INTRODUCTION
Ready-mix concrete is made at a batching
plant and delivered by a cement truck to a work site. This type of concrete
speeds up the construction process by having the concrete already mixed and
ready to pour once it reaches the site. Ready-mix concrete saves builders time
and money.
History:-
Ready-mix concrete was first mixed in a
factory in the 1930s. It wasn't until the 1960s that ready-mix concrete became more
in demand. Since that time, the ready-mix concrete business has continuously
grown and is now a major industry.
APPLICATIONS:-
· This technique is useful in congested
site or diverse work Places and saves the consumer from the botheration of procurement,
storage and handling of concrete materials.
· Ready mix concrete is produced under
factory conditions and permits a close control of all operations of manufacture
and transportation of fresh concrete.
SPECIFICATIONS
Quality
of ready mix concrete is generally specified in terms of two systems
·
Performance parameters
·
Prescriptive specifications
1. Performance parameters:
·
It is the best way to order ready mixed
concrete because RMC producer, who is expert in the field, would design an economical
mix with the desired properties
·
The RMC producer accepts the
responsibility for the design of the mixture for the desired performance
2. Prescriptive specifications:
·
Purchaser specifies aggregate size,
slump, air content, cement content, or weight of cement per cubic meter of
concrete, maximum water content and admixtures required.
The purchaser accepts the responsibility for concrete
strength and its performance.
How Ready-Mix
Concrete is made
Function:-
Ready-mix
concrete is used for small and large construction projects. It is used to pave
driveways and for foundations on homes and large commercial buildings. It is
also used for bridges, roads and sidewalks.
Benefits:-
There
are several benefits to the use of ready-mix concrete. Ready-mix concrete helps
speed up the building process. Construction companies save time and money by
using concrete that is already mixed. It eliminates the need for a crew to mix
concrete at the work site. Ready-mix concrete also helps to reduce
construction-site pollution. A lot of dust is created when concrete is mixed on
site.
Limitations:-
Ready-mix
concrete cannot be transported over long distances. When concrete is mixed at a
batching plant, it must be poured within two to three hours to hold up to
industry standards. This creates the need for central mixing plants to serve
the construction industry.
Speculation:-
The
advantages of ready-mix concrete far outweigh all limitations. The building and
construction industry would be greatly inhibited without it. What used to take
several man-hours to do can now be accomplished in a very short period of time
MAKING OF RMC
Its
simplest form, concrete is a mixture of paste and aggregates (sand & rock).
The paste, composed of cement and water, coats the surface of the fine (sand)
and coarse aggregates (rocks) and binds them together into a rock-like mass
known as concrete.
Within
this process lies the key to a remarkable trait of concrete: it’s plastic and
can be molded or formed into any shape when newly mixed, strong and durable
when hardened. These qualities explain why one material, concrete, can build
skyscrapers, bridges, sidewalks, and superhighways, houses and dams.
Proportioning:-
The
key to achieving a strong, durable concrete rests on the careful proportioning
and mixing of the ingredients. A concrete mixture that does not have enough
paste to fill all the voids between the aggregates will be difficult to place
and will produce rough, honeycombed surfaces and porous concrete. A mixture
with an excess of cement paste will be easy to place and will produce a smooth
surface; however, the resulting concrete will be more likely to crack and be
uneconomical.
A
properly proportioned concrete mixture will possess the desired workability for
the fresh concrete and the required durability and strength for the hardened
concrete. Typically, a mixture is by volume about 10 to 15 percent cement, 60
to 75 percent aggregates and 15 to 20 percent water. Entrained air bubbles in
many concrete mixtures may also take up another 5 to 8 percent
The
character of concrete is determined by the quality of the paste. The strength
of the paste, in turn, depends on the ratio of water to cement. The
water-cement ratio is the weight of the mixing water divided by the weight of
the cement. High-quality concrete is produced by lowering the water-cement
ratio as much as possible without sacrificing the workability of fresh
concrete. Generally, using less water produces a higher quality concrete
provided the concrete is properly placed, consolidated and cured
Other Ingredients:
Although
most drinking water is suitable for use in concrete, aggregates are chosen
carefully. Aggregates comprise 60 to 75 percent of the total volume of
concrete. The type and size of the aggregate mixture depends on the thickness
and purpose of the final concrete product.
