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CONCRETE MIX DESIGN By (R . N. Gupta) Professor/Track machine

CONCRETE MIX DESIGN By (R . N. Gupta) Professor/Track machine. Reference Codes. CBC:2014 – Indian Railway Standard Code of Practice for Plain, R einforced & P restressed C oncrete for General B ridge C onstruction IS 456:2000 –

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CONCRETE MIX DESIGN By (R . N. Gupta) Professor/Track machine

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  1. CONCRETE MIX DESIGN By (R. N. Gupta) Professor/Track machine

  2. Reference Codes • CBC:2014 – • Indian Railway Standard Code of Practice for Plain,Reinforced & PrestressedConcrete for General Bridge Construction • IS 456:2000 – • Plain & reinforced cement concrete- code of practice (fourth revision) Amendments up to 4 • IS 10262:2019– • Concrete Mix Proportioning – Guidelines (Second Revision) • IS 383:2016 – • Specification for coarse and fine aggregates from natural sources for concrete (Third revision)

  3. Concrete • Combination of Cementatious Material,Aggregates and water along with Admixtures, if required. • It is denoted by Grade M-X Here, X is the Characteristic Strength of Concrete in N/mm2 tested on 15 cm cube after 28 days

  4. Grade of Concrete • Ordinary Concrete – M-10, M-15, M-20 • Standard Concrete – M-25 to M-60 • High Strength Concrete – Above M-60 IS:456 applicable upto M-100 CBC for M60 only

  5. Types of Mix • Nominal Mix • When concrete is prepared on the basis of nominal quantities of its constituents. • Used for relatively unimportant and simpler works. • No scope for any deviation by the designer since ingredients are prescribed and their proportions specified. • May be used for M-20 or lower mix. The proportions of materials for nominal mix concrete shall be in accordance with Table-9 of IS:456.

  6. Nominal Mix • Table 9 – IS:456 CBC also provides proportions for M20 nominal mix in Table-5

  7. Types of Mix • Design Mix • When Concrete is prepared by mixing the quantities of its constituents arrived on the basis of design for particular objective of Workability, Strength and Durability. • Choice of ingredients and proportioning are left to designer. • User has to specify requirements of concrete in fresh as well as in hardened state to design the mix

  8. Objective of Design Mix • Design mix is prepared to fulfill three basic objectives-Strength, Workability and Durability • As per IS:10262-2019 “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 proportioningis carried out to achieve specified characteristics strength at specified age, workability of fresh concrete and durability requirements”

  9. Durability Criteria • From durability consideration, depending upon the environment to which the structure is likely to be exposed during its service life, IS:456 prescribes requirement of Minimum Concrete Mix, Minimum Cement Content and maximum W/C ratio. (Table-5) • Also given in CBC Table 4(a) and 4(b)

  10. Environmental exposure Conditions(Based on Table 3 of IS 456:2000)

  11. Environmental exposure Conditions(Based on Para 5.4.1 of Concrete Bridge Code )

  12. Workability requirements • Para 7.1 of IS:456 “The concrete mix proportions chosen should be such that the concrete is of adequate workability for the placing conditions of the concrete and can properly be compacted with the means available”.

  13. Workability requirements • Workability requirements as per CBC Para 5.3.1

  14. Characteristic Strength Para 6.1.1 of IS:456-2000 Characteristic Strength is defined as the strength of material below which not more than 5 percent of the test results are expected to fall.

  15. S.D. (S) = ∑ (x- µ)² n-1

  16. NUMBER OF SPECIMENS IN INTERVAL COMPRESSIVE STRENGTH IN MPa

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

  18. Target Mean Strength 5% 1.65σ 1.65σ f’ck fck Target Mean Strength (f’ck) = Desired Strength (fck) + 1.65 Standard deviation (σ)

  19. Target Mean Strength • Target Mean Strength (f’ck) shall be more than the Desired Strength (Characteristic strength, fck) so that test results falling below Desired strength are not more than 5%. • As per IS 10262-2019 • f’ck= fck + 1.65 σ 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 σ- Standard Deviation in N/mm² X - Factor based on the grade of concrete, as per Table 1 of IS:10262

  20. Standard Deviation • SD Shall be calculated on the basis of minimum 30 nos of samples • When sufficient test results for a particular grade of concrete are not available the value of SD may be assumed as per Table-8 of IS:456, and table-8 of CBC. • However, when adequate test samples become available, SD should be calculated on that basis. • Standard Deviation for each grade of concrete shall be calculated separately. • If significant changes are made in production of concrete in terms of materials, equipment, quality control etc, the SD values shall be calculated again.

