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2. construction

Chapter Five
Batching, Mixing, Handling,
Placing, Compacting, and Curing
Concrete
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Batching
Is the process of weighing or volumetrically measuring and
Introducing into the mixer the ingredients for a batch of concrete
To produce a uniform quality concrete mix, measure the ingredients
accurately for each batch. Most concrete specifications require that
the batching be performed by weight, rather than by volume, because
of inaccuracies in measuring aggregate, especially damp aggregate.
Water and liquid air-entraining admixtures can be measured
accurately by either weight or volume. Batching by using weight
provides greater accuracy and avoids problems created by bulking of
damp sand.
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Mixing Process
Concrete should be mixed until it is uniform in appearance and all the ingredients
are evenly distributed.
The mixing operation consists essentially of rotation or stirring, the objective being
to :
Coat the surface of all aggregate with cement Paste.
Blend all the ingredient of concrete into a uniform mass. This process must
not be disturbed by the process of discharging from the mixer.
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Mixing Fresh Concrete
Hand mixing: Suitable for small jobs.
•Dry materials mixed first
•Water added slowly at the center
•Mixing from outside to inside.
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Machine mixing
1. Electrical or gasoline operated
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2. Portable or staonary
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3. Rotating horizontal drum type or rotating tilting drum type
Tilting drum mixers: are preferable for the mixes of low workability and for
those containing large size aggregate.
Pan mixers: Efficient with stiff and cohesive mixes.
Used for : Precast concrete, Small quantities of concrete and mortar in the
laboratory
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Non tilting drum mixer: the concrete is sometimes susceptible to segregation
Dual Drum:
Used : in the case of highway construction where space or access is
often limited
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Notes:
In the drum - type mixers: No Scraping of the sides takes place. Thus
the first mix will leave behind mortar stuck to the walls of the mixer;
means less mortar in the mix. To solve this problem; two alternatives
are suggested:
Initial batch discarded.
Buttering Procedure: adding Mortar (less CA) prior to mixing
Mixer Sizes
Laboratory use = 0.04m3
Huge mixer for field or mixing plants use = 13 m3
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Correct sequence of mixing
Step 1: Butter the mixer ( extra paste)
Step 2: CA + some water
Step 3: Mix
Step 4: FA + cement + some water
Step 5: Remaining water
Step 6: Wait until full mixing is achieved.
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Mixing Time
The optimum mixing time depends on:
the type and size of mixer (batch volume)
the speed of rotation,
the quality of blending of ingredients.
A minimum me of 1 minute should be assured so as to obtain a
uniform mix.
Recommended Minimum Mixing Times:
Capacity of Mixer Mixing
Time (min.)
0.8
1
.
.
1.6m3
3.25
Little more than minimum does improve nothing
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Under mixing : not homogenous mix.
Over mixing :damages the quality of the concrete, tends to :
• Grind the aggregate into smaller pieces,
• More friction → Increases the temperature of the mix →increase
Evaporation → decrease water ,
• Lowers the slump (loss of workability)
• Decreases air entrainment,
• Decreases the strength of the concrete.
• Loss of time
• Loss of energy
Re-tempering (adding water to regain workability) could:
lower strength
Increase shrinkage ( lower resistance to stresses)
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Ready - Mixed Concrete
•
•
•
•
•
Concrete is batched at the plant.
Better quality control of materials and concrete.
No need for storage space at the site.
Saving time.
Economy.
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Transportation of Concrete
Concrete from the mixer should be transported to the point where it
has to be placed as rapidly as possible by a method which prevents
segregation or loss of ingredients. The concrete has to be placed
before setting has commenced.
During handling concrete should:
Remain cohesive, uniform and of proper consistency
Be without Segregation or separation of materials.
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The principal methods of transporting concrete from the
mixer
Wheel and power barrows (‫)العربات‬, buggies or
dumpers.
Tippers (‫ )القالبات‬and lorries.
Truck mixers and agitator lorries.
Dump bottom bucket on elevator
Belt conveyors (‫)السيور الناقلة‬.
Pumping.
Chutes ( ‫)المزالق‬.
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Formwork
Concrete must be supported by form work until it gain sufficient
strength.
Form removal 3days-3weeks
Requirement for formwork:
Wooden or steel molds.
Proper dimension.
Sufficient strength
Sufficient rigidity.
Easy to assemble and remove
Water tight.
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Placing Fresh Concrete
Placing and compacting are interdependent and are carried out almost
simultaneously.
Placing objective: deposit concrete as close as possible to its final position
so that segregation is avoided and the concrete can be fully compacted.
Concrete be placed in uniform horizontal layers, not in large heaps or
sloping layers. Thickness of layer be compatible by method of vibration so
that entrapped air be removed. (30 cm thick for reinforced concrete, up to
50 cm for plain concrete).
Each layer should be fully compacted before placing the next one. Each
subsequent layer should be placed whilst the underlying layer is still plastic
so that monolithic construction is achieved.
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Compaction Fresh Concrete
Fresh concrete must be compacted so that :
It will spread evenly inside the form.
Entrapped air voids will be minimized and a dense concrete will be
obtained. (Since low slump concrete contains more entrapped air
than high slump one. The former requires more effort to compact
satisfactory. This effort is provided by the use of vibrators.)
