Manufacturing of Concrete Products and Precast Elements - Helmut Kuch - ebook

Manufacturing of Concrete Products and Precast Elements ebook

Helmut Kuch

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Opis

The flexible use of prefabricated concrete products requires a continuously increasing diversity with regard to fresh concrete mix designs and properties, moulding processes, surface finishes and product characteristics. This trend imposes ever-higher requirements on manufacturers of the associated production equipment and on precast plants. The main goal is to implement a flexible production system in all processing stages. The relevant correlations and interactions need to be thoroughly considered and evaluated in order to ensure that concrete products and precast elements are manufactured to the required quality standard. To date, no comprehensive description of these correlations has been published in the relevant literature. This richly illustrated book closes the gap by describing the basic principles of the production processes, the fundamentals of materials, the composition of the concrete mix, and the equipment used for concrete production. Clearly arranged chapters detail the production processes and equipment used to manufacture small concrete products, concrete pipes and manholes, and precast elements. The authors have used their many years of experience in the field of precast technology and their close ties to the industry. Their aim was to integrate modern testing and calculation methods from neighbouring disciplines into precast technology. This includes, for instance, modelling and simulation of the workability behaviour of mixes, implementation of the latest advancements in machine dynamics to the design and engineering of production equipment, and the use of stateof-the-art measuring and automation technology for quality control purposes.

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Manufacturing of Concrete Products and Precast Elements

VLB record Helmut Kuch / Jörg-Henry Schwabe / Ulrich PalzerManufacturing of Concrete Products and Precast Elements Processes and Equipment Verlag Bau+Technik GmbH, 2010ISBN 978-3-7640-0538-2

©  by Verlag Bau+Technik GmbH Produced by: Verlag Bau+Technik GmbH, P.O. Box 12 01 10, 40601 Düsseldorfwww.verlagbt.de Originally published in German in 2009 as: Herstellung von Betonwaren und Betonfertigteilen

Manufacturing of Concrete Products and Precast Elements

Processes and Equipment

Dozent Dr.-Ing. habil. Helmut Kuch

Prof. Dr.-Ing. Jörg-Henry Schwabe

Dr.-Ing. Ulrich Palzer

Foreword

Concrete is one of the most important building materials of our times. Concrete products and precast elements that are prefabricated on an industrial scale fully utilise the performance potential of concrete whilst offering major benefits with regard to the construction process. The flexible use of prefabricated concrete products results in a continuously increasing diversity with respect to

– fresh concrete mix designs and properties,

– external geometry and design,

– surface finishes in terms of colour and design and

– characteristics of the finished product (quality).

These factors impose corresponding requirements on both the manufacturers of the associated production equipment and its operators, i.e. precast plants.

The main objective is to implement a flexible production system with respect to all four components of the production process, i.e.

– material-related aspects,

– technological processes,

– technical equipment and

– characteristics of the finished product (quality).

These components need to be carefully considered and evaluated to ensure that the concrete products and precast elements are manufactured to the required quality standards.

The relevant literature does not include any comprehensive discussions of these relationships to date.

This book is based not only on the authors’ many years of experience gained in the field of precast technology at the Bauhaus University of Weimar and at the Institut für Fertigteiltechnik und Fertigbau Weimar e. V. (Weimar Institute for Precast Technology and Construction), but also on their close ties to the industry.

The authors’ aim was to select state-of-the-art testing and calculation methods from neighbouring disciplines and apply them to precast technology. This includes, for instance, modelling and simulation of the workability behaviour of mixes, application of the latest advancements in machine dynamics to the design and engineering of production equipment, and the use of state-of-the-art measuring and automation technology for quality control purposes.

In the English translation, the system of mathematical symbols and designations used in the German version was intentionally retained. The same applies to the metric units of measurement used for physical parameters.

We thank all those who contributed to the publication of this book, in particular Prof. Dr.-Ing. habil. Dieter Kaysser, Dr.-Ing. Steffen Mothes and Dipl.-Phys. Günter Becker for their active involvement.

We are also grateful to numerous companies for providing photographs.

