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CONCRETE: HARD FACTS. DURABLE STRUCTURES -Part I. Dr Sirion Robertson Concrete is fascinating stuff. The Romans used it at least 500 years before the time of Christ, and they were probably not the only ones or the earliest ones. Although concrete technology has advanced a little since those remote days, it is still used in modern building projects - from foundations for giant telescope mountings, to nuclear power stations, to sky-scrapers in Manhattan and Tokyo, to garage and patio floors, roads and railway sleepers, shore protectors against wave action, dam walls, and settings for the poles of washing lines and chicken runs. Concrete is also used very successfully in boat building. The weight of a well- made concrete boat compares favourably with that of a wooden boat of the same capacity. The fundamental chemistry is much the same in all cases, although the concrete itself may be modified in a variety of ways to adapt its properties to an enormous diversity of modern requirements. This is the first of a two-part article, written specially for SCIENCE IN AFRICA. In the first part we deal with important generalities about concrete, and in the second we will discuss some of the chemistry of concrete, and "technical" or specialised aspects, including reinforcement, and various additives for special purposes. Modern concrete, very much like its earliest form used by our remote ancestors, is a mixture of cement, or a cement-like substance, with sand, stones and water. The water is added to the dry components to initiate the chemical changes leading to hardening, after which the strength and durability of the material is comparable to some of the hardest rocks. But, unlike the ages-long geochemical processes involved in rock formation, concrete can be mixed in a few minutes, and will approach its final hardness within a few weeks - or even a few hours if certain chemical "accelerators" are added during the mixing stage. The "heart" of concrete is of course the cement - the substance that, with water, does the chemical work, and binds the sand and stones into an astonishingly strong, composite material. We'll revisit the subject of cement, and "cement-like substances" in Part Two of this article. The sand and stones are referred to as "aggregate": stones are the "coarse aggregate" and sand the "fine aggregate". The stones are usually between about 10 and 20mm in size. Though less important than the quantity of cement involved, the sizes and proportions of the aggregate components are amongst the factors that determine the final properties of the concrete. Both types of aggregate should include particles with widely-varying sizes. (The difference, incidentally, between "mortar" and concrete is that mortar has only fine aggregate. Nothing larger than about 5mm particle size.) An interesting variety of concrete is sometimes referred to as "cyclopean concrete", which is made by adding massive rocks to ordinary concrete. The rocks form a sort of "super coarse aggregate". This is generally used for the walls of large dams and other massive structures where enormous volumes of concrete are required. Proportions and strengths In determining the proportions of sand and stone, the main considerations are physical rather than chemical. The spaces between the stones (often refered to as the "void volume") should be completely filled by the volume of sand. And the small spaces between the sand grains in turn should be filled by the very much smaller particles of the cement. Because the cement, when mixed with water, undergoes the chemical process of changing into a rock-hard, rigid substance, the amounts of cement and water present in the mixture are the main determinants of final strength. The cement, sand and added water make up a "paste", the volume of which should be slightly more than the void volume of the coarse aggregate. Typically, the volume of coarse aggregate (including its void volume) represents about 70-80% of the volume of concrete finally produced. (One of my less intelligent mistakes, in early work with concrete, was to assume, unthinkingly, that the final volume of concrete would be roughly equal to the sum of coarse and fine aggregates. This resulted in major under-production, and prompted me to approach the subject a little more rationally.) Strictly, the proportion of cement in a mix should be specified by mass. This is because the actual quantity is more reliably specified by mass than by volume. The same is true of sand and stone, but variations in the quantities of these components are less important, and it's more convenient and more common to measure the sand and stone by volume. (In terms of the principle involved, the argument about mass, as the better measure of quantity, applies also to the water, because of temperature-related changes of volume. But the effect here is far too small to have any practical consequence, and quantities of water, like quantities of aggregate, are conventionally specified in litres, or in any other convenient unit of volume.) For ordinary garden work with concrete, and other trivial building projects, where the final strength need not be accurately known and isn't particularly important, the quanties of all components are usually measured by volume - commonly 1:2:3 or 1:3:3. For more important projects the proportions are expressed on a mass:volume:volume basis: kg cement to volume of sand to volume of stone. For "general purpose concrete", local cement manufacturers recommend 100kg of cement (two standard bags) to three-and-a-half wheelbarrows of sand and three-and-a-half wheelbarrows of stone. (A standard builder's wheelbarrow has a capacity of about 40 litres.) Translating this into an all-volume ratio, using the fact that cement is about 1.4 times heavier than water, it works out to about 1:2:2. Another common way of expressing the composition of concrete, which closely reflects its potential strength, is to specify the mass of cement in the final volume of concrete. This