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Key points

  • In EN 206 a concrete class is written like C25/30: the first number is the characteristic cylinder strength, the second the characteristic cube strength, in N/mm² at 28 days.
  • The cylinder figure is lower than the cube figure for the same class.
  • ACI 318 code (per the FHWA summary): 7 days of curing at 10°C or above, or 3 days for high early-strength concrete.
  • Stopping curing and restarting it does not complete the 7-day period (practical advice).
Written for:OwnerContractor

Concrete is the skeleton of the house. Because it's poured once and can't easily be changed, understanding the basics lets you follow the contractor and the lab with confidence.

1) Strength class: what does C25/30 mean?

The European standard EN 206 uses a double notation1:

  • C: normal-weight concrete (LC for lightweight).
  • First number: characteristic compressive strength of a cylinder specimen (150 mm diameter × 300 mm high) in N/mm² at 28 days.
  • Second number: characteristic compressive strength of a cube (150 mm) at 28 days.

Example: C25/30 = characteristic cylinder strength 25 N/mm² and cube strength 30 N/mm². so the cylinder figure is lower than the cube figure for the same class1.

"Characteristic strength" doesn't mean the average. It's a statistical value that the vast majority of results are expected to exceed. So the average of test results must be higher than the class figure, per the acceptance criteria in the standard adopted for the project.

2) Cube and cylinder testing

  • Samples are taken during pouring from the actual mix, cured in the lab, and usually crushed at 7 and 28 days.
  • The 7-day result is an early indicator; formal acceptance is on the 28-day result.
  • The 7-day result is naturally below the design strength because concrete keeps gaining strength; don't judge a pour on it alone.

Owners: ask for a copy of the lab reports for every pour (footings, columns, slabs) and keep them.

3) Curing: the forgotten step

Cement hardens by chemical reaction with water (hydration). If the concrete surface dries early, the reaction stops in the outer layer, which becomes weak and cracks. Therefore:

  • According to the US Federal Highway Administration's summary of American Concrete Institute requirements, the building code ACI 318 calls for 7 days of curing at 10°C or above (or 3 days for high early-strength concrete)2.
  • The surface must stay moist (or protected by a curing membrane) throughout; stopping on day six and restarting on day eight doesn't satisfy the seven-day requirement.
  • The US Federal Highway Administration (FHWA) explains how to verify curing effectiveness and when it can end2.
In hot summers: rapid evaporation and dry wind increase the risk of plastic shrinkage cracks. Plan pours for cooler hours, and have curing means ready (spraying, wet burlap, covers or curing compounds) before pouring starts. See concrete cracks.

4) Pour-day checklist

  1. Check the delivery ticket matches the required class (e.g. C30/37) and cement type.
  2. Slump test and sampling in the supervisor's presence.
  3. No water may be added to the mix on site to ease placing; it lowers strength. See admixtures.
  4. Compact with a vibrator to expel air voids, without overdoing it.
  5. Check concrete cover over the steel (spacers/plastic chairs).
  6. Start curing as soon as surface finishing ends, and continue for at least 7 days.

5) From order to acceptance: who decides what?

StageOwner's roleEngineer's decisionLaboratory's role
OrderMakes sure the order states the class, cement type and quantity in writingClass, cement type and admixtures from the design and soil report—
DeliveryKeeps each truck ticket (class, time). See ready-mix concreteAccepts or rejects the truck—
PourConfirms the steel was signed off before pouring and no water is addedRebar sign-off and supervising the pourTaking samples and the slump test
CuringFollows wetting or covering for 7 days2Curing method and duration for the conditions—
ResultsRequests and keeps the 7- and 28-day reportsAcceptance, or further testing if results fall shortCrushing samples and issuing the report

This list is part of the construction quality checks, and the reports go in the quality file.

Sources

The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.

  1. 1Compressive strength class according to DIN EN 206 — BAUKOBOX
  2. 2Guide for Curing of Portland Cement Concrete Pavements — U.S. Federal Highway Administration (FHWA)
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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.