Showing papers on "Compressive strength published in 1997"

Seventeenth Canadian Geotechnical Colloquium: The effect of cohesion loss and stress path on brittle rock strength

Abstract: Stress-strain curves for brittle rocks show three characteristic stress levels: crack initiation, long-term strength, and peak strength. Damage-controlled testing at low confining stresses has shown that the long-term and peak strengths are sensitive to the amount of induced damage, i.e., the greater the amount of damage, the lower the long-term and peak strengths. These tests also showed that the brittle-failure process is characterized by a loss of cohesion as friction is mobilized. Excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel. The back-calculated strength for the failed rock around the tunnel is approximately one-half of that measured in laboratory tests. Crack-induced damage of Lac du Bonnet granite, both in the laboratory and in situ, begins when the load exceeds approximately one-third of the unconfined compressive strength. However, the stress level associated with failure is a function of loading path. In the laboratory, where the loading path monotonically increases, the ultimate strength of an unconfined sample is 225 MPa. Numerical studies suggest that in situ the loading path around the tunnel is more complex, involving stress increase and decrease and stress rotation. For this loading path, failure initiates at a stress between 100 and 120 MPa. Conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel. The results from the damage-controlled laboratory tests and the microseismic monitoring carried out during tunnel construction indicate a constant-deviatoric-stress criterion is a reliable indicator for predicting the onset of damage. This criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel. The fundamental assumption in the constant-deviatoric-stress criterion is that at low confining stresses, such as those which occur around underground openings, the brittle-failure process is dominated by cohesion loss.

Setting Reaction and Hardening of an Apatitic Calcium Phosphate Cement

Abstract: The combination of self-setting and biocompatibility makes calcium phosphate cements potentially useful materials for a variety of dental applications. The objective of this study was to investigate the setting and hardening mechanisms of a cement-type reaction leading to the formation of calcium-deficient hydroxyapatite at low temperature. Reactants used were alpha-tricalcium phosphate containing 17 wt% beta-tricalcium phosphate, and 2 wt% of precipitated hydroxyapatite as solid phase and an aqueous solution 2.5 wt% of disodium hydrogen phosphate as liquid phase. The transformation of the mixture was stopped at selected times by a freeze-drying techniques, so that the cement properties at various stages could be studied by means of x-ray diffraction, infrared spectroscopy, and scanning electron microscopy. Also, the compressive strength of the cement was measured as a function of time. The results showed that: (1) the cement setting was the result of the alpha-tricalcium phosphate hydrolysis, giving as a product calcium-deficient hydroxyapatite, while beta-tricalcium phosphate did not participate in the reaction; (2) the extent of conversion of alpha-TCP was nearly 80% after 24 hr; (3) both the extent of conversion and the compressive strength increased initially linearly with time, subsequently reaching a saturation level, with a strong correlation observed between them, indicating that the microstructural changes taking place as the setting reaction proceeded were responsible for the mechanical behavior of the cement; and (4) the microstructure of the set cement consisted of clusters of big plates with radial or parallel orientations in a matrix of small plate-like crystals.

Mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete

Abstract: This paper presents basic information on the mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete with compressive and flexural strengths up to 85.4 MPa and 11.8 MPa, respectively. The influence of steel fiber on modulus of elasticity and Poisson's ratio of concrete are investigated, and flexural fracture toughness is calculated. Test results show that the effect of fiber volume fraction (Vf) and aspect ratio (lfdf) on flexural strength and fracture toughness is extremely prominent, compressive strength is only slightly improved, and tensile/compressive strength ratio is obviously enhanced. It is observed that the flexural deflection corresponded to ultimate load increased with the increase of Vf and lfdf, and due to fiber arresting cracking, the shape of the descending branch of load-deflection tends towards gently.

