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  • 1. David Bennett Notes Summary of The C02 Construction Audit The estimated C02 embodied in concrete frame and steel frame structures for a 3 storey and 7 storey structure , shows clearly from this study that a concrete frame building has a greater saving in overall C02 emissions , and by as much as 40%. Further C02 reductions are possible for a steel frame buildings if you adopt the lower C02 emissions output, specifying only recycled steel and not virgin steel. If this was universally adopted , it would in the long term mean closure of existing steelworks in the UK; therefore a balance between the two production types would seem prudent Further reductions in C02 emissions are also possible with concrete framed buildings by (1) adopting a passive ventilation system , thus significantly reducing mechanical ductwork and pipe work and (2) eliminating the need for suspended ceilings . If all the ply formwork for casting insitu concrete was re-used in the permanent construction and not disposed of; quite a lot of C02 could be sequestered to offset the embodied C02 in plywood manufacture and concrete production. On the other hand , by choosing to build a precast composite frame, the need for ply formwork would dramatically reduce and this would also reduce the construction C02 of the frame. With concrete you have a choice of frame options to enable designers to optimise on cost and to minimise on C02 emissions. Concrete which combines low embodied cements such as PFA and GGBS and natural aggregates , is not a high emissions product as has been shown by this audit and the technical notes in 'Sustainable Concrete Architecture . Many designers not familiar with the constituents of concrete mistakenly believe it's mainly composed of Portland Cement giving it a poor green rating . This should not prevail. Reducing waste , recycling formwork and not disposing of timber to a rubbish dump to avoid creating methane , should be part of good working practice. The use of public transport to make home to work journeys will reduce car emissions for city centre projects. The key pointers are: * A detailed C02 construction audit of a building is very feasible * The accuracy of C02 data, the energy source, the collection of transport and production C02 data is essential to make value judgements * The supply chain must be encouraged to provide this data at tender stage and such data must be validated
  • 2. * Knowledge of the constituents of concrete , the material choices, and of the frame construction options - insitu , composite and precast - , will lead to better management of C02 in construction..
  • 3. TABLE 1 General Embodied CO2 of Materials (not validated) kg CO2/m3 Density kg CO2/tonne range kg/m3 range Material Metals Lead 67,137 11,340 5920 Copper 56,725 74,566 8,930 6,352 8,350 brass 61,381 8,600 7,137 Zinc 43,863 7,135 6,148 Aluminium recycled 1,191 2,584 2,800 425 923 Aluminium 23,828 83,168 2,800 8,510 29,703 Steel recycled 12,654 27,763 7,800 1,622 3,559 Steel virgin 26,870 54,497 7,800 3,445 6,987 Oil Based Linoleum 17,831 1,180 15,111 Materials Single ply roof membrane 20,046 Carpet, synthetic 10,030 190 52,789 paint (liquid) 13,882 1,000 13,882 Polycarbonate 16,445 1,200 13,704 PVC 11,060 1,100 10,055 Plastics 20,046 1,100 18,224 Glass Tinted glass 44,356 2,400 18,482 Laminated glass 4,853 2,400 2,022 Toughened glass 7,800 2,400 3,250 Float glass 9,810 2,400 4,088 Clay Clay tiles 648 2,200 295 Materials Bricks, engineering* 860 1,950 441 Bricks, Fletton* 128 1,950 66 Bricks autoclaved* 341 1,950 175 Bricks, non fletton* 624 1,950 320 Bricks* 1,895 1,950 972 Cement Particle board 520 Based Sand cement render 171 2,162 79 Materials Concrete tiles 269 2,200 122
  • 4. kg CO2/m3 Density kg CO2/tonne range kg/m3 range Material Concrete precast 328 2,400 137 Concrete, 1:3:6 256 2,400 107 Concrete 376 2,300 163 Autoclaved blocks 341 900 379 Lightweight blocks 256 600 427 Novacem 300 Insulation Woodwool, loose 384 Products Sheep's wool 13 Foamed glass 320 Plastic 480 Polystyrene 445 Fibreglass 115 Mineral wool 98 Cellulose 57 Harvested Straw bale 4 Materials Glulam 1,300 Local softwood 47 450 104 Local green oak 94 650 145 Plywood** 676 640 1,056 Prepared softwood 427 450 949 Local airdried timber 163 Chipboard 357 700 510 Quarried Gypsum wallboard 696 950 733 Materials Plaster 948 950 998 Stone local 240 2,200 109 Local stone tiles 192 2,200 87 Local slate 230 2,200 105 Lightweight aggregate 128 1,200 107 Sand and gravel 19 2,000 10 Crushed granite 64 2,400 27 Natural Aggregate 110 2,000 55
  • 5. kg CO2/m3 Density kg CO2/tonne range kg/m3 range Material * density range for brick is 1700 - 2200, average 1950 used ** density range for plywood is 500 - 780, average of 640 used kg CO2/tonne=kg CO2/m3xdensity/1000 Source: :- kg CO2/m3 - The Green Building Guide: -Centre for Alternative Technology; Environmental Science Handbook; Pittsburgh Corning; Timber Trade Federation; CIRIA; GreenPro The Environmental Handbook Feilden Clegg Bradley: - Building Research Establishment Approved Environmental Profile (1994); BISRIA Environmental Rules of Thumb and Environmental Code of Practice; University of Wellington; Association of Environmentally Conscious Building Architects Journal 8 June 1997; Canadian Architects; Beyer Plastics Source: Density The Way we Build Now - form,scale and technique - Andrew Orton Simetric www.simetric.co.uk; Engineering Toolbox ; www.engineeringtoolbox.com/metal-alloys-densities-d_50.html; IEM : www:.iem-inc.com/tooldens.html Leeds Metropolitan University CO2 Audit
