Indirect Tensile Stress Research Articles

Cryogenic fracturing efficacy in granite rocks: Fracture toughness and brazilian test differences after elevated temperatures and liquid nitrogen exposure

Hot dry rock (HDR) is a significant contingent as a thermal energy source. Due to its low permeability, considerable depth, high temperature (150–650 °C), and high pressure, the stimulation of HDR is challenging. This research work aims to examine the efficacy of cryogenic fracturing on HDR. We performed fracture toughness and indirect tensile stress (Brazilian tests) on granite specimens with three different initial temperatures before exposure to liquid nitrogen (LN2). Fracture toughness tests were implemented on semicircular bend (SCB) specimens containing a pure mode-I fracture, and Brazilian tests were performed with discs. The experimental outcomes show that, overall, both critical load in Brazilian tests and the fracture toughness of LN2-treated specimens of granites both diminished with increasing initial temperature. When the initial temperature exceeded 300 °C (in this instance, 500 °C), the amplitude of thermally induced fractures in granite moderately increased after LN2 treatment, secondary fractures prolonged, and the plastic characteristics increased. Moreover, permeability measurements indicated amelioration with elevated temperatures and subsequent LN2 exposure. SEM analysis showed that an increase in initial temperatures along with LN2 treatment led to further intensified granite microdamage while the quantity and the magnitude of thermal fractures augmented.

Analysis of raw-crushed wind-turbine blade as an overall concrete addition: Stress–strain and deflection performance effects

End-of-life wind-turbine blades undergo non-selective crushing to produce Raw-Crushed Wind-Turbine Blade (RCWTB), which can be recycled as a raw material in concrete. RCWTB contains fibers from glass fiber-reinforced polymer that can add ductility and load-bearing capacity to concrete. Concrete mixes with percentage additions of between 0.0 % and 6.0 % RCWTB by volume are produced to analyze their compressive stress–strain performance, their deflection under bending forces, and their deformability under indirect-tensile stresses. Higher RCWTB contents increased deformability in the longitudinal direction under compression, the concrete material absorbing energy levels that were up to 111.4 % higher, even though additions of only 6.0 % RCWTB were sufficient to strengthen the load-bearing capacity. RCWTB fiber stitching effect was most noticeable in the transverse direction under compression, as it reduced elastic deformability and failure strain, removed the yield step caused by vertical-splitting cracking, and increased the fracture strain by up to 94.4 %. With regard to deflection, RCWTB fibers conditioned concrete compliance at advanced ages without any dependence on the modulus of elasticity, and percentage additions from 3.0 % provided load-bearing capacity. This advantage was also noted in indirect-tensile stresses for 6.0 % RCWTB. In summary, RCWTB successfully increased the ductility and load-bearing capacity of concrete per unit strength and carbon footprint.

Assessment of indirect tensile stress and tensile–strength ratio and creep compliance in HMA mixes with micro-silica and PMB

Abstract This study aims to ascertain the effects of polymer-modified bitumen (PMB) and to utilize mineral filler with micro-silica (MS) on various properties of hot-mix asphalt (HMA). In this work, three specimens of an asphalt-concrete mixture were prepared. The first specimen represents a control mixture, the second specimen contains MS at rates 15, 30, and 45% by the total weight of the total aggregate as filler, and the third specimen contains MS and PMB with 4% novolac by the weight of bitumen and 15% hexamine by the weight of the novolac. The properties of designed mixes were evaluated using the indirect tensile strength test, tensile–strength ratio, and creep compliance. The test results showed an increase in the resistance of asphalt concrete to moisture damage, a reduction influence of water on asphalt concrete properties, creep compliance values indicating growing stiffness for asphalt mixture, and enhanced fatigue life. In conclusion, using 45% MS and PMB improved the properties of HMA and provided long-lasting mixtures for highway construction.

