Construction Engineering

(1)

(2)

CONSTRUCTION

ENGINEERING

OCTAVIAN G. ILINOIU, Ph.D., C.Eng., Lecturer

Department of Civil, Urban and Construction Engineering

Technical University of Civil Engineering of Bucharest

First Edition

(3)

(4)

2003-i

PREFACE

The construction sector is a major part of the construction industry, with projects rang-ing in size from the small to the very large, all sharrang-ing common factors - workers, machinery and materials, and the required organization and control. The graduate civil engineer must be therefore familiar with the range of these common factors, procedures and techniques in common use, and must be able to plan, and direct construction works.

This course book will be limited to presentation of basic principles and process tech-niques in construction execution. It is intended as an aid and a guide to circumvent some problems encountered in design and execution, outlining present techniques and materials re-lated to construction technology for their evaluation and improvement.

The content of this book is arranged in 11 chapters that are summarized below.

Chapter 1. Industrialization of Construction Works, provides an introduction in the field of Construction Engineering and method of industrialization of construction works.

Chapter 2. Formwork, addresses the design and presents basic information and specifies ma-terials, construction, and removal of formwork, mouldwork and shoring.

Chapter 3. Steel Reinforcement, provides information and specifies the materials and work-manship required for manufacturing concrete reinforcement.

Chapter 4. Concrete, provides an overview of conventional concrete technology form mate-rial science and engineering systems perspective – specifically its structure and composition, processing, properties, performance, and the quality control of it.

Chapter 5. Batching and Mixing Concrete, is devoted to discussing the proper equipment and procedures for batching and mixing concrete by ensuring uniform quality throughout the mix. Chapter 6. Building Material Transport Machinery and Equipment, presents relevant infor-mation regarding proper equipment and procedures for handling and transport of building ma-terials.

Chapter 7. Handling and Placing Concrete, presents fundamental concepts in regard of han-dling and placing techniques required for cast in place conventional concrete and the quality control of it.

Chapter 8. Compaction, Vibration and Concrete Finishing, provides a background on the benefits of compaction of concrete and the techniques for undertaking the process on site. Chapter 9. Curing Concrete, provides basic information in relation to related concepts of concrete curing. A review is presented of various curing requirements and techniques for un-dertaking the process on site and in precast concrete production plants.

Chapter 10. Off-site Prefabrication, provides information concerning precast concrete, ad-vantages and limitations, application, techniques and manufacture requirements for different types of elements.

Chapter 11. Erection of Precast Concrete Elements, presents theoretical back ground regard-ing erection of precast concrete units.

The primary object of this first volume, and those which follow, is to provide a refer-ence guide to Construction Engineering within the framework of the Civil Engineering De-partment – English Section of the Technical University of Civil Engineering of Bucharest.

(5)

ii

ACKNOWLEDGMENTS

These notes are originally based on the ideas of Drs. Radu Popa and Mihai Teodorescu. However, views and comments are the writer's own. The author has presented some positions as starting points for drafting a course book rather than as the only positions that can be adopted.

The author gratefully acknowledges the support of a number of organizations, institutions, trade associations and manufacturers and who have given advice, and literature:

American Concrete Institute - ACI, USA

American Society of Civil Engineers - ASCE, USA

American Society for Testing and Materials - ASTM, USA

American Association of State Highway Transportation Officials – AASHTO, USA ATEX C.V.B.A., Belgium

Bell Engineering Group, England BHS – Sonthofen, Germany

Building Science Insight - BSI, Canada Canadian Building Digest - CBD, Canada

Civil Engineering Corps Washington – CECW, USA Cement and Concrete Association Australia

Center for Innovative Grouting Materials and Technology - CIGMAT, USA EFCO, USA

Elba –Werk, Germany

Heidelberg Cement AG, Germany

International Council for Building Research and Documentation - CIB, Canada Institute for Research in Construction - IRC, Canada

International Committee on Asian Concrete Model Code National Research Council - NRCC, Canada

National Institute of Standards and Technology - NIST, USA Officine Riunite Udine SpA – ORU, Italy

OSCAM S.p.A, Italia PERI, Germany

International Union of Testing and Research Laboratories for Materials and Structures – RILEM

SC SOMACO SA, Romania SBH Tiefbautechnik, Germany

The Engineering Wood Association - APA, USA Thwaites Ltd., England

Tremix, Sweden MACON SA, Romania

MAN Nutzfahrzeuge Aktieengesellschaft, Germany Morgan Manufacturing Co., USA

(6)

iii

TABLE OF FIGURES

Figure 2-1 Typical dimensioned lumber panel.10

Figure 2-2 Typical plywood sheathing panel ...10

Figure 2-3 Steel panel...11

Figure 2-4 Pan forms ...12

Figure 2-5 Plywood panels...13

Figure 2-6 Climbing formwork ...13

Figure 2-7 Sliding formwork...14

Figure 2-8 Formwork table...16

Figure 2-9 Form table assembly ...16

Figure 2-10 Permanent formwork ...17

Figure 2-11 Below grade footing forms ...17

Figure 2-12 Shallow footing...17

Figure 2-13 Wall formwork...18

Figure 2-14 Typical gang panel...19

Figure 2-15 Typical slab-on-grade edge form-work...19

Figure 2-16 Girder form details...20

Figure 2-17 Spandrel beam form details ...20

Figure 2-18 Typical example of column formed with plywood panels stiffened with triangu-lar yokes...21

Figure 2-19 Typical steel column formwork ....22

Figure 2-20 Typical telescopic joists...24

Figure 2-21 Typical adjustable steel shores ...24

Figure 2-22 Form accessories...25

Figure 2-23 Concrete pilot mould ...26

Figure 2-24 Static metal moulds...26

Figure 2-25 Stack wooden mould...26

Figure 3-1 Characteristic stress- strain curves ( σ-ε) for steel ...28

Figure 3-2 Standard types of reinforcing bars PC 52 and PC 60...30

Figure 3-3 Dimensions of wire mesh ...31

Figure 3-4 Typical profile of SBPA ...31

Figure 3-5 Typical reinforcement spacing section ...32

Figure 3-6 Chairs and spacers ...32

Figure 3-7 Pliers for straightening bars attached to winch. ...33

Figure 3-8 Straightening of steel bars delivered in coils with an electrical chain winch...33

Figure 3-9 Typical strengthening and cutting machine...34

Figure 3-10 a. b. Rotating drum with screw die arrangement ...34

Figure 3-11 Rolling-mill arrangements ...34

Figure 3-12 Standard hook and stirrup details for reinforcement...35

Figure 3-13 a. Bending of bars with two keys; b. Bending of bars with three pins and a key; c. Typical-bending plates...35

Figure 3-14 Hook bending machine ...36

Figure 3-15 Typical stirrup bending machine ..36

Figure 3-16 Fixed arm and variable arm mesh-bending machines ... 37

Figure 3-17 Hoop and spiral bending machine 37 Figure 3-18 Typical spiral reinforcement ... 37

