We are sure that you have heard the term ‘non-shrink grout’ and it is often backed up by the term Class C or a reference to ASTM C621.

What Do These Terms Mean?

Well for a start, there is no such thing as ‘non-shrink grout’ and further to that, ASTM C621 doesn’t even exist! It was superseded in 1993 by ASTM C827, C1090 and C1107. (27 years ago).

For interest though, in the old ASTM C621, two different types of shrinkage compensation grouts were defined:

  • Class A Grout: Expands during the plastic state (approximately the initial 8 hours).
  • Class C Grout: Expands in both the plastic and hardening stages (Dual Shrinkage Compensated).

So what testing exists now and which should be selected?

Early Age Expansion and Shrinkage – ASTM C827

ASTM C827 is used to measure volume change in the plastic stage. It also measures expansion and can give us an indication of how well we have compensated the plastic shrinkage. This is a good test to provide an idea of volumetric stability in the first 24 hours.

Autogenous Shrinkage – ASTM C1090

ASTM C1090 is one of the most misused tests in the pre-packaged grout industry.  The results are often referred to in claims that a grout is non-shrink. However, because no moisture can escape from the test sample, this test really only measures autogenous shrinkage and ignores far more significant effects of plastic and drying shrinkage. It is surprising how often it is seen on a data sheet along with a claim of ‘non-shrink’.

Drying Shrinkage – AS1012.13 and AS1478.2

AS1012.13 and AS1478.2 tests are used to measure drying shrinkage. However, it is worth noting that both these tests take the initial measurement at 7 days, therefore ignoring any volume change in the first week of cement and grout casting. It is also relevant to note the difference in environmental conditions between testing and application. The lab will have 50%RH which may or may not reflect conditions on-site. Whilst this is the most popular test for measuring concrete and grout shrinkage, it ignores many important aspects including plastic shrinkage, and any drying shrinkage which occurs in the volatile first 7 days.

Fact: AS1012.13 and AS1478.2 only measure shrinkage that occurs from 7 days to 35 days. Any volumetric change in the first 7 days is ignored. This is where most of the shrinkage may actually be occurring.

To learn more about grout shrinkage, control and measurement, head to the new Webinars page.


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