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Mix Design

Mix Design. Mix Design. References IS 10262 : 2019 – CONCRETE MIX PROPORTIONING – GUIDELINES ( Second revision) First published in 1982 ( Recommended guidelines for concrete mix design), First revision in 2009 ( Concrete mix proportioning – guidelines) .

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Mix Design

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  1. Mix Design

  2. Mix Design References • IS 10262 : 2019 – CONCRETE MIX PROPORTIONING – GUIDELINES (Second revision) First published in 1982 (Recommended guidelines for concrete mix design), First revision in 2009 (Concrete mix proportioning – guidelines). • IS 456 : 2000 – Plain & reinforced cement concrete- code of practice(forth revision) Amendments up to 4 (May 2013) • CBC : 2014 (Up to CS no.7) – Code of practice for plain,reinforced & prestressed concrete for general bridge construction • IS 383:2016 ( Third Revision) - Specification for coarse and fine aggregates from natural sources for concrete (second revisions)

  3. References • IS 2386 (Part 3) : 1963 – Method of test for aggregates for concrete : part 3 specific gravity, density, voids, absorption and bulking. • IS 8112:1989 - Specification for 43 grade ordinary Portland cement (first revision) • IS 12269 - Specification for 53 grade ordinary Portland cement • IS 9103:1999 - Specification for admixtures for concrete (first revision)

  4. IS-10262 : 2019 (Second Revision)CONCRETE MIX PROPORTIONING - GUIDELINES Major modifications made In second revision : 1. The standard has been divided into five sections, as follows: • Section 1 General • Section 2 Ordinary and standard grades of concrete • Section 3 High strength grades of concrete • Section 4 Self compacting concrete • Section 5 Mass concrete 2. Mix proportioning procedure for high strength concrete for M 65 or above (up to target strength of M100) has been included.

  5. IS-10262 : 2019 (Second Revision)CONCRETE MIX PROPORTIONING - GUIDELINES 3. The target mean strength for mix proportioning formula has been refined to include a new factor based on the grade of concrete. This has been done to ensure a minimum margin between the characteristic compressive strength and the target mean compressive strength. 4. The calculations for standard deviation have been detailed. 5. A graph of water-cement ratio versus 28 days strength of concrete has been introduced for different grades and types of cement, as an alternate method for assuming the initial water-cement ratio.

  6. IS-10262 : 2019 (Second Revision)CONCRETE MIX PROPORTIONING - GUIDELINES 6. Illustrative annexes for concrete mix proportioning for PPC, OPC with fly ash, OPC with GGBS, high strength concrete, self compacting concrete and mass concrete have been provided. 7. Guidelines on using/selecting water reducing admixtures have been introduced as an informatory annex (see Annex G). 8. The consideration of air content in design of normal (non-air entrained) concrete mix proportion, has been reintroduced. 9. It is suggested that the concrete mix proportioning in the laboratory may be carried out at a temperature of 27± 2°C, relative humidity of minimum 60 percent, and the temperature of concrete may be 27 ± 3°C.

  7. Types Of Mix • Nominal Mix: a. Used for relatively unimportant and simpler works. b. No scope for any deviation by the designer since ingredients are prescribed and their proportions specified. c. May be used for M-20 or lower. Table 9 - IS 456 Kg

  8. Types Of Mix 2. Design Mix: a. Performance based mix. b. Choice of ingredients and proportioning are left to designer. c. User has to specify requirements of concrete in fresh as well as in hardened state only. Fresh concrete – Workability, Bleeding & Segregation. Hardened concrete–Compressive strength, durability, Impermeability & surface finish.

  9. Defination DESIGN MIX • It is a process of selecting suitable ingredients and determining their relative proportions with the objective of producing concrete of having certain minimum workability, strength and durability as economically as possible. Objective as per IS-10262 : 2009 • The objective of proportioning concrete mixes is to arrive at the most economical and practical combinations of different ingredients to produce concrete that will satisfy the performance requirements under specified conditions of use.

