Transpo problem of the day

[cantaloup]

You are assigned to provide a concrete mix for a small project, after doing some research you come up with the following trial mix:

"Cement : sand : 3/4-in aggregate" ratio is 1 : 2 : 2.5

w/c ratio is 0.45

air entrained for concrete is 5%

cement unit weight is 195 pcf

both sand and 3/4-in unit weight (SSD) is 165 pcf

Question: if you want to batch one cubic yard of concrete, how much water (gallons) you need to add?

A/ 35

B/ 38

C/ 42

D/ 45

 

[owillis28]

You are assigned to provide a concrete mix for a small project, after doing some research you come up with the following trial mix:
"Cement : sand : 3/4-in aggregate" ratio is 1 : 2 : 2.5

w/c ratio is 0.45

air entrained for concrete is 5%

cement unit weight is 195 pcf

both sand and 3/4-in unit weight (SSD) is 165 pcf

Question: if you want to batch one cubic yard of concrete, how much water (gallons) you need to add?

A/ 35

B/ 38

C/ 42

D/ 45

I got 36.57 gallons of water. My answer would be 'B'. Is this correct?

owillis

 

[cantaloup]

To find the amount of water needed for a cy, start with 100 lbs of cement first to see how much volume of concrete produced. Then find the volume of water needed for 1 cy of concrete by proportion

In terms of weight : 100 lbs of cement + 45 lbs of water + 450 lbs of sand & rock (weight of 5% air is negligible)

Materials in terms of volume (cf): 100/195 + 45/62.4 + 450/165 = 3.961 cf

Add air : 3.961 / 0.95 = 4.169 cf

Amt of water needed for 1 cy of concrete: (27/4.169)*45 = 291.43 lbs

Volume of water (gal) = 291.43*7.48/62.4 = 34.94 gal

A is correct