ARCH 111 Strength of Materials I: Concrete

Concrete

• Invented by the Romans, concrete is the universal material of construction and the most widely used material on earth, aside from water.
• Concrete is a rock-like material produced by mixing coarse and fine aggregates, cementing material, water, and other materials (additives and admixtures), and allowing the mixture to harden.
• The properties of concrete are dependent on several factors, including the amount of cementing material, fine and coarse aggregates, water/cement ratio, temperature, and time of mixing.
• Concrete is strong in compression and weak in tension, and is also brittle.
• Concrete shrinks slightly during the hardening process due to excess water evaporation.

Characteristics of Concrete

• Only major structural material commonly manufactured on site.
• Does not burn.
• Does not rot.
• Has no form of its own.
• Has no useful tensile strength.
• One main disadvantage is that members once cast cannot be easily dismantled.

Cement

• Different types of cement are required for specific structures.
• Some cement types have high resistance to corrosion, while others harden faster than others.
• Common types of cement used for construction include:
  • PPC (preferred for mass concreting works that do not require high early strength)
  • White Cement (used for decorative works, flooring, and ornamental concrete products)
  • Coloured Cement (produced by adding ground pigments to white cement or OPC, used for designing coloured floors)
  • Sulphate Resisting Cement (used for structures that will be in contact with water with high sulphate concentrations)

Aggregates

• Aggregates make up roughly three-quarters of the volume of concrete.
• The strength of a concrete is heavily dependent on the quality of the aggregates.
• Aggregates must be:
  • Clean
  • Strong
  • Resistant to freeze-thaw deterioration
  • Chemically stable
  • Properly graded for size distribution
• Size distribution of aggregate particles is important for achieving close packing of the particles.

Fine Aggregates

• Fine aggregates are basically sand or crushed stones.
• The size of fine particles is not bigger than 5 mm.

Reinforcing Materials

• Reinforcement can be:
  • Mild steel bars
  • High yield steel
  • Steel fibres
  • Bamboo reinforcement
• Steel is generally used because of its easy availability, high strength, and good bond with concrete.
• Steel reinforcing bars (rebar) are round in cross-section and deformed with surface ribs for better bonding to concrete.

Steel Reinforcement

• Different types of steel bars are available, including hot rolled bars, cold twisted bars, and hard drawn steel wire fabrics.
• Steel reinforcing bars typically have diameters varying from 5 to 50 mm.

Bamboo Reinforcement

• Bamboo reinforced concrete construction follows the same design, mix proportions, and construction techniques as used for steel reinforced concrete.
• Bamboo has high tensile strength, making it a suitable reinforcing material.
• The structure of bamboo from its origin gives it high resistance against wind forces.

Properties of Fresh and Hardened Concrete

• Fresh Concrete:
  • Properties are important because they influence the quality of the hardened concrete.
  • Temperature of fresh concrete should not be less than 5°C in cold weather conditions or exceed 30°C in hot weather conditions.
  • Air content is typically around 1-2% of the volume of fresh concrete.
• Hardened Concrete:
  • Compressive strength is a major determinant in deciding the particular use of concrete.
  • Compressive strength is typically measured after 28 days.
  • Exposure conditions and durability affect the choice of mix design and the grade of concrete.