Specimen Slenderness Research Articles

The effect of specimen size on the unconfined compressive strength of cement-stabilized granular mixtures made of natural and recycled aggregates

The cement-stabilization technique is employed on natural and recycled granular materials to improve their mechanical properties. The strength of these materials is assessed by the unconfined compressive strength on laboratory compacted specimens, typically after 7 days of curing. Standards and technical specifications specify different values of specimen height and diameter and different loading modes of testing. This makes the comparison between different materials and with the acceptance limits of technical specifications difficult. The research investigates the effect of specimen size and loading mode on the unconfined compressive strength of both natural and recycled cement-stabilized granular materials. The results revealed significant differences in strength due to variations in specimen size and loading mode. As expected, an increase in specimen slenderness resulted in a decrease in compressive strength. A linear regression model was developed to quantify the effect of the experimental variables on the compressive strength of the two cement-stabilized materials.

Influence of Materials of Moulds and Geometry of Specimens on Mechanical Properties of Grouts Based on Ultrafine Hydraulic Binder.

Ultrafine hydraulic binder grout injection is a technique utilised for repairing masonry, either to connect sections, seal joints, or fill voids due to its great capacity for penetration and higher mechanical strength than lime grout. In this research, the mechanical properties of ultrafine hydraulic cement grout are analysed considering the influence of the mould material for preparing the specimens and their geometry characteristics in the context of the specifications set out in several international standards. The test campaign to ascertain compressive and flexural strength in different circumstances is supplemented with a physical and chemical characterisation of both binder and fresh and hardened grout. Significant differences in mechanical properties between specimens prepared with absorbent or non-absorbent-water material are found due to the influence of drying shrinkage and decanting binder during the curing process. Furthermore, the slenderness of specimens is presented as an important factor in determining the compressive strength of mixtures.

Influence of specimen slenderness and stacking on the experimental strength of solid fired clay bricks

This study evaluates the effect of the specimen’s slenderness and stacking on the experimental measurement of the compressive strength of solid fired clay units. The experimental campaign has focused on two different types of solid fired clay bricks, namely mechanically extruded and handmade, with a total amount of 382 specimens tested. The research considers different standard and non-standard specimens with varying width (40, 50, 60, 70, 80, 90, and 100 mm) and height (40, 80, and 120 mm). The experimental results evidence the influence of the stacking procedure on the compressive strength, with different results depending on the manufacturing process of the unit. The influence of specimen’s slenderness on the compressive strength is more evident and regular in handmade brick specimens than in extruded ones.

Axial compressive behaviors of circular steel slag concrete‐filled steel tube slender columns

AbstractThis study aims to investigate the feasibility of using steel slag aggregate in concrete‐filled steel tubes by conducting axial compression tests on five groups of 10 circular steel slag concrete‐filled steel tube (SSCFST) slender columns. The failure mode, load–deformation curve, load–strain curve, axial bearing capacity, initial axial stiffness and ductility of the columns are analyzed. The effects of the steel slag replacement ratio and slenderness ratio on the axial compressive behaviors of the specimens are also quantified. These test results are employed to verify the reliability of a finite element model of the circular SSCFST slender columns established using Abaqus software. Then, a parametric analysis is conducted to explore the factors affecting the axial compressive behaviors of circular SSCFST slender columns. Moreover, the prediction accuracy of the existing design methods for the axial bearing capacity of circular SSCFST slender columns is evaluated. The results demonstrate that the ultimate bearing capacity of the specimens decreases by 2.3% and 3.1% on average, the initial stiffness decreases by 2.8% and 3.5%, and the ductility index decreases by 1.2% and increases by 1.6%, respectively, when the steel slag replacement ratio increases from 0 to 50% and 100%. The ultimate bearing capacity of the specimens decreases by 11.1% and 37.1% on average, the initial stiffness decreases by 6.3% and 28.8%, and the ductility index decreases by 5.1% and 12.2%, respectively, when the specimen slenderness ratio increases from 40 to 60 and 80. The confinement factor and slenderness ratio play a significant role in determining the axial compressive behaviors of SSCFST slender columns. The accuracy of the formulas provided in TCECS 625‐2019 and by Han is demonstrated in predicting the axial bearing capacity of circular SSCFST slender columns.

