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安徽工业大学大学 毕业设计（论文）计算书58CHAPTER 5CODECOMMENTARYbe maintained through the normal compressive strength test requirements provided by 5.6.5.2 Selection of concrete proportionsR5.2 Selection of concrete proportionsRecommendations for selecting proportions for concrete aregiven in detail in “Standard Practice for Selecting Pro- portions for Normal, Heavyweight, and Mass Concrete”(ACI 211.1).5.1 (Provides two methods for selecting andadjusting proportions for normalweight concrete: the esti- mated weight and absolute volume methods. Example cal- culations are shown for both methods. Proportioning of heavyweight concrete by the absolute volume method is presented in an appendix.)Recommendations for lightweight concrete are given in“Standard Practice for Selecting Proportions for Struc- tural Lightweight Concrete” (ACI 211.2).5.2 (Provides a method of proportioning and adjusting structural grade con- crete containing lightweight aggregates.)5.2.1 Proportions of materials for concrete shall beestablished to provide:(a) Workability and consistency to permit concrete to be worked readily into forms and around reinforce- ment under conditions of placement to be employed, without segregation or excessive bleeding;(b) Resistance to special exposures as required byChapter 4;(c) Conformance with strength test requirements of5.6.R5.2.1 The selected water-cementitious material ratioshould be low enough, or in the case of lightweight concrete the compressive strength high enough to satisfy both the strength criteria (see 5.3 or 5.4) and the special exposure requirements(Chapter 4). The code does not include provisions for espe-cially severe exposures, such as acids or high temperatures, andis not concerned with aesthetic considerations such as surface finishes. These items are beyond the scope of the code and should be covered specifically in the project specifications. Concrete ingredients and proportions are to be selected to meet the minimum requirements stated in the code and the additional requirements of the contract documents.5.2.2 Where different materials are to be used fordifferent portions of proposed work, each combination shall be evaluated.5.2.3 Concrete proportions shall be established inaccordance with 5.3 or, alternatively, 5.4, and shall meet applicable requirements of Chapter 4.R5.2.3 The code emphasizes the use of field experienceor laboratory trial mixtures (see 5.3) as the preferred method for selecting concrete mixture proportions.5.3 Proportioning on the basis of fieldexperience or trial mixtures, or bothR5.3 Proportioning on the basis of fieldexperience or trial mixtures, or bothIn selecting a suitable concrete mixture there are three basicsteps. The first is the determination of the sample standard deviation. The second is the determination of the required average compressive strength. The third is the selection of mixture proportions required to produce that average strength, either by conventional trial mixture procedures or by a suitable experience record. Fig. R5.3 is a flow chart out- lining the mixture selection and documentation procedure.Copyright American Concrete InstituteProvided by IHS under license with ACINo reproduction or networking permitted without license from IHSLicensee=Black & Veatch/5910842100Not for Resale, 11/28/2005 18:20:15 MST第 122 页 共 132 页 CHAPTER 559COMMENTARYFig. R5.3Flow chart for selection and documentation of concrete proportionsCopyright American Concrete InstituteProvided by IHS under license with ACINo reproduction or networking permitted without license from IHSLicensee=Black & Veatch/5910842100Not for Resale, 11/28/2005 18:20:15 MSTCODECOMMENTARY60CHAPTER 5CODECOMMENTARYThe mixture selected should yield an average strength appreciably higher than the specified strength f . The degree of mixture over design depends on the variability of the test results.5.3.1 Sample standard deviationR5.3.1 Sample standard deviation5.3.1.1 Where a concrete production facility hastest records, a sample standard deviation, ss , shall be established. Test records from which ss is calculated:(a) Shall represent materials, quality control proce-When a concrete production facility has a suitable record of30 consecutive tests of similar materials and conditions expected, the sample standard deviation, ss, is calculated from those results in accordance with the following formula:dures, and conditions similar to those expected and21 2 changes in materials and proportions within the testrecords shall not have been more restricted than those for proposed work;(b) Shall represent concrete produced to meet awhere:ss=( xi x )-( n 1 )specified compressive strength or strengths within7 MPa of f ;(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 in5.3.1.2.5.3.1.2 Where a concrete production facility does not have test records meeting requirements of 5.3.1.1, but does have a record based on 15 to 29 consecutive tests, a sample standard deviation ss shall be estab- lished as the product of the calculated sample stan-ss = sample standard deviation, MPaxi = individual strength tests as defined in 5.6.2.4x= average of n strength test resultsn= number of consecutive strength testsThe 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:dard deviation and modification factor of Table 5.3.1.2. ( n 1 )( s )22 1 2 + ( n2 1 )( ss 2 )ss =( n+ n 2 )To be acceptable, test records shall meet require-ments (a) and (b) of 5.3.1.1, and represent only a sin-gle record of consecutive tests that span a period of not less than 45 calendar days.where:12TABLE 5.3.1.2MODIFICATION FACTOR FORss= statistical average standard deviation wheretwo test records are used to estimate the sam- ple standard deviationSAMPLE STANDARD DEVIATION WHEN LESSTHAN 30 TESTS ARE AVAILABLEModification factor for sampless1, ss2 = sample standard deviations calculated from twotest records, 1 and 2, respectivelyn1, n2 = number of tests in each test record, respectivelyNo. of tests*standard deviationLess than 15Use table 5.3.2.2151.16201.08251.0330 or more1.00* Interpolate for intermediate numbers of tests. Modified sample standard deviation, ss , to be used to determine required average strength, fcr , from 5.3.2.1.If less than 30, but at least 15 tests are available, the calcu-lated sample standard deviation is increased by the factor given in Table 5.3.1.2. This procedure results in a more con- servative (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 (equiva- lent 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 ofrequired average strength should be developed under condi- tions “similar to those expected” see 5.3.1.1(a). This requirement is important to ensure acceptable concrete.Copyright American Concrete