Showing papers on "Compressive strength published in 1993"

Rubber-Tire Particles as Concrete Aggregate

Abstract: Accumulations of worn‐out automobile tires create fire and health hazards. As a possible solution to the problem of scrap‐tire disposal, an experimental study was conducted to examine the potential of using tire chips and crumb rubber as aggregate in portland‐cement concrete. This paper examines strength and toughness properties of concrete in which different amounts of rubber‐tire particles of several sizes were used as aggregate. The concrete mixtures exhibited lower compressive and splitting‐tensile strength than did normal concrete. However, these mixtures did not demonstrate brittle failure, but rather a ductile, plastic failure, and had the ability to absorb a large amount of plastic energy under compressive and tensile loads. A mathematical model is used to describe the effects of rubber aggregate on the compressive and tensile strength reduction of concrete.

Carbon fiber reinforced concrete for smart structures capable of non-destructive flaw detection

Abstract: Electrically conducting concrete, as provided by the addition of a short carbon fibers (0.2-0.4 vol.%) to concrete, can function as smart structure material that allows non-destructive electrical probing for the monitoring of flaws. The electrical signal is related to an increase in the concrete's volume resistivity during crack generation or propagation and a decrease in the resistivity during crack closure. The linearity between the volume resistivity change and the compressive stress was good for mortar containing carbon fibers together with either methylcellulose or latex as dispersants. However, the linearity was poor for mortar containing carbon fibers together with both methylcellulose and silica fume, as this mortar required a minimum compressive stress for crack closure, whereas the other two mortars did not.

Structural Design Considerations for High-Strength Concrete

Abstract: This paper highlights aspects of structural design in which traditional procedures may need to be modified to account for the different characteristics of high-strength concrete. The stress-strain response of such concretes is considered, as well as the capacity of columns and the minimum reinforcement and ductility in tension and flexural members. The shear strength of beams is also considered.

Material and environmental factors influencing the compressive strength of unsealed cement paste and concrete at high temperatures

Abstract: Results of uniaxial compressive strength measurements taken on 1210 unsealed cement paste and concrete specimens heated to temperatures up to 600°C are reported; they show that both material and environmental factors have a significant influence during the heat cycle and afer cooling. The best results were obtained using firebrick aggregate and a cement blend containing 65 % slag replacement of OPC by weight. At 600°C the “hot” compressive strength of this material was 85% of the value measured before heating.

Bond Performance of Reinforcing Bars Embedded in High-Strength Concrete

Abstract: Because of the lack of test data, ACI-318-89 building code requirements imposed an arbitrary upper limit of 10,000 psi (69 MPa) on specified compressive strength of concrete that may be used in calculating tension development length and tension splice length. A research study was conducted to evaluate the bond performance of reinforcing bars embedded in high-strength concrete. Twelve beam splice specimens using No. 11 reinforcing bars and concrete with compressive strengths exceeding 14,000 psi were tested. The effect of several variables on bond capacity in high strength concrete is discussed and a failure hypothesis explaining the observed behavior during the experimental phase of the study is presented. Additionally, it is concluded that the current trend in ACI of making the splice length longer to compensate for having small cover and spacing may not be an effective approach. A better approach would be to provide a minimum amount of transverse reinforcement.

Brittle strength of basaltic rock masses with applications to Venus

Abstract: Laboratory measurements of rock deformation in the brittle regime provide constraints on the response of rocks to stress. These values are essential parameters in tectonic models of near-surface deformation because they influence both the stress state and the conditions for predicting the types and occurrences of brittle structures such as joints and faults. However, additional parameters must be included before the laboratory values can be used to construct brittle strength envelopes for near-surface materials. The properties of basaltic rock masses provide a more precise estimate of the strengths of basaltic lava flows on the terrestrial planets than other, more widely used approaches (intact rock or frictional strength of a through-going surface). Rock mass strength is defined by three parameters including unconfined compressive strength of intact basalt and two others related to the degree of fracturing of the material. Experimental results for elevated temperature extend the applicability of these parameters to the near-surface environment of Venus. Representative values of strength parameters for intact basalt at ambient temperature (20°C)and negligible confining pressure are: Young's modulus, 73 GPa; Poisson's ratio, 0.25; tensile strength, −14 MPa; unconfined compressive strength, 262 MPa; fracture toughness, 1–3 MPa m½ cohesion, 66 MPa; and coefficient of friction, 0.6. At elevated temperature (∼450°C) and zero confining pressure, reference values for the strength of intact basalt are: Young's modulus, 57 GPa; Poisson's ratio, 0.25; unconfined compressive strength, 210 MPa; and fracture toughness, 2–2.8 MPa m½. Corresponding values for a basaltic rock mass that incorporate the weakening effects of scale (but not elevated temperature) are: Deformation modulus, 5–50 GPa; Poisson's ratio, 0.3; tensile strength, −0.2 to −2 MPa; uniaxial compressive strength, 12–63 MPa; cohesion, 0.5–6 MPa. Values of tensile and cohesive strength for the basaltic rock mass are approximately one to two orders of magnitude lower than corresponding values for intact basalt. Temperatures comparable to those at the Venus surface may slightly increase the deformation modulus but decrease the compressive strength of the rock mass. Brittle strength envelopes for the rock mass as a function of depth are typically stronger in both extension and compression than conventional envelopes that assume a simple frictional strength. These results indicate that the strengths of basaltic rocks on planetary surfaces and in the shallow subsurface are significantly different from strength values commonly used in tectonic modeling studies which assume properties of either intact rock samples or single planar shear surfaces.

