Concrete Technology

Concrete Technology

Concrete is a composite material composed of coarse aggregate bonded together with a fluid cement that hardens over time.

Properties of Fresh Concrete

Consistency

• Slump Test (ASTM C143): Measures consistency.
• Vebe Test: Measures workability of very dry mixes.
• Compacting Factor Test: Measures workability of low slump mixes.

Segregation and Bleeding

• Segregation: Separation of coarse aggregate from the mortar.
• Bleeding: The appearance of water on the surface of concrete after it has been placed and compacted but before it has set.

Properties of Hardened Concrete

Compressive Strength ($f'_c$)

• The most important property of concrete.
• Measured by testing standard cylinders (150 mm x 300 mm) or cubes (150 mm).
• Typical values range from 20 MPa to over 100 MPa.

Tensile Strength ($f_t$)

• Much lower than compressive strength (about 10%).
• Measured by split cylinder test (ASTM C496) or flexural test (modulus of rupture, ASTM C78).

$$f_r = K \sqrt{f'_c}$$

Where $K$ is a constant (typically 0.62 for MPa).

Elasticity and Creep

• Modulus of Elasticity ($E_c$):

$$E_c = 4700 \sqrt{f'_c} \quad (\text{MPa})$$

• Creep: Time-dependent deformation under sustained load. Can be 2-3 times the elastic strain.
• Shrinkage: Decrease in volume due to loss of moisture (Drying Shrinkage).

Mix Design (ACI 211.1 Method)

The process of selecting suitable ingredients and determining their relative quantities to produce concrete with the required properties.

Testing

• Compressive Strength: ASTM C39.
• Flexural Strength: ASTM C78 (Third-point loading).
• Split Tensile Strength: ASTM C496.