R.-R.SCHULZ
Fachhochschule Frankfurt am Main, Germany
Demolition and Reuse of Concrete. Edited by Erik K.Lauritzen. © 1994 RILEM.
Published by E & FN Spon, 2–6 Boundary Row, London SE1 8HN. ISBN 0 419 18400 7.
Abstract
This paper reports about the development of natural aggregate resources and substitute potentials in Germany. It shows which formal prior conditions are necessary to recycle building rubble as concrete aggregate. Steps against the varying properties of the processed rubble as particle density and water absorption are discussed from a practical point of view. It is reported about the first attempts on performing large scale tests concerning the reuse of concrete and masonry as concrete aggregate in Germany.
Keywords: Concrete Aggregates, Priorities for Reuse, Guidelines, Standards, Properties of Aggregates, Bulk Density, Water Absorption, Pilot Projects,
1 Introduction
Recycling of structural material in Germany is still far from getting a real closed-loop-recycling-system. Though building rubble is increasingly recycled more than discarded as sanitary landfill, valuable demolition rubble is still reused for secondary purposes as subbase for roads and noise protection walls. If a closed-loop system is to be achieved, this material has to serve its original purpose. The commonly used procedure can be compared with a declining spiral. That is why the recycled material is decreasing in usefulness and value.
Probably a real closed-loop cannot be achieved. Just in a few exceptional cases the quality of the recycling products will be similar or even higher than the quality of the original material. Nevertheless reuse on the highest level should be aimed at to delay the descending motion of that spiral. How can construction rubble be reused according to its former function?
2 Natural aggregates and potential substitutes
2.1 Natural gravel and sand
One part of the reduced consumption of river gravel and sand in the time between
Fig. 1. Development of aggregate consumption (Data provided by industrial associations)
1970 and 1990 (Fig. 1) may refer to the trade cycle. The other part can only be explained by diminishing resources, which force reduced consumption. Despite the slight increase in the last few years the tendency of reduction is inevitable. Shortage and price rise (Fig.
2) will accelerate as a result of increasing restrictions. Protection of drinking water reservoirs and environmental reasons make it much more difficult to obtain prospect licences. Some owners of gravel-pits are reducing their production in order to keep the deposits on a long-term basis.
2.2 Crushed rock
The most important substitute for gravel and sand is crushed rock (Fig. 1 and 2). That is why its usage has increased nearly by the factor of ten in the last decade. Until then crushed natural stone was preferred for special purposes, which means it was used where a good cohesion of fresh formed concrete products (paving blocks) or an improved abrasion resistance or a high flexure or splitting or impact strength is needed (pavements). Concretes which are continuously exposed to high temperatures need
crushed rock with a reduced quartz content. Because of the scarcity of gravel and sand and meanwhile balanced prices (Fig. 2), crushed stone is used for nearly all purposes in concrete production.
2.3 Lightweight aggregate
Today artificial lightweight aggregate is preferred for the production of porous insulation concrete bricks and walls and is less used for structural concrete. A potential might exist for the substitution of natural aggregate but this is restricted by the inevitable high consumption of primary energy.
Fig. 2. Returns for natural aggregate without carriage and tax, Mergelsberg (1989)
Fig. 3. Demand for concrete aggregates in 1990 compared to the amount of demolition rubble in 1989
2.4 Industrial byproducts and recycled aggregates
The sources of alternative structural material are also restricted. This is true for industrial byproducts (slag, ashes from power and incinerator plants) as well as for recycled building material. In principal, valuable primary supplies can be saved by the usage of those alternatives but we have to consider that the whole demand for concrete aggregate is much larger than the amount of substitutes. Additionally, it must be considered that
industrial byproducts are already being reused at a very high rate (slag: 100%). Fig. 3 shows the difference between the total aggregate demand and the supply of recycled material in the westem parts of Germany (1989/1990). Only a part of the 23 million tons of building rubble per year is capable of being reused as concrete aggregate. It is known that the sand fraction of recycled material will cause problems and cannot be reused for concrete.
