CONCRETE – BLOCKS - BRICKS
It may have been around for more than 6,000 years and may have been used in the construction of the Pyramids, but concrete is still as fundamentally useful in buildings today as it always was. It’s an extremely versatile construction material with a very long list of applications. And it has handy attributes – it’s incombustible, and it can endure extreme temperatures and corrosive environments.
These days, like everything else, it must live up to accepted levels of scientific performance arrived at by means of tests and measurements. The main criteria is its U-value and here it performs very well at the correct thickness and density. However, concrete – which is a mix of water, sand and cement – does have a very poor reputation in relation to its environmental impact: the production process for cement accounts for no less than 5% of global man-made carbon emissions. An alternative carbon-neutral cement is also now available in Ireland.
Here are some details on the two types of cement:
1. Portland cement.
2. GGBS cement.
1. The most commonly used product for more than a century in buildings in this country, Portland cement is convenient and easily handled, and is still the dominant form of cement used to make concrete all over the world.
It draws on some of the earth’s most common and abundant minerals for its raw materials. In the sniping war with the timber industry, the cement industry likes to boast that the amount of land used to extract the materials needed to make Portland cement is only a fraction of that used for timber logging.
2. As an alternative cement, GGBS (ground granulated blastfurnace slag) is now abundantly available in Ireland – it has been manufactured in this country since 2003 – and it has a significantly lower carbon footprint than Portland. Leading users of GGBS are the Netherlands, Belgium, Italy and Germany.
This cement is a recycled product and it replaces the use of Portland cement in concrete on a one-for-one basis by weight. In practice, replacement levels vary from 30% to up to 85%. GGBS cement is used for its environmental benefits, improved appearance in exposed concrete (it is much whiter than the stoney-grey of Portland-type concrete) and technical benefits, including strength, and resistance to sulphate and chloride attack. Its use in Ireland is covered under the Irish Annex to EN 206-1:2002, the EU concrete standard.
Concrete in general has many attributes, including its Fabric Energy Storage properties. Concrete blocks absorb free heat from the sun, mainly through south-facing windows and doors, and store this in the floors and walls of the building before eventually returning it to the air over a period of up to six hours. Concrete is known to improve comfort conditions by preventing rapid temperature swings caused by strong sunlight or large numbers of people entering a room.
You never hear of problems with dry rot in concrete, which also has an incredibly high durability – and it’s fire resistant. It also performs exceptionally well as regards soundproofing and noise control.
In the eternal debate over whether concrete/block houses are better or worse than timber-frame houses, the independent agency SEI (Sustainable Energy Ireland) says that the main consideration in building is how quickly the construction will lose heat through its fabric. It’s essential, it says, to build correctly and ensure that the U-values for walls, floors and roofs are as low as possible, using the optimum combination of building materials and insulation as laid out in Part L of the building regulations. The SEI concludes: “If a timber-frame house is built to the same building regulations as a concrete house (i.e. the U-value of the wall is the same, U-value of the roofs are the same, etc), then both houses will perform identically. After that it is preference that decides which house is for you.”
The most frequently-built external walls in Ireland are masonry cavity walls, consisting of two thin block or brick walls with the space between them filled by insulation. While cavity walls are a boon in our climate for keeping heat in the house, they can also be one of the most troublesome areas of building and they should be constantly inspected (but often aren’t) during the construction process. This is because any breach of the cavity or insulation (the technical term is ‘bridging’) can have a serious effect on the overall performance of the cavity wall system in terms of moisture transference and heat conductivity. Problems can be created very simply: through, for example, excess mortar dropping into the cavity and either piling up at the bottom or landing on the ties used to fix the insulation, thus bridging the gap between the two walls.
A modern variation from the traditional concrete block is the aircrete block, which is lighter, easier to chase and cut, and has superior insulation properties. The interlocking poroton block has a honeycomb core and claims to enable exterior walls to be built in single-leaf form without the need for labour intensive and expensive secondary insulation. The poroton claims a U-value as low as 0.11.
The use of Thin Joint Mortar is a faster way of building. A compound mixed with mortar, which is used thinly between the blocks, sets more rapidly than traditional concrete and is fully cured in two hours.
In recent years, some architects and builders have been using pre-cast concrete panels, which are often delivered to the site already clad in natural stone or brick with pre-cut openings for windows and doors. Insulation is then fixed to the panel on site to give the required U-value to the wall. While this system is mainly used for commercial buildings, it is regarded as a viable proposition for housing too.
Another modern development is ICF (insulated concrete formwork), which entails pouring concrete between two polystyrene insulation panels. These panels stay in place permanently, with various external coverings fixed to them and then plastered. A potential difficulty is the weakness in the joints created when there is more than one concrete pour and where one pour has cured before the second pour is carried out. The ICF system is particularly suitable for curved walls.
Glossary
U-value
This is the expression of measurements of heat loss, so the lower the U-value the better. The SEI (Sustainable Energy Ireland, the independent government agency) puts it this way: One square metre of a standard double-glazed window will transmit about 2.5 watts of energy for each degree difference either side of the window (denoted by the unit W/m2K) whereas a low-emissivity window will do better at about 1.8 W/m2K.
GGBS
Ground granulated blastfurnace slag is known as the carbon-neutral cement. It is a by-product of the steel industry. Molten slag lying on top of the molten iron in the blastfurnace comprises silicates (glass) and this is the raw material for this cement. The molten slag is cooled and then finely ground to form GGBS cement.
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