Enhanced Mechanical Properties of TiNi Shape Memory Fiber/Al Matrix Composite

Abstract: A design concept of shape memory TiNi fiber reinforced/Al metal matrix composite (SM-MMC) was proposed. Mechanical tensile properties such as stiffness and yield strength, were improved by the strengthening mechanisms: back stress in the Al matrix induced by stiffness of TiNi fibers and the compressive stress in the matrix caused by shape memory shrinkage of TiNi fibers. Damping capacity of the composite was also increased. These results suggest that this composite with prestrain can be applicable and is suitable for machinery, especially engine components where the material becomes stronger at higher temperatures owing to the shape memory effect

Influence of high strength concrete on transfer and development length of pretensioning strand

Abstract: Twenty-two precast, pretensioned concrete beam specimens were fabricated and tested to determine the influence of concrete strength on the transfer length and development length of pretensioning strand. The main variables were the concrete compressive strength, varying from 4500 to 12,900 psi (31 to 89 MPa), and the strand diameter, which included 3/8, 1/2 and 0.62 in. (9.5, 12.7 and 15.7 mm) diameters. Expressions are given for the influence of concrete strength on the transfer length and development length of pretensioning strand.

New Aspects of the Setting of Glass-ionomer Cements

Abstract: For many years, glass-ionomer cements have been described as setting by the formation of a poly(acrylate) matrix. Recent research has suggested that a second reaction may be involved, namely, the formation of a silica matrix. So that this hypothesis could be tested, non-polymer cements, based on an ionomer glass plus acetic acid, were prepared and stored for up to six months. They were insoluble in water, and their compressive strength was found to increase rapidly over the period of storage. By contrast, the product of the reaction between ZnO and acetic acid was soluble in water. These results support the idea that there is a secondary setting reaction in glass ionomers and suggest that it is responsible for the increase in strength observed.

Compressive failure of notched carbon fibre composites

Abstract: We present an investigation into the compressive fracture properties of carbon fibre/epoxy laminates. Unnotched and notched strengths are reported for a wide range of ‘lay-ups’, and results are interpreted in terms of existing theories of strength. In all cases, we find that compressive failure is governed by plastic microbuckling of the 0° plies. For unnotched laminates, the failure strain is independent of lay-up configuration, suggesting that a critical strain to failure, or maximum strain criterion can predict the failure point adequately. In tests on notched panels, the growth of damage from the edge of a single hole is examined. For notched materials, the failure strength and damage zone size at failure support the prediction of the Soutis, Fleck and Smith model.

Behavior of Fiber-Reinforced Cemented Sand Under Static and Cyclic Loads

Abstract: Triaxial static compression, cyclic compression, and splitting tension tests were performed to evaluate the effect of randomly distributed fiber reinforcement on the response of cemented sand to load. Test results indicated that fiber reinforcement significantly increases the compressive and splitting tensile strength of cemented sand. An increase in the compressive and tensile strength was found to be more pronounced at higher fiber contents and longer fiber lengths. Peak strength envelopes in compression indicated that both the friction angle and cohesion intercept of cemented sand were increased as a result of fiber inclusion. Inclusion of fibers also contributed to increased brittleness index of cemented sand while increasing its total energy absorption capacity. Fiber reinforcement also affected the response of cemented sand to cyclic load by significantly increasing the number of cycles, and the magnitude of cyclic strain needed to reach failure.

Generation and applications of compressive stress induced by low energy ion beam bombardment

Abstract: Compressive stress is a widespread phenomenon in films subjected to ion beam bombardment. A thermodynamic treatment of materials under a general stress is used to show that preferred orientation will occur as a result of free energy minimization in a biaxial stress field. The equilibrium between structural phases of materials is also affected by stress. Thermodynamics is used to calculate the equilibrium between graphite and diamond under biaxial stress. Recent molecular dynamics studies of the mechanism of compressive stress generation are reviewed in which the phenomena of focused collision sequences and thermal spikes are studied. Experimental work shows compressive stress is linked to preferred orientation in hexagonal boron nitride films and to stabilization of high pressure phases such as cubic BN and tetrahedral amorphous carbon films, in agreement with the thermodynamic analysis.

