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Introduction
Concrete is strong in compression but comparatively weak in tension. In many structural and pavement applications, concrete is also subjected to bending or flexural stresses. The flexural strength of concrete is therefore an important property used to evaluate its resistance to bending.
The flexural strength of concrete is commonly determined by testing a concrete beam specimen under a specified loading arrangement until failure. The result is generally expressed as the modulus of rupture.
The test is particularly relevant to applications such as concrete pavements, industrial floors and other applications where flexural performance is an important specified property.
What is Flexural Strength of Concrete?
Flexural strength is the ability of concrete to resist failure when subjected to bending.
Unlike the compressive strength test, in which a concrete cube or cylinder is compressed directly, the flexural strength test applies a bending load to a concrete beam specimen.
The maximum stress calculated at failure is known as the modulus of rupture.
Flexural strength is generally expressed in:
MPa or N/mm²
Why is the Flexural Strength Test Conducted?
The test is conducted to:
- Determine the flexural strength of concrete.
- Evaluate the resistance of concrete to bending.
- Assess concrete used in pavements and flooring.
- Compare the performance of different concrete mixes.
- Check whether concrete satisfies specified project requirements.
- Evaluate the effect of materials, curing and mix proportions on flexural performance.
IS Code for Flexural Strength Test of Concrete
The flexural strength test of concrete is covered under IS 516 (Part 1/Sec 1):2021, Concrete — Testing of Hardened Concrete — Part 1: Methods of Test for Strength of Concrete.
The test determines the flexural strength, expressed as the modulus of rupture, using a concrete beam specimen under the specified loading arrangement.
The specimen dimensions, loading arrangement, test procedure, calculation, fracture classification and reporting should be followed according to the applicable requirements of the standard and project specification.
Important: Where a project specification refers to an earlier edition of IS 516, the edition specified by the project should be followed.
Concrete Beam Specimen for Flexural Strength Test
A concrete beam specimen is used for determining flexural strength.
The commonly specified specimen sizes are:
100 mm × 100 mm × 500 mm
150 mm × 150 mm × 700 mm
The specimen size, span and loading arrangement must be selected and maintained according to the applicable test standard.
For the commonly used specimens, the corresponding effective span is based on the requirements of the applicable IS 516 test method.
Important: Do not select the beam size, span or loading arrangement independently. They must be used as a consistent test configuration.
Apparatus Required
The main equipment required includes:
- Flexural testing machine or suitable loading frame.
- Beam specimen mould.
- Loading rollers.
- Supporting rollers.
- Measuring scale.
- Weighing equipment.
- Tamping rod or appropriate compaction equipment.
- Curing tank.
- Marking tools.
The testing machine should be properly calibrated and capable of applying the required load smoothly.
Preparation of Concrete Beam Specimens
The concrete is prepared using the approved mix proportions.
The mould should be:
- Clean.
- Properly assembled.
- Free from excessive moisture.
- Coated with a suitable release agent where required.
Fresh concrete is placed into the mould in layers and compacted properly.
Proper compaction is important because voids and inadequate consolidation can significantly affect the measured flexural strength.
Casting of Beam Specimens
The general procedure is:
Step 1 — Prepare the mould
Clean and assemble the beam mould and apply release agent if required.
Step 2 — Place concrete
Fill the mould with freshly mixed concrete in the required layers.
Step 3 — Compact the concrete
Compact each layer properly using the specified method.
Step 4 — Finish the surface
Strike off and finish the top surface so that the specimen is uniform.
Step 5 — Mark the specimen
Identify the specimen with relevant information such as:
- Concrete grade
- Date of casting
- Specimen number
- Mix identification
Step 6 — Initial curing
Keep the specimens under the specified initial curing conditions.
Step 7 — Demoulding
Remove the beam specimens from the mould after the specified period.
Step 8 — Water curing
Place the specimens in the curing tank until the specified testing age.
Curing of Concrete Beam Specimens
Proper curing is essential for obtaining reliable flexural-strength results.
Curing allows cement hydration to continue and helps the concrete develop its required strength.
The curing conditions should be maintained consistently for all specimens.
Flexural strength is commonly assessed at 28 days for concrete strength evaluation, unless another testing age is specified by the project or applicable standard.
Flexural Strength Test Procedure
After curing, remove the beam specimen from the curing tank and prepare it for testing.
Step 1 — Clean the specimen
Remove excess water and clean the specimen surface.
Step 2 — Measure the specimen
Measure the relevant dimensions required for calculation.
Step 3 — Position the beam
Place the beam correctly on the supporting rollers.
Step 4 — Apply the loading arrangement
Position the loading rollers according to the applicable test method.
