Operational and embodied CO2:
The use of concrete in buildings can lead to questions regarding
its embodied CO2, which is often thought to be much higher than
other construction materials. In reality, the difference is typically quite small, and
becomes insignificant when compared to a building's operational CO2
emissions over its life.
In buildings where the inherent thermal mass of concrete forms
part of the cooling strategy, any additional embodied CO2 burden
can in fact be offset many times over. This can be shown quite
simply by comparing the embodied and operational CO2 emissions per
m2 for a typical air-conditioned office with those of a typical
mixed-mode office i.e. one that is cooled using thermal mass in
conjunction with natural and mechanical ventilation.
A source of data for the embodied CO2 of office buildings is
life cycle assessment research on steel and concrete framed
buildings. One of the conclusions of the study was that overall
there is no significant difference between steel composite,
reinforced concrete and precast concrete options with regard to
embodied CO2. For a small to medium rise office, built to a
developer's standard specification, the range of embodied CO2 for
different options was equivalent to only 16 months CO2 emissions
from an energy-efficient, air-conditioned office [1,2].
A source of data for operational CO2 emissions is the BCO Guide
which reports that for a mixed-mode office that utilises thermal
mass, the annual CO2 emissions are around 40-80 kg/m2 per year .
This is applicable to a solid in-situ concrete slab or a precast
hollowcore option. The annual CO2 emissions from an
energy-efficient air conditioned office are around 60-90 kg/m2 .
The range of values arise from variations in building location and
activity within, as well as mechanical design and building
management. Design and management are vital if operational CO2
emissions are to be minimised, but it is clear that the conceptual
design choice of utilising thermal mass gives the best chance of
minimising operational CO2 emissions.
If, conservatively, the 'mixed-mode office' building  is
considered to have the greatest embodied energy from Eaton &
Amato  and the air-conditioned building  is considered to
have the least embodied energy from Eaton & Amato, then the
comparative graph in Figure 1 can be drawn. From Figure 1 it can be
seen that the additional CO2 in the thermally massive building is
offset in only six years. More moderate assumptions would see the
crossover even earlier.
To most effectively design for thermal mass, a slab thickness of
over 200mm can be fully utilised by exposing it on both the soffit
and the top surfaces; the latter by use of hard finishes or
underfloor ventilation. If it can only be exposed on one surface a
thickness of 150mm to 200mm concrete can act effectively in storing
and releasing energy during particularly hot weather. For more
details on thickness of concrete required to maximise thermal mass
Thermal Mass from the Publications Library.
Radiance from thermally-massive surfaces is an important part of
maximising thermal mass benefits. Whilst passing air over 'hidden'
surfaces has a beneficial effect, unless occupants can see the
surface of the thermal mass they do not gain the additional benefit
of the radiance. It is to be noted that the thermal comfort of an
occupant is dependent on air temperature and radiant temperature
(the temperature of surfaces that can be seen). Therefore, exposed
concrete slab soffits and concrete walls should be considered at
the earliest stages of design.
The integration of services into a design where structure and
slabs are exposed offers challenges to the design team.
Utilisation of Thermal Mass in Non-residential Buildings from
The Concrete Centre is a useful tool to designers facing this