Wednesday, August 3, 2011

Week 27: Trying not to add to the concrete jungle...


So much is happening on-site that I do not know where to begin our update. Fortunately, our contractors are good at multi-tasking and can execute separate items of work simultaneously.

An important milestone last week was the completion of all the major planned concrete work on-site. The last item on the list was a small roof slab for the pump room located below the deck. This therefore seems to be an appropriate time for us to talk about the use of concrete in our project.

As I mentioned in my last post, we have tried to reduce the use of concrete to the minimum in our project. The reason is the inherent negative environmental impact of cement use. The production of Portland cement is known to account for 5 - 8% of global greenhouse gas (GHG) emissions which are the major cause of climate change. The high emissions are due to the chemical process that produces cement and as a by-product generates large amounts of carbon dioxide. There is little that can be changed in the cement producing chemical process and hence the resulting emissions. This puts the cement industry at the forefront of global environmental and climate change debate. Link to a NYTimes article on the topic.

View of the living room with 40cm wide basalt load bearing walls and two corner columns to allow for extra-large front and back openings
In India, most buildings are built with a structural frame of concrete beams and columns. After this, the walls are filed in with brick or other building blocks. In our mission to reduce the amount of cement use, we decided very early on to design our structure with load bearing walls. This strategy is more feasible (and actually the more obvious and practical solution) for a building like ours that is mostly built on the ground plus one floor (as a result the primary load on the structure is the roof and not multiple floors above). All our external walls are built with basalt or laterite stone, both locally sourced natural materials that take the weight of the structure above. The only need for concrete columns, total six in number, was to allow for the extra large openings designed in all our rooms.

As a second step, we decided to use fly-ash cement in place of regular Portland cement in our concrete mixes. Fly-ash cement is sometimes called green cement as it replaces between 15-25% of cement with fly-ash. Fly-ash is the by-product from burning coal. Therefore, fly-ash cement reuses industrial waste. The performance of fly-ash cement is also completely structurally safe, as it must follow the national codes for design and construction of concrete, as per IS codes 456-2000. In addition, fly-ash in concrete is known to improve the long-term strength of concrete, reduce corrosion and permeability. Here is some more reading material on Fly-ash cement

After using fly-ash cement in our concrete mixes, the only difference that we found is that a fly-ash concrete mix takes slightly longer to set, so contractors prefer to use the quick drying Portland cement. Other than that the performance and workability has been comparable to regular Portland cement.

While laying the RCC slabs using filler blocks and fly-ash cement
Another construction innovation employed by us in our quest to reduce the amount of cement used in the project 
was the use of filler slabs in place of conventional reinforced concrete (RCC) slabs. The science behind filler slabs allows for a replacement of up to 30% concrete in a RCC roof slab with lightweight filler and non-structural material such as bricks, tiles or earthenware. The idea is that in a RCC slab, the upper part is subjected to compressive forces while the lower part experiences tensile forces. The concrete in RCC is good at supporting compressive forces and steel good at tensile strength. Hence, there is no structural need for concrete in the lower part of an RCC slab. As a result, the gaps between steel reinforcement can be filled in with lighter material (even waste material such as bottles, old roof tiles or broken bricks). Filler slabs also contribute in reducing the load of the slab itself therefore reducing the amount of weight that falls on the vertical structural members (load bearing walls, or columns) and foundations. Read more about filler slabs.

For use as filler material in our project, we made simple mud blocks on site from the earth excavated from foundations. These blocks were sun-dried. As a result, we used no new material or spent any additional energy in making them or transporting them to site. 

Filler blocks made with excavated earth from site being sun-dried
Filler blocks that were made on-site ready to be used
View of filler slab from below after removing the shuttering
In our experience, a big deterrent against the reduction of concrete in building construction is the unwillingness of structural engineers to work with any structural solutions other than the conventional concrete frame structure. The calculation models used by structural engineers don’t seem to be designed to incorporate load bearing capacity of walls and reduction of the slab’s weight by using lightweight filler material. Along with creative architects, the building industry also desperately needs creative engineers who understand the long-term implications of their work and input on the environment.

Finally, I guess the follow-up question is why more people are not building with load bearing walls, fly-ash cement and filler slabs. The only reason that comes to my mind is the inertia to adopt new ideas or perhaps laziness. The building industry fails to educate their clients and together adopt obvious and simple steps towards building a greener building, reducing the pressure on existing resources and ensure the long-term sustainability of our planet.


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