Tensile Strength of Concrete – Tests, Formula and Importance

Ask any civil engineering student what concrete’s biggest weakness is and they’ll say “tension.” But do you actually know why concrete is so weak in tension, how we test it, and what that weakness means for design? If not, this article has you covered. Understanding tensile strength of concrete is essential not just for RTMNU exams but for truly understanding why RCC structures are designed the way they are.

1. Why is Concrete Weak in Tension?

Here’s a thought experiment: imagine a concrete beam loaded at the centre. The top fibres get compressed (squeezed) while the bottom fibres get stretched (tensioned). The bottom side will crack long before the top side crushes. That’s because concrete has a deep structural weakness in tension, and it comes from three causes:

• Pre-existing micro-cracks: Even before any load is applied, tiny cracks already exist at the interface between aggregate particles and the cement paste (called the Interfacial Transition Zone or ITZ). These form during curing due to shrinkage. When tensile stress comes along, it finds these ready-made crack tips and rapidly propagates them.
• Brittle paste matrix: Unlike steel which can deform plastically (stretch and bend before snapping), the cement paste matrix is brittle. Once a micro-crack forms under tension, there is no mechanism to redistribute stress — the crack simply runs through the material.
• Weak ITZ: The interface zone between aggregate and paste is the weakest structural link. It is more porous and has a higher concentration of Ca(OH)&sub2; crystals compared to the bulk paste. This zone fails first under tension.

The result? Tensile strength is only about 1/8 to 1/14 of the compressive strength (roughly 1/10 as a rule of thumb). For M25 concrete with f_ck = 25 MPa, tensile strength is only about 2.5–3.5 MPa. That’s why steel is essential in RCC.

2. Three Types of Tensile Strength

There isn’t just one tensile strength — it depends on how you apply the tension. Each type is measured differently and gives a different value:

• Direct Tensile Strength (f_t): Pure axial tension applied to a specimen. Theoretically most accurate but practically very difficult. No standard IS test exists for concrete.
• Splitting Tensile Strength (f_ct): Indirect method using a cylinder loaded diametrically. Standard method in India. IS 5816:1999. Most commonly used in lab and field testing.
• Flexural Tensile Strength – Modulus of Rupture (f_r): Tensile stress at the bottom fibre of a beam at failure in bending. Always the highest of the three values for the same concrete.

Magnitude ranking: Direct tensile < Split tensile < Modulus of Rupture (MOR)

3. Direct Tensile Test

In this test, a concrete specimen (usually dog-bone shaped or cylindrical) is gripped at both ends and pulled apart. Sounds simple, but in practice it’s extremely difficult to execute reliably:

• Applying perfectly axial tension without any eccentricity (bending) is nearly impossible. Even a tiny misalignment creates bending stress that combines with tension and gives a falsely low result.
• Gripping concrete at the ends causes stress concentrations at the grip locations, which is where the specimen often fails rather than in the gauge length.
• Results are highly variable and difficult to reproduce between labs.

For these reasons, the direct tensile test is not used as a standard test for concrete in India. The split cylinder test (IS 5816) is the preferred indirect method.

4. Split Cylinder Test – IS 5816:1999

IS Code: IS 5816:1999 – Splitting Tensile Strength of Concrete – Method of Test

This is elegant in its simplicity: by applying a compressive line load along the diameter of a concrete cylinder, you create a uniform tensile stress perpendicular to the load direction. The cylinder splits in two halves along the loaded diameter.

Specimen

Standard cylinder: 150 mm diameter × 300 mm length (L/D = 2). Cast and cured the same way as compressive strength cylinders.

Procedure

1. Mark two diametrically opposite lines along the full length of the cylinder as the load application lines.
2. Place a narrow hardboard bearing strip (12 mm wide × full cylinder length) on the lower platen. Set the cylinder on its side on this strip, aligned so the load line is centred.
3. Place a second bearing strip on top of the cylinder along the top load line.
4. Apply compressive load at a rate of 14–21 kN/min continuously and without shock until the cylinder splits along the diameter.
5. Record the maximum load P at failure.

Formula for Splitting Tensile Strength

5. Solved Numerical

Problem: A 150 mm × 300 mm concrete cylinder fails at a load of P = 210 kN in a split tensile test. Calculate the splitting tensile strength.

6. IS 456:2000 Formula for Tensile Strength

Running a split tensile test every time you need the tensile strength is impractical. So IS 456:2000 provides a simple empirical formula to estimate the flexural tensile strength (Modulus of Rupture) directly from the compressive strength:

Example calculations:

• M20: f_r = 0.7 × √20 = 0.7 × 4.47 = 3.13 MPa
• M25: f_r = 0.7 × √25 = 0.7 × 5.00 = 3.50 MPa
• M30: f_r = 0.7 × √30 = 0.7 × 5.48 = 3.83 MPa
• M40: f_r = 0.7 × √40 = 0.7 × 6.32 = 4.43 MPa

7. Typical Values for Common Grades

Note: Direct tensile < Split tensile < Modulus of Rupture for the same concrete grade.

