|SCIENCE & TECHNOLOGY||Thursday, July 10, 2003, Chandigarh, India|
Corrosion problems in RCC structures
Understanding summer monsoon
UNDERSTANDING THE UNIVERSE
Corrosion problems in RCC structures
DECAY of the concrete structures is cancerous and is commonly caused by carcinogenic agents that corrode reinforcement steel embedded deep inside the body of structures. The engineers believe that a dense high-grade concrete will offer nearly a complete barrier protection to the embedded steel even under severe environment. The scientists support the above view on account of alkalinity of concrete and its fairly high electrical resistance.
Many RCC structures have withstood the ravages of time in the past, but in recent past severe damage to the RCC structures has been reported. It is well established that corrosion of reinforcement leads to cracking, spalling, debondment of concrete resulting ultimately in collapse of the structure.
Corrosion of the embedded steel causes damage of high magnitude. The corrosion products formed exert enormous pressures on the surrounding concrete, thus promoting deterioration of structure. Studies have shown that in an aggressive environment, the stress as high as 490 P’s may result.
Corrosion failures of structures is not specific to our country but is found to occur in other countries as well. For example the roof of a swimming pool in Switzerland collapsed recently on getting damaged by corrosion and 12 people lost their lives.
The Central Electro-Chemical Research Institute (CERCRI) Karaikudi (Tamil Nadu) has carried research in the field under discussion, and offered solution to many common problems. Recently, in the April, 2003, issue of the CE&CR (Civil Engineering and Construction Review) a technical journal published from New Delhi the solutions have been listed.
In well-laid concrete, alkality level may be uniform, but as a result of fine cracks that may get formed during exposure to sun and rain, salts solutions reach the steel more readily at those places. Hence the passive-active cell gets set up with the steel below the crack acting as anode and the other steel away from the cracks as cathodes.
This type of cell can have as much potential difference as that exist between steel and copper. The dry concrete has very high resistance, but wet concrete has low resistance. Thus the electrical resistance of concrete will not come in way of operation of the passive-active cell when the concrete is wet.
Most often the repair option is simply laying mortar over the surface flaws. Thus the real problem remains untreated underneath and the decay continues unabated. CECRI has developed the following protective systems for corrosion protection of reinforced and pre-stressed concrete structures.
(i) Inhibited and sealed cement slurry coating to the steel reinforcement bars embedded in concrete.
(ii) Cement polymer composite coating to steel reinforcement.
(iii) Coating to the concrete surface.
The first process listed above is a "field process" which can be applied to steel reinforcement after its bending operations have been completed.
The second process is a "shop process" which has better bendability properties than the earlier described process. This process is done in the factory even prior to bending of reinforcement.
The third process is a simple coating with very good corrosion resistance and reaction to ultraviolet radiations.
A series of investigations carried out by CECRI, (the results of which have been confirmed by the National Building Organisation, New Delhi, and the Central Building Research Institute, Roorkee) show that Indian Pozzolana cements offer less protection to steel subjected to aggressive conditions. Some recent foreign publications also suggest a cautious approach in using different pozzolanic cements. The durability factor for PPC is found to be 0.5 when compared to the OPC concrete. But if the use of PPC cannot be avoided the "Inhibitor" formulation developed by the CECRI may be used to improve the resistance of PPC concrete to a considerable degree.
Now a days the use of alternative materials is gaining importance as the the materials are becoming more economical. One class of alternate material is stainless steel to be used as reinforcement in the concrete.
Stainless steel (SS 316) is commonly used but there is no long-term performance data available regarding its performance. But investigation have shown that the chloride toleration limit is at least one order higher for the SS316 material. But the fabrication operations, such as welding, results in reduction of the toleration limit by 30 to 60 per cent.
Special purpose steel coating materials carrying nickel-3.5%, copper-0.2% and tungeston-0.1%, have been developed for use in concrete. This steel also exhibits more than twice the corrosion resistance compared to mild steel commonly used.
Steel with metallic coating: Bars with metallic coating have been investigated and the coatings tried are that of zinc, cadmium, tin, nickel and aluminium.
The coating of zinc and cadmium were able to provide protection only in the absence of chloride and these coatings were found to only delay the attack on the embedded steel.
