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| SCIENCE TRIBUNE | Thursday, February 22, 2001, Chandigarh, India |
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Protecting buildings from
efflorescence Science
Quiz |
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Earthquakes and collapse of buildings Till now there is no scientific method to predict occurrence of an earthquake on any part of the earth. It is the precautions or the Scientific approach of the designers, engineers and builders which can minimise the loss of man and material. Brick and stone masonry construction is common in almost all parts of the country. The local availability, cheapness of the two materials and requirement of less skilled labour are the factors responsible for their common use. But sometimes their abundant and unthoughtful use leads to great misery. Now a days urbanisation has given rise to multistoreyed/high-rise buildings which brought into existence the framed structures. Frames consisting of foundations, columns, beams and slabs are made of reinforced cement concrete (RCC). Rest of the wall panels are made of brick or ashlar stone facing. While RCC frame of the building provides a good degree of ductility to the building, the infill wall material is brittle and is unable to resist any lateral thrust which is a common force during earthquakes. The hydraulic structures like-barrage, weir, dam, pump house etc and public utility structures such as a railway or a road bridge, electric towers, water supply and sewer pipeline etc are other important kinds of structures, in which masonry and concrete are used isolately or in unison. Bricks or stone masonry exhibits very good results against axial compressive loads. That’s why it is extensively used where it is predominently subjected to compressive stresses. But its weakness in tension becomes a great cause of concern. In areas of high seismic zones the weakness of masonry (brick or stone) and plain concrete against tensile stresses needs to be improved, to prevent any catastrophic failure of the structure during its life time. This could be done by encasing the masonry or plain concrete member with a ferrocement layer. Ferrocement is a composite material in which the inherent properties of two constituent materials are best made use of. The filler material — cement sand mortar called matrix — is reinforced with layers of continuous fibres of reinforcing mesh in both the principal directions. The subdivision and distribution of ductile material (wiremesh) throughout the mass of cement-sand mortar increases the elasticity of the matrix and structural member around which it is wrapped. The technique is simple. The structural member like column, wall beam etc that need to be encased with ferrocement is cleaned with wirebrush. It is wrapped with desired layers of wiremesh. The wiremesh is kept intact with nails etc; and is plastered with rich cement-sand mortar with low water cement ratio. Ferrocement possesses a high degree of toughness, durability, ductility, strength and crack resistance. The basic materials required for ferrocement composite are: cement, sand, water and wiremash of proper size and configuration. For making good mortar for a ferrocement composite, cement-sand ratio in the mortar may vary from 1:1.5 to 1:2.5 and water-cement ratio from 0.35 to 0.50 by weight. Wiremash of different configuration-square woven, hexagonal chicken, welded and expanded metal can be used. The requirement is that it should be flexible and easy in handling so that it could be bent around the sharp edges of structural members. The wiremesh is generally made of 0.50 to 2.00 mm diametre wires spaced at 5 to 25 mm apart. Applications of Ferrocement are numerous ranging from railway sleepers, electric poles/towers, walls, columns beams, slabs to water and oil storage tanks, foodgrain storage bins, canal lining etc. One of its most important application is its use in strengthening and rehabilitation of old and partially damaged buildings/structures and giving a remedial measure to some of the building defects. Ferrocement encased structural members not only impart strength to the members, but for equal strength it helps in reducing the cross sectional area of the member, thereby reducing the dead load of the structure or its member. It also increases flexibility and ductility of a building, which is a sought after quality in the earthquake prone area. If at all during an earthquake of severe magnitude, say more than 6.5 on Richter scale, a building sways and it is unable to resist the sway force ferrocement encasing of its structural elements, like pillars, walls, lintels beams etc; does not let the building collapse. The ferrocement encased building or structure will bend or distort and this excessive bending or distortion could be remedied after the tremors are over. Thus such an engineered building will not only save precious lives but also help in conserving the costly building materials. Laboratory results have proved that percentage increase in ultimate strength of walls and columns encased with ferrocement composite was 90 to 115%. It infers that load carrying capacity of the structural element can be easily increased by 100 per cent with the use of ferrocement composite. It has also been observed from the test results that elements encased with ferrocement, like walls and columns could sustain 70 to 80 per cent of ultimate load at failure, without any substantial increase in deformations. This shows that at worst, even if the structural member distorts or deforms, it can still withstand three-fourths of the failure load without further deformations; which otherwise is not possible in plain masonry elements. In nutshell with the use of ferrocement composite sudden failure of any structure is eliminated. Laboratory results reveal that a failed/damaged column strengthened with ferrocement encasing with two layers of wiremesh took one and half time higher load than the load at which it originally failed. But for practical purposes where laboratory like situation is not possible, it is good enough that the damaged part is strengthened to its original capacity and that is possible with this uniquematerial. Hence with ferrocement composite, damaged and failed structures, buildings can be rehabilitated and could be made functional again with same original safety. Obviously, cost and skilled workmanship are two factors, which might discourage the expansive use of this wonder composite material. But it is pertinent to mention that increase in cost is not in multiples, it is only the cost of wiremesh and a small quantity of cement which will increase the total cost of a building by not more than 5 to 8 per cent. As far as technical skill is required, any skilled mason can easily wrap the wiremesh and then plaster it with a little care to cement-sand and water-cement ratio. Thus both these factors are not burdensome. It is only the awareness and adaptability of the people in general and Engineers, Builders in particular, which could save lakh of people from future tremors and give them safe homes.
