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MIX PROPORTIONING

MIX PROPORTIONING. Construction Technology & Mgt. VOLUME OF FRESH CONCRETE. The volume of the fresh concrete is equal to the sum of the absolute volumes of its components, including the naturally entrapped or purposely entrained air. V = V a + V w + V c + V fa + V ca.

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MIX PROPORTIONING

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  1. MIX PROPORTIONING Construction Technology & Mgt

  2. VOLUME OF FRESH CONCRETE The volume of the fresh concrete is equal to the sum of the absolute volumes of its components, including the naturally entrapped or purposely entrained air. V = Va + Vw + Vc + Vfa + Vca

  3. V = Va + Vw + Vc + Vfa + Vca Where: Va = Volume of the air Vw = volume of the water Vc= absolute volume of the cement Vfa = absolute volume of the fine aggregate Vca = absolute volume of the coarse aggregate

  4. Absolute Volume

  5. Mix Design/Proportioning • Is the selection of mix ingredients and their proportion

  6. Mix proportioning is a function of: • Cement • Type & quality • Aggregate • Coarse and fine • Water • Amount, quality& the way mixed and handled • There are also human and environmental effects (temp,wind speed and relative humidity)

  7. Design Consideration: Obtaining Good Workability

  8. Design Consideration: Obtaining Good Workability

  9. Obtaining Good Workability

  10. Obtaining Good Workability

  11. Design Consideration: Prevent Segregation & Bleeding

  12. Well Graded Aggregates Will Reduce Segregation & Bleeding Well Graded Poorly Graded

  13. Measuring Workability: Slump

  14. The Slump Cone

  15. Goals: Maximize strength = minimize water = control bleeding & segregation Reduce Cost = use largest gravel possible for the job = minimize paste requirement Provide good durability = use well graded aggregates = maximize void packing = reduced segregation Economics of Mix Design

  16. Cost Comparison of Constituents

  17. MIX DESIGN • The design begins by specifying : • The desired workability as defined by a given slump. • Durability requirements taking in to consideration the weather exposure conditions (freeze thawing) • And the required 28 day compressive strength.

  18. Design Method We Will Use:Absolute Volume Method • Assumes air voids in concrete • Amount of concrete is sum of solid & air volumes: 1 CUBIC METER • Cement • Sand • Coarse aggregate • Water • Air

  19. WATER CORRECTION • Any water content in aggregates above SSD water content must be subtracted from the water requirements • Any water requirement of aggregates (below the SSD water content) must be added to the water requirements

  20. Material Values & Constants Needed For Design: • Gradation of aggregate (nominal size) • SSD (Absorption) of Sand • Unit Weight & SG of Sand • SSD (Absorption) of coarse aggregate • Fineness modulus of fine aggregate • Unit Weight & SG of coarse aggregate • SG of Cement = 3.15 • Density of Water = 1000kg/m3

  21. ACI Standard Mix Design Method • The standard ACI mix design procedure can be divided up into 8 basic steps: • Choice of slump • Maximum aggregate size selection • Mixing water and air content selection • Water-cement ratio • Cement content • Coarse aggregate content • Fine aggregate content • Adjustments for aggregate moisture

  22. Step #1: Select Slump Table 9.6

  23. Step #2: Determine Mixing Water and Air Content Table 9.5

  24. Step #3: Max. Agg. Size Check • DEFINITION: Nominal maximum aggregate size is the largest sieve that retains some of the aggregate particles. • ACI Limits: • 1/3 of the slab depth • 3/4 of the minimum clear space between bars/form • 1/5 minimum dimension of non-reinforced member • Aggregate larger than these dimensions may be difficult to consolidate and compact resulting in a honeycombed structure or large air pockets.

