Posted on

What Is The Flexural Strength Of Concrete?

The flexural strength of concrete is one measure of the tensile strength of unreinforced concrete. It refers to the ability of the concrete beam or slab that is being tested to resist bending.

It is measured in Modulus of Rupture (MR), a measurement that is used to inform the design of concrete products. As with compressive strength, the flexural strength of concrete is another crucial property of concrete that must be tested thoroughly before it is used in construction projects. Due to its tendency to crack, the flexural strength of concrete is usually a lot lower than its compressive strength (between 10-20% of the compressive strength) and this is what engineers can expect to see from the results of the test.

Now we’ve covered exactly what it is, let’s take a closer look at how flexural strength is actually measured.

Set up

Under strict laboratory conditions, the flexural strength of concrete is tested by applying an external load to the sample until it fails i.e is unable to resist bending.

Whilst compressive strength can be measured directly, flexural strength must be measured indirectly. This is because an apparatus has not yet been developed which allows the tensile force to be evenly distributed to the sample. As a result, it is measured using either So the ‘one point loading’ or ‘two-point loading’ test.


Now, this is the point at which the Modulus of Rupture (MR) measurement which we mentioned earlier comes in. MR measures the extreme fibre stresses which the sample experiences when placed under a load during the test. As the tensile stress exceeds the flexural strength of the sample, cracks will appear as the sample has reached the maximum point of bending.


Flexural strength Example Image

Observations from the test are then used in a formula to calculate the flexural strength of the concrete which looks something like this:

fb = Pl/bd2 (when a > 13.3 cm)
fb = 3Pa/bd2 (when a < 13.3 cm)
a = the distance between the line of fracture and the nearest support, measured on the centre line of the tensile side of the specimen (cm)
b = width of specimen (cm)
d = failure point depth (cm)
l = supported length (cm)
P = Maximum Load taken by the specimen (kg)

Difficulties That May Arise

Flexural tests are extremely sensitive to testing variations which is why the flexural strength of concrete can be a difficult property to assess accurately. All specimens need to be prepared, handled and cured to an exact standard before testing and this can be hard to manage accurately which can lead to inaccurate results if the test is not regulated correctly.

Can The Flexural Strength Of Concrete Be Improved?

The flexural strength of concrete can be improved by a number of methods, for example by adding pozzolanic additives to the mix. Another option is to reduce the porosity and water: cement ratio of the mix. Concrete that has been prestressed will also have a higher overall tensile strength than other concrete products.

About Al-Manaratain

As one of the leading construction companies in Bahrain, Al-Manaratain is dedicated to ensuring its customers have all the information they could possibly need when it comes to our range of concrete products. Our article section is full of informative articles and is regularly updated with new articles that should interest anybody who’s interested in concrete products or the concrete industry.