The tensile strength of concrete is one of the most crucial mechanical properties that determine how concrete behaves under tension. Although concrete is strong in compression, it is inherently weak in tension — typically only about 8–15% of its compressive strength. Understanding and measuring the tensile strength of concrete helps engineers design safer and more efficient reinforced structures such as beams, slabs, pavements, and water-retaining structures.
What Is the Tensile Strength of Concrete?
The tensile strength of concrete refers to its ability to resist cracking, fracture, or failure when subjected to tensile (pulling) forces. When concrete is stretched or bent, internal tensile stresses develop, and if these exceed its tensile capacity, cracks begin to form.
Because direct tension tests on concrete are difficult to perform due to gripping and alignment issues, the tensile strength is usually determined indirectly through tests such as the split tensile test or flexural strength test.
The tensile strength is a key factor in determining the crack resistance, ductility, and durability of concrete structures.
Typical Tensile Strength Values of Concrete
The tensile strength of concrete depends on factors like mix proportion, curing, aggregate properties, and age.
Typical values for normal-weight concrete are:
| Compressive Strength (fck) | Approx. Tensile Strength (ft) |
|---|---|
| 15 MPa | 1.7 MPa |
| 20 MPa | 2.2 MPa |
| 25 MPa | 2.6 MPa |
| 30 MPa | 2.9 MPa |
| 40 MPa | 3.5 MPa |
| 50 MPa | 4.1 MPa |
As per IS 456:2000, the modulus of rupture (flexural strength) of concrete may be taken approximately as:
fcr = 0.7√fck (MPa)
and the direct tensile strength can be approximated as:
ft = 0.56√fck (MPa)
Importance of Tensile Strength in Concrete
Although the tensile strength of concrete is low, it plays a vital role in structural behavior and design.
- Crack Control: It determines when and where cracks may occur under service loads.
- Flexural Strength: Beams and slabs are subjected to bending, where tensile stresses develop at the bottom fibers.
- Shear Resistance: Helps in evaluating diagonal tension and bond stresses in reinforced members.
- Durability: Affects the resistance to water permeability and environmental deterioration.
- Reinforcement Design: Reinforcement steel is provided mainly to resist tensile forces because plain concrete cannot handle them effectively.
Factors Affecting Tensile Strength of Concrete
- Compressive Strength: Higher compressive strength usually results in higher tensile strength.
- Water-Cement Ratio (w/c): A lower w/c ratio increases both compressive and tensile strength.
- Aggregate Properties: Shape, size, and surface texture of aggregates influence bonding.
- Curing Conditions: Proper curing promotes better hydration, enhancing tensile capacity.
- Age of Concrete: Tensile strength increases with age but at a slower rate than compressive strength.
- Mix Proportion: Richer mixes (more cement content) improve tensile strength.
- Rate of Loading: Rapid loading may reduce the apparent tensile strength.
Methods to Determine Tensile Strength of Concrete
Since direct tension tests are complex, several indirect methods are used in practice:
1. Direct Tensile Strength Test
This is the most straightforward but least commonly used method due to the difficulty in gripping and aligning the specimen.
Procedure:
- A cylindrical or prismatic concrete specimen is gripped at both ends and pulled apart using a testing machine.
- The load is applied axially at a uniform rate until failure occurs.
Formula:
ft=PAf_t = frac{P}{A}ft=AP
Where: - ftf_tft = direct tensile strength (MPa)
- PPP = maximum load at failure (N)
- AAA = cross-sectional area (mm²)
Limitation: Failure often occurs near grips due to stress concentration, making this method less reliable.
2. Split Tensile Strength Test (Indirect Tension Test)
This is the most common and standardized method (as per IS 5816:1999 and ASTM C496).
Principle:
A cylindrical specimen is placed horizontally in a testing machine and loaded along its diameter. The compressive load induces tensile stresses perpendicular to the applied force, causing the specimen to split.
Apparatus:
- Cylinder size: 150 mm diameter × 300 mm height (standard).
- Universal Testing Machine (UTM).
