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Civil Engineering DISCUSSION
We cast a floor slab in C25/30 concrete and the 7-day cubes have come back at an average of 21 MPa. The contractor says that is normal at one week and wants to remove the props. I am trying to judge whether 21 MPa at 7 days is on course for the specified strength.
Why is 28 days the reference age, how much of the final strength should be there after 7 days, and can the props be removed on the basis of the 7-day result?
Concrete gains strength as the cement hydrates, a reaction that is fast in the first days and then slows but continues for months. The 28-day age is a convention: by then an ordinary Portland cement concrete kept moist at about 20 °C has developed most of the strength it will reach, so design codes define the characteristic strength at that age and everything else is referred to it.
As a rough guide for such concrete, expect around two thirds to three quarters of the 28-day strength at 7 days and about 90 percent at 14 days. Mixes containing fly ash or slag gain more slowly at first and keep gaining for longer; rapid-hardening cements are ahead of these figures.
Applying that to your result: 21 MPa at 7 days projects to somewhere between 21 / 0.75 = 28 MPa and 21 / 0.67 = 31 MPa at 28 days. For C25/30 the 30 is the characteristic cube strength, the value that nearly all results must exceed, so the average of a properly designed mix should be several megapascals above 30. A projected mean of 28 to 31 MPa is therefore on the low side, not comfortably normal.
It is not proof of a problem either, since the ratio depends on the cement. Ask the supplier for the strength development data of that particular mix, and look at how the cubes were made and stored, because poorly compacted or badly cured cubes read low.
Eurocode 2 gives a formula you can use for the projection: the mean strength at age t days is the 28-day mean multiplied by exp(s × (1 - sqrt(28 / t))), where s depends on the cement class: 0.20 for rapid, 0.25 for normal and 0.38 for slow-hardening cement. At 7 days the square root is 2, so the factor is exp(-s): 0.82, 0.78 and 0.68 respectively.
The formula assumes curing at 20 °C. Temperature has a large effect on early strength: a slab that spent its first week at 5 to 10 °C will be well behind a laboratory cube stored in water at 20 °C, even though both will eventually reach similar strength.
Prop removal should not be decided from a rule about days, and not from the contractor's judgement alone. The requirement is that the slab has enough strength and stiffness in place to carry its own weight and the construction loads, and the criterion, usually a minimum in-situ strength or a stated fraction of the specified strength, comes from the structural designer or the project specification.
Young concrete is also less stiff and creeps more, so a slab released too early can be strong enough not to fail and still end up with excessive long-term deflection and cracking. If early striking is wanted, the normal route is to agree the criterion with the designer, demonstrate the in-situ strength with site-cured cubes or maturity measurements, and leave back-props in place.
Two points about the numbers themselves. C25/30 means a characteristic strength of 25 MPa on cylinders and 30 MPa on cubes; the same concrete gives roughly 20 percent lower results on cylinders because of the specimen shape. Compare your cube results with the 30, and be careful when reading specifications from countries that test cylinders.
Secondly, whatever the 7-day results say, curing of the slab itself matters more than is usually appreciated. Keeping the surface moist for the first week, with sheeting, wet coverings or a curing compound, lets hydration continue in the surface zone. Letting it dry out in sun and wind, or adding water to the mix on site to make it easier to place, lowers strength and durability in ways no later test can recover.
