CONCRETE AND ITS DURABILITY
Even before the energy crisis of 1973, the failure of modern buildings to cope with a variety of climates was apparent. They relied on technology to provide comfortable conditions – artificial ventilation and air-conditioning. Local and traditional methods that had evolved simple and successful means of achieving comfort were ignored.
The modern movement believed that exploitation of advanced technology would make buildings not only quicker to build and cheaper, but of a higher standard. Standards were cut, bad enough in small buildings, but disastrous in high structures. In this attempt, a few techniques and materials were positively dangerous to health such as asbestos, paints, cavity insulation, air-conditioning etc. Asbestos was once considered to be a wonderful building material to be used as a roofing alternative. Later on, we realized the danger of using materials such as asbestos, certain paints, and the air-conditioning methods were not only causing damage to the occupants, but also the environment at large.
Large areas of glass were employed in all climates – hot or cold. They let in all the unnecessary heat and then we try to air-condition the space to make us comfortable. As time passed, the thrust was to use the more expensive glass which may reflect more heat compared with the traditional glass. Doing elevations became easier, the façade of the building no longer reflected what is happening inside the building.
Two decades ago, everybody believed that reinforced cement concrete (RCC) structures will last forever. But over some time, it has been realised that it is very difficult to do an RCC roof that does not leak. Now it has become the norm to use waterproofing chemicals. Research has shown that even if the concrete is done with good quality materials and workmanship, they will leak after twenty or thirty years.
Very brittle concrete has to expand or contract with every degree change in temperature. Even if the workmanship is perfect, the concrete tends to develop fine cracks which widen over few years (if the workmanship is very bad, then the roof will start leaking in two years) due to the alternate expansion and contraction due to the variations in temperature within a day. Water finds entry through these cracks and reinforcement starts corroding over a period of time. This is the reason why the RCC buildings which have been built in the eighties and nineties are developing leaks now. It has become very expensive to repair these concrete slabs and they require huge maintenance expenditure over some time.
Putting a weathering course with brick jelly concrete and clay tile on top of the RCC roof will protect the concrete layer from direct exposure to sun and rain. As a result, the durability of the concrete will increase. In hot humid conditions such as in Chennai or Kerala etc. this affects the thermal performance. More thickness for the roof means, the higher the heat storage capacity and the concrete roofs start radiating this heat into the room at night when you require the maximum comfort. On the other hand, a light Mangalore tiled roof will start radiating the heat immediately, but at night it is quite comfortable.
In the case of a sloping RCC roof, one can put some kind of roof tiles on top to protect the RCC from being directly hit by sun and rain. What I have used more extensively are the Mangalore tiles. When I use on top of RCC roofs, I use it in such a way that there is an air column in between.
The trend which is happening in Kerala at present is to do a flat RCC roof and then provide a sloping steel truss roof on top. This not only improves the thermal comfort in a house but also increases the durability of concrete since they are not exposed to direct sun and rain. But there is an additional cost, but in the long term, this will avoid a leaking RCC roof. We have learnt about the mistakes over a while.
This is true even in the case of cement plaster and cement flooring. The problem is more serious when it is exposed to sun and rain intermittently.
Importance of Overhang
The overhang of the roofs or coping of the parapets is very important for the durability of the external plaster especially when it is done with cement. Cement develops shrinkage cracks when it sets and over a period of time, this widens allowing the water to enter inside the plaster. This leads to loss of adhesion.
Lack of any overhang of the roof or coping leads to hairline cracks in the cement plaster affecting the durability of the plaster.
Whenever I have done earth buildings, I had thought about the overhang being given to the traditional compound walls in Kerala. There is an old English saying that “Give a mud house a hat and a good pair of boots, it will last forever”. The hat is the roof and boots are the foundation and basement of the building. This is not only true about the mud houses, but also other buildings built with different materials.
The durability of concrete roofs, external plaster and even paints in any building depend on the protection it receives from driving rain and scorching sun.