This book … can be considered as a basic tool to achieve such a goal. It has been conceived and written by three great engineers who have great experience. It presents very clearly what to do, how to do it and when to do it for better concrete practice. The trend towards concrete performance based specifications dealt with in a book perhaps for the first time Mr Day has educated hundreds of concrete quality professionals worldwide The very broad scope for the book and knowledgeable authors makes this book a must buy.
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For both formats the functionality available will depend on how you access the ebook via Bookshelf Online in your browser or via the Bookshelf app on your PC or mobile device. Stay on CRCPress. Exclusive web offer for individuals on all book. Preview this Book. Add to Wish List. Close Preview. Toggle navigation Additional Book Information. Summary The nature of concrete is rapidly changing, and with it, there are rising concerns.
Author s Bio Ken W. Reviews "It is imperative to promote the use of durable and sustainable concretes having adequate and sensitive specifications in order to avoid any waste of materials. Request an e-inspection copy. Share this Title. Recommend to Librarian.
Shopping Cart Summary. Items Subtotal. View Cart. The objective of this book is to provide such knowledge of a small, but intensively used, cor- ner of concrete technology. In so doing, the reader will be made aware of other problems to which more detailed answers should be sought else-where.
It is important to understand the place of computers in the fields of mix design and quality control. Very little knowledge about computers is necessary in order to use them to great effect in these fields and they are so powerful and cost-efficient that they are virtually indispensable.
A computer must not be thought of as an infallible font of all knowledge, rather it should be seen as a microscope which will reveal what is other- wise unseen and inexplicable. It is a data processing unit which can accept vast quantities of information and carry out a very complicated analysis of it in a few seconds. One message of this book is that we are rapidly approaching the point at which complete automation is techni-cally possible, but it must be realized that computers work on the GIG0 principle garbage in, garbage out whether that garbage is inaccurate test data or an unrealistic program.
The pace of technical progress heaps additional pressure and respon-sibility on concrete technologists, but it also provides the means to react swiftly and precisely to change. No longer need filing cabinets be searched for data, yield calculations and statistical analysis done and combined grading curves painfully constructed. The already expert technologist is not disenfranchised but is assisted to work a hundred times more quickly with all the hack work done automatically. The raw amateur is helped and advised not only to a solution but also to an understanding of the situation which may otherwise take years and many costly mistakes to acquire.
Some will object that this book is impractical because its recommendations cannot be implemented within the existing structure of codes of practice and national specifications in their country. There are two replies to this. One is that the book describes how things can be done better and how they are likely to be done in the next decade. The other is that there is little in the book which has not already been done by the author.
Of course it requires co-operation, but if the controller and the controlled both want effective and equitable control then it is possible to implement it particularly on very large projects whilst still paying the necessary lip service to official requirements. Layout and scope It is again emphasized that this book is not intended to be a first or only source of knowledge on concrete technology, nor does it attempt a coverage of all aspects. The author therefore makes no apology for the omission of material which is both non-contentious and readily available in any book which does attempt a comprehensive coverage.
Thus, the book does not describe the production of concrete materials, well- established test methods, or the chemistry of cement and admixtures. The principal matters addressed are in the first six chapters with supplementary material in the remaining chapters. The material in the later chapters is aimed more at providing context and explanation for the views and techniques in the earlier chapters than at providing comprehensive information on the subject of the chapter. This is done partly to avoid any unjustified appearance of originality and partly to show the limitations of other approaches.
Concrete has to be fully understood before it can be effectively controlled. Too many specifications have been written without an understanding of the material and its production. Acknowledgements There are three individuals without whom this book could not have happened and four more without whom it may have been very different. The first group comprises: O. The second group comprises: John Fowler, who wrote the first computer program using my mix design methods, at a time when I had a firm opinion that mix design was partly an art and could never be computerized; D.
A third kind of indebtedness is to those who assisted in the actual production of the book. They have become too numerous to list all of them by name but Hasan Ay and Andrew Travers are especially thanked for their work on figures and tables and, for the second edition, Matt Norman.
A new kind of indebtedness is to those individuals in my major client companies who have not only enabled my company Concrete Advice Pty Ltd to survive and prosper but have also contributed in no small measure to improvements in the system. The Conad computer program has come a long way since the first edition and thanks are due to my staff at Concrete Advice Pty Ltd.
Michael Shallard and Lloyd Smiley wrote the latest program and Andrew Travers, now Manager of the company, knows how to use it better than I. Foreword by John J. Peyton, Director, Connell Wagner Rankin Hill, Consulting Engineers The writer is a consulting engineer based in Melbourne, Australia, and in charge for the last 20 years of the structural work of the office of Connell Wagner, the largest structural office in Australia.
In the s and s, and continuing to the present day, we have been involved in some very large and prestigious building structures, most of which have been in concrete and many of which have pushed back the previous barriers in terms of concrete strength, durability, appearance, and general quality requirement. Outstanding early examples were the Arts Centre and Concert Hall. Collins Place was another landmark project of the early s comprising twin storey towers.
