Wednesday, August 30, 2006
Towards a `green' goal
The building sector is the third largest consumer of energy after manufacturing industry and agriculture. In order to make the building sector socially and ecologically sensitive, we have to incorporate the best practices that result in environment protection, water conservation, energy efficiency, use of recycled products and renewable energy. In short, we have to go green.
Concrete is the material of choice for building modern infrastructure, be it buildings, bridges, tunnels, railways, airports or docks and harbours.
For achieving the "green goal," we may concentrate on three main factors, namely, conserving concrete and building materials, extending life and durability of infrastructure and adopting a holistic approach by maximisation of waste reuse.
Conservation of materials
Carbon dioxide discharge from cement plants deserves attention. For example, in the production of one tonne of cement, nearly one tonne of CO2 gas is discharged into the atmosphere. More than 50 per cent of the discharge is derived from decomposition of the raw material when limestone is changed to CaO in addition to some energy consumption during burning process. The mechanism of discharge is essential for the production of cement.
Aggregates, together with cement and water, are the primary components of the construction industry, but they are not any more freely available.
The use of demolition waste and recycled water and adoption of membrane curing are methods of saving the scarce resources. Recycled water from ready-mixed plants has been satisfactorily used for fresh mixing of water to make concrete.
Our country has huge amounts of disposable coal ash and blast furnace slag and rice husk convertible to admixture RHA. These can be effectively used as cement replacement materials in concrete or for the production of blended cements or as mineral admixtures, thereby conserving the use of clinker and reducing the CO2 emission related to cement production.
Innovative
The next issue is whether large-scale cement replacement is really possible in the construction industry. Successful structural use of high volume fly ash concrete in high rise structures the world over and especially in Canada indicates that this innovative, cheap, and high value fly ash concrete is the best waste material for use by the construction industry.
Urban infrastructure such as bridges and buildings generate environmental loading when they are constructed, served, demolished and disposed of. Emission of CO2 during the construction stage is large mainly due to the consumption of cement and steel.
Life Cycle CO2 emission (LCCO2) is an useful tool to evaluate infrastructure alternative in order to choose a system with low environmental loading during construction. During construction, the energy required for raw materials, processing and transportation is summed up to obtain energy consumption in terms of CO2 discharged into the atmosphere. Quantitative assessment of different counter-measures including utilisation of waste and recycled materials and alternative methods of construction are possible in terms equivalent to LCCO2.
A waste material like coal ash may not meet certain chemical or physical requirements prescribed for standard works but still it can have application where its fine particle size will be an advantage for roller-compacted concrete, concrete pavements, base course in embankments and special applications of controlled low strength slurry. Urban activities have an impact on urban climate. Higher temperature is observed in urban areas. Such temperature shift makes the urban areas more uncomfortable.
Energy consumption for space cooling would increase; in other words, more CO2 is emitted due to locally hotter climate conditions in summer. Passive solar architecture may be an useful alternative to consider.
Durability of infrastructure
Earth's natural resources are conserved if service life is prolonged. Concrete is the material of choice because it is cheap and produced with the simplest of technology. However, concrete should be made durable and a high performance material for the future structures if we want to make it green.
We have to combat corrosion in an effective, environment-friendly way. Concrete mixtures containing highly active pozzolans, high volume fly ash, rice husk ash or silica fume deserve to be considered. These concrete mixtures display dense microstructure free from pores and cracks. When cured well, they are an answer to both corrosion inhibition and sustainable material use.
Holistic approach
Currently, super-plasticisers have become an essential component of concrete. By substantially reducing the amount of water in concrete, they have contributed to the significant reduction in its porosity and consequent increase in strength and durability.
These chemical admixtures have also been responsible for use of other mineral admixtures in high performance concrete. By making possible the use of these mineral admixtures, which are industrial wastes as a partial replacement of cement, these super-plasticizers have contributed to reducing the CO2 emission.
There is no doubt that increased use of supplementary materials such as fly ash is by far the most powerful tool to reduce CO2 emission from production of concrete.
For this, the quantity of fly ash required is available. The technology to use it is known and proven. The investment involved is negligible.
The author is Emeritus Professor, Department of Civil Engineering, IIT Madras and former Dean, Anna University.
