3 — Proportioning on the basis of field experience or trial mixtures, or both

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5



Fig. R5.3—Flow chart for selection and documentation of concrete proportions.

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5.3.1 — Sample standard deviation

5.3.1.1 — Where a concrete production facility has

strength test records not more than 24 months old, a

sample standard deviation, ss , shall be established.

Test records from which ss is calculated:



5



R5.3.1 — Sample standard deviation

The standard deviation established from test records is a

measure of the concrete supplier’s ability to manage variability

of materials, production, and testing of concrete. A test

record obtained less than 24 months before a submittal is

acceptable.



(a) Shall represent materials, quality control procedures, and conditions similar to those expected and

changes in materials and proportions within the test

records shall not have been more restricted than those

for proposed Work;



When a concrete production facility has a suitable record of

30 consecutive tests of similar materials and conditions

expected, the sample standard deviation, ss, is calculated from

those results in accordance with the following formula:



(b) Shall represent concrete produced to meet a

specified compressive strength or strengths within 7 MPa

of fc′ ;

(c) Shall consist of at least 30 consecutive tests or two

groups of consecutive tests totaling at least 30 tests as

defined in 5.6.2.4, except as provided in 5.3.1.2.

5.3.1.2 — Where a concrete production facility does

not have strength test records meeting requirements

of 5.3.1.1(c), but does have test records not more than

24 months old based on 15 to 29 consecutive tests, a

sample standard deviation ss shall be established as

the product of the calculated sample standard deviation

and modification factor of Table 5.3.1.2. To be

acceptable, test records shall meet requirements (a)

and (b) of 5.3.1.1, and represent only a single record

of consecutive tests that span a period of not less than

45 calendar days.



TABLE 5.3.1.2 —MODIFICATION FACTOR FOR

SAMPLE STANDARD DEVIATION WHEN LESS

THAN 30 TESTS ARE AVAILABLE

No. of tests*

Less than 15



Use Table 5.3.2.2



15



1.16



20



1.08



25



1.03



30 or more

*Interpolate

†



Modification factor for sample

standard deviation†



1.00



for intermediate numbers of tests.

Modified sample standard deviation, ss , to be used to determined required

′

average strength, fcr , from 5.3.2.1



2 1/2



Σ ( xi – x )

ss = -----------------------(n – 1)

where

ss =

xi =

x =

n =



sample standard deviation, MPa

individual strength tests as defined in 5.6.2.4

average of n strength test results

number of consecutive strength tests



The sample standard deviation is used to determine the

average strength required in 5.3.2.1.

If two test records are used to obtain at least 30 tests, the

sample standard deviation used shall be the statistical

average of the values calculated from each test record in

accordance with the following formula:

2



2 1/2



( n 1 – 1 ) ( s s1 ) + ( n 2 – 1 ) ( s s2 )

ss = --------------------------------------------------------------------------( n1 + n2 – 2 )



where

= statistical average standard deviation where two

ss

test records are used to estimate the sample standard

deviation

ss1, ss2= sample standard deviations calculated from two

test records, 1 and 2, respectively

n1, n2 = number of tests in each test record, respectively

If less than 30 tests, but at least 15 tests are available, the

calculated sample standard deviation is increased by the

factor given in Table 5.3.1.2. This procedure results in a

more conservative (increased) required average strength.

The factors in Table 5.3.1.2 are based on the sampling

distribution of the sample standard deviation and provide

protection (equivalent to that from a record of 30 tests)

against the possibility that the smaller sample underestimates

the true or universe population standard deviation.

The sample standard deviation used in the calculation of

required average strength should be developed under



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conditions “similar to those expected” [see 5.3.1.1(a)]. This

requirement is important to ensure acceptable concrete.

