Concrete Testing 101
Concrete is one of the most versatile and widely used construction materials on earth. It’s strong, durable, low maintenance, fire resistant, simple to use, and can be made to fit any size or shape. When concrete is poured, it is critical that concrete testing is completed to ensure the mix is up to scratch!
What Does Concrete Testing Tell Us? Concrete testing is a performance check of the mix delivered for the specific project. Thus, concrete is tested in the field and laboratory to ensure the concrete supplied meets the mix design specifications.
Concrete testing in the field and laboratory is conducted following stringent procedures laid out in CSA A23.2:19. Whether the project involves concrete testing in Toronto, Vancouver, or St. John’s, these are the standards that must be followed.
A scoop and container are used to collect samples of the fresh concrete from the point of discharge. The scoop and container must be large enough to collect a sufficient amount of concrete but small enough to ensure easy and safe handling of the fresh concrete. Concrete should be sampled between ten percent and ninety percent of the load volume to ensure the sample is representative. It is important to wear gloves when handling fresh concrete to avoid concrete burns.
A slump cone is used to complete the concrete slump test, which measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete, and therefore the ease which the concrete flows. AllRock has significant experience with concrete and slump testing in Ottawa, Toronto, and our offices in Newfoundland and Labrador.
Air Content Test
A concrete air meter is used to measure the air content of air entrained concrete. The amount of can affect the strength of the concrete, and can determine whether the concrete is protected from freeze-thaw conditions or is prone to weathering, and can affect the finish and cosmetic look of the concrete.
Casting Concrete Cylinders Concrete cylinder samples for compressive strength testing are commonly cast in 100 x 200-mm plastic molds. Typically, a set of three (3) or five (5) cylinders are cast for each concrete pour, or as requested. Metal tamping rods are used to eliminate air voids in the concrete samples collected. Cylinders are cast in three (3) layers and rodded 20 times per layer using a 10mm diameter steel rod. A concrete vibrator may also be required to eliminate air voids for concrete mixes when the slump is less than 40mm. The sides of the mold are tapped 10 to 15 times using a small rubber mallet to release any larger air bubbles that might have been trapped.
The cylinders will be used to test the compressive strength of the concrete over a specified period, which will ensure the concrete meets the specified strength for the project.
Cylinder Curing – Field
After the concrete cylinders are cast, they are placed in a curing box and left onsite to allow for initial curing. Various methods are used to ensure that the initial curing conditions are such that the temperature is regulated between 15 degrees Celsius and 25 degrees Celsius during initial curing and hardening phase. A digital thermometer is placed in the cooler with the cylinders to record the temperatures during the field curing process. The samples are typically retrieved the following day and brought to the laboratory for further curing and testing.
Cylinder Curing – Laboratory
Once the concrete cylinders are delivered to the laboratory, they are removed from the plastic molds and placed in a temperature-controlled limewater bath. The bath ensures that the concrete is kept moist by curing, to prevent it from drying out as it hardens. The bath also ensures that the cylinders are kept at a consistent temperature throughout the curing process. The cylinders are cured in the bath for 7 and 28 days, at which point they will be tested for unconfined compressive strength (UCS) in Toronto. The concrete is expected to achieve approximately 65% strength after 7 days and 99% strength after 28 days.
Cylinder Preparation Prior to compressive strength testing, the cylinders must be prepared. Firstly, the cylinders are removed from the curing bath. Next, the cylinders are either placed in a grinder to make a smooth top and bottom surface, or they can be capped using sulfur.
Compressive Strength Testing Compressive testing of concrete cylinders is used to determine the strength of the concrete after a specified period of curing. Prior to testing, the cylinders are subject to various dimensional verification checks to ensure that there are no irregularities in the cylinder length, shape, diameter, or surface prior to testing. The concrete test cylinders are placed in a concrete compression machine and loaded at a controlled rate until the cylinder breaks. The type of fracture is also analyzed and recorded.
Written by: Chelsey Crawley and Nigel Chaytor