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BEWARE OF EXCESS CEMENT

Cement deteriorates with age, and has an inherent weakness of cracking.

28bgp-greensens_28_2355568gHow many of us know that India is the second largest manufacturer of cement in the world with more than 360 million tonne annual capacity? Is it a matter to be proud of? Majority will say, yes. The price of cement has come to indicate the health of market economy, shifting directions of monetary investments, rate of infrastructure development and such others. Again, an important information for all of us.

How many of us know that every tonne of cement produced causes nearly another tonne of carbon dioxide, a major contributor to greenhouse gases? Is it a matter to be proud of? This time hopefully, majority will say ‘no.’ This is besides the fact that cement also has very high embodied energy, a major concern today in sustainable buildings. If so, how should we treat this wonder material?

Cement is a product of calcium, aluminium, silicon and iron, supplied through mainly limestone, clay and sand. The mix in right proportion with more than 3/4th limestone is grinded, pre-heated and then heated up to 1400 degree centigrade in a rotating furnace kiln, where decarbonation takes place, releasing carbon dioxide, slurry and clinkers. The clinkers are set into a horizontal chamber for final and fine grinding.

Within about 200 years of its discovery, cement has conquered the world of construction, due to its versatility of usage, flexibility in design applications, strength and setting time.

While professionals may handle it more efficiently, even a village mason picks up the skills of working with cement very fast, hence its popularity. However, thanks to its popularity, we appear to be ignoring its drawbacks.

Though lime and cement share common raw material, cement deteriorates with age, while lime stays fit for long. Cement has an inherent weakness of cracking, passing on this trait to concrete too. As such, experts do not guarantee a cement and concrete building to last more than 60 to 80 years without periodic improvements.

Besides, it absorbs heat if directly exposed to sun, with surface cracks. If we checkout any building with only cement mortar plastering without wall paint, hundreds of cracks can be seen on the surface. Normal mortar tends to absorb water, being groscopic in nature, hence requires varied water-proofing applications. Traditional buildings in stone, wood, non-homogeneous roofs and such others could withstand minor settlement in site or even small-scale earthquakes. In contrast, cement as a material is not good to withstand settlements.

Despite knowing about the drawbacks of cement, we end up having cement mortar in all joints and surfaces; concrete in foundation, columns, lintels, beams, slabs, frames and coping bands; cement blocks in walls. What we are building today appears to be a cement building from head to toe.

While it is a dangerous trend in construction, it is equally a shame on us to neglect dozens of appropriate materials available to us. Once cement was a boon, today it may not be. It is time to look outside cement.

Tracking cracks in structures

Expansion and shrinkage due to temperature, humidity and loading happen to all construction materials, resulting in cracks.

18bgp-greensens_18_1723540eWhat discourages people from building with natural materials? Why are there very few buildings using them, despite people knowing much about the cost and performance advantages of mud, stone, bricks, wood, bamboo and clay? Eco-friendly architecture nurtured human settlements for thousands of years, yet our generation seems to be discarding the idea. Are there reasons for rejection other than material scarcity or vanishing skill?

Apprehensions of many kinds are among the major reasons and cracks are among the major apprehensions. All buildings crack, irrespective of them being sustainable or consuming; local or global. Expansion and shrinkage due to temperature, humidity and loading happen to all construction materials, resulting in cracks. Some could be invisible; the visible ones could be small or big; they may appear in wall or window; some may appear and disappear periodically. If so, cracks are not an inherent characteristic of sustainable buildings only built with natural materials or alternative technology. Yet, the pretext of cracks is often sighted to discourage people from constructing an eco-friendly building.

It is not easy to decipher cracks to be structurally dangerous or not. Mostly what we get to see is surface cracks in plastered walls, small angular lines in the corners of window bottoms, some narrow cracks in the edge of lintels, horizontal gaps where concrete beam touches brick wall or slab bottom, vertical cracks along concrete columns and such others, all of which are harmless. There are valid reasons such as shrinkage of material, differential wall weight around window openings, ignoring code of practice, speed of construction, lack of supervision, and non-compatibility of two materials like RCC and bricks when placed adjacent to each other. All these are commonly seen in conventional plastered buildings also.

Contrary to the common belief, in case of exposed materials, we tend to see less of cracks, with the joints absorbing and concealing them. If we try composite wall, i.e. with different materials for outside and inside, people worry that they may separate with a gap in between. This does not normally happen because of material bonding and mortar packing in between.

When wall materials like clay block or stabilised mud block end at a concrete column, they should not be touching it, but be segregated by a mortar joint, which will ensure no visible cracks.

Walls with natural materials, if very long or wide, may develop cracks following the shrinkage in volume. Hence, it is advisable to break the area by openings, lintels, concrete bands and such others to reduce the risk. Filler slabs, if laid across large spans, may develop cracks; hence need expansion joints, just like normal RCC roof. Arches and vaults tend to shrink, hence crack. All these cited cases normally do not fail structurally. Once we refinish it with crack fillers, they get covered up and the cracks will not reappear.

There are specific cracks that denote structural implications, which need to be studied in more detail, lest we may assume all cracks to be harmless.