Mix Design Basics CIVL 3137 1
Mix Design Goals adequate workability adequate strength adequate durability minimum cost CIVL 3137 2
Cost of Materials crushed stone = $ 12/ton concrete sand = $ 9/ton Type I cement = $126/ton Minimum Cost = Minimum Cement CIVL 3137 3
Minimizing Cost Cement minimize the void space between aggregate particles that must be filled with cement paste minimize the surface area of the aggregate particles that must be coated with cement paste CIVL 3137 6
Minimizing Void Space Void content = 48% Void content = 41% Use a gravel-sand blend with a dense gradation to minimize the void content of the aggregate CIVL 3137 7
Minimizing Surface Area 10 " surface area = 11 ft 2 surface area = 22 ft 2 Use the largest NMAS you are allowed to in order to minimize the surface area per cubic yard of concrete CIVL 3137 8
Minimizing Surface Area surface area = 6.0 ft 2 /ft 3 surface area = 4.8 ft 2 /ft 3 Use gravel instead of crushed stone if possible because it has a lower surface area per unit volume occupied CIVL 3137 9
Obtaining Adequate Workability To obtain good workability, you need enough mortar to fill the voids between the gravel particles, enough cement paste to fill the voids between sand particles, and enough water to both hydrate and lubricate the cement particles. The main goal of the ACI mix design method is to get the relative volume proportions of the ingredients right in order to ensured good workability. CIVL 3137 11
Obtaining Adequate Workability gravel Need enough mortar to keep all the gravel particles apart. mortar CIVL 3137 12
Obtaining Adequate Workability sand gravel cement paste mortar CIVL 3137 13
Obtaining Adequate Workability sand Need enough cement paste to keep all the sand grains apart cement paste CIVL 3137 14
Obtaining Adequate Workability sand cement water cement paste CIVL 3137 15
Obtaining Adequate Workability cement Need enough mixing water to lubricate all the cement grains water CIVL 3137 16
Obtaining Adequate Workability cement Air entrainment adds lubrication without adding additional water air bubble water CIVL 3137 17
Obtaining Adequate Strength CIVL 3137 18
Water-Cement Ratio Cement Water 0% Hydration Hydration Products Water Air 100% Hydration CIVL 3137 19
CIVL 3137 21
Obtaining Adequate Strength If the structural engineer designs a beam based on a concrete strength of 4500 psi, you have to design your concrete mix to have a strength much higher than that. WHY? CIVL 3137 22
4500psi f CIVL 3137 23 c
4500psi f CIVL 3137 24 c
4500psi 5700psi f f CIVL 3137 25 c cr
Overdesign Factors Required Average Compressive Strength When Data Are Not Available to Establish a Standard Deviation Adapted from ASTM C94 CIVL 3137 28
ACI Mix Design CIVL 3137 29
Mix Design Example Coarse aggregate = subangular crushed stone CIVL 3137 30
Step 1: Select the slump CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 31
Step 2: Select the NMAS narrowest dimension NMAS 5 depth of slab NMAS 3 NMAS 0.75 clear space CIVL 3137 32
Step 3: Estimate the water and air CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 33
Questions to Ponder 1. Why does the amount of water required to obtain a desired slump decrease with increasing NMAS? CIVL 3137 34
Effect of NMAS on Paste Volume 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 35
Effect of NMAS on Paste Volume 10 " surface area = 11 ft 2 surface area = 22 ft 2 CIVL 3137 36
Questions to Ponder 2. Why does the amount of entrapped air in a concrete mix decrease with increasing NMAS? CIVL 3137 37
Effect of NMAS on Paste Volume 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 38
Questions to Ponder 3. Why does the target air content in an air-entrained mix decrease with increasing NMAS? CIVL 3137 39
Effect of NMAS on Paste Volume 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 40
Air Content Paste Air Content 40% Assume 7% Cement Paste Concrete Air Content 0.4 7% = 2.8% 60% Smaller Aggregate NMAS CIVL 3137 41