Relatively
thin building sections call for small coarse aggregate, while aggregates up to
six inches in diameter have been used in large dams. A continuous gradation of
particle sizes is desirable for efficient use of the paste. In addition,
aggregates should be clean and free from any matter that might affect the
quality of the concrete.
Hydration Begins:-
Soon
after the aggregates, water, and the cement are combined, the mixture starts to
harden. All Portland cements are hydraulic cements that set and harden through
a chemical reaction with water. During this reaction, called hydration, a node
forms on the surface of each cement particle. The node grows and expands until
it links up with nodes from other cement particles or adheres to adjacent
aggregates.
The
building up process results in progressive stiffening, hardening, and strength
development. Once the concrete is thoroughly mixed and workable it should be
placed in forms before the mixture becomes too stiff.
During
placement, the concrete is consolidated to compact it within the forms and to
eliminate potential flaws, such as honeycombs and air pockets. For slabs,
concrete is left to stand until the surface moisture film disappears. After the
film disappears from the surface, a wood or metal hand float is used to smooth
off the concrete. Floating produces a relatively even, but slightly rough,
texture that has good slip resistance and is frequently used as a final finish
for exterior slabs. If a smooth, hard, dense surface is required, floating is
followed by steel toweling.
Curing
begins after the exposed surfaces of the concrete have hardened sufficiently to
resist marring. Curing ensures the continued hydration of the cement and the
strength gain of the concrete. Concrete surfaces are cured by sprinkling with
water fog, or by using moisture-retaining fabrics such as burlap or cotton
mats. Other curing methods prevent evaporation of the water by sealing the
surface with plastic or special sprays (curing compounds).
Special
techniques are used for curing concrete during extremely cold or hot weather to
protect the concrete. The longer the concrete is kept moist, the stronger and
more durable it will become. The rate of hardening depends upon the composition
and fineness of the cement, the mix proportions, and the moisture and
temperature conditions. Most of the hydration and strength gain take place
within the first month of concrete's life cycle, but hydration continues at a
slower rate for many years. Concrete continues to get stronger as it gets
older.
READY MIX CONCRETE MUST BE PREPARED CORRECTLY
Ready
mix concrete is a pre-mixed building material containing Portland cement, sand
and gravel aggregate. It is popular with do-it-yourselfers because the only ingredient
that must be measured is water. When using ready mix concrete, it is important
to properly mix and pour to obtain the best results.
Estimating
the Amount:-
It
is important to know how much concrete you will need before the project begins
to avoid purchasing too much or too little. Multiply the width, length and
depth of the area that you need to fill to obtain the cubic meter. For example,
a space that is 1-by-2-by-3 feet requires six cubic feet of concrete. Bags of
ready mix concrete will tell you how many cubic feet of concrete they contain.
Use this information and your calculations to determine how much to purchase.
Mixing by
Hand:-
Empty
the contents of the bag into a wheelbarrow or mixing trough. Measure the amount
of water instructed on the bag in a 5-gallon bucket and add it to the concrete.
Mix the concrete with a hoe or shovel until you reach the desired consistency.
It should be thick enough to hold its shape but not so thick that it cannot be
mixed. This option works well for small amounts of concrete, but larger amounts
should be mixed with a power mixer.
Mixing by
Machine:-
Check
the mixer to make sure there is no dirt, dried concrete or other foreign matter
before you begin. Pour half the water into a mixer and turn it on. Let the
mixer run for a few minutes and pour in the concrete and the rest of the water.
Let the concrete mix for about three minutes and transfer it to a wheelbarrow.
Wash out the mixer with a garden hose before the concrete can set or the mixer
will be severely damaged.
Pouring and
Curing:-
Ready
mix concrete can be shoveled or poured into a form. These forms are typically
constructed of lumber and hold the concrete within a certain shape while it
cures. Spread the concrete evenly, being careful not to trap air pockets. Use a
screed to level the concrete and use a float to push the aggregate to the
bottom and bring excess water to the surface. Ready mix concrete must be
allowed to cure until it has completely hardened and cooled. The time required
will depend on the thickness of the concrete, but it is typically no less than
five to seven days.
