Concrete on site is usually accepted on a few small specimens: cubes or cylinders made from the delivered concrete and crushed at 7 and 28 days. The results are only as good as the sampling, making and curing behind them. Below we explain the strength class, correct handling and the 7-day result, and what to check before a low result leads to concrete coring.

What C25/30 means: cylinder and cube strength

A strength class such as C25/30 gives two numbers in MPa (N/mm²): the characteristic compressive strength on 150×300 mm cylinders (25) and on 150 mm cubes (30), normally at 28 days. TS EN 206 defines it as the value below which only 5 % of all possible results for that concrete are expected to fall: a statistical lower value, not a figure every specimen must exceed.

Strength class	Cylinder fck,cyl (MPa)	Cube fck,cube (MPa)	Cylinder ÷ cube
C20/25	20	25	0.80
C25/30	25	30	0.83
C30/37	30	37	0.81
C35/45	35	45	0.78
C40/50	40	50	0.80

The cube reads higher mainly because of its shape. Friction at the steel platens restrains the concrete from spreading sideways; in a cube this restraint reaches through the whole specimen, while the middle of a cylinder twice as tall as its diameter fails almost unrestrained (Neville, Properties of Concrete). The ratio in the TS EN 206 class table is not constant and applies to characteristic values, so we never convert a single result with a fixed factor: compare cube results with the cube number and cylinder results with the cylinder number.

Cubes and cylinders: sizes, ends and testing

Shapes and sizes are set by TS EN 12390-1. The reference specimens are the 150 mm cube and the 150×300 mm cylinder, and the standard recommends a specimen size of at least 3.5 times the maximum aggregate size. The specification chooses the type, which is then kept for the whole job. We test both:

Cubes are turned on their side, so the load acts on moulded faces, not on the trowelled top.
A cylinder top is not flat enough to load, so TS EN 12390-3:2019 requires the ends to be ground or capped; a cap that fails before the concrete makes the test unsatisfactory.
We measure each specimen, determine its density to TS EN 12390-7:2019 and test it to TS EN 12390-3:2019 at a constant rate within 0.6 ± 0.2 MPa/s, on our 2000 kN press verified as Class 1 to TS EN 12390-4:2025. The failure pattern is checked against the patterns shown in the standard.

ASTM projects use ASTM C31/C31M-26b and ASTM C39/C39M-26 instead, with their own sizes and curing temperatures; do not mix the two systems on one job.

Sampling at the truck

Sampling follows TS EN 12350-1:2019. A composite sample combines several portions taken through the discharge; a spot sample comes from one part of the load. The very first concrete out of the chute is not used, and where the specification requires it the sample is taken at the point of placing. We record:

the delivery note, truck and batching time, and the class, consistence class and maximum aggregate size stated on the note;
the sampling time and the element being cast;
any water or admixture added on site, and who authorised it.

Slump (TS EN 12350-2:2019), fresh density (TS EN 12350-6:2019) and, where specified, air content (TS EN 12350-7:2019) are measured on the same sample, so the fresh and hardened results describe the same concrete.

Making and curing specimens

We make and cure specimens to TS EN 12390-2:2019:

Clean, tight, lightly oiled moulds; concrete placed in layers, each fully compacted by vibration or rodding without segregating the mix; the top struck off level; a durable mark on every specimen.
At least 16 hours and at most 3 days in the moulds, protected from shock, vibration and drying, at 20 ± 5 °C, or 25 ± 5 °C in hot climates.
After demoulding, curing until the test in water at 20 ± 2 °C, or in a chamber at 20 ± 2 °C and at least 95 % relative humidity.
Careful transport with the specimens kept wet; damaged corners and edges are recorded at the laboratory.

Only standard curing makes results comparable with the strength class. Specimens kept beside the element, for example to decide when formwork can be struck, are not used for conformity.

Common curing mistakes in a Cyprus summer

In our experience, low results often come from the specimens’ first day, not from the concrete:

moulds left uncovered in the sun or on a hot slab;
specimens driven away a few hours after casting;
cubes demoulded on site and stored dry instead of in water;
specimens left on site for more than 3 days, or never labelled.

In a study by Bloem for the NRMCA, summarised by Obla et al. in Concrete International, cylinders that spent their first day in air at 38 °C reached about 88 % of the strength of standard-cured companions, and about 78 % after three days. Plan a shaded, covered and damp place for the moulds, and collection the next day so that water curing starts on time.

7-day and 28-day strength

The 7-day result is an early indicator; the strength gained by then depends on the cement, additions, admixtures and temperature. As an illustration, EN 1992-1-1:2004 (Eurocode 2), clause 3.1.2, gives the development of mean strength at 20 °C for concrete cured to the EN 12390 series. At 7 days it gives:

Cement class in EN 1992-1-1	Cement strength classes	7-day mean as % of 28-day mean
R	CEM 42.5 R, 52.5 N, 52.5 R	about 82 %
N	CEM 32.5 R, 42.5 N	about 78 %
S	CEM 32.5 N	about 68 %

These are design estimates, not acceptance limits. Blended cements, retarders or heat can give quite different ratios, and the producer’s strength development data for the mix is a better guide. Acceptance is at 28 days unless the specification sets another age, so make separate specimens for the 7-day test.

Number of specimens, conformity and low results

The specification sets how often samples are taken (per volume, day or element), how many specimens are made from each and at what ages they are tested. Two different checks then apply:

Producer conformity: under TS EN 206, and TS 13515 where it applies, the ready-mix producer assesses its production statistically from its own results.
Identity testing: TS EN 206 also gives criteria for deciding whether a particular volume, such as one pour, belongs to conforming concrete. They compare the mean and individual results with the characteristic strength, so a single result below the class number is not on its own a non-conformity.

When results are low, first check the specimens’ history: storage on site, transport, curing record, test age, density against the others, failure pattern and, for cylinders, end preparation. If doubt remains, cores can be taken from the structure to TS EN 12504-1:2019 and assessed to TS EN 13791:2019, as our guide Concrete coring and in-situ strength explains. What this means for the structure is for the structural engineer to judge; demolition is not the default response to a low cube.

How we can help

We sample fresh concrete on site, test slump, density and air content, make cubes or cylinders, collect and cure them in our laboratory and test them at the specified ages. Our reports are digitally signed PDFs (see How to read and verify a test report). See our concrete and aggregate testing and concrete coring services, or contact us to plan sampling for your pours.

Frequently asked questions

Can a cylinder result be converted into a cube result?

Not reliably for individual results. The pairs in the TS EN 206 class table, such as 25 and 30 for C25/30, are characteristic values, and their ratio changes from class to class. Agree the specimen type at the start and compare each result with the matching number.

The 7-day result is low. Is the concrete inadequate?

Not necessarily. Check the specimens’ first day and their curing, and compare the result with the producer’s strength development data for that mix. Conformity is judged at 28 days or at the age set in the specification.

Do you test cubes that we made ourselves?

Yes, to TS EN 12390-3:2019, with their condition on arrival recorded. The report states that they were supplied by the client, as sampling and early curing were not ours.