Experimental and Numerical Analysis of High Strain Rate Splitting-Tensile Tests

Abstract: Splitting tensile concrete specimens were tested at strain rates of 10 to the power -7/sec to 10 to the power 2/sec in a low speed material test machine and in a 50.8-mm diameter Split Hopkinson Pressure Bar (SHPB). A comprehensive finite element method (FEM) analysis was conducted on the same test speciments. A high-speed framing camera was used to record the gross deformation and cracking during the fracture process in the SHPB tests. In addition, an ultra high speed image converter camera with equivalent framing rates of 10,000 to 1,000,000 frames per sec was used to record some of the early crack formations during the fracture process in the SHPB tests. Results of tensile strength versus strain rate are presented and compared with compressive strength at similar strain rates. These same tensile data are compared with strength data obtained using a fracture mechanics model. Computer generated crack patterns are presented and compared to experimentally observed crack patterns in the fracture of concrete at high strain rates.

Carbon fibre compressive strength and its dependence on structure and morphology

Abstract: The axial compressive strength of carbon fibres varies with the fibre tensile modulus and precursor material. While the development of tensile modulus and strength in carbon fibres has been the subject of numerous investigations, increasing attention is now being paid to the fibre and the composite compressive strength. In the present investigation, pitch- and PAN-based carbon fibres with wide-ranging moduli and compressive strengths were chosen for a study of fibre structure and morphology. A rayon-based carbon fibre was also included in this study. Structural parameters (L c, La(0), L a(90), orientation parameter Z, and the spacing between graphitic planes d(00, 2)) were determined from wide angle X-ray spectroscopy (WAXS). Fibre morphology was characterized using high-resolution scanning electron microscopy (HRSEM) of fractured fibre cross-sections. The mechanical properties of the fibres, including compressive strength, the structural parameters from WAXS, and the morphology determined from HRSEM are reported. The influence of structure and morphology on the fibre compressive strength is discussed. This study suggests that the width of the graphitic sheets, the crystallite size perpendicular to the fibre axis (L c and L a(0)), and crystal anisotropy play significant roles in accounting for the large differences in compressive strengths of various carbon fibres.

Measurement of interface strength by the modified laser spallation technique. I. Experiment and simulation of the spallation process

Abstract: Part I of this paper presents the modified laser spallation technique for measuring the tensile strength of planar interfaces, using a Doppler displacement interferometer. In this technique, a laser‐produced compressive stress pulse in the substrate, reflecting from the coating’s free surface pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. The current technique is an improvement over the previous one, since the interface stress is determined directly by recording the coating or substrate free‐surface velocities using a laser displacement interferometer. The recorded surface velocity is related to the interface stress via an elastic wave equation simulation. The process of coating spallation is investigated, and the effect of the stress pulse profile and the coating and substrate characteristics on the interface tensile stress is studied using the simulation. Several interface stress charts are given for wider applicability of the modified technique.

Progressive Failure of Laminated Composites with a Hole under Compressive Loading

Abstract: A progressive failure model that was developed earlier for tensile loading is extended for laminated composites under uniaxial compressive loading. This model is capable of predicting the extent of...

Low-cost, high early strength, acid resistant pozzolanic cement

Abstract: A low cost cement composition that can be admixed with water and hydrothermally closed cured to give acid-resistant products of high compressive strength consisting essentially of, in parts by weight, 1 to 1.5 parts of a calcium oxide material containing at least about 60% CaO, 10 to 15 parts of pozzolanic material containing at least about 30% by weight amorphous glass or vitreous silica, and 0.025 to 0.075 parts by weight of an alkali metal catalyst and building materials made therefrom as well as the method of making such building materials by closed curing.

Tensile and compressive shaft capacity of piles in sand

Abstract: In most soil types, it is generally assumed that the shaft capacity of a pile is identical under both tensile and compressive loading. However, there is widespread experimental evidence that in sand the shaft capacity is significantly lower for tensile loading, or uplift, than for compressive loading. This paper describes the results of a parametric study that was conducted to explore the theoretical basis for such differences. It is shown that the primary cause of lower tensile capacity is due to a Poisson’s ratio effect, and that the ratio of tensile to compressive shaft capacity may be expressed as a function of the relative compressibility of the pile and the slenderness ratio. Design recommendations are proposed, and corroborated with results from high-quality field tests.

Micromechanical Modeling of Composite Compressive Strength

Abstract: Composite compressive failure is studied using a micromechanical (beam-on-elastic foundation) model, and the stiffness of the foundation is determined through an elasticity solution of the foundation model problem. An explicit expression has been derived for evaluation of composite longitudinal compressive strengths (CLCS). It is found that this expression predicts relatively lower CLCS compared to classical models, and closely matches some experimental data for carbon fiber/epoxy composites. It is found that a complete matrix slippage will reduce the CLCS by over 50%.

Effects of layer waviness on the compression strength of thermoplastic composite laminates

Abstract: Layer waviness was investigated in T300/P1700 carbon/polysulfone com posite laminates under static compression loading. A three-step procedure was used to fab ricate isolated layer waves into the c...