Showing papers on "Compressive strength published in 1974"

Brittle fracture in compression

Abstract: The initiation and propagation of microfractures and their contribution to material failure in compression are examined. The early part of the fracture process, the lateral and the axial yield points of the stress-strain curves, are identified with the onset of microfracture, respectively at the tensile and the compressive stress concentrations of the elastic flaw boundary. Later stages, including the initiation of inclined shear fractures, the mobilization of total resistance and the reduction of strength to the residual level, are discussed in terms of a modified Coulomb model.

Estimating the strength of rock materials

Abstract: A practical approach for estimating the strength behaviour of rock materials is presented. The demand for data on rock properties in engineering design is considered, and it is shown that, based on the trends observed in the South African mining industry during the last twenty years, simple and easy-to-use methods for the estimation of the uniaxial and triaxial strength of rock materials are needed. It is shown that the uniaxial compressive strength of rock can be conveniently determined from the point-load strength index, which is obtained underground on unprepared rock cores. The triaxial strength of rock can be estimated for most practical purposes from empirical strength criteria. Two such criteria are proposed that allow estimation of the triaxial strength for rock materials to about 10 per cent. The only input required for these criteria is the uniaxial compressive strength. The validity of the practical approach outlined in this paper is confirmed from the experimental tests conducted on some 700 rock specimens representing five rock types. A practical example for the prediction of rock strength is given.

Waste Glass as Coarse Aggregate for Concrete

Abstract: The paper reports on the performance of 34 different concrete mixes containing glass crushed to ¾-in. (19-mm) maximum size as coarse aggregate and six reference mixes made with gravel of the same size. Two cements of alkali equivalent 0.58 and 1.13, classifiable as low and high alkali (ASTM C 150-72), in amounts ranging from 400–900 lb/yd3 (237–534 kg/m3 were used in combination with glass both with the fines removed and in the as-crushed condition. Partial cement replacement with fly ash and mixing of glass with gravel aggregate were included in an attempt to find a suitable method of overcoming the expected adverse effects of the reaction between glass and cement alkalis. On the basis of compressive strength, flexural strength, expansion, and visible surface deterioration recorded up to an age of one year, the results show that in many cases the direct combination of glass with portland cement yields concrete which exhibits marked strength regression and excessive expansion due to alkali-aggregate reaction. The conditions under which performance is satisfactory appear to relate to limiting maximum values of cement content and alkali equivalent. Replacement of 25 to 30 percent by weight of the cement, whether low or high alkali, appears to be an effective and widely applicable method of ensuring good long-term concrete performance, although the minimum required in any given case may be related to cement composition.

Compressive behaviour of Kevlar 49 fibres and composites

Abstract: The low compressive strength of Kevlar 49® unidirectional composites cannot be satisfactorily explained in terms of current theories which assume that failure is due to the matrix material. For a given matrix, Kevlar 49 composites are considerably weaker in compression than those based on other comparable high strength, high modulus filaments. Fracture is found to occur before any plastic deformation of the matrix is observed.

