Question

Design the concrete mix according to the following conditions: Design Environment Caissons for Bridge pier crossing to the Outer Bankers, NC Required design strength=4800psi

Minimum dimension = 12 in.

Minimum space between rebars = 2.5 in.

Minimum cover over rebars = 2.5 in.

Statistical data indicate a standard deviation of compressive strength of 250 psi is expected (more than 30 samples).

Only air entrained is allowed. Available Materials Air entrainer: Manufacture specification is 0.15 fl oz/1% air/100 lb cement.

Aggregate: 1 in. nominal maximum size, crushed stone

Bulk oven-dry specific gravity=2.45,absorption=2.5%

Oven-dry rodded density=125pcf

Moisture content=1.5%

Fine aggregate: Natural sand

Bulk oven-dry specific gravity=2.61,absorption=3.4%

Moisture content=3.8%

Fineness modulus=2.81

1. Calculate the strength requirements.(psi)

2. Part 2: Determine the water to cement ratio.

3: Calculate the volume of coarse aggregate (lb/yd^3)

4: Find the air content percentage (%)

5: Determine the workability.

Answer 1

Ans 1) According to ACI 318, required compressive strength (f'cr),

f'cr = f'c + 1200

where, f'c = specified compressive strength = 4800 psi

=> f'cr = 4800 + 1200

=> f'cr = 6000 psi

Hence, required strength is 6000 psi

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Ans 2) According to PCA manual , table 9-3, water cement ratio (w/c) corresponding to 6000 psi and air entraing concrete = 0.32

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Ans 3) To determine volume of coarse aggregate first calculate nominal maximum aggregate size (NMAS)using ACI 318-19 section 26.4.2.1 . The NMAS should be larger of :

a) one fifth of minimum dimension = 12 /5 = 2.4 in

b) three fourth of spacing of rebar = 2.5 x 0.75 = 1.875 in

Hence, NMAS = 2.4 in , so choose 2 in as rounded off size

According to PCA manual, table 9-4, for NMAS = 2 in and fineness modulus = 2.81, volume of coarse aggregate = 0.74 cy or 19.98 ft3

Hence, amount of coarse aggregate = Dry rodded density x Volume = 125 x 19.98 = 2497.5 lb/cy

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Ans 4) According to PCA manual, table 9-5, for NMAS = 2 in and moderate exposure, air content = 3.5%