concrete materials in constructions .ppt

CE 417, King Saud University 1
Chapter 12
Concrete Construction
Part 2

12-2 CONCRETE CONSTRUCTION PRACTICES
• Concrete construction involves:
– concrete batching,
– mixing,
– transporting,
– placing,
– consolidating,
– finishing, and
– curing.

• The production and transportation of concrete are
described in Section 7-2.
• In this section, we will discuss the equipment and
methods involved in placing, consolidating, finishing,
and curing concrete used for structural purposes.
• Special considerations for pouring concrete during
extremely hot or cold weather are also described.
• The use of concrete in paving is described in Chapter
8.

• Transporting and Handling
• Placing and Consolidating
• Finishing and Curing
• Hot-Weather Concreting
• Cold-Weather Concreting

Transporting and Handling
• A number of different items of equipment are
available for moving concrete from the mixer to
its final position.
• Equipment commonly used includes
wheelbarrows, buggies, chutes, conveyors,
pumps, buckets, and trucks.
• Regardless of the equipment used, care must be
taken to avoid segregation when handling
plastic concrete.

• The height of free fall should be limited to
about 5 ft (1.5 m) unless downpipes or ladders
are used to prevent segregation.
• Downpipes having a length of at least 2 ft (0.6
m) should be used at the end of concrete
conveyors.

• Wheelbarrows have a very limited capacity
(about 1½ cu ft or 0.04 m3
) but are often used
for transporting and placing small amounts of
concrete.
• Push buggies that carry 6 to 11 cu ft (0.17 to
0.31 m3
) and powered buggies carrying up to
½ cu yd (0.38 m3
) are often employed on
building construction projects.

• However, these items of equipment are
gradually being replaced by concrete pumps
capable of moving concrete from a truck
directly into final position up to heights of 500
ft (152 m) or more.
• Truck-mounted concrete pumps equipped with
placement booms such as that shown in Figure
12-15 are widely used in building construction.

FIGURE 12-15: Concrete pump and truck mixer.
(Courtesy of Challenge-Cook Bros., Inc.)

• Concrete conveyors are available to move
concrete either horizontally or vertically.
• Chutes are widely used for moving concrete
from the mixer to haul units and for placing
concrete into forms.

• Truck mixers are equipped with integral
retracting chutes that may be used for
discharging concrete directly into forms within
the radius of the chute.
• When chuting concrete, the slope of the chute
must be high enough to keep the chute clean
but not high enough to produce segregation of
the concrete.

• Concrete buckets attached to cranes are capable of lifting
concrete to the top of highrise buildings and of moving
concrete over a wide area.
• Concrete buckets are equipped with a bottom gate and a
release mechanism for unloading concrete at the desired
location.
• The unloading mechanism may be powered or may be
operated manually.
• The use of remotely controlled power-operated bucket
gates reduces the safety hazard involved in placing
concrete above ground level.

• Although truck mixers are most often
employed for hauling plastic concrete to the
job site, dump trucks equipped with special
concrete bodies are also available for hauling
concrete.
• The bodies of such trucks are designed to
reduce segregation during hauling and provide
easy cleaning and dumping.

• When using nonagitator trucks for hauling
concrete, specifications may limit the truck
speed and maximum haul distance that may be
used.
• Temperature, road condition, truck body type,
and mix design are the major factors that
influence the maximum safe hauling distance.
• Railway cars designed for hauling concrete are
also available but are not widely used.

Placing and Consolidating
• The movement of plastic concrete into its final
position (usually within forms) is called placing.
• Before placing concrete, the underlying surface and
the interior of all concrete forms must be properly
prepared.
• Concrete forms must be clean and tight and their
interior surfaces coated with form oil or a parting
agent to allow removal of the form from the
hardened concrete without damaging the surface of
the concrete.