Air Content 30% Paste Air Content Cement Assume 7% Paste Concrete Air Content 70% Larger 0.3 7% = 2.1% Aggregate NMAS CIVL 3137 42
Step 3: Estimate the water and air CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 43
Step 4: Adjust for Aggregate Shape Aggregate Water Reduction Shape (pounds per cubic yard) Crushed stone (angular) 0 Crushed stone (subangular) 20 Gravel (some crushed) 35 Gravel (well rounded) 45 CIVL 3137 45
Questions to Ponder 4. Why does the water required to obtain a given slump change as a function of aggregate shape? Aggregate Water Reduction Shape (pounds per cubic yard) Crushed stone (angular) 0 Crushed stone (subangular) 20 Gravel (some crushed) 35 Gravel (well rounded) 45 CIVL 3137 46
Minimizing Surface Area surface area = 6.0 ft 2 /ft 3 surface area = 4.8 ft 2 /ft 3 CIVL 3137 47
Mix Design Example Coarse aggregate = subangular crushed stone CIVL 3137 48
Step 4: Adjust for Aggregate Shape Aggregate Water Reduction Shape (pounds per cubic yard) Crushed stone (angular) 0 Crushed stone (subangular) 20 Gravel (some crushed) 35 Gravel (well rounded) 45 CIVL 3137 49
Step 5: Select the w/cm ratio f cr CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 50
Overdesign Factors Required Average Compressive Strength When Data Are Not Available to Establish a Standard Deviation Adapted from ASTM C94 CIVL 3137 52
Step 5: Select the w/c ratio f cr CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 53
Questions to Ponder 5. Why is the w/cm ratio different for air-entrained concrete compared to non-air-entrained concrete? CIVL 3137 56
Effect of Air Content on Strength CIVL 3137 57
Step 6: Calculate the cement weight W water W = w/c ratio cement CIVL 3137 58
Step 7: Estimate coarse aggregate bb o CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 59
What does b/b o represent? b o b 1 Ratio of bulk aggregate volume (b) to bulk concrete volume (b o ) CIVL 3137 61
Mix Design Example Coarse aggregate = subangular crushed stone CIVL 3137 62
Step 7: Estimate coarse aggregate bb o CIVL 3137 Source: Design and Control of Concrete Mixtures (PCA, 2003) 63
Step 7: Estimate coarse aggregate bulk bulk V b b V gravel o concrete dry-rodded bulk bulk W V γ gravel gravel gravel unit weight bulk bulk W b b V γ gravel o concrete gravel CIVL 3137 64
Step 8: Estimate fine aggregate Estimated Weight Method First Estimate of Concrete Unit Mass (lb/ft 3 ) NMAS (in) Non-Air-Entrained Concrete Air-Entrained Concrete ⅜ 142.0 137.5 ½ 144.0 139.0 ¾ 146.5 141.5 1 148.5 143.5 1½ 151.0 146.0 2 153.0 147.5 3 155.5 150.0 6 157.5 152.0 CIVL 3137 65
Questions to Ponder 6. Why does the unit weight rise with increasing NMAS? First Estimate of Concrete Unit Mass (lb/ft 3 ) NMAS (in) Non-Air-Entrained Concrete Air-Entrained Concrete ⅜ 142.0 137.5 ½ 144.0 139.0 ¾ 146.5 141.5 1 148.5 143.5 1½ 151.0 146.0 2 153.0 147.5 3 155.5 150.0 6 157.5 152.0 CIVL 3137 66
Effect of NMAS on Unit Weight 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 67
Effect of NMAS on Unit Weight V V V paste water cement W W W paste water cement RD RD RD paste w water w cement w W W W W water cement water cement RD RD RD paste water cement CIVL 3137 68
Effect of NMAS on Paste Volume Assume w/c = 0.5 1.5 W 0.5 W 1.0 W cement cement cement RD 1.00 3.15 paste RD 1.83 paste RD 2.65 typical aggregate CIVL 3137 69
Effect of NMAS on Unit Weight 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 70
Questions to Ponder 7. Why is the ratio of non-air-entrained density to air-entrained density a function of NMAS? First Estimate of Concrete Unit Mass (lb/ft 3 ) NMAS (in) Non-Air-Entrained Concrete Air-Entrained Concrete ⅜ 142.0 137.5 ½ 144.0 139.0 ¾ 146.5 141.5 1 148.5 143.5 1½ 151.0 146.0 2 153.0 147.5 3 155.5 150.0 6 157.5 152.0 CIVL 3137 71
Effect of NMAS on Unit Weight 30% 40% Cement Cement Paste Paste 70% Larger 60% Aggregate Smaller NMAS Aggregate NMAS CIVL 3137 72
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