How to produce high strength concrete It really takes the best of everything BY PIERRE-CLAUDE AITCIN, PROFESSOR, DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF SHERBROOKE T hough a longtime major aim of the concrete industries has been to provide the least expensive material possible, these industries have also been concerned with constantly improving their product. New vanguard producers of ready mixed concrete are directing their promotion efforts toward commercialization of highstrength concretes (9000 psi and more). Traditionally concrete has been the predominant material for horizontal construction while steel has taken most of the vertical construction market. As a matter of fact, many designers still tend to think of concrete as a heavy material with an effective top limit on compressive strength of about 4500 psi.2 Yet, if we combine the use of structural lightweight concrete in horizontal elements with high-strength concrete in vertical elements, the construction of skyscrapers of a respectable height can be achieved at a lower cost than with steel design. In a field that until now has been exclusively reserved for steel stru c t u re s, the Water Tower Place building in Chicago, with its 74 floors and 859-foot3 height is surely one of the most spectacular breakthroughs among all concrete buildings. Such construction has been made economically possible only because the designer could obtain ready mixed concrete having a guaranteed strength on the jobsite of 9000 psi.1 High-strength concretes offer a technological breakthrough in the building of columns, beams and shear walls because, with concrete two or three times as strong as ordinary, the concrete cross-sectional area or the amount of steel can be reduced, thus lessening the dead weight of the building, relieving the strain on the lower columns and foundations, and reducing costs. Reducing the cross-sectional area of the columns and shear walls also means slightly increasing the rentable space on each floor. Without daring to assert that high-strength concrete will finally supplant ordinary concrete in most of its applications, one cannot deny that in the near future many ready mix producers should be prepared to offer their customers high-strength concretes. This evolution to- wards the commercialization of high-strength concrete should become more marked with the constantly growing and irreversible concern for energy conservation. Even though concrete is a building material with a very low energy content, the fact remains that in industrialized countries the production of cement consumes great quantities of energy, so that each time we use a pound of cement we ought to be certain that we make the most of its structural potential. The purpose of this article is to recall some basic principles that could aid the commercialization of highstrength concrete. These are important because highstrength concrete has not been achieved by chance; it is a concrete in which all the factors that contribute toward an increase in strength must be maximized whereas those that can lessen strength must be reduced to a minimum. Developing a high-strength concrete requires intensive research work, the establishment of an efficient system of quality control and a good knowledge of what helps and what hinders achieving a concrete of good quality. Compressive strength of concrete Before trying to improve the compressive strength of a c o n c re t e, it is worth observing the different possible forms of rupture of a specimen of hardened concrete subjected to a compression test. The specimen will rupture when the shear tension or strains induced by the uniaxial compression load reach a critical value in one of the three following zones: • in the hydrated cement-sand mortar (Photo 1) • along the interface between coarse aggregate and hydrated cement-sand mortar (Photo 2) • in the coarse aggregate (Photo 3) Consequently, in order to make a high-strength concrete we must improve the concrete strength in these three zones. Improvement in strength of cement sand mortar The hardened cement-sand mortar is composed of h yd rated cement grains, sand grains and air. The stronger this hydrated mortar, the stronger the concrete. As a first step, we must therefore optimize the different factors which can affect the strength of the mortar . Photo 1 The hydrated cementsand mortar was the weakest part of this air-entrained concrete. Photo 2. The bond of the aggregate to the hydrated cement-sand mortar was weak in this concrete (made with ceramic balls of 1/2 inch17 maximum size to clearly illustrate the phenomenon. Photo 3. The aggregate is the weakest part of this concrete. This is a close-up view of the concrete of the broken cylinder shown in photo 6. Cement Water The choice of cement is of utmost importance to a high-strength concrete because the success of the operation depends mostly on the binding power of the cement used. Experience shows that not all cements are alike from this point of view, because the standards of quality which all cements must meet are only minimum standards and they leave a rather large working latitude to cement manufacturers. The cement content will have to be rather high, ranging from 850 to 1000 pounds per cubic yard,4 in order to increase the proportion of binder in the mortar. Yet the cement should not exceed such amounts. Too much cement may cause problems by too-rapid liberation of heat during hydration (which lessens the final strength of concrete) and problems with too much drying shrinkage. Whenever economically feasible we should try to reduce the cement content and add a good quality fly ash so that the negative effects of heat liberation and drying shrinkage are reduced to a minimum. Yet it should be remembered