It was the first use of p. In the s and s, 50 to storey buildings have become a commonplace. The increased tempo of construction has caused us to accept such techniques as insitu temperature monitoring to enable earlier prestressing. Gone are the days when conservative nominal stripping and stressing ages could be specified. Consulting engineers in general have considerable difficulty dealing with the vagaries of concrete test results. Excessive severity in dealing with marginal results is a waste of resources, yet any leniency results in continued infringements.
What should be done with the odd set of low results? What if the concrete is already inaccessible, or has a storey or more of subsequent construction on top of it? How can a recurrence be prevented? How can it be explained to the client or the Building Authority? Dare we really use high strength concrete?
In many cases recalculation of the force on the particular offending element permits its acceptance at a lower concrete strength. In other cases the element is jack hammered at con-siderable cost and inconvenience, or perhaps load tested. This may be considered necessary to maintain control, even when it is obvious that the shortfall is insufficient to cause a structural problem. And how do we know that the trucks of concrete which were not tested are any better than those we did?
It is possible to cause endless disruption and more problems by non-destructive testing. The fact is that consulting engineers in general are structural designers rather than concrete technologists.
The more one knows about concrete, the easier it is to admit the truth of this. Only those who have stayed within conservative limits and con-fined themselves to routine work can sustain a belief that they actually know all about concrete. It is a false economy, and a disservice to the client, to soldier on with the same old specifications and limits, ignoring new technology. This brings me to Ken Day and the almost magical disappearance of these problems when he appeared on the scene in the late s.
I referred to him at one stage as our favourite form of cash penalty whenever problems were encoun-tered. Better still, we found that the problems simply did not occur when we required contractors to engage his services in the first place. At last the formality of prior mix approval acquired real meaning. This enabled him to develop his mix evaluation, result analysis and reporting services to a high order. I was particularly impressed by the early stage at which any slight shortfall was detected and resolved before any actual low results were experienced.
His services freed us from concerns as we pushed strengths higher and achieved greater economy when able to rely on full attainment of our design intentions. I am pleased and proud to see that Ken believes I had a hand in the origination and recognition of these skills. I recommend the book as required reading to any person charged with the specification and responsibility for concrete quality, and as an in-depth study to concrete technologists who aspire to assist them effectively. However let us all hope that Ken succeeds in his mis-sion to educate the concrete producers of the world so effectively that there are no longer any problems to overcome.
Wuerpel, taught a course in concrete mix design. I failed to learn to do it—too much arithmetic. This was in Ken Day understands this. I have also argued that one can design structures including pavements for whatever loading is relevant and then select a design concrete strength for that loading compressive, flexural, or whatever and pro-portion a mixture to achieve it; but having done so, it is dumb not to use compressive strength for routine control testing; Ken says this.
He knows that hot weather does not directly increase water demand, he just implies it like everybody else. Dewar J D Dewar Consultancy Former Director, British Ready- Mixed Concrete Association Concrete is moving fast from the stage of an art to that of a science; rule of thumb is being replaced by a blend of theory and experience and the development of expert systems aided by the computer.
While it has been said that almost anyone can design and control concrete because it is such an accommodating material, it still takes an expert to do it economically and consistently. To teach others to do so requires an even rarer brand of expertise. Ken Day has demonstrated his ability to be a leader in this field. Ken is one of the few world citizens working in concrete with his experience drawn from five continents. He can write easily about cubes or about cylinders, about working in the tropics or in temperate climes, operating in advanced cities or in a wilderness.
If you are an expert concrete technologist, there is a wealth of information on well-tried and new systems which you can adopt or adapt as seems right to you. If, on the other hand, you are a novice, this is also the book for you because Ken Day has many things to teach us and he does so with a relaxed style that makes reading this book a pleasure.
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More than this, he is not a mere theoretician suggesting a new approach which might work. You do not need to agree with everything Ken writes or to do all that he proposes. His idea is to make you think before you draw conclusions or make decisions on the situations you face. There are a very few ideas with which I would disagree, specifically: use of specific surface or surface area index as a basic design parameter; and cash penalty specifications.
These are a heaven sent opportunity for the less scrupulous to capitalize from poor testing. The provision of positive incentives is a far more attractive proposition than negative penalties. He identifies that most concepts have their limitations. I hold the view that each day should present a new challenge, a different viewpoint, a change of mind, or an addition to knowledge. Ken obviously shares these views. When Ken stops producing new ideas it will be because either he or the use of concrete has come to an end.
I look forward to the next edition, and the next…. In both cases the question is whether any of them really works. In the case of concrete mix design there is certainly substantial evidence to the contrary. Nearly all systems end by suggesting eye adjustment of a trial mix. Most commercial concrete results from the continued ad hoc modification of existing mixes without any application of formal mix design. If the purpose of a mix design system is to enable ideal materials to be proportioned so as to produce good general purpose concrete of the desired strength then it will have very limited value.