A statistical bonded particle model study on the effects of rock heterogeneity and cement strength on dynamic rock fracture

Numerical modelling and simulation can be used to gain insight about rock excavation processes such as rock drilling. Since rock materials are heterogeneous by nature due to varying mechanical and geometrical properties of constituent minerals, laboratory observations exhibit a certain degree of unpredictability, e.g. with regard to measured strength and crack propagation. In this work, a recently published heterogeneous bonded particle model is further developed and used to investigate dynamic rock fracture in a Brazilian disc test. The rock heterogeneities are introduced in two steps—a geometrical heterogeneity due to statistically distributed grain sizes and shapes, and a mechanical heterogeneity by distributing mechanical properties using three Weibull distributions. The first distribution is used for assigning average bond properties of the grains, the second one for the intragranular bond properties and the third one for the bond properties of the intergranular cementing. The model is calibrated for Kuru black diorite using previously published experimental data from high-deformation rate tests of Brazilian discs in a split-Hopkinson pressure bar device, where high-speed imaging was used to detect initiations of cracks and their growth. A parametric study is conducted on the Weibull heterogeneity index of the average bond properties and the grain cement strength and evaluated in terms of crack initiation and propagation, indirect tensile stress, strain and strain rate. The results show that this modelling approach is able to reproduce key phenomena of the dynamic rock fracture, such as stochastic crack initiation and propagation, as well as the magnitude and variations of measured quantities. Furthermore, the cement strength is found to be a key parameter for crack propagation path and time, overloading magnitudes and indirect tensile strain rate.

Application of Miniaturized Brazilian Disc Tests for the Determination of High-Temperature Strength of Ceramic Filter Materials

Open-cell ceramic foams are used for the filtration of molten metals with the aim of improving the final quality of cast products. Among the various filter materials that have been developed and studied, carbon-bonded alumina (Al2O3-C) have drawn the most attention due to their superior properties. However, to decrease the hazardous effects of coal tar pitch binder Carbores® P, environmentally friendly Al2O3-C based Tannin and Lactose binder systems have recently received much attention. A systematic characterization and assessment of their properties are required to replace the currently used binders. Considering the service temperature of these filter materials, a test method performed at elevated temperatures is needed. Due to the thermal and mechanical loads applied on these filters, the fracture strength of the bulk material is a key indicator of their structural integrity. The Brazilian Disc Test (BDT) is a method of determining fracture mechanical properties of brittle materials with indirect tensile stresses, formed perpendicular to the loading direction. Due to the porous structure of the materials under investigation, the contact points between the specimen and the loading device are susceptible to major deformation and damage. The aim of this study is on the one hand to define the geometry at the contact points in such a way that valid fracture toughness values can be determined even for small specimen dimensions. For this purpose, the deformations on the surface of the specimens are determined at room temperature using digital image correlation during the test. On the other hand, the optimized test geometry is used to perform high-temperature tests to evaluate their strength and fracture toughness. This evaluation is performed with accompanying finite element analyses using cohesive zone models. The required parameters of these models are identified using methods of nonlinear optimization.

Utilization of a Miniaturized Brazilian Disc Test for Strength Measurements of C‐Bonded Alumina Filter Materials

Filtration of molten metals using open cell ceramic filters has proven the premises regarding the improvements of the final product quality. The Collaborative Research Center 920 has made a substantive record in developing carbon‐bonded alumina () filters. Considering the mechanical loads carried by these filters, material strength determination is a necessary step to improve their performance. The Brazilian disc test (BDT) is a method to determine mechanical properties of brittle materials with indirect tensile stresses. The simplicity of the specimen production, test application, as well as results evaluation makes this test attractive. Herein, experimental investigations of different variations of the BDT setup on specimens are conducted. Test variations mainly concern the geometry of the load application to ensure a defined failure of the specimens. Digital image correlation (DIC) technique is used to extract the necessary data for the evaluation of the results. Moreover, within an inverse parameter identification method finite element analysis is used to determine strength and elastic material parameters. Additionally, a high‐speed camera is used for the detection of crack formation. It can be concluded that a special variation of the BDT is a suitable method to evaluate the mechanical properties of .

