**2. Conventional Triaxial Compression Tests of C60 and C70 High-Strength Concrete**

#### *2.1. Test Materials and Equipment*

According to the Specifications for Mix Proportion Design of Ordinary Concrete (JGJ55-2011), high-strength concrete with strength grade of C60 and C70 was prepared. The cement was grade 52.5 ordinary Portland cement, with siliceous river sand with a fineness modulus of 2.73, and crushed basalt with particle sizes in the range of 5–10 mm. The admixture is the NF-F high-efficiency admixture compound, wherein the slag and silicon powder accounted for 73% and 20%, respectively. The mix proportions of high-strength concrete (C60 and C70) are listed in Table 1.


**Table 1.** Mixture proportions of C60 and C70 high-strength concrete.

The concrete mix was poured into 150 × 150 × 150 mm plastic molds. The obtained concrete specimens were demolded after 16 h and immediately transferred into a curing box at 20 ± 2 ◦C and at a relative humidity of 95% for 28 days. Three specimens were subsequently randomly selected for uniaxial compressive strength testing at 28 days. The remaining concrete specimens were finished by coring, cutting, and grinding according to the Standard for Tests Method of Engineering Rock Masses (GB/T50266-2013). The obtained standard cylindrical specimens measured *ϕ*50 × 100 mm. The upper and lower faces of the specimens were ground to be plane and parallel, and ensured a uniform stress distribution in the final cylindrical specimens.

Conventional triaxial compressive tests were conducted with a ZTCR-2000 rock triaxial testing system (Figure 1). The equipment is mainly used for uniaxial, conventional triaxial and creep tests of concrete and rock materials, which is mainly composed of an axial pressure system, confining pressure system, servo oil source, temperature control system, and computer control system. The computer system controls the whole process of the test equipment and operates the test steps, and automatically collects and processes the test data. The axial compression system includes an axial loading frame, three-axis cavity lifting device, controller, sensor, and electro-hydraulic servo valve. The confining pressure system consists of a pressure chamber, pressurization device, pressure transmitter, digital display meter, low temperature liquid filling oil source, air pump, guide rail, controller, and sensor. The computer system can draw the curve of each parameter in real time. The main technical parameters of the test equipment are shown in Table 2.

The circumferential and axial deformation was measured by a linear variable differential transducer (LVDT) attached to a chain wrapped tightly around the sample and an axial LVDT, respectively. Cylindrical samples of C60 and C70 high strength concrete were tested by applying compressive stress *σ*<sup>1</sup> and the constant confining pressures *σ*<sup>2</sup> = *σ*<sup>3</sup> = 0, 5, 10, 15, 20 MPa, respectively. The samples were hydrostatically compressed until the level of desired circumferential pressure was reached. The sample was then vertically compressed at a rate of 0.05 mm/min until failure.

**Table 2.** Main technical parameters of ZTCR-2000 rock triaxial test system.


**Figure 1.** ZTCR-2000 rock triaxial testing system.

### *2.2. Analysis of Test Results*

The stress–strain curves of high-strength concrete specimens (C60 and C70) at the confining pressures of 0, 5, 10, 15, and 20 MPa are shown in Figure 2.

**Figure 2.** Conventional triaxial stress–strain curves of high-strength concrete. (**a**) C60 and (**b**) C70.

According to the test results, the basic mechanical parameters of C60 and C70 highstrength concrete at different confining pressures are shown in Table 3.



During the setting process of concrete, many microcracks and holes will be formed owing to the drying shrinkage and water evaporation of cement slurry as well as the excessive interfacial microdefects (a) between the cement paste and aggregate, (b) among various phases of cement paste, and (c) among hydration products and un-hydrated cement particles. When subjected to uniaxial loading, the original cracks propagate along the interface and their direction is basically the same as the loading direction. The upper and lower parts of the cracks are compressive stress concentration areas, and the side is a tensile stress area. Because the tensile strength is far lower than the compressive strength, the microcracks first produce tensile failure. At this time, owing to the lack of lateral restraints, the rapid development of microcracks eventually leads to the loss of concrete strength [12]. When the confining pressure is 5, 10, 15, and 20 MPa, the high-confining pressure counteracts the tensile stress on the side of the crack, and increases the compressive stress required for the fracture to connect with each other. The macroscopic performance is characterized by the fact that the peak stresses of C60 and C70 high-strength concrete increase as a function of the confining pressure. Therefore, the confining pressure can effectively limit the propagation speed of micro-cracks in concrete samples, slows down the damage degree of samples, and improves the bearing capacity and deformation capacity of concrete samples. Because the conventional triaxial test is carried out in a closed cavity, it is impossible to directly observe the generation and development process of cracks, but it can be studied by means of SEM technology [13].

The failure modes of C60 and C70 high-strength concrete specimens at different confining pressures are shown in Figure 3. When the confining pressure is 0 MPa, the failure mode of the two high-strength concrete specimens is tensile failure. This is because the transverse tensile stress generated by the Poisson effect at axial loading is greater than the tensile strength of concrete, and results in cracks parallel to the maximum principal stress direction of concrete samples. When the confining pressures are 5, 10, 15, and 20 MPa, the failure mode is oblique shear failure. This is attributed to the fact that the larger confining pressure inhibits the expansion of vertical cracks. The shear stress on the inclined section of concrete subject to high-triaxial stress is greater than its shear strength, and the inclined shear failure occurs along a weak plane.

(**a**) (**b**)

**Figure 3.** Failure modes of C60 and C70 high-strength concrete at different confining pressures. (**a**) C60 and (**b**) C70.