The authors particularly appreciate the assistance of the following industry partners in supplying useful information and images during the preparation of this book:

Avermann, Osnabrück; BETA Maschinenbau, Heringen; BHS Sonthofen; Dreßler Bau, Stockstadt; EBAWE, Eilenburg; Eirich, Hardheim; Elematic, Nidda; Fritz Hermann, Kleinhelmsdorf; Hess, Burbach-Wahlbach; HOWAL, Ettlingen; Knauer Engineering, Geretsried; KOBRA, Lengenfeld; Hawkeye Pedershaab, Bronderslev, Denmark; Liebherr-Mischtechnik, Bad Schussenried; NUSPL BETONWERKSEINRICHTUNGEN, Karlsruhe-Neureut; PRAEFA, Neubrandenburg; Prinzing, Blaubeuren; Rampf, Allmendingen; REKERS, Spelle; Ruf, Willburgstetten; Schindler, Regensburg; Schlosser-Pfeiffer, Aarbergen; Schlüsselbauer, Gaspoltshofen, Austria; Sommer, Altheim; Technoplan, Seyda; Vollert, Weinsberg; Wacker, Munich; Weckenmann, Dormettingen; Weiler, Bingen; Wiggert, Karlsruhe; ZENITH, Neunkirchen

Our special thanks go to the following individuals who supported us in many ways in designing and publishing this book:

Heike Becker

Dipl.-Ing. Jens Biehl

Dipl.-Ing. Frank Bombien

Dipl.-Ing. Tobias Grütze

Dr.-Ing. Barbara Janorschke

Dipl.-Ing. Jürgen Martin

Kerstin Meyer

Dr.-Ing. Simone Palzer

Dipl.-Ing. Kerstin Schalling

Dipl.-Ing. Christina Volland

Dipl.-Ing. Markus Walter

The authors

Weimar, August 2010

Introduction

Building with state-of-the-art precast reinforced concrete construction evolved into an industrial construction method only over the last 60 years or so. The first attempts to erect buildings using structural elements made of precast reinforced concrete were made at the turn of the 20th century. Examples include the casino in Biarritz (Coignet) in 1891 and prefabricated gatekeepers’ houses (Hennebique, Züblin) in 1896. This trend continued across Europe and in the United States during the first half of the last century, and precast technology saw its actual breakthrough after World War II. The huge demand for housing confronted the construction industry with an enormous amount of building work. During this period, the systems developed by the French (e.g. Camus, Estiot) and Scandinavians (e.g. Larsson, Nielsen) provided the key momentum towards large-panel construction. The increasing lack of skilled workers shifted the emphasis to factory production and resulted in the breakthrough of precast products. In addition to systems for industrialised housing construction, the increase in related education and training programmes led to the full emergence of skeleton construction based on structural framework using columns, beams and wide-span floor slabs. For both industrial and sports facilities construction, standardised product ranges were developed that included precast columns, prestressed double-T beams and purlins or shed roofs.

Parallel to these processes, other concrete products were developed for the associated infrastructural facilities above and below ground.

Prefabrication of precast elements and the virtually countless variety of small concrete products require the use of appropriate production equipment. The German building materials machinery sector made a particularly significant contribution to respond to this need, which is why German equipment manufacturers are global market leaders today. A major factor that had to be taken into account were ongoing developments in the materials field, which have a significant impact on precast technology.

About 25 years ago, concrete was still a conventional ternary mixture comprising cement, water and aggregate. In addition to these three main constituents, it now contains additives (e.g. workability agents or retarders) and additives (e.g. coal fly ash). This trend enabled significant widening of the performance range of concrete. Modern product ranges include high-strength, fibre-reinforced and self-compacting grades.

Further material developments in the precast sector include optimisation of lightweight concrete by adding suitable lightweight aggregates (e.g. expanded clay, shale or glass, pumice, lava, lightweight sand, perlite) or using artificially introduced pores or foams. New areas of application are opening up for high-performance concretes containing fine-grained aggregates and textile reinforcement in combination with new design and placement principles. Chemical additives play a crucial role in making the material more sustainable, enabling more slender elements, and utilising concrete in a specific and economical manner.

The current state of the art also includes reinforcing fibres that are added to enhance the viscosity, strength and crack resistance of concrete, which would otherwise remain brittle. The use of textile mesh reinforcement or various fibres (carbon, glass, basalt, polymers) is fostering the development of new concrete grades with a better performance in terms of their impermeability, structural design and strength, as well as their material and surface qualities.

Strengthening concrete with fibreglass-reinforced plastics has opened up new markets on account of their new material quality parameters (e.g. corrosion resistance, electrical insulation, non-magnetic properties and resistance to chemical attack).

