Showing papers in "Magazine of Concrete Research in 1965"
TL;DR: In this paper, the mechanism of failure at ultimate shear load is explained in terms of truss analogy, and the ratio between the width of the compression flange and the thickness of the web is shown to have a considerable influence on the inclination of the top chord and of the diagonal struts of the truss and consequently on the tensile forces in the web.
Abstract: Summary The shear tests reported in the present paper were carried out in Stuttgart from 1961 to 1963. The mechanism of failure at ultimate shear load is explained in terms of the truss analogy, and the ratio between the width of the compression flange and the thickness of the web is shown to have a considerable influence on the inclination of the top chord and of the diagonal struts of the truss and, consequently, on the tensile forces in the web. From the results, a simple method for reducing the shear reinforcement is derived. For short beams with a slenderness ratio l/d < 8 or for concentrated loads close to the supports, a further reduction in shear coverage is possible. The determination of the points for cut-off and bent-up bars must be adapted to the variations in the compression members of the truss. The tests showed clearly that stirrups are preferable to bent-up bars for shear reinforcement, particularly if high-tensile steel is used.
61 citations
TL;DR: In this article, the action of 5% solutions of ammonium salts on a Portland, a Portland-pozzolana, and a high alumina cement concrete was reported, showing that the expansion caused by formation of calcium sulpho-aluminate is dominant.
Abstract: Summary Results are reported on the action of 5% solutions of ammonium salts on a Portland, a Portland-pozzolana, and a high alumina cement concrete. Ammonium chloride and nitrate solutions had a strong leaching action, acting as dilute acids owing to loss of ammonia by reaction with lime. Strengths of concrete cubes with all three cements fell by at least a half in four years without outward signs of deterioration. Analysis of the specimens showed large reductions in the lime content of the cements. In the action of ammonium sulphate on Portland cement, the expansion caused by formation of calcium sulpho-aluminate is dominant. High alumina cement was unaffected by ammonium sulphate.
53 citations
TL;DR: In this article, the authors reported on the strain of concrete under sustained uniaxial tensile stress and the effects on this strain of the level of stress, the time under load, the age of loading and the humi...
Abstract: Summary Tests are reported on the strain of concrete under sustained uniaxial tensile stress. The effects on this strain of the level of stress, the time under load, the age of loading and the humi...
48 citations
TL;DR: In this paper, the effect of horizontal restraint on strength in punching shear was obtained by testing pairs of slabs, one simply supported along its edges; the other was similarly supported hut confined within a surrounding steel frame.
Abstract: Summary The results of tests on 22 plain and reinforced concrete square slahs failing in punching shear are reported. The effect of horizontal restraint on strength in punching shear was obtained by testing pairs of slabs. One of each pair was simply supported along its edges; the other was similarly supported hut confined within a surrounding steel frame. The increase in strength due to the restraining frame variedfrom zero to 60%, the extent of increase depending on the state of flexural cracking in the simply supported slabs at failure.
40 citations
TL;DR: In this paper, the authors examined the strain of concrete under sustained compressive stress in terms of three components, instantaneous recoverable, time-dependent recoverable and irrecoverable, defined here as elastic, delayed elastic and flow respectively.
Abstract: Summary The strain of concrete under sustained compressive stress is examined in terms of three components, instantaneous recoverable, time-dependent recoverable and irrecoverable, defined here as elastic, delayed elastic and flow respectively. Experimental evidence is presented of the influence on each of the length of the period of loading and the age of the concrete when the load is applied. It was found that at a given age the rate of flow is virtually independent of any earlier loading and that the delayed elastic strain tends to a limiting value which varies little with the age of the concrete. The rate of occurrence of delayed elastic strain diminishes with the age of the concrete and it can be linked with the corresponding rate of flow.
37 citations
TL;DR: In this paper, the lateral stiffness and strength required of supporting beams and exterior panels of continuous reinforced concrete slab-and-beam floors in order to ensure membrane action in the panels at ultimate load is considered.
Abstract: Summary The lateral stiffness and strength required of supporting beams and exterior panels of continuous reinforced concrete slab-and-beam floors in order to ensure membrane action in the panels at ultimate load is considered. It is shown that tie reinforcement may be required in the supporting beams to carry the membrane forces. Tests on twenty small-scale mortar models of idealized nine-panel slab-and-beam floors with the interior panel uniformly loaded are described. The tests illustrated the modes of failure of exterior panels with insufficient reinforcement to resist the reactire forces due to the membrane action of the interior panel. The ultimate loads of interior panels of the slabs with adequately reinforced exterior panels showed good agreement with theory derived previously (which included the effect of axial strains in the panel and lateral movement of the edges due to extension of the tie steel) once the concrete had cracked.
34 citations
TL;DR: Because of tensile membrane action, reinforced concrete slabs can sustain loads far in excess of the ultimate load calculated by the yield-line theory, and a method of calculating the corresponding load-carrying capacity is proposed.
