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  • Writer's pictureSi Shen

Time-dependent behaviour of structures (3) – strength of concrete

Have you ever wondered, why have we chosen 28 days strength as a basis for concrete design?

This blog post answers this question and explores the relationship between time and strength of concrete.

The reason, or at least one of the key reasons, for choosing 28-day strength as a basis for design is that regardless of the cement class, i.e. how quickly the concrete gains strength, it reaches the 28th day at almost the exact same strength. See the diagram below. This is taken from the book “Properties of Concrete for use in Eurocode”. This is an excellent book with in-depth and yet practical knowledge about concrete.

All concrete continues to gain strength past 28 days, although at different rates depending on the cement class. Typically in certain design scenarios where concrete strength is critical over long term, 90-day strength is used as another important threshold for its strength measurement, and is typically taken as the ultimate strength as further strength gain is really slow and testing samples are typically not kept long enough. The 90-day strength is typically 10% to 20% higher than its 28-day strength.

We know some cement is deliberately made to be quick-setting. This is classified as ‘R’ (for ‘rapid’). This can be very useful for situations where you want to put the concrete to use as soon as possible, so that quick early strength gain is desired. This is typically the case for prestressed concrete or temporary works concrete. However, we must bear in mind that cement that gains strength quickly upfront will have lower ultimate strength. This is because the quicker the chemical reactions go through, the less thorough they are. More haste, less strength – this holds true for concrete.

Some other times, the cement is deliberately made to be slow-setting (class ‘S’, for ‘slow’). This could be for the reduction of heat release, or for prevention of cold joint, or for better long-term strength gain.

The previous two posts under this series can be found here:

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