Prediction of Mortar Compressive Strength Based on Modern Minor-Destructive Tests

The crucial task of the diagnosis of an existing masonry structure is to assess the current values of the mechanical parameters of the materials from which the structure was erected-usually bricks and mortar. The article presents the results of minor-destructive tests carried out on bed joints of three-brick-masonry prisms prepared in the laboratory. Three types of mortars used in the masonry were tested, which differ by the type and amount of binder. In order to determine mortar compression strength, three modern diagnostic methods were used: double punch test (DPT), standard penetrometric test (PT) and torque penetrometric test (TPT). Tests were carried out after 4, 12 and 90 weeks. The mortar strength determined in each of these tests was compared with the mortar reference strength determined on the beam specimen according to the methodology given in EN 1015-11. The results of the conducted tests confirmed the high usefulness of all three diagnostic methods. However, limitations in the application of the PT test were noticed-only lime mortars and weak cement-lime mortars can be tested with this method. In the case of mortars with an increased amount of cement binder, the impact energy is too low to estimate the compressive strength of the mortar in the brick wall joint. Technical limitations in the use of TPT and DPT tests were also indicated-weak lime mortars with low cohesion do not allow for obtaining reliable results. It was shown that DPT results strongly depend on two factors, specimen slenderness and mortar strength. Due to this fact, simple non-parameter conversion from mortar compressive strength according to the DPT test into mortar reference strength may lead to significant overestimation. As the results show, in newly built masonry, proper selection of diagnostic method is crucial due to the strong dependence of mortar curing dynamics on its location in the joint. This paper helps to match diagnostic techniques with the condition and type of mortar in the existing structure.

Strengthening of Recycled Aggregate Concrete Slender Column with CFRP

This piece provides an overview of an experimental program that tests the structural performance of slender recycled aggregate RAC columns that are externally restrained by carbon fiber reinforced polymer (CFRP composite system). To demonstrate that the CFRP strengthening system is one of the recommended effective techniques for making up for the reduction in load-carrying capacity caused by the use of 100% RCA in combination with the slenderness of column specimens, eight slender circular RC columns were modeled and tested. To more accurately predict the structural features of the RC thin column (the ultimate carrying weight, first cracking load, load-displacement curve, and load-strain response), the findings from experiments have been analyzed and monitored. The findings indicated that the type of transverse reinforcement and the amount of external strengthening impact the degree of improvement in column performance. The strength for the tied RC columns with 100% RCA confined by (25, 50, and 100) % CFRP increased by 5.5, 44.97, and 112.85%, respectively, compared to control columns with no CFRP confinement. Similar strength improvements are seen in spirally RC columns with 100% RCA and the same external confinement coverage ratio: 10.32, 42.81, and 113.51%.

2D mesoscale modeling of compressive fracture in concrete using a mesh fragmentation technique

The computational prediction of the failure processes of concrete under compression is still a challenge. Several researchers have proposed mesoscale models to have a better understanding of the influence of the distinct phases of the concrete on the fracture process. In this sense, this work proposes an extension of the mesoscale model proposed by Rodrigues et al. (2016) to describe the complex failure behavior of concrete under compression. In the proposed 2D approach, two layers of interface elements are inserted into the standard finite element mesh to define the potential crack paths using the mesh fragmentation technique. Each layer is formed by a pair of high aspect ratio elements and is responsible for modeling the tensile or frictional shear failure behavior. According to this approach, the compressive failure is a consequence of the combination between tensile and shear failure modes in the mesoscopic scale. The use of these two damage models allows to represent the debonding (opening) between the aggregates and matrix due to local tensile stress concentration, i.e. the fracture propagation in mode-I, as well as the sliding process corresponding to the fracture propagation in mode-II. Furthermore, adopting adequate parameters, these models allow representing the friction condition between the concrete specimen and the steel loading plates. The failure behavior of compression tests with different specimen slenderness as well as the different friction restraints between loading platen and concrete specimen is predicted. The numerical results are compared qualitatively and quantitatively against the experimental results found in the literature, demonstrating that the proposed approach is able to describe the failure process of concrete in compression.