New developments in the concrete and precast industry are driven not only by the rising costs of energy and raw materials, but also by increasingly stringent product quality standards with respect to thermal insulation, durability and resistance of the products to environmental effects and other characteristics that depend on their intended use.

The design options for concrete products will extend their range of application. Such options include various concrete surface finishes that are achieved by washing, fine washing, acid washing, blasting, flame cleaning, grinding and polishing, by applying stonemasonry techniques, by creating coloured surfaces as a result of adding various cements, mineral aggregates and pigments, by painting or by photo-engraving.

This diverse range of design options for the concrete products requires suitable manufacturing processes and equipment.

These aspects are the focus of this book. It has been written for everyone involved in the production of prefabricated concrete products, including:

– manufacturers of production equipment,

– users and operators of such equipment, i.e. concrete and precast plants,

– students enrolled in related degree courses and advanced training,

– researchers and developers of processes and equipment in the field of precast technology.

The current situation is characterised on the one hand by increasingly diverse concrete products, and on the other, by the great degree of variety and numerous control options offered by commercially available equipment.

The aim is to develop a flexible manufacturing process for prefabricated concrete products that conform to a high quality standard. This necessitates clarification of the complex relationships between the various components of the concrete production process, namely:

– material-related aspects,

– technological processes,

– technical equipment and

– characteristics of the finished product (quality).

In many cases, however, these factors are still being dealt with on an empirical basis.

Mastery of these complex processes requires that all parties involved must cooperate as closely as possible. This applies, in particular, to the manufacturers and operators of the production equipment. To achieve this goal, they should have a sound knowledge of the basic underlying principles and interactions.

From their many years of experience gained in close collaboration with industrial partners, the authors concluded that this was exactly where a real gap existed in the literature on precast technology, which is why they decided to write this book.

Chapter 1 outlines the basic principles required to understand the interactions referred to above.

The process for manufacturing concrete products is first described on the basis of the process elements, process layout and process flow. The processing behaviour of concrete is described with particular attention paid to moulding and compaction of the concrete mix. The associated processing parameters are defined.

This chapter also describes the raw materials used to produce the concrete mix whilst also looking at the concrete mix design in greater detail. The evolution from a ternary mixture to the current quinary system is also discussed. The empirical solutions commonly applied in the past will be increasingly replaced by process optimisation and simulation exercises that take account of the properties of the concrete mix, fresh and hardened concrete as well as their testing.

The fundamentals of the products are outlined starting with a clear definition of the concrete products and product groups whose manufacture is described in subsequent chapters. This is followed by a discussion of the requirements for the product properties and a description of the associated testing methods.

In the chapter describing the basic aspects of the equipment, reference is first made to the various types of vibration equipment, which is crucial for the manufacture of concrete products.

The current situation with regard to modelling and simulation of the workability behaviour of mixes is then described. This option to evaluate processing work steps in conjunction with laboratory-, pilot- and industrial-scale testing is becoming increasingly popular. The development of the associated hardware and software will strengthen this trend. The application of these principles is demonstrated in Chapter 2: the processes and equipment required to produce the concrete mix are described for all prefabricated concrete products.

The same applies to the dynamic modelling and simulation of production equipment. Modelling of equipment using

– multi-body systems and

– the Finite Element Method (FEM)

can be used to investigate motion processes as well as stresses generated by dynamic loading. The application of these simulation methods is then described along with the individual equipment components.

The processes and equipment to manufacture precast concrete products are then discussed for the individual product groups:

– small concrete products,

– concrete pipes and manholes,

– precast elements.

The characteristics of the final product are of crucial importance, which is why in-process quality control is becoming increasingly popular. Implementation of a quality control system requires state-of-the-art measuring and automation technology, which is also discussed in this book.

Also addressed are issues associated with appropriate measures for reducing noise and vibration during the manufacture of precast products.

1 Basic Principles

1.1 Process Fundamentals

1.1.1 Production Process

The production process to manufacture concrete products can also be considered a system, just like any other process. The schematic representation shown in Fig. 1.1 indicates the system boundaries of the production process.

Fig. 1.1: The production process I Input parameters O Output parameters E External conditions A Associated effects

As is the case with any system, the basic characteristics of this production process are its function and structure. The function of the production process is the conversion of certain input parameters (e.g. material, energy or information) into the associated output parameters (e.g. semi-finished and finished products). The structure of the production process serves to fulfil the function and includes a set of elements that are interlinked by particular relationships.

