Question

Ex if std no: 1abcde, 161667 (a=6, b=1, c=6, d=6, e=7) In my case std 173009 Q1) Design a concrete mix by using ACI Method of Mix Design having 28-day characteristic mean strength of 2a MPa for a deck of a residential building at Nebraska. Dry weather and gradually warming temperatures may be expected throughout the working week. Fresh concrete composed of a slump of 90 mm and maximum aggregate size of 25 mm. Determine the amount of water reducing admixture to be used for 0.2b % and 0.c5 %. Deck is a flat surface capable of supporting weight, similar to a floor, but typically constructed outdoors, often elevated from the ground, and usually connected to a building. Decks that need piers or footings must have them installed below the frost line. This will keep the piers from heaving up out of the ground from the strength of the freeze. Reinforcing steel will help keep the piers from cracking. The freeze or frost line is the depth to which the soil freezes or has been known to freeze at some point in the past. It is absolutely critical that the foundations of buildings extend below this point in a soil profile. Except for a few arid locations, soils typically contain water and this water expands approximately nine percent in volume when it freezes. The frost line depth is not consistent around the nation because there is such a dramatic difference in climate from state to state. Deck is composed of 2.5 cm reinforcement and there is no need for piers. The material properties are as follows: For Coarse aggregate: SSD bulk specific gravity: 2.75, absorption capacity = 1.5 %, total moisture: 2.5 %, Dry-rodded unit weight: 15de kg/m3. For Fine aggregate: SSD bulk specific gravity: 2.65, absorption capacity = 1.0 %, surface moisture: 3 %, F.M: 2.6. Cement: Type V, Sp.Gravity: 3.23 Probability constant: 1.22 and standard deviation: 2 MPa.

Answer 1

Ans) Let the total volume of design concrete mix be 1 m3 .According to ACI 211.1-91,table 6.3.3 , for 90 mm slump and nominal aggregate size of 25 mm , amount of water required per cubic meter of concrete is 175 kg

=> Amount of water = 175 kg per m3 concrete

Air content for 25 mm aggregate with moderate exposure is 4.5%

Now, required compressive strength as per ACI 318 should be larger of :

fcr = f'c + 1.34S

or

fcr = f'c + 2.33S - 3.45

Standard deviation (S) = 2 MPa

=> fcr = 27 + 1.34(2) = 29.68 MPa

or

=> f'cr = 34 + 2.33(2) - 3.45 = 35.21 MPa

Hence, f'cr = 29.68 30 MPa

Now according to Table 6.3.4 (a), for compressive strength of 30 MPa, water cement ratio is 0.45

Hence, amount of cement = Amount of water / w-c ratio = 175/ 0.45 = 388.89 kg (Hence OK)

Now, according to table 6.3.6 for nominal aggregate size of 25 mm and fineness modulus of 2.6 , volume of coarse aggregate is 0.69 m3

=> Amount of coarse aggregate = dry rodded density x volume = 1502 x 0.69 = 1036.38 kg

Volume of fine aggregate = Total volume of concrete - Volume of all water, cement, coarse aggregate and air

=> Fine aggregate volume = 1 - [(175/1000) + (388.89 / 3.23 x 1000) + (1036.38/ 2.75 x 1000) + 0.045]

=> Fine aggregate volume = 1 - 0.716 = 0.284 m3

=> Amount of fine aggregate = volume x specific gravity x water density = 0.284 x 2.65 x 1000 = 752.6 kg

Now, since both aggregates has moisture and has absorption capacity , amount of mixing water needs to be corrected as shown :

Net Water absorbed by coarse aggregate = (0.015 - 0.025) x 1036.38 = -10.36 kg

Net Water absorbed by fine aggregate = (0.01 - 0.03) x 752.6 = - 15 kg

=> Actual amount of water to be added = 175 - 10.36 - 15 = 149.64 kg 150 kg

Hence, batch weight for trial mix of 1 cubic meter is as follows :

Now,
Admixture dose = 0.24% and 0.75% by weight of cement
Amount of cement = 388.89 kg
=> Amount of chemical admixture required = 0.0024 x 388.89 kg = 0.933 kg
=> Amount of chemical admixture required = 0.0075 x 388.89 kg = 2.92 kg