CON 124 Session 3 - Examples of Concrete Proportioning

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CON 124 Session 3 - Examples of Concrete Proportioning

  1. 1. CON 124Basic Concrete Mix Design ProportioningSession 3Examples of Concrete Proportioning
  2. 2. Examples of Proportioning Concrete Mixtures Absolute volume method Trial mixture using the Water-Cement ratio method
  3. 3. Proportioning Concrete Mixtures Example 1Using the Absolute Volume Method VolAbs Vol Density = Weight/Volume (no voids) Specific Gravity = Abs Vol Density / Density of Water
  4. 4. Proportioning Concrete MixturesAbsolute Volume Method for Example 1 using the equations below: Abs Vol=Wt/(Specific Gravity x Density of Water) Wt=Abs Vol x Specific Gravity x Density of WaterDensity of Water = 62.4 lbs per cu ft ( @ 40C)1 Cubic yard concrete = 27 cubic feet
  5. 5. Conditions for Example 1 Concrete for a building foundation Specified Compressive Strength: f c= 3500 psi Type I cement; Relative Density = 3.15 Design for minimum 3 in of concrete cover Minimum distance between reinforcement bars is 4 in; Only admixture allowed is air entrainment No statistical data on mixes available Coarse Aggregate: ¾ in gravel; OD Relative Density = 2.68; Absorption = 0.5%; Oven dry density = 100 lbs/cuft; Moisture = 2% Fine Aggregate: Natural sand; OD Relative Density = 2.64; Absorption = 0.7%; Moisture = 6%; Fineness Modulus = 2.80
  6. 6. Specifications for Example 1 Strength: No statistical data available; fcr = fc + 1200; therefore, fcr = 3500 + 1200 = 4700 psi (Table 11 CON 124 Proportioning Methods) Water to Cement ratio: No required Max (Table 1 CON Proportioning and Mix Design); Recommended W/C ratio for fcr = 4700 psi , 0.42 interpolated (Fig 1 or Table 3 CON 124 Proportioning and Mix Design)
  7. 7. Specifications for Example 1 (Cont) Coarse Aggregate Size: ¾ in nominal maximum size adequate ¾ distance between reinforcing bars also between reinforcing bars and forms (cover) Air Content: Target air of 6% needed to improve workability and reduced bleeding. Design 6% +/- 1%, Max of 7% Slump: Not specified, range 1 to 3 in adequate (foundations, CON 124 Proportioning and Mix Design)
  8. 8. Specifications for Example 1 (cont) Water Content: 3-in slump, air entrained concrete, ¾ in nominal maximum size aggregate; requires 305 lbs/cu yd (Fig 3 and Table 5,CON 124 Proportioning and Mix Design); Crushed gravel particles reduce water by 35 lbs, Estimated water content of 270 lbs Cement Content: Based on max W/C ratio and water content; 270 lbs/0.42 = 643 lbs
  9. 9. Specifications for Example 1 (cont) Coarse Aggregate Content: Bulk volume CA recommended using a fine aggregate Fineness Modulus of 2.80 is 0.62 (Fig 3 or Table 4, CON Proportioning and Mix Design); CA (oven dry) weighs 100 lbs/cu ft, thus for a cubic yard of concrete(27 cu ft) requires 100x27x0.62 = 1674 lbs/cu yd Admixture Content: 7% air, 0.9 fl oz/100 lbs of cement Fine Aggregate Content: Volume is determined by subtracting absolute volumes of known ingredients from 27 cu ft/ cu yd (see next slide), Absolute Volume = 27.0 – 19.50 = 7.50 cu ft; Weight of dry fine aggregate = 7.50x2.64x62.4 = 1236 lbs
  10. 10. Absolute Volume Computation for Fine Aggregate Content 270 Water = = 4.33 ft3 1 • 62.4 643 Cement = 3.15 • 62.4 = 3.27 ft3 7.0 • 27 Air = = 1.89 ft3 100 Coarse 1674 aggregate = = 10.01 ft3 2.68 • 62.4 Subtotal = 19.50 ft3 Fine aggregate volume=27-19.50= 7.50 ft3Fine aggregate mass =7.50 • 2.64 • 62.4 = 1236 lb
  11. 11. Mixture Design for Example 1 per Cubic Yard Water: 270 lbs Cement: 643 lbs Coarse Aggregate (dry): 1674 lbs Fine Aggregate (dry): 1236 lbs Total weight: 3823 Air-entraining admixture: 5.8 fl oz Slump: 3-in (+/- ¾ in for trial batch) Air Content: 7%