Figure 3-19 Manual shears ... 38

Figure 3-20 Electronic measurement model.... 38

Figure 3-21 Typical mesh cutting machine ... 38

Figure 3-22 Typical arc-welding outfit... 39

Figure 3-23 Typical automatic/tack welding ma-chine... 40

Figure 3-23 Splicing of steel reinforcing... 41

Figure 3-24 Sleeve splice connection ... 42

Figure 3-25 Typical alternate tying procedure of individual reinforcing bars to produce a mesh ... 43

Figure 3-26 Detail of column and slab rein-forcement intersection... 43

Figure 3-27 Column cages being assembled on site... 44

Figure 3-28 Typical procedure of wiring main beam reinforcement to stirrups and ties ... 45

Figure 3-29 Typical wiring procedures ... 46

Figure 3-30 Detail of slab reinforcement... 46

Figure 3-31 Typical storage of wire in spools . 47 Figure 3-32 Typical storage off the ground of reinforcement ... 47

Figure 3-33 Typical wire coil storage... 47

Figure 3-34 Typical arrangement of reinforce-ment manufacturing shop... 48

Figure 4-2 Fresh concrete sliding on chute... 50

Figure 4-3 Measurement of slump from height of slump cone ... 51

Figure 4-4 Entrained air voids in concrete... 53

Figure 4-5 Types of concrete shrinkage ... 54

Figure 4-6 Effects and phases of plastic shrink-age... 54

Figure 4-7 a. A sectioned clinker nodule, b. Un-hydrated ordinary Portland cement parti-cles, field width of 320 microns... 56

Figure 4-8 Speed of cement components heat hydration ... 58

Figure 4-9 Mechanism of cement setting. Figure shows long needle-like crystals and short crystal–like formations of calcium-silicate-hydrate gel... 59

Figure 4-10 Schematic view of cement setting 59 Figure 4-11 Compressive strength gain of min-eral cement components... 59

Figure 4-12 Significance of aggregates grading ... 61

Figure 4-13 Angularity and surface texture ... 62 Figure 4-14 Moisture conditions of aggregate. 62

(7)

iv

Figure 4-15 Main moisture conditions of

aggre-gate ...62

Figure 5-1 Twin shaft batch mixer ...66

Figure 5-2 Single shaft drum mixer...67

Figure 5-3 Tilting drum mixer...68

Figure 5-4 Reversing drum mixer ...68

Figure 5-5 Forward flow mixer ...69

Figure 5-6 Backward flow mixer ...69

Figure 5-7 Vibrating mixer...69

Figure 5-8 General layout of concrete plant...70

Figure 5-9 Mixing tower - Batching plant...71

Figure 5-10 Linear batching center ...72

Figure 5-11 Cement silo ...73

Figure 6-1 Wheelbarrow...74

Figure 6-2 Push chart...74

Figure 6-3 Forklift truck...75

Figure 6-4 Typical types of dump trucks...75

Figure 6-5 Chute and hopper ...75

Figure 6-6 Typical concrete buckets ...76

Figure 6-7 Belt conveyor...77

Figure 6-8 Transit mix truck...78

Figure 6-9 Rail cars ...78

Figure 6-10 Truck mounted concrete pump ....79

Figure 6-11. Boom extensions of pump ...79

Figure 6-12 Typical tower pump detail ...79

Figure 6-13 Pumps for concrete transport ...80

Figure 6-14 Pneumatic pipeline...80

Figure 7-1 a. Placing techniques for flatwork. .83 Figure 7-1 b. Concrete placing techniques for columns and walls ...84

Figure 7-2 Compacting columns ...85

Figure 7-3 Slab concreting ...85

Figure 7-4 Typical types of concreting joints for arches/vaults ...86

Figure 7-5 Typical types of construction joints 87 Figure 7-6 Construction joint formed...88

Figure 8-1 Typical detail of concrete showing sand in a cement paste matrix...89

Figure 8-3 Sinusoidal motion ...91

Figure 8-4 Types of vibration...91

Figure 8-5 Typical aspects of concrete compac-tion during vibracompac-tion ...92

Figure 8-6 Hand tapping tools ...92

Figure 8-7 Typical internal vibrator. ...94

Figure 8-8 a. Internal / poker vibrator. Example of working procedures with vibrator ...94

Figure 8-9 b. Use of poker vibrator ...95

Figure 8-10 a, b Typical surface vibrator...96

Figure 8-10 c. Degree of compaction varies across width when surface vibrators are used...96

Figure 8-11 Typical positioning of clamp vibra-tors in formwork ...96

Figure 8-12 External vibrators...97

Figure 8-13 Vibropress...97

Figure 8-14 Compacting by rolling ... 97

Figure 8-15 Standard set of vacuum dewatering installation... 98

Figure 8-16 Compacting by centrifugal force.. 99

Figure 8-17 Trowels (floats) for surface finishing ... 100

Figure 8-18 Power float ... 100

Figure 8-19 Methods of concrete surface finish ... 100

Figure 9-1 Effect of duration of water curing on the permeability of cement paste ... 103

Figure 9-2 Variation of concrete strength with curing environment (W/C =0,5)... 104

Figure 9-3 Spraying on a curing compound .. 105

Figure 9-4 Temporary shelter ... 108

Figure 9-5 Typical heating enclosure ... 108

Figure 9-6 Framed enclosure ... 108

Figure 9-7. a. Procedures for casting a structural concrete wall in enclosure;... 108

b. Air supported structure ... 108

Figure 9-9 Typical protection of fresh cast con-crete with heating forms ... 109

Figure 9-8 Heating form ... 109

Figure 9-10 Typical solution of concrete protec-tion using insulating blankets... 109

Figure 9-11 a. Heating aggregates before intro-duction in mix; b. heat source underneath the aggregate pile. ... 110

Figure 9-12 Typical curing racks in steam- chamber; Precast concrete curing accelera-tion cycle... 112

Figure 10-1 Reinforced concrete centrifuged pipe ... 116

Figure 10-2 Layout of Stand setting ... 118

Figure 10-3 Typical Multiple Beam Tensioning Stand cross section setting for prestressed-concrete units ... 118

Figure 10-4 Layout of Conveyor setting ... 118

Figure 11-1 Typical trailers ... 121

Figure 11-2 Typical job-site casting yards .... 122

Figure 11-3 Typical stack storage of precast stairs units ... 122

Figure 11- 4 Typical lifting devices for precast concrete members ... 125

Figure 11-5 Crane clearances ... 125

Figure 11-6 Typical tower crane... 126

Figure 11-7 Track-mounted crane ... 127

Figure 11-8 Lorry mounted crane... 127

Figure 11-9 Self propelled crane ... 128

Figure 11-10 Typical gantry crane ... 128

Figure 11-11 Transfer of prefabricated facade to the working level ... 131

Figure 11-12 The precast facade is secured with temporary bracing ... 131

(8)

v

CHAPTER 8. COMPACTION, VIBRATION AND CONCRETE FINISHING...89 8.1. General Considerations ...89 8.2. Vibration...90 8.2.1. Basic Characteristics ...91 8.3. Compaction of Concrete...92 8.3.1. Manual Compaction ...92 8.3.2. Mechanical Compaction...93 8.3.2.1. Vibration ...93 8.3.2.2. Applying Pressure ...97 8.3.2.3. Rolling...97 8.3.2.4. Vacuum Dewatering...98 8.3.2.5. Centrifugal Force ...98

