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5.8 — Mixing

5.8.1 — All concrete shall be mixed until there is a

uniform distribution of materials and shall be

discharged completely before mixer is recharged.



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R5.8 — Mixing

Concrete of uniform and satisfactory quality requires the

materials to be thoroughly mixed until uniform in appearance and all ingredients are distributed. Samples taken from

different portions of a batch should have essentially the

same density, air content, slump, and coarse aggregate

content. Test methods for uniformity of mixing are given in

ASTM C94M. The necessary time of mixing will depend on

many factors including batch size, stiffness of the batch,

size and grading of the aggregate, and the efficiency of the

mixer. Excessively long mixing times should be avoided to

guard against grinding of the aggregates.



5.8.2 — Ready-mixed concrete shall be mixed and

delivered in accordance with requirements of ASTM

C94M or C685M.

5.8.3 — Job-mixed concrete shall be mixed in accordance with (a) through (e):

(a) Mixing shall be done in a batch mixer of

approved type;

(b) Mixer shall be rotated at a speed recommended

by the manufacturer;

(c) Mixing shall be continued for at least 1-1/2

minutes after all materials are in the drum, unless a

shorter time is shown to be satisfactory by the

mixing uniformity tests of ASTM C94M;

(d) Materials handling, batching, and mixing shall

conform to applicable provisions of ASTM C94M;

(e) A detailed record shall be kept to identify:

(1) number of batches produced;

(2) proportions of materials used;

(3) approximate location of final deposit in structure;

(4) time and date of mixing and placing.



5.9 — Conveying



R5.9 — Conveying



5.9.1 — Concrete shall be conveyed from mixer to

place of final deposit by methods that will prevent

separation or loss of materials.



Each step in the handling and transporting of concrete needs

to be controlled to maintain uniformity within a batch and

from batch to batch. It is essential to avoid segregation of

the coarse aggregate from the mortar or of water from the

other ingredients.



5.9.2 — Conveying equipment shall be capable of

providing a supply of concrete at site of placement

without separation of ingredients and without interruptions sufficient to permit loss of plasticity between

successive increments.



The Code requires the equipment for handling and transporting concrete to be capable of supplying concrete to the

place of deposit continuously and reliably under all conditions

and for all methods of placement. The provisions of 5.9

apply to all placement methods, including pumps, belt

conveyors, pneumatic systems, wheelbarrows, buggies,

crane buckets, and tremies.

Serious loss in strength can result when concrete is pumped

through pipe made of aluminum or aluminum alloy.5.14

Hydrogen gas generated by the reaction between the cement

alkalies and the aluminum eroded from the interior of the

pipe surface has been shown to cause strength reduction as



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much as 50 percent. Hence, equipment made of aluminum

or aluminum alloys should not be used for pump lines,

tremies, or chutes other than short chutes such as those used

to convey concrete from a truck mixer.



5.10 — Depositing



R5.10 — Depositing



5.10.1 — Concrete shall be deposited as nearly as

practical in its final position to avoid segregation due to

rehandling or flowing.



Rehandling concrete can cause segregation of the materials.

Hence, the Code cautions against this practice. Retempering

of partially set concrete with the addition of water should

not be permitted unless authorized. This does not preclude

the practice (recognized in ASTM C94M) of adding water

to mixed concrete to bring it up to the specified slump range

so long as prescribed limits on the maximum mixing time

and w/cm are not violated.



5.10.2 — Concreting shall be carried on at such a rate

that concrete is at all times plastic and flows readily

into spaces between reinforcement.

5.10.3 — Concrete that has partially hardened or been

contaminated by foreign materials shall not be deposited

in the structure.

5.10.4 — Retempered concrete or concrete that has

been remixed after initial set shall not be used unless

approved by the licensed design professional.

5.10.5 — After concreting is started, it shall be carried

on as a continuous operation until placing of a panel or

section, as defined by its boundaries or predetermined

joints, is completed except as permitted or prohibited

by 6.4.

5.10.6 — Top surfaces of vertically formed lifts shall be

generally level.

5.10.7 — When construction joints are required, joints

shall be made in accordance with 6.4.



Section 5.10.4 of the 1971 Code contained a requirement

that “where conditions make consolidation difficult or

where reinforcement is congested, batches of mortar

containing the same proportions of cement, sand, and water

as used in the concrete, shall first be deposited in the forms

to a depth of at least 1 in. [25 mm] (Note: see ACI 318M-08).”

That requirement was deleted from the 1977 Code since the

conditions for which it was applicable could not be defined

precisely enough to justify its inclusion as a code requirement.

