Calculation Derivation and Test Verification of Indirect Tensile Strength of Asphalt Pavement Interlayers at Low Temperatures

When the direct tensile test is adopted to determine the interlayer tensile strength of the asphalt pavements, specimen separation or internal cracking often occurs at the bonding area of the loading head, rather than at the interlaminar bonding interface. In view of the tedious and discrete data of the direct tensile test, this paper attempts to introduce an indirect tensile test to determine the interlayer bond strength of asphalt pavement to solve this problem. However, the indirect tensile test method of a binder lacks the corresponding mechanical theory. This paper deduces the calculation formula of the indirect tensile strength of a binder based on elastic theory. A mechanical model of the test was established with the finite element method. In accordance with the two-dimensional elastic theory and the Flamant solution, an analytical solution of tensile stress in the indirect tensile test is proposed through the stress superposition. On this basis, the calculation formula for the indirect tensile strength of the interlaminar bonding is derived according to Tresca’s law. A low-temperature indirect tensile test was designed and conducted to verify the correctness of the formula. By comparing the results of the indirect tensile test and direct tensile test, it is found that the interlaminar strength of the mixture measured by them is similar, and the dispersion of indirect tensile test results is small. The results show that the indirect tensile test can replace the direct tensile test to evaluate the interlaminar tensile strength.

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.

Structural performance and section optimization of precast concrete sandwich panels with pin-type GFRP connectors

A precast concrete sandwich panel (PCSP) offers a good potential in the application of façade wall due to the improved energy efficiency. In this study, the structural performance of PCSP with pin-type glass fiber reinforced polymer (GFRP) connectors was investigated, and an optimization characterized by ribbed structural wythe was proposed and studied. Firstly, the pull-out and shear capacity of the pin-type connector were evaluated through direct tensile test and direct shear test, respectively. Thereafter, seven PCSP specimens were fabricated and tested under four points flexural load. The investigating parameters included the structural wythe thickness, loading direction, insulation bond, and section type of the structural wythe. The load-deflection relationship, crack pattern, failure mode, load-strain relationship, and degree of composite action of the PCSP were studied and compared. It was concluded that: (1) the tested PCSPs presented ductile failures; (2) the structural wythe thickness, loading direction and insulation bond would influence the cracking, yielding, and peak loads of the tested PCSP; (3) the PCSP with pin-type GFRP connectors could be designed as non-composite type owing to the low composite action; and (4) the proposed ribbed structural wythe could achieve a lightweight PCSP while considerable flexural stiffness and capacity could be retained.

Exploring Environmentally Friendly Biopolymer Material Effect on Soil Tensile and Compressive Behavior

The study of the high-performance of biopolymers and current eco-friendly have recently emerged. However, the micro-behavior and underlying mechanisms during the test are still unclear. In this study, we conducted experimental and numerical tests in parallel to investigate the impact of different xanthan gum biopolymer contents sand. Then, a numerical simulation of the direct tensile test under different tensile positions was carried out. The micro-characteristics of the biopolymer-treated sand were captured and analyzed by numerical simulations. The results indicate that the biopolymer can substantially increase the uniaxial compressive strength and tensile strength of the soil. The analysis of the microparameters demonstrates the increase in the contact bond parameter values with different biopolymer contents, and stronger bonding strength is provided with a higher biopolymer content from the microscale. The contact force and crack development during the test were visualized in the paper. In addition, a regression model for predicting the direct tensile strength under different tensile positions was established. The numerical simulation results explained the mechanical and fracture behavior of xanthan gum biopolymer stabilized sand under uniaxial compression, which provides a better understanding of the biopolymer strengthening effect.

Recovery Behavior of Fe-Based Shape Memory Alloys under Different Restraints

This paper presents the experimental results of an evaluation of the recovery behavior of Fe-based shape memory alloys (Fe-SMAs) under different restraints. For the study, three types of Fe-SMA (FSMA-A, FSMA-B, FSMA-C) were produced. As a result of the direct tensile test, the yield strength of the FSMA-A specimen was nearly 34% higher than the strength of FSMA-B and FSMA-C. Under free restraint, the recovery strains are 0.00956, 0.01445, and 0.01977 for FSMA-A, FSMA-B and FSMA-C specimens, respectively, after activation when the pre-strain is 0.04, and the heating temperature 200 °C. Under rigid restraint, the final recovery stresses are 518, 391 and 401 MPa for FSMA-A, FSMA-B, FSMA-C specimens after activation when a pre-strain of 0.04 and heating temperature 200 °C. Additionally, under the rigid restraint, the effect of pre-strain on the final recovery stress was insignificant, whereas the final recovery stress increased as the heating temperature increased. When Fe-SMA was constrained during cooling, the recovery stress is 50% lower than under rigid restraint. Hence, in order to develop a large recovery stress, Fe-SMA must be constrained during heating. In addition, a method for calculating the effective confining stress of the Fe-SMA coupler for pipe joining was proposed based on the experimental results.

Enhancement of Energy Absorption Capacity of Polyethylene Fiber-Reinforced Cementitious Composites According to Admixtures and Curing Conditions

This study aims to enhance the energy absorption capacity of cementitious composites with 2 vol.% of polyethylene fibers, by adjusting mixing ingredients and curing conditions. Ground blast furnace slag, cement kiln dust, limestone powder, and silica fume were incorporated, and two different curing conditions were applied: 72 h of curing at 90 ℃ and 120 h of curing at 40 ℃. Compressive strength test and direct tensile test were performed on 6 mixtures and the test results were compared with those of ultra-high-performance concrete and engineered cementitious composite specimens. The maximum compressive strength of the 6 mixtures was measured to be approximately 117 MPa. The higher cement replacement ratio of the other components resulted in a decrease in the compressive strength of the specimens cured at 90 ℃. In the direct tensile test, the specimens cured at 40 ℃ exhibited lower tensile strength than those cured at 90 ℃, but the strain capacity was increased by approximately 305% and reached 7.7%. This also resulted in an enhancement of the energy absorption capacity from 80%–292% because of the differences in micro-cracking and fracturing behaviors, such as an increase inthe number of micro-cracks and decrease in crack width.