Do I Need to use a 13N Thermal Block in Ireland?

There is some confusion in the Irish market about the requirement for a 13N thermal block and our Technical by Mannok team have received a number of enquiries about a suitable Mannok Aircrete Thermal Block. In our latest Your Questions Answered video, Jason Martin of our Technical by Mannok team addresses these queries.

Why specify a 13N block?

There would be two reasons why a 13N block may be used in a masonry structure.
Firstly, a 13N block maybe specified for structural reasons if the building is subject to high loading, for example a high rise or commercial building. In this case, the Structural Engineer may specify at 13N block for structural reasons, and if that’s the case, you will need to use a 13N block.

The other reason why 13N blocks may be specified is for durability. The higher cement content of a 13N block makes it more durable. So, if you need to use a block with higher durability in certain locations, then a 13N dense block is fine for this purpose. However, we need to give further consideration as to whether a 13N lightweight block is suitable in certain locations.

Suitable blocks below or near ground level: SR 325 Table 14

Table 14 of NSAI publication SR 325 covers durability in masonry in finished construction.
The table has various sections and the relevant section to address this query is section A, which covers work below or near finished ground level.
Table 14 shows various exposure conditions starting with MX 2.1, which is low saturation without freezing, going on to low saturation with freezing, high saturation without freezing and finally, the worst case scenario is MX 3.2, high saturation with freezing.

The table defines conditioned MX 3.2 as being located in an area 150mm above to 150mm below finished external ground level, as highlighted in Fig.1

Fig.1: MX3.2 Location

The table sets out the requirements for aggregate blocks to meet MX 3.2 as:

1. The block must be at least 13N/mm² in strength
2. The block must have a density of at least 1500kg/m³
3. The block must be manufactured using dense Aggregates.

It is the requirement for a 13N block in this scenario which has generated the confusion around 13N thermal blocks.

Currently, there is no thermal block available on the market in Ireland which meets all three requirements.

There is a solution if you need a thermal block in a location of condition MX 3.2. That is the Mannok Aircrete block, which is certified as suitable for use in this condition.

When it comes to the use of Aircrete blocks, the direction of SR 325 Table 14 is to refer to the manufacturer regarding suitability of use.
The key difference between aggregate blocks, which are defined in Table 14, and Mannok Aircrete blocks is that there is currently no freeze/thaw test method available for aggregate concrete blocks, whereas there is a recognised test method for AAC or Aircrete blocks.
All Mannok Aircrete blocks have been tested for freeze/thaw using this method and are certified for use in all ground conditions, up to and including MX 3.2.
The blocks are also covered in standard IS EN 1996-2 2006 specification of masonry for durability. Table B1 clearly states that AAC blocks meet the requirements of MX 3.2.

So, Mannok Aircrete blocks are the only thermal blocks available on the market in Ireland which meet the requirements for use in condition MX 3.2.

Thermal blocks: Location

Looking at the location where condition MX 3.2 occurs (Fig.1), it would not be typical for a thermal block to be specified in this location as a thermal block used here will not significantly improve the thermal performance of the junction.

Thermal blocks are generally specified at the floor to wall junction on the inner leaf to improve the thermal performance of this junction, highlighted in Fig.2, Mannok Aircrete blocks will only need to be specified in this location. Using Mannok Aircrete blocks here will give excellent thermal performance.

Fig.2: Mannok Aircrete block in detail

If you are using a poorer performing thermal block however, they may also be specified below the junction, as highlighted in Fig.3, which may be required to get a similar thermal performance to that of the Mannok Aircrete block.

Fig.3 Alternative thermal blocks to Mannok Aircrete

The requirements set out in Table 14 for blocks used in this location differ to the requirements of MX 3.2 as this location is lower than 150mm below finished ground level. Table 14 states that you do not need a 13N block in this location as a 7.5N block is sufficient. However, it states that the block must have a density of 1500 and it must be manufactured using dense aggregates.
So the requirements for using concrete blocks in this location are:
1. The block must be at least 7.5N/mm² in strength
2. The block must have a density of at least 1500kg/m³
3. The block must be manufactured using dense Aggregates.

There is no thermal block available on the market in Ireland which meets all of these requirements.
So if you wish to specify a thermal block in this location, perhaps using trench blocks from the foundation up, then the Mannok Aircrete block is again the only thermal block on the market which is suitable.

In summary, for any of the conditions outlined in Table 14a, from low saturation without freezing right up to high saturation with freezing, the Mannok Aircrete thermal block is the only thermal block available is certified for use these conditions.

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  Jason Martin

  Jason Martin is the Specification and Product Development Manager at Mannok. Much of his daily focus is on...

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