214_77.pdf

This document has been approved for use by agencies of the Department of Defense and for listing in the DOD Index of Specifications and Standards.

Recommended Practice for Evaluation of Strength Test Results of Concrete (ACI 214-77)* (Reapproved 1997) Reported by ACI Committee 214 V. M. M A L H O T R A Chairman

V . RAMAKRISHNAN HUBERT RUSCH ROBERTO SANCHEZ-TREJO ROBERT G. SEXSMITH V. D. SKIPPER J. DERLE THORPE

RICHARD J. DOERMANN R I C H A R D D. GAYNOR ARNOLD R. KLINE K. R. A . M . NEVILLE ROBER ROBERT T E. PHILLEO FRANCIS J. PRINCIPE

EDWARD A. ABDUN-NUR H O W A R D T . ARNI JOSEPH F. ARTUSO ROBERT M. BARNOFF T. G. CLENDENNING HERBE RT K. COOK

Statistical procedures provide valuable tools for assessing results of strength tests, and such an approach is also of value in refining design criteria and specifications. The report discusses briefly the numerous variations that occur in the strength of concrete and presents statistical procedures which are useful in interpreting these variations. coefficient of variation: variation: compression compression tests: compressive compressive strength strength;; concrete concrete construction construction:: Keywords: coefficient concretes: cylinders: evaluation; quality control; sampling sampling;; standar standard d deviatio deviation; n; statistic statistical al analysi analysis; s; variations.

CONTENTS Chapter I-Introduction

2

.....................................................

Chapter 2-Variations in strength

2

............................................

2.3-Testing methods

2.1-General 2.2-Properties of concrete

Chapter 3-Analysis of strength data

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. . . . . . . . . . . . . . . . . . . . . . .

.

. . . . .

3

3.4-Strength variations 3.5-Standards of control

3.1-Notation 3.2-General 3.3-Statistical functions

Chapter 4-Criteria . . . . . . . . . . . . .

. . .

.

.

4.4-Quality control charts 4.5-Tests and specimens required 4.6-Rejection of doubtful specimens

4. l-General 4.2-Criteria for strength requirements 4.3-Additional information

Chapter 5-References

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CHAPTER I-INTRODUCTION The purposes of strength tests of concrete are to determine compliance with a strength specification and to measure the variability of concrete. Concrete, being a hardened mass of heterogeneous materials, is subject to the influence of numerous variables. Characteristics of each of the ingredients of concrete, depending on their variability, may cause variations in strength of concrete. Variations may also be introduced by practices used in proportioning, mixing, transporting, placing, and curing. In addition to the variations which exist in concrete itself, test strength variations will also be introduced by the fabrication, testing, and treatment of test specimens. Variations in the strength of concrete must be accepted, but concrete of adequate quality can be produced with confidence if proper control is maintained, test results are properly interpreted, and their limitations are considered. Proper control is achieved by the use of satisfactory materials, correct batching and mixing of these materials, correct batching and mixing of sired quality, and good practices in transporting, placing, curing, and testing. Although the complex nature of concrete precludes complete homogeneity, excessive variation of concrete strength signifies inadequate concrete control. Improvement in control may permit a reduction in the cost of concrete since the average strength can be brought closer to specification requirements. Strength is not necessarily the most critical factor in proportioning concrete mixes since other factors, such as durability, may impose lower water-cement ratios than are required to meet strength requirements. In such cases, strength will of necessity be in excess of structural demands. Nevertheless, strength tests are valuable in such circumstances since, with established mix proportions, variations in strength are indicative of variations in other properties. Test specimens indicate the potential rather than the actual strength of the concrete in a struc-

