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TECHNICAL REPORT Continued from page 11 Eurocode 6 (BS EN 1996-1-1) Shortly after the introduction of Eurocode 6 and the UK National Application Document (NAD) giving values to use in the UK for designs to Eurocode 6, it became known that designs to this code and the UK NAD were significantly more conservative than designs to BS 5628. Twelve years after a paper, anticipating Eurocode 6, with me as one of the joint authors, was published in the Structural Engineer making the case for a K factor of 0.7 in the equation: f k = K f m 0.3 . f b 0.7 [the equation to calculate characteristic strength (f k ) when designing to Eurocode 6] the BSI mirror committee for the masonry Eurocode, has finally agreed to adjust the factor of K from 0.55 (their original choice) to 0.75 for Group 1 concrete units and from 0.45 to 0.75 for manufactured stone units. This agreement has taken an inordinate amount of effort analysing and re-analysing test data and putting forward numerous proposals to amend the K factors and unit shape factor corrections to try and get a straight line relationship between the fk values in BS 5628-1 and those calculated according to the Eurocode 6 equation. designs are carried out to BS EN 1996- 1-1 instead of advocating the use of the withdrawn BS 5628. The new UK NAD will also contain some key guidance on calculating fk values for collar-jointed walls and walls with blocks laid flat which the existing UK NAD does not provide. Flooring blocks Following a CBA initiative, it has been agreed that a national foreword can be added to BS EN 15037-2 ‘Precast concrete products. Beam and block floor systems. Part 2 Concrete blocks’ which states that blocks to BS EN 771-3 can be either 7.3N/mm 2 strength with no need to test for transverse strength or alternatively they need to be tested for transverse strength according to that standard. The test method is identical to that used in the UK since blocks were first supplied for this application. This will mean that you will not have to claim compliance with BS EN 15037-2 when supplying blocks for beam and block flooring and have to CE mark to 2 standards. Thermal bridging details We are in the process of putting together some generic thermal bridging details for use by CBA members. There are some general details (Accredited Construction Details known as ACDs) available for downloading from a Government website but these give conservative values and we need to have dedicated thermal bridging details appropriate for constructions involving generic aggregate blocks available as these are needed for SAP calculations. This is specialist work and we are going outside for this modelling work. A typical detail for a floor/wall junction is shown left. It will be appreciated that the permutations of junction type, block type, other product type in the junction, insulation type and thickness together with the combination of positions where the insulation can be placed leads to a vast number of possible thermal bridging details. Initially we will be modelling the 17 most frequently found junctions and add to them as demand dictates. (Below) Once modelled the details will give ψ-values and f-factors for each detail modelled and models will cover λ-values for ultralightweight, lightweight and dense blocks. A revised UK NAD to BS EN 1996-1-1 where the new K factors are to be found has now been accepted and is expected to be published before the summer. We will thus be able to recommend that masonry CBA Website a hit Our website continues to receive 4000 - 5000 visits a month with the average visitor looking at 3 pages. The most sought after pages continue to be the 20 technical data sheets, u-value calculator and Part L 2010 Regs guide. With this in mind many of the technical data sheets have been updated over the last 12 months, for instance the one on sustainability has been comprehensively upgraded and contains more factual detail. A members only area is now up and running which contains all the Spring Update and Monthly Newsround back issues along with the Technical Commitee minutes. A facility to trade plant and equipment member to member has also been set up. Make sure that your staff and customers are aware of the ease of access to information on concrete blocks via our website. Remember that if you print the CBA QR tag ob your literature it will give smartphone users instant access to the site. www.cba-blocks.org.uk 12 www.cba-blocks.org.uk
thermal mass Thermal mass in concrete In an age of growing environmental awareness, the use of concrete in buildings can lead to questions regarding its embodied CO 2 . Often it is perceived as higher than that of many alternative construction materials. In the case of timber frame housing, the difference compared to masonry construction is actually quite small at around 4-6% for the average house. This is because with the exception of the blockwork and timber frame, all the other construction materials used are common to both house types and account for most of the embodied impacts, for example, UPVC windows, brick facade, roof structure, carpets etc. Over the life of a masonry home, this difference becomes even less significant since the CO 2 emissions resulting from heating, lighting and hot water will be many times greater. So, this puts the embodied CO 2 issue into context, but there is another important factor to take account of, which is the affect of the thermal mass provided by the blockwork. Over the life of the building, this has the potential to tip the overall CO 2 balance in favour of masonry construction. Blocks and other heavyweight materials can enhance fabric energy efficiency in well insulated homes by capturing and later releasing free heat from the sun and internal sources such as cookers, electrical appliances and people. This in turn can reduce the load on the boiler and, over time, offset the slightly higher embodied CO 2 in a masonry home. Put another way, these savings can be viewed as payback for the investment in the embodied CO 2 of a block inner leaf. This sounds good in theory, but to establish the credibility of this argument The Concrete Centre commissioned Arup to compare the embodied and operational CO 2 emissions of a masonry home with an equivalent timber frame home, taking into account the likely impact of climate change on their thermal performance over the 21st century. The reason for factoring this in was to include the affects of our warming climate and the ability of thermal mass to avoid or reduce the future need for domestic air-conditioning; another likely source of domestic CO 2 emissions in the coming years. The research showed that the additional embodied CO 2 in an average semidetached masonry home can be offset in as little as 11 years if the design seeks to include the following: High levels of insulation and airtightness Some south facing windows to maximise solar gain in winter Continuous background heating (young family, retired occupants, underfloor heating). The research also showed that increasing the thermal mass further by, for example, including a concrete upper floor will increase the offset period due to the higher level of embodied CO 2 , but further reduces annual emissions, ultimately leading to the lowest whole life CO 2 footprint of all the options considered in the study. To put these findings into context, the overall CO 2 saved (embodied and in-use) over the 21st century when compared to the lightweight home was around 7-10% for a typical masonry home with a timber upper floor, and 10-15% when a concrete upper floor is added. Whilst this study was completed back in 2006, a recent check using the latest embodied CO 2 data for construction materials confirmed the research remains valid. For more information, a technical paper on this research can be downloaded from the Concrete Centre: www.concretecentre. com/Embodied_and_operational_carbon_ dioxide_emissions_from_housing Conclusions The thermal mass in concrete blocks has an important role to play in the 21st century by helping to both mitigate and adapt to climate change. The potential whole life CO 2 savings provided by blocks can outweigh any additional embodied impacts when compared to timber frame construction. Whilst it may not always be possible to realise these savings, thermal mass is nevertheless playing an increasingly important role in fabric energy efficiency and recent changes to construction regulations and standards are beginning to reflect this. Figure 1 – (Reference: “Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change”; Energy and Buildings 40/3, 2008) Tom De Saulles MPA – the Concrete Centre www.cba-blocks.org.uk 13
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