  21. Assumed Standard Deviation Table-8 of CBC IS:456 (Table 1 of IS 10262) 10262

  22. Table 1 of IS-10262

  23. Sampling • Sample - Set of Three Cube Specimen • Minimum 1 sample should be taken for each shift • 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

  24. Cube casting • Manual compaction • Concrete is filled in mould in 3 equal layers. • Each layer is tamped 35 times by bullet headed tamping rod of 16 mm dia and 600 mm long. • Needle/plate vibrator can also be used. • The cube in mould is covered with glass/metel plate. • Marking of date and number for identification. • Stripped after 16-24 hrs and then submerged in water till testing. • Cube should not be allowed to dry till testing.

  25. Cube Testing • Testing should be done as per IS:516 • Smooth faces should face top and bottom during testing. • The rate of loading should not be more than 14 N/mm2 per minute. • The test strength of the sample shall be the average of three specimen. • Individual variation of Cube strength should not be more than +15% of average strength. If so, the test results of sample are invalid Example 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

  26. Acceptance Criteria Compressive Strength The concrete shall be deemed to comply with the strength requirements when Boththe following condition are met: a) The Meanstrength determined from any group of Four Non Overlapping Consecutive test resultscomplies with the appropriate limits in col 2 of Table 11 of IS:456 b) Any Individualtest result complies with the appropriate limits in col 3 of Table 11 of IS:456.

  27. IS 456:2000- Table 11: Characteristic Compressive Strength Compliance Requirement

  28. Acceptance criteria

  29. Acceptance Criteria Flexural Strength The concrete shall be deemed to comply with the Flexural strength requirements when Boththe following condition are met: • The Meanstrength determined from any group of Four Non Overlapping Consecutive test resultsexceeds the specified characteristic strength by at least 0.3 N/mm2 . • The strength determined from any test result is not less than the specified characteristic strength less 0.3 N/mm2 Specified Characteristic Flexural Strength = 0.7√fckN/mm2 Where fck= Characteristic Compressive Strength of Concrete

  30. Acceptance Criteria • Characteristic Strength of Concrete is defined only on the basis of 28 days strength. • Specified 28 days compressive strength shall alone be the criterion for acceptance or rejection of the concrete. • However, in order to get a relatively quick idea of the quality of concrete, optional compressive strength tests at 7 days may be carried out in addition to 28 days compressive strength tests. • Optional Tests on beams for modulus of rupture at 72 hours or at 7 days may be done • For this purpose, the values given in Table 7 of CBC may be taken for general guidance

  31. Table-7: CBC (Clause 8.7.1.1)

  32. Illustrations for Site concrete acceptance In one shifts 4 m3 foundation concrete was done. One sample (3 Cubes) were collected Mix=M25 Fck=25 N/mm2; SD = 4 N/mm2F’ck=29 N/mm2

  33. Illustrations for Site concrete acceptance In 3 shifts 27 m3 foundation concrete was done. One sample (3X3 Cubes) were collected Fck=25 N/mm2

  34. Illustrations for Site concrete acceptance

  35. 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+3 =23 N/mm2 whichever is greater • Individual result should not be less than fck – 3, i.e 20 – 3 = 17 N/mm2

  36. Calculating mean of set of four cubes • Mean of Istset =(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 23, therefore, first criteria is satisfied • and since no test result is having value less than 17 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.