• When concrete is poured directly onto a
subgrade, the subgrade should be moistened or
sealed by a moisture barrier to prevent the
subgrade from absorbing water from the plastic
concrete.
• When placing fresh concrete on top of hardened
concrete, the surface of the hardened concrete
should be rough-ened to provide an adequate
bond between the two concrete layers.

• To improve bonding between the layers, the
surface of the hardened concrete should also
be coated with grout or a layer of mortar
before the fresh concrete is placed.
• Concrete is usually placed in layers 6 to 24 in.
(15 to 61 cm) thick except when pumping into
the bottom of forms.

• When placing concrete in layers, care must be
taken to ensure that the lower layer does not
take its initial set before the next layer is
poured.
• Concrete may also be pneumatically placed by
spraying it onto a surface.

• Concrete placed by this process is designated
shotcrete by the American Concrete Institute but is
also called pneumatically applied concrete, gunned
concrete, or gunite.
• Since a relatively dry mix is used, shotcrete may be
applied to overhead and vertical surfaces.
• As a result, shotcrete is often used for constructing
tanks, swimming pools, and tunnel liners, as well as
for repairing damaged concrete structures.

• Concrete may be placed underwater by the use
of a tremie or by pumping.
• A tremie (see Figure 10-21) is nothing more
than a vertical tube with a gate at the bottom
and a hopper on top.
• In operation the tremie tube must be long
enough to permit the concrete hopper to
remain above water when the lower end of the
tremie is placed at the desired location.

• With the gate closed, the tremie is filled with
concrete and lowered into position.
• The gate is then opened, allowing concrete to flow
into place.
• The pressure of the plastic concrete inside the
tremie prevents water from flowing into the tremie.
• The tremie is raised as concrete is poured, but care
must be taken to keep the bottom end of the tremie
immersed in the plastic concrete.

• Consolidation is the process of removing air
voids in concrete as it is placed.
• Concrete vibrators are normally used for
consolidating concrete, but hand rodding or
spading may be employed.
• Immersion-type electric, pneumatic, or
hydraulic concrete vibrators are widely used.

• However, form vibrators or vibrators attached to the
outside of the concrete forms are sometimes employed.
• Vibrators should not be used to move concrete
horizontally, as this practice may produce segregation of
the concrete mix.
• Vibrators should be inserted into the concrete vertically
and allowed to penetrate several inches into the previously
placed layer of concrete.
• The vibrator should be withdrawn and moved to another
location when cement paste becomes visible at the top of
the vibrator.

Finishing and Curing
• When the concrete has hardened enough so
that a worker's foot makes only a small
impression in the surface, the concrete is
floated with a wood or metal float.
• Floating smooths and compacts the surface
while embedding aggregate particles.

• Finishing is the process of bringing the surface
of concrete to its final position and imparting
the desired surface texture.
• Finishing operations include screeding,
floating, troweling, and brooming. Screeding is
the process of striking off the concrete in
order to bring the concrete surface to the
required grade.

• Troweling with a steel trowel follows floating
when a smooth dense surface is desired.
• A three-unit riding-type power trowel is
shown in Figure 12-16.
• Finally, the concrete may be broomed by
drawing a stiff broom across the surface.
• This technique is used when a textured skid-
resistant surface is desired.

• The completion of cement hydration requires
that adequate moisture and favorable
temperatures be maintained after concrete is
placed.
• The process of providing the required water
and maintaining a favorable temperature for a
period of time after placing concrete is
referred to as curing.

• Methods for maintaining proper concrete temperatures in
hot-weather and cold weather concreting are described in
this chapter.
• Methods used to retain adequate curing moisture include
covering the concrete surface with wet straw or burlap,
ponding water on the surface, covering the surface with
paper or plastic sheets, and applying curing compounds.
• The use of sprayed on curing compounds applied
immediately after finishing has become widespread in
recent years.