The practice, however, has merit and should be incorporated

in job specifications where appropriate, with the specific

enforcement the responsibility of the job inspector. The use

of mortar batches aids in preventing honeycomb and poor

bonding of the concrete with the reinforcement. The mortar

should be placed immediately before depositing the

concrete and should be plastic (neither stiff nor fluid) when

the concrete is placed.



5.10.8 — All concrete shall be thoroughly consolidated

by suitable means during placement and shall be thoroughly worked around reinforcement and embedded

fixtures and into corners of forms.



Recommendations for consolidation of concrete are given in

detail in ACI 309R.5.15 (This presents current information

on the mechanism of consolidation and gives recommendations on equipment characteristics and procedures for

various classes of concrete.)



5.11 — Curing



R5.11 — Curing



5.11.1 — Concrete (other than high-early-strength)

shall be maintained above 10°C and in a moist

condition for at least the first 7 days after placement,

except when cured in accordance with 5.11.3.



Recommendations for curing concrete are given in detail in

ACI 308R.5.16 (This presents basic principles of proper

curing and describes the various methods, procedures, and

materials for curing of concrete.)



5.11.2 — High-early-strength concrete shall be maintained above 10°C and in a moist condition for at least

the first 3 days, except when cured in accordance with

5.11.3.



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5.11.3 — Accelerated curing

5.11.3.1 — Curing by high-pressure steam, steam at

atmospheric pressure, heat and moisture, or other

accepted processes, shall be permitted to accelerate

strength gain and reduce time of curing.



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R5.11.3 — Accelerated curing

The provisions of this section apply whenever an accelerated

curing method is used, whether for precast or cast-in-place

elements. The compressive strength of steam-cured concrete

is not as high as that of similar concrete continuously cured

under moist conditions at moderate temperatures. Also, the

modulus of elasticity Ec of steam-cured specimens may

vary from that of specimens moist-cured at normal

temperatures. When steam curing is used, it is advisable to

base the concrete mixture proportions on steam-cured test

cylinders.



5.11.3.2 — Accelerated curing shall provide a

compressive strength of the concrete at the load stage

considered at least equal to required design strength

at that load stage.

5.11.3.3 — Curing process shall be such as to

produce concrete with a durability at least equivalent

to the curing method of 5.11.1 or 5.11.2.



5.11.4 — When required by the licensed design

professional, supplementary strength tests in accordance with 5.6.4 shall be performed to assure that

curing is satisfactory.



Accelerated curing procedures require careful attention to

obtain uniform and satisfactory results. Preventing moisture

loss during the curing is essential.

R5.11.4 — In addition to requiring a minimum curing

temperature and time for normal- and high-early-strength

concrete, the Code provides a specific criterion in 5.6.4 for

judging the adequacy of field curing. At the test age for

which the compressive strength is specified (usually 28 days),

field-cured cylinders should produce strength not less than

85 percent of that of the standard, laboratory-cured cylinders.

For a reasonably valid comparison to be made, field-cured

cylinders and companion laboratory-cured cylinders should

come from the same sample. Field-cured cylinders should

be cured under conditions identical to those of the structure.

If the structure is protected from the elements, the cylinder

should be protected.

Cylinders related to members not directly exposed to

weather should be cured adjacent to those members and

provided with the same degree of protection and method of

curing. The field cylinders should not be treated more

favorably than the elements they represent. (See 5.6.4 for

additional information.) If the field-cured cylinders do not

provide satisfactory strength by this comparison, measures

should be taken to improve the curing. If the tests indicate a

possible serious deficiency in strength of concrete in the

structure, core tests may be required, with or without

supplemental wet curing, to check the structural adequacy,

as provided in 5.6.5.



5.12 — Cold weather requirements



R5.12 — Cold weather requirements



5.12.1 — Adequate equipment shall be provided for

heating concrete materials and protecting concrete

during freezing or near-freezing weather.



Recommendations for cold weather concreting are given

in detail in ACI 306R.5.17 (This presents requirements and

methods for producing satisfactory concrete during cold

weather.)



5.12.2 — All concrete materials and all reinforcement,

forms, fillers, and ground with which concrete is to

come in contact shall be free from frost.

5.12.3 — Frozen materials or materials containing ice

shall not be used.

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5.13 — Hot weather requirements



R5.13 — Hot weather requirements



During hot weather, proper attention shall be given to

ingredients, production methods, handling, placing,

protection, and curing to prevent excessive concrete

temperatures or water evaporation that could impair

required strength or serviceability of the member or

structure.



Recommendations for hot weather concreting are given in

detail in ACI 305R.5.18 (This defines the hot weather factors

that affect concrete properties and construction practices

and recommends measures to eliminate or minimize the

undesirable effects.)



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