ture. To be meaningful, conclusions on strength of concrete must be derived from a pattern of tests from which the characteristics of the concrete can be estimated with reasonable accuracy. Insufficient tests will result in unreliable conclusions. Statistical procedures provide tools of considerable value in evaluating results of strength tests and information derived from such procedures is also of value in refining design criteria and specifications. This report briefly discusses variations that occur in the strength of concrete, and presents statistical procedures that are useful in the interpretation of these variations with respect to required criteria and specifications. For these statistical procedures to be valid, the data must be derived from samples obtained by means of a random sampling plan designed to reduce the possibility that choice will be exercised by the sampler. “Random sampling” means that each possible sample has an equal chance of being selected. To insure this condition, the choice must be made by some objective mechanism such as a table of random numbers. If sample batches are selected by the sampler on the basis of his own judgment, judgment, biases are likely likely to be introduced introduced that will invalidate results analyzed by the procedures presented here. Reference 1 contains a discussion of random sampling and a useful short table of random numbers. Additional Additional information information on the meaning and use of this recommended practice is given in Realism in the Application of A C I Standard This volume is a compilation of information on AC I 214-65 that was presented at a symposium held at Buffalo, N. Y., in 1971. In addition to the papers from the symposium, it includes reprints of some pertinent papers that were published earlier in the ACI JOURNAL, and of discussion that resulte d from them. Although the information given was AC I 214-65, most of it is still relevant. based on ACI An additional additional source of material material on evaluation evaluation of strength tests is AC I Bibliography No. 2, published in







TABLE 2.I-PRINCIP INC IPAL AL SOURC ES OF STRENGT NG TH VARIA VARIAT TION IO N

Variati Variations ons in the proper properties ties of concrete

Discrepancies in testing methods

Changes in water-cement ratio: Poor control of water Excessive variation of moisture in aggregate Retempering

Improper sampling procedures

Variati Variations ons in water water requir requiree- Variations Variations due to fabricament: tion techniques Aggreg Aggregate ate gradin grading, g, abHandling and curing of sorption, particle shape newly made cylinders Cement and admixture Poor quality molds properties Air content content Delivery time and temperature Variati Variations ons in charac character teristic isticss Changes in curing: and proportions of ingreTemperature variation dients: Variabl Variablee moistu moisture re Aggreg Aggregate atess Delays in bringing cylinCement ders to the laboratory Pozzolans Admixtu Admixtures res Variati Variations ons in transp transport orting ing,, placing, and compaction Variati Variations ons in temper temperatu ature re and curing

Poor testing procedures: Cylinder capping Compression tests

first criterion for producing concrete of constant strength, therefore, is a constant water-cement ratio. Since the quantity of cement and added water can be measured accurately, the problem of maintaining a constant water-cement ratio is primarily one of correcting for the variable quantity of free moisture in aggregates. The homogeneity of concrete is influenced by the variability of the aggregates, cement, and ad-

mixtures used, since each will contribute to variations in the concrete strength. The temperature of fresh concrete influences the amount of water needed to achieve the proper consistency and consequently contributes to strength variation. Construction practices may may cause variations in strength due to inadequate mixing, poor compaction, delays, and improper curing. Not all of these are reflected in specimens fabricated and stored under standard conditions. The use of admixtures adds another factor since each admixture adds another variable to concrete. The batching of accelerators, retarders, pozzolans, and air-entraining agents must be carefully controlled. 2.3-Testing methods

Concrete tests may or may not include all the variations in strength of concrete in place depending on what variables have been introduced after test specimens were made. On the other hand, discrepancies in sampling, fabrication curing, and testing of specimens may cause indications of variations in strength which do not exist in the concrete in the structure. The project is unnecessarily penalized when variations from this source are excessive. Good testing methods will reduce these variations, and standard testing procedures such as those described in ASTM standards should be followed without deviation. The importance of using accurate testing machines and producing thin, high-strength, plane, parallel caps should need no emphasis since test results can be no more accurate than the equipment and procedures used. Uniform test results are not necessarily accurate test results. Laboratory equipment and procedures should be calibrated and checked periodically.

