The NSR-10 accepts the strength of a concrete when two conditions are met: every average of three consecutive tests is equal to or greater than the specified strength (f'c), and no individual test falls more than 3.5 MPa below it when f'c is 35 MPa or less (C.5.6.3.3). Each test is the average of at least two 150 x 300 mm cylinders or three 100 x 200 mm cylinders, normally at 28 days (C.5.6.2.4).
How many samples to take
At least one sample is taken of each class of concrete placed in a day, and no fewer than one for every 40 m³, every 200 m² of slabs or walls, or every 50 batches (C.5.6.2.1). If that frequency gives fewer than five tests for the whole project, at least five batches are tested (C.5.6.2.2).
If a result comes out low
First, check whether the load-carrying capacity is compromised. If the doubt is confirmed, cores are taken: three for each deficient test (C.5.6.5.2). The area is considered adequate if the average of the three cores is at least 85% of f'c and none is below 75% (C.5.6.5.4). If it still does not comply, the authority may order a load test (C.5.6.5.5).
What fails most often on site
Cylinders cured badly, taken from the first discharge of the truck or left in the sun on site; records that get lost; and laboratories without traceability. That is why the technical supervisor approves the laboratory and keeps the record of every pour. See technical supervision.
Source: NSR-10, Title C, section C.5.6.
Frequently asked questions
When does the NSR-10 accept a concrete?
When every average of three consecutive tests is equal to or greater than f'c and no test falls more than 3.5 MPa below it, for f'c up to 35 MPa (C.5.6.3.3).
What happens if a cylinder comes out low?
Three cores are taken for each deficient test; the area is adequate if their average is at least 85% of f'c and none falls below 75% (C.5.6.5).
