About Ready Mixed Concrete
About Ready Mixed Concrete
Learn
how the natural ingredients of concrete combine to form a remarkably strong,
durable and economical building material that has been benefiting mankind for
millennia.
Concrete Basics
In 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.
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.
Portland
cement’s chemistry comes to life in the presence of water. Cement and water
form a paste that surrounds and binds each particle of sand and stone. Through
a chemical reaction of cement and water called hydration, the paste hardens and
gains strength.
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 work ability of fresh
concrete. Generally, using less water produces a higher quality concrete
provided the concrete is properly placed, consolidated and cured.
Besides
Portland cement, concrete may contain other cementitious materials including
fly ash, a waste byproduct from coal burning electric power plants; ground
slag, a byproduct of iron and steel manufacturing; and silica fume, a waste
byproduct from the manufacture of silicon or Ferro-silicon metal. Some of these
cementitious materials are similar to the volcanic ashes the Romans mixed with
lime to obtain their cement binder. Some of these structures still exist today!
The concrete industry uses these materials, which would normally have to be
disposed in land-fill sites, to the advantage of concrete. The materials
participate in the hydration reaction and significantly improve the strength,
permeability and durability of concrete.
Other
Ingredients
Aggregates
for concrete 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
buildings sections can require small coarse aggregates, though aggregates up to
six inches (150 mm) 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.
Almost
any natural water that is drinkable and has no pronounced taste or odor may be
used as mixing water for concrete. However, some waters that are not fit for
drinking may be suitable for concrete.
Excessive
impurities in mixing water not only may affect time and concrete strength, but
also may cause efflorescence, staining, corrosion of reinforcement, volume
instability and reduced durability.
Specifications
usually set limits on chlorides, sulfates, alkalies, and solids in mixing water
unless tests indicate that the water will not negatively impact concrete
properties.
Hydration Begins
After the aggregates,
water, and the cement are combined, the mixture remains in a fluid condition
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. The RMC producer is the expert in selecting the
proportions based on previously developed guidelines and experience.
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 cementitious 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.
How is it produced?
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
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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
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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:
- Faster production capability than a transit-mix plant
- Improved concrete quality control and consistency and
- Reduced wear on the truck mixer drums.
There are several types
of plant mixers, including:
- Tilt drum mixer
- Horizontal shaft paddle mixer
- Dual shaft paddle mixer
- Pan mixer
- Slurry mixer
The tilting drum mixer
is the most common American central mixing unit. Many central-mix drums can
accommodate up to 12 yd3 and can mix in excess of 200 yd3 per hour. They are
fast and efficient, but can be maintenance-intensive since they include several
moving parts that are subjected to a heavy load.
Horizontal shaft mixers
have a stationary shell and rotating central shaft with blades or paddles. They
have either one or two mixing shafts that impart significantly higher
horsepower in mixing than the typical drum mixer. The intensity of the mixing
action is somewhat greater than that of the tilt drum mixer. This high energy
is reported to produce higher strength concrete via to thoroughly blending the
ingredients and more uniformly coating the aggregate particles with cement paste.
Because of the horsepower required to mix and the short mixing cycle required
to complete mixing, many of these mixers are 4 or 5 yd3 units and two batches
may be needed to load a standard truck or agitator.
Pan mixers are generally
lower capacity mixers at about 4 to 5 yd3 and are used at precast concrete
plants.
Slurry mixing
The slurry mixer is a relative newcomer to concrete mixing technology. It can
be added onto a dry-batch plant and works by mixing cement and water that is
then loaded as slurry into a truck mixer along with the aggregates. It is
reported to benefit from high-energy mixing. Another advantage is that the
slurry mixer reduces the amount of cement dust that escapes into the air.
"Mix Mobiles"
- Mobile Volumetric Proportioning Plants
"Mix Mobile" are truck-mounted, volumetric batching and continuous
mixing units. These "plants-on-wheels" often supply small-volume or
specialty pours and offer the convenience of freshly mixed concrete in fairly
precise quantities. The unit consists of a truck with bins of sand, coarse
aggregate, cement, water, and admixtures. The aggregate bins have longitudinal
belts at the bottom of the sand, and as well as coarse aggregate bins that drag
the aggregate to separate adjustable gates at the rear of the bin. The speed of
the belts is connected to a feeder in a cement bin, and all three materials
drop down into a mixer. Flow meters control the introduction of water and
admixtures.
Plant Styles, etc.
Concrete batch plants come in a variety of styles and configurations designed
to accommodate a variety of markets, technical and environmental
considerations.
- Portable Plants In
general, they have a cement silo and an overhead bin for sand or one or
two coarse aggregates.
- Permanent Plants The
plant operates from same location for a relatively long period of time.
Large quantities of materials of greater variety are stored at the plant.
The plant will tend to have larger overhead storage and may have two lanes
to permit batching two trucks at the same time. Plants may be also
classified as
- High profile -
The traditional stack up plant is a tall plant that has aggregate and
cement storage bins that feed into batchers or weigh hoppers by gravity.
- Low profile -
The aggregate weigh hoppers are near the ground with belts to elevate the
aggregate to load the mixer.
Delivery of Ready Mixed Concrete
The ASTM C 94, Specification for Ready Mixed Concrete, indicates that the concrete shall be discharged on the job site within 90 minutes and before 300 revolutions after water was added to the cement. The purchaser may waive this requirement, when conditions permit.
Inspection and Testing of Ready Mixed 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.
Each test helps to
determine the quality of concrete and it should be performed in accordance with
American Society for Testing and Materials (ASTM) standards. A certified ACI
Concrete Field Testing Technician – Grade I, must make the tests.
When there are no formal
job specifications, such as with a homeowner or small contractor, it is
important for the concrete producer to agree to furnish concrete in accordance
with ASTM C 94 or at least certain critical sections of ASTM C 94. This
reference should also be included on the delivery ticket.
ASTM C 94 includes a
number of things that should be a part of any agreement between the producer and
a purchaser. Some of them are:
1.
Define the basis of purchase, cubic yards, and how it is measured.
2.
Define acceptable material specifications and acceptable industry
practice and tolerances.
3.
Define strength testing procedures and acceptance criteria.
4.
Set laboratory personnel qualifications. The testing laboratory
must comply with ASTM C 1077, which is required in ASTM C 94.
Types of 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 prestressed or prestressed
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 precasting.
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 site precasting.
Flow able Fill
Flowable also known as constrolled low-strength material (CLSM) is a
self-compacted, cementitious 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. Flowable
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.
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