1. Introduction
As a widely used economical and practical building material, concrete plays an important role in civil construction, transportation and water conservancy construction, and underground engineering construction. Whether for civil engineering or national defense engineering, the concrete structure will be subjected to a quasi-static load, that is, a normal design load. Moreover, concrete structures are often subjected to violent dynamic loads such as impact loads or blast loads. In the safety assessment, reinforcement, and maintenance of concrete structures after fire or explosion, it is necessary to master the dynamic mechanical response law and fracture law of concrete after high temperature treatment in a complex stress state. Therefore, it is particularly important to carry out in-depth research on this. However, concrete materials are typical heterogeneous brittle materials, including cement colloids, coarse and fine aggregates, and other microcracks, holes, weak media, etc., and materials are often affected by many external factors, such as temperature, moisture content, confining pressure, loading rate, etc. In addition, the impact loading test and analysis methods considering the confining pressure are limited. Therefore, there are still many problems to be solved in the study of the influence of multiple factors on the performance of concrete materials.
At present, the test system used to study the dynamic mechanical properties of materials mainly includes split Hopkinson pressure bar (SHPB), the drop-weight test, and other test systems. Among them, the SHPB test is the most widely used [
1]. Gong [
2,
3] first used SHPB technology to study the dynamic strength of concrete materials. Brara [
4] proposed the corresponding failure criteria. Numerous researchers have also conducted a large number of studies on the effectiveness of the SHPB test, such as the dispersion effect between the elastic pressure bar and the specimen and the inertial effect of the specimen during the test [
5,
6], the friction effect between the pressure bar and the specimen and the matching degree of the cross-section end face [
7], as well as the ultra-high strain rate test technique [
8].
The conventional SHPB test device needs to be modified for use in triaxial impact compression tests. Christensen et al. [
9] improved the SHPB test device and designed a triaxial SHPB test device for rock materials, which realized the dynamic loading of rock materials under confining pressure. Gong et al. [
10] studied the effects of high strain rates and low confining pressures on the dynamic mechanical properties of sandstone, and obtained the results that under approximately the same strain rate, the dynamic triaxial compressive strength linearly increases with the confining pressure. Yeakley et al. [
11] studied the dynamic mechanical properties of basalt at a strain rate of 103 s
−1 and a confining pressure of 69 MPa. The experimental results showed that the dynamic strength of basalt increases by 30% compared with quasi-static strength under confining pressure. Rome and Nemat-Nassei [
12] improved and completed a set of triaxial SHPB test devices. Through the triaxial impact compression loading test of concrete materials, it was verified that the test device can be applied to concrete materials. Gerstle [
13] used the test method to study the strength and deformation of concrete under multiaxial stress conditions, and summarized the concrete test results obtained by many testers under complex stress conditions, which is of great significance. Mier [
14] studied the response of concrete under a triaxial stress state through experimental methods, and found by crack detection technology that the anisotropic stress–strain behavior was caused by the development of weak regions in larger particle aggregation regions during concrete hardening. The difference observed in macroscopic stress–strain behavior was the result of different crack distribution directions. Salami [
15] carried out a relevant experimental analysis on the deformation characteristics of concrete specimens with multiaxial stress states under different loading rates, and further expanded the field of concrete materials in the experimental research direction. With the continuous development of test methods and techniques, the research on the dynamic mechanical properties of brittle materials such as concrete under complex stress conditions is also deepening.
However, the SHPB experiment can only study the macroscopic mechanical properties of the material. The microscopic laws of the stress, strain distribution, and crack propagation of the specimen need to be studied by numerical simulation. Bertholf and Karnes [
16] used two-dimensional numerical analysis to describe the effects of friction, geometry, and strain rate on the test. Zhu [
17] analyzed the failure process of dynamically loaded rock media by simulating the basic principle of the failure process of rock under static and dynamic loads. Besides, the effects of axial static stress and dynamic stress on rock damage and failure process are also studied. Guo [
18] analyzed the temperature-dependent changes of peak stress, peak strain, and elastic modulus of materials under passive confining pressure by numerical simulation. Park [
19] used the finite element method to analyze the dynamic response of concrete and mortar under high strain rates, and discussed the influence of aggregate volume fractions on the dynamic bearing capacity of concrete. The numerical simulation also showed that as the aggregate integral number increased, the energy absorption increases, and energy dissipation occurs with deformation. Zhang [
20] used the HJC (Holmquist-Johnson-Cook) model to simulate the SHPB impact test with confining pressure for concrete, and it indicated that confining pressure increases with the growth of strength, ductility, and toughness.
The current research in this area is mainly for the uniaxial test and numerical analysis of the room temperature specimens. The comparative study on the passive confining pressure test and numerical simulation of the concrete after thermal treatment is relatively rare, and at the same time, there have been relatively few studies on the specimen crushing process and crushing law. Therefore, this study uses the combination of experiment and numerical simulation to analyze the impact mechanical properties and dynamic failure of different loading rates under the passive confining pressure of concrete after thermal treatment.
5. Conclusions
The effects of loading rate and thermal treatment on concrete strength under uniaxial and passive confining pressures were studied by a combination of tests and numerical simulations. Using numerical simulation to analyze the fracture process and fracture state of the concrete specimen, the conclusions of the research are as follows.
The uniaxial impact compression test shows that the compressive strength of concrete increases with the increase of a certain loading rate at the same temperature. At the same loading rate, the compressive strength of concrete increases slightly with the increase of low heating temperatures. When the temperature exceeds 400 °C, the compressive strength decreases sharply as the temperature increases.
The passive confining pressure test shows that with the increase of the loading rate, the passive confining pressure of the concrete is obviously enhanced, and the increase range is 3.29 MPa to 4.31 MPa. The effect of the thermal treatment temperature on the passive confining pressure is not obvious. The sleeve effect of the passive confining pressure significantly improves the deformability of the concrete and increases the compressive capacity of the concrete by 12% to 41%. However, the increase in the strength of the material with the temperature damage effect is reduced, and is particularly obvious above 600 °C and with a high loading rate.
From numerical simulation, it can be found that the fracture form has a strong rate dependence in the crushing process. When the loading rate is low, the fracture form is that of a cleavage-like failure. As the loading rate increases, the fracture form changes to crush failure. In a uniaxial test, as the impact rate increases, the crack initiation time advances, and the degree of fracture increases at the same loading time. The fracture is from the edge to the center, and the core of the specimen remains intact. In the case with confining pressure, the stress gradually decreases toward the diameter edge at the center, and the damage at the center is obvious.