39 Important Technical Points about Ready Mix Concrete
Ø Dusting
Concrete Surfaces
Ø Scaling
Concrete Surfaces
Ø Crazing
Concrete Surfaces
Ø Cracking
Concrete Surfaces
Ø Plastic
Shrinkage Cracking
Ø Joints
in Concrete Slabs on Grade
Ø Cracks
in Concrete Basement Walls
Ø Discrepancies
in Yield
Ø Low
Concrete Cylinder Strength
Ø Strength
of In-Place Concrete
Ø Curing
In-Place Concrete
Ø Hot
Weather Concreting
Ø Concrete
Blisters
Ø Finishing
Concrete Flatwork
Ø Chemical
Admixtures for Concrete
Ø Flexural
Strength of Concrete
Ø Flow
able Fill Materials
Ø Radon
Resistant Buildings
Ø Curling
of Concrete Slabs
Ø Delimitation
of Toweled Concrete Surfaces
Ø Loss
of Air Content in Pumped Concrete
Ø Grout
Ø Discoloration
Ø Synthetic
Fibers for Concrete
Ø Corrosion
of Steel in Concrete
Ø Jobsite
Addition of Water
Ø Cold
Weather Concreting
Ø Concrete
Slab Moisture
Ø Vapor
Barriers under Slabs on Grade
Ø Supplementary
Cementations Materials
Ø Ordering
Ready Mixed Concrete
Ø Concrete
Pre-Construction Conference
Ø High
Strength Concrete
Ø Making
Concrete Cylinders in the Field
Ø Testing
Compressive Strength of Concrete
Ø Structural
Lightweight Concrete
Ø Self
Consolidating Concrete
Ø Pervious
Concrete
Ø Concrete
Maturity
ADMIXTURES OF READY MIX CONCRETE
Hydration
Begins:-
After
the aggregates, water, and the cement are combined, the mixture remains in a
fluid conditi0on for about four to six hours which permits transporting,
placing and finishing in its final location, then the mixture starts to harden.
All Portland cements are hydraulic cements that set and harden through a
chemical reaction with water. During this reaction, called hydration, crystals
radiate outwards from cement grains and mesh with other adjacent crystals or
adheres to adjacent aggregates. The building up process results in progressive
stiffening, hardening, and strength development. Once the concrete is
thoroughly mixed and workable it should be placed in forms before the mixture
becomes to stiff. During placement, the concrete is consolidated to compact it
within the forms and to eliminate potential flaws, such as honeycomb and air
voids.
Proportioning:-
The
proportioning of a concrete mix design should result in an economical and
practical combination of materials to produce concrete with the properties
desired for its intended use, such as workability, strength, durability and
appearance.
The
ready mixed concrete producer may independently select the material proportions
to provide the performance you need or may receive instructions through the job
specifications, such as minimum cement content, air content, slump, maximum
size of aggregate, strength, and others.
Regardless
of the source of instructions, there are established methods for selecting the
proportions for concrete for each batch. The Standard Practice for Selecting
Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) published
by the American Concrete Institute Committee 211 is often referenced for
selecting concrete proportions.
Here are the
basics of a good concrete mix:-
Cement
and water combine chemically to bind the sand and aggregate together. Fly ash
or other cementations materials, which enhance concrete properties, may
supplement some of the cement. The key to quality concrete is to use the least
amount of water that can result in a mixture that can be easily placed,
consolidated and finished.
Fine
and coarse aggregates make up about 70% of the concrete volume and impart
volume stability to the concrete. Concrete aggregates are required to meet
appropriate specifications and in general should be clean, strong and durable.
Admixtures
are generally products used in relatively small quantities to improve the
properties of fresh and hardened concrete. They are used to modify the rate of
setting and strength development of concrete, especially during hot and cold
weather. The most common is an air-entraining agent that develops millions of
tiny air bubbles in concrete, which imparts durability to concrete in freezing
and thawing exposure. Water reducing admixtures enable concrete to be placed at
the required consistency while minimizing the water used in the mixture,
thereby increasing strength and improving durability. A variety of fibers are
incorporated in concrete to control cracking or improve abrasion and impact resistance.
Architectural
masterpieces like bridges, high-rise buildings, and dams require the highest
standards of engineering. With the help of admixtures, concrete is able to
fulfill this!
Applicable
to all types of ready mix concrete, from the very basic to ultra-high strength,
our broad range of admixtures are able to improve slump retention, placing,
pumping, finishing, appearance and in general, performance characteristic as
desired.