To be of real value a system must be able to guide the selection of available materials of whatever quality and proportion them so as to produce the most economical concrete which is suitable for the desired purpose. It is not particularly essential that the first mix produced has exactly the desired strength although it may be essential that it exceeds this strength since it is easy to subsequently adjust cement content.
The first essential is that the most advantageous selection of aggregates be made and the second is that the concrete shall have the desired properties in the fresh state. We are accustomed to categorizing concrete by strength and slump but a further description is necessary. What is really needed is a numerical value covering this property, which is essentially the relative sandiness or cohesion of the mix.
It may be that mix design itself is not the most important problem. A typical large premix concrete producer will have hundreds, possibly thousands, of mixes available. What is needed is a system of mix maintenance to enable all these mixes to be kept tuned to satisfy specifications at the lowest cost as material properties, weather conditions and client requirements vary, and even as it becomes necessary or advantageous to substitute different materials.
It may help the reader to start with the old-fashioned idea that concrete consists of cement, coarse aggregate, fine aggregate and water. Historically the problem of mix design has been seen if as other than using nominal proportions as to select suitable aggregates and determine their optimum relative proportions and the cement requirement to produce a given strength at a given slump.
Early investigators tended to be concerned with how to define and produce ideal concrete. Frequently this meant trying to determine the ideal combined grading of the coarse and fine aggregate and therefore how these materials should be specified and in what proportions they should be combined. Today, our consideration should be: firstly, what aggregates are economically available; secondly, what properties should the concrete have; and thirdly, what is the most economical way of providing these required properties.
Spelling this out more clearly: 1. Use available aggregates rather than searching for ideal aggregates. Recognize that there is no concrete ideal for all purposes, but rather define what is required for a particular purpose. Understand that there will be competition based on price.
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In this new edition, it must be pointed out that grading is again receiving attention, especially for high strength concrete. However, now it is the grading of the fines in the mix, including the cement, which is seen to be important. The use of slump as a criterion requires comment. Slump certainly does not accurately assess the relative workability of two different mixes.
However, it seems likely to survive because it is a good way of checking the relative water content of two nominally identical mixes, as in the case of successive deliveries of the same mix. Actually, Feret in France in Neville, preceded him and proposed a more accurate proportionality, that between strength and the ratio of cement to water plus voids. A primitive way of designing a mix, assuming that only one fine and one coarse aggregate are involved, would be to make a mix of any reasonable proportions say and fairly high slump say mm.
If a sample of this concrete were heavily vibrated for several say, 15 minutes in a sturdy container such as a bucket, not as small as a cylinder mould then any excess of either coarse aggregate or mortar would be left on top. If the top half were discarded, then the proportions of the bottom half would be a reasonable guide to the desirable sand percentage to use. This is a useful exercise for students since it illustrates the concept of filling the voids in the coarse aggregate with mortar and demonstrates that an ideal mix cannot be over- vibrated once it is fully compacted in place in that the remaining concrete will not further segregate however long it is vibrated.
Chemical attack Readers should look elsewhere Neville, ; Biczok, ; ACI SP47, for detailed information on attack by a range of aggressive substances and for details of the mechanisms of chemical attack. Here we shall consider attack by sulphates, chlorides and seawater which combines the two and deterioration by alkali-silica reaction.
The most readily attacked components of hydrated cement paste are those resulting from tricalcium aluminate C3A and the calcium hydroxide which is liberated in substantial quantities during hydration. Sulphate resisting or low heat cements may be expensive and inconvenient to use as they require an additional silo and it should be noted that concrete made with them is actually less resistant to chloride penetration. It has been shown Kalousek et al.
Fly ash produces its beneficial effects by combining with the calcium hydroxide, converting it to more durable calcium silicates, and by reducing permeability through denser packing and reduced water requirement. Alkali-silica reaction is a disruptive expansion of the cement matrix arising from the combination of alkalis usually, but not necessarily solely, from the cement and reactive silica usually in the coarse aggregate.
While relatively rare, the phenomenon can be totally disastrous when it does occur. There are three possible strategies to limit its occurrence. One is to avoid total alkalis sodium and potassium in the cement exceeding 0. Another is to test the aggregate for reactivity. A third possibility is to provide an excess of reactive silica in the form of fly ash, silica fume, or natural pozzolan so as to consume any alkali present in a non-expansive surface reaction product.
Gross voids arising from incomplete compaction, often resulting from segregation. Micro or macro cracks resulting from drying shrinkage, thermal stresses or bleeding settlement. Pores or capillaries resulting from mixing water in excess of that which can combine with the cement, i. Gross voids may be regarded as too obvious a cause to be included. However, they are worth mentioning because they may be made more likely by action which would otherwise reduce porosity, i. Obviously a low permeability concrete must be such that it will be fully compacted by the means available.
It must not depend on unrealistic expectations of workmanship. Thermal stresses are the result of heat generated during hydration, which is an exothermic reaction section 1. To the extent to which the voids left by the excess water are discontinuous, they will not provide easy passage for water. The latest packing theories of mix design have demonstrated that close attention to the packing of fine material of cement size and smaller can reduce total void space in the paste fraction, especially when accompanied by superplasticizers.