Effect of Length on Compressive Strain Softening of Concrete

Abstract: The concept of localization during the postpeak of compressive strain softening is presented. To explore localization in compression, a feedback-control method using a linear combination of displacement and force that partially subtracts the elastic response of the specimen to give a stable feedback signal is used. Results are presented from two test series (45 and 90 MPa) that use this method to test cylinders with length-to-diameter ratios ranging from 2.0 to 5.5 It is shown that compression failure is in fact a localized phenomenon. The compressive fracture energy is divided into energy dissipated in the prepeak and the postpeak portions of the stress-deformation response. It is found that the amount of energy required to propagate the compression failure during postpeak is independent of length for this range of specimens. The compressive fracture energies are compared for the normal and high-strength concretes.

Tissue Engineering of Bone

Abstract: Bone is a dynamic, highly vascularized tissue with the unique capacity to heal and to remodel depending on line of stress (Buckwalter et al, 1995ab). It exhibits the unlikely combination of high compressive strength and tensile strength due to the composite of calcium phosphate salts (hydroxyapatite) and collagen, respectively (Yaszemski et al, 1996a). It is difficult to find materials to mimic such a complex system when filling bone defects. However, current research capitalizes on the dynamic properties of bone by providing a biodegradable scaffold to guide healing.

Mechanical properties and durability of two industrial reactive powder concretes

Abstract: Two reactive powder concretes (RPC) were produced on an industrial scale at the Universite de Sherbrooke and in a nearby precast plant. A 2.6 cubic meter mix was prepared in the central mixer of the precast plant. The ready mix RPC was sampled before and after the addition of steel fibers while the one produced at the precast plant was sampled only at the end of the mixing process. These RPCs were tested for compressive strength, modulus of elasticity, freezing and thawing cycling resistance, scaling resistance to deicing salts, and resistance to chloride ion penetration. The results show that a 200 MPa compressive strength could be achieved in both cases: after curing in hot water at 90 degrees C or in the low pressure steam chambers at the precast plant. Confinement of the RPC in a steel tube greatly increases its compressive strength and its ductility.

Strain-softening of concrete in uniaxial compression

Abstract: 0025-5432/97 © RILEM tory cast its own specimens following a prescribed recipe The pre-peak behaviour was found to be independent of specimen slenderness when low friction loading platens were used However, for all loading systems a strong increase of (post-peak) ductility was found with decreasing specimen slenderness Analysis of the results, and comparison with data from literature, showed that irrespective of the loading system used, a perfect localization of deformations occured in the post-peak regime, which was first recognised by Van Mier in a series of uniaxial compression tests on concrete between brushes in 1984 Based on the results of the Round Robin, a draft recommendation will be made for a test procedure to measure strain softening of concrete under uniaxial compression Although the post-peak stress-strain behaviour seems to be a mixture of material and structural behaviour, it appears that a test on either prismatic or cylindrical specimens of slenderness h/d = 2, loaded between low friction boundaries (for example by inserting sheets of tef lon between the steel loading platen and the specimen), yields reproducible results with relatively low scatter For normal strength concrete, the closed-loop test can be controlled by using the axial platen-to-platen deformation as a feed-back signal, whereas for high-strength concrete either a combination of axial and lateral deformation should be used, or a combination of axial deformation and axial load FOREWORD An extensive Round Robin test programme on compressive softening was carried out by the RILEM Technical Committee 148-SSC “Test methods for the Strain Softening response of Concrete” The goal was to develop a reliable standard test method for measuring strain softening of concrete under uniaxial compression The main variables in the test programme were the specimen slenderness h/d and the boundary restraint caused by the loading platen used in the experiments Both high friction and low friction loading systems were applied Besides these main variables, which are both related to the experimental environment under which softening is measured, two different concretes were tested: a normal strength concrete of approximately 45 MPa and a higher strength concrete of approximately 75 MPa In addition to the prescribed test variables, due to individual initiatives, the Round Robin also provided information on the effect of specimen shape and size The experiments revealed that under low boundary friction a constant compressive strength is measured irrespective of the specimen slenderness For high friction loading systems (plain steel loading platen), an increase of specimen strength is found with decreasing slenderness However, for slenderness greater than 2 (and up to 4), a constant strength was measured The shape of the stress-strain curves was very consistent, in spite of the fact that each laboraRILEM TC 148-SSC: TEST METHODS FOR THE STRAIN-SOFTENING RESPONSE OF CONCRETE