  37. Preparation of design Mix • Strength -M40 (fck) • Calculate Target Mean Strength (f’ck) • f’ck= fck+ 1.65 σ OR f’ck= fck + X whichever is more • Take Standard Daviation (σ) and X values from Table-1 and Table-2 of IS:10262 values of σ are for good degree of site control having proper storage; weigh batching of all materials; controlled addition of water; regular checking of all materials etc.Where there are deviations from the above, the site control shall be designated as fair and the values given in the above table shall be increased by 1 N/mm2.

  38. Preparation of design Mix • Workability - Slump Value • If not given, workability requirements as per CBC Para 5.3.1 ORPara 7.1 of IS:456 is calculate. • Exposure Condition -Severe • Fix w/c ratio for Target Mean Strength (f’ck) from Fig-1 of IS:10262 • For durability, requirement of Minimum Concrete Mix and Maximum w/c ratio should be checked from table-5 of IS:456. • Coarse Aggregate Size • Nominal Size should conform to table-7 of IS:383 • Coarse aggregate Angularity • Angular, Sub angular, Gravel or Round

  39. Preparation of design Mix • Fix requirement of water for aggregate size as per table-4 of IS:10262 • This water requirement is for Angular Aggregates only for 25-50 mm Slump. • Requirement of water as table-4 of IS:10262 will be reduced by 10kg, 15kg and 20 kg for Sub angular, Gravel or Round aggregates respectively • For Slump requirement more than 50mm, water content is increased by 3% for every increase of 25mm in Slump • Water content can be reduced by 5-10% for Water reducing admixtures and 20-30% for Super Plasticizers. • As per w/c ratio, requirement of cement should be calculated and checked from table-5 of IS:456 for minimum requirement from Durability consideration.

  40. Preparation of design Mix • Water absorption of Aggregates • Moisture Content of Aggregates • Difference of Moisture Content and Water Absorption is the amount of free water in aggregates • Amount of Free water should be reduced from final water requirement • Mineral Admixture - Percentage • If mineral admixtures like Fly Ash (IS:1489-1), GGBS (IS:455) etc are used, Requirement of Cementatious material as calculated above should be increased by 10%. • Calculate the Qty of cement and Mineral Admixture

  41. Preparation of design Mix • Fine aggregate Zone- Zone as per Table-9 of IS:383 • For given zone of fine aggregates and nominal size of coarse aggregates, fix the ratio of Coarse and Fine aggregates by volume as per Table-5 of IS:10262 • This is for w/c ratio 0.5. Change it @ -/+0.01 for +/-0.05 change in w/c ratio. • For Pumpable concrete, this ratio should be reduced by 10% • Now fix the final ratio of Coarse and Fine Aggregates.

  42. Preparation of design Mix • Quantity of Super Plasticizer- Say 2% • Specific Gravity of Cement, Mineral Admixture, Coarse Aggregate, Fine Aggregate, Super plasticizer • Calculate entrapped air as per Table-3 of IS:10262 • Find Volume of Cement, Mineral Admixture, Water and Plasticizer and air content in 1 m3 concrete and calculate balance volume i.e. Aggregates • Calculate the separate volume of Coarse and Fine aggregates from their ratio and calculate their weight. • Calculate the quantity of all ingredients like Cement, Mineral Admixture, Water, Coarse Aggregates, Fine Aggregates and Super plasticizer. Amount of Free water should be reduced from final water requirement

  43. Trial Mixes • Workability of Trial Mix TM-0 will be measured. • The mix will be carefully observed for freedom from segregation, bleeding and finishing properties If workability is exactly same as required, i.e. slump, then no further adjustment is made. • If the workability of TM-0 is different from the stipulated value, the water and/or admixture content shall be adjusted suitably. • With this adjustment, the mix proportions will be calculated again keeping same W/C ratio. This will be TM-01 • In addition, 2 more Mix. TM-2 and TM-3 will be made with same water content and varying the W/C ratio by +10%. • 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 Target mean strength (TMS) • However, durability requirement shall be met.

  44. Fig-1 of IS:10262 OPC 53 OPC 43 OPC 33

  45. Table-5 of IS:456 OPC 53 OPC 43 OPC 33

  46. Table-7 of IS:383

  47. Table-9 of IS:383

  48. Table-4 of IS:10262

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