2.5 Priorities for utilization
Valuable rubble, which could be used as concrete aggregate is also attractive for road building purposes. For this reason priorities must be set to distribute the raw materials among the various fields of application. The main task is to save the natural resources and to reuse the alternative materials in the most appropriate way.
At first it has to be found out how the most appropriate method of usage and the optimum level of reuse for all purposes can be achieved. Valuable natural aggregates should not be dissipated for minor purposes (foundation, equalizing beds) in order to keep them for tasks where they are urgently needed; for example, bridges and other high loaded structural elements. In all other cases materials should be used which just comply with the reduced requirements. That is why, for example, in dry environments aggregates with reduced frost resistance can be used. This could also be a typical application for alternative structural material. Normally all this will be governed by the price, but with recycled aggregates, additional difficulties have to be conquered.
2.6 Aspects of performance
Beside questions of distribution and transportation the casting of concrete with different aggregate types requires additional storage capacity. Average ready mixed concrete plants have got limited facilities. They are not able to offer concrete with crushed rock or lightweight aggregate in addition to concrete with gravel and sand. There is a certain risk to frequent changing in the type of bin content because mix-up or colour change may occur. To avoid complicated arrangements and to avoid problems as mentioned above the number of bins has to be increased. Besides the necessary investments there are some particular features which have to be considered when casting concrete by using recycled aggregates (see chapter 4 and 5). For this reason recycled aggregate has to be significantly cheaper than natural aggregate to maintain a profit by using it
3 Specifications
3.1 Standards and permissions
Numerous scientific results prove that the properties of concrete with crushed concrete aggregate can be quite similar to those of normal concrete. Despite this it is very difficult to reuse demolition rubble as aggregate for structural concrete in Germany.
Concrete for structural elements and stiffeners of reinforced or plain, normal or heavy
concrete with dense structure, has to be cast according to DIN 1045. Aggregates have to conform to DIN 4226. Standardized concrete requires standardized aggregate since DIN 1045 has been installed by law as a convention of technic. Except for crushed brick, recycled aggregates do not correspond with the standards. Building material, structural elements and construction types, which are not commonly used and approved (new material), may be used if their practicability has been proved. General permissions or test marks are needed. Alternatively a case permission has to be obtained from the major or assigned authorities. As origin, components and properties of recycled aggregates are changing continuously, there is no basis for a general permission or a test mark. That is why only the case permission procedure remains, which might be justified if the constituents of a larger quantity with nearly uniform origin and/or quality could be evaluated. Nevertheless, even this procedure is time consuming and expensive and has to be repeated in each case if the conditions are changing. Standardization may be considered in case of general permission, but the material must be produced by several companies and used during a longer period.
When will a case license procedure be worth while? Until now processed rubble is not much cheaper than normal aggregate. On the other hand the properties are not better than those of natural aggregate. More likely they are worse. In addition, manufacturing of concrete with recycled aggregates is more difficult. Taking this into account, it is easy to understand why this kind of recycling is not popular. Despite this, political institutions should not only regard economical aspects. They are responsible for environmental interests and for supplies, as well. That is why initiatives should be expected at least from this side. Unfortunately, the present financial problems, which are a consequence of the unification, prevent the government or local authorities from performing experimental projects. As it is obvious that the increasing scarcity and the decreasing land fill areas will strongly influence the price, investments will be encouraged very soon. After all, it has to be regarded that the amount of processed rubble can never substitute the whole demand of aggregate. Additionally, the use of alternative resources as aggregate competes with other uses, for instance, in road construction.
