There is a lot of confusion in the Irish market about the requirement for 13N thermal blocks for durability purposes.

In response to an industry issue relating to the durability and compliance of aggregate concrete blocks, the Association of Consulting Engineers of Ireland (ACEI) released an Advice Note to Structural Engineers, recommending all concrete blocks manufactured to EN 771-3, for use in rising walls and both the inner and outer leaf of external walls, should have a declared mean compressive strength of ≥13N/mm².

It should be noted that this advice is for aggregate concrete blocks only and does not apply to Aircrete thermal blocks.

Declared mean compressive strength

The basis for the ACEI recommendation comes from Table 14 of SR 325:2013+A2:2018, where blocks used below or near external ground level with a high risk of saturation with freezing, and blocks used in unrendered walls, should have a declared mean compressive strength of 13N/mm².  

Higher strength, density and cement content increases the durability of 13N aggregate concrete blocks. However, the confusion arises when we look at thermal blocks, and particularly Mannok Aircrete Thermal Blocks, which are not aggregate concrete blocks and they are not manufactured to EN 771-3, they are manufactured to EN771-4.

When assessing the durability of Aircrete blocks IS EN1996-2-2006+NA-2010, Acceptable Specifications of Masonry Units for Durability, needs to be considered. Table B.1 of this standard sets out the acceptable specification of various masonry units. 

It clearly states that Aircrete blocks manufactured to EN771-4, with a density ≥400kg/m³, are suitable for use in exposure class up to and including MX3.2, high saturation with freezing. Mannok’s 7.5N/mm² Aircrete blocks have a density of 760kg/m³ and are therefore suitable.

In addition, unlike aggregate concrete blocks for which there is no Irish or European test method, there is a relevant European test method for assessing the durability of Aircrete blocks for free/thaw resistance. All Mannok Aircrete Themal Blocks have also been independently tested and certified for use where high saturation with freezing is likely to occur.

For aggregate concrete blocks subject to wetting with freezing, IS EN1996-2-2006+NA-2010 states that these blocks must be freeze/thaw resistant. In the absence of test results, designers rely on SR 325:2013+A2:2018 where a 13N/mm² block is recommended for areas where wetting and freezing is likely to occur.

In addition to the higher block strength, SR 325 also recommends a minimum density of 1,500kg/m³ and that the block is manufactured using dense aggregates, neither of which is an issue for most dense concrete blocks.

However, lightweight aggregate thermal blocks have a density less than 1,500kg/m³ and are manufactured using lightweight aggregates, therefore they are not covered by SR 325.

In summary, when specifying thermal blocks in locations where saturation and freezing is likely to occur, Mannok’s 7.5N/mm² Aircrete block exceeds the requirements set out in IS EN1996-2-2006+NA-2010, it has been independently tested for freeze/thaw resistance and should therefore be considered the most durable thermal block on the market in Ireland. 

Contact Mannok’s Technical Experts for more information: Call: +44 28 67748866 Email: