Question

Q2)What would be the maximum aggregate size used for the given mixture properties by using ACI Method of Mix Design. 28-day specified concrete strength is 12 Mpa, standard deviation is choosen as 3.7Mpa and it is required that no more than 1 test result in 200 will fall below the specified strength.

Project Title:Detoration of wood column of a commercial building

Project summary:Wood deterioration at the lower level of a heavy timber column, which supports five floor levels, was remedied by removing a section of column and the construction of a new concrete pier and footing.

For Coarse Aggregate (Uncrushed):SSD Bulk Specific Gravity: 2.60, Absorption Capacity: 1.8 %, Total Moisture: 2.3 % and Dry Rodded Unit Weight: 1430 kg/m³.

For Fine Aggregate (Crushed):SSD Bulk Specific Gravity: 2.70, Total Moisture: 3.4% and Free Moisture: 1.4%. FM= 2.65, Zone II.

Cement:Portland Composite Cement, Specific Gravity: 3.01. For 1,4 m³cement is calculated as 48 kg.

Q3) use the data in question 2 and determine the amount of chemical admixture required for 0.4%

Answer 1

Ans 2) According to ACI, maximum size of aggregate should bot be larger than :

1) 1/5th of the minimum dimension of structural members

2) 1/3rd thickness of slab

According to given consditions, assuming minimum thickness of floor slab as 100 mm. Therefore,

Maximum aggregate size = 100/3 in   37.5 mm

Hence, use 37.5 mm maximum size of aggregate

Ans 3) According to ACI 211.1-91 , table 6.3.1, for concrete used in wood column, slump required is 25 - 100 mm. According to table 6.3.3 , for 37.5 mm aggregate and 100 mm slump  , required water per cubic meter concrete is 181 kg .

According to ACI 318, required compressive strength (f'cr) should be larger of :

    f'cr = f'c + 1.34 s

or

f'cr = f'c + 2.33 s - 3.45

where, s = stanard deviation

=> f'cr = 12 + 1.34(3.7) = 17 MPa

or

=> f'cr = 12 + 2.33(3.7) - 3.45 = 17.2 MPa

Hence, required strength is 17.2 MPa

According to ACI 211.1-91 table 6.3.4(a), for required strength of 17.2 MPa, water cement ratio (w/c) = 0.75

=>Amount of cement required = 181 kg / 0.75 = 241.33 kg

Admixture dose = 0.4% per kg cement

=> Admixture required = 0.004 x 241.33 kg = 0.965 kg