Showing papers on "Compressive strength published in 1990"

Mechanical properties of trabecular bone from the proximal femur: a quantitative CT study.

Abstract: We have investigated the relationships between trabecular bone compressive strength and elastic modulus and the directly measured apparent density and noninvasively measured CT equivalent mineral density for 49 cylindrical specimens harvested from fresh human proximal femora. Compressive strength demonstrated a high positive correlation with both densities, being proportional to the apparent density raised to the 1.8 power (R2 = 0.93) and equivalent mineral density to the 1.5 power (R2 = 0.89). Similarly, the compressive modulus demonstrated a high correlation with both density measures, being proportional to the apparent density raised to the 1.4 power (R2 = 0.91) and CT equivalent mineral density to the 1.2 power (R2 = 0.90). Though variations in architecture and bone marrow fat were observed to influence trabecular properties, the data presented here demonstrate that apparent density, compressive strength, and elastic modulus can be determined accurately using single energy quantitative CT. We expect that the use of these noninvasive data will result in improved estimates of that component of hip fracture risk that is attributable to bone strength.

Correlating schmidt hardness with compressive strength and young’s modulus of carbonate rocks

Abstract: Schmidt hammer has increasingly been used world-wide as an index test for a quick rock strength and deformability characterization. The reason is mainly due to its rapidity and easyness in execution, non destructiveness, simplicity, portability and low cost.

Mechanical Properties of Hardened Cement Paste Exposed to Temperatures up to 700 C (1292 F)

Abstract: Temperatures of significance are identified in the range 20 to 700 C (68 to 1292 F) for changes in strength and elastic modulus both static and dynamic) of unsealed hardened cement paste measured at the various temperaturues as well as after cooling. These changes are correlated with chemical microstructural changes reported in the literature. The beneficial effects of thermal drying and loading, within limits, upon these mechanical properties are observed. It is concluded that these properties are dependent primarily on the maximum temperature of exposure as opposed to the temperature at testing.

The effect of cell size on the mechanical behavior of cellular materials

Abstract: The Young's modulus, strength and fracture toughness, of a brittle reticulated vitreous carbon foam, was measured as a function of cell size at a constant density and compared to a theoretical model Image analysis was used to characterize the macrostructure of the samples and provided a basis for evaluating the mechanical behavior It was determined that both the compressive and bend strength scale inversely with cell size The change in compressive strength is due to a change in the strut strength with cell size The bend strength behavior may be due to a reduction in the critical flaw size, as well as the increasing strut strength at smaller cell sizes The fracture toughness and elastic modulus were found to be independent of cell size Comparison of these results with previous work on open cell alumina clearly indicates a very different behavior and is attributed to a change in the microstructure of the solid phase with cell size in the alumina materials

Effect of Coarse Aggregate Characteristics on Mechanical Properties of High-Strength Concrete

Abstract: An experimental study investigated the influence of 4 coarse aggregate types available in Northern California on the compressive strength and elastic behavior of a very high-strength concrete mixture. Using identical materials and similar mix proportions, it was found using diabase and limestone aggregates produced concretes with significantly higher strength and elastic modulus than those using granite and river gravel. The mineralogical differences in the aggregate types are considered to be responsible for this behavior.

Effect of coarse-aggregate characteritics on mechanical properties of high-strength concrete

Abstract: An experimental study investigated the influence of 4 coarse aggregate types available in Northern California on the compressive strength and elastic behavior of a very high-strength concrete mixture. Using identical materials and similar mix proportions, it was found using diabase and limestone aggregates produced concretes with significantly higher strength and elastic modulus than those using granite and river gravel. The mineralogical differences in the aggregate types are considered to be responsible for this behavior.

Size effect in concrete specimens with dissimilar initial cracks

Abstract: In most structural members with initial cracks (or initial notches), strength tends to decrease as the member size increases: a phenomenon known as size effect. In contrast to other structural materials (wood, steel etc.), concrete shows the size effect with or without initial crack. Published experimental results indicate that even a large concrete member without initial crack resists some stress, contrary to Bazant’s size effect law. In this study, the size effect for concrete with dissimilar initial cracks is discussed on the basis of non-linear fracture mechanics, and prediction models are proposed for splitting tensile strength, shear strength and uniaxial compressive strength tests. The models are derived using regression analyses, with extensive experimental data relating to specimens of various sizes; it is felt that they perform much better than Bazant model, particularly for very large specimens.

