Direct Tensile Research Articles

Experimental Study on Effect of Temperature on Direct Tensile Behavior of Hydraulic Asphalt Concrete at Different Strain Rates

The tensile mechanical properties of hydraulic asphalt concrete (HAC) play a significantly important role in the safety of the impervious core in embankment dams. This paper aims to develop a direct tensile test setup with temperature control, to devise the direct tensile test method for HAC, and to investigate the direct tensile properties of HAC in a temperature range of 0°C–20°C and a strain rate range of 10−5−10−2 s−1. Through a large number of direct tensile tests, the shape and size of the specimen and the connection mode between the clamp and the specimen were designed. The test results show that the temperature and the strain rate significantly affect the tensile stress–strain characteristics of the HAC. As the temperature decreased from 20°C– 0°C or the strain rate increased from 10−5−10−2 s−1, the tensile strength, the tensile modulus, and the aggregate cracking ratio increased, whereas the peak strain declined. Moreover, the empirical formulas for the tensile strength, tensile modulus, and peak strain were successfully derived by applying the time–temperature superposition principle. The coefficient of determination was more than 0.95, indicating that the modeling data well agreed with the test results. Finally, according to the Mohr–Coulomb criterion, the effect of the temperature on the cohesion and the internal friction angle was analyzed at the various strain rates. The obtained test results can provide a reference for the design of asphalt concrete cores in embankment dams.

Interlayer Bond Strength Testing in 3D-Printed Mineral Materials for Construction Applications.

There are no standards for testing the properties of 3D-printed materials; hence, the need to develop guidelines for implementing this type of experiment is necessary. The work concerns the development of a research methodology for interlayer bond strength evaluation in 3D-printed mineral materials. In additive manufactured construction elements, the bond strength is a significant factor as it determines the load-bearing capacity of the entire structural element. After we completed a literature review, the following three test methods were selected for consideration: direct tensile, splitting, and shear tests. The paper compares the testing procedure, results, and sample failure modes. The splitting test was found to be the most effective for assessing layer adhesion, by giving the lowest scatter of results while being an easy test to carry out.

Eco-Friendly, High-Ductility Slag/Fly-Ash-Based Engineered Cementitious Composite (ECC) Reinforced with PE Fibers

Engineered cementitious composites (ECCs) are a special class of ultra-ductile fiber-reinforced cementitious composites containing a significant amount of short discontinuous fibers. The distinctive tensile strain-hardening behavior of ECCs is the result of a systematic design based on the micromechanics of the fiber, matrix, and fiber–matrix interface. However, ECCs require extensive cement content, which is inconsistent with the goal of sustainable and green building materials. Consequently, the objective of this study is to investigate the mechanical performance of slag/fly-ash-based engineered cementitious composites (ECCs) reinforced with polyethylene (PE) fiber under axial compressive loading, as well as direct tensile and flexural strength tests. The composites’ microstructure and mineralogical composition were analyzed using images obtained from scanning electron microscopy (SEM), X-ray energy diffraction spectroscopy (EDS), X-ray powder diffraction (XRD), and X-ray fluorescence (XRF). The experimental results reveal that a slag-containing composite mixture shows strain-hardening behavior and comparable ductility properties to those of fly-ash-based composite mixtures. A ternary system of binder materials with 5% and 15% slag can increase the compressive strength of ECC by 3.5% and 34.9%, respectively, compared to slag-free ECC composite. Moreover, the microstructural results show that the slag-based cementitious matrix has a more closely cross-linked and dense microstructure at the matrix–aggregate interface. In addition, the concentration of particles on the surface of the fibers was higher in the slag-based cementitious composites than in the fly ash-based composite. This supports the concept that there is a stronger bonding between the fibers and matrix in the slag-based cementitious matrix than in fly-ash-based matrix.

A Comparative Study of LASSO and ANN Regressions for the Prediction of the Direct Tensile Behavior of UHPFRC

Abstract Direct tensile behavior is one of the most relevant properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC). However, the determination of this behavior implies the realization of complex tests that must be carried out by experienced personnel because small variations could invalidate the results. This research purpose was to develop and compare two different algorithmic approaches for the direct tensile behavior of UHPFRC. Thereby, the cracking tensile strength σcc, the tensile postcracking capacity σpc and its associated strains, εcc, and εpc, were modeled by means of least absolute shrinkage and selection operator (LASSO) regression and artificial neural networks (ANN). Four ANN models, one for each response, and four LASSO models were created to produce the most accurate approaches. Results showed the reliability of the developed models through statistical indices such as the root of the mean squared error (RMSE), mean absolute error, normalized mean bias error, the ratio of the RMSE to the standard deviation of measured data, coefficient of efficiency, and coefficient of determination (R2). Besides, the analytical research also showed that the highest accuracy belongs to ANN models, with R2 values of .922, .807, .901, and .858, in forecasting the features of direct tensile behavior of UHPFRC (σcc, εcc, σpc, and εpc).