  10. Sampling 1. One sample comprising of 3 cubes shall be taken from each shift. 2. Samples should be taken at random and should be representative of actual concrete. Factors affecting cube test results

  11. Sampling • Three test specimens (cubes) shall be made for each sample for testing at 28 days. • Additional samples may be required for various purposes such as to determine the strength of concrete at 3 days/ 7 days or at the time of striking the formwork, or to determine the duration of curing or to check the testing error. • Additional samples may also be required for testing samples cured by accelerated methods as described in IS 9103. • The specimen shall be tested as described in IS 516.

  12. Cube Casting (IS 516) 1. Concrete is filled in mould in 3 equal layers. 2. Manual compaction - Each layer is tamped 35 times by bullet headed tamping rod of 16 mm dia and 600 mm long. 3. Needle/plate vibrator can also be used. 4. The cube in mould is covered with glass/metal plate. 5. Stripped after 16-24 hrs and then submerged in pond till testing. 6. Light marking of date and number for identification is done . 7. Cube should not be allowed to dry till testing.

  13. Cube Testing 1. 3 cubes constitute one sample. 2. Smooth faces should face top and bottom during testing. 3. The rate of loading should not be more than 14 N/mm2 per minute. 4. Individual variation of Cube strength should not be more than +15% of average strength. If more, the test results are invalid. e.g. grade of concrete-M30 C1=24, C2=32, C3=37 Average=93/3=31 +15% of 31 = 31+31x0.15 = 31+4.65 = 35.65 • 15% of 31 = 31-31x0.15 = 31-4.65 = 25.65 • Invalid

  14. Importance Of Cube Test 1. Cube strength represents the strength of the structure 2. It indicates potential strength of the mix. 3. It detects variation in quality control at site. 4. It helps in determining the rate of gain of strength. 5. It helps in determining the time of removal of form work.

  15. Statistical Interpretation Of Cube Results • Cube test result is influenced by number of factors like material, their proportions, various processes like mixing, compaction, curing and finally the testing procedure of cubes. • Even the results of different cubes, cast from the same concrete at the same time, cured and tested in similar way may also show different results. • Therefore for evaluation of the test results, the help of statistical approach is required.

  16. Statistical Interpretation Of Cube Results(Histogram/Normal distribution curve)

  17. Understanding Of Normal Distribution Curve • Shows the probability of concentration of test results at this strength. • Concentration at mean value depends upon ‘Quality control’ & S.D. • If S.D. is less, probability of concentration of test results will be higher at the mean value.

  18. NUMBER OF SPECIMENS IN INTERVAL COMPRESSIVE STRENGTH IN MPa

  19. Standard deviation (S.D.) : S.D. (S) = ∑ (x- µ)² n-1 Where S = Standard deviation of the group x = Individual test result n = no. of test results considered µ = Average of n test results considered S.D. will be less if the quality control at site is better.

  20. Target Mean Strength

  21. Target Mean Strength • From the experiments it is found that the area under the ‘Normal distribution curve’ follows certain behavior such that area covered within a distance equal to one S.D. on either side from mean will be equal to 34.1% of total area. Similarly 2nd & 3rd S.D. will cover additional area of 13.6% & 2.2% respectively. Approximately 99.9% area will be covered within distance equal to (3 x S.D) on each side from mean value. • From this it is clear that if we require the strength equal to mean value, probability is that only 50% cubes will have strength equal to or more than the required strength. • If we want to increase the % of cubes having strength equal or more than the required strength, we will have to target for higher strength.

  22. Target Mean Strength

  23. Target Mean Strength • So instead of placing the desired strength (fck) at mean value, it is placed on left side of mean value by a distance equal to (k x S.D), where k will depend upon the % of cubes we want to have strength equal or more than the desired strength. As per IS 456-2000, this % is 95% and therefore desired strength has to be placed at 1.65 times S.D. left of mean. f’ck= fck + 1.65 S OR f’ck= fck + X Whichever is higher Here, fck - Characteristic compressive strength at 28 days in N/mm² f’ck -Target mean compressive strength at 28 days in N/mm² S - Standard Deviation in N/mm² X - Factor based on the grade of concrete, as per Table 1. The value of 1.65 is based upon the provision that 5% of the test results can be accepted having lower than the required strength.