3.2 Specifications for concrete pavements
As opposed to structural and stiffening elements, ‘used material’ has been recently allowed for use in pavement concrete. To support technical progress, the latest edition of ZTV Beton-StB (1991) remarks that recycled material can be used if tender conditions permit that. However, the requirements for concrete aggregate were tightened up. The aggregates for the surface layer must conform to DIN 4226 or TL Min StB and must also meet the extended requirements for frost and thawing agents (eFT). The aggregates must have a reduced content of expanding organic particles (eQ). Road classes SV and I to III require better particle shape and an adequate resistance against abrasion. It will be difficult to meet all these requirements with recycled aggregates. That is why recycled material may be considered as suitable just for the second layer (see chapter 6.3).
4 Properties of recycled aggregate
4.1 Composition
The limitation of main and secondary constituents is an important prior condition for sufficient equal and calculable properties. The classification, testing and evaluation of processed masonry and concrete rubble can be performed according to Stichting voor onderzoek (1984). As the composition of the products varies even during one day to a wide extent the manufactured concrete will vary as well. It is recommended to homogenize at least daily production by preselection, stockpiling, shifting and mixing enough that it could be regarded as a unity. Procedures like this are known for example from the cement production.
To evaluate the homogeneity and to obtain data for mixture design, quick tests are recommended. Necessary tests, simple alternatives and depending parameters are discussed by Schulz (1986 and 1988).
4.2 Contamination
Not only from a technical but also from an economical point of view the question has to be answered how far contaminants have to be removed and which maximum content of contaminants could be tolerated. Recycled aggregates should not contain more harmful constituents than are allowed to normal aggregate (s. DIN 4226). Special kinds of contaminants as asphalt, paper, plastics, glass etc., which frequently occur with recycled rubble, are unusual for normal or lightweight aggregate. That is why DIN 4226 gives no information.
As the legislator demands separate demolition, there are good chances to obtain more clean and uniform material. Another way to influence the amount of contaminants will be the price for the reception of demolition rubble, which has to depend on the quantity of contaminants. As contaminants are concentrated in the fine particles of rubble and water absorption is increased, it is recommended not to reuse fine rubble.
Fig. 4. Correlation between bulk density and particle density
4.3 Particle density and bulk density
The particle density is a very important value for mix design and for the accuracy of batching. Properties like strength and modulus of elasticity can be calculated by the particle density, too. Variations in the composition of recycled material also produce variations in the particle density. Additionally the accuracy of batching is influenced by the moisture content of the particles. This is scattering at wide range and also depending on the composition of the demolition rubble, on its origin and on the kind of storage. That is why mix design has to be adjusted frequently. Consideration of these influences presumes that the particle density and the moisture content are tested in short intervals.
Testing of particle density is a difficult and time-consuming procedure. The disadvantages become apparent when the production is scattering at wide range and the test has to be performed frequently. Compared with this, testing of bulk density is much simpler and faster. That is why it is of great interest whether the accuracy will meet the requirements and whether the particle density can be calculated. According to experiences with lightweight aggregate there is a strong correlation between particle density and bulk density. This is true for aggregate from concrete and masonry rubble, too (see fig 4). Bulk density is used as compliance criteria for the production of lightweight aggregate. Whereas lightweight aggregates have a low and uniform moisture content, this cannot be expected for recycled aggregates. That is why the recycled aggregate has to either be dried before testing or otherwise be soaked with water. Drying is on the one hand time consuming and on the other hand accuracy suffers from soaking, because it is difficult to assure a complete saturation and a surface-dry-state simultaneously. Moisture on the surface will influence the degree of dispersal and the
bulk density.
Fig. 5. Correlation between bulk density and water absorption
4.4 Water absorption
The absorption capacity of the aggregate poses severe problems for the production of concrete. The properties of fresh and hardened concrete cannot be predicted with the needed accuracy. Since the pores are not saturated, they can extract water from the paste, which will impair workability and reduce the water cement ratio. That means, beside the influences of particle strength, particle shape, particle composition, and mix composition by scattering particle densities the properties of the cement paste will also be affected significantly.
