Showing papers in "Magazine of Concrete Research in 1966"
TL;DR: In this article, a quantitative interpretation of results of differential thermal analysis of hydration reactions at 2, 12,23 and 52°C of 3CaO.Al2O3 mixed with 0, 0·25, 2·5, 10 or 20% gypsum was presented.
Abstract: Summary This paper presents a quantitative interpretation of results of differential thermal analysis of hydration reactions at 2, 12,23 and 52°C of 3CaO.Al2O3 mixed with 0, 0·25, 2·5, 10 or 20% gypsum, and compares these with length changes obtained from the hydration of compacts of these mixtures. Estimates of the relative reaction rates from d.t.a. peak areas and measurement of length change during these reactions showed marked agreement, although interpretation had to be made with caution. It was concluded that the formation of ettringite (high sulpho-aluminate) had no direct effect upon reaction rate, but that the reactivitv of 3CaO.Al2O3 was reduced by the sorption of (SO4)2− ions on active sites of its surface. Gypsum reduces the rate of conversion of the hexagonal hydro-aluminates to the cubic hexahydrate, probably also by sorption of (SO4)2− ions, and thus is a second mechanism by which they can retard the normal hydration of 3CaO.Al2O3. The degree to which a given addition of gypsum retards the ...
62 citations
Abstract: Summary Tests on 10 two-way spanning, simply supported, reinforced concrete slabs are reported. The slabs were designed to have similar ultimate loads, but different arrangements of the reinforceme...
57 citations
TL;DR: In this article, the authors examined whether there is a relation between the mix proportions and properties of the constituents and the over-all Poisson's ratio for mortars and concretes.
Abstract: Summary Following recent studies of concrete as a composite material, this paper examines whether there is a relation between the mix proportions and properties of the constituents and the over-all Poisson's ratio for mortars and concretes. From an examination of available and new data, it is apparent that Poisson's ratio is affected by the method of testing, the mix proportions, and the moisture condition and temperature of the specimens. Approximate expressions are proposed for the static and dynamic Poisson's ratios of saturated mortars and concretes made with gravel aggregates in terms of the volume fractions and Poisson's ratios of the constituent aggregate and cement paste phases. Static and dynamic tests do not measure the same properties of concrete, and suggestions are made for the differences between them.
42 citations
TL;DR: In this article, a detailed mechanism is put forward, based on new microscopic and electron-optical evidence, to explain the volume expansion of hardened cement paste due to the presence of dead-burnt CaO.
Abstract: Summary A detailed mechanism is put forward, based on new microscopic and electron-optical evidence, to explain the volume expansion of hardened cement paste due to the presence of dead-burnt CaO. It is shown that CaO first forms colloidal Ca(OH)2 and that subsequently these colloidal crystallites recrystallize to macro-crystals. During their growth from supersaturated solution, these secondary Ca(OH)2 crystals will develop a thrust against the constraining cement paste, thereby causing expansion. This process isfurther helped by the elongated habit of the secondary Ca(OH)2 crystals. The present results and those of other workers are discussed in the light of this and other hypotheses.
26 citations
TL;DR: In this paper, an X-ray diffraction study of the effect of impurities on the structure of tricaldum aluminate was carried out and it was shown that silica and magnesia can also enter the C3A lattice and both cause a decrease in the unit cell size.
Abstract: Summary The paper reports an X-ray diffraction study of the effect of impurities on the structure of tricaldum aluminate. A number of workers have shown that Fe2O3 can replace some of the Al2O3 in C3A, and that this is accompanied by an increase in cell size. Estimates of the limiting composition vary from 2 to 10 molar % C3F/(C3F + C3A); the reasons for these differences are discussed. It is shown that silica and magnesia can also enter the C3A lattice and both cause a decrease in the unit cell size. In all the preceding cases the C3A remains cubic; this is also true for additions of Na2O up to about 3 molar %; with higher concentrations the structure becomes orthorhombic. Some measurements of cell size of C3A extracted from Portland cement are also given. In the light of these findings, a structure is postulated for C3A which differs in important respects from the currently accepted one, of Bussem and Eitel.
16 citations
TL;DR: In this paper, the effects of varying the type and size of the aggregate on the stress-strain characteristics of concrete and micro-concrete in compression and tension are described.
Abstract: Summary The effects of varying the type and size of the aggregate on the stress-strain characteristics of concrete and micro-concrete in compression and tension are described. By applying new experimental techniques, it has been shown that the use of a larger and rounder aggregate yields a concrete which attains a lower maximum strength at a correspondingly smaller deformation.
14 citations
TL;DR: The results of a theoretical investigation carried out in the Department of Civil Engineering of the Indian Institute of Science to determine the stress distribution in the anchorage zone of a post-tensioned concrete beam are described in this article.
Abstract: Summary The results of a theoretical investigation carried out in the Department of Civil Engineering of the Indian Institute of Science to determine the stress distribution in the anchorage zone of a post-tensioned concrete beam are described. The problem was idealized as that of a circular concrete beam with a co-axial cable duct due to the application of circular concentrated load at one of its ends. A three-dimensional elasticity solution satisfying all the equations is given and the resulting numerical results are compared with those of other investigators.
12 citations
TL;DR: In this paper, the influence of such characteristics as spherical seating behaviour, misalinement, homogeneity of specimen, longitudinal stiffness and platen restraint on the strength of cubes and cylinders has been examined.
Abstract: Summary Although the compression test on cubes and cylinders is used very extensively as a measure of the general quality of concrete, there has been increasing evidence to show that such tests are not as reproducible as has normally been assumed There are large differences in the strengths obtained from different testing machines, and often a considerable scatter in strengths from one machine In a series of tests, the influence of such characteristics as spherical seating behaviour, misalinement, homogeneity of specimen, longitudinal stiffness and platen restraint on the strength of cubes and cylinders has been examined Depending upon the behaviour of the spherical seating, differences of about 6% may be obtained with accurately centred specimens If such specimens are also misalined, this difference becomes even larger, as enormous reductions in strength are observed with effectively pinned seatings Cube and cylinder strengths are affected quite differently by the method of end loading, the cylinder
10 citations
TL;DR: The behavior of concretes and mortars subject to tensile strain was examined by measuring strains on the tension face of moist beam specimens during the standard flexural strength test.
Abstract: Summary The behaviour of concretes and mortars subject to tensile strain was examined by measuring strains on the tension face of moist beam specimens during the standard flexural strength test. A wide range of concretes was examined with compressive strengths from 1,500 to more than 11,000 lb/in2 at 28 days. Initial cracking, as indicated by a significant deviation of the load-strain curve, was found to occur at tensile strains ranging from 41 to 158 × 10−6 for the range of concretes tested at 28 days. The corresponding average maximum tensile strains, measured before final crack propagation led to fracture, ranged from 75 to 240 × 10−6. These strains were affected by several factors but generally increased with increasing concrete strengths. The behaviour of mortars and concretes with high mortar content differed from that of normal concretes, and the tensile strains were much greater.
9 citations
TL;DR: In this article, the authors show that the fineness modulus is a measure of the (logarithmic) average particle size of the grading and provide a new interpretation for the relationship between grading and workability.
Abstract: Summary Experimental evidence is used to show that, despite certain weaknesses, the fineness modulus method is applicable within wide practical limits. Then aspects of the fineness modulus and the specific surface not generally known are discussed. These include two mathematical expressionsfor thefineness modulus as afunction of the particle size distribution, which show that the fineness modulus is a measure of the (logarithmic) average particle size of the grading. This, in turn, provides a new interpretation for the relationship between the grading and workability. Similar mathematical expressions are presentedfor the specific surface, and they show that this is also a measure of an average particle size. Specifically, it represents the reciprocal value of the ‘harmonic’ average particle size. However, experimental evidence suggests it is inferior to the fineness modulus where aggregates for concrete are concerned. A further item is the ‘D-m-s method’ in which the fineness modulus and specific surface ...
9 citations
TL;DR: In this article, a convenient and reliable test has been developed for determining the angularity of aggregates, which can be used in modified forms for both coarse and fine aggregates and gives satisfactory results for both spheres and natural aggregates.
Abstract: Summary A convenient and reliable test has been developed for determining the angularity of aggregates. An examination of the existing methods indicated that the determination of the volume of voids in the aggregate when placed in a standard way was the most promising. Two methods, referred to as the flow-rate test and loose bulk density test, were investigated in detail. The latter test was found to be the most satisfactory and can be used in modified forms for both coarse and fine aggregates. It gives satisfactory results for both spheres and natural aggregates.
TL;DR: In this paper, the curvature distributions along the plastic lengths have been studied and the rotations deduced therefrom are compared with those obtained on the basis of the standard moment-curvature relationship proposed by Guyon.
Abstract: Summary Twenty-seven three-span and twelve two-span beams, all of rectangular cross-section, were tested to destruction under various arrangements of concentrated loads. The results of these tests are presented. Redistribution of moments was practically complete in the majority of the beams. The curvature distributions along the plastic lengths have been studied and the rotations deduced therefrom are compared with those obtained on the basis of the standard moment-curvature relationship proposed by Guyon.
TL;DR: In this article, an apparatus for measuring the variation of density along cores and beams of concrete is described, which is determined by measuring the change in the absorption of gamma radiation, and it is shown that with careful operation, variations in density may be measured with a standard deviation of about 1 lb/ft3.
Abstract: Summary The paper describes an apparatus for measuring the variation of density along cores and beams of concrete. The variation of density is determined by measuring the change in the absorption of gamma radiation. The gamma rays emanate from a one millicurie source of caesium 137 and are detected by a scintillation counter. This is connected to a rate-meter and chart-recorder which continuously display the intensity of radiation transmitted as the sample traverses along its depth dimension through the gamma beam. The I'ariation in the intensity of the transmitted radiation is translated into density variations by a suitable calculation. Attention is drawn to the need to retain, for each type of specimen, the same source-sample-detector geometry, but it is shown that, with careful operation, variations in density may be measured with a standard deviation of about 1 lb/ft3. Although designedfor use with concrete specimens, the technique can be applied to any cohesive material.
TL;DR: In this article, long narrow model concrete slabs were bent in such a way that strains were everywhere uniaxial, thus simulating a yield line at different angles to the bars in a layer of mild-steel tensile reinforcement, so that the effect of distortion of steel across cracks could be directly measured.
Abstract: Summary Long narrow model concrete slabs were bent in such a way that strains were everywhere uniaxial, thus simulating a yield line at different angles to the bars in a layer of mild-steel tensile reinforcement, so that the effect of distortion of steel across cracks could be directly measured. Most slabs were tested with zero membrane force normal to the yield line, but others were tested in proportional loading with different ratios of moment to membrane force. Separate analysis of the stresses in the steel and concrete shows that distortion of steel was insignificant at all stages after cracking.
TL;DR: In this article, a static loading test on eleven reinforced concrete T beams was conducted and it was shown that the amounts of shear reinforcement provided were more than adequate to ensure that the flexural capacities of the beams were attained.
Abstract: Summary Static loading tests on eleven reinforced concrete T beams are reported. The shear reinforcement in the beams had been proportioned in accordance with proposals made in an earlier paper, the amount of this reinforcement being appreciably less than that required by the current method of design in C.P.114. The tests indicated that the amounts of shear reinforcement provided were more than adequate to ensure that the flexural capacities of the beams were attained.
TL;DR: In this paper, an extension to lightweight aggregate concrete of earlier work on the influence of cement paste content on the creep of concrete has been presented, where it is shown that there is a simple relation between the creep after a given time under load and the content of the cement paste corrected for unhydrated cement.
Abstract: Summary The paper is an extension to lightweight aggregate concrete of earlier work on the influence of cement paste content on the creep of concrete. It is shown that there is a simple relation between the creep after a given time under load and the content of cement paste corrected for unhydrated cement. An expression is proposed for the creep-time behaviour of concrete of any mix proportions but made with a given type of aggregate; this involves a total of four constants from two creep-time tests.
TL;DR: In this paper, a sample of natural tobermorite from Crestmore, California, was studied by a new extraction technique, which revealed that toberminite can be resolved into two fractions: a soluble species and an insoluble species.