CHAPTER 3-ANALYSIS OF STRENGTH DATA







kgf 199 I

183 I

197 I

211

225

253

267

281

I

239

295

I

209

323

I

I

I

I

I

I

I

68.27%

COMPRESSIVE

Fig. 3.3(a)-Frequency distribution of strength data

STRENGTH

PS

I

and corresponding normal distribution

3.2-General

3.3-Statistical

To obtain maximum information, a sufficient number of tests should be made to indicate the variation in the concrete produced and to permit appropriate statistical procedures to be used in interpreting the test results. Statistical procedures provide the best basis for determining from such results the potential quality and strength of the concrete and for expressing results in the most useful form.

The strength of concrete test specimens on controlled projects can be assumed to fall “into a pattern similar to the normal frequency distribution curve illustrated in Fig. 3.3 (a). (a). Where there is good control, the strength values will be bunched close to the average, and the curve will be tall and narrow. As the variations in strength increase, the values spread- and the curve becomes low and elongated, as illustrated by the (b) . . Because the idealized curves shown in Fig. 3.3 (b) characteristics of such curves can be defined mathematically, certain useful functions of the strength can be calculated as follows: -The average strength of all 3.3.1 Average, X -The individual tests

I

169 I

197 225 I I

263 I

261

309 I

337 36 6 394 I I I

4

functions

340 PSI (23.9

n

(3-1)







area under the curve of the frequency distribution of strength data, such as that shown in Fig. 3.3 (a). (a). The best estimate of based on a finite amount of data, is obtained by Eq. (3-2)) or by its algebraic equivalent, Eq. (3-2a). (3-2a). The latter equation is preferable for computation purposes, because it is not only simpler and more adaptable to desk calculators, but it avoids the possibility of trouble due to rounding errors.

TABLE 3.4.1 l-FACTORS FOR COMPUTING WITHINTEST STANDARD DEVIATION*

Number of specimens

3

4 5

6 7

+

-

-

8 9

(3-2)

or

0.8865 0.5907 0.4857 0.4299 0.3946 0.3698 0.3512 0.3367 0.3249

1.128 1.693 2.059 2.326 2.534 2.704 2.847 2.970 3.078

2

10

*From Table B2, ASTM Manual on Quality Quality Control of Materials, Reference 4.

n

- 1

(3-2a)

-The standard 3.3.3 Coefficient of variation, V -The deviation expressed as a percentage of the average strength is called the coefficient of variation:

crete are required to establish reliable values for The within-test standard deviation and coefficient of variation can be conveniently computed as follows: =

(3-4)

-

2

v=

x 100

(3-3)

3.3.4 Range, R -Range is the statistic found by

=

(3-5)

= within-test standard deviation =

=

=

=

a constant depending on the number of cylinders averaged to produce a test (Table 3.4.1) average range within groups of companion cylinders within-test coefficient of variation average strength

Batch-to-batch variations - These variations reflect differences in strength which can be attributed to variations in (a) Characteristics and properties of the ingredients (b) Batching, mixing, and sampling (c) Testing that has not been detected from companion cylinders since these tend to be treated more alike than cylinders tested at different times 3.4.2

As mentioned previously, variations in results of strength tests can be traced to two different sources: (a) variations in testing methods and (b) properties of the concrete mixture and ingredients. It is possible by analysis of variance to compute the variations attributable to each source.

100

where

subtracting the lowest of a group of numbers from the highest one in the group. The within-test range is found by subtracting the lowest of the group of cylinder strengths averaged to produce a test from the highest of the group. The withintest range is useful in computing the within-test standard deviation discussed in the following section. 3.4-Strength variations

x







The batch-to-batch and within-test sources of variation are related to the overall variation [Eq. (3-3) ] by the following expression: =

+

(3-6)

where

overall standard deviation = within-test standard deviation = batch-to-batch standard deviation Once these parameters have been computed, and with the assumption that the results follow a normal frequency distribution curve, a large amount of information information about about the test results becomes known. Fig. 3.4.2 (a) (a) indicates an approximate division of the area under the normal frequency distribution curve. For example, approximately 68 percent of the area (equivalent to 68 percent of the test results) lies within la of the average, 95 percent within etc. This permits an estimate to be made of the portion of =

