Question: Mortar thickness of steel pipe. Steel used for water pipelines is often coated on the inside with cement mortar to prevent corrosion. In a study of the mortar coatings of a pipeline used in a water transmission project in California, the mortar thickness was specified to be 7/16”. A very large number of thickness measurements produced a mean equal to .635” and a standard deviation equal to .082”. If the thickness measurements were normally distributed, approximately what percentage were less than 7/16”.