  24. Value of X Table 1 of IS-10262

  25. Target mean strength • While designing the mix, it should be targeted for higher strength called ‘Target mean strength’ (f’ck)so that not more than 5% test results fall below a desired strength ‘fck’ i.e. characteristics strength (CS). • Few cube test results may show less strength below CS value. This is in accordance with the design criteria of mix and acceptable. Therefore manipulation of results shall not be attempted. • The mix shall be designed to produce the grade of concrete having the required workability and a characteristic strength not less than appropriate values given in Table 2 of IS456-2000.

  26. Target mean strength • Proportion/grading of aggregate shall be made by trial in such a way as to make densest possible concrete.

  27. Acceptance Criteria (As per IS-456:2000) a. Compressive strength- NOTE- 1. In the absence of established value of standard deviation, the values given in Table 8 may be assumed and attempt should be made to obtain results of 30 samples as early as possible to establish the value of standard deviation. 2. For concrete of quantity up to 30m3 (where the no. of samples to be taken is less than four as per frequency of sampling) the mean of test results of all such samples shall be fck + 4 N/mm2 minimum and the requirement of minimum Individual test results shall be fck - 2 N/mm2 minimum. However when no. of sample is only one as per 15.2.2,the requirement shall be fck + 4 N/mm2 minimum

  28. Acceptance criteria

  29. Acceptance Criteria (As per CBC) 1.Compressive strength- (For M-20 & above grades only) a. The mean of 4 consecutive test results should exceed fck by 3 N/mm2 i.e. fck+3 N/mm2 and b. Any individual test result ≥ fck – 3 N/mm2.

  30. Data for Mix proportioning (IS-10262, 2019) • Grade designation, • Type of cement, grade of cement, • Maximum nominal size of aggregate, • Maximum water cement ratio, • Workability required at the time of placement, • Exposure conditions as per table 4 and 5 of IS-456, • Maximum temperature of concrete at the time of placing, • Method of placing, Transportation time, • Degree of site control (good/fair) or value of established standard deviation, if any; • Type of coarse aggregate & fine aggregate, • Maximum & Minimum cement content, 12. Whether chemical & mineral admixture shall or shall not be used and the type of admixture and the condition of use.

  31. Procedure • Calculation of Target mean strength • Approximate Air Content • Selection Of Water-Cement Ratio • Selection Of Water Content • Calculation of cementitious material content • Estimation of coarse aggregate proportion • Estimation of fine aggregate proportion • Mix calculations

  32. DESIGN MIX STEPSFor Ordinary & Standard Grades concrete 1.Calculation of Target mean strength f’ck = fck + 1.65 S OR f’ck = fck + X Whichever is higher 2. Approximate Air content Table 3 gives the approximate amount of entrapped air to be expected in normal (non-air-entrained) concrete.

  33. Assumed Standard Deviation & Value of X (Table 1&2 of IS-10262)

  34. Approximate Air Content (Table 3 of IS 10262)

  35. 3. Selection Of Water-Cement Ratio 1.Table-5 of IS-456 Min cement content, max W/C ratio and min grade of concrete for different exposures with normal weight aggregates of 20 mm nominal maximum size. 2. Fig-1 of IS 10262 gives Free W/C ratio corresponding to expected compressive strength of concrete for various grade of cement.

  36. Procedure (IS-10262, 2019) 1. The supplementary cementitious materials i.e. Mineral admixtures shall also be considered in water cement ratio calculations. 2. The free water-cement ratio selected should be checked against the limiting water cement ratio for the requirement of durability and lower of the 2 values adopted. (From CBC)

  37. TABLE 4 (a) : MAXIMUM WATER CEMENT RATIO (Clause 5.4.3 of CBC)

  38. 4. Selection Of Water Content Table-4 of IS-10262 Water content per cubic meter of concrete for nominal maximum size of aggregate.

