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Ready mixed concrete is that which is delivered to the customer in
a freshly mixed and unhardened state. Due to the ability to customize
its properties for different applications and its strength and durability
to withstand a wide variety of environmental conditions,
ready mixed concrete is one of the most versatile and popular building
materials.
Concrete mixtures are proportioned to obtain the required properties
for the application. It should have the correct consistency, or
slump, to facilitate handling and placing and adequate strength and
durability to withstand applied loads in the anticipated environment
and service conditions. The design quantities of concrete ingredients
are accurately weighed and mixed, either in a mixer at the
concrete plant or in a concrete truck mixer. It is delivered in a truck
mixer or agitation unit, which keeps the concrete uniformly mixed
until it is discharged at the placement location. Concrete remains
plastic for several hours depending on the type of mixture and conditions
during placement so that there is sufficient time for it to be
placed and finished. Concrete normally sets or hardens within two
to twelve hours after mixing and continues to gain strength for
months or even years if it is properly cured during the first few
days.
WHY Use Ready Mixed Concrete?
Concrete, in its freshly mixed state, is a plastic workable mixture
that can be cast into virtually any desired shape. The properties of
concrete can be customized for almost any application to serve in a
wide variety of extreme environments. Concrete is a very economical
building material that can serve its function for several years
with minimum maintenance, provided the proper mixture relative
to the application and established construction practices are used.
A wide variety of options with color, texture and architectural detail
are available to enhance the aesthetic quality of the concrete
application.
HOW to Order Ready Mixed Concretet
The key to placing an order for ready mixed concrete is to provide
all the basic detailed information and to keep the requirements
as simple as possible and relevant to the application. The
ready mixed concrete producer has several mixture formulations
for a wide variety of applications and can help with deciding the
required mixture characteristics.
Some of the basic requirements to keep in mind when placing a
concrete order are as follows:
- Size of coarse aggregate - the important information is the nominal
maximum size required, which should be smaller than the narrowest
dimension through which concrete should flow, such as the thickness
of the section and spacing of the reinforcing steel, if any. For
most applications, nominal maximum size of coarse aggregate is 3/4
or 1 inch (19.0 or 25.0 mm).
- Slump - Concrete slump, a measure of its consistency, should be
indicated. A stiffer mixture will have a low slump value. Typical
slump range for most applications is 3 to 5 inches (75 to 100 mm).
For slip-form construction a maximum slump of 2 inches (50 mm) is
required, while higher slump to a maximum of 7 inches (175 mm) is
typical for basement walls. The tolerance on the slump as delivered
is ±1 to 11/2 inch (25 to 38 mm). Addition of water at the jobsite to
increase slump is permitted, provided it is not excessive enough to
cause segregation and reduce strength and durability.
- Entrained air - Air entrained concrete should be used if concrete
will be exposed to freezing temperatures in service, or even during
construction. In many locations air-entrained concrete is the default
option. When non air-entrained concrete is required this should be
clearly stated at the time of ordering. Target air content depends on
the size of the coarse aggregate and the typical range is 4 to 6% of
the concrete volume. The tolerance on air content as delivered is
±1.5%. The concrete supplier is permitted to make an adjustment for
air content at the jobsite if, when tested it is lower than the required
amount.
- Quality level required - The purchaser specifies the concrete quality,
in terms of its properties or composition.
The preferred method for ordering concrete is by specifying performance
requirements, which is generally the concrete strength. Other
performance characteristics, such as permeability, shrinkage or various
durability requirements, may be specified when required. The
producer should be made aware of anticipated exposure and service
conditions of the structure. The concrete producer is best equipped
to proportion, mix and furnish concrete for the desired performance.
The strength level is generally dictated by the design of the structure
to withstand anticipated loads during construction and in service. A
minimum strength of 3500 to 4000 psi (25 to 28 MPa) may ensure
durable concrete, such as resistance to wear, abrasion, and freezing
and thawing cycles.
Another option is to order concrete by specifying prescriptive requirements.
The purchaser specifies limits on the quantities and types
of ingredients in the mixture. In this case the purchaser should generally
accept responsibility for concrete strength and performance.