Influence of Freeze/Thaw and Alkaline Environment on the Mechanical Properties of a C-FRCM System Bonded on Masonry Substrate

The Fabric Reinforced Cementitious Matrix (FRCM) system in the last two decades was used to strengthen and rehabilitate historical masonry structures as an alternative of the well-known Fiber-Reinforced Polymer (FRP) system. The FRCM systems consist of different types of fiber in form of fabric mesh immersed in a cementitious/hydraulic or lime matrix; they present better compatibility with the substrate and less limitations in heritage applications. However, the durability and the interaction FRCM-substrate under the artificial aging mechanism in aggressive environment is not well known. The paper reports and discuss the first results of an experimental program planned to investigate the durability of some FRCM systems. The effects of the freeze/thaw and alkaline environment on both the mechanical properties of the FRCM coupons and the bond FRCM-to-masonry substrate were analyzed by the results of direct tensile and single-lap shear tests.

Evaluation on the Performance of Hydraulic Bitumen Binders under High and Low Temperatures for Pumped Storage Power Station Projects.

The high and low-temperature performance of five hydraulic bitumen binders was evaluated using the dynamic shear rheometer (DSR) test, infrared spectrum test and direct tensile (DT) test. These hydraulic bitumen binders were respectively applied for several pumped storage power stations (PSPS) projects that were constructed or under construction. In order to relate the bitumen performance to the mixture performance, the slope flow test, three-point bending test and thermal stress restrained specimen test were carried out on hydraulic asphalt mixtures. The test results indicated the DSR rheological master curves can well distinguish the difference of each bitumen binder as well as the effect of polymer modification. Phase angle master curves, black diagrams and infrared spectra all indicated that several penetration-grade hydraulic bitumen binders were not virgin bitumen binders but were modified with relatively lower SBS polymer content when compared with traditional SBS-modified bitumen. When selecting the commonly used Karamay SG70 hydraulic bitumen as a reference, the normal SBS-modified bitumen was superior to other bitumen in terms of low- and high-temperature performance. Several slightly SBS-modified bitumen binders did not always show consistent results, which indicated that slightly modified bitumen may not really have the desired performance as expected. Therefore, SBS-modified bitumen will be more promising when dealing with extremely low or high temperatures. Bitumen performance was well compared with the mixture performance by using the bitumen creep, relaxation and tensile failure strain corresponding to the asphalt concrete slope flow, the maximum bending strain and the failure temperature, respectively. Compared with the traditional penetration, softening point and ductility test, it indicated that the DSR rheological test, creep test, direct tensile test and stress relaxation test can be used as more powerful tools for the characterization and optimization of hydraulic bitumen binders.

Evaluation of mechanical properties and flexural behavior of FRCM system composed of different types of textile grid and AL powder

This experimental work aims to evaluate the flexural strengthening effectiveness of, as well as to mechanical characterize, the different types of textile grid and AL (Aluminum) powder surface coating. The experiment variables were weaving method (Pillar stitch, Leno weave) type of impregnation (SBR, EPOXY), and AL powder surface coating. The mechanical properties of FRCM systems were evaluated by direct tensile and bending tests of FRCM and flexural tests of RC beams. Direct tensile tests of FRCM were performed with clevis grips (AC434.13). Bending tests of FRCM were performed with ASTM C 1609 M. Flexural behavior and performance were analyzed in flexural tests using reinforced concrete beams strengthened with FRCM systems. For flexural tests, nine reinforced concrete beams 2,500 mm long, 150 mm wide, and 250 mm deep were fabricated. The RC beams were classified into two groups according to the rebar ratio (1.5ρmin, 0.75ρmax), and consisted of two reference specimens and seven specimens externally strengthened with the FRCM system. Test results showed that the mechanical property of FRCM systems and the flexural capacity of RC beams highly depend on the weaving methods and the impregnation types of textile grids.

