Concrete is required for interior reinforced building columns 12 in by 12 in., 1

Question

Concrete is required for interior reinforced building columns 12 in by 12 in., 10 ft high. It is to test 4000 psi at 28 days and is to have 3 to 4 in. slump, with 5 (+or- 1)% air. The relative density of the course aggregate is 2.65, its dry-rodded unit weight is 106 lb per cubic foot, and its maximum size is 1 in. The relative density of the fine aggregate is 2.78 and its fineness modulus is 2.60.

A. Design a trial mix, using 30lb cement.

B. If there are 24 columns, what quantity of cement is required to do the job?

Explanation / Answer

determine mean strength fm

fmin = 4000 psi = 28 MPa

assume standard deviation s = 4 MPa

fm = fmin + ks

    = 28 + 1.64x4

   = 34.56 MPa

2) refer table 11.5 ACI 211.1-91 to estimate w/c ratio

from table 11.5, for 35 MPa, air entrained concrete, w/c ratio is 0.4

3) refer table 11.8 ACI 211.1-91 to determine water content

for a slump of 3inch to 4 inch and size of 25mm air entrained, water content required is 175kg/m3

4) refer table 11.4 of ACI 211.1-91 to determine dry bulk volume of coarse aggregate

from table 11.4 for 25mm size of aggreagate, and fineness modulus 2.6, the dry rodded bulk volume of coarse aggregate is 0.69 per unit volume of concrete

dry rodded bulk unit weight of coarse aggregate = 106 lb/ft3 = 1698 kg/m3

therefore weight of coarse aggregate = 0.69 x 1698

                                                            = 1172 kg/m3

5) refer table 11.9 ACI 211.1-91 to calculate density of fresh concrete

for 25mm size of aggregate, air entrained concrete, the density of fresh concrete is 2315 kg/m3

6) weight of water content = 175 kg/m3

weight of cement = 30 lb = 453 kgs

weight of coarse aggregate = 1172 kg/m3

weight of fine aggregate = 2315 - (175+453+1172)

                                       = 515 kg/m3

density of concrete =175 + 453 + 1172 + 515

                               = 2315 kg/m3

B. For one column cement required is 453 kgs

for 24 columns cement required is = 24 x 453 = 10,872 kgs

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