Concrete is strong in compression and comparatively weak in tension. For most structural work, engineers specify compressive strength from cube tests. But for rigid pavements, airport aprons, industrial floors and other slabs on grade, the design is often based on the tensile behaviour of concrete in bending. That property is measured with a beam test, giving the flexural strength or modulus of rupture.
The standard behind the test
In India, methods of test for strength of concrete are covered by IS 516, which has been revised in parts. The flexural strength test on beams sits alongside the compressive strength test in this series. Internationally, ASTM C78 covers third-point loading and ASTM C293 covers centre-point loading. Always work from the current edition specified in your contract or project documents, since clauses and tolerances are updated over time.
Specimens
The standard beam has a square cross-section. The commonly used sizes are 150 mm × 150 mm × 700 mm for concrete with larger aggregate, and 100 mm × 100 mm × 500 mm where the maximum aggregate size is small enough to permit it. Beams are cast in steel moulds, compacted, cured in water under controlled conditions and tested at the specified age, typically 7 and 28 days.
Because beams are heavy and the test is sensitive to surface condition, handle them carefully. Keep them wet until testing; drying of the surface can lower measured strength significantly.
Third-point loading
The usual arrangement is two-point (third-point) loading. The beam rests on two supporting rollers, and the load is applied through two upper rollers placed at the third points of the span. For a 150 mm beam the span between supports is normally 600 mm; for a 100 mm beam, 400 mm. Between the two load points, the beam is under constant bending moment with no shear, so failure occurs in the weakest section of that middle third.
The rollers should be free to rotate and to align with the specimen so that the load is distributed evenly across the width. The beam is placed so that the load is applied to a side face as cast, not to the trowelled top surface.
Loading and calculation
Load is applied without shock and increased continuously at the rate stated in the standard until the beam breaks. After failure, note where the fracture occurred, measured from the nearer support.
When the fracture lies within the middle third, the flexural strength is calculated as:
fb = (P × l) / (b × d²)
where P is the maximum load, l is the span between supports, b is the width and d is the depth of the beam. The standard gives a modified formula when the fracture lies slightly outside the middle third, and states when the result must be discarded because the fracture is too far outside it.
Centre-point versus third-point loading
| Aspect | Third-point loading | Centre-point loading |
|---|---|---|
| Bending moment | Constant over the middle third | Peak directly under the load |
| Where failure occurs | Weakest section in the middle third | Forced near the centre |
| Typical result | Lower, more representative | Higher than third-point |
Because the two methods give different values, a result is only comparable with another result obtained by the same method. Specify and report the method clearly.
The testing machine
A flexure test needs a machine that can apply load smoothly at a controlled rate and has a suitable bending fixture with the correct roller spacing. The load range should suit the expected breaking load of beams, which is much lower than the load used to crush cubes, so the resolution of the load measurement at the lower end of the range matters. Many labs use a dedicated flexure testing machine; others use a frame with a flexure attachment.
A typical civil engineering laboratory also runs cube tests on a compression testing machine, and keeps moulds and other apparatus from a civil lab equipment range.
Common mistakes
- Testing beams that have been allowed to dry out.
- Applying the load to the trowelled face.
- Rollers that are seized or misaligned, causing uneven contact.
- Loading too fast, which tends to raise the result.
- Using the middle-third formula without checking where the beam broke.
What to report
A report should include the specimen identification and size, age at test, curing conditions, span and loading arrangement, maximum load, position of fracture, the calculated flexural strength and any defects noticed in the specimen. Recording the fracture position is easy to forget but essential for checking which formula applies.
Relating flexural and compressive strength
Design codes give approximate relationships between flexural strength and compressive strength, but these are only estimates. Aggregate type, shape and texture have a strong effect on flexural strength, so two mixes with the same cube strength can have different beam results. Where flexural strength is a design input, test it directly for the mix you will use.
Frequently asked questions
Is flexural strength the same as split tensile strength?
No. Both indicate the tensile capacity of concrete, but they use different specimens and test arrangements and give different values. Use the one your design method specifies.
Why do pavement designs use flexural strength?
Slabs on grade fail mainly in bending under wheel loads, so the modulus of rupture relates more directly to their design than cube strength does.
Can I test beams on a compression testing machine?
Only if the machine has a suitable flexure fixture and a load range fine enough for beam loads. Check the load resolution at the lower end of the scale.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.