Civil's Guide

Reinforced Concrete

Concrete Constituents

Concrete is an engineering material which has been used since the time of the Romans and is comprised of cement,water and aggregate. Steel bars can be added to the concrete to enhance its flexural strength as the tensile capacity of concrete is assumed to be zero when design reinforced concrete.

Portland Cement

Portland cement is the most commonly used cement which originates from limestone and was developed in England in the 19th century. It is an energy intensive manufacturing process where limestone (calcium carbonate)with other materials is heated to 1450 degrees in a kiln, in a process known as calcination,to form calcium oxide (quicklime). This quicklime is then ground with gypsum into powder which is ordinary portland cement.

Water

Water is added to the cement which creates cement paste that coasts the aggregate and chemically reacts with cement (hydration). The volumne of water added affects the strength and workability and the lower the water-cement ratio, the stronger the mix but less workability. A balance is needed between the right mix of concrete but typical water-cement ratios are between 0.35 – 0.5.

Aggregates

Course aggregates are crushed gravel or stone, (particles greater than 5mm) and fine aggregates are sand, (particles less than 5mm). They make up the bulk of the concrete mixture (between 60% – 80%).

Reinforcement

Reinforced concrete is a composite material with steel bars added due to the weak tensile capacity of concrete. Reinforced concrete is generally designed to resist tensile stresses,cracking and failure of concrete members.

Material properties

  • Density of concrete: \(24 kN/m^2\)
  • Density of reinforced concrete: \(25 kN/m^2\)
  • Tensile strength: Weak but generally taken as 10% of compressive strength.
  • Compressive Strentgh: \(fcd = acc fck /gm = 0.85 fck /1.5 = 0.567 fck\)
  • Concrete classes are expressed as C20/25, C30/37, C35/45 in EC2 where the first number is the cylinder strength and the second number is the cube strength
  • Young’s modulus of elasticity: 30 GPa but depends on concrete grade
  • Shrinkage: The loss of moisture from concrete during different stages in it’s life results in the concrete section to shrink. The factors affecting shrinkage are water-cement ratio, environmental condition, time, type of aggregates and admixtures
  • Creep: It is the deflection over sustained loading and is a time-dependent process. The factors affecting creep are the type of aggregate used, the mix proportion of the concrete and age of the concrete when it is loaded.