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Concrete Mix Design

Concrete Mix Design. Ajay Ghosh Asst.Professor Works 7420041131. REFERANCES. IS 10262:2009- Concrete Mix Proportioning – Guidelines (First Revision) (First Published in 1982) IS 456 : 2000 – Plain & reinforced cement concrete- code of practice (forth revision) Amendments up to 4

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Concrete Mix Design

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  1. Concrete Mix Design Ajay GhoshAsst.ProfessorWorks7420041131

  2. REFERANCES • IS 10262:2009- Concrete Mix Proportioning – Guidelines (First Revision) (First Published in 1982) • IS 456 : 2000 – Plain & reinforced cement concrete- code of practice (forth revision) Amendments up to 4 • CBC : 2014 – Code of practice for plain,reinforced & prestressed concrete for general bridge construction • IS 2386 (Part 3) : 1963 – Method of test for aggregates for concrete : part 3 specific gravity, density, voids, absorption and bulking.

  3. REFERANCES • IS 383:1970 ( Reaffirmed 2002 ) - Specification for coarse and fine aggregates from natural sources for concrete (second revisions) • IS 8112:1989 - Specification for 43 grade ordinary Portland cement (first revision) • IS 9103:1999 - Specification for admixtures for concrete (first revision) • USSOR: (Indian Railway Unified Standard Specifications & schedule of rates–Works & Materials–2010) with CS no. 1 & 2 Chapter 3,4 & 26 (Plain Concrete,RCC & Mortars.

  4. Concrete Mix Proportioning Nomenclature: • Concrete Mix or Mixture Proportioning • Concrete Mix Design or Proportioning. • Design or Proportioning of Mixture. • Mixture Proportioning for Concrete.

  5. Concrete Mix Proportioning • Some basic information on Workability, Strength, Durability & Economy of concrete provided in table of IS 456 specification before proceeding with concrete mix proportion.

  6. Grades of Concrete (As per IS 456-2000) (Amendment no.4)

  7. Grades of Concrete As per IS 456-2000 (Amendment no.4)

  8. Concrete Mix Proportioning Characteristic strength Defined as the strength of material below which not more than 5 percent of the test results are expected to fall.

  9. Placing Conditions and Degree of Workability (As per IS 456:2000)

  10. Placing Conditions and Degree of Workability as per para 5.3 of CBC Note: Suitable plasticizer/admixture may be used to achieve workability of the order of 150-200 mm.

  11. ENVIRONMENTAL EXPOSURE CONDITIONS(Based on Table 3 of IS 456:2000)

  12. Minimum Cement content for various Exposure condition

  13. Various Methods of Mix design • Arbitrary proportion • Fineness modulus method • Maximum density method • Surface area method • Indian Road Congress, IRC 44 method • High strength concrete mix design • Mix design based on flexural strength • Road note No.4 (Grading Curve method) • ACI Committee 211 method • DOE method • Mix design for pumpable concrete • Indian Standard Recommended method IS 10262:2009

  14. IS-10262 : 2009 (First Revision)CONCRETE MIX PROPORTIONING - GUIDELINES • This standard applies to ordinary (M-10, 15 and 20) and standard concrete grades (M-25 to 60) only. And not for high strength concrete M65 to M100.(As per IS-456-2000 amendments) • All requirements of IS 456 is so far as they apply, shall be deemed to form part of this standard. • Considering that the air content in normal concrete (Non air entrained) is not of much significance in mix proportioning procedure and it is also not a part of IS 456: 2000, the consideration of air content has been deleted. • A new illustrative example of concrete mix proportioning using fly ash as one of the ingredients has been added.

  15. Definition 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 and as economically as possible.

  16. 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. The proportioning is carried out to achieve specified characteristics strength at specified age, workability of fresh concrete and durability requirements

  17. Types of Mix Design 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

  18. Types Of Mix Design • Design Mix: • Performance based mix. • Choice of ingredients and proportioning are left to designer. • User has to specify requirements of concrete in fresh as well as in hardened state only. Fresh concrete - workability & finishing characteristics Hardened concrete – compressive strength & durability

  19. Sampling One sample of 3-cubes is to be taken from each shift

  20. 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 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.

  21. 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/metel 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.

  22. 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

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

  24. Statistical Interpretation Of Cube Results • Result of cube test 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.

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

  26. Different Degree of Control

  27. Standard deviation (S.D.) : S.D. (σ ) = ∑ (x- µ)² n-1 Where x = Individual cube strength value n = total no. of cubes µ = Arithmetic mean of strength of ‘n’ cubes S.D. will be less if the quality control at site is better.

  28. 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.