Bending strength of lumber

Abstract: This is the description of a model that explains the relationship between the three strength properties, tension, compression and bending. The model which included nonlinear compresion behavior and size dependent brittle tension behavior of wood, has been calibrated and verified with reference to test results. Although it could be used for a single piece of lumber, the model is more useful to predict the bending strength of lumber throughout the distribution of strength, given the distribution of tension and compression strengths for a given size, species, and grade.

Strain-rate sensitive behavior of cement paste and mortar in compression

Abstract: The strain-rate sensitivity of the cement paste and mortar constituents of concrete is studied experimentally. Saturated cement paste and mortar specimens are loaded in compression to 15,000 microstrains, 27 to 29 days after casting, using strain rates ranging from 0.3 to 300,000 microstrains/sec. Water-cement ratios of 0.3, 0.4, and 0.5 are used. Strain-rate sensitivity of the material is measured in terms of the initial elastic moduli, maximum stress, and corresponding strain. The initial elastic moduli and the strength of cement paste and mortar increase by 7% and 15%, respectively, with each order of magnitude increase in strain rate. The strain at the maximum stress is the greatest for the lowest strain rate. With an increase in strain rate, the strain at the maximum stress first decreases and then increases.

Compressional behaviour of carbon fibres

Abstract: Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.

Effect of condensed silica fume on the steel-concrete bond

Abstract: This is a study of the effect of condensed silica fume (CSF) on the mechanical behavior of the steel-concrete bond, and on the microstructure of the steel-cement paste transition zone. The mechanical behavior of the steel-cement was studied by using the pullout test on concrete of varying compressive strength, and varying contents of CSF. Increased additions of CSF up to 16 % by weight of cement showed an improving effect on the pullout strength, especially in the high compressive strength range of the concrete. The presence of CSF affected the morpohology and microstructure of the steel-cement paste transition zone. Thus, both porosity and thicknes of the transition zone were reduced. The observed effect of CSF maybe explained by several mechanisms: reduced accumulation of free water at the interface during castng of the specimens; reduced preferential orientation of the CH crystals at the steel-paste transition zone; and densification of the transition zone due to pozzolanic reaction between CH and CSF.

Factors Affecting the Transverse Tensile Strength of Unidirectional Continuous Silicon Carbide Fibre Reinforced 6061 Aluminum

Abstract: A study of the micromechanics of continuous silicon carbide fibre rein forced 6061 aluminum has been carried out using generalised plane strain non-linear finite element analysis. An interface element has been developed enabling separate shear and tensile strengths to be assigned, with a quadratic interaction equation. Residual stresses due to manufacturing were included in the analysis.The effect on transverse tensile strength of fibre packing geometry, fibre spacing, resid ual stresses, interface strengths and matrix material properties were investigated. It was found that the interface strength is the most important factor. Residual stresses are beneficial, these being largely controlled by the yield strength of the matrix material at the time the residual stresses are set up. Fibre packing and spacing and matrix strength do not significantly affect predicted strength.