8. Why Tensile Strength Matters in Structural Design

Even though we rarely design concrete to carry tension (that’s steel’s job), tensile strength appears in design calculations more often than you might expect:

• Shear design of beams: Diagonal tension causes shear failure in beams. The shear capacity contribution of concrete (v_c) is directly related to tensile strength. That’s why shear reinforcement (stirrups) is always required in beams.
• Crack width calculations: IS 456:2000 uses the modulus of rupture to calculate the cracking moment — the moment at which the first crack appears in a beam. Designers limit crack widths to protect reinforcement from corrosion.
• Rigid pavement design (IRC:58): Concrete road slabs resist vehicle wheel loads through bending. The entire pavement slab thickness design is based on the modulus of rupture, not compressive strength.
• Punching shear: Flat slabs fail by the column punching through in diagonal tension. Concrete tensile strength governs this failure mode.
• Prestressed concrete: PSC design ensures no tensile stress (or limited tensile stress below f_r) develops in the concrete at service loads. That’s the whole point of pre-compression.

9. Diagram – Split Cylinder Test & Tensile Strength Comparison

10. Exam Tips (RTMNU)

• ✅ IS 5816:1999 = split cylinder tensile test. Always cite this IS code in answers.
• ✅ Split test formula: f_ct = 2P ÷ (πDL) — practice at least 2 numerical problems.
• ✅ IS 456 formula: f_r = 0.7√f_ck (Clause 6.2.2) — this is asked in almost every exam. Know it cold.
• ✅ Tensile strength ≈ 1/10 of compressive strength — state this as a key fact in every tensile strength answer.
• ✅ Know the ranking: Direct tensile < Split tensile < MOR. Explain why for full marks.
• ✅ Cylinder for split test: 150 mm dia × 300 mm long — same as for compressive cylinder test.
• ✅ Bearing strips used in split test: 12 mm wide hardboard strips — prevents stress concentration at the load line.

11. Key Takeaways

• Concrete is weak in tension due to pre-existing micro-cracks at the ITZ and the brittle nature of the cement paste matrix.
• Three types: direct tensile, split tensile (IS 5816), and flexural tensile (modulus of rupture). Direct < Split < MOR.
• Split cylinder test: f_ct = 2P ÷ (πDL) on a 150 mm × 300 mm cylinder (IS 5816:1999).
• IS 456:2000 formula: f_r = 0.7√f_ck (Clause 6.2.2) — used in beam crack and pavement design.
• Tensile strength is critical for shear design, crack width calculations, rigid pavement design, and PSC.

12. Frequently Asked Questions (FAQs)

Q1. Why is concrete weak in tension?

Concrete is weak in tension because it is a brittle, heterogeneous material with pre-existing micro-cracks at the aggregate-paste interface (ITZ). When tensile stress is applied, these micro-cracks rapidly propagate through the brittle cement paste, causing failure at stresses only about 1/8 to 1/14 of the compressive strength.

Q2. What is the split cylinder test and what is its IS code?

The split cylinder (Brazil) test is a standard indirect method of measuring the tensile strength of concrete. A standard 150 mm × 300 mm cylinder is placed on its side and loaded diametrically through narrow bearing strips. This induces uniform tensile stress perpendicular to the load direction, splitting the cylinder along its diameter. The IS code is IS 5816:1999. Tensile strength = 2P ÷ (πDL).

Q3. What is the IS 456:2000 formula for tensile strength?

IS 456:2000 Clause 6.2.2 gives the empirical formula for the Modulus of Rupture (flexural tensile strength): f_r = 0.7√f_ck, where f_ck is the characteristic compressive strength in MPa and f_r is in MPa. For M25: f_r = 0.7 × 5 = 3.5 MPa.

Q4. Why is flexural tensile strength (MOR) higher than split tensile strength?

In flexural testing, only the extreme bottom fibre reaches maximum tensile stress. As cracking begins, the concrete can redistribute stress to adjacent fibres. In the split test, the tensile stress is more uniformly distributed across the diameter with less scope for redistribution. This size effect and stress gradient mean MOR is always higher than split tensile strength for the same concrete.

Q5. Where is the tensile strength of concrete used in design?

Tensile strength is used in: (1) shear capacity of beams (diagonal tension), (2) cracking moment calculations for deflection and crack width control (IS 456), (3) rigid pavement slab thickness design per IRC:58 (based on MOR), (4) punching shear of flat slabs, and (5) PSC design (ensuring no tensile stress exceeds f_r).

🔗 Related: Flexural Strength of Concrete – Modulus of Rupture Test

🔗 Related: Compressive Strength of Concrete – Cube Test and Grades

📚 Reference: IS 5816:1999 – Splitting Tensile Strength of Concrete, BIS