Manufacturing of Concrete Products and Precast Elements

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

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 20^th 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.