TABLE 3.4.2-EXPECTED PERCENTAGES OF TESTS LOWER THAN

Average strength,

WHERE EXCEED S AMOUNT SHOWN

Expected

Average strength,

low tests 46.0 42.1

70

80

+ + 0.90

90

+

Percent of average strength

Fig. 3.4.2(b)-Cumulative distribution curves for different coefficients of variation

84.4 60

80

70.3

+

+ 1.7 2.3 1.8

18.4 15.9 13.6 11.5

1.50

422

Expected low tests

38.2 34.5 30.9 27.4 24.2 21.2

+ 3.0

+

5 6 .2 .2

BY THE

28.1

14.1

0.19

0.13

0









TABLE 3.5-STANDARDS OF CONCRETE CONTROL Overall variation Standard deviation for different control standards, psi Class of operation

Excellent

Very good

Good

Fair

Poor

General construction testing

below 400 (28.1)

400 to 500 (28.1) (35.2)

500 to 600 (35.2) (42.2)

600 to 700 (42.2) (49.2)

above 700 (49.2)

Laboratory trial batches

below 200 (14.1)

200 to 250 (14.1) (17.6)

250 to 300 (17.6) (21.1)

300 to 350 (21.1) (24.6)

above 350 (24.6)

Within-test variation Coefficient of variation for different control standards, percent Class of operation

Excellent

Very good

Good

Fair

Poor

Field control testing

below

3.0

3.0 to 4.0

4.0 to 5.0

5.0

to 6.0

above 6.0

Laboratory trial batches

below

2.0

2.0

to 3.0

3.0 to 4.0

4.0 to 5.0

above 5.0

the test results expected to fall within given multiples of of the average or of any other specific value. Table 3.4.2 3.4.2 has been adapted from the normal probability integral of the theoretical normal frequency distribution curve and shows the probability of tests falling below in terms of the average strength of the mix = + to). Cumulative distribution curves can also be plotted by accumulating the number of tests below any given strength expressed as a percentage of the average strength for different coefficients of variation or standard deviations. Fig. 3.4.2(b) and 3.4.2 (c) present (c) present such information. =

3.5-Standards of control

The decision as to whether the standard deviation or the coefficient of variation is the appropriate measure of dispersion to use in any given situation depends on which of the two measures is the more nearly constant over the ‘range of strengths characteristic of the particular situation. Present information indicates that the standard deviation remains more nearly constant particularly at strengths over 3000 psi (211 For within-test variations the coefficient of variation is considered to be more applicable (see References 5-10)







Standard deviation, psi Coefficient of v ariation, percent Fig. 4.1 (a)-Ratio of required average strength to specified strength for various coefficients of variation and chances of falling below specified strength

Fig. 4.1(b)-Excess of required average strength to specified strength for various standard deviations and chances of falling below specified strength

of the test results fall below the design strength, a corresponding large percentage of the test results will be greater than the design strength with an equally large probability of being located in a critical area. The consequences of a localized zone of low-strength concrete in a structure depend on many factors; included are the probability of early overload, the location and magnitude of the low-quality zone in the structural unit, the degree of reliance placed on strength in design, the initial cause of the low strength, and the consequences, economic and otherwise, of structural failure.

sidered to have been complied with if the tests represent either a group of 30 consecutive batches of the same class of concrete or the statistical average for two groups totalling 30 or more batches. “Similar” conditions will be difficult to define and can be best documented by collecting several groups of 30 or more tests. In general, changes in materials and procedures will have a larger effect on the average strength level than on the standard deviation or coefficient of variation. S i g n i f i c a n t changes generallyinclude changes in type and brand of portland cement, admixtures, source of aggregates, mix proportions,