Concrete for background tests to determine sample standard

deviation is considered to be “similar” to that required if

made with the same general types of ingredients under no

more restrictive conditions of control over material quality

and production methods than on the proposed Work, and if

its specified strength does not deviate more than 7 MPa

from the fc′ required [see 5.3.1.1(b)]. A change in the type

of concrete or a major increase in the strength level may

increase the sample standard deviation. Such a situation

might occur with a change in type of aggregate (i.e., from

natural aggregate to lightweight aggregate or vice versa) or

a change from non-air-entrained concrete to air-entrained

concrete. Also, there may be an increase in sample standard

deviation when the average strength level is raised by a

significant amount, although the increment of increase in

sample standard deviation should be somewhat less than

directly proportional to the strength increase. When there is

reasonable doubt, any estimated sample standard deviation

used to calculate the required average strength should

always be on the conservative (high) side.

Note that the Code uses the sample standard deviation in

megapascals instead of the coefficient of variation in

percent. The latter is equal to the former expressed as a

percent of the average strength.

Even when the average strength and sample standard deviation

are of the levels assumed, there will be occasional tests that

fail to meet the acceptance criteria prescribed in 5.6.3.3

(perhaps one test in 100).



5.3.2 — Required average strength



R5.3.2 — Required average strength



5.3.2.1 — Required average compressive strength

′

fcr used as the basis for selection of concrete

proportions shall be determined from Table 5.3.2.1

using the sample standard deviation, ss , calculated in

accordance with 5.3.1.1 or 5.3.1.2.



R5.3.2.1 — Once the sample standard deviation has been

determined, the required average compressive strength, fcr ,

′

is obtained from the larger value computed from Eq. (5-1)

and (5-2) for fc′ of 35 MPa or less, or the larger value

computed from Eq. (5-1) and (5-3) for fc′ over 35 MPa.

Equation (5-1) is based on a probability of 1-in-100 that the

average of three consecutive tests may be below the specified

compressive strength fc′ . Equation (5-2) is based on a

similar probability that an individual test may be more than

35 MPa below the specified compressive strength fc′ .

Equation (5-3) is based on the same 1-in-100 probability

that an individual test may be less than 0.90fc . These equations

′

assume that the sample standard deviation used is equal to

the population value appropriate for an infinite or very large

number of tests. For this reason, use of sample standard

deviations estimated from records of 100 or more tests is

desirable. When 30 tests are available, the probability of

failure will likely be somewhat greater than 1-in-100. The

additional refinements required to achieve the 1-in-100

probability are not considered necessary because of the



TABLE 5.3.2.1 — REQUIRED AVERAGE

COMPRESSIVE STRENGTH WHEN DATA ARE

AVAILABLE TO ESTABLISH A SAMPLE

STANDARD DEVIATION

Specified compressive

strength, MPa



fc′ ≤ 35



fc′ > 35



Required average compressive

strength, MPa

Use the larger value computed

from Eq. (5-1) and (5-2)

′

(5-1)

fcr = fc′ + 1.34ss

fcr = fc′ + 2.33ss – 3.5

′

(5-2)

Use the larger value computed

from Eq. (5-1) and (5-3)

′

(5-1)

fcr = fc′ + 1.34ss

′

(5-3)

fcr = 0.90fc′ + 2.33ss



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uncertainty inherent in assuming that conditions operating

when the test record was accumulated will be similar to

conditions when the concrete will be produced.



5



5.3.2.2 — When a concrete production facility does

not have field strength test records for calculation of ss

meeting requirements of 5.3.1.1 or 5.3.1.2, fcr shall

′

be determined from Table 5.3.2.2 and documentation

of average strength shall be in accordance with

requirements of 5.3.3.

TABLE 5.3.2.2 — REQUIRED AVERAGE

COMPRESSIVE STRENGTH WHEN DATA ARE

NOT AVAILABLE TO ESTABLISH A SAMPLE

STANDARD DEVIATION

Specified compressive

strength, MPa



Required average compressive

strength, MPa



fc′ < 21



fcr = fc′ + 7.0

′



21 ≤ fc′ ≤ 35



fcr = fc′ + 8.3

′



fc′ > 35



fcr = 1.10fc′ + 5.0

′



5.3.3 — Documentation of average compressive

strength



R5.3.3 — Documentation of average compressive

strength



Documentation that proposed concrete proportions

will produce an average compressive strength equal to

or greater than required average compressive strength

′

fcr (see 5.3.2) shall consist of one or more field

strength test record(s) or trial mixtures. The field

strength test records or trial mixtures shall not be more

than 24 months old and shall conform to 5.3.3.1 and

5.3.3.2, respectively.



