This is not going to be one more review of the step by step procedure of casting test cylinders. That procedure was clearly described in the article, " Standard Procedure for Making Concrete Cylinders for Strength Test" in the June, 1957, issue of Concrete Construction. Rather, our purpose here is to clarify some matter which are present by inference only in the ASTM specifications or which are sometimes misunderstood. Perhaps consideration should first be give to the reason for casting test cylinders. Contrary to the thought of many, cylinders are not ordinarily made to determine concrete strength in structures or pavements. Cylinder are cast to verify the potential strength of the concrete being used. In other words, this is a method of checking the quality of work done before actually placing begins. If the cylinders indicate that the concrete was capable of developing the required strength but subsequent developments prove that it was not, then the trouble spot can usually be narrowed down to the placement in the forms after its delivery. If you wish to know the strength of the concrete in place, then an additional three cylinders should be cast. These extra cylinders are handled in exactly the same manner as the concrete on the job and the other three cylinders are man-made and cured in conformance with the ASTM specifications. Even when properly cast and cured, test cylinder can vary surprisingly. In a test, the same six-sack mix concrete was delivered to several contractors who cast cylinders from it. The results from these specimens ranged from 1800 psi to 4200 psi. On another occasion, identical cylinders were delivered to three commercial testing laboratories. Their reports of strengths showed a difference of 27 percent.