  12. 12. Trial Batch Corrections for Moisture in Aggregates Dry Batch Weights need correction for absorbed and surface moisture (MC) Mixing water reduction due to amount of free moisture contributed by aggregates Coarse Aggregate (CA) MC is 2%; Fine Aggregate (FA) MC is 6% CA (2% MC) = 1674 lbs x 1.02 = 1707 lbs FA (6% MC) = 1236 lbs x 1.06 = 1310 lbs Estimated mix water: 270 lbs – (1674 x 0.015) – (1236 x 0.053) = 179 lbs
  13. 13. Estimated Batch Weights for Aggregate Moisture Content Water (to be added): 179 lbs Cement: 643 lbs CA (2% MC,wet) 1707 lbs FA (6% MC, wet) 1310 lbs Total Batch Weight: 3839 lbs/ 1 Cubic Yd
  14. 14. Laboratory Trial Batch (2.0 cu ft; or 2/27 cu yd) Trial batch concrete needed for air, slump, and casting of cylinders for strength Water: 179 lbs x 2/27 = 13.26 lb Cement: 643 lbs x 2/27 = 47.63 lb CA, wet: 1707 lbs x 2/27 = 126.44 lb FA, wet: 1310 lbs x 2/27 = 97.04 lb Total laboratory trial batch: 284.37 lbs Air Entraining admixture: 5.8 fl oz x 2/27 = 0.43
  15. 15. Laboratory Trial Batch Data Lab trial batch concrete testing:  Measured Slump, 4-in  Air Content, 8%  Density (unit weight), 141.49 lbs  Pre-measured water remained unused, original amount of water was 13.26 lbs, only used 13.12 lbs Mixture Design: Water = 13.12 lbs, Cement = 47.63 lbs, CA (2% MC) = 126.44 lbs, FA (6%MC) = 97.04; Total trial batch = 284.23 lbs/ 2.0 cu ft
  16. 16. Yield of Laboratory Trial Batch Total laboratory batch weight : 284.23 lbs Laboratory density (unit weight): 141.49 lbs/cu ft Yield: 284.23lbs/141.49 lbs/cu ft = 2.009 cu ft
  17. 17. Mixing Water Content of Trial Batch for 1- cubic yard Water added: 13.12 lbs Free Water CA: 126.44/1.02 x 0.015 = 1.86 lbs Free Water FA: 97.04 /10.6 x 0.053 = 4.85 lbs Total Water: 19.83 lbs Mix water needed / cu yd for same slump: 19.83 lbs x 27.00 cu ft/cu yd/2.009 cu ft = 267 lbs
  18. 18. 1 yard Concrete Trial Batch Adjustments Measured 4-in slump unacceptable more than 0.75 in above designed 3- in max Trial yield slightly exceeds design Air content of 8% is unacceptable, more than 0.50% above designed 7% max Reestimate amount of air entraining admixture for a 7% max air content, and adjust water to obtain 3-in slump Increase mixing water by 5 lbs for each 1%;reduce mixing water by 10 lbs for 1-in reduction in slump: (5 x 1) – (10 x 1) + 267 lbs = 262 lbs/cu yd With less mixing water needed, less cement required for desired water- cement ratio of 0.42: 262 lbs/cu yd/0.42 = 624 lbs/ cu yd
  19. 19. New Adjusted Concrete Batch Volumes per Cubic Yard Water: 262/1 x 62.4 = 4.20 cu ft Cement: 624/3.15 x 62.4 = 3.17 cu ft CA: 1674/2.68 x 62.4 = 10.01 cu ft Air: 7.0/100 x 27.0 = 1.89 cu ft Total from above: 19.27 cu ft FA: 27.00 – 19.27 = 7.73 cu ft
  20. 20. Adjusted Dry Weights for 1 Cubic Yard FA (dry): 7.73 x 2.64 x 62.4 = 1273 lbs Air dosage; 0.8 fl oz/ 100 lbs of cement to achieve 7% air: 0.8 fl oz x 624/100 = 5.0 fl oz Designed Batch weights: Water = 262 lbs Cement = 624 lbs CA (dry) = 1674 lbs FA (dry) = 1273 lbs Total Batch = 3833 lbs Estimated Concrete Density (unit weight) at SSD (262 + 624 + (1674 x 1.005) + (1273 x 1.007)) divided by 27 = 142.60 lb/cu ft
  21. 21. Example 2 Proportioning by Trial Mixtures Using W/C ratioTrial batching verifiesthat a concrete mixturemeets designrequirements prior touse in construction.
  22. 22. Example 2 Proportioning by Trial Mixtures Using W/C ratio Trial mixtures should use the same materials proposed for the work. Three mixtures with three different water- cementing materials ratios or cementing materials contents should be made. The trial mixtures should have a slump and air content within ±20 mm (±0.75 in.) and ± 0.5%, respectively, of the maximum permitted. Three cylinders for each water-cementing materials ratio should be tested at 28 days.
