lujuusluokka

Lujuusluokka, or strength class, is a label used in construction to denote the mechanical strength capacity of a material under standardized testing. It is a key concept in Finnish building practice and in European structural codes, where materials are grouped into classes that guide design, procurement and quality control.

In concrete, lujuusluokka is most commonly encountered and is usually written as Cx/y (for example C25/30). The first number represents the cylinder compressive strength in megapascals (MPa) and the second the cube strength, reflecting different test methods. These values are obtained from standardized tests on specimens cured under prescribed conditions, typically at 28 days. The chosen strength class affects the design values used in calculations, the required cross‑sectional dimensions, and the overall safety and durability of a structure. The classification is defined by national and European standards, such as EN 206 and its Finnish adaptations (SFS EN 206).

Outside concrete, lujuusluokka is also used in other construction materials to indicate their mechanical capability, though the specific scales and test methods differ by material and code. In all cases, the strength class provides an at-a-glance indication of expected performance and is used alongside other material properties to ensure compliance with building regulations and project specifications.