Compressive strength tests indicate the comprehensive strength by the unit stress required to cause failure of a specimen. This is an important measure to assure quality and profitability in the concrete business. KSA Industries of Portsmouth, Ohio, a manufacturer of concrete railroad ties, tests its mix daily for compressive strength. KSA makes overnight tests, 7-day tests, and 28-day tests. All KSA's testing is done in the company's own laboratory. Orco Block in Riverside, Calif., sends most of its testing out, rather than doing it in-house.