concrete rubble which passes this process does not have significant influence on the compressive strength of concrete made with this recycled aggregate. The guideline specifies periods of time and the frequency for checking the properties fixed in the requirements.

Conformity Certificate

The producer of the recycled aggregate has to make transparent, by which means he fulfils the requirements. This comprises the production equipment as well as the quality management system. Both is to prove against the third party.

Production, Packing, Transport, Storage and Identification

Recycled aggregate is provided and classified in the same way as natural aggregate. It can be supplied neat or mixed together with natural aggregate. If so, the mixing ratio has to be specified in % by volume. Requirements depending packing, transport and storage, do not differ from the requirements for natural dense aggregate. Recycled aggregate has to be identified on the delivery note.

CONCLUSIONS

The new German regulation "Concrete with Recycled Aggregate" allows the use of recycled aggregate derived from concrete rubble for producing concrete in accordance with the German Standard DIN 1045. Due to it, the reuse of a big part of the construction and demolition waste on a high quality level is possible. For the time being for outdoor use of the concrete there is a restriction. The use is only allowed, when the insensitiveness of the recycled aggregate against alkali silica reaction is proven. On this topic further research work has to be done. The resistance of concrete made with recycled aggregate against frost attack seems to be not a severe problem. But to prove this property, a test method with more evidence is needed. The clarifying of both questions is necessary for enhancing the amount of recycled aggregate for the production of concrete. Furthermore to enlarge the volume of reuse of construction and demolition waste, a regulation for the use of recycled aggregate derived from mineral building material rubble is needed, which will be released in the next future.

REFERENCES