Building swimming pools

Applications of steel

Steel is the most widely used and recycled metal material on earth due to its high strength and relatively low production cost. The five main sectors in which steel is used are:

1. Appliances and industry
2. Packaging
3. Energy
4. Transportation
5. Construction

By far, the majority of steel is used in the construction industry due to the fact that structures can be built relatively quickly and inexpensively in comparison to other materials. Structural steel is used in the following construction applications:

• Low- and high-rise buildings;
• Schools and hospitals;
• Bridge deck plates;
• Piers and suspension cables;
• Harbours;
• Cladding and roofing;
• Offices;
• Tunnels;
• Security fencing;
• Coastal and flood defences;
• Reinforced concrete (rebar); and
• Swimming pools.

Reinforcing bar (rebar)

Reinforcing steel (rebar) is made from carbon steel and is the primary type of structural steel used in creating reinforced concrete structural members. Rebar is normally round in cross-section and is ridged (deformed) along its length. These ridges are responsible for transferring loads (stresses) from the concrete to the steel via mechanical bond.

It is normally placed into plastic concrete (the stage at which fresh concrete can be moulded), or plastic concrete is poured around the rebar cage. The concrete flows around and completely covers the rebar. Once hardened, the rebar and concrete essentially move and respond to loading conditions as one. Hence, reinforced concrete is formed.

The concrete primarily provides mass and resistance to crushing loads, while the rebar provides strength in tension and flexure. The concrete also serves to protect the rebar from corrosion by forming a passive layer of protection over the steel as they chemically interact when the plastic concrete is placed onto the rebar.

The alkaline (A_T) environment of the fresh concrete paste makes the steel much more resistant to corrosion than it would be in a more neutral or acidic pH environment. As long as the steel is fully coated by concrete, it is less likely to corrode. However, many structural members get loaded beyond their design strength and as a result develop tension cracks, which allow atmospheric elements to reach the embedded rebar, which initiates the corrosion process.

This is one mode of failure of structural members and structures; once corrosion starts, it will continue to spread and eventually accelerate the complete failure of the structure, unless it is rehabilitated in time.

Rebar sizes and grades

Once the architect or designer comes up with the structure’s overall ‘look,’ engineers use structural analysis and building codes to determine the loads the structure will be subjected to and the internal stresses that will result from this loading and configuration.

Each member of a reinforced structure must be designed based on the following primary parameters: concrete cross-section size (i.e. length and width) of the member, and the location and amount of reinforcing steel.

The size and number of bars specified will be dictated by the size of the load (stresses) to be resisted and the size of the member (cross-section). The primary calculation is to determine the total cross-sectional area of steel required to safely resist the design loading. An engineer will specify the location, size, and number of bars required in the cross-section.

Rebar is specified in both metric (Canadian) and imperial (U.S.) sizes. In Canada, most builders still tend to refer to rebar by the imperial sizes; however, rebar is typically sold in Canada by the metric bar designation. Therefore, it is extremely useful to understand the relationship between both.

In U.S. sizes, imperial bar designations represent the bar diameter in fractions of ⅛ in., such that #8 (8⁄8 in.) represents a 1 in. diameter (see Figure 2).

In Canadian sizes, metric bar designations represent the nominal bar diameter in millimetres rounded to the nearest 5 mm (see Figure 3). For example, a 10M bar actually has a nominal diameter of 11.3 mm, but is rounded down to 10 mm.

By comparing the nominal cross-sectional areas of the above charts, correlations can be derived between metric and imperial bar sizes as follows:

• #3 < 10M < #4;
• 15M = #5;
• #6 < 20M < #7;
• 25M = #8;
• #9 < 30M < #10;
• 35M = #11;
• 45M = #14; and
• #14 < 55M < #18