It is easy to calculate the overall concentration of steel fibres in a concrete batch seeing as the mass of fibres used and the volume of the batch are both known. However, it is more difficult to discern how well the fibres are distributed throughout the batch, which is important to know as poor distribution could lead to weak points in the hardened concrete.
The wash out test is a quick way to test multiple parts of the batch to see if the concentration of fibres is consistent throughout, though it must be noted that test results are not conclusive and should be regarded as indicative of fibre distribution only.
The method is as follows:
The national standards for this test are BS EN 1170 and ASTM C1228 in the UK and USA respectively.