Building Material (Concrete)

 👉 Classification of Concrete

 (a) Based on Bulk density

     Extra light weight ⟶ < 500 kg/m3

     Light weight ⟶ 500 - 1800 kg/m3

     Dense weight ⟶ 1800-2500 kg/m3

    Super heavy weight ⟶ > 2500 kg/m3

(b) Based on Strength

   Low strength concrete ⟶ < 20 N/mm2

   Medium strength concrete ⟶ 20-40 N/mm2

   High strength concrete ⟶ > 40 N/mm2

👉 Curing Temperature of Concrete : 5 to 28 degree centigrade

👉Maturity of Concrete   

• The strength of concrete depends on both period of curing (i.e age) and temperature during curing.
• Maturity of Concrete = (Period x temperature)
• It is measured in °C hours or °C days.
• The maturity of concrete is defined as the summation of product time and temperature.
• Maturity = Σ (time x temperature)

👉Strength of Concrete 

(a) Compressive Strength Test

• Test Specimen : 150 x 150 x 150 mm cubes 

                                  : Cylinder of 150 mm diameter and 300 mm height

• Test specimens are stored at a temperature of 27 ± 3 °C at 90 % humidity for 24 ± 1/2 hour from the time of addition of water to the dry ingredients.
• 7 days strength of concrete should be at least 2/3 of 28 day strength of concrete.
• Average of the three values is taken as the compressive strength of concrete, provided the individual variation is not more than ± 15 % of the average.
• Cube Strength = 1.25 x cylinder strength

(b) Flexural Tensile Strength Test ( Modulus of Rupture Test)

• Direct measurement of tensile strength is difficult.
• Indirect test for assessing the tensile strength of concrete.
• Concrete is filled in the mould size 150 x 150 x 700 mm

Modulus of Rupture = p.l / bd2  when a > 200 mm

                                = 3pa / bd2   when 200 mm > a > 170 mm

Here,

    a = distance between the line of fracture and the nearest support, measured on the centre line of the tensile side of the specimen (cm)

b and d is measured width and depth of specimen respectively

l = length of the span on which the specimen is supported (cm)

p = maximum load applied to the specimen

(c) Split Tensile Strength Test

• Standard test cylinder of concrete specimen of 300 mm x 150 mm diameter is placed horizontally between the loading surfaces of compression testing machine

 

                   Split Tensile Strength (σ ) = 2P / πDL

Where, P = Applied load

            D = Diameter of the cylinder

            L = Length of the cylinder

👉 Cube Strength > Cylinder Strength > Modulus of Rupture > Split Tensile Strength

👉 Generally tensile strength of concrete is 10 % of its compressive strength

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