AXIAL COMPRESSION TESTING OF BAMBOO PLYWOOD-ENCASED THIN-WALLED STEEL TUBE/STONE DUST CONCRETE COLUMNS

A novel structural member, the bamboo plywood-encased thin-walled steel tube/stone dust concrete composite column (BSDCC), was investigated in this study. Axial compression tests were conducted on 10 BSDCC specimens; their failure characteristics and modes were examined, and the effects of the stone-dust concrete content ratio and strength, specimen slenderness ratio, cross-sectional composition and binding bar confinement pattern, and binding bar spacing ratio on the bearing capacity and deformation of the columns were investigated. Two main compressive failure modes were observed: (1) adhesive failure by cracking and debonding between the bamboo plywood boards and between the bamboo plywood and the steel tube and (2) compressive-flexural failure of the bamboo plywood between the binding bars in the middle of the specimen. For specimens with the same cross-sectional dimensions, the cross-sectional content ratio of the stone dust concrete impacted the deformation and failure mode but did not significantly affect the ultimate bearing capacity. The bearing capacity decreased with increasing specimen slenderness and binding bar spacing ratio and increased with increasing stone dust concrete strength and bamboo plywood constraint (in terms of the cross-sectional composition and binding bar restraint pattern). A model for the ultimate bearing capacity of BSDCCs was established through regression analysis.

Comparison of the Compressive and Tensile Strength Values of Rocks Obtained on the Basis of Various Standards and Recommendations

The objective of the current study was to compare results relating to the compressive and tensile strength of rocks obtained during research undertaken according to Polish Standards (as part of the European standards known as Eurocodes), American Society for Testing and Materials (ASTM) Standards, and the recommendations of the International Society for Rock Mechanics (ISRM). A total of 130 experiments for uniaxial compression on axisymmetric samples, point loads, and transverse compression (so-called Brazilian tests) were performed on rock samples comprising granite, limestone, and sandstone. Geometric properties of the samples were selected depending on the applied research method, and the relationship between the specimen’s slenderness and shape, and the obtained values of compressive and tensile strength, were analyzed. The results of the study showed that values of compressive and tensile strength obtained in a laboratory depend significantly on specimen slenderness, different values of which are imposed by various ISRM standards and recommendations, wherein this sensitivity was much higher in the case of compressive strength. The study also raised doubt about the usefulness of the so-called point load test as a method for determination of the compressive strength of rocks and potential estimation of the tensile strength.

Experimental investigation of slender RC columns under horizontal static and impact loads

This paper reports an experimental investigation on the behaviour of slender reinforced concrete (RC) columns under horizontal static or impact loading. Fourteen square RC columns were tested under horizontal impact loading by using a modern horizontal impact facility. These specimens were subjected to the same design impact velocity of 0.8 m/s. The effects of three main parameters, namely the specimen slenderness ratio, the longitudinal reinforcement ratio, and the axial compression ratio, were considered. Based on the impact test results, the descriptions of failure mode, crack pattern, time histories of impact force, deflection, and strain are illustrated. On the other hand, tests on three additional RC columns with different cross-section dimensions subjected to horizontal static loading were undertaken for comparison purpose. In the comparisons of the load–displacement relationships for static and impact load cases, the significant divergence can be observed owing to the existence of the inertial force initiated in the dynamic responses. After the completion of the impact test, six impact-damaged specimens were tested under static loading to evaluate their residual resistance and the corresponding behaviour. It is found that the ultimate capacities of damaged specimens could reduce by 7%–14% compared with those of undamaged ones.

End and shape effects of brittle rock under uniaxial compression

Accurate estimation of rock strength is one of the most important tasks in rock engineering design. We analyzed the mechanical and failure properties of rock specimens affected by the end and shape effects under uniaxial compression through experimental and numerical studies. It was found that the end effect could change the failure mode of rock specimens from splitting failure to shear failure, as the coefficient of friction at the rock specimen-loading platens contacts increased. When the coefficient of friction was increasing, the contour distribution of the equivalent plastic strain of specimen also changed, which simultaneously influenced by the shape of specimen. It can be observed that an increase in the slenderness of specimen caused a decrease in end frictional effect on the uniaxial compressive strength of rock specimen. The numerical results also revealed that the shape effect originated from the end frictional effect as the rock material was homogeneous. The relationship between the uniaxial compressive strength and the aspect ratio of specimen was obtained and fitted by incorporating the end effect, which indicated that the uniaxial compression test should mitigate the coefficient of friction and increase the aspect ratio of specimens to make the observed strength of specimen more meaningful.

