 |
|
| SCIENCE TRIBUNE | Thursday, January 9, 2003, Chandigarh, India |
A hard look at today’s high-grade cements Harnessing the powers of light |
|
|
A hard look at today’s high-grade cements EVER since my earlier write-up, "The Right Cement for your House" appeared on this page on July 11, a large number of queries have been received. Most questions related to the use of high grade cements and in particular their effect on the durability of the structure. This question bothers the builder who approaches a cement dealer for purchase of a few bags for his use. He is offered different grades of cements with alluring qualities. He is at a loss to find the solution to how he can obtain a durable structure by use of high grade cements such as Grade 53 etc. Based on the available data an attempt is being made to answer this question. Durable concrete is one which will withstand in a satisfactory manner the effects of severe conditions like weathering, chemical action and wear to which the structure will be subjected. Though laboratory tests have been developed for evaluating durability of concrete but such tests are labourious and costly and fail to establish the direct correlation between the service records and the laboratory findings. The resistance of concrete to freezing and thawing has been found to depend upon the pore structure of the concrete obtained. The watertightness of a structure affects its durability. The more watertight is the concrete, more difficult it will be for water to gain entrance and fill voids; so the greater resistance to frost action and freezing and thawing action. Deterioration of concrete results from contact with various chemical agents. The attack may lead to corrosion of concrete resulting from the formation of soluble products which may be removed by leaching. When cement and water combine, one of the compounds formed is hydration lime which is readily dissolved by water passing through cracks, and along improperly treated construction joints or through voids. The removal or leaching may seriously impair the quality of concrete. The white deposits or efflorence seen on the concrete surface, which may be on account of leaching and subsequent carbonation and evaporation process. Certain chemical agents combine with cement to form compounds which have low solubility but which disrupt the concrete because their volume is much greater than the volume of the cement paste from which they orignate. High grade cements depend upon the fine powder effect to obtain early high strength and early setting. Higher fineness of cement increases the rate at which the cement hydrates and results in greater early strength and more rapid generation of heat of hydration. Although total heat generation and strength at later age are somewhat greater for finer cements but the effects of higher fineness are manifested principally during the early period of hydration. Because of their extremely small size, the fine cement particles are not susceptible to separation into a single fraction through sieving action, so special tests have been evolved to determine the degree of fineness of cements. The most commonly adopted test is the Blain’s air permeability test’, which obtains the summation of the surface area (in cm2 per gram of cement). The evaluated quantity is called the Specific Surface of cement and the most commonly used cements have the value ranging between 2600 and 5000. Use of cement having value less than 2800 (indicating coarse cement) results in poor workability of concrete and may result in excessive bleeding of concrete. Greater fineness improves the early strength development. However tests and investigation indicate that resistance to freezing and thawing is reduced to some extent when the finely ground cement is used in concrete under conditions similar to those in the field. Grade 43 & Grade 53 The cements selected were manufactured by the same company and similar chemical composition. It was found that the specific surface area of Grade 53 cement was 3290 compared to 2930 for the Grade 43 cement (which is called OPC). The initial setting time of Grade 43 Cement was 155 minutes but the Grade 53 had initial setting time of 120 minutes. Comparison of the 3 days, 7 days and 28 days compression tests of the above two varieties of indicated that the strength was 22% higher at 3 days, 17% higher at 7 days and 5% higher at 28 days test intervals for the Grade 53 cement compared to Grade 43 cement. So it is clear that initial advantage obtained at the early stage got dissipated with increase in age of the concrete. This indicates that we can take advantage of early removal of shuttering if we use higher grade cement and also take advantage of quick setting if concreting is to be carried out in emergency. But advantage of higher strength cannot be taken in effecting a reduction in quantity of cement used. The I.S. 456 (2000) Code of practice for plain and reinforced concrete has been revised recently (in 2000) with main thrust to durability of concrete structures and it has recommended use of higher concrete grades for concrete commonly used. Earlier, use of M15 Grade concrete was recommended which has now been increased to M20 Grade i.e. an increase of 33%. Earlier the nominal mix (volumetic) adopted was 1:2:4 which has now been changed to 1:1.5:3. The change is recommended for ensuring greater durability of concrete structures. The above code of practice makes no mention of use of finer cements i.e. the so called high grade cements. At the present state of
knowledge it is not safe to take advantage of initial high rate of
development of strength to make reduction in volume of cement used.