The prescriptive limits may indicate minimum cement content, maximum
water-cement ratio, and limits on the quantities of pozzolans,
slag or admixtures. Frequently, this approach is used when a particular
prescriptive mixture formulation has worked well in the past.
This approach does not allow the producer much flexibility on the
economy of the mixture or to accommodate changes in material
sources or characteristics that may affect concrete’s performance.
Specifying performance and prescriptive requirements is discouraged
as the performance requirements may conflict with the prescriptive
limits.
Quantity of concrete - Concrete is sold by volume, in cubic yards
(cubic meters), in a freshly mixed unhardened state as discharged
from the truck mixer. The delivered volume, or yield, is calculated
from the measured concrete density or unit weight. One cubic yard
of concrete weighs about 4000 lb. (2 short tons). One cubic meter
(approximately 1.3 cubic yards) weighs about 2400 kg. The typical
capacity of a truck mixer is 8 to 12 cubic yards (5 to 9 cubic meters).
Order about 4% to 10% more concrete than is estimated from a volumetric
calculation of the plan dimensions. This will account for waste
or spillage, over-excavation, spreading of forms, loss of entrained
air during placement, settlement of a wet mixture, truck mixer holdback
and change in volume - hardened concrete volume is 1% to 2%
less than that of the fresh concrete. Reevaluate the needs during placement
and communicate any changes to the concrete supplier.
Disposal of returned concrete has environmental and economical
implications to the ready mixed concrete producer. Make a good
estimate of concrete required for the job before placing an order.
Avoid ordering small clean-up loads, less than 4 cubic yards (2.5
cubic meters).
Additional Items - A variety of value-added options are available
from the ready mixed concrete producer. Chemical admixtures can
accelerate or retard the setting characteristics of concrete to aid in
placing and finishing during hot or cold weather. Water reducing
admixtures are used to increase concrete slump without adding water
to the concrete. Synthetic fibers can reduce the potential for plastic
shrinkage cracking. Using color or special aggregates can enhance
aesthetic characteristics. The concrete contractor can also be a
resource for innovative finishes and textures to concrete.
Scheduling delivery - Schedule the delivery of concrete to accommodate
the construction schedule. Inform the producer of the correct
address, location and nature of the pour, and an estimated delivery
time. Call the ready mixed concrete producer well in advance of
the required delivery date. Concrete is a perishable product and the
construction crew should be ready for concrete placement when the
truck arrives. Notify the producer of any schedule changes or work
stoppage immediately.
Ensure that the truck mixer has proper access to the placement location
location.The concrete truck weighs in excess of 60,000 lbs. (27,000 kg)
and may not be able to maneuver on loose dirt and residential curbs
and pathways.
WHAT are the Responsibilities
The responsibilities of the various parties involved in the construction
process should be addressed at a pre-construction meeting, especially
on a large job. These responsibilities should be documented
and distributed to all concerned for reference during the construction.
Some items are discussed below.
- The concrete producer is responsible for the concrete slump as
specified for a period of 30 minutes after the requested time or
the time the truck arrives at the placement site, whichever is later.
- The concrete producer is required to deliver concrete at the requested
slump and air content, within the accepted tolerances
addressed above, as measured at the point of discharge from the
transportation unit.
- When placing procedures can potentially alter the characteristics
of the fresh concrete, it is the responsibility of the purchaser
to inform the producer of changes to the mixture requirements to
accommodate these effects. An example is pumping concrete in
place.
- When a job uses more than one type of concrete mixture, it is the
purchaser’s responsibility to verify the mixture delivered and
direct it to the correct placement location.
- The purchaser should check and sign the delivery ticket and document
any special occurrences on the ticket.
- The concrete producer cannot be responsible for the quality of
concrete when any modification or additions are made to the
mixture at the jobsite. These include addition of excessive water,
admixtures, fibers or special products, or if the truck has to wait
for an extended period before discharging the concrete.
- When strength tests are used for acceptance of concrete, the
samples should be obtained at the point of discharge from the
transportation unit. The purchaser or his representative should
ensure that proper facilities are available for curing the test specimens
at the jobsite and that standard practices are followed for
subsequent curing and testing. Certified personnel should conduct
the tests. Test reports should be forwarded to the producer
in a timely manner to ensure that deficiencies are rectified.
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