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July, 2001 CONCRETE can be singled out as one construction material that has been most used as well as abused by the construction industry. For ages, scientists and engineers have kept themselves engaged in studying its behaviour and reaction with other materials so as to make it more economical strong, durable, and compatible with other materials. At the same time, it has been played with, tempered with and downgraded in quality to satisfy the greed of those having vested interests. A major handicap has been our inability to test it to be as per specified parameters without destroying it. “Rogues” have been taking maximum benefit of this handicap. To test a structure under doubt by “destroying” it is an extremely bold step for an engineer who is supposed to be a creator and not a destroyer. If the test results of the structure harmed for testing purposes, so as to remove the doubt, are found satisfactory, the guilt that sets in has no treatment. Under such conditions an engineer longs for such testing methods that may indicate the strength and soundness of a structure without destroying or harming it. It is due to this reason that the focus has been over non-destructive testing for the past few years and now significant breakthrough has been made in this direction. Such tests and instruments can now be listed which can check the strength, soundness or say, full anatomy of a structure without disintegrating it. What needs to be practised by the engineers is that they should become fully conversant with these instruments and should avail them whenever in doubt. A few of them that may find maximum use and may come most handy to the engineers are described below. Checking corrosion of steel reinforcement; Once the concrete is poured in position and reinforcement is covered, it is not possible to check whether the steel used for reinforcement was free of rust and scales or not. Moreover if the concrete poured is not of sound quality, the steel may get corroded with time. The condition of steel can now be checked by using Corrosion Analysis instrument which given out printouts showing the condition of steel. Use of rod and wheel electrodes known as half-cells enable this instrument to detect the condition of steel. This instrument can display 240 measurements at a time and can store up to 1,20,000 values. Another instrument called Resistivity Meter can now be used to measure the electrical resistance of a reinforced concrete structure. This resistance helps in assessing the degree of risk of corrosion of steel embedded in concrete. Checking cover-depth: Providing adequate cover to steel reinforcement embedded in concrete is one of the most important requirements of concrete structures to save steel from corrosion. Less than prescribed cover may lead to deterioration of steel with the passage of time. To find our whether sufficient cover depth has been kept or not without actually chipping off the concrete, cover Meters have now been developed. These can detect the cover depth up to 300 mm while the normal cover to reinforcement varies from 15mm to 50mm. Light in weight, weighing just 200gm and very much portable, these instruments study the variation in electromagnetic field due to presence of steel bars in concrete and work out the cover to reinforcement. Checking reinforcement bar spacing and size: Nothing can be said to be hidden and beyond reach with the development of instruments that can detect the size and even the spacing of reinforcement bars embedded in concrete. These instruments called Digital Depth Detectors can measure bar size to an accuracy of +1mm. In these instruments, a path measuring device having an integrated computer scan software is used which makes the embedded reinforcement bars visible. Profometers are the other instruments developed for this purpose. These instruments use magnetic or radar technique to detect the spacing and size of reinforcement bars. Use of an integrated cyber scan software by these instruments makes the reinforcement bars visible on the display screen. An area of 10 sq ft can be displayed in just one minute. Checking quality of concrete: No-uniformity of concrete, honeycombing in concrete, cavities and cracks in concrete are such defects which may lead to its deterioration due to poor strength, lesser density and ingress of moisture. If finishing is done properly, as most of the builders do, the concrete surface may look fine and smooth and these defects may not at all come to anybody’s notice. To check these defects in concrete, Ultrasonic Testers have been developed. These instruments send electro-acoustic pulses through the concrete pured in position. Total time taken by a pulse to travel from a transducer fixed micro-seconds. More is the time taken, denser and better is the concrete. Further, the strength and density of concrete can now be calculated from the graphs created by developing a relationship between the time taken by the pulse to travel and the strength and density of concrete. Ultrasonic testers are portable instruments and site checking can easily be performed. Checking location of cracks in concrete: While the Ultrasonic testers tell about the health, density and strength of concrete, exact location of cracks hidden below concrete surface and supposed to appear later in an alarming way can be detected by use of Echo Testers which again pass a wave through the concrete. As soon as the wave encounters a crack or void in concrete, it sends a signal to the monitor on which its movement is being watched. These instruments may be of great help in detecting hidden defects and consequent repair of concrete. Checking Adhesive strength of concrete and mortar: It has been seen many times that the plaster applied to the walls comes off with time. similarly the plastic coatings or paint finishes applied to the plastered or concrete surfaces come off. Such instruments have now been developed which can measure the adhesive strength of a coating or mortar applied upon a surface. These instruments are called Pull of Testers. These may prove highly useful in work as the adhesive strength of a coating can be measured by applying a sample coating and remedial measures can be taken if adhesive strength is found to be insufficient. Checking Permeability of concrete: Concrete is supposed to be an impervious material. That’s why it is used in damp-proof course also. In actual, the concrete poured may not be fully impervious due to a variety of reasons such as poor workmanship, loos of excessive time in pouring, use of lesser than designed quantity of cement and so on. If concrete is not impermeable, it will not remain durable and may succumb under severe weather conditions. To avoid such a mis-happening, it is important to check the permeability of concrete. Lesser the permeability, better is the quality of concrete. To check concrete permeability, instruments have now been developed called permeability Testers. These instruments apply a water column pressure upon concrete surface for a fixed period and then check the drop in water pressure. More is the drop, more is the permeability of concrete. Checking Asphalt content of concrete: Asphalt concrete is finding more common use in construction of pavements today. For the pavements to be durable and resistant to wear and tear, a correct asphalt mix is extremely important. To determine the quantity of asphalt in asphalt concrete, Asphalt content gauges can now be used. These gauges can indicate the aphalt content of concrete at the site of pavement, at the mixing plant site or in the laboratory to an accurate extent and within minutes.
With the advent of so many non-destructive and accurate testing instruments which are easy to operate, give quick results and are not expensive, it now becomes imperative that a structure must be evaluated with the help of these instruments before it is put to use(if newly constructed) or before dismantling (if outlived). Visual assessments and guess-work should now be minimised.
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