Step 5 — Check alignment
Ensure that the specimen is properly aligned and the loading points are correctly positioned.
Step 5A — Check the rollers and bearing surfaces
Ensure that the supporting and loading rollers are clean and correctly positioned. The specimen should sit properly on the supports without improper packing or misalignment.
Step 6 — Apply load
Apply the load gradually and continuously at the specified rate.
Step 7 — Observe failure
Continue loading until the beam fails.
Step 8 — Record maximum load
Record the maximum load carried by the specimen at failure.
Step 9 — Determine the failure location
Observe and record the location of the fracture in relation to the specified loading region.
Step 10 — Calculate flexural strength
Calculate the modulus of rupture using the appropriate formula for the actual failure and loading arrangement.
Flexural Strength Formula
The formula depends on the loading arrangement and the location of the fracture.
For the applicable loading arrangement, the flexural strength is calculated using the relevant expression involving:
- Maximum applied load
- Span
- Width of specimen
- Depth of specimen
- Location of fracture, where applicable
Important
Do not use one formula for every flexural test arrangement.
The correct equation must be selected according to the loading arrangement and failure location specified by the applicable test standard.
Modulus of Rupture Formula
The flexural strength of the beam is expressed as the modulus of rupture.
The applicable equation depends on the type and location of fracture.
Case 1 — Fracture within the specified central portion
For the applicable failure condition, the modulus of rupture is:
Where:
- fb = flexural strength or modulus of rupture, in MPa
- P = maximum load at failure, in N
- L = span of the specimen, in mm
- b = measured width of specimen at the point of failure, in mm
- d = measured depth of specimen at the point of failure, in mm
The equation is the standard expression for the applicable middle-third failure condition.
Case 2 — Fracture outside the central portion but within the permitted range
If the fracture occurs outside the central portion but remains within the permitted range, the modified equation is:
Where:
- P = maximum load at failure, in N
- a = distance between the line of fracture and the nearer support, measured along the centre line of the tensile side, in mm
- b = measured width of specimen, in mm
- d = measured depth of specimen at the point of failure, in mm
The permitted fracture ranges depend on the beam size and the applicable IS 516 provisions. If the fracture occurs outside the permitted range, the result should be treated according to the standard rather than applying the formula blindly.
Important
Do not use the same formula for every fracture location. First examine the fracture pattern and determine which calculation condition applies.
Fracture Location and Selection of Formula
The fracture location must be checked after the beam fails because the calculation depends on the type and location of fracture.
For the applicable standard test arrangement:
- A fracture within the specified central portion is calculated using the PL/bd2 expression.
- A fracture outside that portion but within the permitted range is calculated using the 3Pa/bd2 expression.
- A fracture outside the permitted range is treated according to the standard and should not be accepted simply by applying one of the above equations.
Therefore, the maximum load alone is not sufficient to calculate the final flexural strength.
Worked Example of Flexural Strength Calculation
Consider a beam specimen having:
- Width, b=150 mm
- Depth, d=150 mm
- Span, L=600 mm
- Maximum failure load, P=20,000 N
Assuming the applicable fracture condition uses:
Substituting:
Therefore:
Flexural strength = 3.56 MPa
Note: This is an illustrative calculation only. In an actual laboratory test, the specimen dimensions, span, loading arrangement and fracture condition must comply with the applicable IS 516 requirements.
Failure Pattern in Flexural Strength Test
The location of the crack or fracture is important.
After failure, examine the beam and record the location of the fracture relative to the loading points.
The applicable calculation depends on whether the fracture occurs within the specified region.
Therefore, the operator should not calculate flexural strength solely from the maximum load without checking the failure location.
Factors Affecting Flexural Strength of Concrete
Several factors influence the flexural strength of concrete:
1. Water-Cement Ratio
An increase in water-cement ratio generally reduces concrete strength when other factors remain unchanged.
2. Aggregate Quality
Aggregate strength, grading, shape and surface characteristics can influence flexural performance.
3. Cement Content
Cement content and paste quality influence the bond between cement paste and aggregate.
4. Compaction
Poor compaction can produce voids and weak zones.
5. Curing
Insufficient curing can reduce strength development.
6. Age of Concrete
Concrete generally gains strength with age under suitable curing conditions.
7. Specimen Preparation
Incorrect mould filling, inadequate compaction or improper handling can affect test results.
8. Loading Arrangement
Incorrect positioning of the specimen or loading rollers can produce unreliable results.