Lateral Confinement Effects on the Axial Compressive Strength of Mortar Specimen

Many researchers have confirmed that the lateral confinement increases the dynamic strength of concrete-like material in compression. It has been found that the factors responsible for the lateral confinement involve in various material and structural parameters, e.g. material inertia, hydrostatic dependence, interfacial friction, specimen slenderness etc. This paper reported quasi-static and dynamic experimental results on the compressive strength of mortar specimens. A range of compression tests were conducted for cylindrical specimens with different slenderness ratios under various end constrains with difference coefficients of friction. It was found that for slender specimens, end face friction has large effect on the lateral confinement, which enhanced the axial compressive strength of mortar. Numerical modeling was also performed to understand the influence of these structural factors on the compressive strength of concrete-like materials.

Investigation of scale effect of numerical unconfined compression strengths of virtual colluvial–deluvial soil–rock mixture

A series of numerical unconfined compression tests for virtual pseudo three-dimensional colluvial–deluvial soil–rock mixture (CDSRM) specimens with different areal block proportions (BPs), sizes and slenderness ratios were conducted to study the variations of strength, deformability and failure process. Surrogate models of the specimens were generated on the basis of statistical characterization of the mesostructures extracted from the outcrop images of Baishuihe landslide site located in Three Gorges Reservoir Area of China. We find that the strength exhibits reduction for higher BPs and exhibits enhancement for lower BPs when the size increases and become asymptotic eventually. The deformability shows a monotonically increasing linear correlation with the specimen size for most BP scenarios. Simultaneously, the strength and deformability values decrease with the specimen slenderness monotonously. These findings are different from the previous assumption that the geo-mechanical behavior of bimrock is scale-independent. Comparing the observations of this study with the reported scale effect of other geotechnical materials, we conjecture that the scale effect is due to the block size and distribution.

Influence of Slenderness on Stress-Strain Behavior of Concrete-Filled FRP Tubes: Experimental Study

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) have received significant research attention over the last two decades, with recent experimental studies identifying significant benefits of CFFTs filled with high-strength concretes (HSC). However, many test parameters of high-strength CFFTs, such as specimen slenderness, remain experimentally limited. This paper presents an experimental investigation on the axial compressive behavior of 33 monotonically loaded circular normal-strength and high-strength CFFTs (NSCFFTs and HSCFFTs). The influence of specimen slenderness is investigated on test specimens with height-to-diameter ratios (H/D) of 1, 2, 3, and 5. The CFFT specimens were instrumented with numerous lateral strain gauges to examine the development of hoop strains along the specimen height and around specimen perimeter. The experimentally recorded stress-strain relationships are presented graphically and the ultimate axial stresses and strains, and FRP tube hoop strains at rupture are tabulated. The results indicate that specimens with a height-to-diameter ratio (H/D) of 1 outperform specimens with a H/D ratio of 2 to 5, with significantly increased strength and strain enhancements. The influence of slenderness on specimens with a H/D ratio between 2 and 5 was found to be significant in regards to axial strain enhancement, with a decrease observed as specimen slenderness increased. Conversely, the influence of slenderness on axial strength enhancement of specimens with a H/D ratio between 2 and 5 was found to be negligible. The strain results indicate that hoop rupture strains along the height of CFFTs become more uniform for specimens with higher amounts of confinement. On the other hand, the variation of hoop strains around the perimeter of CFFTs was not observed to be significantly influenced by slenderness, concrete strength or amount of confinement.

Effects of Specimen Slenderness Ratio and Diameter Ratio Between Specimen and Pressure Bars on Stress Evaluation by Split Hopkinson Pressure Bar Technique

Effects of Specimen Slenderness Ratio and Diameter Ratio Between Specimen and Pressure Bars on Stress Evaluation by Split Hopkinson Pressure Bar Technique

Variation of Hoop Strains in Concrete-Filled FRP Tubes with Concrete Strength, Amount of Confinement and Specimen Slenderness

This paper presents an experimental investigation on the variation of FRP hoop strains around specimen perimeter and along specimen height. A total of 24 concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) with circular cross-sections were tested under monotonic axial compression. The CFFT specimens were instrumented with numerous lateral strain gauges attached to the FRP tubes to examine the development of hoop strains along the specimen height and around specimen perimeter. Specimens were manufactured with height-to-diameter ratios (H/D) of 2 or 5, with all specimens maintaining a nominal diameter of 150 mm. Additional test parameters selected for this study included amount of confinement and concrete compressive strength. This paper focuses on the experimentally recorded hoop strains and their variation around specimen perimeter and along specimen height. The results indicate that hoop rupture strains along the height of CFFTs become more uniform for specimens with higher amounts of confinement. On the other hand, the variation of hoop strains around the perimeter of CFFTs was not observed to be significantly influenced by height-to-diameter ratio (H/D), concrete strength or amount of confinement.