Abstract: Summary Because of tensile membrane action, reinforced concrete slabs can sustain loads far in excess of the ultimate load calculated by the yield-line theory. To take advantage of this, it is necessary to establish de flexion criteria for ultimate load conditions and to develop a method of calculating the corresponding load-carrying capacity. Such a method is proposed, and the possible advantages are indicated.
30 citations
TL;DR: In this paper, small specimens of mortar and paste were cured for 28 days and then subjected to various conditions of drying and carbonation, in general the extent of carbonation for a given CO2 concentr...
Abstract: Summary Small specimens of mortar and paste were cured for 28 days and then subjected to various conditions of drying and carbonation. In general the extent of carbonation, for a given CO2 concentr...
28 citations
TL;DR: In this article, the authors discussed the time-dependent effects of thermal stressing of prestressed concrete beams with reference to long sustained temperature crossfalls and proposed an iterative numerical method to allow for creep in the calculation of the variation with time of stresses, strains and displacements.
Abstract: Summary The paper briefly discusses the time-dependent effects of thermal stressing of prestressed concrete beams with reference to long sustained temperature crossfalls. Certain principles and assumptions are presented and an iterative numerical method is introduced to allow for creep in the calculation of the variation with time of stresses, strains and displacements. The results of tests on heated simply supported and continuous prestressed beams are compared with the predictions of the theory. The measure of agreement shows that the method can indicate the magnitude of the severe time-dependent effects with reasonable accuracy. Finally, it is demonstrated in general terms that under a prolonged temperature crossfall, a prestressed concrete beam will, in general, reach a stable stress distribution. This distribution can be calculated without reference to the elastic modulus and coefficient of expansion of the concrete. The limiting stresses thus calculated have been verified in various examples by the ...
21 citations
TL;DR: In this paper, a modification to the cube test enables the uniaxial strength to be determined very readily, and the extremely convenient cube specimen can therefore now be used for normal compressive strength determination as well as for normal control purposes.
Abstract: Summary The cube is undoubtedly the most convenient specimen to use when large numbers of crushing tests are required for concrete control purposes. The cylinder, or prism, on the other hand can give a much better estimate of the uniaxial compressive strength of concrete. The recent tendency has been to favour the adoption of the cylinder as the standard specimen, despite its shortcomings regarding convenience. The paper shows that variable differences can occur between the cube crushing strength and the uniaxial compressive strength, and then indicates how these differences can be greatly minimized by a suitable modification of the testing technique. This simple modification to the cube test enables the uniaxial strength to be determined very readily. The extremely convenient cube specimen can therefore now be used for uniaxial compressive strength determinations as well as for normal control purposes. The continued use of the cube as the standard form of test specimen is therefore recommended.
17 citations
TL;DR: In this paper, a study of the behavior of neat cement pastes and fresh cement mortars under shear in a rotational viscometer, especially designed for this purpose, is described.
Abstract: Summary In this paper a study of the behaviour of neat cement pastes and fresh cement mortars under shear in a rotational viscometer, especially designed for this purpose, is described. It is shown that a considerable structural breakdown occurs in the cement paste under the shearing action and a comparison between the results obtained from the viscometer and those associated with the compaction of concrete under vibration seems to suggest that a breakdown under shear does take place during the vibration of concrete. It is also shown that the rate of energy input to the paste is not constant, and the effect of this on the Vebe test is discussed. A method of measuring the workability of cement pastes and mortars is presented.
TL;DR: In this article, it was shown that at the limit of solid solution at 1,350°C the sodium calcium aluminate which is formed has the composition 91·0 mol % C3A, 9·0mol % N3A and has been indexed on an orthorhombic unit cell.
Abstract: Summary Conwicke and Day showed that partial solid solution occurs in the system C3A-NC8A3 at temperatures below the liquidus. Their results indicated a marked similarity between the X-ray powder diffraction data for a preparation containing more than 3·0% Na2O and the data given by Moore for a new phase in Portland cements. The present work was almost complete when Conwicke and Day's paper was published. It generally confirms their results but gives more information on the solid solution. At the limit of solid solution at 1,350°C the sodium calcium aluminate which is formed has the composition 91·0 mol % C3A, 9·0 mol % N3A and has been indexed on an orthorhombic unit cell with a = 15·314, b = 15·394, c = 15·137 A.
TL;DR: In this paper, the results of two-point loading shear tests on 50 pre-tensioned concrete I beams without web reinforcement were presented, and it was concluded that failure should have occurred when the web is cracked in shear.
Abstract: Summary This paper presents the results of two-point loading shear tests on 50 pre-tensioned concrete I beams without web reinforcement. Although many of the beams carried a shear load greater than the cracking shear load, the amount of the excess could not be predicted, and it is concluded that failure should be considered to have occurred when the web is cracked in shear. An expression is developed for the diagonal cracking shear force. Test results already published are shown to give satisfactory agreement with this expression, and comparisons are made, for specific cases, between the ultimate shear load predicted by it and by other design equations and code rules.
TL;DR: In this paper, a yield criterion for quasi-isotropic reinforced slabs is proposed, where the possibility of kinking of reinforcement bars on the open fracture line is taken into account and a detailed analysis of yield moment on the plastic hinge is made.