8.4. Under-Vibration and Over-Vibration ...99

8.5. Revibration ...99

8.6. Concrete Surface Finishing ...100

CHAPTER 9. CURING CONCRETE...102

9.2. Basic Curing Requirements...102

9.3. Duration of Curing ...102

9.4. Curing Under Temperature Ranging from 5 o_{C to 30}o_C...103

9.4.1. Methods of Concrete Curing ...103

9.5. Curing Concrete in Extreme Weather Con-ditions ... 105

9.5.1. General Considerations ... 105

9.5.2. Placement of Concrete in Extreme Temperatures ... 106

9.5.3. Hot Weather Concreting... 106

9.5.4. Cold Weather Concreting ... 106

9.6. Accelerated Concrete Curing... 110

9.6.1. General Considerations ... 110

9.6.2. Effects of Accelerated Curing on Ce-ment and Concrete Structure ... 110

9.6.3. Classification of Concrete Accelerated Curing Procedures ... 111 CHAPTER 10. OFF-SITE PREFABRICATION... 114 10.1. General Considerations ... 114 10.2. Industrialization of Factory-manufactured Systems... 114

10.4. Off-site Precast Manufacturing Technolo-gies... 115

10.4.1. Code Marking... 115

10.5. Concrete Products Precasting Methods 116 10.5.1. Concrete Pipe ... 116

10.5.2. Manufacturing Procedures... 117

10.6. Quality Assurance, Product Certification ... 119

CHAPTER 11. ERECTION OF PRECAST CONCRETE ELEMENTS ... 120

11.2. Job Planning ... 120

11.2.1. Preliminary Execution Works ... 121

11.2.1.1. Manufacturing, Transport and Storage of Precast Units ... 121

11.2.1.2. Inspection of Units After Trans-port and Storage ... 123

11.2.1.3. Unit Preparation ... 123

11.2.1.4. Selection of Lifting Devices and Equipment ... 123

11.2.1.5. Scheduling... 128

11.2.1.6. Health, Safety and Welfare Regu-lations ... 129

11.3. Erection of Precast Units ... 130

11.3.1. Sequence, Schemes and Procedures for Unit Erection... 130

11.3.3. Unit Erection Detailing Sequences131 11.3.3. Connections ... 132

11.4. Inspection of Erection and Correction of Dimensional Tolerances ... 132

REFERENCES ... 134

(10)

7

CHAPTER 1. INDUSTRIALIZATION OF

CONSTRUCTION WORKS

1.1. GENERAL CONSIDERATIONS

The history of technology, in the field of construction, represents man's efforts to

con-trol his material environment for his own benefit. Man has been able to do this using tools and applying reason to the properties of matter and energy. For many thousands of years, …his progress in technology was made by trial and error, which made possible impressive results. It was only toward the end of the 18th century, once with the Industrial Revolution, that technology started to transform itself form craft skills…. to applied science. (Encpl.

Bri-tannica)

1.2. CONSTRUCTION INDUSTRIALIZATION

Definition of industrialization: The application of scientific principles to the optimal conversion of natural resources into structures, machines, products, systems, and processes (DEX 1998).

Industrialization is concerned with both on-site and off-site methods of construc-tion organized in a systematic way in such that erecconstruc-tion can proceed as a continuous opera-tion. This is achieved by careful planning of activities carried out and by setting up a produc-tion line to provide an organized flow of components.

Much traditional building procedure remains site-bound and labor-intensive. Tradition is still used in the field of “wet” construction. However, by incorporating factory produced units and components into traditional in situ concrete construction, and by employing me-chanical plant and equipment, erection time has been shortened considerably, particularly on work of a repetitive nature. Sometimes the contractor uses part of the site as a workshop or temporary factory for the production of woodwork or pre-cast components.

Whether traditional or industrialized, on-site organization of materials, components, and labor is vital in construction procedure; pre-planning of each stage is essential, and ade-quate time should be allowed for working out details before operations can commence. Pre-planning of activities should cover - site layout, work sequence, design, manufacture, and fix-ing of standardized components, mechanical plant.

1.3. BASIC PRINCIPLES OF INDUSTRIALIZATION

The efficiency of industrialized production results from the careful, systematic appli-cation of the ideas and concepts outlined above. The following summary lists the basic prin-ciples of mass production:

- Prefabrication of building components. Many prefabrication technologies deliver a bet-ter product because building is done in a quality controlled shelbet-tered environment. Just as importantly, prefabrication can dramatically improve productivity. The method controls construction costs by economizing on time, labor, wages, and materials.

- Developing new construction methods like industrial type production to constantly im-prove efficiency in the scope of improving labor productivity (minimizes the amount of human effort required).

(11)

8

- Careful division of the production operations into specialized tasks comprising of rela-tive simple, highly repetirela-tive human motion patterns and minimal handling or positioning of the workpiece that can be easily learned and rapidly performed with a minimum of un-necessary motion or mental readjustment.

- Developing new and improved construction materials in factory type prefabrication up to the total elimination of the time consuming operations (reinforcement bending, tying wires, welding etc.) within the construction site. These operations must improve construc-tion quality.

- Simplification and standardization of component parts through:

1. dimensional co-ordination – agreement made between the manufacturers of buil-ding units and the designers in order to simplify assembly by standardizing sizes; 2. modular design – a technique that uses a standard size module (1M = 100 mm) as

the fundamental unit for space planning. Larger spaces comprise multiple modules (n x M), while smaller spaces sub modules (M/n).

to allow large production runs of parts that are without difficulty fitted to other parts without adjustment. The imposition of other standards (e.g. dimensional tolerances, parts location, material types) on all parts of the product further increases the economic benefit that can be achieved.

Carefully designed, construction engineering and management, projects are required to achieve the maximum benefits that application of these principles can provide. Planning be-gins with the original design of the product; raw materials and component parts shall be adaptable to production and handling by mass techniques. The entire production process is planned in detail, including the flows of materials and information throughout the process.

For the industrialization of construction to be efficient, the production flow of compo-nents/materials shall be:

- Carefully estimated because the selection of techniques depends upon the volume to be produced and anticipated short-term changes in demand.

- Large enough, first, to permit the task to be divided into sub-processes assigned to differ-ent individuals; second, to justify the substantial capital investmdiffer-ent often required for spe-cialized machines and processes; and third, to permit large production runs so that human effort and capital are efficiently employed.

- Planed in detail because the large, continuous flow of product from the factory requires distribution and marketing operations to bring the product to the client.

