Concrete Formwork Systems

Awad S. Hanna

• Publisher: CRC Press
• Year: 1998
• eBook ISBN: 9781135550424
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Offers insights on currently-used concrete formwork structures, from classification, system components and materials' properties to selection and construction requirements and procedures, while considering product quality, labour, safety and economic factors throughout. The text details hand-set, crane-dependent and crane-independent systems.

1
Concrete Formwork: An Introduction

1.1 CONCRETE CONSTRUCTION

A quality reinforced concrete structure offers many advantages over structures made with other building materials. Concrete is a durable material that reduces building maintenance costs and provides a longer service life. A concrete structure will reduce energy usage because of its mass and high resistance to thermal interchange. The use of concrete will lower insurance costs by virtue of its high resistance to fire. Buildings made of concrete are also more secure against theft and vandalism. Concrete floors and walls reduce the transfer of noise, yielding a quieter environment and happier occupants. Reinforced concrete possesses considerable strength for resisting seismic and wind loads. These factors and others make the selection of reinforced concrete an economical alternative.

1.2 CONCRETE FORMWORK

The construction of a concrete building requires formwork to support the slabs (horizontal formwork) as well as columns and walls (vertical formwork). The terms concrete formwork and concrete form carry the same meaning and are used interchangeably in this book. Formwork is defined as a temporary structure whose purpose is to provide support and containment for fresh concrete until it can support itself. It molds the concrete to the desired shape and size, and controls its position and alignment. Concrete forms are engineered structures that are required to support loads such as fresh concrete, construction materials, equipment, workers, various impacts, and sometimes wind. The forms must support all the applied loads without collapse or excessive deflection.

1.2.1 Formwork System

A formwork system is defined as “the total system of support for freshly placed concrete including the mold or sheathing which contacts the concrete as well as supporting members, hardware, and necessary bracing.” Formwork system development has paralleled the growth of concrete construction throughout the twentieth century. As concrete has come of age and been assigned increasingly significant structural tasks, formwork builders have had to keep pace. Form designers and builders are becoming increasingly aware of the need to keep abreast of technological advancements in other materials fields in order to develop creative innovations that are required to maintain quality and economy in the face of new formwork challenges.

Formwork was once built in place, used once, and subsequently wrecked. The trend today, however, is toward increasing prefabrication, assembly in large units, erection by mechanical means, and continuing reuse of forms. These developments are in keeping with the increasing mechanization of production in construction sites and other fields.

1.3 FORMWORK ECONOMY AND SIGNIFICANCE

Formwork is the largest cost component for a typical multistory reinforced concrete building. Formwork cost accounts for 40 to 60 percent of the cost of the concrete frame and for approximately 10 percent of the total building cost. Figure 1.1a, b presents a breakdown of different cost categories for conventional concrete slab and wall formwork. A large proportion of the cost of conventional formwork is related to formwork labor costs. Significant cost saving could be achieved by reducing labor costs.

Formwork costs are not the only significant component of the formwork life cycle. Other important aspects of the formwork operation include speed, safety, and quality.

Figure 1.1 Distribution of costs for cast-in-place concrete slab wall: (a) slab; (b) wall.

1.3.1 Speed

Speed of construction is defined as the rate in which concrete building is raised and can be expressed in terms of number of floors erected per week or months. Speed of construction can be also measured in terms of inches or millimeters of concrete poured per hour. Formwork operations can control the pace of construction projects. Formwork is typically supported by several levels of shores and reshores that carry the loads until the concrete gains enough strength to support its own weight and all other externally applied loads. Shores are vertical members made of wood that support recently built concrete that have not developed full design strength. On the other hand, reshoring occurs when the original shoring is removed and replaced in such a manner as to avoid deflection of the cured concrete. As a result, several floors may be blocked, preventing the progress of any other construction activities. Faster formwork cycle from erection to stripping would allow for faster removal of shoring and reshoring and faster overall project progress.

1.3.2 Safety

Formwork operations are risky, and workers are typically exposed to unsafe working conditions. Partial or total failure of concrete formwork is a major contributor to deaths, injuries, and property damages within the construction industry. Another common hazard occurs during stripping of formwork in which loose formwork elements fall on workers under the concrete slab being stripped.

Structural collapses and failures involving concrete structures account for 25 percent of all construction failures. More than 50 percent of concrete structure failure during construction is attributed to formwork failure. Formwork failures result from faulty formwork structural design, inadequate shoring and reshoring, improper construction practices during construction, inadequate bracing, unstable support or mudsills, and insufficient concrete strength to sustain the applied load after construction.

Contractors are generally responsible for stability and safety of concrete formwork. Contractors are guided by several federal, state, and local codes and regulations that regulate formwork safety. Most of these documents provide general guidelines for safety but provide no guarantee against failure. Contractors typically are trying to achieve fast removal of formwork elements without compromising the safety and integrity of structures.

1.3.3 Quality

The quality of the resulting concrete is dictated by the quality of formwork materials and workmanship. Many concrete-related problems such as discoloration, stains, and dusting are attributed to concrete formwork. Also, some deformed concrete surfaces are due to deformed formwork systems caused by repetitive reuse and inadequate support of formwork.

1.4 AN INTEGRATED CONCRETE/FORMWORK LIFE CYCLE

The purpose of this section is to introduce formwork operation as an integrated part of the whole building process and to explain some of the terminology used in concrete and concrete formwork. The process of providing formwork and concrete is highly integrated. The left circle in Figure 1.2 represents the formwork life cycle, while the right circle represents the concrete construction life cycle. The two intersecting points represent the beginning and the end of the concrete construction life cycle.

