Formwork For Concrete PDF

FORMWORK FOR CONCRETE Formwork refers to the temporary Formwork: molds / frameworks / supports which are used to shape and support newly can be erected quickly poured concrete until it becomes self- is highly loaded for a few hours during supporting. t...

FORMWORK FOR CONCRETE Formwork refers to the temporary Formwork: molds / frameworks / supports which are used to shape and support newly can be erected quickly poured concrete until it becomes self- is highly loaded for a few hours during supporting. the concrete placement Formwork development has and within a few days disassembled paralleled the growth of concrete for future use. construction throughout the 20th century. The connections, braces, tie anchorages, and adjustment devices The increasing acceptance of which forms need are also temporary. concrete as a major construction material presents the form builder a new range of problems in the development of appropriate sheathing materials and maintenance of rigid tolerances. FORMWORK FOR CONCRETE Formwork, hardware and accessories Typical Formwork Section can be used repeatedly, which necessitates durable and maintainable materials. Formwork disassembly is severely restricted by concrete bond, rigidity, and shrinkage. FORMWORK FOR CONCRETE Lumber was once the predominant form material. Newer uses include metal, plastics, and other materials, along with the use of specialized accessories. Formwork was formerly constructed in place, used once and then discarded. The trend today is toward increasing prefabrication, assembly in larger units, erection by mechanical means such as placement by crane, and reusing the forms. FORMWORK FOR CONCRETE The purpose of the forms is to mold the concrete into a desired size and shape while controlling its alignment and position. Formwork needs to support its own weight and newly placed concrete, and endure construction live loading from other materials, equipment and personnel. Objectives of Building Forms: Quality: Strength, rigidity, position, dimensions. Safety: For the workers and structure. Economy: Lowest cost after consideration for quality and safety. TYPES OF FORMWORK Timber. Built on site using lumber and plywood. Time-consuming to produce for large structures. Plywood has a short lifespan. Has advantages in markets where labor costs are lower than those for obtaining reusable formwork. It is also the most flexible type of formwork. Prefabricated, modular. Modules which are prefabricated and can be easily assembled and disassembled. Made of steel, aluminum or high-density plastics. These are typically lightweight and durable, and known to improve the finished concrete surface. The various panels can be interconnected to form different shapes and sizes. TYPES OF FORMWORK Stay-In-Place Structural Systems. Assembled on-site, utilizing prefabricated fiber- reinforced plastic forms, metal or concrete. Typically for columns and piers, also for bridge decks and other uses. Stays in-place after the concrete has cured. Flexible. Lightweight, high strength sheets of fabric or other “softer” materials to take advantage of the fluidity and plasticity of concrete by creating customized building forms and shapes allowing for architectural features. FORMWORK FOR CONCRETE When designing and building formwork, a contractor should achieve economy without sacrificing safety or quality. The concrete structural elements depend on accurate form construction. The forms must be: Rigid under the construction loads to maintain the designed shape of the desired concrete. Stable enough to maintain large members in alignment. Strong enough to maintain position. Constructed to endure handling / reuse without losing integrity. *Note: Formwork must remain in-place until concrete can carry its own weight. FORMWORK FOR CONCRETE A significant percentage of concrete cost is the formwork. Contractors must absolutely plan for Material formwork at the time of bid. Impacts to: schedule and stripping times Placement capacity of available equipment to handle form sections / materials capacity of mixing and placing equipment Labor construction joints reuse of forms job- , shop- built forms vs. ready made construction milestones / weather FORMWORK FOR CONCRETE How to economize… 1. Design for maximum reuse A form designed for maximum reuse may be stronger and more expensive, but can save on total cost 2. Design economical form construction use shop-built-forms– provides the greatest efficiency in purchase and use of materials and tools create shop area on the site− to form sections too large or transportation cost too high use job-built− for small jobs, or where forms must be fitted buy prefabricated forms (maximum reuse!) rent prefabricated forms 3. Set and strip repetition of the same functions to increase crew efficiency as the job progresses use metal clamps or special wedge pin connections that are secure and easy to assemble / dismantle add extra features that make handling, erection, stripping easier (for example… handles) FORMWORK FOR CONCRETE 4. Cranes and Hoists size of form sections should be limited to the capacity of the largest crane planned construct stair towers early if it aids the schedule in moving workers and materials plan for gaps in construction to permit crane and vehicle access 5. Reinforcement form design may allow rebar to be pre-assembled before installation (cages) 6. Concrete Placement avoid high lifts in wall construction due to placement and vibration difficulties rate of placing concrete is limited by form design FORMWORK FOR CONCRETE Form Materials Plywood Wales Provide Support for the Studs Studs Provide Support for the Plywood Wale Spacing Equals Stud Spacing FORMWORK DESIGN Ties Anchors hold the sides of concrete forms embedded in previously placed together against the fluid pressure of concrete or rock to attach or support flowing concrete formwork are loaded in tension Hangars have an end connector that attaches for attaching formwork to steel or them to the sides of the form precast concrete framing members may be designed for partial removal Clamps or complete removal used for surround column forms, supporting lateral pressure BASIC FORCES ON WOOD FORMWORK DESIGN Allowable Stresses for Lumber Load Duration CD : Bending stresses can be increased by 25% for short duration loads (~7 days or less) on materials intended for limited reuse. Repetitive Members Cr : One of three members, spaced no more than 2 feet apart, and joined by a load- distributing element such as plywood. Bending stress can increased by 15% when these requirements are all met. *Note: ACI recommends ignoring the repetitive member allowance if the load has already been adjusted for a short duration. FORMWORK DESIGN Allowable Stresses for Lumber Moisture CM : Affects wood above 19%. Softens wood, less able to carry stresses Size CF : Increase in cross sectional size reduces stresses Perpendicular to Grain Compression Cb : Perpendicular to grain bearing stress at the ends of the beam may be adjusted for length of bearing… Cb = (lb + 0.375) / lb where lb is parallel to the grain FORMWORK DESIGN Allowable Stresses for Lumber Horizontal Shear Constant CH : Condition of lumber at ends of beams (limited defects, increase factor) Temperature CT : Adjustment for concrete temperatures above 100ºF Stability CP : Adjusts base stress for buckling based on slenderness FORMWORK DESIGN Plywood economical in large panels available in various thicknesses creates smooth, finished surfaces on concrete has predictable strength is manufactured in more than 40 surface textures that can provide various architectural finishes available in two types: interior and exterior (waterproof glue) various thickness depends on veneer layers cross banding produces panels with desirable properties in both directions FORMWORK DESIGN Plywood Grade and Section Properties FORMWORK DESIGN No need to calculate moment of inertia or section modulus for plywood. The tabulated section properties can be used for calculating flexural stresses, shear stresses, and deflections. Moment of Inertia will be used to calculate deflection. Section Modulus will be used to calculate bending stress. Rolling Shear Constant will be used to calculate the rolling shear stress in plywood having loads applied perpendicular to the panel. FORMWORK DESIGN Loads on Concrete Formwork Flowing concrete (fluid) – hydrostatic pressure Fresh concrete can only approximate a fluid Internal friction of particles (aggregate, water, cement, air) Self-supporting properties (pressure reduces as concrete hardens) Factors to Consider Regarding Lateral Pressure of Concrete density of the mixture the temperature of the mixture the rate of placement of the concrete in the forms use of admixtures or cement replacements effect of vibration or other consolidation methods NOT aggregate size and shape, size of form cross section, consistency of the concrete, amount and location of the reinforcement, and the smoothness of the surface of the forms