69

197

2400

2800

225

253

281

309

4ooo 4400 3200 3600 Compressive Strength, psi

Fig. 4.1(c)-Normal frequency curves for coefficients of variation of

Whenever the average of a certain n umber of tests is involved in the specification, Eq. (4-l) is modified as follows: (4-lb) Vn

Percentages of tests falling within the limits to 40 50

(4-lc)

Fig. 4.1 (c) demonstrates that as the variability increases must increase and thereby illustrates the economic value of good control. The requirement of at least 30 test results mentioned previously is based on the fact that 25 to 30 randomly selected test results from a normally

3

5200

10, 15,

and

20 percent

TABLE 4.1-VALUES OF t

and Vn

366

337

95.45

98

Chances of falling below lower limit 3 in 2.5 in 2 in 1 in 1.5 in 1 in 1 in 1 in 1 in 1 in 1 in 1 in

10 10 10

6.3 10 10 20

40 44

t

0.52 0.67 0.84 1.00

1.04

1.28 1.65 1.96

2.00

100

2.33

200

2.58 3.00

741

For small jobs that are just getting started, where no prior information is available, the con-













TABLE 4.3-EVALUATION OF CONSECUTIVE LOW STRENGTH TEST RESULTS 1

2

5

Probability of averages less than percent

Averages less than indicated require investigation* Number of consecutive

Criteria for original selection of

tests averaged

1 test in 10 below

For V = 15, percent

For given

1 test in 100 less than 500 psi (35.2 -

1 test in 10 below

For given

-- 500 0.76 0.88 500 + 1.30 1.41 - + 1.49 500

+

10.0

+

3.5

-

-

500 500 500

+

+

0.1

*The probability of averages less than the levels indicated is approximately 2 percent if the population average equals and the standard deviation or coefficient of variation is at the level assumed. and the standard deviation or coefficient of variation the population average equals is at the level assumed.

4.2.4 Criterion No. 4- A certain probability that a random individual strength test will be less than

a certain percentage of As an example consider a probability of 1 in 100 that a strength test will be less than 85 percent of an of 4000 psi (281 kgf Standard deviation method Using Eq. (4-la) (4-la) and Table 4.1 and a standard we have deviation of 750 (53 = 0.85 + = 0.85 (4000) + 2.33 (750) = 5145 psi (361 As a result the concrete mixture should be proportioned for an average strength of not less than 5145 psi (361 Coefficient of variation variation method Using Eq. Using Eq. (4-l) and Table 4.1 and 4.1 and a coefficient

tests should not normally fall. These values are based on the premise that the concrete is proportioned to produce an average strength equal to The values in Column 2 are theoretically correct only for concrete with a coefficient of variation of 15 percent. Those in Columns 3 and 4 apply to any known standard deviation. In either case the probability of their being exceeded when the concrete is properly controlled is only about 0.02. Thus, failure to meet the tabulated limits in a larger proportion of cases than that stated may be an indication that the current average strength is less than or that or V has increased. This could be caused by lower strength or poorer control than expected, or both. The possibility should not be overlooked that the low tests may be caused by errors in sampling or test-















4.5-Tests and specimens required

For any particular job, a sufficient number of tests should be made to insure accurate representation of the variations of the concrete. Concrete tests can be made either on the basis of time elapsed or cubic yardage placed and conditions on each job will determine the most practical method of obtaining the number of tests needed. A test is defined as the average strength of all specimens of the same age fabricated from a sample taken from a single batch of concrete. A project where all concrete operations are supervised by one engineer provides an excellent opportunity for control and for accurate estimates of reliability with a minimum of tests. Once operations are progressing smoothly tests taken each day or shift, depending on the volume of concrete produced, are sufficient to obtain data which reflect the variations in the concrete of the structure. In general, it is advisable to make a sufficient number of tests so that each different type of concrete placed during any one day will be represented by at least one test which is an average of two standard 6 x 12 in. cylinders tested at the required age. Single specimens taken from two different batches each day will provide more reliable information on overall variations, but it is usually desirable to make companion specimens