  • 6. Visual Concrete: OUTLINE Summary of Presentation given by David Bennett Introduction: We all want to create smooth, uniform, defect free concrete finishes. The precast industry with their experience and skilled craftsmanship are able to achieve it every time so why it is so difficult when it comes to site cast concrete. After all the concrete mix is the same, the site conditions are perhaps more onerous but are not too dissimilar The explanation for this is simple: its lack of understanding about(1) the factors which control the surface appearance of concrete, (2) how to specify architectural concrete and(3) how to handle it on site. We should not expect the engineering specification to cater for visual concrete compliance because such a specification places greater emphasis on concrete strength and durability, allowing the mix constituents to be adjusted to maintain these criteria. This is totally incompatible for visual concrete where the mix constituents are not permitted to be adjusted as they will alter the surface appearance and colour. An architectural concrete specification needs to be drafted to guide and direct contractors and suppliers of these requirements. Discussion What determines the surface colour of concrete? Is it the sand, the coarse aggregates, the cement or the water? Typical mix proportions: cement 350kg, sand 700kg stone 1200kg water 150kg, typical particle size: cement 60- 90micron, sand 50micton- 3mm, stone 20-5mm, the answer is the cement, but why? The finest particles dominate the surface colour which are the cement fines plus pigments in that sand that have a particle size of 50micron or less. Mix Specification Must have enough cement powder to saturate the surface and coat all the other particles The key points of the architectural concrete mix cement content not less than 350kg/m3 cement from the same source water cement ratio not to exceed 0.5 sand 150micron content to be constant sand zone M sand colour consistent cement/aggregate ratio not to exceed 6 sand: cement ratio not to exceed 2 coarse aggregate not more than 20% to pass the 10mm sieve Ready Mixed Concrete Trail mixes, proportions fixed after lab trials, no change to mix constituents permitted thereafter, wet batched, single size bins preferred for 10mm and 20mm aggregates, computer print out, target slump between125mm and 175mm, sample panel to check widest variation of mix, check compatibility of admixture with release agent, check consistency of cement colour. Quality control: no water added once mixed, small changes in water content alter tone of surface colour, higher water
  • 7. content gives lighter tone. Journey time to site , check for loss of slump, if unacceptable may have to increase plasticiser dosage or introduce retarder. Formwork What type of surface finish required, define how it is to be achieved: smooth as struck - mat or shiny textured - grit blasted , acid etched or retarded board marked - size of panels point tooled - type of mechanical finish profiled - form liners Form Face: Veneer overlays: MDO, HDO , PSF give different finishes depending on the amount of resin in the veneer – less resin, less shiny the surface, but less robust the veneer. Formliners and GRP can be very expensive, so ensure high re-use factor best for precast work. Formwork: cost indicators range from £19/m2 – 50/m2 plus ( formliners/GRP £200 -500/ m2), ensure at least 4 re-uses of selected formwork, check cost, inspect example of finish and check re-use factor with supplier. Formwork preparation: cutting sheets and drilling holes, clean contact surface with plastic and wooden faced tools, seal all cut edges, make grout tight joints; architect prepares joint and tie bolt layout and not leave it to the contractor. Finish tolerances: state surface tolerances, abrupt edges, out of plumb, rigidity of formwork (negligible movement), and design under full liquid head. Release Agents: specify active, non staining type, does not wash off in rain, remains active for a week or more, ensure compatible with concrete admixture and formwork type - petroleum based, mineral based - apply sparingly. Striking Times: must be kept constant throughout – say between 24 and 36 hrs, the longer the forms remain in place after 24 hrs, the darker the concrete and the greater the potential risk of discolouration from release agent and imprint from backing ply; dense veneer overlays and metal faces give shiny surface with greater risk of surface discolouration; long term carbonation will mask early difference in tone of surface colour, exception slab soffit. Concrete Workmanship Architectural concrete compaction: 500mm max layers set by rate of travel of air bubble along form face, radius effect of poker critical to determine insertion points for even compaction , use constant amplitude electrical vibrators for correct compaction , re-vibration of top layer. Check rebar details and cover spacer orientation to maximise working space in slender wall forms. Handling and placing: reduce free fall height of concrete to 1m, tremie walls and columns, pour at rate 2m per hour to avoid cold joints, have standby equipment in case of break down. Surface Sealant To maintain dirt free, water repellent surface, apply siloxane coat which is transparent and non- shiny; it must be vapour permeable and not degrade in sunlight, nor darken the concrete or go straw coloured in sunlight
  • 8. Reference Panel Full scale with construction joints to assess quality of construction joinery, grout tight joints and sharp arrises. As the concrete colour settles after 6 months the trial panel is not for assessing colour. Publication References Sustainable Concrete Architecture ,David Bennett, RIBA Publications Architectural Insitu Concrete, David Bennett, RIBA Publications The Art of Precast Concrete, David Bennett, Birkhauser Exploring Concrete Architecture, David Bennett, Birkhauser Concrete Architecture, Catherine Croft, Laurence King Concrete Design, Sarah Gaventa, Mitchell Beazely Appearance Matters, .Bill Monks, BCA Publication Cast In Concrete, Susan Dawson, ACA Technical Report 52 , Plain Formed Concrete Finishes, Concrete Society Visual Concrete Specification EN 206 -1/BS8500 All available from RIBA Bookshop and The Concrete Centre