Improved Mechanical Properties of Cement-Stabilized Soft Clay Using Garnet Residues and Tire-Derived Aggregates for Subgrade Applications

The growth of the global economy in recent years has resulted in an increase in infrastructure projects worldwide and consequently, this has led to an increase in the quantity of waste generated. Two recycled materials, namely garnet residues (GR) and tire-derived aggregates (TDA), were used to improve mechanical properties of soft clay (SC) subgrade in this study. GR was evaluated as a replacement material in SC prior to Type I Portland cement stabilization. TDA was also studied as an elastic material in cement-stabilized SC–GR. The laboratory tests on the cement–TDA-stabilized SC–GR included unconfined compressive strength (UCS), indirect tensile stress (ITS) and indirect tensile fatigue (ITF). Microstructural analysis on the cement–TDA-stabilized SC–GR was also performed to illustrate the role of GR and TDA contents on the degree of hydration. The UCS of cement-stabilized SC–GR increased when cement content increased from 0% to 2%. Beyond 2% cement content, the UCS development was slightly slower, possibly due to the presence of insufficient water for hydration. The GR reduces the specific surface and particle contacts of the SC–GR blends to be bonded with cementitious products. The optimum SC:GR providing the highest UCS was found to be 90:10 for all cement contents. Increased amounts of GR led to a reduction in UCS values due to its high water absorption, resulting in the insufficient water for the cement hydration. Moreover, the excessive GR replacement ratio weakened the interparticle bond strength due to its smooth and round particles. The TDA addition can enhance the fatigue resistance of the cement-stabilized SC–GR. The maximum fatigue life was found at 2% TDA content. The excessive TDA caused large amounts of micro-cracks in cement–TDA-stabilized SC–GR due to the low adhesion property of TDA. The SC:GR = 90:10, cement content = 2% and TDA content = 2% were suggested as the optimum ingredients. The outcome of this research will promote the usage of GR and TDA to develop a green high-fatigue-resistant subgrade material.

Safety factor calculation of a road structure with cement-modified loess as subgrade

The deformation and failure parameters of six mixtures of cement-modified loess have been determined based on direct tensile stress testing, indirect tensile test testing and unconfined compressive stress testing. Results have been used to determine the parameters of the Mohr-Coulomb failure criterion and the parameter of the Hoek-Brown failure criterion. The Hoek-Brown model clearly shows that results from indirect tensile stress measurement must be weighted to describe materials properly. Both models’ results were then used to calculate the safety factor of a road structure with cement-modified loess used as subgrade material using reduction techniques. Specifically, for the Hoek-Brown model, a new calculation adapted to structures with tensile stresses is proposed. Results vary by around 8.9 for the Mohr-coulomb model and around 3.2 for the Hoek-Brown model.This shows, at the laboratory scale, that resource-based economies could be expected if cement-modified loess mechanical parameters were considered in the design of structures. It also shows that the Hoek-Brown model is a promising tool to study tensile stressed structures made with cement-modified materials.

Reinforcing rammed earth with plant fibers: A case study

This paper is about an experimental study on the mechanical behavior of rammed earth under compression and indirect tensile stresses. Local raw earth and sand are the base materials used. As stabilizers, cement, lime, barley straw and date palm fibers were used to manufacture the specimens tested. In all, five different mixtures were prepared and tested. The cylindrical specimens used were prepared under optimum compaction references using the Proctor test procedures.Two types of mechanical tests were performed: the uniaxial compression test and the splitting tensile strength test. For each compression test, four parameters were determined: the compressive strength, the initial tangent modulus, the secant modulus at maximum stress and the peak axial strain corresponding to the maximum compressive stress.The results obtained show that, plant fibers (barley straw or date palm fibers) can provide improvements in rammed earth strengths such as those that cement or lime can provide, especially in tensile strength. The results also show that plant fibers decrease the stiffness of rammed earth while cement and lime increase it. Nevertheless, a coefficient of proportionality of about 0.6 was observed between the secant modulus and the initial tangent modulus for all the mixtures tested. In addition, a linear correlation between tensile strength and compressive strength has been established in the cases of both unstabilized and cement-or-lime-stabilized rammed earth. However, no similar correlation has been found when considering fiber-reinforced rammed earth, because the adhesion of the fibers used to the sand-earth matrix has more effect on tensile strength than on compressive strength.

