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Water - Cement Ratio

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Water/Cement Ratio

- The number of pounds of water per pound of cement.
- A low ratio means higher strengths, a high ratio means lower strengths.
- For NCDOT, the ratio depends on the class of concrete, whether an air agent is used or not, and the shape of the stone - rounded or angular.

W/C Ratio Cont.

- Example:
W/C = 0.500, and Water = 250 pounds

How much cement is needed?

250 / 0.500 = 500 pounds of ``````````````````````cement

W/C Ratio Cont.

- Example:
W/C = 0.500, and Cement = 600 pounds

How much water is needed?

0.500 X 600 = 300 pounds of water

300 pounds / 8.33 = 36.0 gallons

Water/Cementitious Problem

- Cement Used in Mix – 436 pounds
- Fly Ash in Mix – 131 pounds
- Maximum Water – 36.0 gallons
- Total Water – 33.5 gallons
- Metered Water – 27.5 gallons
- Free Water in aggregates – 50 pounds
- Determine the design w/c ratio and the batched w/c ratio

SOLUTION: Design W/C

Add Cement And Fly Ash:

436 + 131 = 567 pounds

Convert Design Water Into Pounds:

33.5 X 8.33 = 279 pounds

Plug Into Formula W/C = Ratio:

279 / 567 = 0.492

(carry answer to three places after decimal)

Batched W/C Ratio

Add Cement And Fly Ash:

436 + 131 = 567 pounds

Convert Metered Water Into Pounds:

27.5 X 8.33 = 229 pounds

Add free water 229 + 50 = 279 Lbs

279 / 567 = 0.492

W/C Ratio with Ice

- Determine the W/C ratio if 68 pounds of ice is used to lower the temperature of the concrete.
- The W/C ratio remains the same because the quantity of total water does not change.

% Solids And Voids

- In determining mix designs, you must use an aggregate dry rodded unit weight.
- This weight is determined at the lab.
- In the procedure for determining this weight, only the coarse aggregate is used.
- Therefore, there is a % of solids and a % of voids in the container.

Formula : % Solids & Voids

% Solids:

Dry Rodded Unit Weight

(Spec. Gravity) X (62.4)

The Answer Is Then Multiplied Times 100

To get % Voids:

Subtract % Solids from 100

Example:

Dry Rodded Unit Weight:96.6 pcf

Specific Gravity Of Agg.:2.80

% Solids = 96.6 = 0.553 (2.80 X 62.4)

0.533 X 100 = 55%

% Voids = 100 - 55 = 45%

Terms I Should Know….

- Abrasion Resistance of an Aggregate
- Durability
- Hydration
- Ph
- Saturated Surface Dry
- Set Retarder
- Unit Weight
- Water / Cement Ratio
- THAT IS ENOUGH FOR A MONDAY!!

PROBLEM SOLUTION

1.Water: 209 + 15 = 224 gals

224 X 8.33 = 1866 pounds

- Add all material:
4060+7733 +13,586 +1866 = 27,245

- Divide by unit weight:
27,245 = 7.1 cu. yd.

(142.10 X 27)

PROBLEM SOLUTION

2. Water / Cement = Ratio

1866 / 4060 = 0.460

3. (A) % Solid

88.6 = 0.508 X 100 = 51%

(2.79 X 62.4)

(B) % Void = 100 – 51 = 49%

PROBLEM SOLUTION

4. Wet Sand:

5.9 - 0.5 = 5.4%5.4 / 100 = 0.054

0.054 X 1102 = 59.5 pounds

1102 + 60 = 1162 pounds (batch weight)

Dry Sand:

0.5 / 100 = 0.005

0.005 X 1102 = 5.5 pounds

1102 - 6.0 = 1096 pounds

PROBLEM SOLUTION

- Gallons of Water from Wet Sand
- 1720 – 1620 = 100 pounds / 8.33 = 12.0 gallons