It’s obvious that we do not need to over reinforce the structural element than the required strength. It is straight a waste of steel. However, another important reason is hidden inside the practice.

At the end of the concrete element designing, we need to calculate the required reinforcement. Providing more than the required amount is a must!. The over reinforcement we are discussing today is about providing rebars more than the requirement. It’s about improving tensile capacity by adding rebars more than the compression capacity in the element.

Compression and tension zone of a beam under vertical loads

Following is the general model we consider during the design of a beam. The top part of the beam is subjected to compression while the bottom half is in tension. Compressive force due to bending will carryout the concrete and tensile force is depending on the steel reinforcement.

Think about a situation of adding rebars to control the deflection of a beam. If the beam is already designed for the load, adding some more reinforcement will be beneficial. But increasing tensile capacity may pass the compression capacity of the beam. Then the beam is weak in compression. Let’s see what are the possible causes of reinforcement we can find.

Over reinforced case

Compressive strength capacity is higher than tensile strength capacity

When the tensile capacity is higher than the compression capacity of the bending element concrete will fail before the steel reinforcement. Compression failure in concrete is sudden. Concrete will crush within a snap of time allowing us no time to react. The crushing failure will not show any significant crack or displacement like in tensile failure.

See the bending failure of over reinforced beam

Bending failure of an over reinforced concrete beam: Materials Lab on-line

Balanced reinforced case

Compressive and tensile strength capacities are same

A balanced reinforced case displays similar tensile and compression capacities in the element. When the load increases, steel reinforcement, and concrete will fail at the same time.

Under reinforced case

Tensile strength capacity is higher than compressive strength capacity

in this case, the tensile capacity of the element is less than the compression capacity. So the failure will start at tensile reinforcement. This way we got enough warnings before failures. Tensile cracks at the bottom half of the beam and too much deflection are the most common signs before a tensile failure.

Additionally, the rebar yielding time will allow the users to evacuate the area before a collapse.

See how the failure happen in an under reinforced beam

Bending failure of an under reinforced concrete beam: Materials Lab on-line

Summery

Always we should try to avoid compression failures when designing concrete structures. The sudden burst in the elements can cause a severe impact on the structure as well as the users.

On the other hand, tensile failures are slow compared to compression failures. It takes a fair time to fail in addition to the prior warning cracks and deflections.

kalhara

Kalhara Jayasinghe is a civil engineer currently engage with hydropower construction works in Sri Lanka. He has completed his bachelor's degree & master's in structural engineering from the University of Peradeniya and achieved chartered engineer title in 2019 from the Institute of Engineers Sri Lanka.

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