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Fiber Bragg Grating (FBG) sensors, with excellent properties, have been widely adopted to monitor the mechanical parameters in civil engineering in recent years. On the other hand, the current study on fatigue performance of corroded prestressed steel strands is still limited, and this is mainly because the long-term strain conditions monitoring is difficult to conduct. Based on the aforementioned considerations, a total of six beam specimens were fabricated in this study. The loading mode of four points bending was adopted in the form of sinusoidal waves in the experiments. On basis of the experimental results, it can be concluded that the fatigue life of the beam decreases sharply with the increase of the corrosion rate of steel strands. Besides, with the increase of the maximum fatigue load, the fatigue life of the beam will decrease significantly. Furthermore, the existing fatigue damage of steel strand inside the beam before corrosion may further accelerate its fatigue failure. As a result, the fatigue life of the beam is reduced because of the stress concentration. Under the same external load, the strain increment and the residual strain of steel strands in the stages of loading and unloading after corrosion increase significantly compared with other stages, while the existing residual strain always shows an increasing trend at various static loading stages. Therefore, the corrosion of steel strand seriously affects not only its mechanical properties, but also its fatigue performance. Finally, the FBG sensors are capable of measuring the steel strand strain, as well as the long-term strain conditions.

To study the improvement effect of graphene modified epoxy coating on reinforcement corrosion , the epoxy resin with different content of graphene (the content of graphene was 0%, 0.05% and 0.1%) was prepared, and the anti rust effect of epoxy coating with different content of graphene was studied through experiments. To study the bonding performance between graphene/epoxy coated steel bars and concrete, the central pull-out experiment was carried out. Four different diameters (14 mm, 18 mm, 22 mm, 25 mm), four kinds of coated steel bars (three kinds of graphene modified epoxy coatings and one uncoated) were selected to make the center pull out test specimens. For the three kinds of graphene modified epoxy coatings, the graphene content is 0, 0.05%, 0.1% respectively. The results show that adding a certain amount of graphene into the epoxy coating can improve the corrosion resistance of the coating. The bonding performance between graphene/epoxy coated steel bars and concrete is worse than that of ordinary specimens without coated steel bars. The different content of graphene in epoxy coating has little effect on the bonding performance of epoxy-coated steel bars and concrete. In practical engineering, graphene/epoxy coated steel bars can be applied according to the specifications of ordinary epoxy steel bars.

The bond behavior (failure modes, bond strength, bond stress-slip curves) between steel bar and recycled aggregate concrete after different cycles of freeze–thaw was studied using pullout tests. One recycled aggregate concrete, two types of steel bars: plain steel bars with diameter 12 mm, deformed steel bars with diameter 14 mm, 18 mm, 22 mm and 0, 15, 25, 50, 75 cycles of freeze–thaw were considered in the experiment. Based on the experimental results, the influence of number of freeze–thaw cycles, type of steel bar, and diameter of deformed steel bar on the bond behavior between steel bar and recycled aggregate concrete was studied and analyzed. It was found that the bond strength decreases as the freeze–thaw cycles increased, the drop of bond strength between plain steel bar and recycled aggregate concrete is larger than that between deformed steel bar and recycled aggregate concrete as the freeze–thaw cycles increased, the pullout force and bond strength for deformed steel bar are larger than that for plain steel bar after the same cycles of freeze–thaw.

The strength of concrete under multiaxial tension–compression (biaxial tension–compression (T–C) and triaxial tension–compression–compression (T–C–C)) in the direction of tensile stress and compressive stress was lower than the uniaxial tensile strength and uniaxial compressive strength, respectively. This reduction is affected by many factors. Among these factors, type of concrete, method of friction-reducing in the direction of compressive load, stiffness of testing machine and loading rate etc. are the main factors. The mechanical behavior of normal air-entrained concrete subjected to biaxial T–C, triaxial T–C–C was studied in this paper. The experimental results of normal air-entrained concrete under multiaxial tension–compression and mechanical behavior of other types of concrete under multiaxial tension–compression conducted in the same testing machine was compared and analyzed. The failure criterion of various types of concrete under biaxial T–C and triaxial T–C–C was proposed. It provides the experimental and theory foundations for strength analysis of various concrete structures.

The strength of concrete under biaxial compression and triaxial compression is higher than the strength under uniaxial compression, but the level of increase is affected by factors such as the type of concrete, differences in testing machines, loading rate, etc. Based on this, an experimental investigation into the strength of normal air-entrained concrete (NAEC) under biaxial and triaxial compression loading conditions was carried out. The experimental results on NAEC were compared with the multiaxial compression strengths of other types of concrete obtained using the same testing machine. According to the test data, failure criteria and multiaxial ultimate strength envelopes in principal stress space were proposed. The test results indicated that biaxial compressive strength is higher than uniaxial compressive strength and the maximum increase of ultimate strength occurred at a biaxial compression stress ratio of 0·5 for all types of concrete. A relationship between biaxial compressive strength, triaxial compressive strength and stress ratio was developed. This work provides theoretical and experimental foundations for strength analysis of concrete structures.