The writer is a civil engineer specialising in structural engineering and constructional techniques. |
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Protecting buildings from
efflorescence EFFLORESCENCE in buildings is a serious problem. It plays havoc with walls and masonry due to poor quality of bricks and water used in the construction. Temporary whitish deposits on the surface of a wall is known as efflorescence (Kallar or Shora local names) whereas rise of the ground water in walls is termed as dampness. If proper care is not taken during or after the construction, the building becomes inhabitable and unhygienic after some time. Due to the use of saltish water during construction and through rains, the salts (like sulphates or magnesium, sodium, calcium and calcium carbonate) present in the body of wall get dissolved. On evaporation the previously absorbed water along with salts is brought to the surface of the wall in concentrated patches either as white powder or as translucent crystals, resulting in serious disfigurment on inner and outer faces of walls. Another cause of efflorescences in pyrites in clay or in soils the source of sulphur dioxide which contribute efflorescence in the form of sulphate salts. All these salts react with certain compounds of cement causing the weakening, cracking and clumbling of cement mortar and wall plaster of buildings. Building owners with reinforced brick roofing generally comes with complaints of appearance of some white patches on the ceilings of their buildings. The bricks used in the roof contain some type of lime which causes white patches (lime efflorescence). Best way to avoid this is to use Reinforced Cement Concrete roofing instead of Reinforced Brick roofing. Attempts to seal back efflorescence are not usually successful and it is advisable to allow the efflorescence to expand itself fully before applying treatment for its removal. Exterior efflorescence in existing buildings can be removed to a great extent by firstly washing the wall with water and then treating with dilute solution of one part of Hydrochloric acid with 33 parts of water followed by again washing with water and finally brushing surface efflorescence when wall gets dry. It can also be checked by repeated washing of walls with clean potable or canal water and then brushing the efflorescence from the surface of the walls. Interior efflorescence can be checked partially by first allowing it to develop fully and then removing the same with stiff wire brush or to reverse the evaporation gradient initially, so that evaporation takes place from the exterior surface of the wall. This reversal can be attained by applying a suitable colourless water proofer to the interiors to stop evaporation from the inner face. For finding out the presence of efflorescence in the bricks, five bricks are selected at random out of the stack of bricks. Then all bricks are immersed on end in a shallow dish containing one inch deep clean distilled/potable water. The bricks are allowed to stand in this position in a warm and well ventilated room until all the water in the dish is evaporated. The same process is repeated for the second time also on the same bricks. At the end of second process the bricks are visually examined for efflorescence. The first class bricks shall not show any appreciable sign of efflorescence (whitish deposits) on its surface after the second process. Bricks containing efflorescence should be avoided in construction of buildings. Efflorescence is directly linked with moisture. So by preventing the entry of moisture from walls by using impervious coping on top of walls and parapets, sealing of all joints, avoiding joints in window sills, providing adequate drips on all copings, projections and cornices. Most importantly by using efflorescence free bricks and providing dampproof course 6” or 9” above ground level in all the walls during the construction, so that wall may not get saturated with moisture carrying the soluble salts derived from soils. By using clean and potable or canal water free from suspended material, organic impurities, acids, alkalies and salts for the construction and especially curing work, we can have our newly constructed buildings free from efflorescence and other related problems. |
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Unseen world of atoms Final checks are being made to one of the world’s most powerful mass spectrometers, capable of detecting all elements known to man to levels as minute as one part in one billion in solid samples. Called the VG 9000, the British-developed machine is the first one of its kind to use a new technique known as “glow discharge” to examine samples. This avoids the complexity which surrounds many other methods for solids analysis and brings remarkable ease of operation to the procedure, making the glow discharge mass spectrometer suitable for both demanding research and process control, such as on a semiconductor production line. The sample is bombarded by excited argon gas molecules which remove atoms from the surface of the material to be analysed. The charged atoms, which have been produced in the source, are accelerated through combined magnetic and electrostatic fields in the mass spectrometer which separates the atoms by atomic weight. The separate atoms are then collected and identified by a detector to reveal the amount of each element in the sample. The VG 9000 is ideal for investigation into the purity of semiconductors, high purity metals and high temperature metals such as jet engine turbine blades. It also has a direct application for any process where impurities in the end-product can have critical effects on performance.