may vary between about 200 kg and 550kg per cubic metre. (The mix we've just mentioned is about 285kg/cubic metre.) Cement is the most expensive component of concrete, so there is often a tendency to economise by reducing the cement to a very small proportion. In important building projects the proportions of the components are very carefully specified and controlled, and samples of the hardened concrete are tested to ensure that its stength is appropriate to the job. In addition to the proportions of cement to aggregate, there is a close and important relationship between the amount of water used in the mix, and the final strength of the concrete. A sloppy, runny mix produces weak concrete. Usually the volume of water is about the same as the volume of cement in the mix. The problem with making too dry a mix is that it isn't easy to work with, and is likely to have cavities in it when hardened. This of course will greatly reduce its strength. The "runniness" or stiffness of the newly-mixed concrete (the best word here is plasticity) is expressed by a "slump" value. A cone-shaped, metal container of standard dimensions, open at both ends, is stood base-down on a solid surface and carefully packed with a sample of the freshly-mixed concrete. The container is then lifted off the conical mound of wet concrete. The more runny it is, the more will it collapse into a low-profile "glob". The slump is found by measuring the loss of height of the conical pile of concrete after it has "sagged". Normal slump values are about 7 to 9 cm. The height of the cone itself is about 30 cm. When "throwing" or "placing" concrete - that is, when introducing it into foundation trenches or moulds for setting - it's important to ensure that it occupies all the volume intended for it. "Voids" - the term given to gaps or cavities in the concrete - can seriously impair the finished structure. The concrete must be tamped or rammed down into its bed with a spade or rod. On large projects mechanical vibrators of various sorts are used to help consolidate the wet, newly-thrown concrete and eliminate voids. "Curing" As concrete sets, the water in the mixture enters into a chemical reaction with the cement, and new chemical substances are formed. Although the concrete "dries", in the sense that no liquid water remains, the water is still there, as a very important part of its structure. This is different from the drying of mud, for example, in which the water simply evaporates and leaves the remaining solid material, without having changed it chemically. For this reason we talk of "curing" (or "setting") of concrete rather than "drying". In fact it's very important to keep concrete wet during the early stages of curing. This is normally done by spraying it regularly for the first week or so, and keeping it covered with sacks, leaves or any other convenient materials. The chemical process involved in curing is called "hydration" (which simply means combining with water), and keeping plenty of water on the surface ensures that the concrete doesn't lose water by evaporation. If water is lost from the wet concrete in this way there may be insufficient to allow the hydration process to go to completion, and the result will be reduced final strength. Another, related reason for keeping it wet while curing is that there is always some contraction of the concrete volume as it cures. This can lead to small cracks forming. Contraction, and crack-formation, are minimised if plenty of surface water is present. (Because it is only the cement and water that enter into chemical reaction, mixes with high concentrations of cement tend to contract more than do weaker mixtures.) In addition to drying by evaporation to the air, wet concrete may lose water to its surroundings - such as dry ground, when poured into trench foundations. To guard against this, the earth, and any other porous structures that will come into contact with the concrete should be thoroughly sprayed with water before the concrete is placed. While setting, the concrete gains hardness and srength, as the process of hydration slowly permeates the entire body of material, and new chemical bonds extend their fingers throughout the structure. Curing should be allowed to progress for several days before subjecting the new concrete to significant stress. The rate of curing depends on the temperature (as the rates of all chemical reactions are dependent on temperature), and for this reason the "safe curing time" is less in hot weather than in cold, and generally less in tropical climates than in higher latitudes. In the same way that curing concrete should be prevented from drying, it should also be protected from extreme cold. If you have an option, it's better to work with concrete in warm, humid weather than in cold, windy, dry weather. Trying to get advice from experts on safe curing times for concrete can be frustrating. An authoritative (British) textbook on building science says that trench foundations, for example, should be left for a week before wall-building begins. Having read this, I asked the Rhodes University Architect whether he agreed with it. Perhaps not a week, he said, but a minimum of three days, certainly. A little later I asked a local building contractor whether he left trench foundations for three days before building on them. Perhaps not as long as that, he answered, but 24 hours, certainly. You must leave them for 24 hours. But in the event it didn't happen like that. I subsequently gave him a small building contract: I wanted my garage extended. His men finished throwing a trench foundation for the new wall at 5 one afternoon. At 7 the next morning - 14 hours later - they arrived to start building. They weren't especially pleased when I told them to go away and come back at a later date, but in view of what the contractor himself had told me I felt justified. So views on acceptable curing times vary quite widely - and actual building practises vary more widely still. The problem, of course, is that leaving concrete for