Aircrete blocks

Cliff Fudge, technical director of H+H, describes how aircrete blocks can play a key role in providing sustainable solutions and help to deliver the housing stock that is desperately needed.

[edit] Introduction

When a product is as familiar as an aircrete block it should be safe to assume that its characteristics, potential uses and product development are widely understood. However, aircrete remains one of the most misunderstood and undervalued products on the building site, so it is worth being reminded of why this grey block has become such a staple.

The defining characteristic of aircrete is the air voids formed inside the structure so that the masonry blocks are not dense but instead are punctuated with pockets of air throughout — decreasing the weight and increasing thermal insulation of the block.

This structure is produced in a manufacturing process. The constituent elements of aircrete are: pulverised fuel ash — a by-product of the energy generation industry — water, cement, lime and a very small amount of aluminium powder. These products are mixed into slurry and poured into moulds. It is the chemical reaction caused by the aluminium and lime combination that creates thousands of air bubbles — a process similar to yeast used to make dough rise.

Curing takes place in an autoclave where the aircrete is heated under pressure. The resultant blocks are provided in a range of sizes and strengths — aircrete blocks such as High Strength Grade and Super Strength Grade Celcon Blocks give compressive strengths of 7.3N/mm2 and 8.7N/mm2 respectivel; they are typically used in the foundations and lower storeys of three-storey buildings, piers under high vertical loads and in multi-storey buildings.

For most contractors, however, the most common use of aircrete blocks will be in the construction of brick-and-block structures — primarily houses. Aircrete is generally used for the inner leaf of a cavity wall, although it is also well suited to solid wall designs where it is likely to be faced with an insulating render.

For site workers, the most immediate appeal of aircrete is its ease of use. The cellular structure reduces the weight of standard blocks: aircrete weighs around a third of an equivalent dense aggregate block — typically a 100mm block weighs around 7kg. The fact that the blocks can be cut with a standard handsaw ensures that workers can cope with fine detailing on site.

[edit] Energy efficiency

The building designer, however, is likely to place a higher value on the performance characteristics and thermal efficiency is of particular importance. Recent changes to Building Regulations Approved Document L have focused attention on the thermal efficiency of the building fabric, requiring energy efficiency targets to be achieved by the structure rather than by the use of renewable energy generation technology such as solar panels or heat pumps.

It is entirely possible to meet the exacting U-value requirements of Part L with a standard cavity wall construction using aircrete blocks on the inner wall, high-performance insulation in the cavity and a brick facing. However, even better performance can be achieved by using the thin-joint mortar method — a process that has been widely used in Europe for decades, but which has been relatively slow to make ground in the UK market. H+H’s thin-joint system combines the use of large-format, accurately dimensioned aircrete blocks and quick-setting thin layer mortar to create a highly productive and cost-effective building system. It can be used for solid or cavity walls in all types of buildings, including houses, apartments, commercial buildings, schools and offices.

Thin-joint refers to the thickness of the mortar joint. Mortar joints are usually 10mm, but when using the thin-joint system this is reduced to 2mm. Instead of using traditional sand/cement mortar, H+H’s thin-joint system uses Celfix mortar which was developed specifically for use with the H+H aircrete blocks. Celfix is cement-based and supplied as a dry premixed powder. The main advantages of using this method of building are speed and increased air tightness.

Speed of build is dramatically increased as the Celfix mortar sets extremely fast. Traditionally, block layers have only been able to build six courses per day, limited by the need to allow the relatively thick mortar joints to reach full strength. However, the rapid setting of thin-joint mortar allows for continuous laying until storey height is reached.

Thin-joint enables the inner and outer leaves of a cavity wall to be constructed independently of each other, allowing for a weathertight shell to be constructed much faster and enabling the follow-on trades to get started in the interior while the outer leaf is still under construction.

The other significant advantage of the thin-joint system relates to the technical performance of the structure, specifically in relation to considerations of air permeability and thermal bridging.

Air permeability is a significant factor in the construction of homes to current Building Regulations, but is particularly significant when it comes to high-performance structures such as Passivhaus designs.

Building Regulations stipulate an airtightness of a maximum of 10m3/hr.m2 for houses. Aircrete blockwork typically results in values of 5-6m3/hr.m2. However, blockwork laid using thin-joint can achieve an airtightness level of 4m3/hr.m2 without the need for a parge coat — a level of airtightness that reaches the performance requirements for Passivhaus construction.

