Category CONCRETE FORMWORK

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.

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.

Place Concrete

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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 deliv-

ery via gravity to lower levels, and by pumping for horizontal and vertical delivery of concrete.

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 form­work 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 form­work 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. Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.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.

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.

Fabricate Formwork

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.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.

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. Noncon­ventional or crane-set systems have gained increasing popularity because of low labor costs and their ability to achieve faster con­struction cycle.

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.

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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.

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.

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.

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.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.

Speed

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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.