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Hong KongConcrete testing and strength
Compressive strength is tested at 28 days on cylinders. The slump test measures workability, not strength. Air content, temperature and unit weight are tested separately at placement.
Four tests measuring four different things, and candidates routinely conflate two of them.
The tests
| Test | Measures | When |
|---|---|---|
| Compressive strength | Strength, in pounds per square inch | At 28 days, on cylinders |
| Slump | Workability and consistency | At placement |
| Air content | Entrained air, for freeze-thaw durability | At placement |
| Temperature | Concrete temperature | At placement |
It measures how the mix flows. A high slump often correlates with more water and therefore lower strength, but the test itself tells you about workability. Candidates who half-remember it as a strength test lose the question.
The 28-day standard
Concrete continues gaining strength for a long time, and 28 days is the conventional age at which the specified strength must be reached. It hydrates.
Seven-day breaks are commonly taken as an early indicator, typically reaching a substantial fraction of the 28-day strength.
Water-cement ratio
The single largest determinant of strength. More water makes concrete easier to place and weaker when cured.
Adding water on site to improve workability is the classic field mistake, and it is why admixtures exist.
Curing
Maintaining moisture and temperature so hydration continues. Seven days of moist curing is a common minimum for normal concrete.
Concrete does not dry to gain strength; it hydrates. Letting it dry out stops the reaction and the strength never arrives.
Weather thresholds
Forty degrees Fahrenheit for cold weather provisions and 90 degrees as a common hot weather threshold.
Common questions
When is concrete strength tested?
At 28 days, on cylinders, for compressive strength.
What does the slump test measure?
Workability and consistency, not strength.
What determines concrete strength most?
The water-cement ratio. More water means easier placement and weaker concrete.
How long should concrete be cured?
Seven days of moist curing is a common minimum for normal concrete.
Does concrete dry to gain strength?
No. It hydrates. Letting it dry out stops the reaction and the strength never develops.