# Experimental Investigation on the Dynamic Mechanical Properties and Microstructure Deterioration of Steel Fiber Reinforced Concrete Subjected to Freeze–Thaw Cycles

^{1}

^{2}

^{*}

## Abstract

**:**

^{−5}/s, 10

^{−4}/s, 10

^{−3}/s and 10

^{−2}/s). Performance was also evaluated, including triaxial compressive strength, peak strain, the relationship between stress and strain, failure mode and microstructure. The results show that with the increase in F–T cycles, the compressive strength and energy absorption capacity of concrete gradually decrease. The mechanical properties of concrete increased with the addition of steel fibers during F–T cycles, and the optimum amount of steel fiber to enhance resistance to F–T cycles is 1% within the evaluation range. In this study, the effects of strain rate and confining pressure on the strength and failure mode of concrete after fiber addition are studied. Both the dynamic increase factor and the concrete strength increase linearly with the increase of strain rate, the dynamic increase factor is characterized by an increase in intensity caused by strain rate. When there is no confining, the crack direction of the concrete specimen is parallel to the stress loading direction, and when there is confining, it is manifested as oblique shear failure. The results of scanning electron microscopy analysis of the microstructure demonstrate the performance results at the macroscopic level (compressive strength and peak strain).

## 1. Introduction

^{−5}/s, 10

^{−4}/s, 10

^{−3}/s and 10

^{−2}/s). To determine the impact of steel fiber content, strain rate, F–T cycle and confining pressure on dynamic compressive properties, such as triaxial compressive strength, peak strain, stress–strain behavior, failure mode and dynamic increase factor (DIF), which was introduced to further understand the enhancement of dynamic compressive strength, SFRC specimens with three steel fiber contents (0 to 2% by volume) were tested under dynamic triaxial servo-hydraulic testing equipment. Finally, the microstructural feature of SFRC under F–T cycles was investigated via scanning electron microscopy (SEM).

## 2. Experimental Program

#### 2.1. Materials

#### 2.2. Experimental Methods

^{−5}/s, 10

^{−4}/s, 10

^{−3}/s and 10

^{−2}/s).

## 3. Results and Discussion

#### 3.1. Triaxial Compressive Strength

^{−5}/s strain rate, the compressive strength of the specimens with 0%, 1% and 2% steel fiber are 66.34 MPa, 68.15 MPa and 67.03 MPa, respectively. The specific reasons are analyzed below. The relative compressive strength of specimens with a 5 MPa confining pressure at various F–T cycles is shown in Figure 6. It is evident that as the number of F–T cycles rises, the compressive strength of concrete gradually decreases. Lu et al. [35] reported similar outcomes. The decrease in the compressive strength of concrete is due to the fact that the pore solution in the concrete undergoes repeated freeze–thaw processes after each F–T cycle and causes expanding internal cracks, surface fouling and loss of overlay thickness, and adding fibers to a concrete sample can reduce the number of cracks and increase strength [30,31]. Furthermore, concrete specimens that were subjected to F–T cycles showed the following two stages in their triaxial compressive strength: (I) a slow decline phase from the start of F–T cycles to 50 F–T cycles, and (II) an accelerated descent stage from 50 F–T cycles. As an illustration, the triaxial compressive strength of the concrete specimen SF0 with a strain rate of 10

^{−5}/s fell by 4.7% and 40.5% after 50 and 200 F–T cycles, respectively. The gradual decline in the triaxial compressive strength may be due to a gradual loosening of the aggregate and mortar of the concrete specimen and internal structure as the number of cycles increases.

#### 3.2. Peak Strain

^{−5}/s as an example, the axial peak strain increases by 42.1%, 39.3% and 40.5% for SF0, SF10 and SF20, respectively, from 0 F–T to 200 F–T cycles. This result is analogous to that of Zhao et al. [31], who discovered that the F–T cycles damage the concrete and therefore reduce its dynamic performance, thereby leading to concrete deformation after F–T to a large degree under the same energy. Additionally, it can be found that peak strain of the specimens subjected to F–T cycles is similar to compressive strength which exhibits following two phases: (I) there is a gradual increase phase from the first immersion through 50 F–T cycles and (II) an accelerating increase phase from 50 to 200 F–T cycles.

