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I was shocked to learn that the construction sector, operation, and maintenance of the built environment is responsible for 45% of the total UK's carbon emission from the recent study published in the UK. The report went on to add that 32% of landfill waste comes from construction and demolition business, and nearly 13% of it is useful stuff.
How can we reduce the carbon footprint in the Construction Sector? What steps are being taken up by construction companies and the government to curb this emission?
One such step towards this effort could be - to target concrete, which is a significant component in the building construction.
Cement Manufacturing and Co2
Cement, being one of the main ingredients in concrete, is responsible for significant C02 emissions because, during manufacturing, a massive amount of C02 is released into the atmosphere. One metric ton (MT) of CO2 gets released into the atmosphere when a metric ton of cement is manufactured.
India, being 2nd largest manufacturer, produced about 320 million tons in 2019, and globally the manufacturing touched 4.2 billion metric tons in 2019.
Current global Co2 emission is 36.81 billion tons, while in 1997, this figure was 21.6 billion tons recording 42% increase.
From the above, it is clear that cement in 2019 contributed to 11.4% of the global emission.
My own experience
Although I was aware of blended cement, I did not experience the use of cement replacement until 2003, when I went to the Middle East. This is because of the projects that I worked on, specified use of fly ash (PFA), silica fume ( SF), and GGBS ( Ground Granulated Blast furnace Slag) as a cement replacement. Batching plants designed the mixes and supplied the durable concrete using cement replacements.
On this project, we poured 2.5 million m3 of concrete, out of which 1 million m3 of structural concrete was produced using cement replacement. Cementitious material used was 451Kg per m3, out of which 136 Kg was the of cement replacement.
So in this case carbon emission was reduced by 136kg x 1,000,000m3 / 1000kg =136,000 Metric Ton.
So the project substantially reduced the carbon emission during the construction phase of the project.
My 2300 Sq Ft house used 70m3 of concrete, contributing 28 MT of Co2. I could have used Fly Ash and reduced some amount of C02 emission. But then I had no idea of using Fly Ash.
I leave it to your imagination for emissions happening in a concrete jungle called Mumbai.
Since there is no end to this menace, which is only growing year by year, it's time we do something about it in slowing down the increase in emission levels.
Yes, I am referring to cement substitutes or replacements to slow down the rate of emission.
Waste by-products such as fly ash (PFA), silica fume ( SF), and GGBS ( Ground Granulated Blast furnace Slag) coming from coal using power plants and steel production facilities serve as cement replacement.
While we can discuss the chemistry part at a later date to explain this, but it is proven beyond doubt that these industries by-products when mixed with water and other concrete ingredients behave the same as cement. So actually you are incorporating waste product which emits no CO2, unlike cement.
On the job I worked, project specification permitted replacing of cement by 8 to 10% of Silica Fume, 25% to 40% of PFA, and 30% to 70% of GGBS by the weight of total cementitious materials.
PFA confirmed to ASTCM C618, Class F.
GGBS conformed to ASTCM C989, Grade 100, and 120.
Silica Fume Conformed to ASTM C1240.
Another advantage of substituting cement with cement replacement is the savings due to the low cost of substitutes. For example, I googled to check the price of fly ash to be Rs 1400 per ton in comparison to Rs 8000 per ton for that of cement. GGBS, on the other hand, costs Rs 2800 per ton. So, the use of PFA and GGBS is cheaper by 60 to 70%.
However, in the Middle East, these substitutes need to be imported, thereby raising the cost. I also found Silica Fume to be costlier than cement.
Consumers should ask batching plants to make provisions to reduce the prices for the use of substitutes.
Many projects in other countries, including the US, Canada, and the Middle East are using high-volume fly ash concrete for many years now.
Fly Ash Advantages
Concrete with high fly ash content has the following advantages:
- Extended age strength
- Mitigates sulfate attack, corrosion, and chemical attacks.
- Less permeable concrete
- More durable
- Less heat of hydration, reduces cracks, higher structural integrity
- Longer concrete Life
- It can produce much better architectural exposed concrete free of porosity, holes, and honeycombing.
What is stopping us?
It could be a lack of awareness, outdated code standards, or contract specifications, not mentioning the use of substitutes. Even the best of the codes like ACI 318 limits the use of Fly Ash to 25%, and hence many designers and batching plant owners design mixes up to a maximum 25% fly ash. However, ACI 318 suggests a maximum limit of 50% for the use of slag (GGBS).
For some reason, the batch plants are less eager to move ahead with low cost, high performance, high-volume fly ash concrete as the alternative to traditional concrete.
Designers /Consultants/Structural engineers are not keen enough to design concrete mixes differently than they have been doing it traditionally over the years or to increase their awareness about high-content fly ash concrete.
Another disadvantage is a slower rate of gain in strength. Cube crushing may not result in concrete gaining designed 28-day strength for high fly ash mixes and may fall short of it. This is where the designers and specification writers should make some hard decisions.
Specify the acceptance criteria to be 56 days. Some projects in the US are already adapted to this approach.
We need to take some bold steps and prepare contract tenders and specifications in such a way that contractors practice environment-friendly construction and, at the same time, building structures that are more durable and have extended Life.