The End of Blockwork?

ABOVE: H+H Celcon are one of only a few manufacturers of aircrete blocks, which are about 50% air, making them easier to handle than denser blocks (hhcelcon.co.uk).

Let’s be straight from the start: blockwork isn’t dead. Blockwork is just too damn flexible and for - giving to die; too damn useful to be heading for the knacker’s yard anytime soon. But, on the other hand, the future of blockwork may look a little different to the recent past.

Bear in mind that blockwork was once itself an innovation. It was really a development of the cement industry, which came to dominate the building trade in the 20th century. Cement wasn’t widely used before the 1920s: before that if you wanted a solid masonry wall you used brick, stuck together with lime mortar. Laying brick in lime is a much slower process because bricks are tiny and lime takes longer to set.

So when cement production really took off, it didn’t just replace lime in mortar, it got used to make concrete, which in turn was used to make all manner of things like concrete blocks and concrete roof tiles. Cheap cement fuelled a housing boom in the 1930s, and the humble concrete block was the backbone on which these homes were built.

The next stage in the evolution of the block came about in the 1980s as insulation standards were introduced. The trade switched from ordinary concrete blocks to a lightweight alternative, known as ‘aircrete’. Aircrete blocks promised much better insulation levels because they are about 50% air by volume. They are also much easier to handle so generally bricklayers (and especially their labourers) liked them. Making aircrete blocks is a much more expensive process, involving baking the blocks in pressure cookers known as autoclaves, and so the industry became dominated by just a few players — H+H Celcon, Thermalite (now part of Hanson) and Durox/Tarmac. (By the way, if you want to know if the blocks you have are aircrete, then try dropping them one a bathful of water. If it floats, it’s aircrete. If not, then you’ll need a new bathtub!).

Most of the innovation in blockwork comes from the aircrete manufacturers. You can make aircrete up into large panels, or into thick foundation blocks. Despite their relatively light weight, they can be ‘extra-strength’ grade for use in foundations. They can be engineered to reduce sound transmission or to accept a paint finish without the need for plaster. But perhaps the cleverest innovation, and the one of most interest to self-builders, is the advent of thinjoint aircrete blockwork.

Thin-joint (IMAGE 2 BELOW), as its name suggests, does away with the conventional 10mm beds of cement mortar and replaces it with a 2-3mm glue mortar, similar in characteristics to tile adhesive. It sets very rapidly and thus enables blocklayers to use much larger blocks and to build them into walls much faster. The larger the project, the bigger the labour savings from employing thin-joint methods, and when used wisely, it can bring many of the benefits of offsite construction methods (like timber frame or SIPs) on site. The big manufacturers all offer thin-joint solutions, each of them subtly different.

Innovation in blockwork doesn’t stop with thin-joint. Beavering away from the main - stream are a number of businesses who are determined to reinvent blockwork using a wide range of materials.

• Take Durisol (1). It’s an entirely novel form of hollowed-out block which is best described as a wood-based concrete. It is stacked together dry (without mortar) and then filled with ready-mix concrete.

• Or any number of polystyrene blocks, known as ICFs (insulated concrete forms) (4) which are based on a similar idea of using hollow blocks, stacked dry, and then filled with concrete. Leading suppliers of polystyrene blockwork systems include Beco Wallform and Logix. In all fairness, ICFs are normally thought of as a building system quite separate to blockwork, and not all ICF systems are based around the idea of a polystyrene block (preferring panels instead).

• Have a look at ThermoPlan (5) fired-clay insulating blocks, available from Natural Building Technologies. Clay-block building has been widely used on the continent but is now being manufactured in England, which should make the price more reasonable. Essentially, it’s another thin-joint system, but this time based on clay block rather than aircrete (which not everybody loves).

But in all this excitement, let’s not forget where it all started — the humble (if heavy) concrete block; because it’s still a long way from being killed off. Still available at less than £5/m², which makes it much the cheapest walling material you will come across, its very heaviness is seen as an advantage by some designers, keen to use high thermal mass as a heat battery in low-energy designs. Pack enough insulation around it, and the very basic quality of the concrete block becomes an advantage. Many of our more innovative buildings, like the wellknown BedZED (3) development in South London, are based on the idea of using heavy concrete blocks packed behind bales of insulation.

1. Durisol’s ‘wood-based’ concrete blocks are dry-stacked before being filled with readymixed concrete
2. Tarmac’s Durox thin-joint system uses a glue mortar instead of cement, allowing for the laying of bigger blocks
3. The BedZED environmentally friendly housing development in South London uses heavy concrete blocks packed behind bales of insulation to create a high thermal mass
4. ICFs are hollow polystyrene blocks which are dry-stacked before being filled with concrete
5. ThermoPlan’s fired clay blocks have a tongue-and- groove design allowing for dry vertical joins

How blockwork became green

The blockwork industry has been busy coming up with solutions which enable blockwork builders to meet the new Code for Sustainable Homes (CSH) requirements and in particular, reduce U-values (the measure of heat loss, for instance, through a wall; the lower the Uvalue the better). In simple terms, the six levels of the CSH award points for meeting a suite of energy and lifestyle measures, and the most points are awarded for building homes with very low U-values. So to meet Code Level 3 – now a requirement in some local authorities – your home will need to exceed current Part L of the Building Regulations by 25%. Assuming other features are installed (energy-efficient lighting, mechanical ventilation with heat recovery, airtightness etc.), according to the Concrete Centre a U-value of 0.26 can be achieved with a wall construction of 100mm aircrete block/50mm phenolic insulation/50mm air gap/100mm brick. If you want to reduce U-values to, say, 0.15, you’ll need to increase the cavity to 150mm (with 100mm of insulation). In essence, it’s possible to achieve a value as low as that achieved by SIPs construction, using blockwork — but you’ll end up with 350mm-thick walls. It’s worth noting that improving U-values and airtightness alone may not be sufficient to achieve Code Level 3 without enhancements to the heating and ventilation systems. See more blockwork solutions at homebuilding.co.uk/jasonorme.

Further reading:

• Timber Frame or Blockwork?
• SIPs Examined
• A Guide to Construction Systems