Flexural Strength vs Compressive Strength
| Property | Compressive Strength | Flexural Strength |
|---|---|---|
| Main resistance | Compression | Bending |
| Common specimen | Cube/cylinder | Beam |
| Main test | Compression test | Flexural test |
| Result | MPa | MPa |
| Typical application | General structural concrete | Pavements, floors and bending-related applications |
| Failure mode | Crushing | Bending/fracture |
Important: Flexural strength should not be calculated from compressive strength using one universal conversion factor. The relationship depends on the concrete mix, materials, age and testing conditions.
Flexural Strength vs Split Tensile Strength
Both tests provide information related to the tensile behavior of concrete, but they are different tests.
| Property | Flexural Strength | Split Tensile Strength |
|---|---|---|
| Loading | Bending | Diametral compression |
| Common specimen | Beam | Cylinder |
| Result | Modulus of rupture | Split tensile strength |
| Main behavior | Flexural failure | Indirect tensile failure |
| Common application | Pavements and flexural applications | Concrete tensile-property assessment |
For the detailed split tensile test, readers can refer to your existing:
Split Tensile Strength Test of Concrete
Common Mistakes During Flexural Strength Testing
Avoid the following mistakes:
- Incorrect beam dimensions.
- Poor compaction.
- Improper curing.
- Incorrect roller position.
- Misalignment of the specimen.
- Applying load incorrectly.
- Recording the wrong maximum load.
- Ignoring fracture location.
- Using the wrong calculation formula.
- Using a poorly calibrated testing machine.
- Rough handling of specimens before testing.
Practical Site and Laboratory Recommendations
For reliable results:
- Use representative concrete.
- Follow the approved mix proportions.
- Prepare moulds properly.
- Compact concrete adequately.
- Maintain proper curing conditions.
- Check beam dimensions before testing.
- Verify machine calibration.
- Position the specimen accurately.
- Apply load at the specified rate.
- Record the maximum failure load carefully.
- Record the fracture location.
- Use the correct formula for the applicable loading arrangement.
Why Flexural Strength is Important for Concrete Pavements
Flexural strength is particularly important for concrete pavements because pavement slabs are subjected to bending stresses from traffic loads and support conditions.
For pavement applications, the specified flexural strength should be assessed according to the applicable pavement specification and test standard.
The required strength should not be assumed solely from a general relationship with compressive strength.
Frequently Asked Questions
1. What is the flexural strength of concrete?
Flexural strength is the ability of concrete to resist failure when subjected to bending. It is commonly expressed as the modulus of rupture in MPa.
2. Which specimen is used for the flexural strength test?
A concrete beam specimen is commonly used. The exact specimen dimensions should follow the applicable test standard.
3. What is the modulus of rupture?
The modulus of rupture is the calculated flexural tensile stress at which the concrete beam fails under the specified flexural loading arrangement.
4. What is the formula for flexural strength?
The formula depends on the loading arrangement and fracture location. For the applicable loading arrangement, one commonly used expression is:
The applicable standard should be followed for the exact test configuration and fracture condition.
5. At what age is flexural strength tested?
Flexural strength is commonly assessed at 28 days, unless another testing age is specified by the project or applicable standard.
6. Is flexural strength the same as tensile strength?
No. Flexural strength is determined from a bending test, while direct tensile and split tensile strength are determined using different test methods.
7. Is flexural strength related to compressive strength?
Yes, concrete strength properties are related, but the relationship is not a universal fixed conversion. The relationship depends on the concrete mix and testing conditions.
8. Why is flexural strength important in pavement concrete?
Pavement slabs experience bending stresses due to traffic loads and support conditions. Therefore, flexural strength is an important performance parameter for pavement concrete.
9. What happens if the beam is improperly positioned?
Incorrect positioning can affect the stress distribution and failure pattern and may lead to an unreliable result.
10. Why should the fracture location be recorded?
The applicable calculation can depend on where the specimen fractures. Therefore, the fracture location must be checked before selecting the calculation method.
11. What is the difference between flexural strength and modulus of rupture?
Flexural strength and modulus of rupture refer to the flexural strength obtained from the specified beam test. The modulus of rupture represents the calculated extreme-fibre stress at failure under the prescribed loading arrangement.
Key Takeaways
- Flexural strength measures the resistance of concrete to bending.
- It is commonly determined using a concrete beam specimen.
- The result is generally expressed as modulus of rupture.
- Proper casting, compaction and curing are essential.
- The beam must be correctly positioned in the testing machine.
- The loading arrangement and fracture location are important when selecting the calculation formula.
- Flexural strength is particularly important for concrete pavements and other flexural applications.
- Flexural strength should not be treated as a simple fixed conversion from compressive strength.
- Always follow the applicable IS standard and project specification for specimen dimensions, loading arrangement, testing procedure and acceptance requirements.
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