• Vacuum dewatering may be employed to reduce the
amount of free water present in plastic concrete after the
concrete has been placed and screeded.
• The dewatering process involves placing a mat having a
porous lower surface on top of the concrete and applying
a vacuum to the mat.
• Vacuum within the mat causes excess water from the mix
to flow into the mat and eventually to the vacuum source.
• Removal of excess water results in a lower water/cement
ratio and a denser mix.

• Floating and troweling then follow as usual. In
concept, vacuum dewatering permits placing
concrete with a high water content (for good
workability) while obtaining the strength and
durability of concrete with a low water/cement ratio.
• Other advantages claimed for concrete placed by this
method include high early strength, increased
ultimate strength and wear resistance, reduced
shrinkage, reduced permeability, and increased
resistance to freeze/thaw damage.

• While the vacuum dewatering process was
invented and patented in the United States in
1935, it has not been widely used in this
country.
• Recent improvements in the equipment used
for the process have led to increased use of
the process in both Europe and the United
States.

• When constructing large slabs and decks,
concrete may be placed by chutes, buckets, or
side discharge conveyors.
• Mechanical finishing may be supplied by roller
finishers, oscillating strike-off finishers, large
power floats, or other types of finishers.
• Figure 12-17 shows a large slab being poured
directly from a truck mixer and finished by a
roller finisher.

Hot-Weather Concreting
• The rate of hardening of concrete is greatly
accelerated when concrete temperature is
appreciably higher than the optimum
temperature of 50 to 60°F (10 to 15.5°C).
• Ninety degrees Fahrenheit (32°C) is considered a
reasonable upper limit for concreting operations.
• In addition to reducing setting time, higher
temperatures reduce the amount of slump for a
given mix.

• If additional water is added to obtain the desired
slump, additional cement must also be added or the
water-cement ratio will be increased with
corresponding strength reduction.
• High temperatures, especially when accompanied by
winds and low humidity, greatly increase the
shrinkage of concrete and often lead to surface
cracking of the concrete.
• Several steps may be taken to reduce the effect of
high temperatures on concreting operation.

• The temperature of the plastic concrete may be
lowered by cooling the mixing water and/or
aggregates before mixing.
• Heat gain during hydration may be reduced by using
Type IV (low-heat) cement or by adding a retarder.
• Air-entraining agents, water-reducing agents, or
workability agents may be used to increase the
workability of the mix without changing
water/cement ratios.

• It is also advisable to reduce the maximum
time before discharge of ready-mixed concrete
from the normal 1% to 1 h or less.
• The use of shades or covers will be helpful in
controlling the temperature of concrete after
placement.
• Moist curing should start immediately after
finishing and continue for at least 24 h.

FIGURE 12-16: Roller finisher being used on large slab pour.
(Courtesy of CMI Corp.)

Cold-Weather Concreting
• The problems of cold-weather concreting are
essentially opposite to those of hot-weather
concreting.
• Concrete must not be allowed to freeze during
the first 24 h after placing to avoid permanent
damage and loss of strength.

• Specifications frequently require that, when
air temperature is 40°F (5°C) or less, concrete
be placed at a minimum temperature of 50°F
(l0°C) and that this temperature be
maintained for at least 3 days after placing.
• Type III (high early strength) cement or an
accelerator may be used to reduce concrete
setting time during low temperatures.

• Mix water and/or aggregates may be heated
prior to mixing to raise the temperature of the
plastic concrete.
• The use of unvented heaters inside an
enclosure during the first 36 h after placing
concrete may cause the concrete surface to
dust after hardening.

• To avoid this problem, any fuel-burning
heaters used during this period must be
properly vented.
• When heat is used for curing, the concrete
must be allowed to cool gradually at the end
of the heating period or cracking may result.

12-3 CONCRETE FORMWORK
• General Requirements for Formwork
• Typical Formwork
• Minimizing Cost of Formwork
• Construction Practices
• Formwork Safety

General Requirements for Formwork
• The principal requirements for concrete
formwork are that it be safe, produce the desired
shape and surface texture, and be economical.
• Procedures for designing formwork that will be
safe under the loads imposed by:
– plastic concrete,
– workers and other live loads, and
– external forces (such as wind loads) are explained in
Chapter 13.