The total amount of pore space is not the only factor determining permeability. Another important factor is the distribution of the pores and their discontinuity. Bleeding is a source of continuous or semi- continuous pores. Bleeding is initiated by the settlement of cement particles in the surrounding mixing water, after compaction in place. This tends to leave minute pockets of water under fine aggregate grains. There may be enough water to allow the fine aggregate grains to settle slightly and the water to escape around them and rise up through the concrete.
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The process occurs on a larger scale under the coarse aggregate particles and eventually the whole mass of the concrete settles slightly, leaving a film of water on the surface. The process can happen very gently without having a great effect on the concrete properties. If bleeding is severe, the rising water tends to leave well defined capillary passages and it is then known as channel bleeding. Water penetration of the hardened concrete is obviously greatly facilitated by both the vertical channels and the voids formed under the coarse aggregate and even fine aggregate particles.
Pore blocking after they have formed takes place as cement continues to hydrate and extends gel formation into the pores. Another means is to line the pores in the concrete with hydrophobic material. Hydrophobic material may provide a temporary benefit but lose its effectiveness in the longer term.
Factors affecting bleeding are: 1. Amount of fine material including cement, slag, fly ash, silica fume and natural pozzolans. Air entrainment. Water reduction through admixtures or lower slump. Continuity of grading especially including fine aggregate grading. The use of methyl cellulose or other gel-forming admixtures mainly in grouts. Retardation, whether due to low temperature or chemical retarders, delays gel formation and so extends the period of bleeding.
Table 1. The better the particles pack together, the more difficult it will be for water to pass through the mass. Cement, slag, fly ash, entrained air, rice hull ash and silica fume in increasing order of effectiveness are good inhibitors of bleeding. Silica fume is the most effective inhibitor of bleeding. It is many times finer than cement and particles of it fill the interstices between the cement particles. It should be noted that the effectiveness of the fume is greatly reduced if it is incompletely dispersed.
Essentially, this means that silica fume should always be used in conjunction with a superplasticizing admixture and given adequate mixing time. It should be noted that eliminating or greatly reducing bleeding can create problems with evaporation cracking. Such concrete may require careful attention to preventative measures such as the use of aliphatic alcohol evaporation retardant or polythene sheeting, mist sprays etc. A blockage pumping failure occurs when the mix segregates under pressure.
It can also result in water being squeezed out of the concrete, either to move further along the pipe and out through a leaking joint or into incompletely saturated coarse aggregate. A particular risk arises when pumping is not continuous. Concrete may pump quite readily when fresh, but if it bleeds during any small delay, restarting may prove impossible. Segregation of mortar occurs at a gap in the coarse aggregate grading or, more frequently, between the coarse and fine aggregate gradings. Bleeding of water or segregation of cement paste can occur at a gap in the fine aggregate grading.
Pumpability is best assured if all sieve sizes with the exception of the largest, of which there will be more are present in approximately equal quantity. The inclusion of silica fume and a high-solids superplasticizer were considered to be essential to achieve this. Cracking results when shrinkage is restrained and differential shrinkage can give rise to such problems as curling of slabs and sloping floors in multistorey buildings differential shrinkage of central core and peripheral columns. Shrinkage also causes stress losses in prestressed members and failure of joints which have not been designed to cope with the amount of shrinkage experienced.
Shrinkage generally means drying shrinkage although further shrinkage takes place during carbonation and is caused by the contraction of hardened cement paste when it loses water. The contraction is resisted by the aggregates in the concrete, especially the coarse aggregate. It is therefore to be expected that shrinkage will be largely dependent on the total amount of water in the original mix and on the elastic modulus of the coarse aggregate and its proportion, although the latter two are relatively minor effects except in the case of lightweight aggregates.
Many coarse aggregates are subject to moisture movement which is obviously directly passed on to the concrete. With some aggregates this effect can be as large as that due to the use of an oversanded pump mix. Another factor which influences shrinkage is the gypsum content of the cement. Gypsum is calcium sulphate and the SO3 content of cement is limited by most national codes, to avoid the risk of excessive expansion. Not all the SO3 is in an active form, so these limits may be below the optimum level for shrinkage reduction and admixtures are available to supplement it.
At least in the USA, shrinkage compensating or even prestress producing cements are available, having such a material interground with normal cement. Rather they produce an expansion which continues so long as the concrete is kept wet and the normal drying shrinkage then occurs when it is permitted to dry. The mechanism is for the expansion to be resisted by reinforcing steel so as to produce a precompression in the concrete. Subsequent shrinkage then merely dissipates this compression without producing tensile stresses.
There is some tendency for a threshold effect in which inadequate doses of shrinkage compensator, or inadequate curing, produce an expansion tendency that is entirely dissipated in creep and has little or no effect on the final situation. Such shrinkage occurs much more quickly than normal drying shrinkage and cannot be prevented by measures such as curing compounds or polythene sheeting.