  29. Target Mean Strength

  30. Target Mean Strength • So instead of placing the desired strength 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. i.e. mean = desired strength + 1.65 x S.D. f’ck = fck + 1.65 σ Here, desired strength is called ‘Characteristic strength’(fck) & mean value is called ‘Target mean trength’(f’ck) f’ck & fck at 28 days in N/mm², S.D. in N/mm² 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.

  31. Target Mean Strength

  32. 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.

  33. Acceptance Criteria (As per IS-456:2000) • 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.

  34. Acceptance Criteria (IS-456-2000) • Compressive strength- a)The mean of 4 consecutive test results should be • For M-15 Mean ≥ fck + 0.825 x S.D. or ≥ fck + 3 N/mm2 whichever is greater. (ii) For M-20 and above Mean ≥ fck + 0.825 x S. D. Or ≥ fck + 4 N/mm2 whichever is greater. b)Any individual test result should not have value less than fck- 3 N/mm2 for M-15 and fck- 4 N/mm2 for M-20 and above.

  35. Acceptance Criteria (IS-456-2000) Clause 16.2 of IS-456 • Flexural strength- 1. The mean determined from any group of 4 consecutive results > fck+ 0.30 N/mm2. 2. The individual strength ≥ fck - 0.30 N/mm2

  36. Check the following test results according to Acceptance Criteria assuming fck = 20 N/mm2 and established standard deviation of 3.65 N/mm2 • 25, 25.5, 26, 19, 25.75, 24.75, 26.50, 19.50, 25.9, 25.2,26.75, 18.5 (All in N/mm2) • The mean of 4 consecutive test results should be ≥ fck + 0.825 x 3.65 or ≥ 23.01 N/mm2 or 20+4 =24 n/mm2 whichever is greater • Individual result should not be less than fck – 4, i.e 20 – 4 = 16 N/mm2

  37. Calculating mean of set of four cubes • Mean of I-stset =(25 + 25.5 + 26 + 19)/4 = 24.87 • Mean of 2nd set=(25.75 + 24.75 + 26.50 + 19.50)/4 = 24.12 • Mean of 3rd set=(25.9+25.2+26.75+18.5)/4 = 24.08 • The mean of all the three sets are more than or equal to 24, therefore, first criteria is satisfied • and since no test result is having value less than 16 N/mm2, the second criteria is also satisfied. • Therefore, even if the three results are having value less than 20 N/mm2, the above results have passed the criteria for acceptance.

  38. Fine Aggregate • Grading - On the basis of particle size, fine aggregate is graded into four zones. • 26.1.3.1 General - Aggregate most of which passes through 4.75mm IS sieve is known as fine aggregate. • The grading when determined in accordance with the procedure prescribed in Annexure 26.4 of USSOR shall be within the limits given in Table 26.1 of USSOR below. • The higher the Grading Zone, the finer the sand, with Grading Zone I – coarsest and Grading Zone IV-Finest. • Where the grading falls outside the limits of any particular grading zone of sieves, other than 600 micron IS sieve, by a total amount not exceeding 5 per cent, it shall be regarded as falling within that grading zone.

  39. Fine Aggregate • This tolerance shall not be applied to percentage passing the 600-micron IS Sieve or to percentage passing any other sieve size on the coarse limit of Grading Zone I or the finer limit of Grading Zone IV. • Since the values of 600 µ size are not overlapping for different zones, hence it is used for conforming the zone of a sample of FA. • It is recommended that fine aggregate conforming to Grading Zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportions.

  40. Fine AggregateTable 26.1of USSOR & Table 4 of IS 383

  41. GRADED STONE AGGREGATE OR GRAVEL Table 3.1 of USSOR & Table 2 of IS 383

  42. SINGLE SIZED(UNGRADED) STONE AGGREGATE OR GRAVEL Table 3.2 of USSOR & Table 2 of IS 383

  43. Degree of absorption Diagrammatic representation of moisture in aggregates

  44. Concrete Mix Proportioning Indian Standard Method of Concrete Mix Proportioning (Step – by – Step procedure as per IS 10262 specification) • Given data for Mix Design • Target Strength and Standard Deviation • Selection of Mixture proportions • Trial Mixtures.

  45. Step 1: Data for Mix proportioning (IS-10262, 2009) • Grade designation, • Type of cement, • Maximum nominal size of aggregate, • Minimum cement content, • Maximum water cement ratio, • Workability, • Exposure conditions as per table 4 and 5 of IS-456, • Maximum temperature of concrete at the time of placing, • Method of transporting and placing, • Early age strength requirements, if required, • Type of aggregate, • Maximum cement content, • Whether an admixture shall or shall not be used and the type of admixture and the condition of use.

  46. Step 2a: Target Strength Target Strength for Mix Proportioning f’ck = fck + 1.65 s Where f’ck = target mean compressive strength at 28 days in N/mm2 fck = Characteristic Compressive Strength at 28 days in N/mm2 and S = Standard Deviation N/mm2

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