Tensile Strencth of Concrete Formula |
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\( f_t \;=\; 7.5 \cdot \sqrt{ f_{c}{'} } \) (English units) \( f_t \;=\; 0.7 \cdot \sqrt{ f_{c}{'} } \) (Metric units) |
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| Symbol | English | Metric |
| \( f_t \) = Tensile Strength of Concrete | \(lbf \;/\; in^2\) | \(MPa\) |
| \( f_{c}{'} \) = Concrete Specified Cylinder 28-day Compression Strength | \(lbf \;/\; in^2\) | \(MPa\) |
Tensile strength of concrete is the maximum tensile stress that concrete can resist before cracking or failure in tension. Concrete is a heterogeneous composite material composed primarily of hydraulic cement, fine aggregate, coarse aggregate, and water. While it exhibits relatively high compressive strength, its resistance to direct tensile stress is comparatively low. This low tensile capacity is a fundamental and well-established material property, and it governs many aspects of structural concrete design, particularly the need for reinforcement in tension zones.
Tensile strength of concrete is determined through standardized test methods rather than simple uniaxial tension testing, which is difficult to perform reliably due to gripping and stress concentration issues. Commonly used methods include the splitting tensile test, and the flexural strength test. Direct tensile testing procedures also exist, though they are less common for structural concrete. The splitting tensile test applies a compressive line load along the diameter of a cylindrical specimen, inducing tensile stresses perpendicular to the load direction until failure occurs.
For normal weight concrete, the tensile strength is significantly lower than the compressive strength. Empirical relationships established in structural engineering practice relate tensile strength to the square root of the compressive strength. For example, building code provisions express the splitting tensile strength of normal weight concrete as proportional. This reflects extensive experimental data demonstrating that tensile strength increases with compressive strength but at a slower rate.
The tensile strength of concrete is critical in the analysis of cracking, serviceability, shear capacity, bond strength, and the design of reinforced and prestressed concrete members. Because plain concrete has limited tensile resistance, reinforcing steel or other tensile reinforcement is provided in structural members to carry tensile stresses after cracking. The measured tensile strength is therefore a key parameter in determining cracking loads, spacing of reinforcement, and durability performance in service.