Review of a new shear-strength criterion for rock joints

Abstract: Direct shear tests performed on model tension fractures have provided a very realistic picture of the behavior of unfilled joints at the roughest end of the joint spectrum. The peak shear strength of rough-undulating joints such as tension surfaces can now be predicted with acceptable accuracy from a knowledge of only one parameter, namely the effective joint wall compressive strength or JCS value. For an unweathered joint this will be simply the unconfined compression strength of the unweathered rock. However in most cases joint walls will be weathered to some degree. Methods of estimating the strength of the weathered rock are discussed. The predicted values of shear strength compare favourably with experimental results reported in the literature, both for weathered and unweathered rough joints. The shear strength of unfilled joints of intermediate roughness presents a problem since at present there is insufficient detailed reporting of test results. In an effort to remedy this situation, a simple roughness classification method has been devised which has a sliding scale of roughness. The curvature of the proposed strength envelopes reduces as the roughness coefficient reduces, and also varies with the strength of the weathered joint wall or unweathered rock, whichever is relevant. Values of the Coulomb parameters c and o fitted to the curves between the commonly used normal stress range of 5-20 kg/sg.km appear to agree quite closely with experimental results. The presence of water is found in practice to reduce the shear strength of rough unfilled joints but hardly to affect the strength of planar surfaces. This result is also predicted by the proposed criterion for peak strength. The shear strength depends on the compressive strength which is itself reduced by the presence of water. The sliding scale of roughness incorporates a reduced contribution from the compressive strength as the joint roughness reduces. Based on the same model, it is possible to draw an interesting analogy between the effects of weathering, saturation, time to failure, and scale, on the shear strength of non-planar joints. Increasing these parameters causes a reduction in the compressive strength of the rock, and hence a reduction in the peak shear strength. Rough-undulating joints are most affected and smooth-nearly planar joints least of all.

Mechanical properties of human cancellous bone in the femoral head.

Abstract: The ultimate compressive strength of human cancellous bone derived from femoral heads was found to range between 21 and 1963 Ibf/in2 (145-13535 kN/m2). A correlation coefficient of 0·70 was found for the relationship between compressive strength and apparent density of cancellous bone; however, no statistically significant correlation was found between compressive strength and location of the cancellous bone within a femoral head. The stress relaxation of cancellous bone, measured at widely differing initial stresses, demonstrates the viscoelastic nature of cancellous bone, and consequently the tendency to flow or creep under time-dependent stresses.

A Study of Sintered Apatites

Abstract: Hydroxyapatite, fluorapatite, and chlorapatite materials were synthesized and their sintering characteristics were studied. The compressive strength of the apatites was determined and is given as a function of density. Scanning electron micrographs of the typical material structure are given. The degradation of strength of the hydroxyapatite after an extended acid soak also is reported.

No-fines concrete - its properties and applications

Abstract: No-fines concrete consists solely of normal portland cement, water and coarse aggregate. It has been used in Europe and the United Kingdom since the 1930s for the building of single story and multistory dwellings, but had found little acceptance in North America. In recent years, however, due to increased awareness of the need for conservation of nonrenewable mineral resources, increased consideration is being given to the use of no-fines concrete in Canada and the United States. The compressive strength of no-fines concrete is considerably lower than that of conventional portland cement concrete and varies between 200 to 2000 psi (1.37 to 13.73 MN/square meters ). Young's modulus of elasticity is usually between 1.0 x 10,000,000 to 1.5 x 10,000,000 psi (0.7 x 100,000 to 1.2 x 100,000 MN/square meters) depending on the strength level of the concrete. The ratio of modulus of rupture to compressive strength expressed as a percentage varies between 10.8 and 42.0 percent. The shrinkage of no-fines concrete made with crushed limestone or river gravel, is of the order of 200 x 0.000001. This is about half for that of conventional concrete. Investigations at CANMET have indicated that no-fines concrete prisms with no air-entraining agent had poor resistance to freeze-thaw cycling; the corresponding prisms incorporating an air-entraining agent were able to withstand up to 274 freeze-thaw cycles compared with 56 for prisms without an air-entraining agent. The principal advantages claimed for no-fines concrete are economy in materials, somewhat higher thermal insulating values, lower shrinkage, and lower unit weight. The major disadvantages are its low compressive, flexural, and bond strength, and higher permeability. The principal applications of no-fines concrete are for load-bearing cast-in-place external walls of single story and multistory housing, small retaining walls and as a dampproofing subbase material for concrete floors cast on grade. This type of concrete is also eminently suitable for construction in northern Canada because of its somewhat higher thermal insulating property and low cement content.