Methods :
1. Roding: suitable for laboratory work and small jobs.
2. Vibrang :
a. Internal Vibrators (inside concrete)
b. External Vibrators ( on formwork)
c. Vibrating Tables
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a. Internal Vibrators
The poker is immersed in concrete and thus applies
approximately harmonic forces; hence, the alternative
names of poker vibrator or immersion vibrator. They are
useful for heavily reinforced and relatively inaccessible
sections.
It is good because all work done directly on the concrete.
Using of Vibrator:
Immersed in concrete, Moved from one place to
another, and vibrated every 0.5 to1m for 5 sec to 2 min;
depending on concrete consistence. Stop: when a
surface of concrete appears. Concrete should be neither
honeycombed nor contain an excess of mortar. The
vibrator should be withdrawn slowly.
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b. External Vibrators
Vibrators are rigidly clamped to the framework which rests on an elastic support,
so that both the form and the concrete are vibrated. Therefore, the framework
should be strong. The vibraon frequency ranges from 50 to 150Hz.
External vibrators are used for precast or thin in situ sections having a shape or
thickness which is unsuitable for internal vibrators.
Concrete should be placed in layers of suitable depth so as to be able to expel
air.
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c. Vibrating Tables
Used mostly in:
Compaction of precast concrete units (uniform vibration).
In laboratory works.
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For 1m3 concrete ( 3-4) minutes of vibraon sufficient.
Proper compaction is obtained when concrete surface becomes shiny with
film of water and paste appearing.
Under compaction: more entrapped air, more voids, layer concrete→ not
dense concrete.
Over compaction: excess bleeding, segregation, loss of entrained air, loss
of time and energy.
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Finishing
Final concrete surface must be finished.
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Curing
• Producing a moist environment around concrete for hydration to continue
at suitable temperatures.
• Methods of curing:
1. Ponding or immersion in water.
2. Spraying or fogging (sprinkling) periodically.
3. Wet covering ( Burlap, plasc sheet).
4. Steam curing: supply heat and addional moisture to accelerate strength
development (for precast members in precasting plants and prestressing
plants.
Curing affect greatly the strength of concrete, reduce the liberate of heat and
allowed the chemical reaction to take place normally.
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Development of Strength and Curing
Curing is the name given to procedures used for promoting the
hydration of cement, and thus, the development of strength of
concrete.
The curing procedure being control of the temperature and of the
moisture movement from and into the concrete.
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Normal Curing
The object of curing at normal temperature is to keep concrete saturated as
nearly saturated as possible, until the originally water-filled space in the
fresh cement paste has been occupied to the desired extent by the product
of hydration of cement.
Curing is essential in concrete with low w/c ratio, to prevent self-desiccation
due to consumption of water.
The water existence aims to provide appropriate condition to achieve as
much hydration as possible, yet the amount of total hydration is depended
also on w/c ratio of the mix.
In case of site concrete, active curing nearly always ceases long before the
maximum possible hydration has taken place. Tensile or compressive
strength are affected in similar manner.
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Methods of Normal Curing
There are many methods and materials that can be used for moist curing
of concrete.
These can be divided into two groups:
1. Water Curing
2. Sealed Curing
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1. Water Curing
Those that supply additional moisture as well as prevent moisture loss.
Supply of water is accomplished by:
Ponding : maintain a layer of water on horizontal surfaces.
Spraying
Sprinkling: Fine spray (keep concrete wet)
Saturated coverings: Two steps:
A. Wet concrete with water
B. Prevents or reduces evaporation of water using water
saturated burlap or other absorbent materials
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2. Sealed Curing
Prevent evaporation of moisture using:
Waterproof paper: applied on soon as the surface has hardened, and
after concrete is sufficiently wetted.
Plastic sheeting: Used to cover more complex shapes; more flexible
than water proof paper.
→ High convenience and Lower labor needed
Curing membranes: Created by compounds such as resins, waxes or
synthetic rubbers which are dissolved in volatile solvent or emulsified
in water.
Application should happen after some water curing took place.
Used for vertical surface.
Not entirely prevent evaporation ( less effective than plastic sheets).
Different colors could be used to reduce heat absorption.
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Curing at (Elevated) High Temperature
A. Low-Pressure Steam Curing
Curing in live steam at atmospheric pressure dramatically increases the rate of
strength development of concrete. Thus, helping speeding the rate of production. The technique is
used for:
Curing prestressed beams and wall panels
Curing precast concrete units.
Appropriate temperature should chosen to achieve satisfactory rate of strength gain
and ultimate strength.
Pre-steaming (Cured for some time at room temperature before exposed to steam
curing (2-6 hours) ) → Rise temperature up to 33°C/h.
No pre-steaming → Temperature is raised at a rate of less than 11°C/ h.
Cooling: cooling should be done at a rate of 22-33 °C /h so as to reduce thermal
shock.
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B. High - Pressure Steam Curing
Curing at temperature in excess of 100°C and a steam
pressure of 6 to 20 atm.
Effect on concrete:
Products are ready for use within 24h.
Strength is generally equivalent to 28-day strength.
Cause less creep and Shrinkage.
Better sulfate resistance.
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