that this change in ingredients always lowers the short-term strength of concrete . Experience has shown that the only way to find the best cement to make high-strength concrete is to test mortar cubes of all the types and brands of cement economically available as a basis for comparing the comp re s s i ve strengths to be expected in the concretes. It should be remembered, howe ve r, that the compressive strengths of standard cubes of mortar do not always vary in the same way as compressive strengths of cylinders of concrete. The amount of mixing water used should be minimized; that is, the water-cement ratio should range from 0.28 to 0.30. Still, for such a concrete to be placed easily in the forms a superplasticizer must be used. It is desirable also to use the Coldest water possible in order to limit the temperature of Concrete during setting. Superplasticizers The commercialization of superplasticizers promises to greatly improve the production of high-strength concrete; it is now possible to make concrete with a watercement ratio of only 0.28 at a slump of 8 inches.5 It is necessary to use a superplasticizer at a higher dosage than recommended by the manufacturer, though not so high as to adversely affect the final strength of the conc re t e. A field concrete of 14,000 psi6 has thus been delivered in Japan. The ratio of cement to superplasticizer that will yield the strongest concrete has to be determined experimentally. Air Since air voids reduce the effective surface area of the concrete section that resists compressive stresses, the amount of air entrapped in the cement mortar during the mixing and placing of the concrete should be reduced as much as possible. The use of a superplasticizer makes this easier. (In this article we obviously are not considering the production of air-entrained highstrength concrete, which has already been discussed in the article by Weston Hester in the February 1977 issue Photo 4. The bond of smooth grains of coarse sand, A, was weak in this high-strength concrete. Particle A is about the largest that would pass a Number 418 screen. Photo 5. Coarse aggregate, A, in this high-strength concrete ruptured by slipping along a cleavage plane while other aggregate, B, was split. These two particles are about 1/2 inch17 size. of CONCRETE CONSTRUCTION, page 77.) Sand Since the mix is already rich in fines, it is advisable to use a relatively coarse sand so that the concrete will not be too stiff. Usually a sand having a fineness modulus of about 3.0 is suitable. The proportion of sand should be kept as low as possible, but the mix should not be made too harsh, which could cause problems when the concrete is placed at the jobsite. Finally, it is worth remembering that the mineralogical composition of the sand can be of some importance too. Improvement in cement aggregate bond The bond strength of cement to sand affects the strength of the mortar fraction. The bond strength of cement to coarse aggregate affects the concrete. These bond strengths can be of two kinds: mechanical and chemical. The surface roughness creates fixing points for the hydrated cement that help prevent any movement of either material relative to the other. That is why it is not advisable to use natural, rounded gravels as coarse aggregate in the production of high-strength concrete (Photo 4). In certain circumstances bonding can also be mineralogical when some kind of chemical adhesion forms between the hydrated cement and the minerals included in the aggregates. It is plain that this adhesion can appreciably augment the mechanical bond strength. On the other hand, crushed granite aggregates, even if they are intrinsically quite strong, are not very desirable be- cause the hydrated cement will not adhere to any extent to the micaceous and quartz grains present on the surfaces of such aggregate. It is not enough, howe ve r, simply to improve the bond of hydrated cement to the aggregate. When the interface of the hydrated cement and aggregate is subjected to stress, the two materials must yield the same amount in order to avoid a decrease in the bond strength. Suppose that in the production of concrete we used an aggregate whose Young’s modulus is 11⁄2 times that of the h yd rated cement. This would mean that, for a given increase in stress, the hydrated cement will be deformed roughly ll⁄2 times as much as the aggregate, inevitably leading to the movement of one surface over the other. Ac c o rd i n g l y, to obtain a high-strength concrete we should preferably use freshly crushed fine-grained metamorphic limestones (for the surfaces to be more effective). Metamorphic limestones with grains that are too coarse can yield too low a strength in the direction of cleavage planes, very quickly causing rupture of the aggregate in that direction as shown in Photo 5. Optimization of coarse aggregate characteristics The characteristics of coarse aggregates that are of most importance to high-strength concrete are compressive strength, shape and maximum size. Compressive strength of coarse aggregate To make high-strength concrete we must obviously use coarse aggregate that has a high compressive strength to prevent rupture from occurring in the coarse aggregate, as it has done in Photo 3. We must therefore find coarse aggregates that come from quarries that produce rocks with compressive strengths above 16,500 psi7 and absolutely avoid rocks that are too soft or which present cleavage planes. So before making laboratory trial batches, we should determine the compressive strengths of all the coarse aggregates economically available. Yet, as already noted, it is not necessarily the strongest coarse aggregate which will produce the strongest concrete, since the bond of the hydrated cement to that same