We’ve spent some time learning about the role of concrete in the construction process, so now let’s learn about what goes into testing it!
The strength of a particular batch of concrete depends on many factors: temperature, age of the concrete, water-to-cement ratio, and the type of cement used. When the safety of a structure depends on its strength, it’s important to know just how much strength the structure’s concrete has.
Concrete samples are taken from a jobsite, typically mid-pour, and placed into cylindrical molds. Following ASTM standards, the technician “rods” the concrete (uses a steel rod to consolidate the concrete in the mold), and taps the outside of the mold with a mallet to remove air bubbles (much like you would tap a brownie pan on the counter to remove the air bubbles before baking). Following these procedures ensures that you get consistent results with each specimen and test.
The cylinders labelled with the date, project, location, and number, and are capped and placed where they are going stay until the concrete sets up, for up to 48 hours. The temperature of the curing environment must be regulated during this time, with consistent temperature and moisture levels. Some technicians use thermostatically controlled curing boxes to achieve this. The specimens should be kept out of direct sunlight and away from jobsite heaters in the winter.
Once the initial curing is completed, the cylinders are transported to the lab. There is a timeframe – transport can take no more than four hours – so it’s important to choose a testing company who can meet that restriction. At the lab, the molds are removed from the cylinders, and the test specimens are stored submerged in a water tank, or in a room with controlled temperature and humidity for final curing. Encorus has a “fog room” dedicated to curing concrete cylinders. The temperature is maintained at 70° – 77°F and humidity at 95% or greater in this room to ensure optimal conditions, and adherence to the ASTM standards.
All of this care and regulation culminates in the test cylinder being removed from the water tank or curing room, capped with a very high strength sulfur compound or unbonded cap, and broken, usually using a compression machine which applies a steady load of pressure to the sample until it breaks.
Do you need your concrete tested? Call Encorus’s Civil Laboratory Supervisor Jeremy Lake at (716) 592-3980, ext. 133, or email him at jlake@encorus.com.
