Materials and Structures最新文献

Corrosion of reinforcing bars deteriorates the bond between the rebar and concrete, consequently leading to the degradation of the strength and stiffness of reinforced concrete. Because structural deterioration due to corrosion may cause failure and large-scale disaster, Query various methods of measuring the corrosion ratio (electrochemical methods, fiber optic sensors, strain gauge, ultrasonic wave, and infrared thermography) have been proposed so far. The electrochemical methods are the most widely used for estimating corrosion ratios but have the limitations that the measurement results are qualitative, and are affected by climatic parameters. To overcome these limitations, fiber Bragg grating sensors to directly detect the expansion displacement of reinforcing bars due to corrosion have been developed. However, these sensors are susceptible to strain induced by heat and external loads, and they also present challenges in terms of miniaturization and installation. This study developed a new optical sensor that can overcome the limitations of existing sensors. The developed sensor detects the expansion displacement of the sacrificial steel plate due to corrosion in order to indirectly estimate the corrosion ratio of rebar. A linear relationship between the displacement and the sensor response was observed, and the sensor was capable of measuring the displacement up to half the depth of the sensor. The sensor also showed a reversible response to repeated displacement and was stable even in alkaline and salty environments. Its practicality was also verified by accelerated corrosion tests.

Adequate development of the substrate-to-overlay bond is crucial in repaired concrete structures. Poorly developed bonds may facilitate crack propagation, a concern evaluated through fracture tests. However, the scarcity of fracture tests, especially for in-plane shear-type cracks (Mode II), coupled with the reliance on strength-based bond characterizations in field tests, emphasizes the need to understand the relationship between fracture and strength behavior. Therefore, this study compares tensile, shear, and Mode I and Mode II fracture tests in characterizing shotcrete-to-concrete interface bonds with different substrate surface preparation techniques (chipped (C), sandblasted (SB), pressure-washed (PW), and as-cast (AC)). Results indicate that all three test methods are sensitive to the substrate surface preparation technique. The shear bond strengths in C, SB, and AC specimens are over two times the corresponding tensile bond strengths. In contrast, the shear bond strength of PW specimens is about 73% of the corresponding tensile strength. It is also evident that the Mode II fracture and shear behavior closely resemble each other and are more sensitive to surface roughness than the tensile bond strength. The comparative tests conducted in this study can assist in screening surface preparation techniques for cementitious overlays.

This paper seeks to provide a better understanding of the performance of calcium sulfoaluminate cement (CSA) in comparison with calcium aluminate cement (CAC) and Portland-limestone cement, CEM II A/L 52.5 N (CEM II) in live sewer environments, thereby providing rich field data for enhancing sewer concrete design. Three concrete mixes using these binders, with 0.34 w/b, siliceous pit sand, and dolomite aggregates, were prepared and exposed for 26 months in two sewer sites. Visual observations, measurements of concrete surface pH, and mass and thickness change were conducted regularly to observe deterioration. After exposure, microstructural analyses based on SEM, QEMSCAN, and XRD were conducted to clarify the deterioration mechanisms further. Regardless of sewer exposure conditions, the results indicated that CAC concrete had superior performance, followed by CSA, then CEM II. Sewer hydraulic action and high H₂S gas concentrations (max. > 300 ppm) caused accelerated corrosion rates. Binder performances were primarily related to their chemistry, mineralogy, and aggregate interaction.

Seawater sea-sand concrete (SSC) structures reinforced with fiber reinforced polymer (FRP) bars were proposed to capture CO₂ by means of carbonation curing in this study. FRP-SSC structures allowed sufficient carbonation to occur since the steel corrosion in traditional reinforced concrete structures would not exist. Herein, the pore structure of CO₂-cured SSC with sufficient carbonation was examined, and the mechanical behaviors under uniaxial compression were also investigated. MIP testing was employed, and surface fractal dimension in various pore-size regions was calculated. The results indicate that CO₂ curing leads to a more significant variation in smaller mesopores of SSC than CC. Regarding middle capillary pores, the surface fractal dimension in almost all CO₂-cured specimens ranges from 2.6617 to 2.8124, which means that these pores show distinct fractal characteristics, but this phenomenon does not be observed in water-cured specimens. This indicates that CO₂ curing can greatly reduce ink-bottle pores in concrete. Furthermore, the compressive strength gain of CO₂-cured SSC with sufficient carbonation is above 30% at the 180-days age. The compressive strength gain can be attributed to the improvement in the surface fractal dimension. Moreover, CO₂-cured specimens exhibit higher peak stress, smaller peak strain, and greater elastic module, resulting in lower plasticity. Consequently, CO₂ curing renders SSC and CC more brittle.

A bending test was selected by modern codes as a reference test for fibre-reinforced concrete (FRC) mechanical characterization. However, specimen dimensions, lack of laboratories adequately equipped, and its complexity hinder its use. This study aims to evaluate the so-called Montevideo (MVD) test as an alternative to the results of EN14651 bending tests, simplifying FRC mechanical evaluation. A strong correlation was obtained using the results of experimental campaigns carried out in three countries. Using two linear transformations, MVD loads can be converted to the EN14651 ones, both for the limit of proportionality and for the residual loads, which are valid for all the CMOD reported in EN14651. These general rules seem valid for different types of concretes (conventional, Self-Compacting, Ultra High-Performance, Micro and Sprayed concrete), blended with different fibre types (plastic and steel) and a wide range of contents, which show both softening and hardening behaviour.

It is insufficient to enhance the strength of shotcrete by simply relying on the early strength component aluminum sulfate in the liquid accelerator. The introduction of organic additives can effectively compensate for this deficiency. In this study, three different structures of organic ethers, 18-crown-6 (ECS), tetraethylene glycol monomethyl ether (Tg), and ethylene glycol monomethyl ether (Eg), were added to an aluminum sulfate alkali-free liquid accelerator, and their effects were comprehensively investigated. The results revealed that these organic ethers were capable of bolstering the strength of the mortar after 28 d. The efficiency of these ether organics on the strength of cement mixed with the accelerator followed the order: ECS > Tg > Eg. Specifically, when the ECS content reached 0.3%, the compressive strength of the mortar at 1 d was measured at 8.07 MPa. In addition, changes in the phase composition and micromorphology were scrutinized through X-ray diffraction and scanning electron microscopy, culminating in the establishment of a mechanistic model elucidating the hydration process of organic ether accelerator cement.