What is the Flexural Strength of Concrete?

Engineers spend a lot of time talking about compressive strength when it comes to concrete. Makes sense - concrete is excellent at being crushed, and testing compressive strength is straightforward. Most concrete specifications focus heavily on this single number.

However, for a concrete structure to be truly durable, another property of concrete is just as critical. This property is its flexural strength.

Flexural strength is the ability of concrete to resist bending. This is an essential quality for many structural elements that must support loads over a span, such as beams and structural slabs.

This guide will explain what flexural strength is, how it is tested, and why it is so important for the safety and durability of concrete.

Understanding Flexural Strength

The flexural strength of concrete represents the point where bending stresses overcome the material's tensile capacity. Most people don't realise that bending failure is actually a tension problem in disguise.

Take a simple beam with a load applied at the center. The top gets compressed while the bottom gets stretched. The bottom portion experiences tension, and concrete is notoriously weak in tension.

This creates an interesting situation. A beam might have plenty of compressive capacity remaining when the tensile stresses on the bottom cause cracking and failure. The flexural strength essentially measures this tensile limit, but it's expressed in terms of the bending stress that caused it.

Reinforced concrete addresses this problem by adding steel bars in the tension zone. Plain concrete structures, however, must rely entirely on the concrete's limited tensile strength. This is why flexural strength testing becomes so important for unreinforced elements like pavements and some precast products.

How to Check Flexural Strength?

The process for measuring this property is a standardized laboratory procedure known as flexural strength testing.

Engineers cast a standard-sized, unreinforced concrete beam from the same batch of concrete that will be used in the actual concrete structure. After this beam has cured for a specific period, typically 28 days, it is taken for testing.

The beam is supported at two points near its ends. A load is then applied to the top surface, usually at the center or at two points. The load is gradually increased until the beam breaks. The flexural strength is then calculated based on the load that caused the failure and the dimensions of the beam.

Read Also: How to Test the Compressive Strength of Concrete Effectively

What is the flexural strength formula?

While the exact calculation depends on the testing method, in India, the flexural strength can be estimated based on the concrete's compressive strength. According to the Indian Standard code IS 456:2000, the flexural strength formula is

fcr = 0.7 * √fck:

Here, fcr is the flexural strength, and fck is the characteristic compressive strength of the concrete in N/mm².

Using this formula, we can estimate the flexural strength for common concrete grades:

• What is the flexural strength of M20 concrete? For M20 grade, fck = 20 N/mm². The flexural strength is approximately 3.13 N/mm².

• What is the flexural strength of M30? For M30 grade, fck = 30 N/mm². The flexural strength is approximately 3.83 N/mm².

Achieving the required flexural strength is essential for the long-term performance of any concrete element that will face bending forces. The durability of concrete also depends heavily on using high-quality cement, a proper mix design, and correct curing procedures.

FAQs:

1. Why does flexural strength matter in slabs and roads?

Flexural strength matters because slabs and concrete roads often face bending. When a slab or pavement carries load, one side of the concrete is compressed while the other side is stretched.

Since concrete is weaker in tension, cracks can develop on the stretched side. This is why flexural strength is important for roads, floor slabs, beams, and other concrete members that carry bending loads.

2. What is the difference between compressive strength and flexural strength?

Compressive strength measures how much crushing load concrete can resist. It is usually tested on cube or cylinder specimens.

Flexural strength measures how much bending stress concrete can resist before cracking or failure. It is tested on beam-shaped specimens. In simple terms, compressive strength checks resistance to crushing, while flexural strength checks resistance to bending.

3. How is flexural strength of concrete tested?

Flexural strength is tested using a concrete beam specimen. The beam is cast, cured properly, and then placed on supports in a testing machine.

Load is applied gradually from the top until the beam cracks or fails. The failure load and beam dimensions are then used to calculate the flexural strength, also called the modulus of rupture.

4. Can flexural strength be checked directly at site?

Flexural strength is not checked through a simple on-site visual inspection. It requires a properly cast beam specimen, curing, controlled loading, and calculation.

At site, the focus should be on following the approved mix design, maintaining proper water content, placing concrete correctly, compacting it well, and curing it properly. If flexural strength verification is needed, beam samples should be tested in a lab.

5. What site-level mistakes can affect flexural strength?

Common mistakes include adding extra water, poor compaction, improper placing, delayed curing, inadequate curing, and not following the approved mix design. These mistakes can reduce concrete quality and increase the risk of cracking under bending loads. For slabs and pavements, proper mix control, workmanship, and curing are especially important.