The International Concrete Abstracts Portal is an ACI led collaboration with leading technical organizations from within the international concrete industry and offers the most comprehensive collection of published concrete abstracts. Title: Highlights of ACI standard Recommended practice for evaluation of strength test results of concrete. Publication: Concrete International. Keywords: coefficient of variation; compression tests; compressive strength; concrete construction; con-cretes; cylinders; evaluation; quality control; samp-ling; standard deviation; statistical analysis; varia-tions.
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PRINCIPE 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.
Keywords: coefficient of variation: compression tests: compressive strength; concrete construction: concretes: cylinders: evaluation; quality control; sampling; standard deviation; statistical analysis; variations. This recommended practice for evaluation of strength test results has been developed from data derived from tests performed on concrete limited to a compressive strength of psi or less.
Whairman during development of the revision. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. Concrete, being a hardened mass of heterogeneous materials, is subject to the influence of numerous variables.
Characteristics of each of the ingredi- ents 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, plac- ing, 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 con- crete of adequate quality can be produced with confidence if proper control is maintained, test results are properly interpreted, and their limi- tations are considered.
Proper control is achieved by the use of satis- factory 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 com- plex 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 require- ments. Strength is not necessarily the most critical fac- tor 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 de- mands. Nevertheless, strength tests are valuable in such circumstances since, with established mix proportions, variations in strength are indi- cative 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.
Insuf- ficient tests will result in unreliable conclusions. Statistical procedures provide tools of consider- able value in evaluating results of strength tests and information derived from such procedures is also of value in refining design criteria and speci- fications. This report briefly discusses variations that occur in the strength of concrete, and presents statistical procedures that are useful in the inter- pretation of these variations with respect to re- quired criteria and specifications.
For these sta- tistical 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. 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, biases are likely to be 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 information on the meaning and use of this recommended practice is given in Realism in the Application of ACI Standard ? This volume is a compilation of information on ACI that was presented at a symposium held at Buffalo, N. Although the information given was based on ACI , most of it is still relevant.
Differences in strength can be traced to two fundamentally different sources as shown in Table 2. TABLE 2. 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 temperature of fresh concrete influences the amount of water needed to achieve the proper consistency and con- sequently contributes to strength variation. Con- struction practices may cause variations in strength due to inadequate mixing, poor com- paction, 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 con- trolled. On the other hand, discrepancies in sampling, fabrication cur- ing, and testing of specimens may cause indica- tions 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 ma- chines and producing thin, high-strength, plane, parallel caps should need no emphasis since test results can be no more accurate than the equip- ment and procedures used.
Uniform test results are not necessarily accurate test results. Lab- oratory equipment and procedures should be cali- brated and checked periodically. 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. 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 be- comes low and elongated, as illustrated by the idealized curves shown in Fig.
Because the characteristics of such curves can be defined mathematically, certain useful functions of the strength can be calculated as follows: 3. X, are the strength results of individual tests and n is the total number of tests made. A test is defined as the average strength of all specimens of the same age fabri- cated from a sample taken from a single batch of concrete.
This statistic is known as the standard deviation and may be considered to be the radius of gyration about the line of symmetry of the 5. The best estimate of CI, based on a finite amount of data, is obtained by Eq.
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.
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 within- test range is useful in computing the within-test standard deviation discussed in the following sec- tion. It is possible by analysis of variance to compute the variations attributable to each source.
It is reasonable to assume that a test sample of concrete is homogeneous and any variation between companion cylinders fabricated from a given sample is caused by fabricating, curing, and testing variations. A single batch of concrete, however, provides insufficient data for statistical analysis and com- panion cylinders from at least ten batches of con- TABLE 3. The batch-to-batch and within-test sources of variation are related to the overall variation [Eq.
For example, ap- proximately 68 percent of the area equivalent to 68 percent of the test results lies within 3t la of the average, 95 percent within t 20, etc. TABLE 3. Table 3. 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.
In these figures, the ordinate indicates the per- cent of the population of strength values which may be expected to exceed the strength indicated by any abscissa value for a selected coefficient of variation or standard deviation. For within-test variations the coefficient of varia- tion is considered to be more applicable see Ref- erences These values are not applicable to other strength tests. Because of the possible disparity between the strength of test cylinders and the load-carrying capacity of a structure it is unwise to place any reliance on inadequate strength data.
The number of tests lower than the desired strength is more important in computing the load- carrying capacity of concrete structures than the average strength obtained. It is impractical, how- is always the possibility of even lower strengths, even when control is good. It is also recognized that the cylinders may not accurately represent the concrete in each portion of the structure. Fac- tors of safety are provided in design equations which allow for deviations from specified strengths without jeopardizing the safety of the structure.
These have been evolved on the basis of construction practices, design procedures, and quality control techniques used by the construc- tion industry. Coefficient of variation, percent Fig. The consequences of a localized zone of low-strength concrete in a structure de- pend on many factors; included are the probability of early overload, the location and magnitude of the low-quality zone in the structural unit, the de- gree of reliance placed on strength in design, the initial cause of the low strength, and the conse- quences, economic and otherwise, of structural failure.
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