Strine Environments

Concrete and Embodied Energy – Can using concrete be carbon neutral

 

Concrete is the most widely used building material in the world. There is now approximately 2 tonnes of concrete for each person on the planet earth. The small amount of embodied energy (carbon) in one tonne of concrete, when multiplied by the huge amount of concrete used, results in concrete being the material that contains the greatest amount of carbon in the world.

The embodied energy of a material represents the amount of carbon (carbon dioxide) embodied in that material.

Justification for higher embodied energy in buildings

A higher embodied energy level in buildings can be justified if it contributes to lower operational energy over the life of the building. For example, large amounts of thermal mass, high in embodied energy, can significantly reduce cooling and heating needs in well designed and insulated passive solar buildings, particularly In climates with greater cooling or heating requirements and significant day/night temperature variations (like Canberra and region). (1)
As the operational energy of a building over its life cycle far exceeds its embodied energy, using the high thermal mass of concrete to virtually eliminate heating and cooling energy requirements results in saving lots of energy that creates a carbon neutral outcome over the life of the building.

How to reduce the impact of embodied energy

The single most important factor in reducing the impact of embodied energy (EE) is to design long life, durable and adaptable buildings. Buildings should aim to use materials that have lower EE. (1)
The choice of construction material should depend on all of the benefits it contributes to optimising the buildings performance over its life cycle.

Sustainability

Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. (Bruntland, UN, 1987)
Sustainability includes environmental, social and economic considerations, not just the single issue of greenhouse gas emissions.

Embodied energy

Embodied energy is the energy consumed by all of the processes associated with the production of a material or an assembly like a building, from the mining and processing of natural resources to manufacturing, transport and product delivery. EE does not include the operation and disposal of the building material, which would be considered in a life cycle approach. EE is the ‘upstream’ or ‘front-end’ component of the life cycle impact of a material or building. It occurs only once when a material or building is produced, and can be 10% to 20% of the energy used in a home. It is difficult to assess the EE of a material and even more difficult for an assembly of materials and estimates can vary by a factor of up to ten. Figures quoted for EE are generally Process Energy Requirements (PER) and are usually in MJ/kg, although some figures quote CO2 emissions per tonne (CO2/T). EE figures can only be broad guidelines. (1)
As energy inputs entail greenhouse gas emissions, there is a direct relationship between EE and carbon content.

Operational energy

Operational energy accumulates over time throughout the life of a building. Operational energy consumption depends on the efficiency of the building envelope and the occupants’ behaviour and can be up to 90% of the energy used in a home. (1)

Life cycle assessment

Life cycle assessment (LCA) examines the total environmental impact of a material or assembly over its whole life. It is necessarily complex.

Choosing sustainable building materials

The criteria for choosing sustainable building materials are:
Embodied energy
Resource depletion
Recyclability
Life cycle contribution
Environmental impact
Embodied energy may not be the most significant factor, depending on the location, design and available energy.

Embodied energy of common materials

Embodied energy ( EE ) content varies greatly with different materials and construction types.

Typical figures for some Australian materials are given in the tables that follow. Generally, the more highly processed a material is the higher its embodied energy. However, materials with the lowest EE, such as concrete, bricks and timber, are usually consumed in large quantities compared to materials with high EE. As a result the greatest amount of EE in a building can be from either low EE materials such as concrete or from high EE materials such as steel. (1)

Material PER embodied energy MJ/kg
Kiln dried sawn softwood 3.4
Kiln dried sawn hardwood 2.0
Air dried sawn hardwood 0.5
Particleboard 8.0
MDF 11.3
Plywood 10.4
Laminated veneer lumber 11.0
Plastics – general 90.0
PVC 80.0
Synthetic rubber 110.0
Acrylic paint 61.5
Stabilised earth 0.7
Plasterboard 4.4
Fibre cement 4.8
Cement 5.6
Insitu Concrete 1.9
Precast tilt-up concrete 1.9
Clay bricks 2.5
Concrete blocks 1.5
AAC Hebel 3.6
Glass 12.7
Aluminium 170.0
Copper 100.0
Galvanised steel 38.0
(2)

The EE for a component or assembly is more useful than an individual material. For example the PER EE for an elevated timber floor is 293 MJ/m2 compared to 645 MJ/m2 for a 110mm slab on ground.
CARBON DIOXIDE
Carbon dioxide CO2 is one of several greenhouse gases that cause global warming by trapping the sun’s radiant energy in the atmosphere.
The production of energy generates CO2, meaning that EE equates to carbon content.

Green house gas emissions and global warming

Increased greenhouse gas emissions (GGE) are contributing to global warming.
The key contributors to GGE are from energy production, transportation, industry and agriculture.

Cement

Cement is a dry grey powder, one ingredient of concrete used as a binding agent or glue.
Normal concrete contains 7.5% to 15% of cement.
Of all the material used to make concrete, cement has the highest EE of 5.6MJ/kg. This is still low compared to MDF or glass, and very low compared to plastics, rubber, aluminium, steel and copper.
Cement production accounts for 2% to 3% of human generated CO2 production and consumes about 0.5% of total energy consumption.
Cement substitutes like flyash or slag can reduce its EE.

Concrete

What is concrete
A liquid that turns solid
A plastic material, that assumes any formed shape
A brittle solid with high compressive strength and almost no tensile strength

Characteristics of concrete:
Very massive, heavy and dense: 2.3 -2.5 Tonne/m3
Brittle, and always shrinks, never grows, which means all concrete has cracks (not defects)
Cheap: say $200/m3 or $20/m2 for 100mm thick
High conductivity, low resistivity means low insulation value
High thermal capacity to absorb energy (heat), slow to lose energy gives a time lag of 4 hours per 100mm thickness (a good thermal fly wheel)
High decrement factor: the slow response of concrete to temperature variations can reduce the internal temperature variation of a building by more than 50%
Very high fire resistance
Very good acoustic insulation
Very strong when combined with steel tensile reinforcement

Composition of concrete by weight
Cement 10%
Water 10%
Air 5%
Coarse aggregates (gravel) 50%
Fine aggregates (sand) 25%

How does concrete set
Concrete sets due to a chemical reaction between the cement and water, called hydration. It is rapid at first, then decreases but continues for years.
Good hydration results in good concrete strength, and requires water. It also requires heat to start the process.
The conditions that concrete is subjected to as it sets are known as its curing conditions. Curing affects the rate and degree of hydration, which affects the concrete strength.
Concrete that is properly cured with controlled moisture levels and temperature is 3 times stronger than air cured concrete.

Concrete strength
The characteristic or structural design strength of concrete is its 28 day strength.
It is measured in Mega Pascals (MPa) eg 25 MPa for basic concrete.
Typical concrete strengths:
Footings 15-20 MPa
Slabs 25-32 MPa
Columns 32-40 MPa
Precast 32-50 MPa

Sources
(1) Your Home -http://www.yourhome.gov.au/materials/embodied-energy
(2) Dr Bill Lawson ‘Buildings, Materials, Energy and the
Environment’ (1996)