The aim of this study is to characterize the behavior of air-concrete under triaxial compression with stress ratio α 2 = σ 2 : σ 3 = 1 : 1, while changing stress ratio α 1 = σ 1 : σ 3 from 0 to 0.2 after different cycles of freeze–thaw. 100 mm × 100 mm × 100 mm concrete cubes were tested in the experiment. Based on the experiment data, the influence of the stress ratio α 1 = σ 1 : σ 3 and the number of freeze–thaw cycles on the ultimate compressive strength σ 3 are analyzed respectively. A failure criterion expressed in terms of octahedral stress with consideration of the influence of the freeze–thaw cycles is proposed.

An experimental study of C20, C25, C30, C40, and C50 big mobility concrete cubes that came from laboratory and construction site was completed. Nondestructive testing (NDT) was carried out using impact rebound hammer (IRH) techniques to establish a correlation between the compressive strengths and the rebound number. The local curve for measuring strength of the regression method is set up and its superiority is proved. The rebound method presented is simple, quick, and reliable and covers wide ranges of concrete strengths. The rebound method can be easily applied to concrete specimens as well as existing concrete structures. The final results were compared with previous ones from the literature and also with actual results obtained from samples extracted from existing structures.

Properties of air-entrained concrete under compressive loads with constant confined stress after 0, 100, 200, 300 and 400 cycles of freeze–thaw were studied. The failure characteristic of specimens and the direction of the crack are observed. Based on the test data, the influence of freeze–thaw cycles and lateral compressive stress ratio on the ultimate compressive strength is analyzed, respectively. The relationships between the ultimate compressive strength and freeze–thaw cycles, lateral compressive stress ratio are given, respectively. The unified failure criterion with consideration of the influence of freeze–thaw cycles and stress ratio is proposed. It provides the experimental and theory foundations for strength analysis of air-entrained concrete structures subject to complex loads in cold environment.

The behavior of strength and deformation of plain concrete under triaxial compression after 0, 25, 50 and 75 cycles of freeze-thaw are experimentally studied using the static and dynamic triaxial experimental machine. The compressive strength, strain at the peak of stress and stress–strain relationship under triaxial compression were measured. The failure modes of concrete specimens are also described. The experimental results showed that the triaxial compressive strength decreased as the freeze-thaw cycles were repeated for plain concrete. The influence of the number of freeze-thaw cycles and the stress ratio on the principal compressive stresses σ 3 and corresponding strain e 3 , stress–strain relationship was analyzed. On this basis, the failure criterion of concrete under triaxial compression after freeze-thaw cycles is proposed. It can serve as a reference for the maintenance, design and the life prediction of ocean structures, hydraulic structures, marine structures and offshore platform in cold regions.

Nondestructive testing technology is essential in the quality inspection of repair, alteration, and renovation of the existing engineering, especially for concrete structure in severe environment. The objective of this work is to deal with the behavior of ultrasonic velocity and mechanical properties of plain concrete and air-entrained concrete subjected to freeze-thaw cycles (F-T-C). The ultrasonic velocity and mechanical properties (tensile strength, compressive strength, cubic compressive strength, and splitting strength) of C30 air-entrained concrete and plain concrete with different water-cement ratio (water-cement ratio was 0.55, 0.45, and 0.50, resp.) after F-T cycles were measured. The influences of F-T cycles on ultrasonic velocity and mechanical properties of C30 air-entrained concrete and plain concrete were analyzed. And the relationship between mechanical properties and ultrasonic velocity was established. The experimental results can be useful for the design of new concrete structure, maintenance and life prediction of existing concrete structure such as offshore platform and concrete dock wall.

Freezing-thawing resistance is a very significant characteristic for concrete in severe environment (such as cold region with the lowest temperature below 0°C). In this study, ordinary-air-entrained (O-A-E) concrete was produced in a laboratory environment; the compressive strength, cubic compressive strength of C50, C40, C30, C25, and C20 ordinary-air-entrained concrete, tensile strength, and cleavage strength of C30 ordinary-air-entrained concrete were measured after fast freeze-thaw cycles. The effects of fast freeze-thaw cycles on the mechanical properties (compressive strength and cleavage strength) of ordinary-air-entrained concrete materials are investigated on the basis of the experimental results. And the concise mathematical formula between mechanical behavior and number of fast freeze-thaw cycles was established. The experiment results can be used as a reference in design, maintenance, and life prediction of ordinary-air-entrained concrete structure (such as dam, offshore platform, etc.) in cold regions.

For use in fire resistance calculations, the relevant thermal properties of high-performance concrete (HPC) with fly ash were determined through an experimental study. These properties included compressive strength, cubic compressive strength, cleavage strength, flexural strength, and the ultrasonic velocity at various temperatures (20, 100, 200, 300, 400 and 500∘C) for high-performance concrete. The effect of temperature on compressive strength, cubic compressive strength, cleavage strength, flexural strength, and the ultrasonic velocity of the high-performance concrete with fly ash was discussed according to the experimental results. The change of surface characteristics with the temperature was observed. It can serve as a reference for the maintenance, design, and the life prediction of high-performance concrete engineering, such as high-rise building, subjected to elevated temperatures.