InstituteProvided by IHS under license with ACINo reproduction or networking permitted without license from IHSLicensee=Black & Veatch/5910842100Not for Resale, 11/28/2005 18:20:15 MSTCHAPTER 561CODECOMMENTARYConcrete 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) ora change from non-air-entrained concrete to air-entrained concrete. Also, there may be an increase in sample stan- dard deviation when the average strength level is raised bya 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 devia- tion used to calculate the required average strength should always be on the conservative (high) side.Note that the code uses the sample standard deviation inpounds per square inch 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 devia-tion are of the levels assumed, there will be occasional tests that fail to meet the acceptance criteria prescribed in 5.6.3.3(perhaps 1 test in 100).5.3.2 Required average strengthR5.3.2 Required average strength5.3.2.1 Required average compressive strengthfc used as the basis for selection of concrete propor- tions shall be determined from Table 5.3.2.1 using the sample standard deviation, ss , calculated in accor- dance with 5.3.1.1 or 5.3.1.2.TABLE 5.3.2.1REQUIRED AVERAGE COMPRES- SIVE STRENGTH WHEN DATA ARE AVAILABLE TO ESTABLISH A SAMPLE STANDARD DEVIATIONR5.3.2.1 Once the sample standard deviation has beendetermined, the required average compressive strength, fc ,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 f 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 speci- fied compressive strength f . Equation (5-2) is based on a similar probability that an individual test may be more than3.5 MPa below the specified compressive strength fc. Equa-Specified compressive strength,MPaRequired average compressivestrength, MPaUse the larger value computed from Eq. (5-1) and (5-2)tion (5-3) is based on the same 1-in-100 probability that anindividual test may be less than 0.90f . These equations assume that the sample standard deviation used is equal tofc 35f = f + 1.34s(5-1)the population value appropriate for an infinite or very largecrfcrccssnumber of tests. For this reason, use of sample standard = f +2.33s 3.5(5-2)Use the larger value computeddeviations estimated from records of 100 or more tests isfc 35from Eq. (5-1) and (5-3)desirable. When 30 tests are available, the probability of fail-fcr = fc + 1.34ss(5-1)ure will likely be somewhat greater than 1-in-100. The addi- = 0.90f + 2.33s(5-3)fcrcstional refinements required to achieve the 1-in-100probability are not considered necessary, because of the uncertainty inherent in assuming that conditions operating when the test record was accumulated will be similar to con- ditions when the concrete will be produced.Copyright American Concrete InstituteProvided by IHS under license with ACINo reproduction or networking permitted without license from IHSLicensee=Black & Veatch/5910842100Not for Resale, 11/28/2005 18:20:15 MST62CHAPTER 5CODECOMMENTARY5.3.2.2 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 5.3.2.2REQUIRED AVERAGECOMPRESSIVE STRENGTH WHEN DATA ARE NOT AVAILABLE TO ESTABLISH A SAMPLE STANDARD DEVIATIONSpecified compressivestrength, MPaRequired average compressivestrength, MPa = f + 7fc 35fcrc5.3.3 Documentation of average compressivestrengthR.5.3.3 Documentation of average compressivestrengthDocumentation that proposed concrete proportionswill produce an average compressive strength equal to or greater than required average compressive strengthf (see 5.3.2) shall consist of a field strength testrecord, several strength test records, or trial mixtures.Once the required average compressive strength fc 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 special exposure requirements of Chapter 4. The documentation may consist of a strength test record, several strength test records, or suitable laboratory or field trial mixtures. 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 inter- polation between the strengths and proportions of two such records of consecutive tests. All 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 are used to demon-strate that proposed concrete proportions will produce fcr (see 5.3.2), such records shall represent materials and conditions similar to those expected. Changes inmaterials, 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 ofFor strengths over 35 MPa where the average compressivestrength documentation is based on laboratory trial mix- tures, it may be appropriate to increase f calculated in Table 5.3.2.2 to allow for a reduction in strength from labo-ratory trials to actual concrete production.Copyright American Concrete InstituteProvided by IHS under license with ACINo reproduction or networking permitted without license from IHSLicensee=Black & Veatch/5910842100Not for Resale, 11/28/2005 18:20:15 MSTCHAPTER 563CODECOMMENTARYless 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 pro- portions shall be permitted to be established by inter- polation 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 testresults is not available, concrete proportions estab- lished from trial mixtures meeting the following restric- tions shall be permitted:(a) Materials shall be those for proposed work;(b) Trial mixtures having proportions and consisten-cies required for proposed work shall be made using at least three different water-cementitious materials ratios or cementitious materials contents that will produce a range of strengths encompassing f ;(c) Trial mixtures shall be designed to produce aslump within 20 mm of maximum permitted, and for air-entrained concrete, within 0.5 percent of maximum allowable air content;(d) For each water-cementitious materials ratio orcementitious materials content, at least three test cylinders for each test age shall be made and cured in accordance with “Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory”(ASTM C 192M). Cylinders shall be tested at 28 days or at test age designated for determination of fc;(e) From results of cylinder tests a curve shall beplotted showing the relationship between water- cementitious materials ratio or cementitious materials content and compressive strength at designated test age;(f) Maximum water-cementitious material ratio orminimum cementitious materials content for con- crete to be used in proposed work shall be that shown by the curve to produce f required by 5.3.2,unless a lower water-cementitious materials ratio or higher strength is required by Chapter 4.5.4 Proportioning without fieldexperience or trial mixturesR5.4 Proportioning without fieldexperience or trial mixtures5.4.1 If data required by 5.3 are not available, con-crete proportions shall be based upon other experi- ence or information, if approved by the registered d