fischer fixing-systems main-catalogue Page 644
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Basic knowledge of fastening technology Building material – Concrete. The substrate’s quality is crucial when selecting the correct fixing: Concrete A differentiation is made between concrete, masonry and panel building materials. Concrete is made from a mixture of cement, aggregates and water. 16 Concrete’s main properties are: Concrete is mainly split into two categories: · High level of compressive strength, but only low tensile strength (≈ 10 % of the compressive strength). · Inserting individual rebars and or mats will increase tensile strength (steel + concrete = reinforced concrete). · Structure easily reproducible as it is regulated by standards, therefore it is an ideal anchor base. · Standard concrete and lightweight concrete. While standard concrete contains gravel; lightweight concrete contains additives such as pumice, expanded clay or polystyrene® usually with a lower compressive strength or bulk density. This leads to unfavourable conditions for anchor fixings. · The load bearing capacity of heavy duty fixings depends on the concrete’s compressive and tensile strength. This is indicated by the numbers in the abbreviations: e. g. the most commonly used concrete compressive strength is C20/25 with a cube compressive strength of 25 N/mm². Expert tip Standard concrete qualities: C12/15 to C50/60, even higher grades are also available for special applications. The majority of anchors approved for concrete may only be used from concrete quality from C20/25 up to a max. of C50/60. In the past, designations from DIN 1045 were used in Germany: B25 (~ C20/25) to B55 (~ C45/55). C = concrete 20 = compressive strength fck or fck, cyl of a concrete test cylinder (Ø 150 mm, height 300 mm) in N/mm2 25 = compressive strength fck, cube of a concrete test cube (edge length 150 mm) in N/mm2 644 Normal concrete without accelerating additives reaches its nominal strength after 28 days. Only then can the fixing be installed in compliance with the approval /assessment. Fresh concrete: still workable up to approx. one hour after pouring. Green concrete: starts to harden after four hours, no longer workable. New concrete: is hardened after 28 days, however minimum compressive strength not yet reached. Hard concrete: more than 28 days old, nominal strength reached.
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