1.7 Design strength concrete

Concrete Strength and Design Considerations

Compressive Stress Measurement

• The compressive stress in concrete can be expressed in terms of 150 and the characteristic strength at 28 days, or as a function of cylinder dimensions (150 mm diameter, 300 mm height) .

• Cylinder strains are typically smaller than cube strains, with cube strain being about 60% of the cube strength. This is crucial for design calculations in Europe .

Characteristic Strength Definitions

• In European standards, the characteristic compressive strength is denoted as fck. The first number indicates the characteristic strain while the second number represents the characteristic cube strength of concrete used in design .

• For example, if fck = 35, this indicates a specific grade of concrete based on its characteristics .

Design Strength vs. Characteristic Strength

• Design strengths are generally lower than characteristic strengths due to safety factors considered in structural design. This relationship is mathematically represented by equations involving FCD (design compressive strength) and FCTD (design tensile strength) .

• The factor of safety for reinforced concrete structures (RCC) is influenced by both characteristic compressive and tensile strengths .

Safety Factors and Eurocode Guidelines

• The partial safety factor for concrete, γC, is set at 1.5 according to Eurocode guidelines. This value ensures adequate safety margins during construction and use .