Formula:
ft=2PπLDf_t = frac{2P}{pi LD}ft=πLD2P
Where: - ftf_tft = split tensile strength (MPa)
- PPP = load at failure (N)
- LLL = length of the cylinder (mm)
- DDD = diameter of the cylinder (mm)
Typical Range: 2–5 MPa for normal concrete.
Advantages:
- Simple and reliable.
- Provides a uniform tensile stress field.
- Recommended by most codes for quality control.
3. Flexural Strength Test (Modulus of Rupture)
Also called the modulus of rupture test, this method measures the tensile strength of concrete under bending. It is suitable for pavements and slabs.
Test Standards: IS 516:1959, ASTM C78.
Specimen: Beam of 150 mm × 150 mm × 700 mm.
Procedure:
The beam is placed on two supports and loaded either at the center (three-point loading) or at one-third points (two-point loading).
Formula:
For two-point loading:
fcr=Plbd2f_{cr} = frac{Pl}{bd^2}fcr=bd2Pl
For center-point loading:
fcr=3Pl2bd2f_{cr} = frac{3Pl}{2bd^2}fcr=2bd23Pl
Where:
- fcrf_{cr}fcr = flexural tensile strength (MPa)
- PPP = failure load (N)
- lll = span length (mm)
- bbb, ddd = width and depth of beam (mm)
Typical Range: 3–5 MPa for ordinary concrete.
Relationship Between Tensile and Compressive Strength of Concrete
The tensile strength of concrete is usually proportional to the square root of compressive strength.
Empirical relationships:
- IS 456:2000:
ft=0.7fckf_t = 0.7 sqrt{f_{ck}}ft=0.7fck (for modulus of rupture) - ACI 318:
ft=0.56fc′f_t = 0.56 sqrt{f’_c}ft=0.56fc′ - BS 8110:
ft=0.6fcuf_t = 0.6 sqrt{f_{cu}}ft=0.6fcu
Where:
- fckf_{ck}fck, fc′f’_cfc′, fcuf_{cu}fcu = characteristic compressive strength (MPa).
Example:
For M25 concrete (fck = 25 MPa),
ft=0.5625=0.56×5=2.8 MPaf_t = 0.56 sqrt{25} = 0.56 × 5 = 2.8 text{ MPa}ft=0.5625=0.56×5=2.8 MPa
Stress-Strain Behavior of Concrete in Tension
When subjected to tension, concrete behaves linearly up to about 75–85% of its tensile strength. Beyond this limit, microcracks form and rapidly propagate, leading to brittle failure. Unlike compression, where concrete exhibits strain-hardening before failure, in tension it fails suddenly with little warning.
The stress-strain curve for tension is almost linear up to failure, indicating low ductility.
Improving the Tensile Strength of Concrete
Since concrete is weak in tension, various techniques are used to enhance its tensile capacity:
- Reinforced Concrete (RC): Adding steel bars to resist tension.
- Fiber Reinforced Concrete (FRC): Incorporating fibers like steel, glass, or polypropylene to improve toughness and crack control.
- Polymer Concrete: Using polymers instead of cement for better bonding and tensile strength.
- Prestressed Concrete: Introducing pre-compression in concrete to offset tensile stresses during service.
- High-Performance Concrete (HPC): Using admixtures and low w/c ratios for improved performance.
Applications of Tensile Strength in Design
- Beams and Slabs: Helps in determining reinforcement requirements.
- Pavement Design: Flexural strength governs the thickness of concrete pavements.
- Water Retaining Structures: Crack control to ensure watertightness.
- Bridges and Dams: Ensures safety against bending and uplift forces.
- Tunnels and Tanks: Helps in evaluating tensile cracking resistance.
Conclusion
The tensile strength of concrete is a vital mechanical property that governs the cracking behavior, durability, and structural safety of concrete structures. While concrete excels under compression, its tensile weakness necessitates the use of reinforcement or special techniques to improve performance. Tests such as the split tensile test and flexural strength test are indispensable for evaluating tensile capacity in both design and quality control. Understanding and considering tensile strength in structural design ensures that concrete structures remain strong, stable, and crack-resistant throughout their service life.
