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Mix Design Review

Mix Design Review. Review Questions. The weight of a cubic foot of water is 62.4 cu ft Gallons of water in a cubic foot 62.4 cf. Review Questions. Submit a mix design to NCDOT for review on form 312U

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Mix Design Review

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  1. Mix Design Review

  2. Review Questions • The weight of a cubic foot of water is • 62.4 cu ft • Gallons of water in a cubic foot • 62.4 cf

  3. Review Questions • Submit a mix design to NCDOT for review on form • 312U • A process in which an aggregate is separated into its various sizes by passing it through a nest of sieves is • gradation

  4. Review Questions • The minimum number of revolutions at mixing speed required for concrete batched into a transit mixer is • 70 • Solid volume of cement is • .48

  5. Review Questions • The water cement ratio needed to hydrate cement is • .250 • The smallest seive opening through which the entire amount is permitted to pass is • Nominal Maximum size

  6. Review Questions • The pH of mixing water used in concrete should be between • 4.5-8.5

  7. Definitions • Cement Yield – the volume of concrete produced from one bag of cement for one cubic yard. • Water to cement ratio – the number of pounds of water from all sources per pound of cement.

  8. Definitions • Unit weight – the number of pounds of concrete per cubic foot of volume. • Specific gravity – ratio of the weight of a given volume of material to the weight of a given volume of water. • Cement factor – number of bags or pounds of cement in a cubic yard.

  9. Definitions • Absolute or solid volume – volume of a material in a voidless state. • Durability – resistance of concrete to deterioration caused by weathering, chemicals, and abrasives. • Consistency – a condition of concrete which relates to its cohesion, wetness, or flow. Slump is a good measure of consistency.

  10. Cementitious Materials - Cement • Portland cement – bonding agent • Portland cement vary by color, fineness of grind, and chemical composition. • Type I – general all purpose • Type II – low heat of hydration • Type III – high early strength

  11. Cementitious Materials- Cement • 1 bag = 94 pounds • A 6 bag mix = 564 lb • 1 barrel = 4 bags = 376 pounds • Specific gravity of portland cement = 3.15

  12. Cementitious Materials - Fly Ash • Flyash – a by product from a coal powered power plant • A good flyash is fine with a low carbon content • Substitute flyash for cement on a 1.2 to 1 ratio up to a 20% replacement of ash for cement

  13. Cementitious Materials • Slag and silica fume – by products of different furnaces • Substitute them for cement on a 1 to 1 ratio up to a 50% replacement of cement

  14. Aggregates • Abrasion resistance – ability to resist wearing or deterioration. Resistance is measured by the L.A. Abrasion test • Gradation – relative amounts of aggregate particles of consecutively larger and smaller sizes

  15. Aggregates • Well-graded – a uniform percentage of material retained on each standard sieve where the gradation change is uniform from coarse to fine • Sieve analysis – a process to separate an aggregate into its various sizes by passing a sample through screens with different sized openings

  16. Aggregates • The purpose of sieve analysis is to determine the distribution of the separately sized particles • Saturated surface dry – SSD is a condition where an aggregate will neither absorb nor contribute water to the mix

  17. Fine Aggregate • Sand gives workability to the mix and fills the voids between the coarse aggregate particles. • Workability – a property which is measured in terms of ease of placing, handling, and finishing.

  18. Fine Aggregate • Fineness modulus – an index to the fineness or coarseness of an aggregate: the range for medium sand is 2.50 to 2.80.

  19. Coarse Aggregate • Stone is a filler material and aids in strengthening the concrete mix. • Dry Rodded unit weight – the weight per unit volume of dry aggregate compacted in a container by rodding in 3 layers. • Maximum size- the sieve size on which the largest particles are permitted to pass.

  20. Coarse Aggregate • Nominal maximum size- the smallest sieve opening through which the entire sample will pass. • Rounded aggregate is from a pit. • Angular aggregate is from a quarry. • The size and shape of the coarse aggregate can affect the strength of the mix.

  21. Angular shaped aggregate is generally stronger than rounded shaped aggregate.

  22. Water • Water causes a chemical reaction with the cement called hydration. • Bleed water – the excess water that accumulates on the surface of plastic concrete when aggregates settle.

  23. Water • For a concrete with a given cement factor, an increase in total mixing water will cause the strength to decrease. • Specific gravity of water = 1. • The ph of mixing water is 4.5 to 8.5.