Water absorption after 30 minutes is normally used for lightweight concrete mixtures to estimate how much mixing water will be absorbed by the particles and to determine the supplementary water addition. This part of water is not available for the cement paste.
Water absorption after 24 hours of storage in water provides the necessary information about the saturation behaviour for lightweight aggregate concrete which has to be pumped.
Information about the water absorption of recycled material is less useful because the soaking velocity will be retarded with increasing particle size. Additionally it is depends on the consistence and from the existing water content of the aggregates, which scatters at a wide range, as well. Only if the aggregate is dry, test results may be useful estimates for the maximum quantity of expected water absorption. In these cases water can be batched up to this limit to improve consistence. This will reduce the danger to exceed maximum water/cement ratio. The values of the initial tests are likely to be exceeded if
aggregates have an unknown moisture content.
5 Effects on concrete technology
5.1 General
As recycled aggregates are porous, experiences from the lightweight aggregate concrete technology can be adopted to a wide extent. Water absorption of the aggregate, influence of the particle strength on the concrete strength, increased deformability, that means reduced modulus of elasticity and the retarded but in total higher shrinkage have to be mentioned.
5.2 Workability of fresh concrete
Experiences show that the efficient design of consistence and the observance of the effective water/cement ratio suffers from changing water absorption. The water absorption capacity and absorption rate of processed rubble depends from the kind, quantity and particle size of each constituent and from the quantity of free water.
5.3 Consideration of water absorption in practice
The following methods are considered by using experiences from lightweight concrete.
1. Increasing the supplementary water addition according to the results of water absorption test after 30 minutes.
2. Saturation of the aggregate by pre-soaking.
3. Water addition on the construction site according to the required workability.
Further modifications or combinations are possible but all of them have both advantages and disadvantages, Moser (1991).
Method 1: Experiences with lightweight aggregates show that the water absorption after 30 minutes is a characteristic value of sufficient accuracy, which has proved itself in practice. The supplementary water addition is increased by this quantity. As mentioned above, the existing water content of recycled aggregate cannot be predicted with the same accuracy during production. For this reason free water may exceed the calculated maximum content. More than lightweight aggregate the absorbed quantity of water depends on consistence. For this reason tested water absorption coefficients do not directly correspond to the absorption behaviour in concrete. The more liquid the fresh concrete becomes, the more water will be absorbed. If the total amount of supplementary water is added at once into the mixer, the concrete becomes too liquid at first. Then there is a certain risk that concrete spills out of the mixing unit each time truck mixers accelerate
Method 2: This method is theoretically the best and most certain one. But it is not the most liked in practice because several necessary arrangements have to be made. Though pre-soaking of lightweight aggregate has not proved itself because the water is kept in the
pores over a long period and affects the insulation behaviour of the concrete, it may be suitable for recycled aggregate concrete because this material will not be used for insulation purposes. Besides this, the pore structure of crushed rubble will help to support the drying process. The aggregate must be completely saturated but surface dry. This will avoid any water exchange with the fresh paste. Under these circumstances water absorption tests can be abandoned. Nevertheless additional equipment and storage capacity will be necessary to prepare the aggregate during a longer period and to keep it in a homogeneous saturated condition. Spraying of the aggregate requires a sufficient shape and size of the stockpiles and a drained underground. Alternatively the aggregates could be stored in water basins. Though water absorption may be more thorough, there are some severe disadvantages. First the efforts for filling and discharging the basins have to be considered and the efficiency depends on the size and capacity of the basins. The aggregate may be too wet, which could influence the effective water/cement ratio. The latter method is recommended in connection with wet cleaning techniques. In any case the moisture content has to be adjusted up to the batching process. That means the aggregate must be sprayed with water on the way from the stockpile to the batch bin.
Problems may occur in winter when the water on the particle surface and the spraying equipment is going to freeze, Moser (1991). Compared with other methods the weight of
Problems may occur in winter when the water on the particle surface and the spraying equipment is going to freeze, Moser (1991). Compared with other methods the weight of