The production process is subject to certain conditions that must be considered during the planning, preparation and execution stages. These are:

– on the input side, conditions that restrict the degree to which the function can be fulfilled; these include environmental factors, available equipment and conditions of supply.

– on the output side, conditions associated with the fulfilment of the function; these include emissions and by-products generated by the process.

1.1.2 Components Determining the Structure of the Production Process

The process to manufacture concrete products is a complex, dynamic system made up of technical and organisational elements.

Process elements are basic processes or workflows that can no longer be sub-divided from a macro-technological perspective. These process elements are linked by temporal, spatial and quantitative relationships that are determined by the process function.

These relationships govern the process layout and flow with respect to both space and time.

Therefore, the following parameters need to be determined to describe the production process fully:

– process elements

– process layout

– process flow

1.1.2.1 Process elements

Like any other process, the basic operation, as a process element, has both a function and a structure.The function of the basic operation is a fundamental change in the state of the target object towards the final product and aims to achieve a certain intermediate state.

All existing objects can be assigned to one of the following main categories:

material - energy - information

They are modified by basic operations of the various types of change, all of which can be assigned to the following categories:

production - transport - storage

Depending on the relevant type of change, the basic operations are elements that determine the production process and can most generally be described, from a functional point of view, as:

a) production elements

b) transport elements

c) storage elements

With regard to the overall production process, the characteristics of its elements also form the basis for its constituents:

1 Manufacturing technology and organisation 1.1 Production technology and organisation 1.2 Transport technology and organisation 1.3 Storage technology and organisation.

2 Manufacturing-related technology and organisation 2.1 Supply and disposal technology and organisation 2.2 Maintenance technology and organisation 2.3 Safety and security technology and organisation 2.4 Control technology and organisation.

The structure of the basic operations forms part of the technological microstructure (Fig. 1.2).

Fig. 1.2: Structure of the basic operation

Structural elements thus include:

– the object of change (Xe, Xa),

– the technological method (Vt),

– technical means (Mt),

– human workforce ().

In the basic operation, a human being uses a technical means to affect the object of change directly or indirectly, thus modifying it with a certain aim or purpose. The technological method governs the basic way in which this proceeds. Technological methods thus represent the approach usually applied in practice to implement scientific effects and to thus modify the object in accordance with the intended purpose. The technological method is not an object itself, it is inherent to the technical means that fulfils its function within the technological process.

The technical means represents a technical object that can be considered a system, i.e. a technical entity (technical equipment). The function of the technical means is to implement one or several technological methods within the technological process.

In accordance with this function, it is useful to classify these means analogously to the functional relationships between the basic operations (i.e. according to the type of change):

– production means

– transport means

– storage means

Furthermore, the technical means may also be categorised according to the object of change:

– material-related means

– energy-related means

– information-related means

The set of material-related means comprises all technical means that serve to change the state of materials in the most general sense. These include all pieces of equipment (such as machines, apparatus, devices and systems) that are used to manufacture products from the materials.

Energy-related means comprises all technical means that convert or transform energy, such as drive motors, steam generators, transformers or energy distribution systems.

Fig. 1.3: Technological line (plant) to manufacture concrete products 1 Pallet buffer 2 Block machine 3 Elevator 4 Transfer car with top finger car 5 High-bay rack/curing chamber 6 Lowerator 7 Quality control 8 Re-arranging and stacking unit 9 Strapping system 10 Cleaning, turning and stacking of pallets 11 Storage of products ready for dispatch 12 Transport equipment 13 Mix processing and feed

Information-related means comprise all technical means that serve to process information. These include, for instance, IT systems, signalling installations, measuring equipment as well as weighing and batching units.

The coupled set of technical means used in the production process represents the production line, which is an overall entity (technological line) and is also a prerequisite to carry out the production process (Fig. 1.3). The technical means are thus at the very heart of the various processes.

