Cubes Be Gone: Why Cubes Shouldn’t Be Used To Measure Compressive Strength
The traditional method of assessing the compressive strength of concrete on-site is through collecting sample cubes, and sending them to a lab to be tested. However, companies are quickly realising that there are many limitations to this practice which make it inferior to modern day methods.
There are a number of reasons why it’s time companies improve their method for measuring compressive strength.
Predetermined crush dates make schedules incredibly inflexible. Assuming crush dates are scheduled every 7 days, if, for example, your cube reaches strength on day 8, the test will show a slab that has not reached strength. As a result, permission to strike will not be granted until the next strike date, day 14, creating a large gap of time where you could be progressing your programme.
Moreover, despite mixes being over-designed in order to meet the required strength, often the cubes aren’t prepared properly, resulting in inaccurate strength results. This is commonly due to under-compaction of the test cube. Compaction removes air voids from the sample — Any remaining air left will result in a lower strength (1% of air left in the concrete can reduce the strength by 5%). Consequently, the strength reading isn’t representative of the actual slab, and permit to strike is withheld unnecessarily.
In addition, the inaccuracy of cubes stem from them being kept in standard lab conditions which do not reflect the temperature the in-situ concrete. In labs, the temperature is kept steady at 20℃, whereas concrete in-situ often reaches peak temperatures of 40–50℃. Strength is achieved following a chemical reaction within the concrete which is accelerated in higher temperatures. Therefore, the cube is not indicative of when the slab reaches strength.
Although there is no replacement for cubes for compliance purposes, replacing early strike cubes with other more accurate, and more frequent methods of measuring can make a big impact in keeping schedules efficient.