Coke strength is an indicator of physical strength of a coke made from a particular coal. Coke needs to be strong to support the iron ore and coke mix above it in the blast furnace.

The test is based on a procedure developed by Nippon Steel Corp in the 1970's as an attempt to get an indication of coke performance and is used widely throughout the world evaluate coke hot strength. The test studies the effects of one of the key coke reactions in the blast furnace on the strength of the coke. In the test a 200g sample of 21mm +19mm square hole coke (usually prepared by crushing 10kg of +25mm coke) is heated at 1100 C under 1atm pressure of carbon dioxide for 2 hours. The coke is cooled under nitrogen and the weight loss during reaction is determined. The percentage weight loss is known as the reactivity (CRI). The reacted coke is placed in an I drum (no lifters) and subjected to 600 revolutions. The percent of material removed from the drum that is +10mm square hole is known as the coke strength after reaction (CSR). The test procedure is shown the figure below.   

Generally a high CSR indicates a strong coke.

Reifenstein (2003)  examined the standard, with particular reference to a set of variables which were possible sources of variation within the coke reactivity test.