The life cycle of formwork starts with the “choose formwork” activity. The physical activities in the formwork life cycle are represented by these steps: (1) fabricate formwork; (2) erect formwork; and (3) remove formwork. The concrete construction life cycle starts after the “fabricate formwork” activity and ends before the “remove formwork” activity. The function of the formwork life cycle is to provide the structure with the specified shape and size, while the function of the concrete construction life cycle is to provide the structure with concrete of specified strength, durability, and surface texture. A brief description of each stage of both the concrete and formwork life cycles is given below.

Figure 1.2 Integrated concrete formwork life cycle.

1.4.1 Choose a Formwork System

The choose formwork system activity includes the process of selecting formwork systems for different structural elements. It also includes the process of selecting accessories, bracing, and a release agent for the selected formwork system. There are several forming systems used in the construction of reinforced concrete structures. For example, formwork systems for concrete slabs can be classified as hand-set or conventional systems and crane-set systems. Conventional systems are still the most common and popular formwork systems. Their popularity stems from their ability to form different shapes and elements. However, conventional formwork usually results in high labor and material cost. Non-conventional or crane-set systems have gained increasing popularity because of low labor costs and their ability to achieve faster construction cycle.

1.4.2 Fabricate Formwork

The second step in the formwork life cycle is fabricate formwork. This activity includes receiving formwork materials, cutting and stockpiling the materials by sizes and types, assembling the pieces into the desired shapes and sizes, and storing the forms near the lifting devices. The contractor may also choose between building forms on the job site by setting up a special fabrication area, or building many forms in a central yard facility and transporting them to the site. The contractor may also choose between building the forms themselves and buying or renting them. Many contractors find that renting forms for specific usage allows them more flexibility in controlling the volume of work they are able to perform.

1.4.3 Erect Formwork, Place Inserts, and Reinforcement

The method and sequence of erecting formwork may vary depending on the availability of lifting equipment and whether reinforcing cages are available. Forms are usually handled manually, by small derrick, or by crane. The erect formwork activity includes the process of lifting, positioning, and aligning the different form-work elements. This activity also includes the process of applying the form release agent or coating that prevents bonding of concrete to forms. The concrete life cycle starts after the erect form-work activity is finished with placing inserts and reinforcement activity. The logical sequencing of erecting formwork and its relation to placing inserts and reinforcement is:

1. Set lines—a template is generally set in place on the floor slab or footing to accurately locate the column floor
2. Erect scaffolding
3. Install column reinforcement
4. Provide forms for column
5. Erect outside forms for walls
6. Install wall reinforcement
7. Erect inside forms for walls
8. Install ties
9. Provide bracing for walls
10. Erect forms for beams
11. Install beam reinforcement
12. Erect forms for slabs
13. Place inserts for mechanical and electrical connections, openings for ducts and conduits, and supporting bars for reinforcement
14. Place secondary and main reinforcement

Figure 1.3 shows inserts and reinforcement installed above the forms.

Figure 1.3 Reinforcement and inserts installed above forms.

A form coating or release agent is often applied to the inside surface of formwork to prevent the concrete from bonding to the formwork elements. Coating can be applied by spraying, brushing, or by a roller. Form coating facilitates the operation of removing the formwork after the concrete has gained enough strength to support itself. Another function of the formwork coating is sealing the surface of the wooden elements which prevent the water in freshly placed concrete from being absorbed by wood. Form release agent should not affect or react with the finished concrete in any way.

1.4.4 Place Concrete

This activity includes mixing, transporting, pumping, and placing of the concrete. The concrete used in most projects is truck-mixed. Concrete is usually transported by belt conveyers for horizontal applications, by buckets for delivery via cranes, by chutes for delivery via gravity to lower levels, and by pumping for horizontal and vertical delivery of concrete.

1.4.5 Consolidate Concrete

Consolidation is the process of compacting or striking the concrete to mold it within the forms, around embedded inserts and reinforcement. It is also done to remove the humps and hollows. Consolidation of concrete is usually performed with hand tools or mechanical vibrators to guarantee a dense structure.

1.4.6 Finish Concrete

This activity includes the process of treating the exposed concrete surfaces to produce the desired appearance, texture, or wearing qualities. Finishing of concrete is usually performed by moving a straight edge back and forth in a sawlike motion across the top of the concrete.

1.4.7 Cure Concrete

The hardening of concrete is a chemical process that requires warmth and moisture. This activity involves curing concrete with water, steam, or any other method to prevent shrinkage and allow the concrete to gain sufficient early strength. Steam curing is used where early strength gain of concrete is important. After the concrete is cured, the rest of the formwork life cycle continues with the strip forms activity. The cure concrete and strip forms activities are interchangeable depending on the type of structural element. For example, columns and walls are cured after stripping of the forms, while slabs and beams are cured before and after the forms are stripped.

1.4.8 Strip Forms

As soon as concrete gains enough stren...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• Acknowledgments
• 1 Concrete Formwork: An Introduction
• 2 Horizontal Formwork Systems: Hand-Set Systems
• 3 Slab Form Design
• 4 Horizontal Formwork Systems: Crane-Set Systems
• 5 Selection Criteria for Horizontal Formwork System
• 6 Vertical Formwork Systems: Crane-Dependent Systems
• 7 Wall Form Design
• 8 Vertical Formwork Systems: Crane-Independent Systems
• 9 Selection Criteria for Vertical Formwork System
• References
• Appendix