Avaliação da transferência de fissuras em sistemas de impermeabilização aderidos

Sistemas de impermeabilização aderidos em estruturas de concreto formam barreiras protetoras contra a entrada de fluídos e agentes agressivos. No entanto, se fissuras surgirem no substrato e passarem para a camada impermeável aderida, a estanqueidade do sistema pode ser comprometida. Esse trabalho tem como objetivo a avaliação experimental do comportamento de duas membranas impermeabilizantes, de base acrílica, quando aderidas em substrato de concreto sujeito a fissuração. É proposto um método de ensaio mecânico de tração indireta associado a análise por correlação de imagem (DIC) para avaliação da transferência de fissuras de substratos de concreto para membranas impermeabilizantes aderidas. Os resultados mostraram que esta metodologia permite diferenciar o comportamento das membranas com relação a transferência de fissuras. Além disto, foi possível mensurar quantitativamente, com a técnica DIC, a magnitude da fissura no concreto que resultou em fissuração nas membranas. A membrana de acrilatos puros resultou menor capacidade de absorver os esforços provenientes do substrato do que a membrana de copolímeros acrílicos sem cimento. Esse comportamento pode ser relacionado com a diferença no alongamento dos materiais. A aderência das membranas ao substrato foi elevada e muito superior aos requisitos normativos, e pode ter resultado na dificuldade das membranas de absorver os esforços do substrato, fissurando com maior facilidade.

Implication of Nondestructive Test NDT for Evaluation of Crack Healing in Asphalt Concret

Asphalt concrete practices heavy loading and environmental impacts through the service life of the pavement. Micro cracks usually initiate and accumulate to form various types of distresses. However, asphalt concrete has the ability of self-healing under rest period and environment conditioning. Asphalt concrete is a composite material consisting of aggregates, bitumen, and air voids. Its mechanical behavior is complex due to its dependency of temperature, loading frequency, and strain level. In this investigation, asphalt concrete specimens of wearing course have been prepared in the laboratory and subjected to repeated indirect tensile stresses to initiate the micro-cracks. The test was stopped after 1200 load repetitions, and the specimens were stored in an oven at 60°C for 120 minutes to allow the crack healing process by external heating to start. Specimens were returned to the testing chamber and were subjected to another round of load repetitions. Specimens were tested before and after load repetitions and crack healing with the aid of ultrasonic pulse velocity traversing the specimen using pundit instrument. The healing indicator was the change in pulse velocity before and after the repeated load test and before and after healing process. It was concluded that the velocity decreases as the loading cycles proceeds indicating the start of damage, while the ultrasonic pulse velocity increases after the micro-crack healing process.

Monitoring Micro Crack Healing Phenomena in Recycled Asphalt Concrete

Asphalt concrete usually practices cracking and distress during its service life. However, recycling process as well as crack healing phenomena can enhance the physical properties and extend the pavement service life. This investigation is concerned with monitoring the microcrack healing of recycled asphalt concrete through its influence on deformation measurement under three testing techniques, repeated (compressive, tensile and shear stresses). Reclaimed asphalt concrete mixture was recycled with [carbon black-asphalt] and [Styrene Butadiene Rubber (SBR)-asphalt] rejuvenators. Specimens have been tested with the aid of the pneumatic repeated load system (PRLS). The repeated loading of the three testing techniques was terminated after 1000 load repetitions, and the specimens could heal by external heating at 60°C for 120 minutes. Specimens were returned to the (PRLS) and subjected to another run of repeated stresses application after healing. It was concluded that the deformation increases after recycling process while it declines after microcrack healing regardless of the testing technique adopted. The deformation declines by (31.8, 5.8 and 19)%, (43 ,49 and 24 )% and (44 ,10 and 13 )% under three level of repeated compressive stresses of (0.068, 0.138, and 0.206) MPa at (40 °C) after healing cycle for reclaimed and recycled mixture with (carbon black-asphalt) and (SBR-asphalt) respectively when compared with permanent deformation before the healing cycle. The permanent deformation under repeated punching shear and indirect tensile stresses declines after microcrack healing by (32, 26 and 25.7) % and (73, 78.7 and 78.2) % for reclaimed, recycled with (carbon black-asphalt) and recycled with (SBR-asphalt) mixtures respectively.