The nickel and chromium coatings were found to offer the barrier type protection only. Once the barrier got broken, i.e. the coating got damaged, severe localised corrosion was observed.
The coating system varied in tolerance limit for chloride for specific thicknesses. Among these the chromium was found to exhibit the maximum tolerance, A specific treatment developed by the CECRI based on chromium coating was found to offer excellent resistance to corrosion upto 10,000 ppm of chloride.
Aluminium coating was found to corrode rapidly due to high alkalinity. The presence of chloride was found to accelerate the corrosion process.
Even though High Strength Low alloy steel was found to offer considerable resistance along with high strength but long-term performance data against corrosion is not available yet.
Understanding summer monsoon
LAST year’s monsoon, which was forecast as "the 14th normal monsoon in succession" turned out to be a drought, with a 17 per cent rainfall deficit for the country as a whole.
Over the long term, the Indian summer monsoon is stable: even the droughts and floods are a part of its normal variability. However, the year-to-year variations in the monsoon rainfall have a profound bearing on industrial and agricultural production. Hence, forecasting the monsoon rainfall at least a season in advance becomes vital.
Laymen, as well as scientists, often wonder why numerical weather forecasting has not made much headway, despite technological advances. It is not easy to predict the weather, because the atmosphere is unstable and the elements responsible for such events as clouds or a monsoon depression involve non-linear interactions over distances of kilometres or hundreds of kilometres.
A "normal" monsoon is defined as rainfall that is within 10 `B1 per cent of the long-period average: for the whole of India, this is 88 centimetres. The departure from normal rainfall in the whole of India during 1900 — 1981 has varied from — 30 per cent to + 30 per cent. But, even within 2 to 3 years period, there is variability: years of heavy rainfall tend to be followed by years of reduced rainfall. Large and persistent winter snow-cover over Eurasia, following a strong monsoon can delay and weaken the spring and summer heating of the Asian landmass that is necessary for the establishment of large-scale monsoon flow.
The Indian monsoon has marked vagaries. A late or weak monsoon, or one with long gaps between showers spells widespread failure of crop. When the rainfall in the first half of the season tend to be low, the rain in the second half is also low.
Up to 1945, individual experts made weather forecasts. Today, some of the largest and most powerful computers are used to process the large volumes of data involved.The National Centre for Medium Range Weather Forecasting, Delhi, use a CRAY-XM24 supercomputer. Mathematical models of the atmosphere have been developed. As a result, medium-range (5 to 10 days) prediction has improved dramatically in quality. The India Meteorological Department (IMD) generates the short-range and long-range forecastes. IMD short-range forecasts are valid only for 24 hours, with a qualitative forecaste for the next 48 hours. This will not do the farmers, who wants to know how much rain may be expected in his area in the next week or next month. Countrywide forecasts are not very helpful. However, no country has come up with region-wise and time-wise forecasts.
The first seasonal forecast in the world was issued on June 14, 1886 by H.F. Blandford, based on the inverse relationship between the Himalayan snow cover and the Indian summer monsoon. Later, it was realised that the ‘El Nino — Southern Oscillation’ (ENSO) phenomenon had a strong effect on the Indian summer monsoon. ‘El Nino is a term used for anomalously high sea-surface temperatures (SST) in the equatorial east Pacific Ocean. "Southern Oscillation" refers to the large-scale "see-sawing" of sea-level pressure between the eastern and western sides of the tropical Pacific. An associated phenomenon is "El Nina — a cooling of the Pacific waters. ENSO triggers the movement of warm water from the western Pacific eastward every 2 to 10 years, causing drought and floods worldwide.
Although the Indian Ocean’s summer monsoon was strongly linked to the ENSO from 1960 to 1988, this link has weakened since then, according to Dr Peter Webster, director of the University of Colorado Programme in Atmospheric and Ocean Sciences. Says Webster: "Our research indicates that the Indian Ocean has its own El Nino phenomenon charaterised by east-west oscillation of warm water that affects other parts of world". The weakening of the ENSO link could have been on reason why the IMD forecast for the 2002 monsoon went awry. The other reason is that the IMD forecast issued on May 25, could not have taken account of some major changes in atmospheric, circulation, which occurred in May.
