Question

Perform an ACI mix design using the following data (make assumptions as needed)

The 28-day compressive strength should be 4,000 psi. The concrete is to be used under exterior condition subject to deicer.

The slump should be between 3 and 4 in. and the maximum aggregate size should not exceed 1 in.

The coarse and fine aggregates in the storage bins are wet.

The properties of the materials are as follows:

Cement- Type I, specific gravity = 3.15

Coarse Aggregate:

Bulk specific gravity (SSD) = 2.70; absorption capacity = 1.1%; dry-rodded unit weight = 105 lb./ft.³

Moisture content = 2.1%

Fine Aggregate:

Bulk specific gravity (SSD) = 2.67; absorption capacity = 1.3%; fineness modulus = 2.70;

Moisture content = 2.8%

Answer 1

Ans) Let the total volume of trial mix be 1 cubic yard (27 )  then according to ACI 211.1-91 , table 6.3.3 , for 1 in aggregate and 3 to 4 in slump , amount of water required per cubic yard of concrete is 295 lb/cy

=> Water requied = 295 lb/cy

Since, concrete is exposed to deicing chemicals, class of exposure is moderate so according to ACI 211.1-91, table 6.3.3 ,for moderate exposure and 1 in aggregate, volume of air = 4.5%

Cement content :

According tom ACI 318, required compressive strength (f'cr) when data from previous mix is not avilable can be given as :

f'cr = f'c + 1200

=> f'cr = 4000 + 1200

=> f'cr = 5200 psi

According to ACI 211.1-91, table 6.3.4(a) , for required compressive strength of 5200 psi, required w/c ratio is 0.38

Also, Amount of cement = Amount of water / w-c ratio

=> Amount of cement required = 295 lb / 0.38 = 776.31 lb

Volume of cement = Weight / (Specific gravity x 62.4) = 776.31 / (3.15 x 62.4) = 3.95 cu.ft/cy concrete

Calculation of Coarse aggregate :

Now, according to ACI 211.1-91 table 6.3.6 , for nominal aggregate size of 1 in and fineness modulus of 2.70, volume of coarse aggregate is 0.68 cy or 18.36 ft3

=> Amount of coarse aggregate = Dry rodded density x volume

=> Amount of coarse aggregates = 105 x 18.36 = 1927.80 lb

Calculation of Fine aggregate :

According to absolute volume method,

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

=> Fine aggregate volume = 27 - [(295/62.4) + 3.95 + (1927.8/ 2.7 x 62.4) + 0.045(27)]

=> Fine aggregate volume = 27 - 21.33 = 5.67 ft3

=> Amount of fine aggregate = Volume x Specific gravity x Water unit weight = 5.67 x 2.67 x 62.4 = 944.66 lb

Correction in mixing water :

Now, since both aggregates has moisture and absorption capacity, so they will absorb some water as well as provide some water to concrete so in order to maintain constant water cement ratio, amount of mixing water needs to be corrected

Water provided by coarse aggregate = (0.021 - 0.011) x 1927.8 lb = 19.28 lb

Water provided by fine aggregate = (0.028- 0.013) x 944.66 lb = 14.17 lb

=> Actual amount of water to be added = 295 lb - 19.28 - 14.17 = 261.55 lb

Hence, mix design for 1 cy batch is as follows :

Component	Amount (lb)
Cement	776.31
Water	261.55
Coarse aggregate	1927.8
Fine aggreate	944.66