Shearing strengths of kaolinite, illite, and montmorillonite

Abstract: During the years 1957-1970, over 400 S and R triaxial compression tests were performed using sedimented, slurried, or remolded specimens of kaolinite, illite, montmorillonite, prepared with several adsorbed cations and ranges in pore- water electrolyte concentration and pH, to study the relative effects of physicochemical variables on the shearing properties. The strengths were found to be controlled mainly by physical effects related to the size and shape of individual particles. Diffuse double-layer forces were never found to be significant for calcium clays. They were also insignificant for sodium kaolinite but were effective in reducing the strength of sodium illite, and caused major loss in strength for sodium montmorillonite by causing a great decrease in particle thickness thus leading to highly flexible particles of low physical strength. /ASCE/

Effect of Size on Tension Perpendicular-To-Grain Strength of Douglas-Fir

Abstract: The strength of wood in tension perpendicular-to-grain has been studied by several authors and found to depend on specimen geometry In this paper, the weakest-link concept has been applied to predict the relationship between specimen volume and load-carrying capacity for Douglas-fir specimens loaded in uniform tension perpendicular-to-grain The theory allowed the prediction that logarithm of maximum strength should decrease linearly with logarithm of volume Experimental data taken from the literature were used to evaluate the theoretical model and agreement was found to be high (R 2 ≥ 085) Average strength of a unit volume is approximately 460 psi, whereas the predicted strength of a 10- X 10- X 20-inch specimen (2000 inches 3 ) is approximately 100 psi The magnitude of the size effect may depend on the quality of material in the specimens, but certainly any rational development of working stresses for tension perpendicular-to-grain must consider effects of specimen (or structural component) size

Magnesium phosphate concrete compositions

Abstract: The setting time of quick-setting magnesium oxide-ammonium phosphate concretes is extended by the addition of specified oxy-boron compounds such as sodium borate. The incorporation of the oxy-boron compound in the composition has also been found to increase the extended or overall compressive strength of the composition.

Creep of Frozen Silt and Clay.

Abstract: Unconfined compressive creep strengths and strains were measured for remolded saturated frozen Hanover silt and Suffield clay. The creep tests were conducted at the approximate stress levels of 60, 35, 20 and 5 percent of the conventional unconfined compressive strength. Testing temperatures were 15, 25, 29, and 31 degrees F (minus 9.45, minus 3.89, minus 1.67, and minus 0.56 degrees C). An equation was found for the variation of unconfined compression peak strength with temperature. Unconfined compression creep strength could be estimated by Vialov's strength equation. Long-term creep strength was less than 45 percent of unconfined compression strength and could be as low as 10 percent of this strength. An equation for the increase in long-term creep strength with the decrease in soil temperature was also found. An expression was derived for obtaining a close estimate of the long-term creep strain. The results of this investigation were in general agreement with findings of other investigators for frozen silt and clay except that Hanover silt and Suffield clay did not display the classical creep curve shape.

Imparting strength and toughness to brittle composites

Abstract: Appropriate intermittent coatings of fibers can produce areas of low and high toughness in brittle composites. Experiments using silicon vacuum grease (SVG) and polyurethane varnish (PUV) coatings that achieve weakly and strongly bonded interfaces are described. Tensile strength and edge-crack fracture toughness for both SVG and PUV coatings were plotted against the percentage coating (C). Both coating materials maintain tensile strengths in the order of the rule of mixture strength values up to a large C. It is suggested that both materials produce similar coated interfacial shear strengths while producing different effects on toughness.

Microbuckling of Lamina-Reinforced Composites

Abstract: Compressive strength and failure modes of unidirectionally lamina-reinforced composites were studied both theoretically and experimentally. The failure modes investigated included microbuckling and compressive strength failure of the reinforcement. The influence of the following parameters on compressive strength was investigated experimentally: lamina volume fraction, lamina end fixity, lamina thickness, specimen geometry, and matrix properties. The two-dimensional microbuckling theory showed good correlation with test data from specimens designed to fail by microbuckling. As the Young's modulus of the matrix increased, the failure mode changed from microbuckling to compressive failure of the reinforcement.