Modeling the Influence of Surface Free Energy on Moisture Damage of Recycled Asphalt Concrete

Recycling is a sustainable process for restoration of the pavement quality. In this investigation, aged binder was recycled by digestion with (0.5, 1.0, and 1.5) % of polyethylene and crumb rubber. The recycled binder was implemented in the preparation of Marshal specimens. The surface free energy of the control and recycled binder was determined using the Sessile drop method. Specimens were tested under repeated indirect tensile and double punching shear stresses with the aid of pneumatic repeated load system (PRLS). Another group of specimens was tested for moisture damage, then subjected to the same loading sequence. Specimens were subjected to 1200 load repetitions under stress level of 0.138 MPa at 25 °C. The load was sustained for 0.1 second followed by 0.9 seconds of rest period. The permanent deformation was measured before and after moisture damage process for each testing technique. Regression analysis is used to develop a model for the influence of surface free energy on the deformation and the moisture damage using the SPSS Software. It was concluded that the obtained model can explain 82 % of the variation in moisture damage due to the influence of surface free energy.

Influence of Microcrack Healing Techniques on Tensile Strength of Asphalt

Asphalt concrete pavement practices repeated loading and environmental impacts and suffers distresses. Microcracks usually occurs at early stage of pavement life while the pavement can heal itself under controlled conditions. Microcrack healing process can be controlled and accelerated using various techniques. In this investigation, iron filling was implemented as partial substitute of sand to support the healing process while both induction and external heating techniques were adopted to control the healing using microwave and oven. Asphalt concrete specimens have been prepared with different percentages of iron filling and the corresponding required asphalt content. Specimens were subjected to repeated indirect tensile stresses for 600 and 1200 load repetitions. Specimens were allowed to heal using microwave and oven heating techniques. Specimens were subjected to another cycle of stress repetitions and the variation in the indirect tensile strength was detected before and after healing. The healing performance of the asphalt mixture specimens at different healing conditions was investigated by observing and testing the recovery of indirect tensile strength after healing. It was concluded that implementation of iron filling and induction heating by microwave can improve the healing and tensile properties of asphalt concrete as compared to the oven heating technique.

Mechanical Behavior of Hot-Mix Asphalt Made with Recycled Concrete Aggregates from Construction and Demolition Waste: A Design of Experiments Approach

The present work is a re-evaluation of previous research on the durability of hot-mix asphalt made with recycled concrete aggregates from construction and demolition waste (CDW) with a different approach. Response surface methodology (RSM) was used to conduct this study. The kind of natural aggregates (schist and calcite-dolomite), the recycled concrete aggregates percentage (0%, 20%, 40% and 60%) and the water saturation (0% and 100%) were the pertinent factors for this methodology. Indirect tensile stress (ITS) was determined in mixtures fabricated with 0%, 20%, 40% and 60% recycled concrete aggregates. According to the results, the ITS of the bituminous mixtures increases as the percentage of recycled concrete aggregate increases. This behavior is more significant when calcite-dolomite is used as a natural aggregate. Water saturation has the same influence in both natural aggregates. The indirect tensile strength ratio (ITSR) was calculated to evaluate the stripping potential. According to the Spanish specifications, the results suggest that the percentage of CDW that can be used for hot mixes is 17% when schist is used as natural aggregate and 14% for calcite-dolomite.

Design methodology for in situ cold recycled mixtures with emulsion and 100% rap

At present there is no consensus on the most suitable methodology for the design of cold recycled mixtures with emulsion, due to its complexity. Therefore, design is generally based on compliance with one or more volumetric or mechanical parameters established in the highway regulations, which does not guarantee the optimization of the mixture. The present work proposes a design methodology for cold recycled mixtures with emulsion, in which 6 parameters are evaluated: Air voids, Indirect Tensile Stress, Indirect Tensile Strength Ratio, Wheel Tracking, Stiffness Modulus and Fatigue life. Performance indexes were established to evaluate the values obtained from the tests according to behavior graphs with the maximum value of 1. These graphs evaluate the results of the tests, considering as a maximum value the most favorable results for the optimal performance of the mixture. Finally, for the analysis of the mixtures the authors propose a Global Performance Index (GPI) that shows the average of the 6 parameters measured. The result is a number that represents each job mix formula, which is sensitive to changes in the values of the properties and is obtained using a systematic methodology. Therefore, the choice is based on technical criteria and according with the GPI; the influence of the materials or the formulas used for each mixture can be ranked.

