Pity the poor concrete test cylinder. All it does is lie around for 28 days then get crushed. During its brief life, it seems only fair that we treat it well. How often have you seen concrete test cylinders sitting in the sun or rolling around in the back of a pickup truck? Those poor abused cylinders could end up costing you money, since they can easily break at 15% lower than those that are treated well. Even though the cylinders may not be your responsibility technically, it's to your advantage to make sure they are treated right.
ASTM C 31, “Standard Practice for Making and Curing Concrete Test Specimens in the Field,” is the bible on handling cylinders. The advantage of an ASTM standard is that when the procedures are properly followed, the results will be consistent. Even if you don't like the number, at least it will be the same for all cylinders made with that concrete.
One important requirement of C 31 is that the testing technician making and curing the cylinders should be certified by the American Concrete Institute as a Field Testing Technician, Grade I. Armed with those credentials, we can be pretty sure that the technician knows how to do the work according to the standard.
Theron Tobolski at Prairie Material, Bridgeview, Ill., removes a cylinder from the final cure tank.
C31 starts by referring you to another ASTM standard, Practice C 172, “Sampling Freshly Mixed Concrete.” C 172 tells you to wait until any water has been added at the jobsite; then collect “two or more portions taken at regularly spaced intervals during discharge of the middle portion of the batch.” Finally the technician must mix those samples together.
Concrete is then placed into the cylinder molds. The molds need to be on a “level, rigid surface, free of vibration.” Concrete is put into the molds in three equal layers, and a steel rod is jabbed into each layer of concrete 25 times (rodding) to consolidate the concrete in the mold. The outside of the mold is also tapped with a mallet to remove any air bubbles. Most molds used today are single-use plastic, much more convenient than the old steel molds.
The top of the concrete cylinder is then struck off cleanly with the rod, and the cylinder is capped and marked. The cylinders are immediately placed where they are going to stay until the concrete sets up. During this phase, what C 31 calls initial curing, the cylinders can stay in this location for up to 48 hours, but must be kept at a temperature between 60° and 80° F and in a moist environment. C 31 gives us several ideas on ways to achieve this moist environment, including boxes, plastic bags, and simply capping the plastic cylinder mold. To control temperature, C 31 suggests using ventilation, ice (when the weather's hot), or thermostatically controlled boxes. Curing boxes are by far the best way to store cylinders onsite, and there are some good ones available (for example, the Thermocure II from Engius). Keeping cylinders out of the sun, or away from radiant heaters on a winter site, is top priority in proper curing.
The cylinders can be transported to the test lab no sooner than “eight hours after final set.” They should be padded, not allowed to freeze, and kept moist during transport, which cannot take longer than four hours. When the cylinders arrive at the lab, the molds are removed, and the cylinders are stored submerged in a water tank or in a room with 100% humidity—final curing. The temperature has to be kept at 73±3° F.
Finally, when the big moment arrives, the cylinders are taken out of the water tanks, capped with a very high strength sulfur compound, and tested. Testing always means that they are broken, most commonly in direct compression. So then we know the strength of the concrete, which is hopefully greater than the specified strength, and that ends the short but meaningful life of a concrete test cylinder.