Mix Design Basics CIVL 3137 1 Mix Design Goals adequate - PowerPoint PPT Presentation

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