^{−4}/s, 10

^{−3}/s and 10

^{−2}/s are decreased by 4.4%, 8.0% and 10.6%, respectively, compared with that under the strain rate of 10

^{−5}/s. As a result, with the same number of F–T cycles, the axial peak strain of concrete specimens without steel fiber falls as the strain rate increases. Figure 9b, c reveal that the concrete with 1% and 2% steel fiber shows a similar trend.

^{−5}/s strain rate. Furthermore, for SF10 at a 10

^{−5}/s strain rate, the axial peak strain (Figure 9b) increased from 0.0057 to 0.0113 with the strain rate was raised from 0 MPa to 10 MPa. For concrete with 2% steel fibers (SF20), the results are consistent with those for SF0 and SF10 shown in Figure 9c. This result is in line with research by Wang et al. [44], who found that the peak strain of the specimen rises with increasing confining pressure.

^{−5}/s strain rate.

#### 3.3. Axial Stress–Strain Behavior

^{−5}/s strain rate is shown in Figure 10. The peak value (i.e., compressive strength) and initial slope (i.e., elastic modulus) of the specimen gradually decrease as the number of F–T cycles increases. The shape of the stress–strain curve also gradually changes. In comparison to the thaw–thaw specimen, the descending portion of the curve was shorter and steeper after 200 F–T cycles. As F–T cycling increases, the ability of the specimen to absorb energy decreases, as shown by the narrow area under the curve.

^{−5}/s strain rates is shown in Figure 11. For the concrete specimens, the slope of the curve (i.e., elastic modulus) continues to increase as the confining pressure increases, and the stress–strain curve compaction stage becomes obvious. In addition, the energy absorption capacity increases significantly with the increase in confining pressure. This observation can be interpreted as the fact that confining pressure can inhibit the propagation of concrete cracks [45], thereby fundamentally improving the performance of concrete and increasing macroscopic characteristic parameters such as strength and elastic modulus.

^{−5}/s): the pore compaction stage (AB1), elastic stage (B1C1), unstable cracking stage (C1D1) and subsequent failure stage. The AB1 stage reflects the closing process of the pores in concrete at the initial stage of loading, in which an upward concave shape is shown. The initial section of the stress–strain curve is approximately straight as the strain rate increases. The analytical object was chosen to be the strain rate of 10

^{−2}/s. Compared with the quasi-static strain rate, this value does not show the stage of pore compaction. The cause could be that at high strain rates, micro void in specimen has not been fully closed and then directly enters the elastic stage (AB2), in which the pores of concrete are further compacted, and the relationship between stress and strain in specimen follows Hooke’s law. The unstable cracking section (B2C2) occurs at this stage. Under high strain rate loading, the crack development speed in the specimen is accelerated, new cracks will appear, the damage degree of concrete is intensified the curve gradually convexes from the previous straight line section, and the slope is reduced. When the stress increases, the curve convexes from the previous straight line section. When it reaches the peak point, it will enter the stage of failure.

#### 3.4. Failure Mode

^{−3}/s and confining pressures of 0 and 5 MPa. Obviously, the failure mode of SFRC has changed greatly due to lateral restraint pressure. Under 0 MPa confining pressure, the main cracks parallel to the direction of loading force appear on the surface of the plain concrete specimen when it reaches its ultimate compressive strength. The specimen subsequently fails as a result of a reduction in axial bearing capacity, demonstrating vertical splitting as the mode of failure (Figure 16a). The fracture surface was observed as a whole with good flatness and no adhesion. However, the cracks on the surface of specimen are localized in the center of the concrete and hardly any through cracks form when steel fiber is introduced. The fracture surface also shows the aggregate cement matrix connected by steel fiber, and the spalling degree in the middle part of the concrete is light and exhibits expansion because of the high content of steel fiber (the content is 2.0%). Jin et al. [1] reported extremely strong bond strengths between steel fibers and concrete, consistent with the results of the above studies. When cracks appear in the matrix, the steel fiber mesh connects the small pieces of concrete with its good bonding properties, thereby preventing the matrix from being crushed.

