PROPERTIES OF HARDENED CONCRETE

Properties

• The principal properties of hardened concrete are

Strength Shrinkage and Creep deformationsResponse to temperature variationPermeability and Durability

Strength of concrete (IS:516-1959)

• 1) Compressive Strength:Most common test on hardened concrete,

because easy to perform and properties of concrete are qualitatively related to compressive strength.

• Percentage strength of concrete at various ages:The strength of concrete increases with age. Table shows

the strength of concrete at different ages in comparison with the strength at 28 days after casting.

• Compressive strength of different grades of concrete:

Compressive strength

Compressive strength vs W/C Ratio

Compressive strength

Strength of concrete• 2) Flexural Test:

Concrete is strong in compression and weak in tension. Steel reinforcing bars are provided to resist all tensile

forces in concrete.However tensile stress are likely to develop in concrete

due to drying shrinkage, rusting of steel reinforcement and many other gradients.

Flexural Test

• Third point loading

Flexural test

• Principles of flexural Testing

Flexural Test

• Load transfer for Both types of loading

Flexural test• The flexural strength of specimen is expressed asmodulus of rupture.• The Flexural Strength or modulus of rupture (fb) is given by

fb = pl/bd2 (when a > 20.0cm for 15.0cm specimen or > 13.0cm for 10cm specimen)

• or fb = 3pa/bd2 (when a < 20.0cm but > 17.0 for 15.0cm specimen or < 13.3

cm but > 11.0cm for 10.0cm specimen.)• Where,• a = the distance between the line of fracture and the nearer support,

measured on the center line of the tensile side of the specimen• b = width of specimen (cm)• d = failure point depth (cm)• l = supported length (cm)• p = max. Load (kg)

Split Tensile strength

• This test is carried out by placing a cylindrical specimen horizontally between the loading surfaces of a compression testing machine and the load is applied until the failure of cylinder.

Split Tensile strength

• Positioning of cylinder

Split Tensile strength

• Formula :

Creep and Shrinkage• Creep is the time dependent increase in strain and

deformation due to an applied constant load - Reversible creep and irreversible creep

• Shrinkage is made up of plastic shrinkage and drying shrinkage – Plastic shrinkage occurs when the concrete is plastic and is

dependent on type of cement, w/c ratio, quantity and size of aggregates, mix consistency etc.

– Drying shrinkage occurs when water is lost from cement gel - Smaller than 1500 x 10-06 (strain)

Durability of Concrete• Durability of concrete is defined as its ability to resist weathering

action, Chemical Attack or any other process of deterioration.• Durability of concrete depends on

1. alkali aggregate reaction, 2. freeze-thaw degradation 3. sulphate attack Alkali-aggregate reaction :

Certain aggregates react with the alkali of Portland cement (released during hydration), in the presence of water, producing swelling - Form map-like cracks - Use low alkali cement to prevent this effect - Use of fly ash minimizes

Durability of Concrete Freeze-thaw process:

Water stored in voids of concrete expands as a result of freezing - Generates stresses that tend to crack the concrete after a number of cycles - Air entrainment improves resistance to freezing-thaw cracking

Sulphate attack: Sulphates in soil and seawater react with aluminates in cement to

produce compounds that increase in volume - Leads to cracking - Use low alumina cement - Fly ash reduces sulphate attack

Carbonation of concrete: Carbon-di-oxide from the air penetrates the concrete and reacts with

Ca(OH)2 to form carbonates; this increases shrinkage during drying (thus promoting crack development) and lowers the alkalinity of concrete, which leads to corrosion of steel reinforcement

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