On the effect of rheology on seasonal sea-ice simulations

Abstract: On the seasonal time scales relevant to numerical investigations of climate, the rheology used in large-scale sea ice models significantly affects the ice thickness build-up and ice velocity fields. Plastic rheologies with a normal flow rule have been used to-date in seasonal dynamic thermodynamic simulations. These rheologies have proved useful in simulating discontinuous slip near the coast while still supplying relatively robust velocity fields in the central Arctic Basin. However, as indicated by limited numerical sensitivity studies with different types of elliptical yield curves, the amount of shear strength significantly affects the ice build-up and can possibly cause a stoppage of the ice outflow through Fram Strait. In addition to the shear strength issue, there is also the possibility that non-normal flow rule rheologies, such as the Mohr Coulomb failure criterion used in soil mechanics, may cause somewhat different types of flow patterns, especially in the Fram Strait region. However, to date no seasonal simulations with such non-normal flow rule rheologies have been carried out. In order to investigate the role of different rheologies on the large-scale flow patterns in the Arctic Basin, a more general numerical scheme than that of Hibler (1979) is developed, which allows the simulation of the dynamic thermodynamic behavior of sea ice with a wide variety of different non linear rheologies. Using this numerical scheme, comparative simulations are carried out to seasonal equilibrium with several variations of the Mohr Coulomb rheology and compared to the more standard Elliptical yield curve results. In particular, the main control Mohr Coulomb case is a capped rheology in which the shear strength is taken to be proportional to the compressive stress. In this capped case only shear flow is allowed until a maximum allowable compressive stress is reached. This cap strength is parameterized to be a function of the ice thickness and compactness. For comparison, a simulation with a very large cap strength is also carried out, and an experiment with a similar compressive cap but much lower shear strength. Overall the results are analyzed to determine the sensitivity of the ice build-up to flow rule and shear strength magnitude. In addition special attention is given to the character of the flow and stoppage (if any) through Fram Strait.

Deterioration in Strength of Stud Connectors in Composite Bridge Beams

Abstract: It is a common practice for the stud shear connections in composite bridge beams to be designed for both strength, to resist the ultimate loads, and for endurance, to resist the numerous applications of the serviceability loads. These two calculations are normally done independently of each other and the upper bound of the two connector distributions is used in design. This technique therefore assumes that there is no interaction between the strength of the shear connection and the fatigue loads. However, new experimental tests now show that the monotonic strength of stud shear connectors reduces under fatigue loads, and hence present design techniques do not simulate the actual behavior of the shear connection. A method is given of designing the shear connection to allow for the reduction in the monotonic strength due to fatigue loads that can also be used to estimate the residual strength of the shear connection in existing bridge beams.

High-Strength and Flowing Concrete with a Zeolitic Mineral Admixture

Abstract: Zeolitic mineral admixture (ZMA) is made of the finely divided powder of natural zeolite with a bit of other agent such as triethanolamine. When ZMA is used to displace about 10% (by mass) of the ordinary portland cement (OPC) (strength grade No. 525) in concrete and mixed with a suitable amount of superplasticizer (W/C = 0.31 to 0.35), then a high-strength concrete with compressive strength of about 80 MPa and a slump of about 180 mm can be obtained. The strength of this concrete is about 10 to 15% higher than that of the corresponding concrete mixed with pure OPC, and its bleeding decreases greatly. It also results in no segregation or separation of the mix, and thus it satisfies the requirement of pumping concrete in construction. The ZMA is suitable not only for the OPC but also for the slag portland cement. The strengthening effect of the ZMA is somewhat similar to that of silica fume. But the cost is only two thirds that of OPC. Thus, when ZMA is used to displace a certain amount of the cement in the concrete, the cost of the concrete thus made will be 3 to 5% cheaper than that of the corresponding concrete with pure cement. The ZMA can increase the amount of micropores (d 938 A) in the cement paste. Hence, the strength of concrete is increased and its other properties are also improved. Furthermore, ZMA can raise the SiO2/CaO weight ratio in the transition zone to increase its C-S-H phase and decrease its calcium hydroxide content. Thus, the structure of the transition zone is improved. Consequently, the strength and resistance to permeability of the concrete are increased.

Strength and deformation characteristics of reinforced concrete walls under load reversals

Abstract: The effects of loding history and repair methods on the structural characteristics of reinforced concrete walls was investigated. Large scale slender wall models were tested to failure, then unloaded, repaired, and retested to destruction under various regimes of cyclic horizontal loading. It was found that, while repairing only the damaged regions of the compressive zone was sufficient to fully restore wall strength, the additional use of epoxy resins to heal major flexural and inclined web cracks led only to a marginal improvement of the structural characteristics, the latter being distinctly inferior to those of the original walls. Such results are in compliance with the concept of the compressive force path and demonstrate that, in contrast to widely held views, the compressive zone is the main contributor to shear resistance.