(NF)
EPR puzzle solved A six decade old puzzle in physics called Einstein-Podolsky-Rosen (EPR) non-locality problem has been solved with required experimental support from a scientist at the Tata Institute of Fundamental Research (TIFR). Considered a breakthrough, the scientist, Dr CS Unnikrishnan of Gravitation Group of TIFR, told PTI that resolving of EPR puzzle has opened up the understanding of the Objective Reality in the microscopic world as “we took into account the wave nature of particles in addressing the problem”. It also supports quantum theory without sacrificing concept of “locality”. The new solution removes the irrational notion of non-locality that existed for over a century among the physicists, he said. The EPR non-locality puzzle is one of the most discussed fundamental problems in physics. The EPR analysis was motivated by the desire to assign an Objective Reality to measureable properties in the microscopic world independent of the observer or apparatus. Unnikrishnan said this breakthrough in understanding also ruled out instantaneous action at a distance when a microscopic object is observed at a particular locality. This introduced a notion of objective reality which is different from Einstein’s understanding. All technologies based on Quantum Mechanics will remain the same but their interpretation will change drastically, Unnikrishnan indicated. For example, in Quantum Teleportation which is used in Crytography, there is no real quantum teleporation, or no influence which is transmitted faster than light.
(PTI)
Fungus to treat effluent A white rot fungus, Ganoderma lucidun, can be used to decolourise effluents from paper mills, which have been recognised as environmental hazards. Scientists at the Centre for Advanced Studies in Botany, University of Madras, evaluated the efficacy of G. lucidum for reduction of colour of paper mill effluents under various growth conditions. The discharge of untreated effluents from the pulp and paper mills into water bodies damages the water quality. The brown colour imparted to water due to addition of effluents is detectable over long distances. The effluents have a high biological and chemical oxygen demand (BOD and COD), lingnin compounds and their derivatives. The dark brown colour is due to the formation of lignin-degradation products during the processing of lignocellylosics for paper and pulp manufacture. As the lignin derivatives are highly resistant to microbial attack, they escape the waste water treatment. Because of their ability to degrade lignin, several microorganisms have been tried for biological treatment of such effluents.
(PTI) |
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Science
Quiz 1. The research conducted by this Austria-born U.S. pathologist led to the process of transfusion of blood which has helped save millions and millions of lives. Name this winner of 1930 Nobel Prize for physiology or medicine who also did pioneering work on polio. What was his main contribution to medical science? 2. Recent evidences have shown that a satellite of Jupiter may have a hidden ocean under its surface. Name this satellite which is not only the Jupiter’s biggest but also the largest satellite in the solar system. 3. By this new surgical technique, the pitch and quality of a person’s voice can be modified according to his/her wishes. What is this technique called? 4. WYSIWYG is a common and important term in computer technology. What does it mean? 5. Elements such as iodine, selenium, copper and fluorine are present in our body in very low concentration. What are such elements known as? 6. “The total energy radiated from a black body per unit time per unit normal area is directly proportional to the fourth power of its absolute temperature.” What is this law called? 7. Bone-meal consisting of ground animal bones is used as a fertiliser. Which chemical element present in bones mainly acts as fertiliser? 8. The world’s smallest bird has a length of only 2.5 inches from the tip of its bill to the end of its tail. This can fly forward, backward, straight up and down, and can also hover in midair. Which is this tiny bird that is found in Cuba and on the Isle of Pines in the South Pacific? 9. Atoms the muclei of which have same number of protons but different number of neutrons are called isotopes. What are the atoms called the nuclei of which have the same number of neutrons but different number of protons? 10. IRRI is an international organisation engaged in the development of new productive varities of rice. What is its complete name and where is it located? Answers |
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