Advantages of industrialization in construction: In addition to lowering cost, the application of the principles of industrialization has led to major improvements in uniformity and quality. The large volume, standardized design, and standardized materials and processes facilitate statistical control and inspection techniques to monitor production and control qual-ity.

Limitations of industrialization in construction - the resulting system is inherently inflexible, because maximum efficiency is desired; tools, machines, and work positions are often quite precisely adapted to details of the parts produced but not necessarily to the work-ers involved in the process. Changes in product design may converge toward high costs.

Usually, a production line is designed to operate most efficiently at a specified rate. If the required production levels fall below that rate, operators and machines are being ineffi-ciently used; and if the rate goes too high, operators must work overtime, machine mainte-nance cannot keep up, breakdowns occur, and the costs of production rise. Proper planning can eliminate the problems encountered; flexibility to accommodate changes economically must be planned into the system.

(12)

9

CHAPTER 2. FORMWORK

2.1. GENERAL CONSIDERATIONS

If concrete is to be poured, in place-monolithically, on the job site some means of support, known as formwork, is necessary to shape, to position it precisely (level and loca-tion) and to retain it until it sets.

In other words, formwork is a temporary “mould” into which fresh concrete and rein-forcement are placed to form a particular reinforced concrete element with a predetermined strength.

A typical breakdown of total construction percentage costs, to produce the required structural element, could be as follows: concrete (materials 28%; labor 12%) = 40%; rein-forcement (materials 18%; labor 7%) = 25% and formwork (materials 15%; labor 20%) = 35%.

To ensure that the formwork is economical and practical to build, the designer puts forward the following basic technical, economical and functional requirements that should be kept in mind when designing and constructing formwork.

a. The economic requirements of formwork are:

- Manufacture of forms must lead to low cost of materials, energy, and labor.

- Formwork should be as repetitive and as adaptable as possible. They must be able to withstand a good number of reuses without losing their shape.

- Designed so that the whole formwork can be assembled and dismantled with unskilled or semi-skilled labor.

- Formwork care and maintenance should be done according to specifications. b. The technical requirements of formwork are:

- Forms should assure the attainment of the desired shape, size and location of the member in the structure according to the drawings.

- Careful selection of finish surface and linings to produce the desired concrete surface re-sult direct from the formwork.

- Forms must be strong enough to withstand the pressure of fresh concrete and working loads; and to maintain their shape during the concrete placing operation.

- Formwork must be capable of supporting the designed loads any other applied loads dur-ing the construction period.

- The design must be made so that the forms may be removed without damage to the con-crete or to themselves.

- Panels should be tightly connected so to minimize gap at the formwork connection to pre-vent leakage of cement paste.

c. The functional requirements of formwork are:

- Form sections must be of a size that can be lifted into place without too much difficulty and transported from one job site to another, if necessary.

- Formwork must be dismantled and moved as easily as possible so that construction of the building advances.

- Units should be interchangeable so that they can be used for forming different members. - Forms must be made to fit and fasten together with reasonable ease.

(13)

10

- Forms must be as light (without any strength reduction) as possible so that one or two workers can handle them. The weight of the panels should not exceed 30-40 kg for those lifted by one worker and 60-70 kg for those lifted by two workers.

- Forms must be made so that workers can handle them in regard of safety, respecting the Health, Safety, and Hygiene Regulation in effect.

A balance of the above requirements should be achieved, preferably at pretender stage, so that an economic and competitive cost can be calculated.

2.2. FORM MATERIALS

Materials of formwork shall be selected and the formwork system shall be designed and constructed so that the concrete structure has the satisfactory performance required as per design and the safety of workers are guaranteed.

Desirable materials for making formwork (e.g. wood, plastic, aluminum, steel, insulat-ing materials) shall have the followinsulat-ing properties: sufficient strength, required stiffness, dura-bility, lightweight, reusable, and/or recyclable and volume stability during application.

2.2.1. WOOD

Figure 2-1 Typical dimensioned lumber panel

Caption: 1. Longitudinal frame (stud); 2.

Trans-verse frame; 3. Lumber planks.

Source: Popa R., Teodorescu M., 1984.

Figure 2-2 Typical plywood sheathing panel

Caption: 1. Plywood sheathing; 2. Longitudinal

frame (stud); 3. Braces; 4. Edge framing; 5. Holes in edge framing for tie insertion; 6. Noggins.

(14)

11

Wood in the variety of dimensioned lumber or plywood sheathing is a widely used material for formwork panels because of its good strength, lightweight, workability, relative low cost, better flexibility, easier to repair and reusability.

Dimensioned lumber (Figure 2-1) used in formwork is usually of the softwood va-riety (spruce, pine, fir etc.) because of its availability and good strength. Although dimen-sioned lumber has been used as sheathing material in the past, plywood sheets have replaced it in this application, while dimensioned lumber is now used primarily for framing, bracing, and shoring.

Plywood (Figure 2-2) is a sheathing product made of several wood veneers with their grain lying (normal to one another) at right angles and firmly glued together under pressure, producing a panel that has uniform properties in both directions. Plywood produces smooth concrete surfaces and can be used repeatedly, having excellent strength properties, minimiz-ing deflection durminimiz-ing concrete pour.

2.2.2. STEEL

Steel angles and bars are used as sup-porting members for form panels faced with plywood or steel sheathing.

Steel forms have the following advan-tages: very good du-rability and easy to clean, low cost of erection/ stripping, no distortion with moisture changes, non-inflammability, and limitations: heavy, more difficult to assemble and re-pair.

Figure 2-3 Steel panel Caption: 1. Frame; 2.

Braces; 3. Welded steel sheathing; 4. Pipe; 5. Connecting pipe piece; 6. Socket.

Source: Popa R., Teodorescu M., 1984.

(15)

12

Figure 2-4 Pan forms Source: ATEX, 1999.

Pan forms (referred to also as Waffle Moulds) are used in one- or two-way ribbed slabs. Advantages of using pan forms: low cost, speed of erection and striping, simplicity.

2.2.3. PLASTIC AND ALUMINUM

Plastic reinforced fiberglass is a desired material for sheathing do to its reusability, mouldability, lightweight, strength, and toughness.

Aluminum expensive compared with the other materials is used as supporting, I and U-shaped, members for form panels. Its advantages are its lightweight and strength. Alumi-num sections are used also as beams for supporting slab formwork or as wales on wall form-work.

2.3. CLASSIFICATION OF FORMWORK

The basic components of a formwork are: form panel (comprised of panel sheathing and panel frame), shoring members and form accessories.

Forms can be classified in accordance with a number of criteria, such as by: structure and use/reuse of formwork, final destination, materials used, quality of panel sheathing.

2.3.1. CLASSIFICATION BY STRUCTURE AND USE

2.3.1.1. DISMOUNTABLE FORMWORK

Prefabricated forms (referred to also as Traditional Wall Formwork) consists of standard size framed panels tied together over their backs with horizontal members called wales that provide resistance to the horizontal pressure of fresh concrete.