Influence of Autogenous Shrinkage on the Uniaxial Direct Tensile Behavior of Uhpc with Different Reinforcement Ratios

Influence of Autogenous Shrinkage on the Uniaxial Direct Tensile Behavior of Uhpc with Different Reinforcement Ratios

The Influence of Loading Rate on Direct and Indirect Tensile Strengths: Laboratory and Numerical Methods

To investigate different responses of direct and indirect tensile strengths to loading rate, direct and indirect tension tests were performed on sandstone, rust stone, and granite specimens. Typical load curves indicate that a peak tensile stress frequently appears before the second peak stress, used to calculate the tensile strength in indirect tension tests. As expected, increase in the loading rate increases the tensile strength. In addition, the calculated tensile strengths of the indirect tension tests are frequently higher. Interestingly, the increase ratio of the tensile strength with the increase in the loading rate in indirect tension tests is higher. To verify the above results, crack propagation and stress evolution in direct and indirect tension tests were dynamically monitored using PFC 3D. For direct tension tests, specimens fail at the peak tension point, corresponding to the tensile strength. However, for indirect tension tests, minor cracks, composing of continuous microcracks, form before the peak stress and accompany with the decreased slope of the compression curve. At the peak point, tensile stresses significantly concentrate at the crack tips and further cause large-scale crack propagation. In addition, the initiation stress instead of the peak tensile stress is closer to the tensile strength, obtained from the direct tests for the same loading rate.

Study of Interactions between Induced and Natural Fracture Effects on Hydraulic Fracture Propagation Using a Discrete Approach

Abstract The interaction mode of induced fracture and natural fracture plays an important role in prediction of hydraulic fracture propagation. In this paper, a two-dimensional hydromechanical coupled discrete element model is first introduced in the framework of particle flow simulation, which can well take into account mechanical and hydraulic properties of rock samples with natural fracture. The model’s parameters are strictly calibrated by conducting numerical simulations of uniaxial compression test and direct tensile and shear tests, as well as fluid flow test. The effectiveness of coupled model is also assessed by describing hydraulic fracture propagation in two representative cases, respectively, rock samples with and without preexisting fracture. With this model in hand, the effects of interaction between induced and natural fractures with different approach angles and differential stresses on fluid injection pressure and fracture propagation patterns are investigated and discussed. Results suggest that the interaction modes mainly involve three basic behaviors including the arrested, captured with offset, and directly crossing. For a given differential stress, the captured offset of hydraulic fracture by natural fracture gradually decreases with the approach angle increase, while for a fixed approach angle, that captured offset increases with differential stress decrease.

Effect of Heterogeneity in Micro-Structure and Micro-Strength on the Discrepancies Between Direct and Indirect Tensile Tests on Brittle Rock

Effect of Heterogeneity in Micro-Structure and Micro-Strength on the Discrepancies Between Direct and Indirect Tensile Tests on Brittle Rock

Experimental and Numerical Investigation on the Size Effect of Ultrahigh-Performance Fibre-Reinforced Concrete (UHFRC).

In the last few years, there has been increasing interest in the use of Ultrahigh-Performance Fibre-Reinforced Concrete (UHPFRC) layers or jackets, which have been proved to be quite effective in strengthening applications. However, to facilitate the extensive use of UHPFRC in strengthening applications, reliable numerical models need to be developed. In the case of UHPFRC, it is common practice to perform either direct tensile or flexural tests to determine the UHPFRC tensile stress–strain models. However, the geometry of the specimens used for the material characterization is, in most cases, significantly different to the geometry of the layers used in strengthening applications which are normally of quite small thickness. Therefore, and since the material properties of UHPFRC are highly dependent on the dimensions of the examined specimens, the so called “size effect” needs to be considered for the development of an improved modelling approach. In this study, direct tensile tests have been used and a constitutive model for the tensile behaviour of UHPFRC is proposed, taking into consideration the size of the finite elements. The efficiency and reliability of the proposed approach has been validated using experimental data on prisms with different geometries, tested in flexure and in direct tension.

Distribution of dissipated pseudo-strain energies and temperature effect on fatigue cracking resistance in asphalt mixtures under destructive tensile loading

This paper studies the effect of temperature on the resistance of asphalt mixtures to fatigue cracking by calculating the parameters of the modified Paris’ Law more accurately at multiple temperatures in the basis of separating the DPSE components. Firstly, the repeated direct tensile (RDT) tests with two strain levels were performed on four types of asphalt mixtures to obtain the mechanical properties in the non-destructive and destructive stage under 5 sets of temperatures including 0 °C, 10 °C, 20 °C, 30 °C and 40 °C.The results show that energy ratio to develop fatigue cracking (DPSEF) increases first and then decreases with the temperature increasing, while energy ratio to develop permanent deformation (DPSEP) decreased continuously. If only considering for the relative proportion of the two destructive effects, it was found that the dominant damage mode is fatigue cracking in the 0 °C to 20 °C interval. The dominant damage mode at 40 °C is permanent deformation, and the dominant damage mode at 30 °C depends on material types. This finding helps to understand the development and distribution of fatigue cracking and permanent deformation at different temperatures.Then the DPSEF is further introduced to the modified Paris’ Law to derive the coefficients AJR and nJR. The logarithmic form of AJR and nJR at different temperatures follow the same linear relationship while remaining consistent model fitting coefficients. The damage density follows a good linear relationship with the logarithmic form of the parameter nJR which is independent of temperature. Parameter nJR can reflect the level of cracking resistance performance which is not limited by material type and temperature conditions.