#### 3.5. DIF

_{d}is the dynamic ultimate compressive strength, f

_{s}is the quasi-static ultimate compressive strength, ε

_{d}is the current strain rate, ε

_{s}is the quasistatic strain rate (ε

_{s}= 1 × 10

^{−5}/s) and k is a parameter.

#### 3.6. Microstructure Analysis

## 4. Conclusions

- (1)
- With increasing F–T cycles, the triaxial compressive strength of concrete specimens declines, the stress–strain curve area decreases with time, and the corresponding energy absorption capacity gradually weakens. However, the failure mode of the concrete did not change, only the deterioration of the specimen gradually increased.
- (2)
- The F–T damage of SFRC shows two stages: (I) from 0 to 50 F–T cycles, the compressive strength of specimens slowly decreases and the peak strain slowly increases; and (II) from 50 to 200 F–T cycles, the compressive strength and peak strain of specimens acceleratingly decreases and increases, respectively. Similarly, mortar shedding and crack extension on the specimen surface were also accelerated in the second stage.
- (3)
- Under F–T cycles, steel fiber can enhance the dynamic mechanical properties of concrete. Adding steel fibers to concrete under F–T cycles increases the triaxial compressive strength, peak strain, and energy absorption capacity. However, increasing steel fiber content to 2.0%, the triaxial compressive strength of concrete decreases because the excessive steel fiber causes a small amount of agglomeration.
- (4)
- As the strain rate increases, the compressive strength of the SFRC subjected to F–T cycles increases gradually, the peak strain decreases slowly and the DIF of strength increases linearly. In low strain rate, SFRC specimens have sufficient time to select the path, most of which are along the mortar interior or the weak surface of aggregate mortar. However, when the strain rate increases, the shape of failure surface of the SFRC changes and a large number of coarse aggregate fractures occur.
- (5)
- Under F–T cycles, specimens with no confining pressure exhibit crack directions parallel to loading stress directions, and the cracks are concentrated in the middle of the specimen. However, the steel fiber reinforced concrete specimens change to inclined shear failure under the action of confining pressure. In addition, as confining pressure increased under F–T cycling, SFRC’s triaxial compressive strength increased as well as its peak strain and energy absorption capacity.
- (6)
- The SEM tests conducted on concrete specimens after the F–T cycles show that steel fibers enhance the ITZ between aggregates and mortars, compaction and microstructure improvement of concrete. The microstructure analysis results are accordant with the laws of macroscopic properties (compressive strength and peak strain).

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**Test setup: (

**a**) servo-hydraulic static and dynamic triaxial testing system; (

**b**) schematic of the system.

**Figure 5.**Compressive strength of concrete specimens with 5 MPa confining pressure without F–T cycles.

**Figure 6.**Relative compressive strength of concrete specimens with 5 MPa confining pressure under different numbers of F–T cycles. (

**a**) SF0, (

**b**) SF10 and (

**c**) SF20.

**Figure 8.**Peak strain of concrete specimens with 5 MPa confining pressure under different number of F–T cycles. (

**a**) SF0, (

**b**) SF10 and (

**c**) SF20.

**Figure 9.**Peak strain of concrete specimens with 100 F–T cycles under different confining pressures. (

**a**) SF0. (

**b**) SF10. (

**c**) SF20.

**Figure 10.**Stress–strain curve of SF10 specimens with 5 MP confining pressure and 10

^{−5}/s strain rate under different numbers of F–T cycles.

**Figure 11.**Stress–strain curve of SF10 specimens with 100 F–T cycles and 10

^{−5}/s strain rate under different confining pressure.

**Figure 12.**Stress–strain curve of SF10 specimens with 100 F–T cycles and 5 MPa confining pressure under different strain rates.

**Figure 13.**Stress–strain curve of different concrete specimens with 5 MPa confining pressure and 10

^{−5}/s strain rate under 100 F–T cycles.