Abstract: Summary A specimen of natural tobermorite from Crestmore, California, was studied by a new extraction technique. This investigation revealed that tobermorite can be resolved into two fractions. One of them is a soluble species and the other insoluble. The infra-red spectrum of the soluble polysilicate derived from tobermorite is not identical with the infra-red spectrum of the soluble polysilicate derived from a Portland cement paste. The insoluble derivative has the same morphology as the original tobermorite.
TL;DR: In this article, the point at which the alite phase in clinker starts to accelerate and the cement paste begins to turn solid can be determined with a copper-lead electrode, which is clearly indicated by the distinct increase of potential difference when the surface of copper oxidizes to cuprous oxide as air penetrates to the capillary space.
Abstract: Summary The point at which the rate of hydration of the alite phase in clinker, which is very slow in the beginning, starts to accelerate and the cement paste begins to turn solid can be determined with a copper-lead electrode. This point is clearly indicated by the distinct increase of potential difference when the surface of copper oxidizes to cuprous oxide as air penetrates to the capillary space. Air dissolved in the mixing water does not cause oxidation, nor does entrained air present under practical working conditions. Because of the total absence of the disturbing influence of the aggregate, this method of determining the setting time of mortar or concrete is more accurate than the commonly used mechanical methods and it also yields results which better correspond to the actual situation. The reaction vessel need not be of any specific shape, and therefore direct placement of the electrode, for instance, in a unit mould is possible. In principle, this method is also applicable to hydraulic bonding ...
TL;DR: In this paper, the authors extended the range of the curves for use in mix design and showed that the optimum specific volume ratio of coarse aggregate for the Vebe consistometer test is similar to that for the compacting factor test.
Abstract: Summary The best proportions in which to combine any given coarse andfine aggregates are considered with particular reference to concrete mix design. Earlier work, which was restricted to ¾ in. maximum size of aggregate and used the compacting factor test to indicate the workability of the concrete, is extended to ⅜ in maximum size of aggregate using the Vebe consistometer test. In addition to extending the range of the curves for use in mix design, the present work shows that the optimum specific volume ratio of coarse aggregate for the Vebe consistometer test is similar to that for the compacting factor test. This is a most important result since it shows that the application of the concept of particle interference is not restricted to the compacting factor test.
TL;DR: In this paper, a lower bound solution for the simply supported square slab under uniform load based on the assumption of elastic-plast ic behavior of the slab material is presented. But this lower bound is not applicable to the case of a simple square slab.
Abstract: Summary The paper describes a lower-bound solution for the simply supported square slab under uniform load based on the assumption of elastic-plast ic behaviour of the slab material. It is shown th...
TL;DR: In this article, the authors found that a 0·010 in. total variation from the plane in the surfaces of a concrete cube can cause a reduction in compressive strength of up to 15 %.
Abstract: Summary After the return of some cube moulds from site early in 1965 and their absorption into the general laboratory stock of moulds, excessive variations were found in the strength of some specimens made from carefully controlled concrete mixes. At the same time cubes coming into the laboratory for testing showed surfaces that were excessively convex and it was found that some of the cube moulds being returned from site were very worn and had surfaces which varied from plane by up to 0·025 in. Several cube moulds were examined and measurements taken to select those with surface variations between 0·001 and 0·015 in. from plane. Cubes were cast in these moulds and, after curing for 7 and 28 days, measured for surface planeness and tested for compressive strength. The results indicate that a 0·010 in. total variation from the plane in the surfaces of a concrete cube can cause a reduction in compressive strength of up to 15 %.
TL;DR: In this article, it is suggested that concrete research has tended to follow a traditional pattern, continuing to examine factors well into unrewarding regions according to the law of diminishing returns and ignoring areas which could have a major effect on the over-all cost of construction.
Abstract: Summary Recent events have indicated a growing concern about the effectiveness of research for the construction industry. These are reviewed, and provocative comments are offered, to stimulate a critical appraisal of the value of the subjects chosen for study. It is suggested that concrete research has tended to follow a traditional pattern, continuing to examine factors well into unrewarding regions according to the law of diminishing returns and ignoring areas which could have a major effect on the over-all cost of construction. This situation is attributed largely to the varied way in which the results of different types of investigation are received by the industry. The research worker is exhorted to spend more time in applying his critical faculties to examine how best he can serve the industry and in using his unbiased judgement to incorporate his findings into the common experience of the practitioners he serves.