Observed Variability in Cylinder and Core Strength from FHWA Highway Materials Engineering Course

This paper reports within-batch variability of 3-day concrete compressive strength from multiple tests, each with 90 lab- and field-cured cylinders and cores. In each case, all specimens were made with concrete sampled from the same single truckload. These tests were repeated 12 times over 15 years as an integral component of the Highway Materials Short Course, portland cement concrete module, conducted as a hands-on demonstration of the variability of measured strength within a single load of concrete. Participants sampled concrete from the truck, made specimens, and cured them under both laboratory and field conditions. Cylinders were tested per ASTM C39 by two separate, qualified laboratories to investigate interlaboratory variability. Each team also cast one concrete slab (4 ft × 4 ft × 8 in. thick) from which three cores were extracted. Compilation of the multiyear data set demonstrates broadly scattered test results. The range between maximum and minimum strength in any given year could be as much as one-third to one-half of the average. Relationships are shown between the mean and variance of 6- × 12-in. versus 4- × 8-in. cylinders, interlaboratory precision, applicability of field-cured cylinders, and the relative precision of core tests at 3 days.