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Monday, 5 February 2018

Concrete Formwork

A quality reinforced concrete structure offers many advantages
over structures made with other building materials. Concrete is
a durable material that reduces building maintenance costs and
provides a longer service life. A concrete structure will reduce energy
usage because of its mass and high resistance to thermal
interchange. The use of concrete will lower insurance costs by
virtue of its high resistance to fire. Buildings made of concrete are
also more secure against theft and vandalism. Concrete floors and
walls reduce the transfer of noise, yielding a quieter environment
and happier occupants. Reinforced concrete possesses considerable
strength for resisting seismic and wind loads. These factors
and others make the selection of reinforced concrete an economical
The construction of a concrete building requires formwork to support
the slabs (horizontal formwork) as well as columns and walls
(vertical formwork). The terms concrete formwork and concrete
form carry the same meaning and are used interchangeably in this
book. Formwork is defined as a temporary structure whose purpose
is to provide support and containment for fresh concrete until
it can support itself. It molds the concrete to the desired shape

and size, and controls its position and alignment. Concrete forms
are engineered structures that are required to support loads such
as fresh concrete, construction materials, equipment, workers,
various impacts, and sometimes wind. The forms must support all the
applied loads without collapse or excessive deflection.
·        Formwork System
A formwork system is defined as ‘‘the total system of support for
freshly placed concrete including the mold or sheathing which
contacts the concrete as well as supporting members, hardware,
and necessary bracing.’’ Formwork system development has paralleled
the growth of concrete construction throughout the twentieth
century. As concrete has come of age and been assigned increasingly
significant structural tasks, formwork builders have had
to keep pace. Form designers and builders are becoming increasingly
aware of the need to keep abreast of technological advancements
in other materials fields in order to develop creative innovations
that are required to maintain quality and economy in the face
of new formwork challenges.
Formwork was once built in place, used once, and subsequently
wrecked. The trend today, however, is toward increasing
prefabrication, assembly in large units, erection by mechanical
means, and continuing reuse of forms. These developments are in
keeping with the increasing mechanization of production in construction
sites and other fields.
Formwork is the largest cost component for a typical multistory
reinforced concrete building. Formwork cost accounts for 40 to 60
percent of the cost of the concrete frame and for approximately
10 percent of the total building cost. Figure 1.1a, b presents a
breakdown of different cost categories for conventional concrete
slab and wall formwork. A large proportion of the cost of conventional
formwork is related to formwork labor costs. Significant cost
saving could be achieved by reducing labor costs.

Formwork costs are not the only significant component of
the formwork life cycle. Other important aspects of the formwork
operation include speed, safety, and quality.
·        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.
·        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.
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.
·        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.
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.
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.
·        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
·        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. 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
·        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 delivery
 via gravity to lower levels, and by pumping for horizontal and
vertical delivery of 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.
·        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.
·        Cure Concrete
The hardening of concrete is a chemical process that requires
warmth and moisture. This activity involves curing concrete with
water, steam, or any other method to prevent shrinkage and allow
the concrete to gain sufficient early strength. Steam curing is used
where early strength gain of concrete is important. After the concrete
is cured, the rest of the formwork life cycle continues with
the strip forms activity. The cure concrete and strip forms activities
are interchangeable depending on the type of structural element.
For example, columns and walls are cured after stripping of the
forms, while slabs and beams are cured before and after the forms
are stripped.
·        Strip Forms
As soon as concrete gains enough strength to eliminate immediate
distress or deflection under loads resulting from its own weight
and some additional loads, formwork should be stripped to allow
other construction activities to start. The operation of removing
the forms is called stripping or wrecking the forms. Formwork can
either be partially stripped by removing small areas to prevent the
slab from deflecting or completely stripped to allow the slab to
deflect. As a general rule, formwork supporting members should
not be removed before the strength of concrete has reached at
least 70 percent of its design value.

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