extended curing times - however desirable it may be - can involve expensive delays. "Time is money". In the same way that builders - amateur or professional - may be tempted to use too little cement in their concrete mixes, so too they may be tempted to allow insufficient time for adequate curing of new concrete. In some cases, where speed is essential, but standards of strength cannot be compromised, chemical additives are used to accelerate the process of curing. We'll say more about them in the second part of this article. Where practical (such as in the manufacture of concrete bricks) the curing may be accelerated simply by raising the temperature. An interesting rate-temperature relationship, well-known to biologists, but with far wider applications than biology alone, is the so-called "Q10" value. This is the proportion by which the rate of a chemical reaction is raised by an increase in temperature of 10 degrees on the Celsius scale. In many cases - including the rates of enzyme-catalysed reactions in living cells - the Qten value is very close to 2. In other words, the rate doubles for every 10 degrees increase in temperature. So, applying this to concrete, if adequate curing is achieved in six days at 20 degrees, you could reduce this to three days if the temperature were kept at 30 degrees. (Remembering, in relation to this, that the curing time recommended in Britain is about a week for trench foundation concrete, this might very well be equivalent to about three days in the warmer South African conditions.) In the concrete industry, fully-cured blocks are produced at a very high rate by subjecting them to temperatures of about 200 degrees Celsius, in a water-saturated environment to prevent drying. The process is exactly the same as pressure-cooking food, and in these conditions adequate curing is achieved in a few hours. As we've said, the chemically active component of concrete is the cement, and this is where the strength comes from. The word "cement" refers, in its widest sense, to anything that holds materials together. Glues are "cements". The particular type of cement used in concrete, and nearly all building work, is called "hydraulic cement" because water is required for the chemical reactions that lead to hardening. There are several types of hydraulic cement, but the most important and commonly-used is "portland cement", so-called because its colour, when set, resembles that of a type of stone found on the Isle of Portland, in the English Channel. Portland cement is made by crushing, heating, and crushing again a mixture of rocks and soil- like substances, the main ones being types of limestone, chalk and clay. These are complex substances, in the sense that they contain several chemical elements in various combinations. The most important actual chemicals are calcium, silica, aluminium, iron and oxygen. The purpose of the first crushing and grinding is to bring the chemicals into sufficiently close proximity to enable them to react with each other. Heating, to about 1 500 degrees C, provides the necessary energy for these reactions to occur, and new compounds are formed. The final grinding again produces an extremely fine powder and close proximity of potentially reactive substances. When water is added, new compounds are formed, some contraction in volume occurs, heat is given off, and the individual cement particles fuse into a continuous "matrix" which locks the sand and stone components into a hard, rigid mass. Although modern cements are made in factories under carefully controlled conditions, there are also so-called "natural cements" in many areas of the world. It was these "natural cements" that our forebears used in Rome, Greece and elsewhere. The most common are mixtures of limestone and clays. They are prepared by burning and then crushing into powdered form. Attaining strength - and measuring it The graph to the right shows increasing strength of a sample of concrete as a function of curing time. Notice that strengthening is quite rapid at first: the strength after one week is more than half that attained at the end of four weeks. Also, although the graph doesn't continue beyond 28 days, the shape of the curve makes it quite clear that strength continues to increase well beyond a month. Indeed tests show that, under favourable conditions, concrete is still "maturing" after 18 months! ("Favourable" here means under warm and humid conditions.) The strength unit on the graph is MegaPascals. The Pascal is the basic unit of pressure, and pressure is defined as force per unit area. The details on the graph were obtained empirically - in other words, from experiments. In this case the experiment involves using a hydraulic press to crush specimen cubes of the Concrete, and measuring the pressure at which the cube breaks. The test we've shown in the graph is for "compressive strength". Another sort of test is for "tensile strength". You can best visualise this by imagining a vertical rod of concrete, with a mass suspended from it. Here the force tends to pull the substance apart, rather than crushing the components closer together. As we increase the hanging mass, the rod will eventually break, and the force that finally breaks it will be a measure of the material's tensile strength. The main "Achilles heel" of concrete is its relatively low tensile strength: only about one tenth of its compressive strength. This has important implications for concrete as a mass-bearing structure. Any horizontal beam, supported at both ends, will tend to sag under its own weight. With additional loading it will eventually break. Notice the image to the right, that there are both compressive and tensile stresses acting in the beam. The tensile force acts along the lower part, where the beam tends to stretch. And if tensile strength is less than compressive strength, this is where it will eventually break. Because of their low tensile strength, concrete beams and slabs aren't good at carrying heavy loads unless they're "helped". Reinforced concrete: Steel to