Air tightness is not the only performance characteristic that needs examination, however. The close focus on the thermal efficiency of wall structures in Part L requires every element of the wall to perform to optimal levels and achieving this requires an understanding of the impact of thermal bridging.

[edit] Thermal bridge effect

Every junction in a building has a linear thermal bridge effect — a “psi” value. This is the rate of heat flow per degree per unit length of the thermal bridge that is not accounted for in the U-value of the plain elements. This is taken into account in SAP calculations, but if the designer decides not to calculate the thermal bridges then a default Y-value is used. This effectively adds a significant penalty in the design — if the default value is used it suggests that 50% of the heat loss through the fabric is via junction losses.

Using the Y-value under the regulations, as opposed to an assessed psi value, will more than likely result in the building design not achieving the thermal performance levels in a cost-effective way. For smaller builders that perhaps have limited in-house capabilities and utilise the services of external architects and SAP assessors it is vitally important that they are aware of the potential pitfalls relating to thermal bridging.

Fortunately, there is a shortcut. For aircrete, there are independently published psi values for around 20 common junctions that can be found on the Constructive Details website or, for more common forms of aircrete construction, on the accredited construction section of the LABC website.

These junctions have been determined for full fill and partial-fill cavity walls as well as a solid wall construction. The benefit of using these in the design will result in achievable, cost-effective solutions, with external cavity wall U-values in the range of 0.22 to 0.18W/ m2K. In practice, this will mean 100- 150mm cavities.

Every time Building Regulations are revised, or new targets set for carbon reduction and energy efficiency in new build, questions are raised about whether familiar building materials can meet the higher performance standards. And on each occasion, aircrete has demonstrated its ability to meet or exceed the most exacting standards.

The current issue of most concern is the ability of the UK construction industry to meet ambitious government targets for housebuilding. Consequently there is much discussion around the ability of UK manufacturers to meet product demand and the capacity of the workforce to build the numbers of houses required.

Aircrete manufacturers, with a track record of adapting to meet changing market demand, have comprehensive answers to both these issues. As discussed earlier, H+H aircrete blocks are made primarily of pulverised fuel ash (PFA). This is a by-product of the coal-fired energy generation industry and it is no secret that, in a drive to reduce carbon emissions, the number of coal-fired power stations is in steady decline. Add to this the fact that recent warm winters have reduced energy demand and it is clear that the quantity of fresh fuel ash available for the aircrete — and concrete — industry is at a historically low level.

Aircrete manufacturers can address this challenge in two ways. It is possible to manufacture aircrete using sand in place of PFA. However, a significant advantage of the PFA-based product is the environmental benefit of using what is effectively a waste product.

[edit] Long-term supply

For this reason, rather than switching to an alternative constituent material, H+H has secured a long-term supply of stockpiled PFA. When coal-fired power stations provided the majority of energy generation in the UK, the fuel ash residue was simply buried in vast pits as there were not sufficient manufacturers using it as a constituent. There is sufficient quantity of this stockpiled ash to support our manufacturing output for the foreseeable future.

When it comes to the challenge of building capacity, aircrete also has answers. The thin-joint process discussed above greatly increases the speed of build on site — adding capacity to the building process.

However, the sector also embraces the need to innovate and H+H is poised to roll out a method of construction that combines the speed and efficiency of offsite manufacturing with the performance advantages of traditional masonry build.

We first introduced our Vertical Elements into the UK market in 2007. Storey-high blocks of aircrete, these are simply craned into place and fixed with thin-joint mortar, providing structural walls in hours rather than days and using minimal site labour. The potential for these Elements was proved by the construction of the Barratt Green House at the Building Research Establishment in 2007 and by further prototype buildings trialled by various house builders. However, the recession of 2008 reduced the appetite for innovation and Vertical Elements, while the basis for standard building design across Europe, did not gain a strong foothold in the UK market.

H+H has reintroduced the concept of storey-high aircrete elements as part of a system-build package which is set to propel the material in the form of Celcon Elements into the offsite manufacturing arena for the volume housebuilding sector.

We believe that homeowners prefer a property built of masonry — not only for its performance attributes but also for its proven longevity. Providing we continue to invest in product development, we are confident that aircrete will remain a product of first choice for house building in the UK.

To find out more about H+H and its range of aircrete products and solutions, go to https://www.hhcelcon.co.uk/

--CIOB

[edit] Related articles on Designing Buildings Wiki

• Blockwork.
• Bricks.
• Cement.
• Mortar.
• Parge coat.
• Passivhaus.
• Pulverised fuel ash.
• Thermal bridge.
• U-value.