1.1.2.2 Relationships between process elements

Within the technological process, certain relationships exist between the process elements that are determined in space and time. As a result, the set of relationships between the process elements represents the spatial and temporal organisation of the technological process, i.e. the process layout and flow. Both sides of the structure are governed by the following underlying conditions that must be met by an appropriately designed structure:

1 Fulfilment of the function 1.1 Ensuring both quality and quantity of products 1.2 Implementing the required functional sequence

2 Process efficiency 2.1 High reliability 2.2 Lowest possible outlay for process installation and implementation

3 People-driven nature of the process 3.1 Best possible working conditions 3.2 Lowest possible emissions.

Process layout and flow comprise the set of arrangements and couplings between process elements.

These arrangements determine the position of process elements in space and time.

The spatial arrangement is thus defined by the allocation of the process elements to the required functional sequence and the associated flow of materials, as well as by the options that exist with respect to the set-up and positioning of the technical means. The arrangement within the production space depends on the functional and geometrical/structural characteristics of the technical means, on the requirements for their assembly, operation and maintenance, and on the characteristics of the production space.

The temporal arrangement of the process elements is determined by the required functional sequence and by factors associated with the output parameters and work scheduling.

Couplings are the links that permit transfer of the object of change (material, energy, information) between process elements.

The overall set of couplings comprises:

– spatial-geometric couplings

– temporal couplings and

– quantitative couplings.

Certain compatibility conditions must be met in order to fulfil the coupling function. To achieve compatibility, the output variables of the preceding operation must correspond to the input variables of the subsequent operation with respect to space, time and quantity. If this condition is not met, the operations cannot be coupled. In such a case, either a modification of the elements to be coupled or the integration of additional elements is required.

In this model, a spatial coupling refers to a spatial-geometrical relationship between process elements. This requires geometrical compatibility at the spatial points where objects of change are transferred. For this purpose, the three-dimensional coordinates of the boundaries of the process elements (the technical means) are aligned with each other in such a way that the objects of change can be transferred. A spatial coupling must fulfil the following conditions:

– transfer of the object of change must be ensured

– mobile technical means must have enough space to manoeuvre

– sufficient space must be provided for assembly, repair and maintenance.

This leads to specific coupling distances (Fig. 1.4).

Fig. 1.4: Spatial coupling of process elements: concrete mix spreader above the pallet

Fig. 1.5: Serial process: pallet circulation

A temporal coupling refers to the alignment of process times of the various process elements. Two process categories can be distinguished with respect to their temporal characteristics:

– serial processes

– parallel processes

Serial processes require that a process element must have been completed before the following element can commence. In this case, the time gap amounts to t1, 2 ≥ 0 (Fig. 1.5).

Parallel processes require that all parallel processes involved must have been completed at the lateral nodes so that they can be merged into a common process. In this model, the co-determinative processes must be adjusted to the determinative process (Fig. 1.6).

The following factors are relevant to quantitative couplings:

– The majority of process elements that are coupled to create a production process provide varying capacities, which results in different mass flows.

– With respect to their capacities, technical means of a single type are mainly composed of discrete increments.

– The required mass flows can be achieved either by a large-capacity process element or by several process elements whereby each of these elements provides a lower capacity.

– Process elements that include various types of flow (i.e. continuous vs. discontinuous) may have to be coupled within a single production process.

Fig. 1.6: Parallel processes: block machine and sub-processes

The following compatibility condition applies to the quantitative coupling of two consecutive process elements:

Technical or organisational adjustments need to be made if the mass flow lines diverge (M1 > M2) or converge. (M1 < M2)

Options for technical adjustments are:

– modification of factors that determine the capacity of the process elements to be coupled by changing the material quantity or the production or conveying speed,

– integration of additional intermediate or parallel elements. Process elements to be integrated as intermediate elements mainly include storage elements that are introduced for compensation purposes and which put a certain number of objects of change on hold for a defined period (Fig. 1.7).

A parallel arrangement is required if there are process elements with varying flow increments. In this case, a single, larger-flow element is coupled to several elements with smaller flows in such a way that an alignment is achieved.

Fig. 1.7: Insertion of storage elements: storage system for baseboards

1.1.2.3 Process layout and flow

The process layout determines the spatial structure.

The spatial structure refers to the three-dimensional arrangement and coupling of the process elements. It represents the spatial organisation of the technological process and thus of the production line as the entity that comprises all technical means [1.1]. Its configuration can be varied according to the following types of spatial organisation:

– basic types of arrangement

– types of motion

a) Basic types of arrangement

Basic types of arrangement are distinguished according to the process- or product-driven nature of the spatial arrangement.