the rescue Incorporating steel into concrete produces a composite building material sometimes called "ferrocrete". (It is this material, or the related "ferrocement" that is used for boat-building.) In spite of the fact that concrete has been used since the earliest civilisations, ferrocrete dates from as recently as the mid-nineteenth century. With the great increase in strength that it gives, its effect on architectural styles and potentialities has been enormous. Steel is the ideal material to embed in concrete as a strengthening agent, mainly for three reasons. Firstly, steel has extremely high tensile strength; secondly, by a fortunate coincidence, the extent to which steel and concrete change their dimensions in response to temperature change (that is, their coefficients of expansion) are very similar. Thus even with very great temperature changes, the concrete and the embedded steel more or less keep pace with each other in their responses, and no severe stresses are set up by one material "trying" to change its length more than the other. The third thing that makes steel so appropriate is that - perhaps surprisingly - its elasticity is less than that of concrete. When a concrete beam or slab bends (See the image above), the material on the outer part of the curve stretches. Due to its elasticity, a small amount of stretching can safely occur without breaking the concrete, and it is this very small amount of bending that puts the steel rods under tension - the concrete "tries" to stretch them. The fact that steel has a very much smaller tendency to stretch, under a given force, allows it to come to the rescue and prevent any further deformation of the concrete member. (The tendency of concrete to contract slightly while curing is in one way an advantage. This "shrinkage" causes the concrete to grip very tightly onto the steel. As a precaution, to ensure that the concrete-steel interface doesn't break down under load, the steel members are usually bent in various ways to give even more effective bonding.) From the foregoing you can see that it's extremely important where, in the concrete beam or slab, the steel reinforcing rods are placed. To put the steel near the top of the beam in Figure 2 would be a complete waste. The resulting structure would be no stronger than competely unreinforced concrete. Where would you place the reinforcing rods in the beam shown in the image to the left. The amount, type and positioning of steel in reinforced concrete is calculated very carefully by structural engineers, taking into account the relative tensile strengths and elasticities of the two materials, so that the resulting structure will carry any loads that might be placed on it - with a good safety margin. The size of the "safety margin" built into the design depends on what the engineers see as a "worst case scenario". The main weakness of steel, as a structural material, is its tendency to corrode. Concrete, on the other hand, is highly resistant to corrosion, and therefore makes an excellent shield for the embedded steel. It's important that no steel members come closer than about 5cm from the surface of the concrete, and that the concrete itself is of low permeability. If cracks or incorrect building procedures do allow the reinforcing steel to corrode, it slowly expands and may eventually cause parts of the surrounding concrete surface layers to break off. The condition is known as "spalling". It's clearly a progressive and potentially dangerous situation, and requires careful attention. A Final Miscellany: Accelerating; Retarding; Air entrainment; Colouring. We've covered what are certainly the most important and perhaps the most interesting aspects of concrete technology. Here are some closing remarks on other aspects of this extraordinarily versatile medium. Accelerating of the curing process is achieved either by raising the temperature, as we've discussed, or by adding substances during the mixing. Calcium chloride or sodium chloride are the most commonly-used accelerators. It's inappropriate for large quantities which must be processed at a single session, because hardening shouldn't begin while work is still in progress. The final strength of rapidly-cured concrete is usually a little less than equivalent mixes which haven't been accelerated. Retarding is achieved by adding calcium sulphate, sodium bicarbonate, or various other organic compounds. Air-entrainment is important because this type of concrete, in addition to being somewhat lighter, is considerably more resistant to damage by very low temperatures. Regular freezing and thawing of concrete can lead to severe deterioration. The air-entrained variety is also a very much better insulator against temperature change. Air entrainment is achieved by mixing in, with the cement, a compound that react with the cement and water to produce small bubbles. Animal fats, or various fatty acids are amongst the substances used to cause bubble formation. Colouring is done by mixing one or more of several coloured oxides into the newly-mixed concrete. This adds quite significantly to the cost of the project, and where appropriate it is more common to "float" the pigment onto the surface of the wet concrete immediately after placing, so that only the top few millimetres are pigmented. The colouring is more effective if the concrete is made with a white or nearly-white variety of cement instead of the ordinary grey stuff. This too adds substantially to the cost. Concluding remarks Concrete remains one of the most commonly-used, durable and versatile structural materials: a direct descendant of the stone age, that has kept pace with our most modern and pressing needs. There is good reason to believe that if life exists elsewhere in the cosmos, it is based on carbon compounds. For similarly fundamental reasons we may assume that advanced life- forms elsewhere would have discovered, and used, cement-like substances - including concrete. As we said: fascinating stuff.
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