Properties of some cement stabilised compressed earth blocks and mortars

Abstract: Findings from an on-going investigation into the effects of soil properties and cement content on physical characteristics of compressed earth blocks and soil mortars are presented. A series of test blocks were fabricated using a range of composite soils, stabilised with 5% and 10% cement, and compacted with a manual press. Results for saturated compressive strength, drying shrinkage, wetting/drying durability, and water absorption testing are presented in the paper. In conjunction with the block tests, workability and compressive strength characteristics of suitable soil: cement and cement: lime: sand mortars were also studied. Mortar consistency was assessed using cone penetrometer and slump tests. Water retention properties of the mortars were also measured. For a given compactive effort, the strength, drying shrinkage, and durability characteristics of the compressed earth blocks improved with increasing cement and reducing clay content. Slump testing proved the most reliable means of assessing soil: cement mortar consistency. Both the flow table and cone penetrometer tests were found to be unsuitable. Water retention properties of soil: cement mortars appear well-suited to typical unit water absorption characteristics. Mortar strengths were closely related to cement and clay contents, but as expected were less than the average unit strengths.

The influence of aggregate on the compressive strength of normal and high-strength concrete

Abstract: This paper presents a comprehensive theory describing the influence of aggregate on the compressive strength of concrete. A first distinction is made between topological and mechanical aspects. The former, called confining effect, includes the effect of the volume and the maximum size of the aggregate, which are best described by means of a single physical parameter, the maximum paste thickness (MPT). MPT is defined as the mean distance between two adjacent coarse aggregates. Equations are given to calculate MPT and its effect on compressive strength. The second type of effect concerns the bond between past and aggregate (bond effect), and a limitation of the strength that originates in the intrinsic strength of the rock (ceiling effect). Experiments have been performed on 13 mixtures made up with five sources of aggregate, in order to compare the strength development on pure paste versus composites (mortar and concrete). The quantitative assessment of the three effects of aggregate on compressive strength gives an accuracy close to 2.2 MPa on the studied mixes. Such a model is suitable to be incorporated into software for computer-aided mixture-proportioning and quality-control of structural concrete, up to the high-performance concrete range.

Effects of hydroxypropyl methylcellulose and other gelling agents on the handling properties of calcium phosphate cement

Abstract: The calcium phosphate cement (CPC) used in this study was formed by combining equimolar amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). This powder, when mixed with water, sets to a hard cement in about 30 min. However, the water-based CPC paste is not highly cohesive and is vulnerable to washout until hardening occurs. The objectives of this study were to investigate the effects on handling properties, washout resistance, cement hardening behavior, and mechanical properties of adding several gelling agents to CPC paste. Aqueous solutions that contained a mass fraction of 2-4% of hydroxypropyl methylcellulose (HPMC), carboxyl methylcellulose (CMC), chitosan acetate, and chitosan lactate were used as cement liquids. Hardening time was measured by the Gilmore needle test; resistance to washout was evaluated by the disintegration of the cement specimen in water with agitation; and mechanical strength was evaluated by the measurement of diametral tensile strength and compressive strength. Handling properties were greatly improved by the addition of HPMC, CMC, chitosan acetate, and chitosan lactate. Hardening time was retarded by the additions of HPMC and CMC, and mechanical strength was weakened by the addition of either the chitosan lactate or the chitosan acetate.

Statistical tensile strength of Nextel TM 610 and Nextel TM 720 fibres

Abstract: The properties of fibre-reinforced composites are dependent not only on the strength of the reinforcement fibre but also the distribution of fibre strength. In this study, the single filament strength of several lots of NextelTM 610 and NextelTM 720 ceramic fibres was measured. Fracture statistics were correlated with the effects of gauge length and diameter variation, and the Weibull modulus was calculated using several different techniques. It was found that the measured Weibull modulus at a single gauge length did not accurately predict either the gauge length or diameter dependence of tensile strength.

