A low result does not mean the structure is wrong. The first thing is to look at the age at which the cylinder was tested; then, apply the NSR-10 acceptance criterion (C.5.6.3.3), and only if the doubt remains, extract cores (C.5.6.5). Of the 20 cylinder tests of the Bodegas Centurión structure, 16 exceeded f′c at 28 days; the other four show the typical cases.
1. At what age was it tested?
At 7 and 14 days, a normal concrete has not yet reached its design strength, which is measured at 28 days. At a site committee in January 2024 a 14-day result of 78% was reported: not a reason for alarm, but for waiting for the 28-day result.
2. Does it meet the code criterion?
The NSR-10 does not require every cylinder to exceed f′c: it requires every average of three consecutive tests to equal or exceed it, and no individual test to fall more than 3.5 MPa below it, for f′c up to 35 MPa. At Centurión, the column test gave 26.15 MPa at 28 days against 28 MPa specified: 93%, but within the 3.5 MPa margin (the limit was 24.5 MPa). At 56 days, that same concrete reached 31.65 MPa, 113%.
3. Where did the concrete come from?
The three truly low results at Centurión have something in common: they were concrete mixed on site, without a plant delivery ticket, for specific elements: the crown beam of a facade (12 MPa, 43%), the bottom of the wastewater treatment plant (14.3 MPa, 51%) and some f′c 21 MPa manholes (12.45 MPa at 28 days and 17.65 MPa at 56). Site-mixed concrete depends on that day's proportions, water and mixing; that is why the oversight samples and tests it just like ready-mix.
| Element | f′c | 28 days | 56 days |
|---|---|---|---|
| Columns | 28 MPa | 26.15 MPa (93%) | 31.65 MPa (113%) |
| Facade crown beam | 28 MPa | 12 MPa (43%) | — |
| Wastewater plant bottom | 28 MPa | 14.3 MPa (51%) | — |
| Manholes | 21 MPa | 12.45 MPa (59%) | 17.65 MPa (84%) |
| The other 16 tests | 21 and 28 MPa | 103% a 163% | — |
4. If the doubt remains: cores
When a test fails and calculations show that load capacity may be affected, three cores are taken for each deficient test. The area is considered adequate if the core average reaches 85% of f′c and none falls below 75% (C.5.6.5.4). Non-destructive tests, such as the rebound hammer or ultrasound, help delimit the doubtful area, but do not replace cores for accepting the concrete.
The full sampling and acceptance rules are in concrete cylinder tests; floors are controlled separately, with beams.
Sources: NSR-10, C.5.6.3.3 and C.5.6.5; concrete control record of the DDES oversight at Bodegas Centurión.
Frequently asked questions
What should I do if a concrete cylinder comes out low?
First check the age of the test, because at 7 and 14 days it is normal not to reach f′c. Then apply the NSR-10 criterion: averages of three consecutive tests of at least f′c and no test more than 3.5 MPa below. If it fails and capacity may be affected, cores are taken.
When is concrete accepted with cores?
When the average of the three cores is at least 85% of f′c and none is below 75% (NSR-10, C.5.6.5.4).
