Laboratory Testing of Hardened Concrete

Laboratory testing of hardened concrete is a crucial part of assessing concrete’s mechanical properties, particularly its compressive, tensile, and flexural strength. Different specimen types—cubes, cylinders, and prisms—are used to measure these strengths. Specimen dimensions vary according to standards, with nominal sizes for the main dimension, d, set at 100 mm, 150 mm, 200 mm, 250 mm, and 300 mm. Cylindrical specimens are typically twice as tall as their diameter, while prisms are at least 3.5 times longer than their width.

Compressive Strength

The compressive strength test uses both cube and cylinder specimens. To conduct this test, the specimen is placed with its load-bearing direction perpendicular to the direction in which it was cast. This ensures that the load is applied in the same orientation as it would bear in actual structural use. The load is applied at a stress rate of 0.2 to 1.0 MPa per second until the specimen fails. The compressive strength, fc, is calculated by dividing the maximum load F by the cross-sectional area A of the specimen. Based on the sample size, standards provide guidance on the expected mean strength, fcm , in relation to the characteristic strength, fck , with adjustments for different sample sizes.

Tensile Splitting Strength

The tensile splitting strength test, also known as the "Brazilian" test, specifically evaluates a cylindrical specimen. For this test, the cylinder’s longitudinal axis is centered and aligned perpendicular to two upper and lower rollers. This orientation ensures that the load is applied horizontally across the diameter of the cylinder, indirectly creating tensile stress. The stress rate is controlled within a range of 0.04 to 0.06 MPa per second, and loading continues until failure. The tensile splitting strength, fct, is determined by the formula fct=2F/πLd, where F is the maximum load, L the length, and d the diameter of the specimen. For sample sizes of 15 or more, a correction factor is added to the mean strength, fctm , using a factor of 1.48σ to account for variability.

Flexural Strength

Flexural strength testing employs a prismatic specimen and can use either a two-point loading setup or a center-point loading setup. In both setups, the longitudinal axis of the prism is positioned perpendicular to the upper and lower rollers. This setup tests the specimen’s ability to resist bending. The stress rate is controlled within a range of 0.04 to 0.06 MPa per second, similar to the tensile splitting strength test. In the two-point loading setup, flexural strength fcf  is calculated with the formula fcf=Fl/bh2, while for the center-point setup, it is fcf=3Fl/2bh2. Here, F represents the maximum load, l the distance between supporting rollers, b the specimen width, and h its depth.

Standards and References

These testing procedures adhere to British Standards Institution (BSI) guidelines, including BS EN 12390 for concrete testing and BS EN 206 for concrete specifications. BS EN 12390 provides specifications for specimen shapes, compressive strength, tensile splitting strength, and flexural strength, ensuring consistent methods and reliable results across different labs and applications. The standards not only define the shape and dimensions of specimens but also specify acceptable ranges for loading rates, providing a consistent basis for comparing results.