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How to Specify: Blocks & Blockwork

Blockwork plays a key role in the construction industry. This guide looks at some of the key considerations when selecting blocks for your project.

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Blocks are the common name for concrete masonry units (CMUs), and are sometimes also called concrete bricks, cement blocks, breeze blocks or cinder blocks.

The idea of making blocks of out concrete, mimicking the structural effectiveness of natural stone blocks or bricks at a much lower cost, emerged in the 19th century in America. Throughout the 20th century and on to today, concrete blocks are so ubiquitous as to be reasonably considered the most common building material in the world – by a mile.

While they have fallen out of favour with some architects who consider them outdated, they continue to be used throughout the world in a wide variety of applications where reliability and cost are a factor.

Concrete blocks are literally the workhorse of the building world: with the exception of tall buildings, they are used extensively in all areas of buildings, including foundations, walls and flooring.

Please note: This article will only address blocks made out of concrete, not blocks made out of clay such as honeycomb (Ziegel) or unfired clay bricks.

Technical Details for Blockwork

Concrete blockwork can generally be divided into three categories: dense aggregate, light aggregate, and cellular aircrete, sometimes known as aerated concrete.

Dense aggregate blocks consist of cement, sand and various aggregates, such as barite, magnetite, iron or lead pellets, and has a typical thermal conductivity of 0.70 – 1.30 W/mK.

Light aggregate blocks consist of cement, sand and lightweight natural aggregates such as volcanic pumice, slate, or shale, or industrial byproducts such as fly ash, slag, or FBA (fluorosilicate-based admixture), with a typical thermal conductivity of 0.10 – 0.20 W/mK. Aerated concrete blocks, or aircrete, first invented in the 1920s,

How to Choose the Correct Blockwork

The main consideration to take into account when specifying concrete blocks is the structural loading that can be expected upon them.

In doing so, you should consult a structural engineer, particularly if the building is more than two stories. For buildings under two stories, any of the three main types of concrete block mentioned above will be suitable from a structural point of view.

The second issue to consider is the insulation value of the blocks you specify.

You should carefully consider the heating strategy of your building, and decide whether insulation or thermal mass is more important. While light aggregate or aerated concrete blocks will provide their own inbuilt insulation in the form of air trapped into the concrete (requiring less, or no cavity insulation, depending on regulatory reguirements), these lighter forms of blocks have the disadvantage of having less thermal mass than heavy aggregate blocks.

Lower thermal mass will make it harder to utilise the natural heat from the sun’s light, but this will not be taken into account by building control.

More insulation will keep your house warmer for longer, and will be considered by building control, but may stop some of the natural heat from the sun getting into the structure.

You should also consider the buildability of whatever type of block you specify.

Lighter blocks are significantly easier to handle for builders, but may be difficult to plaster directly onto if they are not completely even.

Heavier blocks will be much more work to build with, but may be desirable for structural or thermal reasons.

Its also worth considering the sustainability of whatever concrete blocks you choose to specify.

Aircrete blocks are the most sustainable type of concrete blocks, and can help meet the Code for Sustainable Homes, with up to 80% recycled content, and significantly reduced transportation costs due to lower weight to volume ratio.

Light aggregate blocks can also contain recycled contents as their aggregate, usually called secondary-aggregates because they are the secondary byproducts of various manufacturing processes.

Regulations to Consider when Specifying Blockwork

Although the building regulations don’t directly address the use of concrete blocks in buildings, wall constructions utilising them must adhere to the Building Regulations Approved Document Part E Resistance to the passage of sound and Approved Document Part L Conservation of fuel and power.

Both of these documents set out minimum requirements for both the passage of sound and energy through walls and other building elements in new buildings and existing buildings being renovated.

For simplicity, only the regulations applying to residential dwellings will be discussed here.

For new residential buildings, the minimum airborne sound transmission for walls is 45dB, while for existing buildings,  the minimum airborne sound transmission for is 43dB.

Part E requires that 10% of all dwellings undergo Pre-Completion Testing (PCT) for acoustical compliance on site. This testing must be done by a test body with the correct accreditation.

Alternatively, specifiers may use a Robust Detail (RD) separating floors, to demonstrate Part E compliance and eliminate the need for pre-completion testing.

For new buildings, the thermal performance of external walls must be 0.30 W/m2.K, while for renovations, external walls with cavity insulation with a heat-loss of over 0.70 W/m2.K should be upgraded so as to have a heat-loss of of 0.50 W/m2.K, while external walls with external/internal insulation ith a heat-loss over 0.70 W/m2.K should be upgraded so as to have a U-value of 0.30 W/m2.K.

You should be aware that the building regulations stipulate further energy saving measures that go above and beyond maximum heat-loss values for walls.

Finally, its also important to check that the products you’re specifying are BBA (British Board of Agrement) Certified, so that they comply with building regulations and can be signed off by a building control officer.

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