Lecture4 ACI MIX DESIGN and Steps in.pdf
- 2. OUTLINES VOLUME OF FRESH CONCRETE CONCRETE MIX DESIGN 2
- 3. 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. VCon = Va + Vw + Vc + Vfa + Vca Where: Vcon = Volume of the fresh concrete 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 3
- 4. MIX DESIGN Mix design is the selection of mix ingredients and their proportions. The purpose of a concrete mix design is to have economical mix proportions for the available concreting materials which has adequate workability to be placed in its final position on site. 4
- 5. 5 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
- 6. Information required for mix design Sieve analysis of fine and coarse aggregates. Unit weight of coarse aggregate. Specific gravities of fine, coarse aggregates and Portland cement and other cementations materials, if used. Water absorption value of aggregates. Moisture content of aggregates. 6
- 7. ACI Standard Mix Design Method The standard ACI mix design procedure can be divided into 8 basic steps: 1. Choice of slump 2. Maximum aggregate size selection 3. Mixing water and air content selection 4. Water-cement ratio 5. Cement content 6. Coarse aggregate content 7. Fine aggregate content 8. Adjustments for aggregate moisture 7
- 8. Step #1: Select Slump 8
- 9. 9 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 Aggregate larger than these dimensions may be difficult to consolidate and compact resulting in a honeycombed structure or large air pockets. Step #2: Max. Agg. Size Check
- 10. Step #3: Determine Mixing Water and Air Content 10
- 11. 11 Step #4: Select W/C Ratio
- 12. 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 12
- 13. 13 Step #6: Coarse Agg. Content
- 14. Step #7: Fine Agg. Content 14
- 15. 15 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.
- 16. 16 Mix Design Example: 25cm Thick Unreinforced Pavement Slab
- 17. Properties of Footing Concrete Specified By Engineer: Slump = 2.5cm 28-day strength of 34.5MPa Air content: 4.5 - 6.5 percent 17
- 18. 18 Information About Materials: Coarse aggregate : nominal maximum size = 37.5mm dry-rodded weight = 1600 kg/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
- 19. 19 Step #1: Select Slump Engineer Specified 2.5cm (correlates w/table)
- 20. 20 ACI Limits: 1/3 of the slab depth 250mm/3 =83.33mm > 37.5mm OK Step #2: Max. Agg. Size Check
- 21. 21 Step #3: Determine Mixing Water and Air Content 37.5mm Stone 2.5 cm Slump
- 22. 22 Weight of Water = 148 kg/m3 Volume of Water =148 kg/m3 = 0.148m3 1000 kg/m3 Volume of Water = 148liters per cubic meter of concrete
- 23. 23 Step #4: Select W/C Ratio
- 24. Step #5: Cement Content 24 W/C= Wt. of Water Wt. of Cement Wt. of Cement = 148 kg/m3 0.40 =370kg/m3 Volume of Cement = 370 kg/m3 (Concrete) 3.15 x 1000kg/m3 Volume of Cement = 0.117m3 per cubic meter of concrete
- 25. 25 Step #6: Coarse Agg. Content
- 26. Weight (Dry) =.71 x 1600 kg/m3 = 1,136 kg Volume = 1,136 kg = 0.42 m3 2.68 x 1000kg/m3 26 Dry Rodded Unit Wt of Stone SG Stone
- 27. 27 Step #7: Fine Agg. Content 1m3 Cubic meter of Concrete 0.148m3 Water 0.055m3 Air 0.117m3 Cement 0.42m3 Stone 0.26m3 Sand Wt of Sand(Dry) = 0.26m3 x 2.64 x 1000kg/m3 = 686.4 kg. SG Sand
- 28. 28 Step #8: Aggregate Batch Weights & Water Adjustment Wt of Stone(Wet) = 1,136 kg x 1.01= 1,147.4kg Wt of Sand(Wet) =686.4 kgx 1.05= 720.7kg 1% Moisture 5% Moisture
- 29. 29 Mixing water needs to be adjusted. Both the coarse and fine aggregate are wet and will contribute water to the cement paste. Water from Stone = 1,136 kg x (.01-.005) = 5.68kg Water from Sand= 686.4 kg x (.05-.007) = 29.52kg Water = 148kg – 5.68kg– 29.52kg= 112.8kg Moisture Dry Wt. Absorption Dry Wt. Moisture Absorption
- 30. 30 Final Batch Wts. (1 Cubic meter) Ingredient Weight Water 112.8kg Cement 370kg Stone(agg.) 1,147.4kg Sand 720.7kg
- 31. 31 Thank you!