Once the required average compressive strength fcr is known,

′

the next step is to select mixture proportions that will produce

an average strength at least as great as the required average

strength, and also meet requirements for applicable exposure

categories of Chapter 4. The documentation may consist of

one or more field strength test record(s) in accordance with

5.3.3.1, or suitable laboratory or field trial mixtures in accordance with 5.3.3.2. Generally, if a test record is used, it will be

the same one that was used for computation of the standard

deviation. However, if this test record shows either lower or

higher average compressive strength than the required average

compressive strength, different proportions may be necessary

or desirable. In such instances, the average from a record of as

few as 10 tests may be used, or the proportions may be established by interpolation between the strengths and proportions

of two such records of consecutive tests. All field test records

for establishing proportions necessary to produce the average

compressive strength are to meet the requirements of 5.3.3.1

for “similar materials and conditions.”



5.3.3.1 — When test records in accordance with

5.3.1.1 or 5.3.1.2 are used to demonstrate that

′

proposed concrete proportions will produce fcr (see

5.3.2), such records shall represent materials and

conditions similar to those expected. Changes in

materials, conditions, and proportions within the test

records shall not have been more restricted than those

for proposed Work. For the purpose of documenting

average strength potential, test records consisting of

less than 30 but not less than 10 consecutive tests are

acceptable provided test records encompass a period

of time not less than 45 days. Required concrete

proportions shall be permitted to be established by

interpolation between the strengths and proportions of

two or more test records, each of which meets other

requirements of this section.

5.3.3.2 — When an acceptable record of field test

results to document the required average strength is not

available, concrete proportions established from trial

mixtures meeting the following requirements shall be

permitted:



For strengths over 35 MPa where the average compressive

strength documentation is based on laboratory trial

mixtures, it may be appropriate to increase fcr calculated in

′

Table 5.3.2.2 to allow for a reduction in strength from

laboratory trials to actual concrete production.

R5.3.3.2 — This section of the Code was modified in ACI

318-08 to clarify the requirements for making trial batches.

(b) For concrete made with more than one type of

cementitious material, the concrete supplier must estab-



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STRUCTURAL CONCRETE BUILDING CODE (ACI 318M-11) AND COMMENTARY



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(a) Materials shall be those for proposed Work;

(b) Trial mixtures with a range of proportions that will

produce a range of compressive strengths encom′

passing fcr and meet the durability requirements of

Chapter 4;

(c) Trial mixtures shall have slumps within the range

specified for the proposed Work; for air-entrained

concrete, air content shall be within the tolerance

specified for the proposed Work;

(d) For each trial mixture, at least two 150 by 300 mm or

three 100 by 200 mm cylinders shall be made and cured

in accordance with ASTM C192M. Cylinders shall be

tested at 28 days or at test age designated for fc′ ;

(e) The compressive strength results, at designated

test age, from the trial mixtures shall be used to

establish the composition of the concrete mixture

proposed for the Work. The proposed concrete

mixture shall achieve an average compressive

strength as required in 5.3.2 and satisfy the applicable durability criteria of Chapter 4.



71



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lish not only the water-cementitious material ratio but

also the relative proportions of cementitious materials and

admixtures, if any, that will produce the required average

compressive strength and satisfy the durability requirements

of Chapter 4. This will require multiple trial batches with

different mixture proportions. The exact number of

batches will depend on the number of cementitious

materials and the range of their relative proportions.

(d) Starting with the 2008 Code, 5.6.2.4 permits two

cylinder sizes for preparing test specimens for field

acceptance testing.

(e) The compressive strength test results may be analyzed

graphically or using regression models to determine the

water-cementitious material ratio and the relative

proportions of cementitious materials, if other materials

in addition to portland cement are used, that will produce

concrete satisfying the required average compressive

strength fcr .

′



5.4 — Proportioning without field

experience or trial mixtures



R5.4 — Proportioning without field

experience or trial mixtures



5.4.1 — If data required by 5.3 are not available, concrete

proportions shall be based upon other experience or

information, if approved by the licensed design profes′

sional. The required average compressive strength fcr

of concrete produced with materials similar to those

proposed for use shall be at least 8.3 MPa greater

than fc′ . This alternative shall not be used if fc′ is

greater than 35 MPa.



