Abstract: Disclosed herein is a concrete material comprising between 0.5% and 10% ferromagnetic fibres. Also disclosed herein is a method for measuring the strain state of a concrete material, the method comprising forming solid concrete containing between 0.5% and 10% ferromagnetic fibres in a random distribution throughout the concrete, applying an oscillating EM current to the concrete, and detecting the associated EM fields within the concrete. Also disclosed herein is the use of an oscillating EM current field to measure the strain state within a concrete material comprising between 0.5% and 10% ferromagnetic fibres.