General Requirements for Formwork
• Construction procedures relating to formwork
safety are discussed later in this section.
• Requirements for the shape (including deflection
limitations) and surface texture of the finished
concrete are normally contained in the construction
plans and specifications.
• Since the cost of concrete formwork often exceeds
the cost of the concrete itself, the necessity for
economy in formwork is readily apparent.

Typical Formwork
• A typical wall form with its components is
illustrated in Figure 12-18.
• Sheathing may be either plywood or lumber.
• Double wales are often used as illustrated so
that form ties may be inserted between the
two wales.

FIGURE 12-18: Typical wall form.

Typical Formwork
• With a single wale it would be necessary to drill
the wales for tie insertion.
• While the pressure of the plastic concrete is
resisted by form ties, bracing must be used to
prevent form movement and to provide
support against wind loads or other lateral
loads.
• Typical form ties are illustrated in Figure 12-19.

FIGURE 12-19: Typical form ties.

Typical Formwork
• Form ties may incorporate a spreader device
to maintain proper spacing between form
walls until the concrete is placed.
• Otherwise, a removable spreader bar must be
used for this purpose.
• Ties are of two principal types, continuous
single-member and internally disconnecting.

Typical Formwork
• Continuous single-member ties may be pulled out after the
concrete has hardened or they may be broken off at a
weakened point just below the surface after the forms are
removed.
• Common types of internally disconnecting ties include the
coil tie and stud rod (or shebolt) tie.
• With internally disconnecting ties, the ends are unscrewed
to permit form removal with the internal section left
embedded in the concrete.
• The holes remaining in the concrete surface after the ends
of the ties are removed are later plugged or grouted.

Typical Formwork
• Column forms are similar to wall forms except
that studs and wales are replaced by column
clamps or yokes that resist the internal
concrete pressure.
• A typical column form is shown in Figure 12-20.
• Yokes may be fabricated of wood, wood and
bolts (as shown), or of metal.

FIGURE 12-20: Typical column form.
(U.S. Department of the Army)

Typical Formwork
• Commercial column clamps (usually of metal) are
available in a wide range of sizes (Figure 12-3).
• Round columns are formed with ready-made fiber
tubes or steel reinforced fiberglass forms.
• Openings or "windows" may be provided at several
elevations in high, narrow forms to facilitate
placement of concrete.
• Special fittings may also be inserted near the bottom
of vertical forms to permit pumping concrete into the
form from the bottom.

FIGURE 12-3: Pumping concrete into bottom of column form.
(Courtesy of Gates & Sons, Inc.)

Typical Formwork
• Figure 12-21 illustrates a typical elevated floor
or desk slab form with its components
identified.
• Forming for a slab with an integral beam is
illustrated in Figure 12-22.
• Forming for the one-way and two-way slabs
described in Section 12-1 is usually
accomplished using commercial pan forms.

FIGURE 12-21: Form for elevated slab.
(Courtesy of American Concrete Institute)

FIGURE 12-22: Beam and slab form.

Typical Formwork
• Figure 12-23 illustrates the use of long pans for a
one-way joist slab.
• Figure 12-24 shows a waffle slab formed with
dome pans.
• Such pan forms may be made of metal or plastic.
• Wooden stairway forms suitable for constructing
stairways up to 3 ft wide are illustrated in Figure
12-25.

FIGURE 12-23: One-way slab form.

FIGURE 12-24: Two-way slab form.

FIGURE 12-25: Wood form for stairway.
(U.s. Department of the Army)

Minimizing Cost of Formwork
• Since formwork may account for 40 to 60% of
the cost of concrete construction, it is
essential that the formwork plan be carefully
developed and thoroughly evaluated.
• A cost comparison should be made of all
feasible forming systems and methods of
operation.

