Formwork For Concrete

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Formwork For Concrete

• INTRODUCTION

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Temporary Structures (TS)

• Definition
• Any means or methods which provide temporary
  support, access, enhancement, or otherwise
  facilitate the construction of permanent
  structures.
• Necessity
• TS form the interface of design and construction.
  Most permanent structures simply could not be
  built without TS.

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Temporary Structures (continued)

• Impact on Schedule, Cost, and Quality
• Losses in time and money will occur if the TS are
  not planned and coordinated with the same degree
  of thoroughness as the permanent structures.
• Safety
• Failure of TS have been responsible for hundreds
  of deaths on construction sites. Safety should be
  the overriding priority of contractors and
  designers responsible for implementing TS.

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Categorizing Temporary Structures

• Several common types of work which occur on
  construction sites are
• Formwork
• Falsework
• falsework generally serves to support massive
  structural members until such time as these have
  attained sufficient load-bearing capacity and to
  sustain the loads which occur in the course of
  the erection or removal of structures, as imposed
  by structural members, construction and handling
  equipment, and by temporary storage of building
  materials, components and equipment.
• Rigging
• Scaffolding
• Excavation support

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Responsibility

• The norm in the construction industry is to place
  the responsibility for TS solely on the general
  contractor. However, architects and engineers
  must at least have formulated their own method of
  construction.
• Cast in-place
• Precast
• Tilt-up
• Coordinating the design of permanent structure
  with the TS that will be required can lead to
  more efficient and cost effective construction.

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Design Consideration

• Safety
• Designers must place the first priority on
  safety. OSHA codes, as well as other codes in the
  industry, provide stringent performance
  specifications (how the system should work)
  regarding TS.
• Cost
• TS can be the most expensive part of some
  construction projects. Designing cost-effective
  solutions to TS problems could easily be the
  competitive advantage of a contractor over
  others. The designer must have a thorough
  knowledge of all the options which will
  sufficiently solve the TS problem.

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Design Consideration (continued)

• Unique Design Challenges
• TS are subject to unique loading conditions which
  do not apply to a permanent structure
  (fluctuating or dynamic loads, impact loads,
  loads which change position).
• Working within spatial constraints- Cramped sites
  require the most efficient TS so that workers
  still have room to maneuver safely.
• Uncertainty of soil conditions- It is always
  possible that an unforeseen condition could arise
  during an excavation. Designers must include an
  appropriate factor of safety in their
  calculations or they may consider contingency
  plans for changing soil conditions.

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The Contractor

• In many cases the contractor is the only member
  of the construction team with considerable
  experience and practical knowledge of TS.
• The contractor must hire his or her own engineer,
  if the specifications or building codes require
  one, or self perform the design of TS.
• The most complex TS are often handled on a
  design-build basis. The design-build situation is
  optimal because it guarantees coordination
  between design and construction.

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Concrete Formwork (FW)

• Forms are TS that provide containment for the
  fresh concrete and support it until it can
  support itself.
• Forms must be designed to support loads of the
  fresh concrete, equipment, workers, impact of
  various kinds, or sometimes wind without collapse
  or excessive deflection.
• The cost of FW is between 40 to 60 of the cost
  of concrete structure. Design of a good forming
  system could both expedite a project as well as
  reduce costs.

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Formwork Requirements - 1

• Safety FW must be
• Strong ( to carry the full load and side pressure
  from freshly placed concrete, together with
  construction traffic and equipment).
• Sound (made of good quality, durable materials).

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Formwork Requirements - 2

• Quality FW must be
• Accurate (within specified tolerances for form
  dimension)
• Rigid (adequately braced and tied to prevent
  movement, bulging, or sagging during
  construction).
• Tight Jointed (to prevent cement paste leakage
  which disfigures the surface of concrete).
• Properly Finished (to provide a concrete surface
  of good appearance).

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Formwork Requirements - 3

• Economy FW must be
• Simple (simple to erect and dismantle)
• Easily handled (the sizes of units should not be
  too heavy to handle)
• Standardized (ease of assembly and possibility of
  reuse)

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Causes of Failures

• Improper stripping and shore removal
• Inadequate bracing
• Vibration
• Unstable soil under mudsill, shoring not plumb
• Inadequate control of concrete placement.
• Rate of vertical placement of concrete can
  develop excessive lateral pressure????

