Flexural Strength of Concrete: Beam Test, Formula and IS 516
Flexural strength of concrete measures how well an unreinforced concrete beam resists bending before it cracks. Labs then test it on a beam under IS 516 and report the result as the modulus of rupture in N/mm². The property matters most for pavements and slabs, so engineers check it directly. This guide covers the beam test, the formula and the link between flexural strength and concrete grade.
Key takeaways
- Flexural strength of concrete is the bending stress at which an unreinforced beam fails, reported in N/mm² as the modulus of rupture.
- A 150 × 150 × 700 mm beam is standard, so labs use a 600 mm span with loading at the third points.
- Load rises at 0.7 N/mm² per minute, which is about 4 kN per minute for a 150 mm beam.
- When the beam breaks in the middle third, fb = P × L ÷ (b × d²).
- IS 456 estimates flexural strength as 0.7 times the square root of fck, but a test gives the real value.
- IS 516 (Part 1/Sec 1):2021 supersedes IS 516:1959.
What is the flexural strength of concrete?
The flexural strength of concrete is the stress at which a plain concrete beam fails in bending. It is also called the modulus of rupture, or the flexural tensile strength of concrete. Concrete is weak in tension, so this test shows how it behaves when a slab or pavement bends under traffic.
How is the flexural strength test for concrete done?
The flexural strength test for concrete loads a beam at two points until it breaks. The method comes from IS 516 (Part 1/Sec 1):2021, and it has five steps.
- Cast and cure the beams. Cast 150 × 150 × 700 mm beams, then cure them in water until the test age, usually 28 days.
- Set up the supports. Place the beam on two supporting rollers 600 mm apart. Then position two loading rollers at the third points, 200 mm apart.
- Apply the load. Load the beam continuously at 0.7 N/mm² per minute, so that no shock reaches it.
- Record the failure. Note the maximum load, and also the distance of the fracture from the nearer support.
- Calculate the result. Then use the formula that matches the fracture position.
IS 516 also allows a smaller 100 × 100 × 500 mm beam on a 400 mm span. That beam loads at about 1.8 kN per minute.
What is the formula for flexural strength of concrete?
The concrete flexural strength formula depends on where the beam breaks, so first note the fracture position. Then, if the fracture falls inside the middle third of the span, use:
fb = P × L ÷ (b × d²)
Here fb is the modulus of rupture in N/mm², P is the maximum load in newtons, L is the span in mm, b is the beam width and d is its depth, both in mm. However, if the fracture falls outside the middle third, use:
fb = 3 × P × a ÷ (b × d²)
In this case, a is the distance from the fracture to the nearer support. For a 150 mm beam, discard the result if a is less than 170 mm. The limit is 110 mm for a 100 mm beam.
Case | Maximum load | Fracture position | Modulus of rupture |
A: 150 mm beam, 600 mm span | 22.5 kN | Middle third | 4.0 N/mm² |
B: 150 mm beam, 600 mm span | 20 kN | a = 180 mm | 3.2 N/mm² |
Both rows are worked examples, not lab data. In case A, fb = 22,500 × 600 ÷ (150 × 150²) = 4.0 N/mm². In case B, fb = 3 × 20,000 × 180 ÷ (150 × 150²) = 3.2 N/mm², because the fracture sits outside the middle third.
How does IS 456 link flexural strength to grade?
IS 456:2000 estimates flexural strength from the characteristic compressive strength, fck. The estimate is 0.7 × √fck, in N/mm². Use it for design, but do not treat it as a test result.
Grade | fck (N/mm²) | Estimated flexural strength (N/mm²) |
M20 | 20 | 3.13 |
M25 | 25 | 3.50 |
M30 | 30 | 3.83 |
M40 | 40 | 4.43 |
Pavements and slabs on grade rely on this property, so a lab beam test is the direct check.
Which IS 516 edition applies?
IS 516 (Part 1/Sec 1):2021 applies today. It supersedes the older IS 516:1959, which many templates and textbooks still cite. As a result, check that your test report names the current edition. Also, the Bureau of Indian Standards publishes both.
Where can you get a flexural strength test done?
Jancy Labs tests concrete at its Madurai laboratory. See the concrete testing lab page for the tests on offer, or request a quote for your samples. Also, more guides sit on the Jancy Labs blog.
Get Your Concrete Tested at Jancy Labs
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Frequently Asked Questions
What is the flexural strength of concrete?
Flexural strength of concrete is the stress at which an unreinforced concrete beam fails in bending. Labs report it in N/mm² as the modulus of rupture.
What size beam is used for the flexural strength test?
The standard beam is 150 mm × 150 mm × 700 mm, supported on a 600 mm span. IS 516 also allows a 100 mm × 100 mm × 500 mm beam on a 400 mm span.
What is the formula for flexural strength of concrete?
When the beam breaks in the middle third of the span, fb = P × L ÷ (b × d²). When it breaks outside the middle third, but not too near a support, fb = 3 × P × a ÷ (b × d²).
How does IS 456 estimate flexural strength?
IS 456:2000 estimates flexural strength as 0.7 times the square root of the characteristic compressive strength, in N/mm². For M25 concrete, that gives 3.5 N/mm².
Which IS 516 edition applies today?
IS 516 (Part 1/Sec 1):2021 applies today. It supersedes the older IS 516:1959, which many references still cite.
Can I courier concrete beams to Jancy Labs?
Yes. Jancy Labs collects samples across Tamil Nadu and accepts courier samples from anywhere in India.