The only effective recourse is actual water curing since the concrete needs to take up additional water. Sometimes very high costs have been incurred by using specially produced flake ice in or instead of mixing water, by casting cooling coils in which brine is circulated into the concrete, or by injecting liquid nitrogen into the mixing truck as a means of lowering the concrete temperature.
It should be noted that normal crushed ice is not suitable for direct addition to concrete. This is because it can take some time to melt and so make slump control extremely difficult. In the extreme, all the ice may not have melted prior to final compaction into place of the concrete. This has been known to produce low strengths in test cylinders. It is certainly true that the higher the supply temperature of concrete, the more rapidly it will generate heat. The use of cooled concrete is therefore beneficial in spreading the heat generation period, allowing more time for heat to escape and therefore reducing peak temperatures.
It also reduces the necessary water content and therefore the necessary cement content. In tropical climates it is generally worthwhile to take all available inexpensive measures to produce concrete at a reduced temperature. These may include shading aggregate stockpiles and perhaps sprinkling them with water.
For a permanent installation it probably also includes evaporative cooling of the mixing water. However, whether the expense of refrigerated cooling, or the addition of ice or liquid nitrogen, is really justified is more doubtful and depends on the circumstances and alternatives. It is generally more economical firstly to reduce heat generation to a minimum and secondly to insulate the outside of the mass so that differential temperatures are reduced by allowing the whole mass to heat up together.
Essentially heat generation can only be reduced by reducing the amounts of C3A tricalcium aluminate and C3S tricalcium silicate present. Some amelioration is possible at a given strength by reducing water content by either mix design or admixtures and so reducing the amount of cement necessary.
Silica fume has been used in this way since, although it generates as much heat per kilogram as cement, and generates it just as quickly, it replaces approximately three times its own mass of cement. Low heat cement produces less heat than OPC and also spreads its heat generation over a longer period, so allowing it to dissipate better. It should be noted that, while low heat cement may be sulphate resisting, sulphate resisting cement is not necessarily low heat.
Both cements limit the amount of C3A, which is the worst generator of heat per unit mass, but there is no limitation of C3S in sulphate resisting cement and this in fact is the largest source of heat, being present in much larger proportion than the C3A. If fly ash is available, it may be both more economical and more effective to use fly ash to reduce cement content than to use low heat cement. This is especially the case since the ash will also reduce bleeding settlement and will directly reduce permeability. High early strength is rarely a requirement for mass concrete, and in any case the generated heat will accelerate strength gain.
However, fly ash unless type C, high calcium ash requires calcium hydroxide released by the cement to form cementitious compounds and cannot be used in as high a proportion as slag. Blast furnace slag is also an excellent way of slowing heat generation in normal conditions, but caution should be exercised when very large sections are involved. Although the rate of heat generation is reduced, the amount of heat generated may even increase.
If the heat cannot escape, a higher final temperature may be reached. It should be emphasized that adiabatic conditions are only approximated in very large masses of concrete, such as a raft foundation more than three metres thick. Whilst smaller masses of concrete may generate substantial heat, the heat may be able to escape quickly enough for the slower heat generation of slag blend cements to result in a substantially lower peak temperature than with OPC. The mix design process will deal with a target average strength.
The average strength selected must take into account: 1. The degree of variability anticipated. The degree of certainty of avoiding rejection required. The required durability. The author has found this assumption to be well justified in practice except that only about half the results theoretically expected to be below the mean minus 1.
Some quite experienced persons, including a number of ACI committees, believe that coefficient of variation, which is standard deviation divided by average strength, is a more appropriate measure of variability than standard deviation itself. There is certainly an increase in testing error at higher strengths, which adds to apparent variability.
ISBN 13: 9780419243304
However, having personally produced very high strength concrete at very low variability, the author is not in favour of coefficient of variation and believes that those who favour it are deluding themselves as to the degree of control achieved on their high strength concrete. The truth lies somewhere between constant standard deviation and constant coefficient of variation for high strength concrete and everyone is therefore entitled to their own choice. However, the author has routinely analysed, month by month, many thousands of test results from many different suppliers, on many different projects, and in several countries.
These results, from any one plant, almost invariably show very little difference in standard deviation in grades from, and including, 20 to 40 MPa. Therefore he may decide to add a safety margin of say 1 or 2 MPa or psi to avoid such problems. However, the cost of such an additional margin would reduce his competitiveness and some of the expenditure may be more usefully directed to reducing variability.
In the UK it is normal to use a target strength two standard deviations above the specified strength. This is all the more onerous since standard deviations of 4 to 6 MPa are apparently normal there, compared to 2 to 3 MPa for normal strength concrete in Australia. The cost of a safety margin may be unattractive to the producer, as being a large proportion of his profit margin. However, the cost of such a margin may be close to negligible compared to the total cost of the structure and the owner of the structure may be well advised to allow a margin by specifying a higher grade of concrete than strictly required section Amongst other advantages, this avoids any competitive disadvantage in the use of a high strength margin.