Brands under this segment:-
A
new generation of versatile and high-performance admixtures based on
polycarboxylate ether for hyper plasticized concrete. Used effectively for high
performance, self compacting and high strength concrete.
GLENIUM
admixtures not only provide answers to old problems but unearth exciting
opportunities for the construction industry for groundbreaking and cost
effective solutions for today’s concrete. GLENIUM has an excellent track record
of being used in the world’s most exciting architectural wonders including the
present tallest Burj-Al-Arab Dubai!
The
admixture used in local ready mix concrete plants is RHEOBUILD 918 it is a
company product of BASF.
TYPES OF MIXING THE READY MIX CONCRETE
A. Transit
Mixed (or "truck-mixed") Concrete:-
In
transit-mixed concrete, also called truck mixed or dry-batched, all of the raw
ingredients are charged directly in the truck mixer. Most or all water is
usually batched at the plant. The mixer drum is turned at charging (fast) speed
during the loading of the materials. There are three options for truck mixed
concrete:
Concrete
mixed at the job site. While travelling to the job site the drum is turned at
agitating speed (slow speed). After arriving at the job site, the concrete is
completely mixed. The drum is then turned for 70 to 100 revolutions, or about
five minutes, at mixing speed.
Concrete mixed in the yard. The drum is turned
at high speed or 12-15 rpm for 50 revolutions. This allows a quick check of the
batch. The concrete is then agitated slowly while driving to the job site.
Concrete
mixed in transit. The drum is turned at medium speed or about 8 rpm for 70
revolutions while driving to the job site. The drum is then slowed to agitating
speed. (More information on ready mixed concrete trucks can be found in the
Delivery section).
B. Shrink
Mixed Concrete:-
Concrete
that is partially mixed in a plant mixer and then discharged into the drum of
the truck mixer for completion of the mixing is called shrink mixed concrete.
Central mixing plants that include a stationary, plant-mounted mixer are often
actually used to shrink mix, or partially mix the concrete. The amount of
mixing that is needed in the truck mixer varies in these applications and
should be determined via mixer uniformity tests. Generally, about thirty turns
in the truck drum, or about two minutes at mixing speed, is sufficient to
completely mix shrink-mixed concrete.
C. Central
Mixed Concrete:-
Central-mixing
concrete batch plants include a stationary, plant-mounted mixer that mixes the
concrete before it is discharged into a truck mixer. Central-mix plants are
sometimes referred to as wet batch or pre-mix plants. The truck mixer is used
primarily as an agitating haul unit at a central mix operation. Dump trucks or
other non-agitating units are sometimes be used for low slump and mass concrete
pours supplied by central mix plants. About 20% of the concrete plants in the
US use a central mixer. Principal advantages include:
Ø Faster
production capability than a transit-mix plant
Ø Improved
concrete quality control and consistency and
Ø Reduced
wear on the truck mixer drums.
Ø There
are several types of plant mixers, including:
Ø Tilt
drum mixer
Ø Horizontal
shaft paddle mixer
Ø Dual
shaft paddle mixer
Ø Pan
mixer
Ø Slurry
mixer
TYPES OF READY MIX CONCRETE
Precast:-
Precast concrete is concrete cast elsewhere
than its final position. Precast construction can be used to produce a wide
variety of structural elements such as wall panels, bridge girders, pipes,
poles, crash barriers, lift stations, etc. The concrete can be reinforced,
lightly pre-stressed or pre-stressed depending upon its requirements. Precast
concrete elements are typically transported from the plant to the job site
through highways, rail, or barges. Tilt-up concrete is a form of precast
concrete and is generally referred to as site pre-casting.
Tilt-Up:-
Tilt-Up construction is "a construction technique of casting concrete elements in a horizontal position at the jobsite and then tilting and lifting the panels to their final position in a structure". The greatest advantage of tilt-up construction over cast-in-place concrete is the ease and speed of construction. Generally the concrete should attain a high early age flexural strength to facilitate lifting and handling. Tilt-up concrete is a form of precast concrete and is generally referred to as sit pre-casting.
