It seems like low-strength breaks always happen at the worst time, when your schedule is already delayed and you are trying to move your project along. Under ACI and CSA Standards, a break is considered to be low when an individual compressive strength test (average of two cylinders) is more than 500 psi below the specified strength or if the average of three consecutive sets of tests does not equal or exceed the specified strength. These requirements account for the typical variability of concrete strength tests.
Understanding what test cylinders represent is important: Cylinders that are standard cured—according to ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field—are not intended to represent the in-place, early-age strength of the concrete even though they are often used that way. Field-cured cylinders under C 31 have to be kept at the jobsite under conditions as near as possible to the concrete they represent until they are tested. That’s difficult, so we are often stuck with strength results from lab-cured cylinders. Understanding what can contribute to failing strength tests either for standard- or field-cured cylinders is important. The six most common causes of low-strength concrete breaks are:
1. Concrete Mix Design: The concrete mix does not gain strength in the time that had been specified, making compressive strength testing lower than expected. This may happen if the wrong mix has been sent or if water is added to a mix to make it easier to place, resulting in a higher water-cement ratio.
2. Sample Production: When casting concrete cylinders, the test specimens are not properly prepared and rodded.
3. Improper Handling: The concrete samples are not handled or transported properly, resulting in cracking.
4. Curing Conditions: The ambient conditions of field-cured specimens do not reflect those of the in-situ concrete. If this is not done properly, the strength value will not represent the strength in the structure.
5. Cylinder Preparation: After the concrete specimen has been cured, it is prepped in the lab for testing by grinding or capping the ends of the cylinder. If the cylinder is not prepared with care and attention, it will not break properly under the applied load.
6. Calculation Errors: Before a lab can break the concrete cylinders, the compression test machine much be properly calibrated.
So what can you do when concrete cylinder breaks are low? The most common solution is to wait and let your concrete continue curing, then test strength again on your next break day (that is, day 7 or 14). If your concrete is still underperforming after that second break, then you have a real problem. You can look at the causes listed above, you can core the structure, or, as a last resort, you can conduct load tests to check the capacity of the structural elements.
But another option is to ditch cylinders for early-age strength testing altogether and switch to wireless maturity sensors to monitor strength over time. These sensors are embedded in the concrete element and monitor strength constantly in real-time using an app on a mobile device. This means that you can check the performance of your mix at any time. Standard-cured cylinders are still made and tested on day 28 for acceptance purposes, but all other early-age strength is determined from the app, saving a lot of time spent on labor that would normally be used making, transporting, and testing the cylinders. CC
The content of this article was created by Giatec Scientific.