There main advantage is that they are can be reused many times at a convenient cost. A standard procedure for site formwork assembly is as follows:

- Forms shall conform to the shape and dimensions shown on the drawings and shall be accurately set to line and grade. All sheathing in contact with concrete surfaces shall be sized to uniform thickness and free from wane, warp, splits, loose knots or other defects which will prevent obtaining a smooth, tight form.

- Forms shall be erected one side of the wall formwork, ensuring its correct alignment, plumbing, and/or strutting.

- Forms shall be tightened by means of slotted wedge that passes through the lower end of the slot. Joints in the lining shall be filled with patching plaster or other plastic filler. Lin-ing material may be re-used if it is in satisfactory condition, well cleaned and re-oiled - Insertion and positioning of steel reinforcement cage before the formwork for the other

(16)

13

- Correct spacing of forms at specified distance from one another by using plastic spacer tubes in which ties are inserted.

- Positioning of horizontal members (wales) to increase the overall rigidity of the formwork panels and to align them.

- Insertion of ties between wales, covering them at the outside with plate washers to ensure that the loads are evenly distributed over the wales.

- Forms for walls, etc., shall have large cleanout openings at their lowest points, which shall not be closed until just before placing concrete. All forms shall be thoroughly cleaned and soaked with water immediately before filling.

Figure 2-5 Plywood

panels

Caption: 1. Plywood

panel; 2. Base plate; 3. Wale; 4. Plate washer; 5. Nut; 6. Tie rod, 7. Pipe spacer; 8. Plastic cone; 9. Shoe; 10. Clamp; 11. Wedge; 12. Concrete kicker. Source: Popa

R., Teodorescu M., 1984.

Climbing Formwork is a method of casting a concrete wall in known vertical lift heights (approx. 1m) using the same forms in a repetitive fashion to obtain maximum usage from a mini-mum number of panels.

A standard procedure, for site operations, is as follows:

- Positioning of the first concrete lift against a 300 mm high kicker.

- Casting the concrete and allowing it to harden after which the forms are removed.

- Resumption of the same operations (casting and curing) for the next 1 m lift until the re-quired height has been reached.

Figure 2-6 Climbing formwork

Caption: 1. Pair of steel studs; 2. Working platform;

3. Adjustable brace; 4. Tie; 5. Loop tie; 6. Plumbing member (strut); 7. Plywood panel; 8. Clamp. Source:

(17)

14

2.3.1.3. NON DISMOUNTABLE FORMWORK

a. Sliding formwork (referred to also as Slip Formwork) represents a formwork system that slides continuously up the face of the concrete wall that is being cast. The climbing operation is possible do to a series of hydraulic jacks that operate on jacking rods.

The formwork system presents the following advantages: they cast the structure monolithically and jointless, very good durability and easy to clean, no distortion with mois-ture changes, and limitations: heavy, more difficult to assemble and repair, the wall should have a uniform thickness (with a minimum number of openings), and a height of at least 20 m to make the cost of equipment, labor and planning economical.

Because of these factors, this method is suitable for constructing water towers, chimneys, bins, silos, and multi-story buildings that have repetitive floors.

Figure 2-7 Sliding formwork Caption: 1. Form panel with

steel face; 2. Steel framed yoke; 3. Hydraulic jack; 4. Jacking rods; 5. Upper inte-rior platform; 6. Upper exte-rior platform; 7. Lower inte-rior platform; 8. Lower exte-rior platform; 9. Buck; 10. Window opening; 11. Control equipment for horizontality inspection; 12. Control equipment for vertical plumb-ing inspection; 13. Electrical installation; 14. Water instal-lation. Source: Popa R.,

Teodorescu M., 1984.

The sliding formwork is comprised of the following basic parts:

1. Side panels forms, made of timber or steel, usually 1,20 m in height, with an overall slid-ing clearance of 6 mm by keepslid-ing the external panel plumb and the internal panel tapered so that it is 3 mm in at the top and 3 mm out at the bottom, giving the true wall thickness, in the center position of the form.

2. Horizontal wales stiffen the side forms along to resist the lateral pressure of concrete and transfer the loads of working platforms to the supporting yokes.

3. Yokes assist in supporting the suspended working platforms and transfers the platform and side form loads to the jacking rods.

4. Working platforms are usually provided to ease the work of the concrete team, for storage of materials, for finishing operations and to carry jacking and control equipment.

5. Hydraulic jacks are anchored at the base of the structure and embedded in the concrete below the forms. The jacks may be hydraulic, electric, or pneumatic.

(18)

15

The jacks used are usually specified by their load bearing capacities and consist of two clamps operated by a piston. The clamps operate on a jacking rod of 25 to 50 mm diameter. The upper clamp grips the jacking rod and the lower clamp, being free, rises, pulling the yoke and platforms with it until the jack extension has been closed. The lower clamp now grips the climbing rod while the upper clamp is released and raised to a higher position when the lifting cycle is recommenced.

If the jack rod is to be reused, it is withdrawn from the wall after the forming is com-plete. This is made possible by sheathing the rod with a thin pipe, which is attached at its top end to the jack base and moves up with the forms. The sheath prevents concrete from bonding to the jack rod and leaves it standing free within the hardened concrete. In some cases, the rod is left unsheathed and remains as part of the reinforcing. The 2,5 to 4,0 m lengths of rod are usually joined together with a screw joint arranged so that no joints occur at the same level.

A standard procedure regarding site operations is as follows:

- Formation of a concrete, 300 mm high, kicker incorporating the wall and jacking rod starter bars.

- Anchorage of vertical reinforcing rods at the base of the structure that extends upward between the inner and outer form. As the form rises and reaches the top of the first set of rods, new lengths are added as concreting continues.

- Assembly of wall forms fixed together with yokes, upper working platforms, and jacking device.

- Placement of first concrete lift. The commencing rate of climb must be slow (150…450 mm/hr.) to allow time for the first batch of concrete to reach a suitable maturity before emerging from beneath the sliding formwork.

- When openings are required to be produced in the wall bucks are inserted in the section of the wall.

- If a concrete projection from the wall is required, it must be added after the forming is complete. A pocket is formed in the wall with dowels bent in so as not to interfere with the operation of the forms. After the forming is complete, the dowels can be bent out, the forms for the projection built around them, and the structure cast.

The success of a slip-forming operation depends on good planning, design, and super-vision so that the operation may in fact, be as continuous as possible. Some of the major fac-tors contributing to successful slip form construction are:

- Round-the-clock working which will involve shift working and artificial lightning to en-able work to proceed outside normal daylight hours.

- Careful control of concrete supply to ensure that stoppages of the lifting operation are not encountered. This may mean having standby plant as an insurance against mechanical breakdowns.

- Suitably trained staff accustomed to this method of constructing in-situ concrete walls. - The proper concrete mix design and careful control of the concrete to maintain the proper

slump and set, in spite of changing temperatures.