Mechanical Properties of Modified Reactive Powder Concrete

Abstract: Original Reactive Powder Concrete (RPC) - in form of a superplasticized cement mixture with silica fume, steel fibers and ground fine quartz was studied in comparison with a modified RPC where a graded natural aggregate (max size 8 mm) was used to replace the fine sand and/or part of the cementitious binder. Original and modified RPC were manufactured at a plastic-fluid consistency, cast by vibration and cured at three different conditions: a) room temperature; b) steam-curing at 90 C; c) high pressure steam-curing at 160C. The addition of the graded aggregate does not reduce the compressive strength provided that the quality of the cement matrix, in terms of its water-cement ratio, is not changed. This result is in contrast with the model proposed to relate to high compressive strength level of RPC (200 MPa) to the absence of coarse aggregate. Both the original and modified RPC (with coarse aggregate addition) perform better - in terms of higher strength and lower drying shrinkage or creep strain - when they are steam cured rather than cured at room temperature. This improvement was related to a more dense microstructure of the cement matrix, particularly in the RPC specimens steam cured at 160 C. The main purpose of the present investigation was to modify RPC including some coarse aggregate in the mixture and to study the influence of the coarse aggregate on the properties of cement mixtures in terms of required mixing water, compressive and flexural strength, shrinkage, swelling and creep.

Parametric Study of Beams with Externally Bonded FRP Reinforcement

Abstract: FRP reinforcement may be externally bonded to the soffit of existing flexural members in order to increase their strength and rigidity. A parametric analysis is conducted to investigate the effects of FRP reinforcement on serviceability, strength, and failure mechanisms of repaired RC beams. FRP reinforcement parameters considered in the analysis are: stiffness, bonded length, thickness, and the adhesive stiffness. The choice of the repair material parameters is important in the design phase in order to obtain the desired results of strengthening or stiffening without other unforeseen effects. In this paper, three typical RC beam cross sections are considered with height-to-width ratios of 0.5, 1, and 4. Two characteristic compressive strength levels (20 and 30 MPa), and two shear span-to-reinforcement depth ratios (4.5 and 7) are considered. All other parameters related to material and geometry of the beams are maintained constant. The results of the analysis are shown in terms of repaired-to-un-repaired strength and deflection ratios. They indicate that brittle failure mechanisms can develop at loads much lower than expected when considering only flexural performance controlled by concrete crushing and FRP tensile rupture. The analytical model used for the parametrization accounts for brittle failure mechanisms induced by debonding of the FRP reinforcement or shear-tension failure in concrete in the plane of the main longitudinal steel reinforcing bars. Even when considering the limitation of the RC member due to its un-modifiable shear resistance, it is shown that the application of FRP reinforcement can considerably increase load resistance capacity and limit deflection at service.

In situ mechanical properties of wall elements cast using self-consolidating concrete

Abstract: The use of self-consolidating concrete (SCC) can facilitate the placement of concrete in congested members and in restricted areas. Given the highly flowable nature of such concrete, care is required to ensure adequate stability. The objective of this paper is to evaluate the uniformity of in situ mechanical properties of SCC used to cast experimental wall elements. Eight optimized SCC mixtures with slump flow values greater than 630 mm and a control concrete with a slump of 165 mm were investigated. The SCC mixtures incorporated various combinations of cementitious materials and chemical admixtures. The water-cementitious materials ratios ranged between 0.37 and 0.42. Experimental walls measuring 95 cm in length, 20 cm in width, and 150 cm in height were cast. No consolidation was used for the SCC mixtures, while the medium fluidity control concrete received thorough internal vibration. Cores were obtained to evaluate the uniformity of compressive strength and modulus of elasticity along the height of each wall. Bond strengths were also determined for 12 horizontal reinforcing bars embedded at various heights of each wall. All SCC mixtures exhibited small variations in compressive strength and modulus of elasticity in relation to height and were similar to those obtained with the medium fluidity control concrete. Considerable reductions were, however, obtained between compressive strength values determined on core samples and those of cast cylinders. Such reduction was approximately 10% and 20% for SCC mixtures made with 10- and 20-mm maximum size aggregate, respectively, and 10-15% for the control concrete. The top-bar factor for reinforcing bars positioned approximately at 140 cm from the bottom of the experimental walls was 1.4 plus or minus 0.2 for seven of SCC mixtures and approximately 2.0 for the control concrete and one SCC. The optimized SCC mixtures are therefore highly stable despite their flowing nature and can ensure uniform in situ properties when cast in deep structural elements.