Flow able Fill:-
Flow able also known as cons trolled
low-strength material (CLSM) is a self-compacted, cementations material used
primarily as a backfill in lieu of compacted fill. The compressive strengths
must be lower than 1200 psi and often is lower than 200 psi. Since the material
flows into place no compaction is required which is a significant advantage
over compacted back fill. Flow able fill can be made with very high amounts of
non-standard materials.
Pervious Concrete:-
Pervious concrete is concrete that does
not contain fine aggregate. It typically has a voids content of 15% to 35%.
This ensures that the concrete has the unique ability to allow storm water to
pass through its mass into the ground underneath. Pervious concrete offers
significant environmental benefits as it reduces the requirement for drainage
facilities. Further it facilitates the recharge of ground water and the
filtration process purifies the water as it percolates below.
TESTING & INSPECTION OF READY MIX CONCRETE
Concrete
is a manufactured product. Specific control tests and evaluations are required
during the manufacturing process to produce predictable high-quality concrete.
The customer may want to verify that concrete meets specifications. Some of the
important properties of concrete that are measured by basic quality control
tests are strength, temperature, slump, air content, and unit weight. In
general concrete is tested at a frequency of 1 in 150 cubic yards
Ø Each test
helps to determine the quality of concrete and it should be performed in
accordance with standards. A certified Concrete Field Testing Technician –
Grade I, must make the tests.
Ø Define the
basis of purchase, cubic yards, and how it is measured.
Ø Define
acceptable material specifications and acceptable industry practice and
tolerances.
Ø Define
strength testing procedures and acceptance criteria.
Set laboratory personnel
qualifications. The testing laboratory must comply with ASTM C 1077, which is
required in ASTM C 94.
DELIVERY
OF RMC
While
ready mix concrete can be delivered to the point of placement in a variety of ways,
a overwhelming majority of it is brought to the construction site in truck mounted,
rotating drum mixers. Truck mixers have a revolving drum with the axis inclined
to horizontal. Inside the shell of the mixer drum are a pair of blades or fins
that wrap in a helical configuration from the head to the opening of the drum. To
load or charge the raw materials from a transit mixed plant or central mixed
plant into the truck, the drum must be turned very fast in the changing
direction. After the concrete is loaded and mixed, it is normally hauled to the
job site with the drum turning at the speed of less than 2 rpm. The truck mixer
having front discharge units are more popular than the traditional ones having
rear discharge units. Commonly used specifications for RMC stipulate that the
concrete shall be discharged on the job site with in 90 minutes and before 300
revolutions after water is added to the cement. In certain situations, air
entraining, water reducing, high range water reducing (HRWR) admixtures may
need to be added to concrete prior to discharge, to compensate for the loss of
air, high temp., or long delivery times
ADVANTAGES OF READY MIX CONCRETE OVER SITE MIX CONCRETE
Ø A
centralized concrete batching plant can serve a wide area.
Ø The
plants are located in areas zoned for industrial use, and yet the delivery
trucks can service residential districts or inner cities.
Ø Better
quality concrete is produced.
Ø Elimination
of storage space for basic materials at site.
Ø Elimination
of procurement / hiring of plant and machinery
Ø Wastage
of basic materials is avoided.
Ø Labor
associated with production of concrete is eliminated.
Ø Time
required is greatly reduced.
Ø Noise
and dust pollution at site is reduced.
Ø Reduce
cost
DISADVANTIGES OF READY MIX CONCRETE
Ø The materials are batched at central plant, and the mixing begins at the plant, so the travelling time from the plant to the site is critical site over longer distances. Some sites are just too far away, though this is usually a commercial rather than technical issue.
Ø Generation of additional road traffic; furthermore, access roads, and site access 2.5 tone per m3. This problem can be overcome by utilizing so-called ‘minimix’ companies, using smaller 4m3 capacity mixers able to access more restricted sites.
Ø Concrete’s limited time span between mixing and going-off means that ready-mix should be placed within 120 minutes of batching at the plant.
Ø It is cost effective
Conclusion
Ø It
is Costlier then site mix concrete.
Ø Due
Taxies it is rear for Small construction
Ø It
is useful in the pre-stressed construction
Ø Good
in quality control concrete.
Ø It
have high strength with low w/c ratio
Ø It
requires skilled labors
Ø This
method help for rapid construction compared to site mix concrete
It
is useful in large construction, It helps to save the time for the
construction. And labor should less as compare to site mixed concrete. Then
better to use in large construction.
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