R5.4.1 — When no prior experience (5.3.3.1) or trial

mixture data (5.3.3.2) meeting the requirements of these

sections is available, other experience may be used only

when permission is given. Because combinations of

different ingredients may vary considerably in strength

level, this procedure is not permitted for fc′ greater than

35 MPa and the required average compressive strength

should exceed fc′ by 8.3 MPa. The purpose of this provision

is to allow work to continue when there is an unexpected

interruption in concrete supply and there is not sufficient

time for tests and evaluation or in small structures where the

cost of trial mixture data is not justified.



5.4.2 — Concrete proportioned by this section shall

conform to the durability requirements of Chapter 4

and to compressive strength test criteria of 5.6.



5.5 — Average compressive strength

reduction

As data become available during construction, it shall

be permitted to reduce the amount by which the

′

required average concrete strength, fcr , must exceed

fc′ , provided:

(a) Thirty or more test results are available and

average of test results exceeds that required by

5.3.2.1, using a sample standard deviation calculated

in accordance with 5.3.1.1; or

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(b) Fifteen to 29 test results are available and

average of test results exceeds that required by

5.3.2.1 using a sample standard deviation calculated

in accordance with 5.3.1.2; and

(c) Requirements

Chapter 4 are met.



for



exposure



categories



of



5

5.6 — Evaluation and acceptance of

concrete



R5.6 — Evaluation and acceptance of

concrete

Once the mixture proportions have been selected and the job

started, the criteria for evaluation and acceptance of the

concrete can be obtained from 5.6.

An effort has been made in the Code to provide a clear-cut

basis for judging the acceptability of the concrete, as well as

to indicate a course of action to be followed when the results

of strength tests are not satisfactory.



5.6.1 — Concrete shall be tested in accordance with

the requirements of 5.6.2 through 5.6.5. The testing

agency performing acceptance testing shall comply

with ASTM C1077. Qualified field testing technicians

shall perform tests on fresh concrete at the job site,

prepare specimens required for curing under field

conditions, prepare specimens required for testing in

the laboratory, and record the temperature of the fresh

concrete when preparing specimens for strength tests.

Qualified laboratory technicians shall perform all

required laboratory tests. All reports of acceptance

tests shall be provided to the licensed design

professional, contractor, concrete producer, and,

when requested, to the owner and the building official.



R5.6.1 — ASTM C10775.3 identifies and defines the duties

and minimum technical requirements and qualifications of

testing laboratory personnel and requirements for testing

concrete and concrete aggregates used in construction.

Inspection and accreditation of testing laboratories is a

process that ensures that they conform to ASTM C1077.

Laboratory and field technicians can establish qualifications

by becoming certified through certification programs. Field

technicians in charge of sampling concrete; testing for

slump, density, yield, air content, and temperature; and

making and curing test specimens should be certified in

accordance with the requirements of ACI Concrete Field

Testing Technician—Grade 1 Certification Program, or an

equivalent program. Concrete testing laboratory personnel

should be certified in accordance with the requirements of

ACI Concrete Laboratory Testing Technician, or Concrete

Strength Testing Technician.

The Code requires testing reports to be distributed to the

parties responsible for the design, construction, and

approval of the work. Such distribution of test reports

should be indicated in contracts for inspection and testing

services. Prompt distribution of testing reports allows for

timely identification of either compliance or the need for

corrective action. A complete record of testing allows the

concrete producer to reliably establish the required average

′

strength fcr for future work.



5.6.2 — Frequency of testing



R5.6.2 — Frequency of testing



5.6.2.1 — Samples for strength tests of each class

of concrete placed each day shall be taken not less

than once a day, nor less than once for each 110 m3 of

concrete, nor less than once for each 460 m2 of

surface area for slabs or walls.



R5.6.2.1 — The following three criteria establish the required

minimum sampling frequency for each class of concrete:

(a) Once each day a given class is placed, nor less than

(b) Once for each 110 m3 of each class placed each day,

nor less than



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