Concrete is the most used construction material in the whole world. More than that, behind water, it’s the most used material, all categories combined. Worldwide, about 10 billion tons of concrete are produced per year. Obviously, the strength of the concrete produced varies, but in most projects, it is crucial to have an idea of this measurement.
During the last few decades of significant progress for concrete, today’s most widely used compressive strength standard emerged: the cylinder concrete break tests. It is a method that has allowed construction to prosper for so long, building the modern world that we live in. For that reason, we must all tip our hats towards this compressive strength test method.
However, it is also a method that allows for errors, which on numerous occasions can have significant safety consequences. Moreover, this is proportionally a very slow method for such a fast-paced industry, generating unnecessary costs and project extensions to general contractors.
Many errors can originate from this testing method, either giving a result that is higher or lower than the real mix design properties. In both cases, we are mostly aware of the problems causing these errors in strength values. However, there are too many variables to control for break tests to always yield perfect results. It is first necessary to understand the two types of cylinder break test methods, or rather the curing difference between standard-cured and field-cured.
Laboratory-Cured vs. Field-Cured
The standard-cured method, or laboratory-cured method, is used for quality control of concrete. The concrete cylinders are cast onsite and brought to a third-party laboratory, where the curing temperature is set at 23 ± 2°C (73 ± 4°F) and the humidity at a minimum of 95%. On the other hand, the field curing method is used to resemble, as close as possible, the conditions of the structure. To do so, the cylinders are kept close to the structure, hoping to mimic the temperature and humidity of the in-place concrete.
In both cases, the cylinders are cast onsite, with different layering and rodding methods depending on the country and the standard test methods used. By experience, I know that casting concrete cylinder samples is a repetitive task and it is hard to always prevent human errors. There could be mistakes made with the rodding depth, the size of the layers, the lack of tampering, and so on.
Once the cylinders are cast, they will remain onsite for one to three days for the initial curing to be over. The initial curing is a major factor in strength development. The cure must be done between 16 and 27°C (60 and 81°F) for concrete with specified strength less than 40 MPa (5800 psi) and between 20 and 26°C (68 and 79°F) for those with greater strength according to ASTM C31/C31M – Standard Practice for Making and Curing Concrete Test Specimens in the Field. Also, high humidity of the samples should be maintained at all times.
Unfortunately, those measures can be often disregarded, and the initial curing is non-standard in many cases. The worst aspect is when those non-standard cures are not reported, thus making them impossible to track when low breaks happen. In the future, this may encourage unnecessarily using concrete with higher specified strength to make sure the 28-days results are within the acceptable range, often at a much higher cost. Finally, these curing errors increase the variation between strength results. This sometimes leads to higher chances of having to drill concrete cores from the structure, creating massive delays on the job site as well as generating unnecessary costs.