Therefore using a higher grade than really needed for day strength can show a double benefit in its effect on earlier prestressing, stripping or de-propping. If inadequate moist curing is anticipated, this may effectively mean that the concrete should be designed to develop its strength early since it will not continue to increase in strength once dry.
Similarly in cold countries, where concrete needs to be protected until it reaches a critical strength, it may be necessary to design for this criterion rather than day strength. It was pointed out in section 1. The use of entrained air or of special cementitious materials is not ensured by specification of a strength level and must be specified in addition to the strength. A strength of 30 or 32MPa say psi may be the minimum required to provide good durability of reinforced concrete in external structures.
In aggressive circumstances, this may need to be increased to 40 or 50 MPa to psi.
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The use of large aggregate concrete, except for special uses such as dams, is becoming rare. The British Standard BS nominates this ratio as 1. It is important to be clear about the relationship between this strength and tensile strength. The flexural test uses both a special test specimen and a special testing machine or a special fitting to a compression testing Table 1. There are differences between different national codes, but the test specimen is normally a beam of either mm or mm square cross-section and long enough to test on a span three times its depth.
The mm beam is suitable for maximum aggregate sizes up to 20 mm and the mm beam should be used for larger sizes of aggregate. The beams are usually required to be tested under either a central point load or third point loading Fig. In either case, failure is initiated in direct tension at the extreme bottom face in the test, and this is usually required to be a side face in casting so that both the top and bottom faces in testing will be smooth moulded faces to provide an even bearing for the loading rollers.
In centre point loading, the stress will be a maximum immediately below the central loading point. The occurrence of a defect at other locations along the beam may therefore not cause a reduced failure load. In third point loading, the bending moment, and therefore the bottom face stress, is essentially constant over the middle third of the span.
Failure is therefore likely to occur at the weakest point of the middle third. It is not obvious which is the better test. The third point loading puts a much larger amount of concrete under effective test. This may be considered fairer, and more likely to give a true result.
However, if the failure is seen as at all likely to be at a defect in the beam which is of a type not anticipated to occur in the actual slab or structure, then it may be better to get a point value strength which is less likely to be affected by such a defect. It will be pointed out later that any consideration of flexural strength must take into account the likely scatter of test results, i. The flexural stress at failure is calculated on the basis of an assumed triangular stress distribution. This is not what happens in practice Fig. The extreme fibres exhibit plastic flow as they near failure, and so distribute load to higher layers of concrete.
This increases the total load at failure and so gives an inflated value of tensile strength i.
Concrete Mix Design, Quality Control and Specification
It can be seen that the apparent strength could be influenced by the rate of loading. Failure in tension takes time to occur, if the load is not increased above the minimum needed to cause eventual failure. This would cause the beam to fail at a lower load if tested very slowly.
The strength level, the age, and the condition wet or dry at test also have a bearing on the ability of the concrete to exhibit plastic flow. Thus concrete of a higher strength grade, older concrete, and dry concrete, is more brittle than their converses. It has already been noted that flexural or tensile strength tests tend to be more affected by defects or imperfections than do compressive strengths.
The imperfections may arise in manufacture or in handling. Small areas of honeycombing or segregation are one kind of possibility. A different possibility is that a failure to maintain the specimens in a saturated condition may permit the development of microcracking. A microcrack in a bottom face location will have a much larger effect on flexural strength and tensile strength than it would on compressive strength. A direct measurement of tensile strength has been obtained on a research basis by casting specially shaped test specimens or using various types of friction grip but is not generally practicable as a control test.
Research work has also been done on applying a tensile stress by applying an internal gas pressure Clayton and Grimer, The usual procedure is to use a cylindrical specimen placed sideways in a compression testing machine so that the compressive force is applied across a diameter. The effect is to generate, across the vertical cross-section, a substantial compressive stress immediately under the loading, but a uniform tension over almost the entire area Fig 1.
The test is known as the Brazil test for the double reason of its similarity to cracking nuts in a nutcracker and the part played in its initial development by Carniero, working in Brazil although the test was also independently developed in Japan. The tensile stress is evaluated by the formula: where L and D are the length and diameter of the cylinder, and P the applied force. There is no universally agreed relationship between flexural, tensile and compressive strengths. Indeed there would be little interest in any other test than a compression test if such a fixed relationship existed.
The reason for taking an interest in directly measured flexural strength is the possibility that some factor may cause a significant change in the relationship. For example the possibility exists that a coating of fines on the coarse aggregate could reduce the bond of mortar to it. This may cause a large reduction in flexural strength without making as much difference to the compressive strength. So concrete specified and controlled on the basis of compressive strength may not ensure a particular load-carrying capability for concrete paving.
Although not the subject of general agreement, readers with a practical rather than a research interest will find it to be a workable assumption that both flexural and tensile strength are related to compressive strength by an equation of the form: The value of K will vary as noted above but will be of the order of 0.
