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MIX PROPORTIONING

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  1. MIX PROPORTIONING Prepared by Marcia C. Belcher Construction Engineering Technology

  2. Design Consideration: Obtaining Good Workability

  3. Design Consideration: Obtaining Good Workability

  4. Obtaining Good Workability

  5. Obtaining Good Workability

  6. Design Consideration: Prevent Segregation & Bleeding

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

  8. Measuring Workability: Slump

  9. The Slump Cone

  10. Approximate Required Slump Values (PCA)

  11. 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

  12. Cost Comparison of Constituents

  13. Guidelines We Use For Mix Design • PCA Manual • Tables for w/c ratio based on compressive strength requirement & slump (workability) • Volume of stone required based on max. agg. size and sand fineness. • Water required based on max. agg. size, slump & w/c ratio (compressive strength)

  14. Design Method We Will Use:Absolute Volume Method • Assumes no air voids in concrete • Amount of concrete is sum of solid volumes: 1 CUBIC YARD • Cement • Sand • Coarse aggregate • Water • Air

  15. 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

  16. Material Values & Constants Needed For Design: • SSD (Absorption) of Sand • Unit Weight & SG of Sand • SSD (Absorption) of Stone • Unit Weight & SG of Stone • Density of Cement = 195 pcf • SG Cement = 3.15 • Density of Water = 62.4 pcf • 1 Cubic Foot Water = 7.48 gal • 1 Gal. Water = 8.34 lbs

  17. 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

  18. Step #1: Select Slump Table 9.6

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

  20. 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.

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

  22. 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

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

  24. Step #7: Fine Agg. Content

  25. 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.

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

  27. Properties of Concrete Specified By Engineer: Slump =1.0 inch 28-day strength of 5000 psi Air content: 4.5 - 6.5 percent

  28. Information About Materials: • Coarse aggregate we are using (ODOT #467): • nominal maximum size = 1.5 inch (see Agg. Size Table) • dry-rodded weight = 100 lb/ft3 • 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

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

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

  31. Step #2: Determine Mixing Water and Air Content • Weight of Water = 250 lbs/yd3 • Volume of Water = 250 lbs/yd3= 4 ft3 62.4 lbs/ft3 Volume of Water = 4 ft3 per cubic yard of concrete

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

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

  34. SG Cement Step #5: Cement Content • W/C= Wt. of Water Wt. of Cement • Wt. of Cement = 250 lbs/yd3 .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

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

  36. 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

  37. 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

  38. 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

  39. 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

  40. Final Batch Wts. (1 Cubic Yard)