Modeling Compression Failure after Low Velocity Impact on Laminated Composites Using Interface Elements

Abstract: Low velocity impact damage can significantly reduce the residual strength of laminated composites. This kind of damage (mostly delaminations) is very dangerous for the structures because it is not apparent to the naked eye and, in some cases, it can reduce the compressive residual strength up to 60%. In this work, a numerical model for predicting the compression failure of laminated composites containing delamination caused by low velocity impact was developed. An interface finite element, previously developed by the authors, was used. This element is compatible with twenty-seven node isoparametric hexahedral elements and enables modeling the behavior of the damaged interface, taking into account a three-dimensional stress state, the interpenetration constraint and the propagation of delamination. In order to verify the numerical model, some experimental work was done. The experimental work, performed on carbon-epoxy (0 4 , 90 4 ), and (90 4 , 0 4 ), laminates, included low velocity impact tests using a drop weight testing machine, followed by X-Ray damage characterization and compression tests using a fixture system similar to IITRI system. The numerical and experimental results were compared and good agreement was obtained.

Shear behaviour of fiber reinforced concrete beams

Abstract: This paper presents the results of shear/flexure tests on steel and polypropylene fiber reinforced concrete beams. In addition to analyzing the influence of fibers on the structural performance in situations of different ratios of shear reinforcement, some aspects of the properties of fresh and hardened concrete are introduced. Fourteen square-section beams were tested. The beams were prepared from seven different mix proportions, varying the type and the volume of fiber added. There were two beams for each composite mix: one model with and the other without stirrups. The main alterations resulting from the use of fibers were increased shear strength, stiffness (particularly after first cracking stage) and ductility. Other parameters used in analyzing performance were the properties of the hardened concrete (compressive strength, tensile strength, and modulus of elasticity), and stresses in the stirrups, in the longitudinal reinforcement and in the concrete (at the web and compression zone).

Axial Load Behavior of LargeScale Spirally Reinforced HighStrength Concrete Columns

Abstract: The axial load behavior of 8 large.scale spirally-reinforced concrete columns was investigated. The 559 mm (22 in.) diameter columns were designed according to ACI 318 Code requirements and tested in concentric axial compression. The columns were made with concrete compressive strengths ranging from about 34.5 MPa to 69 MPa (5 ksi to 10 ksi). The influence of concrete strength, longitudinal reinforcement, and spiral reinforcement size/pitch on the strength and ductility of the columns was evaluated. The higher strength concrete columns displayed less ductility than the lower strength concrete columns. Columns with a higher longitudinal reinforcement ratio were able to maintain peak resistance for a large displacement, but exhibited less ductility as compared to columns with a relatively lower longitudinal reinforcement ratio. For the columns tested in this study, an increase in the spiral size and pitch, while maintaining a constant volume ofspiral reinforcement, lead to an increase in the column ductility. For the high-strength concrete columns, first cracking of the cover concrete was observed at a lower load relative to the peak load as compared to the low-strength concrete columns. Two failure modes were observed in the 8 specimens tested. The lower-strength concrete columns exhibited a bulging type failure mode, and the higher strength concrete columns exhibited an inclinedfailure plane.

Size effect on tensile strength of carbon fibers

Abstract: Tensile strength of carbon fibers exhibits statistical Weibull type distribution and significant size dependence. In the present work, ten types of polyacrylonitrile based and mesophase-pitch based carbon fiber monofilaments were tested for two or three different gauge lengths. Size effect in both axial and radial directions were analyzed based on the two parameters Weibull statistics. It was found that the size effect in axial direction was almost similar for all fibers tested. This result suggests that the tensile strength obtained for a certain gauge length is a meaningful measure as a representative strength of the fiber strength. In radial direction, the size effect of the tensile strength was larger than that in axial direction. The tensile strength of the carbon fibers seemed to have unisotropic statistical characteristics. Size dependence in diameter was numerically simulated with an assumption of unisotropic distribution of Reynolds-Sharp type defects.