You are planning to place a concrete floor slab for an exterior parking garage.

Question

You are planning to place a concrete floor slab for an exterior parking garage. The slab will be subject to freeze/thaw and de-icing in the winter. Design the concrete mix.

Design Criteria:

              Design strength = 3,000 psi Minimum space between rebar 4 in.

              Slab thickness 4.5 in.                  Minimum cover over rebar 1.5 in.

Standard deviation of fc’ obtained from 20 samples = 200 psi

Available Materials:

Cement specific gravity = 3.15

Coarse aggregate: crushed (angular) gravel

Bulk oven-dry specific gravity = 2.55

Absorption = 3.6%

Oven-dry rodded density = 110 lb/ft3 Fine aggregate: natural sand

Moisture content = 2.5 %

Bulk oven-dry specific gravity = 2.66

Absorption = 0.5%

              Moisture content = 2%                      

Fineness modulus = 2.47

Available Materials:

Cement specific gravity = 3.15

Coarse aggregate: crushed (angular) gravel

Bulk oven-dry specific gravity = 2.55

Absorption = 3.6%

Oven-dry rodded density = 110 lb/ft3 Fine aggregate: natural sand

Moisture content = 2.5 %

Bulk oven-dry specific gravity = 2.66

Absorption = 0.5%

              Moisture content = 2%                      

Fineness modulus = 2.47

Explanation / Answer

Step 1: selection of water cement ratio:

Minimum water cement ratio for mild exposure condition = 0.55. Based on experience adopt water cement ratio as 0.5

0.5 less than 0.55, hence OK

step 2: selection of water content:

Maximum water content for nominal maximum size of aggregate 20mm = 186kg

Step 3- selection of cement content:

From table 5 of IS 456,

Minimum cement content = 300 kg/m3

step 4 - coarse aggregate proportion:

for every change in w/c , the coarse aggregate proportion is to be changed by 0.01. If w/c is less than 0.5 ( standard value) volume of coarse aggregate is required to be increased to reduce the fine aggregate content

From table 3 of IS 10262-2009, volume of coarse aggregate per unit volume of total aggregate = 0.62

For pump able concrete or congested reinforcement the coarse aggregate proportion may be reduced upto 10%

Hence,

Volume of coarse aggregate per unit volume of total aggregate = 0.62×90% = 0.558

Volume of fine aggregate = 1 - 0.558 = 0.442

step 5 - estimation of the mix ingredients :

a) volume of concrete = 1 m3

b) volume of cemwnt = ( mass of cement/ specific gravity of cement) × (1/100) =( 300/3.15) × (1/100)

= 0.9323m3

c) volume of water = ( mass of water / specific gravity of water ) x (1/1000) = (186/1)×(1/1000)

= 0.186m3

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