If compressive strength does not necessarily define flexural strength, it would seem reasonable to specify and control concrete for paving and anywhere else where flexural strength is the controlling factor on the basis of flexural strength. This is done by many, but not all, bodies such as roadbuilding authorities concerned with such concrete. The case for doing this becomes a little less obvious when the author relates his experience of being able to predict day flexural test results more accurately from 7-day compressive tests than from 7-day flexural tests.
It is important to understand the restricted conditions to which this experience related. Firstly, the results were being analysed by computer, with the exact current average value of the coefficient K in the above equation being automatically fed back and used at all times. Secondly, the aggregates in use were being rigorously controlled. There was no change of source, shape or contamination.
The experience therefore does not rule out the possibility that, if such a change had occurred, it may have been missed by the compressive test and not by the flexural test. What it does show is that the flexural test is too inaccurate to detect the small batch to batch variations still occurring in excellently controlled concrete. The test data involved which are unpublished and not available for publication also included indirect tensile tests.
The author found that this data was intermediate in reliability between the flexural and compressive data. The flexural test is a more expensive test to carry out than the compression or indirect tensile tests. It seems therefore that, for major paving projects such as roads and airports, the ideal is to specify the concrete on the basis of flexural strength but to control it largely on the basis of compressive strength.
The initial concrete supplied would be tested intensively to ensure that the required flexural strength was being provided and also to establish the relationship between compressive and flexural strength for the particular mix. It would also be required that the relationship be reconfirmed from time to time, and particularly in the event of any changes in mix or ingredients. It seems unlikely that an event such as contaminated aggregate could affect flexural strength without showing any effect whatever on compressive strength.
It also seems extremely unlikely that anything could affect flexural strength without having an equally large effect on indirect tensile strength, so this offers an intermediate alternative. There is a good reason for this in that concrete used to be produced using bagged cement and volume batched aggregates. Care should be taken in interpreting old data as they do not necessarily make clear whether the relative proportions e. The phenomenon is due to the surface tension of water increasing the friction between particles in contact. It follows that dry sand and inundated sand will occupy the same bulk volume, that a peak value will be reached well short of the moisture content to which a sand drains, that fine sands will be more affected than coarser sands and that it will take more water to cause peak bulking the finer the sand.
They proposed the division of sands into four grading zones instead of two classes. The grading zone concept has now been dropped in favour of the BRE system see below. Bolomey modified the Fuller and Thompson formula to include cement and to vary the grading according to the desired workability and the aggregate particle shape section 7. The weakness of the ideal grading approach is that it is rarely possible or economical to replicate exactly the ideal grading in the field.
Also the grading may be ideal for one use but could not simultaneously be ideal for all uses. Stewart The technique is to use a large, often single sized, coarse aggregate often 40 mm and a relatively fine sand. With such a combination it becomes valid to measure the voids in the coarse aggregate and provide just sufficient mortar to fill them, with a small surplus. There is no doubt that gap-graded concrete compacts more rapidly under vibration Plowman, and a given strength can usually be obtained more economically at least if cement content is the only cost criterion with a low slump, gap-graded mix.
However, several factors often militate against such mixes. The first, as with ideal continuous gradings, is that suitable aggregates may not be economically available. The second is that gap-graded mixes have a strong tendency to segregate at anything more than low say, 50 mm slump. Although such concrete is easier to consolidate than a continuously graded mix of similar slump, it is sometimes difficult to convince workmen of this and water is frequently added with disastrous effects.
In short, gap-graded mixes can be unbeatable when used by those familiar with such mixes, and in suitable conditions, but are not to be recommended for general use. Another property of gap-graded mixes is that, with a very stable coarse aggregate, very low drying shrinkage is attainable. This technique involves filling the formwork to be concreted with a large single-sized aggregate and then pumping in an appropriate mortar from the bottom up.
Since the coarse aggregate is everywhere in contact, shrinkage is not possible except as aggregate moisture movement. Such concrete is very suitable for use as a foundation block for large pieces of machinery, the concrete often being placed after the machine has been set in position vibration being unnecessary. It offered tabulated data based on an extensive trial mix series at the Harmondsworth Road Research Laboratory Road Research Laboratory, Four alternative gradings were included so that the user could choose to use a harsher or sandier mix.
The tabulated data not only covered four gradings but also three different maximum sizes of aggregate 40 mm, 20 mm and 10 mm and two different particle shapes. The system was purely empirical and so could not be readily adapted when admixtures came into use and cement properties changed. As coarse sand became less readily available, it became harder to match the grading curves. However the tabulated or graphed gradings have long survived the demise of the actual system, being generally used including by the author as a frame of reference as to what constitutes harsh and soft gradings Fig.
See Chapter 7 for further detail of sand grading zones. The system is attributed to D. Franklin and H. The latest version DOE, does allow for air entrainment and the use of fly ash and ground granulated blast furnace slag ggbfs but does not provide a choice of harsher or softer mixes or readily give an accurate yield or density. It is, therefore, interesting to examine the techniques used in some detail and assess the relative advantages and disadvantages of the two approaches.
