What Makes Concrete Strong? – Pre-tensioned and Post-tensioned

Concrete, despite its high compressive strength, only has about a tenth of its compressive strength in tensile strength. This means that without some kind of reinforcement, concrete will fail in tension long before it can ever be useful for most building applications. Today, concrete is used for a wide variety of building and civil applications, such as highways, parking lots, bridges etc.

As you probably know, these building projects have high tensile requirements, and as such, any concrete used must be reinforced accordingly. Although reinforced concrete has been in use as far back as the 19th Concrete Reinforcementcentury (Francois Coignet), reinforcement alone is often insufficient for many civil engineering applications today; ie, bridges and highways.

Today, we have what is known as prestressed concrete, and in short, is the secret to our more demanding construction applications. Prestressed concrete can be made using one of three methods: pretensioned concrete, bonded post-tensioned concrete, and unbonded post-tensioned concrete. All three methods introduce a compressive force which must first be overcome before there is any serious deflection.

Pretensioned Concrete

Pretensioned concrete is when concrete is cast or poured around a tendon (reinforcement) already under Strengthened Concrete Diagramtension. After the concrete has cured to an acceptable strength, the tension is slowly released, transferring the compressive force to the surrounding concrete. The tendon is placed near the lower face so as to counter the tensile forces induced by the bending load.

Properly installed, quality pretensioned concrete requires that there is sufficient bond between the tendon and surrounding concrete. This is typically done by roughened, corrugated tendons, as well as hooks. Thermal compatibility between tendon and concrete is important as well, or temperature related stresses will form, eventually decoupling the tendon from the concrete.

Watch a video explaining how a prestressed concrete beam is made and how it works.

Bonded and Unbonded Post-tensioned Concrete

Post-tensioned concrete is when the tendon is pulled into tension after the concrete is poured and hardened to a specified degree. Bonded post-tensioned concrete is where the tendon is pushed through a duct which follows the theoretical path of tension, and after concrete is cast around it, the tendon is Unbonded Post-tensioning Diagrampulled into tension and the duct is filled tightly with grout to prevent corrosion, bonding the tendon with the concrete in the process.

Unbonded post-tensioned concrete is when the tendon is coated with grease and covered with a plastic sheathing. This essentially decouples the tendon from the concrete and compression is induced only from the two anchored ends. This method allows flexibility in maintenance, repairs, and non-standard field conditions, where tendons may need to be shifted, destressed, etc.

How and Why Reinforced Concrete Fails

Whether prestressed or not, steel-reinforced concrete of all kinds have an Achilles’ heel. Corrosion. Since the overall strength of the concrete specimen is based on its reinforcement, if this reinforcement were to fail, or even simply unbond from the surrounding concrete, this would translate into an imminent failure event.

Corrosion, or practically speaking – rust, sets in due to an acidic environment. Generally speaking, an Concrete Carbonationacidic environment is one in which the pH is lower than 10. Portland cement starts out with a highly alkaline environment, with a pH of about 13, but slowly decreases to about 8.5 – the pH of water in equilibrium with calcite.

Though at the beginning the steel was protected by the alkaline environment, passivated by a protective film, in time, as carbon dioxide reacts with the Portland cement, it will “carbonate”, and corrosion will become a concern. Measures that can be taken include anti-corrosion additives that are mixed into the cement, surface sealants, galvanizing the steel-reinforcement, and adequate cover depth (distance between reinforcement and surface).

For more detailed information on prestressed concrete, I’m linking to a page I found helpful: FHWA

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