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

* 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..

TABLE 1 General Embodied CO2 of Materials (not validated)

Material

Density

kg/m3

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

kg CO2/m3

range

kg CO2/tonne

range

Material

Density

kg/m3

kg CO2/m3

range

kg CO2/tonne

range

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

Material

Density

kg/m3

kg CO2/m3

range

kg CO2/tonne

range

* 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

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

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/m

2

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

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