aggregate must be taken into account. Shape of coarse aggregate Because the bond between the coarse aggregate and the hydrated cement is more of a mechanical type at the beginning, to make high-strength concrete we ought to use a cubically shaped crushed stone rather than a natural gravel or a crushed gravel. The type of crusher used by the aggregate producer is important in this respect. Fu rt h e rm o re, the surfaces of the coarse aggregate must be clean and free of any dust which would impair mechanical bonding. In certain cases, washing of the aggregate may prove necessary. Careful examination of aggregate samples from local quarries is sufficient to choose the coarse aggregate that offers the most useful characteristics from this point of view. Photo 6. A champion cylinder (5- by 10-inch).19 Maximum size of coarse aggregate Curing of concrete We could show that for a given aggregate there is a relation between its maximum diameter and the maximum compressive strength possible from concrete made with it. The absolute maximum strength seems to be obtained with aggregates having a maximum size of 3 ⁄8 or 1⁄2 inch.8 Standard coarse aggregates of Number 4 to- 3⁄8-inch 9 or Number 4-to-5⁄8-inch10 sizes are the most suitable. The ideal temperature for the curing of concrete ranges from 50 to 60 degrees F.” The hydration reaction seems to reach its fullest development in this temperature range. It is obviously very hard to limit the temperature of field concrete to such a low level, particularly when we deal with cement contents of 850 to 1000 pounds per cubic yard.4 At higher temperatures the heat of hydration raises the temperature of concrete excessively; this can lead to: • cracks due to thermal shrinkage if the concrete does not cool down uniformly Quality control It is obvious that after having so carefully chosen his sources of raw materials, the concrete producer should control no less closely the constancy of their quality. The p ro p o rtioning and mixing of a high-strength concrete obviously requires much more care than that of ordinary concrete. Once a production procedure has been perfected, we should not depart from it lest we lose the last psi of strength gained with so much difficulty. Delivery and placement—particularly placement—should not be left to partially qualified personnel. As a rule, in order to control the fabrication, delivery and placing of a highstrength concrete, it should be sufficient to increase the frequency of the routine tests normally performed (air content, slump, temperature, unit weight) in the quality control of ordinary concretes so as to immediately detect the least variation and apply the necessary correction. • tension strains during cooling at the aggregate-hydrated cement interface if the aggregate and cement have different coefficients of thermal expansion • a too-rapid and incomplete hydration of cement grains Once concrete has hardened, its temperature should be controlled as much as possible by adequately spraying or immersing it. Measurement of compressive strength To measure the compressive strength of high-strength concrete it is best to use rigid steel cylinder molds. These molds enable us to produce cylinders whose compressive strengths are 10 or 15 percent higher than those obtained with cardboard cylinder molds—and the results are more reproducible. The rigidity of the walls of steel cylinder molds ensures better reproducibility of the compaction of the concrete and less deformation of the mold walls during Casting. The Capping material should be strong enough and the Caps as thin as possible so that it does not alter the test results. The Compression test machine should have a Capacity at least twice the maximum load at which the Concrete cylinders will break. Design strength When high-strength concretes are to be used, it is desirable to make the designer aware that design strength should be based on the compressive strength at 56 or even 90 days rather than 28 days. Very few concrete structures are subjected to loads equivalent to their full design capacity before such dates. This lengthening of the period of evaluation of concrete is in harmony with energy conservation since a greater saving can be derived from using later-age strength data, thus using most efficiently the amount of energy necessary to make cement. The ratio, psi obtained/Btu spent,l2 can thus be increased. Conclusion These reflections do not offer any miraculous solution for the production of concrete of 9000 psi1 and higher, for only an intensive research program and an efficient system of quality control can enable a concrete producer to obtain such concretes. Howe ve r, having specified the main points on which research work and quality control should be based, we hope to help concrete producers who are willing to launch into such an enterprise and at the same time show the most skeptical of them that after all it is not difficult to make a concrete of 9000 psi’ when you know how to manage it. As proof, the writer, as part of an underg ra d u a t e course at Sherbrooke University, organizes a contest for his students each year for the purpose of obtaining the strongest concrete possible with a minimum slump of 4 inches13 and a maximum cement content of 1000 pounds per cubic yard. 14 During the 5 years this competition has taken place, all the students have easily gone beyond the desired 9000 psi, the absolute record being 14,390 psi15 at 42 days, slightly higher than the previous record of 14,350 psi16 illustrated in Photo 6. The students, who are all new in the field, only apply studiously the few principles developed above. PUBLICATION #C800222 Copyright © 19980 The Aberdeen Group All rights reserved
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