  25. Step #4: Select W/C Ratio Table 9.3

  26. Step #5: Cement Content • The calculated cement amount is based on the selected mixing water content and water-cement ratio. • W/C= Wt. of Water Wt. of Cement

  27. Step #6: Coarse Agg. Content Table 9.4

  28. Step #7: Fine Agg. Content

  29. Step #8: Batch Weight & Water Adjustment • Aggregate weights.  • Aggregate volumes are calculated based on oven dry unit weights, but aggregate is batched in the field by actual weight.  • Any moisture in the stockpiled aggregate will increase its weight. • Without correcting for this, the batched aggregate volumes will be incorrect. • Amount of mixing water.  • If the batched aggregate is anything but saturated surface dry it will absorb water (if dry) or give up water (if wet) to the cement paste.  • This causes a net change in the amount of water available in the mix and must be compensated for by adjusting the amount of mixing water added.

  30. Mix Design Example: 10” Thick Unreinforced Pavement Slab

  31. Properties of Concrete Specified By Engineer: Slump =25.4 mm 28-day strength of 34.5Mpa Air content: 4.5 - 6.5 percent

  32. Information About Materials: • Coarse aggregate we are using • nominal maximum size = 38.1 • dry-rodded weight = 1600kg/m3 • specific gravity = 2.68 • moisture content = 1.0 percent • absorption = 0.5 percent • Fine aggregate: • fineness modulus = 2.80 • specific gravity = 2.64 • moisture content = 5 percent • absorption = 0.7 percent

  33. WE ARE DESIGNING BATCH WEIGHTS FOR ONE CUBIC YARD • Step #1: Select Slump • Engineer Specified 1” (correlates w/table) Table 9.6

  34. Step #2: Determine Mixing Water and Air Content 1.5” Stone Table 9.5 1” Slump

  35. Step #2: Determine Mixing Water and Air Content • Weight of Water = 150kg/m3 or 250 lbs/yd3 • Volume of Water = 250 lbs/yd3= 4 ft3 62.4 lbs/ft3 Volume of Water =3.06 m3per cubic meter of concrete

  36. Step #3: Max. Agg. Size Check • ACI Limits: • 1/3 of the slab depth • 10”/3 = 3.33 inches > 1.5” OK

  37. Step #4: Select W/C Ratio Table 9.3

  38. SG Cement Step #5: Cement Content • W/C= Wt. of Water Wt. of Cement • Wt. of Cement = 250 lbs/yd3 or .40 =625 lbs/yd3 • Volume of Cement = 625 lbs/yd3 (Concrete) 3.15 x 62.4 lbs/ft3 Volume of Cement = 3.18 ft3 per cubic yard of concrete

  39. Step #6: Coarse Agg. Content Table 9.4

  40. Step #6: Coarse Agg. Content Weight(Dry) =.71 x 27 ft3/yd3 x 100 lb/ft3 = 1,917 lbs Volume = 1,917 lbs = 11.46 ft3 2.68 x 62.4 lbs/ft3 Dry Rodded Unit Wt of Stone SG Stone

  41. Step #7: Fine Agg. Content Wt of Sand(Dry) = 6.87 ft3 x 2.64 x 62.4 lbs/ft3 = 1,131.7 lbs. SG Sand

  42. Step #8: Aggregate Batch Weights & Water Adjustment Since there is moisture in both coarse & fine aggregate, their batch weights must be adjusted 1% Moisture 5% Moisture • Wt of Stone(Wet) = 1,917 lbs x 1.01= 1,936.2 lbs • Wt of Sand(Wet) =1,131.7 lbs x 1.05= 1,188.3 lbs

  43. Step #8: Aggregate Batch Weights & Water Adjustment Mixing water needs to be adjusted. Both the coarse and fine aggregate are wet of SSD and will contribute water to the cement paste. Water from Stone = 1,917 lbs. x (.01-.005) = 9.59 lbs Water from Sand= 1,131.7 lbs x (.05-.007) = 48.66 lbs Water = 250 lbs – 9.59 lbs – 48.66 lbs = 191.75 lbs Dry Wt. Moisture Absorption Dry Wt. Moisture Absorption

  44. Final Batch Wts. (1 Cubic Yard)

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