Fig. 1.8: Process-driven arrangement

Fig. 1.9: Product-driven arrangement

Fig. 1.10: Stationary production

Process-driven arrangement (process principle):

Technical means that implement identical processes are grouped together in a spatial arrangement and treat various types of objects of change (Fig. 1.8).

Product-driven arrangement (product principle):

Technical means that implement different processes are grouped together in a spatial arrangement according to the work sequence required for a certain type of object of change (Fig. 1.9).

b) Types of motion

Types of motion can be distinguished according to the state of motion between objects of change and technical means:

Stationary production

The objects of change (OC) remain at the same manufacturing station during the determinative basic operations. The technical means (Mt) are mobile. They are moved towards the object of change, where they act on it, and are then moved to the next manufacturing station (Fig. 1.10).

The principle of stationary production is used by a number of different systems, of which the following are of particular relevance to the production of wall and structural framework elements:

– single-mould systems

– battery mould systems

– continuous moulding systems

– extrusion systems

– prestressing line systems

Fig. 1.11 shows a battery mould.

Sequential production

The objects of change (OC) move from one manufacturing station to the next. The technical means Mt are stationary (Fig. 1.12).

One or more work steps are carried out at each of the stations (manufacturing units), which is why these work steps run parallel to each other [1.2]. Fig. 1.13 shows a typical example of the carousel manufacturing principle: a pallet circulation system. Block machines used to manufacture concrete products are another example of this manufacturing principle.

Fig. 1.12: Sequential production

Fig. 1.13: Pallet circulation system

1.1.3 Processes for the Industrial Manufacturing of Concrete Products

Concrete products include durable goods made of concrete, reinforced concrete and prestressed concrete [1.3].

Finished concrete is made in the following stages:

concrete mix => fresh concrete => hardened concrete.

Fig. 1.14: Production steps

In accordance with these stages, concrete products are manufactured in the following sequence (Fig. 1.14):

– production of the concrete mix

– fabrication of the reinforcement

– production of moulds and formwork

– production of concrete elements

– finishing and completion (partly integrated in element production)

– storage of precast elements and products

Fig. 1.15: Sub-processes in the manufacture of concrete products

In this workflow, the production of concrete elements is the main process to shape and manufacture the concrete products. Fig. 1.15 shows the work steps that are required for this purpose.

The steps of mix production as well as fabrication of reinforcements, moulds and formwork may be allocated to one or several element production processes. They may also be located outside the boundaries of the factory; however, this would increase outlay for organisation and transportation.

1.1.4 Processing Behaviour of Concrete

1.1.4.1 Classification of processing behaviour

The manufacture of concrete products requires a number of changes in the state of the material to achieve a defined manufactured state at each of these stages. During these changes in the state or condition, which are brought about by the intentional action of the technical means, the respective object (i.e. concrete constituents or concrete at each of its stages) exhibits a certain behaviour. In other words, this constitutes the reaction of the material to the action of the technical means. The processing behaviour is thus process-driven. In accordance with the main classes defined for the types of change, main processing behaviour classes can also be established (Table 1.1).

Table 1.1: Processing behaviour classes

1.1.4.2 Compaction behaviour of the concrete mix

Just like finished concrete, the initial concrete mix is a very versatile material. With respect to its mechanical properties, it takes an intermediate status between a bulk material and a suspension. These mechanical characteristics undergo substantial changes during the compaction process, which thus alters the compaction behaviour.

Compaction is closely related to the moulding behaviour of the concrete mix to produce the concrete product. Moulding and compaction serve to transform the concrete mix into a quasi-solid geometric body of fresh concrete [1.4]. This process creates an artificial stone that has a low initial strength, the so-called green strength.

The aim of the moulding process is to produce an accurately shaped concrete product. The concrete mix is poured into the mould so that it completely fills all the corners and edges. The placement behaviour of the concrete is crucial to achieve this goal and depends on the flow properties of the concrete mix.

For most types of concrete mixes used to manufacture concrete products, natural compaction effects are also utilised to support the placement process. Highly flowable mixes, such as self-compacting concretes (SCCs), show a very good pouring behaviour because any remaining pores are removed by the gravity effect and the motion of the mix during the placement process. As these concretes are already self-compacted, additional compaction is neither necessary nor possible.

Compaction serves to largely eliminate the external porosity of the concrete mix. The reduction in the void volume should lead to higher densities and thus improve the strength and dimensional stability.

