Mannok Aircrete Thermal Blocks Do Not Require 13 N/mm² Compressive Strength for Durability

9th March 2023
There is currently some confusion in the Irish market about the requirement for 13N thermal blocks for durability purposes.
Recent advice note for durability and compliance of concrete blocks in Ireland

In response to an industry issue relating to the durability and compliance of concrete blocks, the Association of Consulting Engineers of Ireland (ACEI) released an Advice Note to Structural Engineers with their recommendations.

The basis for this 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.

Considering the significant durability issues with blockwork in recent years, ACEI have taken this a step further and now recommend that all concrete blocks manufactured to EN 771-3, for use in rising walls, both the inner and outer leaf of external walls, should have a declared mean compressive strength of ≥13N/mm². Unless a documented control system is in place to prevent the mixing of blocks on site, the advice is that internal walls should also be built using blocks with a declared mean compressive strength of ≥13N/mm².

The higher strength, density and cement content increases the durability of 13N blocks and in the absence of updated national standards, it gives designers and specifiers a level of comfort that the blocks are fit for purpose.

Why do Aircrete Thermal Blocks not Require 13N/mm² Compress Strength for Durability?

The confusion arises when we look at thermal blocks, and particularly Mannok Aircrete Thermal Blocks.

Mannok Aircrete Thermal Blocks are not Aggregate Concrete Blocks and they are NOT manufactured to EN771-3. Instead, they are Autoclaved Aerated Concrete (AAC) blocks manufactured to EN771-4 and are therefore not covered in the ACEI advice note.

When assessing the durability of AAC blocks manufactured to EN771-4, I.S. 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 AAC units 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.

Mannok Aircrete blocks have been independently tested for freeze thaw resistance and have been certified for use where high saturation with freezing is likely to occur.

For aggregate concrete blocks subject to wetting with freezing, I.S. EN1996-2-2006+NA-2010 states that these blocks must be freeze/thaw resistant. Here lies the problem. Unlike AAC blocks, there is no approved test method for freeze / thaw action in aggregate concrete blocks within current Irish or European standards. In the absence of this, designers therefore rely on SR 325:2013+A2:2018 where a 13N/mm² block is required where wetting and freezing is likely to occur. In addition to the higher block strength, SR 325 also recommends a minimum density of 1500kg/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 1500kg/m³ and are manufactured using lightweight aggregates, therefore they are not covered by SR 325.

Mannok Aircrete Thermal Blocks – the no.1 choice in Ireland for thermal bridging

In summary, when designers are specifying thermal blocks in locations where saturation and freezing is likely to occur, they can take comfort from the fact that Mannok’s 7.5N/mm² Aircrete block exceeds the requirements set out in I.S. EN1996-2-2006+NA-2010 and have also been independently tested for freeze/thaw resistance.

This, in addition to the obvious thermal advantage they have over alternative thermal blocks, makes them the no.1 choice in Ireland for thermal bridging.

Read our technical article on specifying blocks in locations with low saturation without freezing right up to high saturation with freezing HERE.


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