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CON 124 Session 3 - Examples of Concrete Proportioning
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CON 124 Session 3 - Examples of Concrete Proportioning

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CON 124 Session 3 - Examples of Concrete Proportioning CON 124 Session 3 - Examples of Concrete Proportioning Presentation Transcript

  • CON 124Basic Concrete Mix Design ProportioningSession 3Examples of Concrete Proportioning
  • Examples of Proportioning Concrete Mixtures Absolute volume method Trial mixture using the Water-Cement ratio method
  • Proportioning Concrete Mixtures Example 1Using the Absolute Volume Method VolAbs Vol Density = Weight/Volume (no voids) Specific Gravity = Abs Vol Density / Density of Water
  • 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
  • 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
  • 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)
  • 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)
  • 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
  • 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
  • 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
  • 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%
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Example 2 Proportioning by Trial Mixtures Using W/C ratioTrial batching verifiesthat a concrete mixturemeets designrequirements prior touse in construction.
  • 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.
  • 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)
  • 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
  • Relationship betweenstrength and water tocement ratio based onfield and laboratory datafor specific concreteingredients.
  • 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
  • Relationship between: Slump Agg. Size W/C Cement content 27
  • 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
  • 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
  • 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.
  • Example 2Trial Mixture Data Sheet 31
  • 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
  • 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
  • Proportioning Concrete MixesOrdering Concrete by ASTM C94 Option A -  manufacturer assumes responsibility for proportions  purchaser specifies:  strength requirements  durability, placeability, density requirements
  • 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)
  • 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)