  24. Air Content • Entrained air is added to a concrete mix to improve it’s resistance to freezing and thawing. • Entrained air also improves workability. • The air content measured at the site for structural or incidental concrete will be from 4.5 to 7.5 %.

  25. Air Content • To much air entraining agent will result in to many air voids which will reduce the strength of the concrete mix. • The volume occupied by 6% air is 1.62 cubic feet. • You can calculate the air content by the unit weight method.

  26. Air Content • Increasing the cement will cause the air content to decrease. • Increasing the temperature of the mix causes the air content to decrease.

  27. Retarding Admixtures • Retarders are chemicals that are added to a concrete mix to slow down the set and allow more time to place and finish the concrete. • To much retarder will cause a longer delay in the initial set of the concrete. It will have little affect on the long term (28 day) strength of the concrete.

  28. Retarding Admixtures • According to NCDOT table 1000-2, using a retarder will allow about 45 extra minutes of placement time.

  29. Mix Design ReviewProblem Solving

  30. How many cubic yards did this mix produce? • 3055 Lb Cement • 5660 Lb. Wet Sand ( 5390 Lb. SSD), at 5.0% Free Moisture • 10,205 lb. Wet Stone ( 10,095 Lb SSD) at 1.1% Free Moisture • 150 Gal. Metered Water • 6% Design Air • 20 Gal. Water added at jobsite • 5.8% air by pressure meter • 147.65 pcf field unit weight

  31. Solution: Find the Total Weight of All Materials in the Mix and Divide by the Field Unit Wt. • 3055 + 5660 + 10205 + ( 150 gal x 8.33) +(20 gal x 8.33) = 3055 + 5660 + 10205 + 1250 + 167 = 20,337 lbs. • 20,337 lbs / 147.65 pcf = 137.74 cu.Ft. • 137.74 cu.Ft / 27 = 5.1 cu yds. • Design air not a factor because the air is taken into account in the field unit weight.

  32. Find Yield, Cement Factor, and the Water Cement Ratio for This Same Mix • Cement Yield: 3055 lb cem / 94 = 32.50 bags 137.74 cu.ft. / 32.50 bags = 4.24 • CF: 27 / 4.24 = 6.37 bags 6.37 x 94 = 599 lb cement • W/C: (5660- 5390) + (10,205 – 10,095) + (150 gal x 8.33) + (20 gal x 8.33) = 1797 lb • 1797 lbs water / 3055 lbs cem = .588

  33. Given: 1050 lb SSD sandSpecific Gravity = 2.61Total Moisture = 6.4%Absorbed Moisture = 0.6 % • Find wet weight of sand. • 6.4% - 0.6% = 5.8 % 5.8 / 100 = .058. • .058 x 1050 lb = 61 lb free water. • 1050 lb + 61 lb = 1111 lb batch sand. • Find dry weight of sand. • .006 x 1050 lb SSD = 6 lb absorbed water. • 1050 lb SSD – 6 lb = 1044 lb dry sand.

  34. Find absolute volume of SSDsand(same problem) • 1050 lb SSD = 6.45 cu.ft. 2.61 x 62.4

  35. Fly Ash Mix Problem • Given: Class B non vib mix 15% fly ash replacement. Angular aggregate is used. • Minimum cement from table is 545 lbs. • What is the new cement and cementitious wt? • .15 x 545 lb = 82 lb cement replaced. • 82 lb x 1.2 = 98 lb ash for cement. • New cement is 545 – 82 = 463 lbs. • 463 lb cem + 98 lb ash= 561 lbs cemetitious

  36. Water(same problem) • What is the new water? • W/C = .545 for angular aggregate. • .545 x 561 = 306 lb of water • 306 lb / 8.33 = 36.7 gallons.

  37. Find the % solids and voids in a coarse aggregate. • Given: Dry rodded unit weight = 95 pcf • Given: Specific gravity = 2.79 • 2.79 x 62.4 = 174.10 pcf. • (95 pcf / 174.10 pcf) x 100 = 55% Solid. • 100 –55 = 45% voids

  38. Trial Mix • Class A, vib,AE, rounded aggregate, 3.5 inch slump, 37.5 gallons water. How much cement is needed? • Maximum W/C = .488 • 37.5 gal x 8.33 = 312 lb water • 312 lb water / .488 = 639 lb. cement

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