  39. 5. Calculation of cement/cementitious materials content . 1. The cement and supplementary cementitious materials content per cubic meter can be calculated from free water cement ratio and the quantity of water per cubic meter. 2. The cementitious material should be checked against minimum content for the requirement of durability and greater of the two should be adopted.

  40. MIN. CEMENTITIOUS MATERIAL CONTENT (As per CBC Table 4(c) Clause 5.4.5) Maximum cement content As per IS-456-2000 (Para 8.2.4.2) - Not to exceed 450 kg/m3 when OPC is used As per CBC – 2014 (Clause 5.4.5) - Shall be limited to 500kg/m3.

  41. 6. Estimation of coarse aggregate proportion Table-5 of IS-10262 Volume of CA per unit volume of total aggregate for different zones of FA.

  42. 7. Estimation of Volume of CA+FA Volume- • Volume of entrapped air b. Cement = Mass x 1 SG 1000 c. Water = Mass x 1 SG 1000 d. Chemical Adm. = Mass x 1 SG 1000 Volume of CA+FA = (1-a) - (b+c+d) m³

  43. 8. Estimation of mass of CA & FA proportion e. The value so obtained is divided into CA and FA fractions by volume in accordance with CA proportion already determined. • Mass of CA = Volume x SG x 1000 g. Mass of FA = Volume x SG x 1000

  44. Quantities per 1 m³ of concrete: • Cement - kg/m3 • Water - kg/m3 • CA - Kg/m3 • FA - Kg/m3 • Chemical admixture - Kg/m3 • Water cement ratio - This is first trial mix, TM – 0 Note - • Aggregate should be used in SSD condition. • If otherwise, allowance shall be made for free(surface) moisture contributed by the CA and FA. 3. On the other hand, if the aggregates are dry, the amount of mixing water should be increased equal to the moisture likely to be absorbed. • Necessary adjustments are also required in mass of aggregates. • The surface water and percentage water absorption of aggregates shall be determined according to IS 2386.

  45. 9. Trial Mixes - 1. Workability of TM-0 will be measured. 2. The mix will be carefully observed for freedom from segregation, bleeding and finishing properties. 3. If the workability of TM-0 is different from the stipulated value, the water and/or admixture content shall be adjusted suitably. 4. With this adjustment, the mix proportions will be calculated again keeping same W/C ratio. This will be TM-01. 5. In addition, 2 more TM no. 2 and 3 will be made with same water content and varying the W/C ratio by +10%. 6. A graph between 3 W/C ratios and their corresponding strengths at 28 days will be plotted to work out the mix proportions for the given Targeted mean strength TMS. 7. However, durability requirement shall be met.

  46. Example - M40 grade STIPULATIONS FOR PROPORTIONING a) Grade designation - M40 b) Type of cement - PPC 43 grade conforming to IS 1489(Part-I) c) Maximum nominal size of aggregate - 20mm d) Minimum cement content as per table 3 & Maximum water-cement ratio as per table 5 of IS 456 to be adopted. (320 kg/m3 & 0.45) f) Workability - 75 mm (slump) g) Exposure condition - Severe (for reinforced concrete) h) Method of concrete placing – Chute (Non pumpable) j) Degree of supervision - Good k) Type of aggregate - Crushed angular aggregate m) Maximum cement content - 450 kg/m3 n) Chemical admixture type - Superplasticizer

  47. TEST DATA FOR MATERIALS • Cement used - PPC 43 grade conforming to IS 1489(Part-I) b) Specific gravity of cement – 2.88 c) Chemical admixture - Superplasticizer conforming to IS 9103 d) Specific gravity of: 1) Coarse aggregate (at SSD condition) – 2.74 2) Fine aggregate (at SSD condition) – 2.65 3) Chemical Admixture – 1.145 e) Water absorption: 1) Coarse aggregate - 0.5% 2) Fine aggregate – 1.0%

  48. f) Moisture content of aggregate (As per IS 2386 (Part-3)): 1) Coarse aggregate - Nil 2) Fine aggregate - Nil g) Sieve analysis: 1) Coarse aggregate - Conforming to Table 7 of IS 383 2) Fine aggregate - Conforming to grading Zone II of Table 9 of IS 383

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