Unlike other approaches, the IMD model gives quantitative forecasts. For 14 years (1988 to 2001) the forecast in most years stayed a reasonable margin of error. According to Dr Vasant Gowarikar, former Secretary to the Department of Science and Technology, no forecast model developed for the prediction of the Indian monsoon has come anywhere near the accuracy of the IMD’s model. For almost 44 years it performed flawlessly. (that is, till 2002). Writing in Current Science (October 25, 2002), he points out that the Long-range Weather and Crop Forecasting Group formed in Canada in 1997 is using modelling success achieved in India for forecasting the Canadian spring wheat crop and possibly the US corn crop.
The IMD has recently developed an eight parameter ‘power regression’ model, according to which there is a likelihood that, this year, the country as a whole will have monsoon rainfall which will be 96 per cent of the long period average, with model error of `B1 5 per cent.
There is little
indication of cyclic change in the Indian monsoon pattern. However,
global warming due to greenhouse gases may create a greater summertime
disparity between the land and ocean temperatures, according to Dr
David M. Andersen of the University of Colorado. This could increase
the monsoon intensity, which could in turn mean fewer crop failures
but also flooding and soil erosion that could endanger the livelihood
UNDERSTANDING THE UNIVERSE
Why does a smooth floor feel cooler than a rough one?
I will assume that we are talking about a floor that is made of marble, marble chips or smooth spread of cement. In other words, because of its compaction it is likely to be a better conductor of heat than mud; this is because mud has air pockets enclosed within. Therefore while standing on a marble or smoothly cemented floor there would be more transfer of heat from the sole of the foot to the floor if the ambient temperature were less than the body temperature.
This is true in winter months, as also at other times of the year unless it is very hot. Therefore, usually we do feel that a marble or cemented floor is rather cool.
Now we come to the question of smoothness or roughness. When the surface is rough the total area of contact with the floor is significantly less than when it is smooth. Therefore the rate of heat flow from the foot to the floor is lower. Hence we feel less cold. It is also possible that the rough surfaces are associated with materials that are less conducting. In very cold climates the preferred floors are those made of wood. This is not because of its roughness but the superior heat insulating quality of wood.
Incidentally, marbled or cemented floors out under the hot sun would not feel cold but hot. The reason is the same. This time the heat flow would be from the floor to the foot.
Does a compass show the directions on another planet?
A compass is nothing but a tiny pivoted magnet. It aligns itself along the direction of the existing magnetic field. It can be used to find directions on the earth because horizontal component of earth’s magnetic field is approximately parallel to the geographical north-south direction.
If we go to another planet and the planet does have a magnetic field the compass will show directions as on earth. There also the directions will be with respect to the alignment of the magnetic field, if any. A compass will be no use on the moon because it does not have a magnetic field. The Jupiter, on the other hand, does have a strong field and the compass should work.
A compass is essentially a way of detecting a magnetic field, including its direction. For more accurate work these days scientists use rather sophisticated detectors called magnetometers.
Why the electrons cannot exist in the nucleus of an atom?
In quantum mechanical
terms the size of a particle is inversely proportional to its mass!
The electron is 1840 times lighter than the proton and therefore the
spread of its wave is that much bigger than that of the proton.
Therefore, the electron cannot exist in the nucleus because it is much
bigger than the nucleus. Also a nucleus is a tightly bound structure.
The operating force is the nuclear force. The electron does not have
any nuclear force of short range.
New products & discoveries
New materials from viruses
BONE. Nerve. Muscle. Horn. Hide. Silk. With ingenious assemblages of atoms and molecules, biology produces fantastic substances that have long inspired scientists to develop the synthetic materials of the modern landscape. Lately, materials scientists have turned to biology’s smallest individuals — viruses, bacteria, and fungi. Not only can these microbes be coaxed to produce high-tech components, but they can also themselves serve as valuable ingredients in new classes of materials.
Scientists are beginning to employ microbes, for example, to organise crystals into complicated geometries or provide living templates for growing crystals. Since the structure of materials is intimately linked to their behaviour, a new means of controlling crystal organisation creates a buzz among materials scientists, says Science News.
Microbes have several advantages
as laboratory reagents. Some microorganisms, such as viruses, measure tens of
nanometers in length. Researchers can’t make uniform synthetic particles at
this scale, but microbes are readily available, uniform in size, and easy to