Chapter 15 - Formwork

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Chapter 15 Formwork Guide to Concrete Construction Chapter 15 Formwork Chapter 15 Formwork Formwork has a dual function in concrete INTRODUCTION 15.2 construction. It supports the plastic concrete until the latter is sufficiently strong to support the actions/loads imposed upon it and it imparts a finish to the concrete surface. This Chapter describes the different types of formwork used in modern concrete construction and outlines the requirements which must be met for formwork to perform
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    Chapter 15 Formwork   Guide to Concrete Construction      Chapter 15 Formwork   Guide to Concrete Construction   15.1 Chapter 15 Formwork Formwork has a dual function in concrete construction. It supports the plastic concrete until the latter is sufficiently strong to support the actions/loads imposed upon it and it imparts a finish to the concrete surface. This Chapter describes the different types of formwork used in modern concrete construction and outlines the requirements which must be met for formwork to perform satisfactorily. The special requirements associated with the achievement of visually satisfying surface finishes are discussed in Chapter 13 Control of Surface Finishes . Supplementary information on formwork is contained in CCANZ publications IB 29 Formwork for Concrete  and IB 41 Formwork Detailing  . INTRODUCTION 15.2  Relevant New Zealand and Australian Standards 15.1 BASIC COMPONENTS OF FORMWORK   15.2   15.2 REQUIREMENTS   FOR   FORMWORK   15.2  15.2.1 General 15.2.2 Strength 15.2.3 Stiffness 15.2.4 Accuracy 15.2.5 Watertightness 15.2.6 Robustness 15.2.7 Ease of Stripping 15.2.8 Standardisation 15.2.9 Safety 15.3 MATERIALS FOR FORMWORK   15.5  15.3.1 General 15.3.2 Choice of Materials 15.4 FORMWORK SYSTEMS   15.6  15.4.1 Modular Formwork 15.4.2 Gang Forms 15.4.3 Table Forms 15.4.4 Jump/Climb Forms 15.4.5 Slipforms 15.4.6 Permanent Formwork 15.5 DESIGN OF FORMWORK   15.7  15.5.1 General 15.5.2 Loads on Formwork 15.6 FALSEWORK   15.9  15.6.1 Design of Support Structures 15.6.2 Undisturbed Shores 15.6.3 Reshoring Systems 15.7 CONSTRUCTION   OF   FORMWORK   15.10  15.7.1 Erection of Formwork 15.7.2 Preparation for Concreting 15.7.3 Stripping of Formwork SUMMARY   15.13      Chapter 15 Formwork   Guide to Concrete Construction   15.2 INTRODUCTION Formwork is the temporary structure which moulds concrete into the desired shape, and holds it in the correct position until it is able to support the loads imposed upon it. It also imparts the required surface finish. Formwork and its supports (falsework) is a structural system and must be designed and built accordingly. The actions (loads) on it may be temporary but they can be extremely large. Frequently they are different in nature to those imposed on the finished concrete structure. Concrete is an extremely plastic and mouldable material which will accurately reflect the shape, texture and finish of the surface against which it is cast. Any imperfection or inaccuracy in this surface will be indelibly inscribed on the concrete surface. Form-face materials must therefore be chosen both to achieve the required surface finish and, in conjunction with all the supporting elements, to maintain accuracy and stability under all the loads imposed during erection and concreting, and for some days into the life of the concrete structure.  At early ages, the concrete will not be able to support the loads imposed on it. Until it is able to do so, the formwork (and falsework) will therefore continue to be a loadbearing structure. Only when the concrete has achieved sufficient strength can the formwork be removed without any detrimental effect to the concrete structure. Failure to meet the accuracy, stability and strength requirements will lead to formwork failures in the form of bowing, warping, misalignment, etc. reflected in the final structure. It could even lead to the catastrophic collapse of part or all of the formwork. New Zealand does not have a specific standard for formwork but has requirements in NZS 3109 and NZS 3114. NZS 3109 also cross-references to the Formwork for Concrete  AS 3610 document. The cost of formwork is often a very significant item in the overall cost of a project. The formwork system should be the most economical available but cost should never be permitted to overrule the criteria governing safety, strength and stability. Indeed, the first cost of formwork may be a very poor guide to its suitability for a project. Multiple uses of good quality formwork can result in substantial overall economies. Formwork design and selection of materials should therefore always be approached on the basis of cost per use. Relevant New Zealand Standards NZS 3109 Concrete construction NZS 3114 Concrete surface finishes Relevant Australian Standards  AS 3600 Concrete structures  AS 3610 Formwork for concrete Supplement 1 Blowhole and colour evaluation charts Supplement 2 Commentary 15.1 BASIC COMPONENTS OF FORMWORK The basic components of formwork for typical concrete elements are shown in Figures 15.1 to 15.4 (page 15.3). It will be noted that the basic structure of almost all formwork is the same. It comprises:    formface, e.g. a metal or plywood sheet, sawn timber;    studs, or joists, lengths of sawn timber or, sometimes, metal sections, which support the formface and prevent it from bulging or bowing in one direction; and    walers, or bearers, which brace the studs or support the joists, and prevent bulging or bowing in the other direction.  An important facet of formwork design and construction is the choice of spans (or centres) between studs, and then centres between walers or bearers, to prevent bulging and bowing. 15.2 REQUIREMENTS FOR FORM-WORK 15.2.1 General  Although formwork is constructed only to contain and support concrete until the cast structure is strong enough to support the imposed loads, it must provide a safe environment for all those working on or around it. In addition to being strong enough, it must also be stable against overturning, uplift, and sideways movements. It must also meet all statutory requirements for access ladders, guardrails, working platforms, etc. The requirements for formwork are summarised in Table 15.1  (page 15.3). A great deal of further information is contained in AS 3610 and the two Supplements to that Standard. Supplement 2 is particularly useful for assessing the safety of formwork.    Chapter 15 Formwork   Guide to Concrete Construction   15.3   Figure 15.1 Wall forms   Figure 15.2 Beam form and supports   Figure 15.3 Column forms Figure 15.4 Typical soffit forms and falsework (diagrammatic only, bracing not shown) Table 15.1 Requirements for formwork   Property Purpose Strength Carry imposed loads.   Stiffness Maintain specified shape and avoiddistortion of concrete elements.    Accuracy Ensure shape and size of concreteelements. Ensure specified cover to reinforce-ment.   Watertightness Avoid grout loss and subsequenthoneycombing of the concrete.   Robustness Enable re-use.   Ease of stripping Avoid damage to concrete surfaces.   Standardisation Promote economy.   Safety Ensure a safe working environment.   NZS 3109 and NZS 3114 also contain specific requirements. With new materials, these requirements may be readily met. With re-use, all materials, except perhaps metal components, may be weakened; and even metal components may become loose fitting, or broken, due to wear. All formwork materials and components should be checked regularly to ensure that they are sound and safe. 15.2.2 Strength  All components should be designed to cater for the most severe loads that are likely to be imposed on the formwork. To achieve this, the design should be carried out by a person experienced and competent in formwork design. Care should then be taken to ensure that the design details are met and that the construction
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