- Adequate facilities for supplying concrete to the forms at any height and an adequate con-crete supply.

- A supply of reinforcing steel at hand and experienced workers to do the fabricating as work progresses.

- Reliable forms, designed to stand the stresses placed on them by the constant heat of liq-uid concrete.

b. Table formwork is used when casting large repetitive floor slabs in high-rise structures. There main objective is to reduce the time factor in erecting, striking and re-erecting slab

(19)

16

formwork by creating a system of formwork which can be struck as an entire unit, removed, hoisted and repositioned without any dismantling.

Figure 2-8 Formwork table

Caption: 1. Steel adjustable props; 2. Horizontal struts; 3. Inclined

bracing; 4. Stringer; 5. Deck joists; 6. Plywood decking; 7. Wheel; 8. Handling loops; 9. Base plate. Source: Popa R., Teodorescu M., 1984.

Figure 2-9 Form table assembly Source: EFCO 2001.

The procedure for assembling a form table is as follows (Figure 2-9): - Positioning the stringer on the ground as required by the shop drawings. - Bolting the deck joists to the stringer with bolts.

- Laying the plywood on the joists and nailing them tighter by driving them directly into the lightweight steel.

- Positioning the adjustable steel props as support members (suitably braced) to carry the framed formwork decking (a framed wheeled arrangement can be fixed to the rear end of the table form so that the whole unit can be moved forward with ease).

- Maneuvering the form into position by attaching the handling loops to the crane hook. - Balancing in horizontal position and lowering on to the recently cast slab for

reposition-ing.

- Adjustment for aligning and leveling of form.

- Casting the new concrete slab and after hardening – removal of formwork.

- Final extraction of forms by maneuvering them clear of the structure to a point where they can be attached to the crane that lifts and repositions them to there new location.

- Provisions regarding existence of a working platform at the external edge of the slab that means elimination of independent scaffolds.

2.3.1.4. PATENT FORMWORK

Patent formwork (referred to also as System formwork) is usually identified by the manufacturer's name. It has the same common aim and similarity as traditional formwork, sat-isfying most of the technical, economical and functional requirements by the simplification, standardization and dimensional co-ordination of forms and by easy methods of positioning, securing and bracing them.

(20)

17

2.3.1.5. PERMANENT FORMWORK

In certain circumstances, formwork is left permanently in place because of the difficulty and cost of removing it once the concrete has been cast. Other times, it is used as both formwork and outer cladding especially in the con-struction of in-situ reinforced concrete walls. The external face or cladding is supported by the conventional internal face formwork, which can in certain circumstances over-come the external support problems often encountered. This method is, however, generally limited to thin small modular facing materials (insulating board, gypsum board, precast stone or concrete), the size of which is gov-erned by the supporting capacity of the internal formwork.

Figure 2-10 Permanent formwork

Source: ACI – Construction Engineering Journal.

2.3.2. CLASSIFICATION BY FINAL DESTINATION

2.3.2.1. FOOTING FORMS

Figure 2-11 Below grade footing forms Caption: 1. Poaling boards; 2.

Horizon-tal sheathing; 3. Vertical sheathing; 4. Spreaders; 5. Wales. Source: Suman

R.,1988.

Figure 2-12 Shallow footing

Continuous footing forms cast against excavations

Caption: 1. Traditional wooden panel;

2. Wood pegs; 3. Spreader, 4. Battens; 5. Plywood panel; 6. Wales.

A standard procedure for execution of footings is as follows:

- Positioning according to design to avoid loss of bearing area and eccentricity.

- Excavation of the last 100 mm of a footing cast in earth immediately before the concrete is placed.

- Lining of trenches with wax paper or polyethylene film to prevent earth-absorbing water from the concrete.

- Because the footing is below grade, no surface finish is necessary.

(21)

18

2.3.2.2. WALL FORMS The basic wall forms components are:

- Panel sheathing, to shape and retain the concrete until it sets.

- Studs, to form a framework and support the sheathing or Wales, to keep the form aligned and support the studs.

- Braces, to hold the forms erect under lateral pressure.

- Ties and spreaders, to hold the sides of the forms at the correct spacing.

Several basic methods are available which will enable a wall to be cast in large quanti-ties, defined lifts or continuous from start to finish: built in place forms, prefabricated forms and giant panels respectively gang forms.

a. Built-in-Place Forms are built in place when the design of the structure is such that prefab-ricated panels cannot be adapted to the shape or when the form is for one use only and the use of prefabricated panels cannot be economically justified.

Figure 2-13 Wall formwork

Caption: 1. Formwork panel; 2. Base plate; 3.

Adjust-able steel prop; 4. Working platform; 5. Plate washer; 6. Tie rod; 7. Spacer; 8. Guardrail; 9. Reinforcement; 10. Sole plate. Source: Popa R., teodorescu M., 1984.

a. Sheathing; b. Studs; c. Wales.

Note: When studs are used in form construction, wales are placed outside of them and held in place by nails, clips, or wale brackets nailed to the studs. When there are no studs, wales are placed against the plywood sheathing. In such a case, strongbacks-vertical members tied to-gether in pairs with long ties through the form-are set and braced to provide vertical rigidity.

A standard procedure to assemble a built in place wall form may be as follows: - Proper location on the foundation or slab from which the wall will rise.

- Sole plate anchoring on either the foundation or slab with preset bolts.

- Fastening of plywood sheathing to the studs. The first panel should be set and leveled at the highest point of the foundation to establish alignment for the remainder

- Insertion of ties as sheathing progresses, between the double-sided wales.

- When one side of the form has been completed, the other may be built in sections and set in place, with the tie ends being threaded through predrilled holes.

(22)

19

- Braces will be attached to the sole plate, form the wales or strongbacks. Braces may act in compression only, in tension only or in both, as when forms are braced on one side only heavy wire or cable is suitable for bracing that will be in tension only.

- Tightening of ties where possible and where not or not allowed, external bracing must be provided to securely support both forms.

- Plumbing of formwork, by adjusting the braces. If braces are not adjustable, the wall must be plumbed as the braces are installed and anchored. If one form is plumbed as soon as it is built, there is no need to plumb the opposing one. The ties and spreaders will plumb the second form automatically.

b. Giant Panels and Gang Forms. High walls, in which the concrete will have to be placed in two or more stages or lifts, will nor-mally be formed by the use of giant panels (panels much larger than the normal standard size) or by gang forming. These large forms are built or assembled on the ground by fasten-ing together a number of steel (wood)-framed panels and bracing them strongly to withstand crane handling.

Figure 2-14 Typical gang panel

Caption: 1. Form sheathing; 2. Stud; 3. Wales; 4.

Steel strut. Source: Popa R., Teodorescu M., 1984.

2.3.2.3. FLOOR FORMS

The design of forms for concrete floors depends a great deal on whether the floor is a slab-on-grade or a structural slab supported on a steel or concrete structural frame.