Some engineering properties of slag concrete as influenced by mix proportioning and curing

Abstract: This paper presents a simple method to obtain a 50 MPa 28-day strength concrete having 50 and 65 % by weight cement replacements with slag having a relatively low specific surface. The method produces slag concrete with strengths comparable to ordinary portland cement concrete from 3 days onward. The compressive and flexural strengths and the elastic modulus of these two concretes as affected by curing conditions are then presented. Prolonged dry curing is shown to adversely affect tensile strength and elastic modulus, and to create internal microcracking, as identified by pulse velocity mearurements. High swelling strains at high slag replacement levels show the need for longer wet curing for such concretes. The results emphasize that even 7-day wet curing was inadequate for high levels of slag replacement, and that continued exposure to a drying environment can have adverse effects on the long-term durability of inadequately cured slag concrete.

Performance of epoxy and polyester polymer concrete

Abstract: The behavior of epoxy and polyester concretes was studied under various curing conditions, temperatures, and strain rates. The influence of aggregate size and distribution on the mechanical properties of polymer concrete (PC) was also investigated. The strain rate was varied between 22 and 120 C (72 and 248 F). The strength, failure strain, modulus, and stress-strain per minute, and the temmperature between 22 and 120 C (72 and 248 F). The strength , failure strain, modulus, and stress-strain relationship of polymer concrete systems are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of polymer concrete systems are quantified. Compared to the uniformly graded fine aggregates, the gap-graded aggregates produced polymer concrete with better mechanical properties. The compressive modulus and splitting-tensile strength of polymer concrete are related to their compressive strength. It was found that the property relationsips recommended by ACI codes and others specifically developed for high-strength cement concrete are not directly applicable to the PC systems investigated in this study. A new constitutive model is proposed to predict the complete compressive stress-strain behavior of epoxy and polyester polymer concrete systems. Analytical expressions relating the parameters of the constitutive model to the testing temperature and strain rate are derived.

Axial compressive strength of carbon fiber

Abstract: Efforts were made to estimate the axial compressive strengths of carbon fibers from the fiber fragment lengths produced by subjecting to a strain greater than the fiber ultimate strain for PAN-based and pitch-based carbon fibers. The estimated compressive strength of carbon fibers decreases with increasing temperature in a temperature range from room temperature to 100°C. This decrease in compressive strength may be accounted for by a decrease in the radial compressing force. The real compressive strength, determined by extrapolating a linear relationship between the estimated compressive strength and the radial compressing force, is approximately 25–60% of tensile strength for PAN-based fibers, while it is approximately 10–35% for pitch-based fibers.

Femoral neck fracture fixation. Comparison of a sliding screw with lag screws.

Abstract: The rigidity of a sliding compression screw and three cannulated lag screws in the treatment of subcapital fractures was compared in five pairs of female cadaver femora. There were no significant differences between the compressive strength, bone density, cortical thickness or Singh index of the bones in each pair. A subcapital fracture was standardised using a perpendicular saw cut across the femoral neck. A uniaxial 'load test system' with force and length measurement facilities was used to mimic cyclical stressing applied in vivo at a frequency of 0.5 Hz from 0 to 3 times body-weight. There was no significant difference between the fixation afforded by the sliding compression screw and three lag screws. Bone quality was the single most important factor in the stability of the bone implant unit.

Ultimate shear strengths of copper-silica and nickel-silica interfaces

Abstract: A technique developed recently at Cornell University to measure the ultimate shear strength of metal-ceramic interfaces is used to measure the relative strength of the copper-silica and nickel-silica interfaces. The shear strength of the copper-silica interface is found to be 0.9 GPa in comparison with 1.4 GPa for the nickel-silica interface. The difference is significantly greater than the margin of error in the technique, which is estimated to be less than 20%. The technique consists of depositing a thin film of the ceramic on a thick metal substrate. When the metal is deformed plastically in tension, the ceramic film develops nearly equispaced cracks that are aligned normal to the loading axis. The spacing between the cracks is inversely proportional to the shear strength of the interface. For a given interfacial strength, the spacing varies linearly with the thickness of the film; data which confirm this relationship are also presented. The possible effect of film thickness on delamination vs. shear yielding at the interface is discussed.