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Planning for Formwork

• The contractor should plan FW at the time of
  making bid considering the following factors
• Placing schedule and stripping time requirements
• Capacity of equipment available to handle form
  sections and materials
• Capacity of mixing and placing equipment
• Construction joints
• Reuse of forms as affected by stripping time
• Relative merits of job-built, shop-built and
  ready-made forms.
• Weather (protection requirements and stripping
  time)
• Compare alternative methods to determine the most
  efficient plan.

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Key area of Cost Reduction - 1

• Planning for maximum reuse
• A form designed for max reuse is stronger and
  more expensive, but it can save on the total form
  cost.
• Economical form construction
• Shop-built (greatest efficiency in working
  conditions and in the purchase and use of
  materials and tools)
• Shop area on the site (form sections too large or
  transportation cost too high)
• Job-built (for small jobs, or where forms must be
  fitted to terrain)

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Key areas of Cost Reduction - 2

• Buying prefabricated forms(large number of
  reuses)
• Renting prefab forms(better flexibility in
  regulating volume of work)
• Setting and stripping
• Repetition of the same functions to increase the
  crew efficiency as the job progresses
• Use of metal clamp or special wedge pin
  connections that are secure, yet easy to assemble
  and dismantle
• Add extra features that make handling, erection,
  and stripping easier (handles, lifting eyes)

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Other costs affected by FW - 1

• Cranes and Hoists
• Size of form sections should be limited to the
  capacity of the largest crane planned for the
  job.
• Stair towers may be completed early in the
  schedule to be used for moving men and materials.
• Leave one bay open to permit mobile crane and
  concrete truck movement.
• Bar Setting
• Form design can permit the rebar to be pre
  assembled before installation (more favorable
  condition)

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Other Costs affected by FW - 2

• Concrete Placement
• High lifts in wall construction make placing and
  vibration difficult.
• Placing rate is limited by form design.
• Other Trades
• The plan should permit other trades to perform
  their work efficiently and minimize interruptions
  in placing.

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Loads and Pressures on Forms

• Lateral pressure exerted by plastic liquid
  concrete
• Rate of Placement
• Temperature of concrete
• Low temperatures produce delayed set, greater
  lateral pressure at base of form
• Placement height
• Hydrostatic pressure
• Vibration increases lateral pressure

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• Maximum lateral pressure at any elevation
• Walls rate of pour less than or equal to 7 ft
  per hour
• P 150 9000R/T
• Walls rate of pour greater than to 7 ft per
  hour
• P 150 43,400/T 2800R?T
• Maximum P lesser of 2000 psf or 150h
• P max lateral pressure, psf
• R rate of placement, ft/hr
• T temp. of concrete in forms, F
• h max. ht. of fresh concrete in form, ft

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Column Forms

• Maximum lateral pressure at any elevation
• Columns
• P 150 9000R/T
• Maximum P of 3000 psf, a minimum of 600 psf, but
  in no case greater than 150h.
• P max lateral pressure, psf
• R rate of placement, ft/hr
• T temp. of concrete in forms, F
• h max. ht. of fresh concrete in form, ft
• Maximum ht. Of a single lift recommended for a
  column pour is 18 ft. within a two-hour period.

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Example- Rate of Placement

• Calculate the rate of pour for a section of shear
  wall on the 12th floor of a high rise building.
  Assume the distance from the ground to the top of
  the wall is 168 ft. A tower crane using two
  buckets, each with a capacity of 1.5 cy, has a
  rate of travel of 90 ft/min up and 120 ft/min
  down. Assume a pick-up time of 20 sec. and a
  dump time of 5 minutes
• Wall height is 14 ft, wall thickness is 10-in.
  and wall length is 60 ft.

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Example- Concrete Pressure Calculation

• The shear wall of height 14 ft, wall thickness of
  10-in. and wall length of 60 ft. is poured at a
  rate of 5.65 ft/hr. Calculate the maximum
  pressure developed in the concrete form assuming
  a concrete temperature of 55F.
• Determine the height to which this maximum
  pressure will extend.