The Pool Building Bible – Perth WA edition

Reinforcement

Sprayed concrete is reinforced by conventional steel rods, steel mesh, and/or fibers. Jason from Rapid Shotcrete here in Perth is shown in the following video. The video shows the application of Shotcrete onto rebar.

Reinforced concrete can be classified as precast concrete and cast in-situ concrete.

Concrete is a mixture of cement (usually Portland cement) and stone aggregate. When mixed with a small amount of water, the cement hydrates to form a microscopic opaque crystal lattice structure encapsulating and locking the aggregate into its rigid structure. Typical concrete mixes have high resistance to compressive stresses (about 4,000 psi (27.5 MPa)); however, any appreciable tension (e.g. due to bending) will break the microscopic rigid lattice resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists compression but also bending, and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.

Depending on the type of concrete mix and steel employed, reinforced concrete structures can support 300 to 500 times their combined weight and behave, according to general mechanics, as a single structural entity. Although concrete and steel would appear to have a weight disadvantage, this support ratio is competitive with student balsa-wood bridges.

that is very strong in compression, but virtually without strength in tension. To compensate for this imbalance in concrete’s behavior, rebar is cast into it to carry the tensile loads.

While any material with sufficient tensile strength could conceivably be used to reinforce concrete, steel and concrete have similar coefficients of thermal expansion: a concrete structural member reinforced with steel will experience minimal stress as a result of differential expansions of the two interconnected materials caused by temperature changes.

Physical characteristics

Steel has an expansion coefficient nearly equal to that of modern concrete. If this weren’t so, it would be useless for reinforcing concrete.^[1] Although rebar has ridges that bind it mechanically to the concrete with friction, it can still be pulled out of the concrete under high stresses, an occurrence that often precedes a larger-scale collapse of the structure. To prevent such a failure, rebar is either deeply embedded into adjacent structural members, or bent and hooked at the ends to lock it around the concrete and other rebar. This first approach increases the friction locking the bar into place while the second makes use of the high compressive strength of concrete.

Common rebar is made of unfinished steel, making it susceptible to rusting. As rust takes up greater volume than the iron or steel from which it was formed, it causes severe internal pressure on the surrounding concrete, leading to cracking, spalling, and ultimately, structural failure. This is a particular problem where the concrete is exposed to salt water, as in bridges built in areas where salt is applied to roadways in winter, or in marine applications. Epoxy-coated rebar or stainless steel rebar may be employed in these situations at greater initial expense, but significantly lower expense over the service life of the project. Fiber-reinforced polymer rebar is now also being used in high-corrosion environments.z

STEEL FIXING

Steel Positioning – Cover

Steel must be placed correctly or the strength of a structure may be greatly weakened. Cover is the distance from the outside face of the concrete to the nearest surface of reinforcing steel.

If this distance is insufficient, the steel will rust. As already mentioned above, as the steel changes to rust, the diameter of the actual steel decreases and the strength provided by the steel decreases. This can seriously shorten the life of a pool.

Furthermore, when steel rusts, it expands to 2.2 times its original volume. This expansion bursts the concrete open thus exposing the steel to attack by the weather and pool water which increases the corrosion.

Cover in pool varies from 30 to 60 mm. The actual value depends on the concrete strength, the location of the concrete such as below the ground or inside a wall, and the soil type. Other areas of Australia my use higher covers for these reasons.

Spacers

Spacers are used to maintain cover. They keep the steel the required distance away from the edge of the concrete and soil. Typically pool construction can use both plastic , steel or concrete spacers. Bricks, and pavers that give you the correct spacing can also be used.  Only materials that will not expand should be used as spacers.  An example of incorrect spacers would be any forms of wood.

Fixing

Fixing is fastening the steel reinforcing bars so that they stay in the correct place between the spacers and relative to each other.

The steel must stay in place:

• while workers walk on it,
• while concrete is being placed,
• and during compaction.

Tie-wire is used at the junctions of bars to hold them together.

• It must be tied tightly.
• Free ends must not protrude into the cover space.
• All pieces of tie-wire must be removed from the formwork before concreting otherwise rust-stains soon occur on the finished product.

Bending

Too small a diameter weakens the steel. Too large a diameter may cause problems such as lack of anchorage (a form of bond failure) or create difficulties in keeping other steel bars in the correct place.