1. Slab-on-Grade Forms are forms for concrete slabs placed on grade are usually quite sim-ple. A standard procedure to produce a good slab-on grade from is as follows:

Figure 2-15 Typical slab-on-grade edge formwork Caption: 1. Screed; 2. Slab edge form; 3. Cast

con-crete. Source: Suman R., 1988.

- Concrete will be placed on com-pacted earth or gravel leveled base (the granular material serving as a capillary break under the slab where moisture in the subgrade be a prob-lem).

- Plank, plywood, or steel forms will be required for forming the edges (steel edge forms are commonly used on larger jobs and for highway work). - The forms will be held in place by

wooden pegs.

- Usually a vapor barrier will be placed under the slab on grade.

- Reinforcement in slabs on grade may or may not be specified depending on the slab area and the use of control joints. Normally the amount of steel that is specified for slabs on grade is for crack control rather than to increase the strength of the slab. Proper placement of the reinforcement in the slab is important if it is to be of value. If a single layer of

(23)

rein-20

forcement is specified, its location should be 50 mm below the top surface of the slab to help control cracking in the top of the slab.

- The reinforcement will be placed on its proper location according to the drawing on chairs, bolsters, and spacers made of either metal or concrete.

- Temporary wooden guides named screeds will be positioned in the slab area to help in bringing the concrete to the correct grade.

- When concrete has been placed to the correct level, the screed is removed and the depres-sion filled.

- If the slab is to be placed in sections, construction joints must be made between them, which will transmit shear from one to the other (see Concrete Placement).

2. Structural Slab Form general procedure assembly is as follows:

Figure 2-16 Girder form details

Caption: 1. Girder; 2. Panel end support; 3. Stringer; 4.

Prop; 5. Ledger; 6. Brace; 7. Prop; 8. Shore head; 9. Panel sheathing. Source: Andres C., 1998.

- Positioning of the girder or beam form bottom.

- Girder side forms will overlap the bottom form and rest on the shore heads and the sides of the column form.

- Side forms will be held in place by ledger strips nailed to the shore heads with double-headed nails. - Larger girders will have the side

forms vertically stiffened to prevent buckling.

- When constructing the girder and beam forms each part must be removed without disturb-ing the remainder of the form; strike-off formwork will commence with the beam and girder sides, followed later by the column forms, and finally by the beam and gird bot-toms.

Spandrel Beam Forms (deep beams that span openings in outer walls) need to be carefully formed. Form alignment must be accurate to produce an attractive wall. Shore heads are often extended on the outside to accommodate the knee braces used to keep the forms aligned. The ex-tended shore head also frequently supports a catwalk for workers.

Figure 2-17 Spandrel beam form details

Caption: 1. Stud; 2. Tie back; 3. Plywood sheet; 4. Ledger; 5. Joist; 6. Slab for sheathing; 7. Tie; 8.

(24)

21

2.3.2.4. COLUMN FORMS

Column forms are often subjected to greater lateral pressure than wall forms because of their comparatively small cross sec-tion and relatively high rates of placement. It is therefore neces-sary to provide tight joints and strong tie support. As column sizes increase, either the thickness of the sheath-ing must be increased or vertical stiffeners must be added to pre-vent sheathing deflection. In Figure 2-19, horizontal stiffening are used (ties of this type are generally referred to as yokes).

Figure 2-18 Typical example of column formed with plywood panels

stiffened with triangular yokes.

Source: Pestisanu C., 1995.

A standard procedure to assemble a column form may be as fol-lows:

- Locate column forms accurately by using templates, they are carefully located by chalk line or paint and anchored in posi-tion.

- Mark the location of each yoke on the side of the panel.

- Assemble three sides together, set the partially completed form in place, and add the fourth side later (this would probably be done in setting column forms for a job where the reinforcement is already in position).

- Anchor them at their base, and keeping them in a vertical position are prime considera-tions by using braces.

- Provision of a cleanout opening at the bottom of the form so that debris may be removed before concrete placing begins and to allow the placing of concrete in the bottom half of the form without having to drop it from the top.

- Final check of column forms position, plumbing, bracing, and ready to support the ends of the girder and the beam forms that will be built to them.

Note: The length of the column form is determined be subtracting the thickness of the bottom of the girder form that the column is to carry from the column height indicated on the plans or in the column schedule.

Modular steel panel forms provide several ways to form columns of various shapes and sizes. Generally, modular panels provide a fast and more accurate column form than job-built forms.

The form consists of four panels, of various widths and lengths (maximum forming height of 7.20 m and maximum edge of column 95 cm, 1.5 to 3 mm steel sheathing) that are fastened together at each corner with wedge bolts or clamps.

A standard procedure to assemble a steel column form may be as follows: - Selection of proper type and thickness of sheathing.

- Location of forms accurately by using templates, they are carefully located by chalk line or paint and anchored in position.

- Marking the location of each yoke on the side of the panel.

- Inspection of yokes to ensure that they withstand bending and shear and that deflection will not exceed 1,5 mm.

- Four-panel assembly by fastening them together on each corner, which sets partially, completed form in place.

(25)

22

- Stacking the rest of the panels until reaching the desired height and positioning the rest of the yokes on their upper frame.

- Final check of column forms position, plumbing, bracing, and ready to support the ends of the girder and the beam forms that will be built to them.

Figure 2-19 Typical steel column formwork

Caption: 1. Steel panel; 2. Inclined brace; 3. Yoke; 4. Filler panel. Source: IPC.

2.4. FORM LINERS

Form liners serve two purposes:

- To improve stripping of the form from the concrete surface without damaging the sheath-ing material or the concrete.

- To produce a desired texture on exposed concrete surfaces.

For a smooth finish plywood, steel, and fiberglass are usually used, while for textured finish, plastic and rubber liners are used but are usually limited to a single application.

2.5. RELEASE AGENTS

Release agents (referred to also as Bond Preventives) have traditionally been used as coatings to formwork to prevent adhesion of the concrete. These include oils, emulsions, chemical release agents, and waxes. Liquid bond preventives can be applied by hand or power-operated sprayer. Sprayer application requires less material and produces a smoother, uniform coating.

Main limitations of different agents used: discoloration, residual deposits, or failure to prevent bond conducting towards possible destruction of the concrete.

(26)

23

2.6. FORMWORK REMOVAL

The removal (referred to also as strike-off or stripping) of formwork shall be carried out by ensuring the strength of concrete and the stability of the remaining formwork.

The rate of hardening is temperature dependent and affects the timed removal of formwork, which will more than double in winter conditions. A special attention should be given, after form removal, to the construction member because it will bear the whole design load, which is very important especially for long-span members in flexure.

The following values of concrete hardening levels are recommended for striking off: 2,5 N/mm2 – for the lateral parts of the formwork;

70% of the concrete class for the inferior formwork parts of slabs and beams, with a span of minimum 6,0 m;

85% of the concrete class for the inferior formwork parts of slabs and beams, with a span of maximum 6,0 m.