  23. 23. Laboratory Trial Mixtures Using the Water-Cement Ratio Conditions:  Foundation wall requiring air-entrained concrete exposed to moderate sulfate soils  Compressive strength, fcr, 4000 psi @ 28 days, Type II cement  Minimum thickness of wall is 10-in; concrete cover over ½-in diameter reinforcing bars 3-in Specifications:  Water-cement ratio vs compressive strength relationship based on field and previous lab data  Test records of materials used, std dev is 300 psi  Max W/C ratio for the above conditions should be 0.50 (Table 1 CON 124 Concrete Proportioning and Mix Design)
  24. 24. Concrete Trial Mixture Designed Compressive Strengths Standard Deviation (S) of 300 psi fcr = fc + 1.34S = 4000 + 1.34 (300) = 4402 psi Or fcr = fc + 2.33S – 500 = 4000 + 2.33 (300) – 500 = 4199 psi Therefore fcr = 4400 psi W/C ratio for air-entrained concrete is 0.55 for an fcr of 4400psi; Exposure Conditions requirements govern; Thus W/C ratio of 0.50 used producing higher strengths than needed to satisfy structural needs
  25. 25. Relationship betweenstrength and water tocement ratio based onfield and laboratory datafor specific concreteingredients.
  26. 26. Example 2 Concrete Trial Batch Mixture Data Aggregate: 1½-in max size is satisfactory; less than 1/5 wall thickness; less than ¾ the clear distance between reinforcing bars, as well as forms Air Content: Exposure conditions and workability require moderate level, target concrete air with an 1½-in aggregate is 4.5%+/- 1%, aim for 5.5%+/- 0.5% in trial batch Slump: 1-3-in for placing a reinforced concrete foundation wall; trial batch for 3-in +/- 0.75 in
  27. 27. Relationship between: Slump Agg. Size W/C Cement content 27
  28. 28. Example 2 Batch Quantities Batch size contains 20 lbs of cement Mixing Water: 20 lbs x 0.50 = 10 lbs (values entered as initial weights, Col 2) FA & CA (SSD) proportions used from mixes used to develop graph above Mixing continues until a workable concrete of 3-in slump is reached Results of slump, air, unit weight, and description and workability are noted on data sheet
  29. 29. Example 2 Trial Batch Data Workability: (from data sheet) The amounts of FA & CA not used are recorded, Col 3, Masses of aggregates used (Col 2 minus Col 3) are recorded in Col 4 Additional information on slump, water required, cement quantities are recorded
  30. 30. Example 2 Mixture Proportions for 1 Cubic Yard From Col 5 of data sheet, use batch yield (volume) & density (unit weight) for mixture proportions Example, Cement lbs/cu yd = 27 cuft//volume of concrete in batch x lbs of cement in batch; % of FA is calculated; cement content is 539 lbs/cu yd; FA aggregate is 33.5% of total aggregate Air content and slump were acceptable; 28-day compressive strengths were 4950 psi (greater than fcr ) Mixture in Col 5, with slump and air content, 1-3-in and 3.5% to 5.5%,respectively, now ready for submission.
  31. 31. Example 2Trial Mixture Data Sheet 31
  32. 32. Test Results of Laboratory Trial Mixtures for Example 2 at a W/C Ratio of 0.50, economy & workability Fine aggregate, Air Cement percent ofBatch Slump content Density content, total Worka- no. , in. percent lb/ft3 lb/yd3 aggregate bility 1 3 5.4 144 539 33.5 Good 2 2¾ 4.9 144 555 27.4 Harsh 3 2½ 5.1 144 549 35.5 Excellent 4 3 4.7 145 540 30.5 Excellent Inch-Pound
  33. 33. Common Mix Design Mistakes Not varying water-cement ratio (3 point curve) Not monitoring slump loss during mix design to identify false setting tendency in cement Not monitoring early age concrete temperatures to identify retardation effects of water reducers
  34. 34. Proportioning Concrete MixesOrdering Concrete by ASTM C94 Option A -  manufacturer assumes responsibility for proportions  purchaser specifies:  strength requirements  durability, placeability, density requirements
  35. 35. Proportioning Concrete MixesOrdering Concrete by ASTM C94 Option B -  purchaser assumes responsibility for proportions  purchaser specifies:  cement content  max allowable water content  admixtures (type and dose)
  36. 36. Proportioning Concrete MixesOrdering Concrete by ASTM C94 Option C -  manufacturer assumes responsibility for proportions with minimum allowable cement content specified by purchaser  purchaser specifies:  required compressive strength  minimum cement content  admixtures (type and dose)

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