Abstract: Summary This paper deals with the problem of formulating a yield criterion for quasi-isotropic reinforced slabs. The possibility of the kinking of reinforcement bars on the open fracture line is taken into account and a detailed analysis of yield moment on the plastic hinge is made. Both the “square” yield criterion and the criterion for perfect kinking of reinforcement prove limiting cases of the proposed partial-kinking theory. Values of plastic moments obtained from this criterion are more optimistic than those obtained by Johansen's criterion. The proposed theory enables one to describe analytically the results of some former experimental investigations which have shown that the ultimate moment in an initially isotropic slab is not independent of the plastic hinge inclination.
TL;DR: A series of tests to measure the strains in the endzones of pre-tensioned prestressed concrete I beams is described in this article, where the variables were limited to the web breadth and the arrangement of prestressing wires.
Abstract: Summary A series of tests to measure the strains in the endzones of pre-tensioned prestressed concrete I beams is described The variables were limited to the web breadth and the arrangement of prestressing wires The maximum stress on the end face of a beam when horizontal cracking occurred was found to bear a linear relationship to the cube strength of the concrete at transfer The strain at cracking was virtually constant Peak tensile stresses on the end faces as predicted by various methods are compared with stresses obtained from measured strains, and suggestions are made for approximate predictions of stresses A procedure is proposed for determining whether cracking is likely
TL;DR: In this article, the authors describe experimental work to verify the yield criterion for an isotropically reinforced concrete slab based on the concept of kinking of reinforcement bars across a fracture line.
Abstract: Summary This paper describes experimental work to verify the yield criterion for an isotropically reinforced concrete slab based on the concept of kinking of reinforcement bars across a fracture line. Sixteen model slabs with various arrangements of reinforcement were tested in one-way bending and experimental results have been compared with theoretical predictions. It appears that the concept of kinking is well confirmed although more tests on the behaviour of a fracture line arbitrarily inclined to the direction of the bars seem desirable.
TL;DR: In this paper, a moving-coil vibrator coupled to the upper face of the beam at the centre of the top face is used to measure the frequency of resonance and the strain in the beam.
Abstract: Summary The method and apparatus enable vibratory flexural stresses of magnitudes comparable with those used in conventional quasi-static tests to be applied to concrete beams. The beam is mounted horizontally on rigid supports and is vibrated inflexure by a moving-coil vibrator coupled to the upper face of the beam at the centre. The frequency of resonance and the strain in the beam at the centre of the top face are measured and, in normal operation, the driving current is adjusted to maintain a constant value of strain whilst changes of frequency with time are observed. A decrease in frequency indicates a decrease in the dynamic Young's modulus of elasticity of the concrete, and is associated with a development of micro-cracks in the concrete. The cracks which form at sufficiently high levels of strain fatigue have been observed by a photo-elastic technique before the appearance of visible cracks. Strains between 60 and 120 × 10−6 ultimately led to fatigue of the concretes tested. Stress-strain relation...
TL;DR: In this paper, it was shown that small rises in temperature can reduce and often prevent deterioration of Portland cement mortars in sulphate media, even though increases in temperature cause loss of strength in other cements due to changes in crystal structure.
Abstract: Summary The results indicate that small rises in temperature can reduce and often prevent deterioration of Portland cement mortars in sulphate media, although rises in temperature cause loss of strength in other cements due to changes in crystal structure. The resistance of mortars to the more aggressive magnesium and ammonium sulphates at higher temperatures and the small expansion which occurs above 40°C are conclusively demonstrated.
TL;DR: In this article, the occurrence of crystals of calcium hydroxide and of ettringite (calcium sulphoaluminate) in hardened Portland cement, reported previously by others, is confirmed.
Abstract: Summary The occurrence of crystals of calcium hydroxide and of ettringite (calcium sulphoaluminate) in hardened Portland cement, reported previously by others, is confirmed. Samples of hardened Portland cement paste prepared half a century ago at one of Canada's earliest cement plants were, on examination, found to contain voids in which well developed crystals had formed. The crystals occurred as thin plates and needles that were identified by optical microscopy and X-ray diffraction as calcium hydroxide and ettringite, respectively. It is of interest that the calcium hydroxide had not been altered to carbonate even though the cement samples had been stored for a long time under conditions that would be expected to promote this change.
TL;DR: In this article, it was found that the removal of polar grease lubricating the ball-seat controlling the top platen of a compression testing machine reduced variation to one-third of previous values while increasing the mean strength of a group of specimens without the calibration of the machine being changed, and a recommendation given that polar greases should not be used to lubricate ball-seats in compression testing machines.
Abstract: Summary The unsatisfactory behaviour of a commercial compression testing machine led to an investigation into the cause of variation in testing, which in turn led to the discovery that the removal of polar grease lubricating the ball-seat controlling the top platen of the machine reduced variation to one-third of previous values while increasing the mean strength of a group of specimens without the calibration of the machine being changed. The reasons for this behaviour are discussed and a recommendation given that polar greases should not be used to lubricate ball-seats in compression testing machines.