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