Fresh concrete may thus be considered dense if it is largely free of pores.

Concrete can be considered strong if an almost homogeneous body held together by adhesive and cohesive forces was created due to the high packing density of the concrete constituents.

Concrete can be considered dimensionally stable if no significant dimensional changes occur under ambient conditions in both the loaded and the unloaded states.

a) Moulding and compaction methods

Concrete can be compacted by a number of different methods (Fig. 1.16).

Despite numerous attempts to find alternative methods, vibration – alone or in combination with other processes – continues to be the most popular method for moulding and compacting concrete mixes in order to manufacture both concrete products and precast elements [1.5].

Moulding and compaction aims to:

– match the process and equipment parameters to the respective concrete mix and to implement these parameters in the compaction equipment,

– uniformly transfer the required compaction energy into the concrete mix from all points or areas of introduction,

– ensure, by the selection of the appropriate vibration parameters, that the compaction energy in the concrete mix is transferred in such a way that the concrete or precast product has a uniform density throughout.

Fig. 1.16: Compaction methods

The type of action on the concrete mix is a crucial factor that determines the moulding and compaction behaviour. As shown in Fig. 1.17, the various actions can be grouped according to:

– the point of action

– the function of the vibration action

– the intensity of the action

– type, location and number of simultaneous actions

– the phase position of the exciter functions in relation to each other in the case of several simultaneous actions

With respect to the location of the action, and thus its direction, a fundamental distinction can be made between horizontal, vertical and three-dimensional actions.

As regards the function of the vibration action, harmonic and anharmonic modes of excitation can be distinguished. Both directional (counter-acting) and non-directional (circular) exciters can be used to introduce vibration into the concrete. Anharmonic exciter functions can be sub-divided further into periodic and pulsed actions. For instance, a periodic exciter function can be a multi-frequency action that consists of several harmonic components. Pulsed excitation, also known as shock vibration, is generated by shock-like processes. This triggers the inherent oscillation of all system elements capable of vibration, i.e. an entire frequency spectrum.

Fig. 1.17: Actions on the concrete mix

Parameters that characterise the intensity of the action on the concrete mix are discussed in Section 1.1.5.2.

The type of exciter function, the mode of action and number of exciters and, in particular, their phase position in relation to each other have a major influence on the moulding and compaction behaviour of the concrete mix. For example, a phase coincidence of the harmonic vibration components of the vibrating table and tamper head would hardly achieve a good compaction effect.

The crucial factor is the generation of a dynamic pressure gradient between the layers of the mix that enables relative motion of these layers and mutual rotation of the mineral aggregate particles. These requirements must be met by state-of-the-art processes. When producing large-scale precast elements, for example, the low-frequency action on the fresh concrete is complemented by a higher-frequency vertical excitation. In such a set-up, the frequency of the required vibrators is usually controlled via frequency converters.

When producing concrete products from stiff mixes, modern processes often combine vibration with pressing, as in block machines (through the tamper head) or in concrete pipe machines (through a suitable arrangement of packer heads with several level counter-acting rollers).

A special type of action on the concrete mix is created by the use of internal vibrators (Fig. 1.17; excitation force F4), where horizontal, non-directional vibration is introduced when the vibrator comes into direct contact with the mix.

b) The moulding and compaction process

The actual compaction process, from start to finish, can be considered a dynamic process with a gradual transition from one rheological state to the next [1.7]. This concept is illustrated in Fig. 1.18, which is based on investigations carried out by Afanasiev [1.6]. In this model, the entire compaction process is divided into three phases that are described in more detail in [1.7] and [1.8], for example. Each of these three phases represents a compaction stage where, according to [1.6], its rheological state is characterised by the dry friction model, the Bingham model and the Kelvin-Voigt model.

Both duration and delimitation of the individual phases, as well as the associated rheological body models, depend on the type of material mix to be compacted. It can thus be concluded that each concrete mix requires specific process and equipment parameters for the individual phases of its compaction in order to come as close as possible to an ideal compaction state in the shortest possible time [1.8].

I-III Rheological state of the mixes and ther models

Fh horizontal excitation

Fv vertical excitation

Fig. 1.18: Rheological compactibility curves and corresponding actions on the concrete mix

1.1.4.3 Fundamentals of vibration

a) Kinematics of vibration