• Such an analysis must include the cost of
equipment and labor required to install
reinforcing steel and to place and finish the
concrete, as well as the cost of formwork, its
erection, and removal.
• The formwork plan that provides the required
safety and construction quality at the
minimum overall cost should be selected for
implementation.

• In general, lower formwork cost will result
from repetitive use of forms.
• Multiple-use forms may be either standard
commercial types or custom-made by the
contractor.
• Contractor-fabricated forms should be
constructed using assembly-line techniques
whenever possible.

• Flying forms, large sections of formwork
moved by crane from one position to another,
are often economical in repetitive types of
concrete construction.
• Where appropriate, the use of slip forms and
the tilt-up construction techniques described
earlier can greatly reduce forming costs.
• A flying form is pictured in Figure 12-26.

FIGURE 12-26: Repositioning flying form.
(Courtesy of Lorain Division, Koehring Co.)

Construction Practices
• Forms must be constructed with tight joints to
prevent the loss of cement paste, which may
result in honeycombing.
• Before concrete is placed, forms must be
aligned both horizontally and vertically and
braced to remain in alignment.
• Form alignment should be continuously
monitored during concrete placement and
adjustments made if necessary.

• When a vertical form is wider at the bottom
than at the top, an uplift force will be created as
the form is filled.
• Such forms must be anchored against uplift.
Inspect the interior of all forms and remove any
debris before placing concrete.
• Use drop chutes or rubber elephant trunks to
avoid segregation of aggregate and paste when
placing concrete into high vertical forms.

• Free-fall distance should be limited to 5 ft or less.
• When vibrating concrete in vertical forms, allow
the vibrator head to penetrate through the
freshly placed concrete about 1 in. (2.5 cm) [but
not more than 8 in. (20 cm)] into the previously
placed layer of concrete.
• It is possible to bulge or rupture any wall or
column form by inserting a large vibrator deep
into previously placed, partially set concrete.

• However, revibration of previously compacted
concrete is not harmful to the concrete as long as it
becomes plastic when vibrated.
• When pumping forms from the bottom, it is
important to fill the forms rapidly so that the
concrete does not start to set up before filling is
completed.
• If the pump rate is so low that setting begins,
excessive pressure will be produced inside the form,
resulting in bulging or rupturing of the form.

Formwork Safety
• The frequency and serious consequences of
formwork failure require that special attention
be paid to this aspect of construction safety.
• The requirements for safe formwork design
are explained in Chapter 13.
• The following are some safety precautions
that should be observed in constructing
formwork.

Formwork Safety
1) Provide adequate foundations for all formwork. Place
mudsills under all shoring that rests on the ground.
o Typical mudsills are illustrated in Figure 12-27.
o Check surrounding excavations to ensure that formwork
does not fail due to embankment failure.
2) Provide adequate bracing of forms, being particularly
careful of shores and other vertical supports.
o Ensure that all connections are properly secured, especially
nailed connections.
o Vibration from power buggies or concrete vibrators may
cause connections to loosen or supports to move.

FIGURE 12-27: Mudsills.

Formwork Safety
3) Control the rate and location of concrete placement so that
design loads are not exceeded.
4) Ensure that forms and supports are not removed before the
concrete has developed the required strength.
o The process of placing temporary shores under slabs or structural
members after forms have been stripped is called reshoring.
o Reshoring is a critical operation that must be carried out exactly as
specified by the designer.
o Only a limited area should be stripped and reshored at one time.
o No construction loads should be allowed on the partially hardened
concrete while reshoring is under way.
o Adequate bracing must be provided for reshoring

Formwork Safety
5) When placing prefabricated form sections in
windy weather, care must be taken to avoid
injury due to swinging of the form caused by
wind forces.
6) Protruding nails are a major source of injury on
concrete construction sites.
o As forms are stripped, form lumber must be promptly
removed to a safe location and nails pulled.

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