Investigation on the piezoresistive response of carbon fiber-graphite modified asphalt mixtures

The correlation between the electrical properties of carbon fiber-graphite composite conductive asphalt mixes and their cracking damage can provide an early warning function for cracking. This study is devoted to exploring the force-electrical sensitivity of conductive concrete to find new solutions for asphalt pavement cracking warning. Firstly, a carbon fiber-graphite composite modified asphalt mixture was prepared; Secondly, the force-electrical sensitivity of the mixture was verified by direct and indirect tensile creep test. The tests showed that the resistivity and its cracking degree had a strong correlation. As the crack increases, so does the resistance. When the cracking reaches a certain level, the sudden change in resistance indicates that the mixture has been destroyed internally. Its resistivity and strain show a clear correlation before the mixture is broken. With the increase of strain, the resistivity gradually decreases. At the stage of large fluctuations in resistivity, it can be assumed that the mixture has reached the load carrying capacity limit, and at the stage of increasing resistivity abruptly, it can be judged that the mixture has undergone damaged and lost strength. Finally, the road performance was verified. A comparative study found that the high temperature, water stability and crack resistance are better than those of ordinary and graphite, and the fatigue performance is better than that of graphite. The research of the carbon fiber-graphite composite conductive asphalt concrete provides technical support for the application of the next crack warning function.

Mechanical characterisation for numerical simulation of extrusion-based 3D concrete printing

Extrusion-based 3DCP is experiencing exponential advancement in process, control, material, and fresh-state analysis technologies, strategically poising 3DCP to become an industrial manufacturing process for infrastructure development. To elucidate the mechanical characterisation and numerical simulation of 3DCP elements synergy among experimental and computational mechanics is required. Such activities should characterise the unique material properties of printed components to permit limit state design procedures and the correct prescription of finite element (FE) model parameters. In this research, mechanical characterisation procedures that experimentally evaluate the anisotropic material characteristics of a fibre-reinforced printable concrete (FRPC) are presented. These procedures are comprised of direct tensile, uniaxial compression, Young's modulus, and four-point bending (FPB) crack mouth opening displacement (CMOD) tests. The results portrayed anisotropic non-linear and similar elastic behaviour (in terms of Young's modulus) in all experimental tests conducted. From the mechanical characterisation tests two elastic parameters, seven strength parameters and five inelastic parameters are ascertained. The experimental findings relating to the material characteristic parameters are validated via supplementary numerical evaluation, and suitable constitutive relations are selected. The mechanical parameters are implemented in an anisotropic Rankine-Hill continuum multi-surface plasticity FE model, and the FPB-CMOD fracture response of 3DCP specimens is simulate with respectable agreement.

Tensile Behaviour of FRCM Composites for Strengthening of Masonry Structures-An Experimental Investigation.

This paper presents the results of direct tensile tests performed on six different FRCM (fabric reinforced cementitious matrix) strengthening systems used for masonry structures. The emphasis was placed on the determination of the mechanical parameters of each tested system and a comparison of their tensile behaviour in terms of first crack stress, ultimate stress, ultimate strain, cracking pattern, failure mode and idealised tensile stress-strain curve. In addition to the basic mechanical tensile parameters, accidental load eccentricities, matrix tensile strengths, and matrix modules of elasticity were estimated. The results of the tests showed that the tensile behaviour of FRCM composites strongly depends on the parameters of the constituent materials (matrix and fabric). In the tests, tensile failure of reinforcement and fibre slippage within the matrix were observed. The presented research showed that the accidental eccentricities did not substantially affect the obtained results and that the more slender the specimen used, the more consistent the obtained results. The analysis based on a rule of mixtures showed that the direct tensile to flexural tensile strength ratio of the matrixes used in the test was 0.2 to 0.4. Finally, the tensile stress–strain relationship for the tested FRCMs was idealised by a bi- or tri-linear curve.