Influence of Slenderness on Behavior of High-Strength Concrete-Filled FRP Tubes under Axial Compression

This paper presents an experimental investigation on the influence of specimen slenderness on axial compressive behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs). A total of 18 aramid FRP- (AFRP) confined high-strength concrete (HSC) specimens with circular cross-sections were tested. Specimens with height-to-diameter ratios of 1, 2, 3 and 5 were manufactured and tested, with all specimens maintaining a nominal diameter of 150 mm. The results indicate that specimens with an H/D of 1 exhibit significantly higher strength and strain enhancements compared to specimens with H/D ratios of 2 to 5. The influence of slenderness on specimens with H/D ratios between 2 and 5 was found to be significant in regards to axial strain enhancement, with a decrease observed as specimen slenderness increased. On the other hand, the influence of slenderness on axial strength enhancement of specimens with H/D ratios between 2 and 5 was found to be negligible.

Water retention curve for hydrate‐bearing sediments

AbstractThe water retention curve plays a central role in numerical algorithms that model hydrate dissociation in sediments. The determination of the water retention curve for hydrate‐bearing sediments faces experimental difficulties, and most studies assume constant water retention curves regardless of hydrate saturation. This study employs network model simulation to investigate the water retention curve for hydrate‐bearing sediments. Results show that (1) hydrate in pores shifts the curve to higher capillary pressures and the air entry pressure increases as a power function of hydrate saturation; (2) the air entry pressure is lower in sediments with patchy rather than distributed hydrate, with higher pore size variation and pore connectivity or with lower specimen slenderness along the flow direction; and (3) smaller specimens render higher variance in computed water retention curves, especially at high water saturation Sw > 0.7. Results are relevant to other sediment pore processes such as bioclogging and mineral precipitation.

Specimen Slenderness and the Influence of Joint Orientation on the Uniaxial Compressive Strength of Singly Jointed Rock

Limitations on the joint orientations for which a joint is free to fail exclusively by sliding (without the need for rupture of intact material), for a given specimen height-to-diameter ratio (L/D), can influence the results obtained from uniaxial compressive strength (UCS) testing of jointed rock. For example, a jointed specimen with L/D=2 can only possibly fail exclusively by sliding for joint angles up to ∼63° from the plane perpendicular to the axial direction. For joints greater than ∼63°, intact material rupture is required to accommodate failure and UCS is likely to be overestimated. Experimental work was performed to determine the UCS of jointed rock containing various joint orientations, for sample sets with L/D=2 and 4. Only joint orientations that allowed joint formats for which failure could occur by sliding on the joint for the L/D=4 sample set (i.e., up to 75°) were considered. For joint orientations greater than ∼63°, the L/D=2 sample set produced significantly higher UCS values than the L/D=4 sample set. Additionally, specimens with joint orientations in excess of 63° displayed failure mechanisms involving intact-material rupture for the L/D=2 sample set. Only sliding failure was observed for the same tests on the L/D=4 sample set. Care must be taken to ensure methodologies adopted for UCS testing of jointed rock do not influence the mechanisms by which failure can occur. In some cases, strict adherence to methods set out in popular testing standards may need to be reconsidered.

Specimen Size, Slenderness Ratio and Loading Rate Effect on Dynamic Simulation of HPC

The length to diameter ratio (L/D), loading rate, size and specimen geometry are significant factors that affect the compressive strength of high performance concrete (HPC) under dynamic loading conditions. A numerical study is carried out on selected conventional laboratory size prismatic specimen between the slenderness ratio 0.5 to 3.5 to evaluate the variation of stress-strain by performing finite element (FE) solving procedure. The results show that the maximum stress is found in specimen with slenderness ratio one. The stresses gradually decreased with the increase of the value of slenderness ratio (from one and above). The HPC dynamic strength dependency on strain rate and L/D is correlated using multiple regression equation. The accuracy of the predicted regression equation is verified with published experimental data and the result is 3% and 25% deviation under the applied strain rate 100 and 300 s -1 respectively. The regression equation yields significant result within the limited stain rate 150 s -1 . The size effect analyses also satisfy existing published experimental data of concrete.