Formwork For Concrete

1. 1 Formwork For Concrete INTRODUCTION

2. 2 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.

3. 3 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.

4. 4 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

5. 5 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.

6. 6 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.

7. 7 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.

8. 8 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.

9. 9 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.

10. 10 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).

11. 11 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).

12. 12 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)

13. 13 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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18. 18 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.

19. 19 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)

20. 20 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)

21. 21 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)

22. 22 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.

23. 23 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

24. 24 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

25. 25 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.

26. 26 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.

27. 27 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.