Physical, Mechanical and Durability Properties of Soil Building Blocks Reinforced with Synthetic Fibre

Earth construction is the most economic way to solve housing problems, particularly in case of low cost building, with the limitation of resources. The simplicity in production of earth blocks justifies by the availability of soil as raw material and by the less energy in production. Generally, fibre inclusion increases the strength and performance of the earth blocks. Synthetic fibre such as AR glass fibre, polypropylene fibre were used and investigated experimentally with different percentages of fibre 0.25 %, 0.50 %, 0.75 % and 1 % by weight of earth. A sequence of test was conducted with and without addition of fibre to soil building blocks. Physical properties, mechanical properties and durability properties specifically density test, water absorption by capillarity test, linear shrinkage, compressive test, indirect tensile stress, wearing test and erosion test were conducted and compared with different fibre ratio to determine the optimum fibre content in each mix order to produce blocks that will provide the maximum strength.

Influence of fibre reinforcement on the post-cracking behaviour of a cement-stabilised sandy-clay subjected to indirect tensile stress

An experimental campaign was carried out to determine the influence of polypropylene fibre content and length on the post-cracking response of a sandy-clay stabilised with different cement contents. Three main sets of specimens were prepared: cement-stabilised specimens with two cement contents (5% and 10%); fibre-reinforced specimens with three fibre contents (0.1%, 0.2% and 0.3%) and cement-fibre-reinforced specimens combining the mentioned fibre and cement contents. Tensile tests on the fibres and indirect tensile tests and triaxial compression tests on the prepared specimens were conducted. Results show that the post-cracking behaviour is strongly affected by the combination of fibre and cement content as well as fibre length. Pull-out was the governing failure mode. Post-peak tension loss rate increased with fibre content, as a result of the loss of influence of the fibres on the post-peak behaviour. On the contrary, an increase in fibre content resulted in higher pre-peak strength gain rates and higher peak stresses.

Design Improvements of Segmental Linings Due to Unfavorable Bedding Situations

With mechanized shield tunnelling the quality of the backfill is of significant importance for the stability of the lining. The used material affects the interaction between support and rock mass. Therefore, the choice of a proper backfilling material is of great importance for the system behaviour.Scaled model tests have shown the relocation behaviour of pea gravel within the annular gap. The tests have shown that especially with Double shielded TBMs a distinct relocation mechanism is triggered by the regripping process. This leads to an unfavourable bedding situation immediately behind the TBM shield.Based on a concept study two design improvements have been developed to improve the bedding situation. The first one is based on using geotextile tubes around the lining segments which also prevents a large-scale relocation process of pea gravel within the annular gap. The basic idea of the second concept is the use of ductile support elements with yielding potential in radial direction. Both systems solely differ in the fixation leading to different load cases of the lining. Due to the clamping mechanism, the forces acting on the yield elements are transformed into indirect tensile stresses around the element within the lining.

Impact of Repeated Load on Crack Healing Cycles of Asphalt Concrete

Asphalt concrete pavement is designed to under take repeated loading for its design life with minimal maintenane requirements. Under such loading mode, micro cracks appears in the flexible pavement structure, while it exhibit self healing ability of micro cracks. In this work, the impact of repeated tensile and shear stresses on accumulation of permanent strain of asphalt concrete and micro crack healing cycles have been investigated. Asphalt concrete specimens of 100 mm diameter and 63 mm height have been prepared with optimum asphalt requirement and with extra 0.5% asphalt above and below the optimum. Specimens exhibits permamemt strain under repeated tensile and shear stresses using three stress levels (69, 138 and 207) kPa at 25°C environment. The loading cycle consists of load repetitions application for 0.1 second followed by 0.9 seconds of rest period. Specimens were allowed to heal by external heating at 60°C for 120 minutes after each 1000 load repetitions, then subjected to another load repetition cycles. The healing cycle was repeated twice. It was concluded that as the crack healing cycles proceeds, the resilient modulus increases while the permanent strain decreases. The impact of asphalt content on resilient modulus is variable through the crack healing cycles among tensile and shear stresses. After crack healing cycles, the permanent strain was reduced by an average of (45, 36 and 23)%, for (69, 138, and 207) kPa respectively as compared to reference mix under punching shear stress (PSS), while it was reduced by an average of (5, 23 and 21)%, for (69, 138, and 207) kPa respectively as compared to reference mix under indirect tensile stress (ITS).