**Figure 14.**Failure mode of SF10 specimens with 5 MPa confining pressure under different numbers of F-T cycles.

**Figure 15.**Typical failure modes of SF10 specimen with 0 MPa confining pressure at different strain rates.

**Figure 16.**Typical failure modes of SFRC specimen under different confining pressures: (

**a**) 0 and (

**b**) 5 MPa.

**Figure 17.**Model diagram of SFRC under different stress states. (

**a**) Uniaxial compression state and (

**b**) triaxial compression state.

**Figure 18.**Relationship between DIF and strain rate of concrete subject to 100 F–T cycles under different confining pressure levels. (

**a**) SF0, (

**b**) SF10 and (

**c**) SF20.

**Figure 20.**SEM images of specimen under a different number of F–T cycles: (

**a**) SF0 with 0 F–T cycles, (

**b**) SF0 with 200 F–T cycles, (

**c**) SF10 with 0 F–T cycles and (

**d**) SF10 with 200 F–T cycles.

**Figure 23.**Micromorphology of ITZ between steel fiber and matrix of SF10 specimen. (

**a**) ITZ micromorphology and (

**b**) C–H gels.

Contents | Cement |
---|---|

SiO_{2} (%) | 21.45 |

Al_{2}O_{3} (%) | 6.45 |

CaO (%) | 61.5 |

Fe_{2}O_{3} (%) | 3.09 |

MgO (%) | 1.21 |

K_{2}O (%) | 1.38 |

Na_{2}O (%) | 0.25 |

SO_{3} (%) | 2.01 |

Loss on ignition (%) | 4.05 |

Specific gravity (g/cm^{3}) | 3.15 |

Properties | Natural Sand | Coarse Aggregate |
---|---|---|

Water absorption (%) | 0.79 | 0.76 |

Loose bulk density (kg/m³) | 1678 | 1430 |

Length (mm) | Diameter (mm) | Aspect Ratio (l/d) | Elastic Modulus (GPa) | Tensile Strength (MPa) | Density (kg/m^{3}) |
---|---|---|---|---|---|

30 | 0.5 | 60 | 200 | 1195 | 7.85 |

Specimens ID * | Water | Cement | Sand | Coarse Aggregate | SUPERPLASTICIZER | Steel Fibers |
---|---|---|---|---|---|---|

SF0 | 150 | 375 | 765 | 1135 | 2.63 | 0 |

SF10 | 150 | 375 | 730 | 1095 | 2.63 | 78 |

SF20 | 150 | 375 | 710 | 1045 | 2.63 | 156 |

**Table 5.**Classification of CaCO

_{3}produced by microorganisms and its suitable environmental conditions in concrete structures.

Specimens ID | Confining Pressure (MPa) | k | R^{2} |
---|---|---|---|

SF0 | 0 | 0.0981 | 0.9813 |

5 | 0.0430 | 0.9942 | |

10 | 0.0369 | 0.9912 | |

SF10 | 0 | 0.0938 | 0.9604 |

5 | 0.0406 | 0.9780 | |

10 | 0.0331 | 0.9868 | |

SF20 | 0 | 0.0874 | 0.9813 |

5 | 0.0433 | 0.9944 | |

10 | 0.0296 | 0.9795 |

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**MDPI and ACS Style**

Li, Y.; Zhang, Q.; Wang, R.; Xiong, X.; Li, Y.; Wang, J.
Experimental Investigation on the Dynamic Mechanical Properties and Microstructure Deterioration of Steel Fiber Reinforced Concrete Subjected to Freeze–Thaw Cycles. *Buildings* **2022**, *12*, 2170.
https://doi.org/10.3390/buildings12122170

**AMA Style**

Li Y, Zhang Q, Wang R, Xiong X, Li Y, Wang J.
Experimental Investigation on the Dynamic Mechanical Properties and Microstructure Deterioration of Steel Fiber Reinforced Concrete Subjected to Freeze–Thaw Cycles. *Buildings*. 2022; 12(12):2170.
https://doi.org/10.3390/buildings12122170

**Chicago/Turabian Style**

Li, Yang, Qirui Zhang, Ruijun Wang, Xiaobin Xiong, Yan Li, and Jiayu Wang.
2022. "Experimental Investigation on the Dynamic Mechanical Properties and Microstructure Deterioration of Steel Fiber Reinforced Concrete Subjected to Freeze–Thaw Cycles" *Buildings* 12, no. 12: 2170.
https://doi.org/10.3390/buildings12122170