If these changes were made, the DOE system would work a little more accurately than it now does, in interpolating values from graphs and tables. This clearly illustrates the point that computerization allows an elaboration of the technological basis without detriment to the ease of use. However, if the author were required to produce a new manual system, he would graft the specific surface technique onto the ACI bulk density system section 2.
However, although the theory is still the fundamental basis of both systems, the author and the DOE team have gone in different directions from using exact specific surface. The DOE system used sand grading zones and the version substitutes percentage passing the micron sieve as their simplified approximation. Obviously this cannot be as accurate as true specific surface but was selected as a balance between simplicity and accuracy. Even in a manual system, the additional effort involved is minuscule and certainly does not justify the DOE simplification.
It may be concluded that the DOE simplification was considered worthwhile because true specific surface still did not provide great accuracy so that little was lost by the simplification. The mechanism of selection of fine aggregate percentage is illustrated in Fig.
This figure is for 20 mm maximum aggregate size. The BRE booklet also provides similar charts for 10 mm and 40 mm maximum sizes. The difference between the recommended percentages of a given fine aggregate differs more between the different maximum sizes than this author would consider desirable. It can be seen that a higher fine aggregate percentage and therefore a higher surface area, giving greater cohesion is used for higher slumps.
Fine aggregate percentage is also related to cement content, i. The tabulated water contents are shown in Table 2. This is partly of interest for comparison purposes and partly to show the treatment of pulverized fuel ash pfa , also called fly ash. The remaining interesting technique used is that of combining the tabulated strength data Table 2.
The technique is to enter the graph on Table 2. The same graph can also be used for adjusting values in accordance with actual test results. The table offers no opinion on strengths at earlier or later ages than 28 days but presumably these would be lesser and greater respectively, than those for normal Portland cement. It is not necessary to forego the more precise assessment provided by modified specific surface just because a computer is not available.
Calculation of the modified specific surface of each aggregate using a calculator is little more arduous than fineness modulus calculation section 3. If the effect of varying cement and entrained air contents are to be neglected, as in most mix design systems, the determination of the desirable fine aggregate percentage is extremely simple.
The designer may have a particular combined grading curve in mind e. The specific surface of the desired grading can be determined in exactly the same way as for an individual aggregate. With experience, what will be in mind will be a direct value of combined specific surface taking into account all circumstances including desired slump, cement content, air content, etc. The fine aggregate percentage is then calculated as: 2. Equation 2. Before the advent of pocket calculators, the author had designed many mixes in the field, literally on the back of an envelope, from no more information than a sand grading.
The process took about five minutes. A computer enables all these to be brought to bear in less than the five minutes for manual use of the original basic concept. As many as desired of these features may be added manually but to add them all would extend the process to an hour or more.
The point is that the basic concept already provides as much or more accuracy and much more flexibility than most other mix design systems and only a direct assumption, such as a mix, is quicker to use without a computer. A more accurate way of estimating water content and a more accurate strength formula are given in Chapter 3 or tabulated values can be selected from other systems. The required specific surface is not an estimate but a selection by the designer to suit the particular job conditions.
If desired, selection can be via the tabulated values of mix suitability factor in section 3. The process described above is simpler than most published systems whilst still providing accurately for the effect of varying fine aggregate grading and permitting the designer to select the type of concrete desired. The principal such feature is the use of the bulk density or unit weight of the coarse aggregate as a starting point.
This very neatly allows, in one number, for the combined effect of grading, specific gravity particle density and particle shape of the coarse aggregate on the desirable sand content. The sand content is further varied on the basis of the fineness modulus of the sand Chapter 7 and the absolute volume of cement, water and entrained air.
In effect the volume of all other ingredients is established and the balance is taken as sand Table 2. Water content prediction takes into account only slump, maximum aggregate size and whether or not air is entrained Table 2. Given accurate specific gravity figures, yield is automatically exact by this system. The system can be quite readily computerized and the author as a member of ACI Committee , the revising committee for the document has been advocating for several years that the committee do this officially.
What is missing from the system is a recognition that different degrees of sandiness cohesion are appropriate for different uses. There would be no difficulty in replacing the fineness modulus of the fine aggregate by specific surface in deciding upon i. It should also be noted that the latest version of ACI ACI, , which deals with high strength mixture proportioning, contains an adjustment for predicted water requirement based on percentage voids in the fine aggregate. This has yet to flow through to ACI ACI, , which deals with normal mixture proportioning, but could be an important improvement.
This adjustment is further discussed in section 3. Table 2. The fine to coarse aggregate ratio is adjusted by eye until optimum plastic properties are obtained. A range of mixes with varying cement contents is then prepared, and water requirements and strength obtained at a given slump are determined. The data is then plotted to enable interpolation of properties at 5 or 10 kg increments of cement content.
While the above sounds crude, the actual detailed process is very carefully specified and has been found to give repeatable results. Drawbacks of the process are: 1.