Reinforcing effect of recycled polypropylene fibers on a clayey lateritic soil in different compaction degrees

The present research investigates the reinforcing effect of recycled polypropylene (R-PP) fibers on a compacted clayey lateritic soil in different compaction degrees. R-PP fibers of 12 mm length were mixed with the soil in the contents of 0.1 and 0.25% of soil dry weight. Unconfined compression strength tests (UCS), direct shear tests and indirect tensile strength (ITS) tests were conducted. Fibers addition showed no significate alterations in the optimum compaction parameters. The study evidenced increases in UCS, changing the soil behavior from a brittle failure to a ductile failure, while fiber contribution was most effective for 0.25% R-PP fibers content and 95% compaction degree. The use of fibers improved the shear stress-strain behavior of the composites and soils compacted at different degrees of compaction showed similar shear behavior, which is coherent to the soil water retention curves (SWRC) results. Significant increases in the tensile behavior of soil-mixtures for both fiber contents used were observed, and fibers increase was more significate than increase in soil degree of compaction. The stretching of the fibers and fibers orientation at the sheared interface in direct shear tests and the fiber “bridge” effect in ITS tests could be observed.

A New Flattened Cylinder Specimen for Direct Tensile Test of Rock.

In recent decades, researchers have paid more attention to the indirect tensile test than to the direct tensile test (DTT) of rocks, mainly due to difficulties in the alignment and the stress concentration at the end of an intact cylindrical specimen. In this paper, a new flattened cylinder specimen and a clamp device were designed to obtain the true tensile strength of the rock in DTT. Stress distributions of the specimen with different lengths (l) and cutting thicknesses (t) were analyzed, and damage processes of the specimen were monitored by the Digital Image Correlation (DIC), the fractured sections were also scanned. Different mechanical parameters were also obtained by the DTT of the flattened cylinder specimens and the intact cylinder specimens, as well as the Brazilian disc. Research results show that the tensile strength obtained by DTT is smaller than the Brazilian disc and is slightly greater than the intact cylindrical specimen. The flattened cylinder specimen with 0.20 ≤ 2t/D < 0.68 and 0.10 ≤ l/D ≤ 0.20 is recommended to measure the true tensile strength of rock material in DTT. This new shape of the specimen is promising to be extended in the uniaxial or triaxial direct tension test.

Determining Tensile Strength of Rock by the Direct Tensile, Brazilian Splitting, and Three-Point Bending Methods: A Comparative Study

To accurately obtain the tensile strength of rock and fully understand the evolution process of rock failure is one of the key issues to the research of rock mechanics theories and rock mass engineering applications. Using direct tensile, Brazilian splitting, and three-point bending test methods, we performed indoor and numerical simulation experiments on marble, granite, and diabase and investigated the tensile strength and damage evolution process of several typical rocks in the three different tests. Our experiments demonstrate that (1) the strength is about 10% greater in the Brazilian splitting than in the direct tensile, while the tensile modulus is lower; it is the highest in the three-point bending, which is actually subjected to the bending moment and suggested as one of the indexes to evaluate the tensile strength of rock; (2) the strength in splitting tests is strikingly different, while the strain law is basically similar; the direct tensile test with precut slits is more attainable than that with no-cut slits, with an uninfluenced strength; (3) the failure modes of rocks using different methods are featured by different lithology, while their final modes are basically the same under the same method; (4) PFC and RFPA numerical simulation tests are effective to analyze the internal crack multiplication and acoustic emission changes in the rock as well as the damage evolution process of rock in different tests.

Degradation of glass fiber reinforced polymer (GFRP) bars in concrete environment

Degradation of tensile, shear and flexure properties of glass fiber reinforced polymer (GFRP) bars is investigated in the paper. Glass fibre reinforced polymer bars, when used in structures exposed to aggressive environments, can significantly increase the lifetime, as compared to structures reinforced with conventional steel. However, the progress in utilization of GFRP bars in concrete structures is hampered due to limited information on durability of these materials. Since long-term durability data are not readily available, accelerated aging tests have been used in this research to study GFRP bar degradation. The bars were kept in a highly alkaline solution heated to 50, 60 and 70 °C for 1, 3 and 5 months, respectively, and after each immersion period, bars were taken out and tested in tension, shear and flexure. The experimental results show that immersion in high pH of the alkaline solution reduces the strength of the bars. The speed of degradation depends on the temperature of the solution. The degradation is then studied using the models for solution ingress into the GFRP bars. The conclusions proposed are that the shear test is a good indicator of the tensile strength degradation, bent bars deteriorate faster than straight bars, and smaller diameter bars deteriorate faster than larger diameter bars; finally, bars under flexural (tensile) strength deteriorate faster than those under direct tensile and shear strengths.