The use of greater fineness cements have found to give some reduction
in durability in the form of freezing and thawing resistance. |
|
|
Harnessing the powers of light LIGHT and optical technologies will develop into a major key technology of the 21st century covering a broad number of scientific disciplines. They will open up new markets and create jobs in particular with small and medium-sized enterprises. Optical sensors use light for precision measurements and allows, for instance, the measurement of extreme temperatures or the flow of velocity. They make tooth decay visible and enable the dentist to remove it painlessly. CD players have been commercially used for 20 years they are nothing but a small laser that reads the data stored on the very small surface of the disc.The photons — the smallest "particles" or units of light — are the only way to visualise biological functions of the living cell without changing the cell. It goes without saying that this leads to countless applications in health care and biotechnology. Light can be used for the cutting, welding and drilling of extremely small holes and precision exposure in lithography. It thus allows the precision production of all kind of products, be it computer chips or cars. Even in nano dimensions it has become possible to work on various materials. The complexity of computer chips doubles every 18 months. Only light permits the productions of structures within silicium which are a thousand times smaller than the human hair. These sophisticated production techniques will eventually lead to chips that generate automatic programmes for the online translation of texts from one language into another. Quite a different application of light is the precise description of the function of the genes. Light is fast enough to observe very short biological processes and allows the discovery of the causes of many diseases. Another project is the therapy of allergies which are based on malfunctions of only 5000-10000 genes. Optical technologies also support researchers in finding new medication without side-effects. New drugs are developed in so-called "transparent cells" with the help of microscopes, observing structures and reactions in nano dimensions. Millions of combinations of drug components are analysed with the help of light. Only those which have no side effects are selected. Another example is the development of state-of-the-art illumination of cities, workplaces, homes etc. Taking into account that 8 per cent of the entire electricity production is used for lighting purposes and knowing that energy efficiency hovers around 10-30 per cent, the detrimental effect for our environment becomes clear. The development of new illumination systems with an energy efficiency of up to 70 per cent will not only save a lot of money but will also help reduce the CO2 production and help save the atmosphere. The author is Science
Counsellor, German Embassy. |
|
|
UNDERSTANDING THE UNIVERSE How do the stars glitter? I think what you want to find out is: "Why do stars scintillate?" Stars are very far and can be treated as point sources of light. Light from any object outside the atmosphere has to pass through our atmosphere for getting to the surface of the earth. It is true that the atmosphere is essentially transparent to white light that we see. But the speed of light in air is slightly less than that in vacuum. This means that light is refracted in passing through air. In other words it changes direction by a tiny weenie amount. This bending is dependent on the density of air. We might think that on the average and in crude terms the atmosphere is the same everywhere. This is not strictly true. We know that our atmosphere is an active and mobile beast. For example, if you light a fire the air immediately above becomes rarer. There are other turbulent things that keep happening that make our atmosphere full of a large number of ever changing pockets of changing density. Therefore a narrow beam of light, or a ray, coming in from outside keeps shifting its direction by a very small angle. This is the reason that when we look at a star we see it scintillating, or glittering as you say. We see the star, then we do not, then we see it again and then not and so on and on. That is what we call scintillation. While stars scintillate, planets do not. The reason for this unreasonable behaviour of planets is that they are not point sources of light. When the ray of light from one spot bends away another one from a neighbouring spot takes its place. Since all natural products are from plants why do they differ so much in taste? All plants are not the same even though the basic molecules and atoms that go into making them might be the same or similar. We might even wonder why the plants are plants and not animals. Both are living. Information that goes into growth and architecture makes the difference. Using the same bricks and stone you can build a Taj Mahal, a temple or the Pyramids of Egypt! Structure and design matter a great deal. Let us now come to taste. Sense of taste is a sophisticated
discriminating power given to us. Our palate has a large number of receptors of
different kind. We sometime call them taste buds. They can be thought of as
sockets of complicated design into which only the molecules of a compatible
design or shape can fit — this is like keys fitting into, and opening,
specific locks. When a fit occurs a signal goes out to the brain giving the
feeling of a specific taste. Several different types of receptors working in
concert give us sensations of rich variety of tastes. Sometimes even the
receptors in our nose that give us our sense of smell, work together with the
taste buds to give us the flavour of what we eat! Try eating your favourite
dish with your nose pinched close and you will find out. You will also learn
why food does not taste so good when you have a cold with a stuffy nose. |
|
|
SCIENCE & TECHNOLOGY CROSSWORD Clues Across : 1. This ray is in-fact an electron ejected from an atom of a fast moving ionizing particle. 5. Dark thick sulphurous fog polluting the atmosphere of industrial cities. 8. Abbr. for initial teaching system that helps people in learning English. 9. A unit of mass in fps system equal to 32.174 lbs. 11. A circuit with only one output but more than one inputs. 14. Plant of genus including holly. 15. Symbol for Gadolinium. 16. Intensely poisonous substance produced by certain bacteria. 18. Simple lipid of various glycerides of fatty acids. 19. This generator has two Van der Graaff generators in series thus producing double energy. 22. Symbol for Nobelium. 24. Hard fibrous substance obtained from trunks of trees. 26. A standard such as a bye-law. 27…logy deals with relationship of organisms with their surroundings. 28. Difference in days between length of a solar year and a lunar year. 29. A periodic increase or decrease in loudness heard when two notes of nearly same frequency are sounded together. Down : 1. One twelfth of diameter of Sun or moon, used to denote the extent of eclipse. 2. An interferometer used to study fine spectrum lines. 3. A milky fluid produced by certain plants. 4. Symbol for Arsenic. 5. An irrational quantity. 6. Symbol for Magnesium. 7. This star has high luminosity, low density and size many times that of sun. 10. Non conducting cover of a boiler. 12. An opening in a building to go out. 13. A finely ground oil paint having a resin. 17. Prefix denoting one thousand millionth. 18. Outer circle of a wheel attached by spokes. 20. Point of zero displacement in a system. 21. A hypothetical unit controlling individual characteristics of an organism. 23. This is formed when malt is mashed during brewing process. 25. Short for Optimum Moisture Content as used in Soil Mechanics. Solution to last week’s Crossword:
|
 |
|