The shores will be removed when the following values of concrete strength percentage is achieved:

95 % for members with maximum spans of 6,00 m; 112 % for members with spans of 6,0...12,0 m; 115 % for members with spans grater than 12,0 m.

Forms shall be designed so that removal may be made in the following order: side of beams and girders, slab and joist forms and beam and girder bottoms.

During formwork strike off the following rules must be followed:

- The parts and connections of the forms shall be so arranged that removal will be simple, protecting the concrete from damage and the form panel so that it may be reused without extensive repair.

- The procedure will be supervised by the engineer, when casting defects are seen (honeycombing, caverns etc.) that can influence the stability of the structure the works will be stopped until repair and rehabilitation.

- When forms adhere to the concrete, separation should be achieved by inserting wooden wedges and not by forcing the crowbars against the concrete.

- Beam and joist bottoms shall remain in place until final removal of all shoring under them.

- Formwork shall not be removed until the concrete maturity has developed sufficiently so to support all loads placed upon it. The time varies depending on the structural function of the member and the rate of strength gain of the concrete (concrete class, type of cement, w/c (mass ratio of water to cement), temperature during curing).

- Joist forms shall be designed and removed so that the shores may be removed temporarily to permit removal of joist forms but must be replaced at once. The shores and joists will be dismantled beginning from the middle of the members span, continuing symmetrically up the supports.

- The Engineer shall approve the sequence and pattern for removal of shores and for re-shoring before any of this work is done. Shores and reshores shall be in the same position on each floor to provide a continuous support from floor to floor; at no time shall large ar-eas of new construction be required to support their own weight even temporarily.

- The unfastening of accessories will be done steadily without shocks.

2.7. SHORING MEMBERS

Shoring members are used to support concrete forms and their contents. They can be divided into two major categories: horizontal shoring and vertical shoring.

(27)

24

Figure 2-20 Typical telescopic joists

a. Horizontal shores (also referred to as telescopic joists or centers) have the following characteristics:

- Are manufactured from wood or high-tensile steel with clear spans from 1,8 to 9m.

- According to their load and span, they can bear a specified load.

- They can be precam-bered to compensate any deflections when loaded. - They require shoring

and bracing.

- An assembled unit is lightweight and can be carried by one laborer.

Source: Chudley R., 1999.

b. Vertical shores are those that support the horizontal ones from a firm base below (e.g. con-crete slab).

- Vertical wood shores may be single wood posts, with wedges at the bottom to adjust the height, double wood posts, two-piece adjustable posts, or T head shores.

- Vertical steel shores may be adjustable pipe shores or shores made up of prefabricated metal scaffolding. Scaffold-type shoring, is usually assembled into towers by combining a number of units into a single shoring structure.

Figure 2-21 Typical adjustable steel shores Source: Teodorescu M. 1998, EFCO 2002.

(28)

25

Figure 2-22 Form accessories

Caption: a. Panel clamp; b. Scaffold tubes secured with wire lashing; c.

Cone; d. Tie spreader units; e. Plate washer with tie clamp; 1. Plywood panel; 2. Steel clamp; 3. Wood wedge; 4. Steel shaped clamp; 5. Bolt screw; 6. Lash with bolt; 7. Shore; 8. Cross bracing. Source: Popa R., Teodorescu M., 1984.

Along side the ba-sic components of formwork a number of products are available to aid in making forms stronger and erect-ing them faster. These products in-clude the following items: ties, spread-ers, wedges, corner brackets, clips, keys, column clamps, shores, form rods, concrete inserts, and many others.

2.9. MOULDS

All concrete sections made with poured-in-place concrete require some temporary means of support for the freshly mixed, plastic concrete. As in the case of precast sections, some means of support is necessary to hold the concrete in place during its curing period; this temporary framing is known as a mould.

2.9.1. CLASSIFICATION OF MOULDS

a. By structure and use - dismountable (some components of the mould can be dismantled for removal) and non-dismountable (removal is achieved by griping or expulsion of precast units).

b. By the position it has in the technological flow:

- Stationary: used primarily in the stand prefabrication technology.

- Portable: it moves from one workstation to another like in conveyer prefabrication tech-nology or flow of aggregates.

c. According to the loads that they bear:

- Non-bearing moulds. The only loads they bear are weight of mould, concrete, pressure of concrete on lateral faces and weight of reinforcement.

- Bearing moulds. They bear both the loads stated above and those of the tensions given by the prestressed reinforcement.

d. According to the solution adopted of heating the concrete - heating moulds and non-heating moulds (that are introduced after formation into steam rooms).

e. According to the number of units that are formed in the same mould - individual moulds (1 element), coupled moulds (2 elements) and batteries of moulds (several elements). d. According to the materials used for fabrication: The choice of materials is mainly a question of economical justification on individual projects. They can be of metal, wood, plas-ter, concrete etc.

2.9.2. BASIC COMPONENTS AND MAIN TYPES OF MOULDS

The basic mould components are: form sheathing, frame, and shoring elements.

(29)

26

In designing such details an effort is made to: select standard shapes and sizes for economy of mouldwork, lower operational costs, limit size variations, withstand the required number of reuses within permissible tolerances without excessive maintenance.

Concrete moulds - referred to also as pilot moulds because when precasting individual moulds a concrete replica (pi-lot model) of the final mould is con-structed. Several intermediate models, cast from the pilot model, are used to produce the required number of finished moulds.

Figure 2-23 Concrete pilot mould

Caption: 1. Mould concrete base; 2.

Com-pacted sand; 3. Concrete mould; 4. Concrete face; 5. Steel angle shape; 6. Edge form; 7. Metal plate; 8. Bolt.

Metal moulds - are sometimes used in place of milled woodwork, especially if a detail is repeated.

Figure 2-24 Static metal moulds

Caption: 1. Brace frame; 2. Striking of

de-vice; 3. Thermal insulation; 4. Heating duct; 5. Prefabricated element; 6. Mould sheet; 7. Sheet frame. Source: Popa R., Teodorescu

M., 1984.

Plaster moulds are used for various architectural or ornamental details. The moulds are made of casting plaster containing jute fiber and further reinforced by rods, where necessary. A mould can be used only once, as it is broken in stripping.

Wooden moulds – presents the limitation given by the woods tendency of the wood to swell. For this reason and ease of stripping, it is best not to recess deep into the concrete mass.

The most common method of casting concrete using moulds is by using the stack method. It consists of casting one element on top of another, with each successive element utilizing the preceding element as a casting bed.

The stack may be started in an excava-tion to permit a greater number of ele-ments to be poured direct from transit-mix trucks. All castings in one stack should have identical dimensions and openings of identical size and location. The stack method conserves space, per-mits castings to gain added strength be-fore removal, simplifies curing, and eliminates extra handling.

Figure 2-25 Stack wooden mould

Caption: 1. Mould base; 2. Mould

sheath-ing; 3. Stud; 4. Tie rods. Source: Popa R.,