for fo r good control (Table 3.5) 3.5),, and the estimate of the corresponding average range will be: (0.05 x = for fo r groups of two companion cylinders = (0 . 0 5 x = for groups of three companion cylinders. A cylinder of concrete 6 in. in diameter and 12 in. high which has been moist cured for 28 days at 21 C is generally considered a standard specimen for strength and control of concrete if the coarse aggregate does not exceed 2 in. in nominal size. Many times, particularly in the early stages of a job, it becomes necessary to estimate the strength of concrete being produced before the 28-day strength results are available. Concrete cylinders from the same batch should be made and tested at 7 days, or at earlier ages utilizing accelerated test procedures. The 28-day strength can be estimated by extrapolating early test data. The strength of concrete at later ages, particularly where a pozzolan or cement of slow strength gain is used, is more realistic than the standard 28-day strength. Some structures will not be loaded until concrete has been allowed to mature for longer periods and advantage can be taken of strength gain after 28 days. Some concretes have =







4.6-Rejection of doubtful specimens

The practice of arbitrary rejection of test cylinders which appear “too far out of line” is not recommended since the normal pattern of probability establishes the possibility of such results. Discarding tests indiscriminately could seriously distort the strength distribution, making analysis of results less reliable. It occasionally happens that the strength of one cylinder from a group made from a sample deviates so far from the mean as to be highly improbable. It is recommended that a specimen from

a test of three or more specimens be discarded if its deviation from a test mean is greater than 30, and should be accepted with suspicion if its deviation is greater than If questionable variations have been observed during fabrication, curing, or testing of a specimen, the specimen should be rejected. The test average should be computed from the remaining specimens. A test (average of all specimens of a sample) should never be rejected unless the specimens are known to be faulty, since it represents the best available estimate for the sample.

CHAPTER 5-REFERENCES 1. Natrella, M. G., “Experime “Experimental ntal Statist Statistics,” ics,” HandHand-

book No. No. 91, U. S. Department of Standards, National Bureau of Standards, Washington, D. C., 1963, pp. l-4 to l-6. 2. Realism in the Application of ACI Standard 214-65, SP-37, American Concrete Institute, Detroit, 1973, 215 pp. 3. “Evaluation of Strength Tests of Concrete,” AC I Bibliography No. 2, American Concrete Institute, Detroit, 1960, 13 pp. 4. ASTM Manual on Quality Control of Materials, STP 15-C American Society for Testing and Materials, Philadelphia, Jan. 1951, 127 pp. 5. Neville, Neville, A. M., “The Relation Relation Between Between Standard Standard Deviation and Mean Strength of Concrete Test Cubes,” Magazine of Concrete Research (London), V. 11, No. 32, July 1959, pp. 75-84. 6. Metcalf, Metcalf, J. B., “The Specif ication of Conc rete Strength, Part II, The Distribution of Strength of

Concrete for Structures in Current Practice,” RRL Re port No. LR 300, Road Research Laboratory, Crawthorne, Berkshire, 1970, 22 pp. 7. Murdock, C. C. J., J., “The Control Control of Concrete Concrete Quality,” Quality,” Proc eedi ngs, Institution of Civil Engineers (London), V. 2, Part I, July 1953, 1953, pp. 426-453. 8. Ernt Erntro roy, y, H. C., C., “The Variation Variation of Works Test Cubes,” Research Report No. 10, Cement and Concrete Association, London, Nov. 1960, 28 pp . 9.

H., “Statistical Quality Control of Concrete,” (Dusseldorf), V. 6, No. 11, Nov. 1964, pp. 387-394. 10. “Tentative Recommended Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods for Construction Materials,” (ASTM C 802-74T), 1975 Annual Book of ASTM Standards, Part 13, American Society for Testing and Materials, Philadelphia, pp. 414-443.

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