Reinforcing Bar

The theory behind concrete reinforcement is simple: concrete is a strong building material when compressed, but shearing forces or extreme tension can cause it to crack or buckle. The answer to these problems is to provide concrete with supplementary support in the form of a metal “skeleton” which sits inside the concrete. Such a skeleton is made of “reinforcing bar,” or rebar for short.

To illustrate the IL Centre’s architectural structure, several sections of concrete around the building have been drilled out to reveal the internal reinforcements contained within the structure. Different kinds and makes of rebar are visible in the shear wall, in a section of concrete slab, and in a support column.

Rebar in Columns

 A column being formed in layers.A column being formed.Concrete is strong when compressed, which makes it perfect for columns, since they’re meant to support loads pushing down from above. However, columns also need to be somewhat elastic, so that they can regain their height after being compressed by “live loads” like people, and regain their position if they are turned or twisted by dynamic loads like wind or earthquake. (Wind loads are not as important for shorter buildings like the IL Centre, but tall buildings oscillate, so their dynamic load changes constantly and dramatically).

Although long strips of rebar are used, concrete columns are poured in smaller sections. Thinner rebar is tied in circles around the vertical bars, to hold the long bars in place. If this isn’t done, the rebar could be be pushed outwards by compression forces when the column is finished. The column is then wrapped in sheeting and surrounded by wooden boxing, and the concrete is poured in.

The IL Centre has a special cut-out column demonstrating the rebar’s placement within a typical column inside the building. Also demonstrated is a column footing.

Materials

Civil engineers can choose from several different kinds of rebar when designing a concrete structure. Which kind is appropriate depends on the conditions that the reinforced concrete will be exposed to. Some rebar is too expensive to use in most applications: stainless steel rebar in non-magnetic alloy is used in MRI rooms, for example, because stainless steel does not contain iron. However, its price is much higher than even epoxy-coated rebar. Black rebar is used in almost all standard buildings, except in marine environments.

  • Black Steel Rebar
    • Cheapest kind of rebar
    • Most common rebar in buildings today
    • Excellent where there is no moisture
    • Quite susceptible to rust
  • Epoxy Coated Rebar
    • More expensive than black steel rebar
    • It has difficulty bonding to concrete
    • Far less susceptible to rust
    • Easily damaged, which could isolate and magnify any corrosion
    • Best used in marine environments such as dock pilings
  • Stainless Steel Rebar
    • Good corrosion resistance
    • Capable of withstanding shipping, handling, bending
    • Comes in magnetic or non-magnetic alloy
    • Very expensive

FRP

One of the slab displays in the IL Centre showcases two different kinds of fibre-reinforced polymer (FRP) rebar. FRP is used when added strength and durability is required; it resists changes in temperature, and can be used in chemically aggressive environments.

Depending upon the material, FRP is transparent to both radio frequency and magnetic fields, and can be used in electrical substations and hospital MRI rooms. It’s also lightweight, typically one-quarter to one-sixth the weight of standard steel rebar, but the cost is much higher than traditional rebar.

The three types of FRP are glass, carbon, and aramid. All are much more expensive than traditional steel rebar. The newest kind of reinforcing rods are made from aramids, a family of nylon that includes Kevlar.

  • Glass (GFRP)
    • Highest density
    • Lowest tensile strength
    • Has the longest ultimate elongation
    • Quite lightweight, one-quarter the weight of steel
    • Non-condutive, non-magnetic
    • Non-corrosive, impervious to chloride ion and chemical exposure
    • Transparent to magnetic fields or radio frequencies
  • Carbon (CFRP)
    • Medium density
    • Highest tensile strength
    • Smallest ultimate elongation
    • Semi-conductive (thermal, electrical and RF energy.)
    • Very lightweight, one-fifth the weight of steel
    • Modulus of elasticity close to steel
    • Impervious to chloride ion and other chemical exposure.
  • Aramid (AFRP)
    • Lowest density
    • Extremely lightweight, one-sixth the weight of steel
    • Medium tensile strength
    • Medium ultimate elongation
    • Excellent abrasion resistance
    • Good thermal isolation
    • UV exposure can cause degredation of aramid fibres

Slab and rebar