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In order to examine the compressive dynamic performance of polypropylene fiber reinforced concrete to be used for the retaining structure under the automobile collision magnitude, an experimental study was carried out by using hydraulic servo on concrete specimens with 4 different polypropylene fiber contents under 6 loading strain rates. The failure modes and stress-strain curves of concrete under different loading conditions were obtained. Then, by comparatively analyzing the mechanical characteristic parameters of polypropylene fiber reinforced concrete under different loading conditions, the following conclusions are drawn: with the increase of the polypropylene fiber content, the integrity of concrete upon compressive failure is gradually improved. When the polypropylene fiber content is relatively high, the static and dynamic failure modes are basically similar. With the increase of the loading strain rate, the peak compressive stress and elastic modulus of the polypropylene fiber reinforced concrete gradually increase. The increase in the polypropylene fiber content gradually intensifies the effect of loading strain rate on the peak compressive stress dynamic improvement coefficient. The peak strain of polypropylene fiber reinforced concrete is gradually increased with the increase of polypropylene fiber content, while the effect of the loading strain rate on the peak strain shows an obvious discreteness characteristic. Meanwhile, we proposed a relationship equation for describing the peak compressive stress dynamic improvement coefficient based on the coupling effect of the polypropylene fiber content and loading strain rate and further discussed the underlying stress mechanism in detail. Our research findings are of important research significance for the application and promotion of polypropylene fiber reinforced concrete in the engineering practice of retaining structures.

Polypropylene fiber reinforced concrete is a kind of concrete material formed by adding polypropylene fiber into ordinary concrete in accordance with a certain proportion. It is featured with the advantages of high corrosion resistance, strong chemical stability, and high impact resistance. Meanwhile, polypropylene fiber reinforced concrete is able to suppress the generation of early shrinkage cracks to a certain extent so as to improve the ductility of concrete. At present, polypropylene fiber reinforced concrete has been widely concerned and applied in engineering structures, such as dams, retaining walls, and culverts [

Research on the static and dynamic performance of polypropylene fiber reinforced concrete is usually carried out by considering the effects of the size and mix proportion of polypropylene fiber and the influence of loading strain rate [

In this research, we carried out an experimental study on the effect of strain rates at the automobile collision magnitude on the compressive dynamic performance of retaining structures that were made of concrete reinforced with different contents of polypropylene fiber. By applying the hydraulic servo, the failure modes and compression stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions were obtained. Then, by comparatively examining the mechanical performance characteristic parameters under different loading conditions, we comprehensively analyzed the compressive dynamic performance of the polypropylene fiber reinforced concrete from both qualitative and quantitative perspectives and investigated the corresponding stress mechanism in detail.

Our experimental study was mainly carried out on the compressive dynamic performance of concrete with 4 different polypropylene fiber contents. The concrete containing 0% of polypropylene fiber (i.e., ordinary concrete) was taken as the reference working condition. The designed strength of ordinary concrete mix proportion is 30 MPa. The raw materials used for ordinary concrete are as follows: ordinary Portland cement PO 42.5, natural aggregate gravels with particle size ranged from 4 to 16 mm (coarse aggregates), natural river sands with an apparent density of 2645 kg/m^{3}, a fineness modulus of 2.6 (fine aggregates), and city tap water. The concrete mix did not contain any additives. In accordance with the “Specification for Mix Proportion Design of Ordinary Concrete” (JGJ55-2011), the mix proportion of ordinary concrete (0% fiber) in this study was shown as in Table

Mix proportion (0% fiber) of ordinary concrete.

Concrete strength grade | Mass of each component per cubic meter/ (kg) | |||
---|---|---|---|---|

Cement | Water | Coarse aggregate | Fine aggregate | |

C30 | 461 | 175 | 1252 | 512 |

The fiber used in this study is polypropylene fiber with physical characteristics as shown in Table

Physical properties of polypropylene fiber.

Diameter (mm) | Length (mm) | Tensile strength (MPa) | Elastic modulus (GPa) | Elongation at break (%) | Density (kg/m^{3}) |
---|---|---|---|---|---|

0.026 | 19 | 641 | 4.5 | 40 | 0.91 |

The effects of different static and dynamic actions are usually realized by the approach of loading strain rate. The relevant literature [^{−8}∼10^{−6}/s), static load (10^{−6}∼10^{−5}/s), automobile collision magnitude (5 × 10^{−5}∼5 × 10^{−3}/s), earthquake (10^{−3}∼10^{−2}/s), impact (1∼10^{2}/s), and explosion (10^{2}∼10^{3}/s) [^{−8}∼10^{−5}/s, the medium strain rate is 10^{−5}∼10^{−1}/s, and the high strain rate is higher than 1/s. In this study, we aim to carry out an experimental study on the dynamic performance of polypropylene fiber reinforced concrete under the magnitude of automobile collision, which corresponds to the loading strain rate ranged from 5 × 10^{−5}/s to 5 × 10^{−3}/s. Considering the need of comparative analysis with the mechanical performance of polypropylene fiber reinforced concrete under the quasi-static condition, the static loading strain rate was set to be 1 × 10^{−5}/s. Based on all the conditions above, we determined a total of 6 loading strain rates for our experiment: 1 × 10^{−5}/s, 5 × 10^{−5}/s, 1 × 10^{−4}/s, 5 × 10^{−4}/s, 1 × 10^{−3}/s, and 5 × 10^{−3}/s.

Ranges of strain rate and static/dynamic action.

The hydraulic servo, as shown in Figure

Loading equipment.

During the experiment, we performed antifriction treatment on the concrete loading surface in order to control the impact of friction between the loading surface of equipment and the compression surface of specimen on the experimental data [

In accordance with our experimental plan for the dynamic performance of polypropylene fiber reinforced concrete, we obtained the failure modes of concrete with different polypropylene fiber contents under different loading strain rates. Based on the examination of the failure mode, we analyzed the dynamic mechanical performance of the concrete from a macroscopic perspective. In view of space limitation, two loading strain rates (1 × 10^{−5}/s and 5 × 10^{−3}/s) were selected for the analysis of each working condition, as shown in Figure

Compressive failure modes of polypropylene fiber reinforced concrete under different loading conditions. (a) 1 × 10^{−5}/s. (b) 5 × 10^{−3}/s. (c) 1 × 10^{−5}/s. (d) 5 × 10^{−3}/s. (e) 1 × 10^{−5}/s. (f) 5 × 10^{−3}/s. (g) 1 × 10^{−5}/s. (h) 5 × 10^{−3}/s.

Based on Figure

In accordance with our predetermined compressive loading scheme for different polypropylene fiber contents and loading strain rates, the compressive stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions were obtained, as shown in Figure

Compressive stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions. (a) 0%. (b) 0.2%. (c) 0.4%. (d) 0.6%.

In accordance with Figure

From the compressive stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions, we extracted the peak compressive stress when the loading strain rate is equal to 1 × 10^{−5}/s, in order to examine the effect of polypropylene fiber content on the peak compressive stress under the static loading strain rate, as shown in Figures

Relationship between polypropylene fiber content and peak stress.

Relationship between polypropylene fiber content and stress variation coefficient.

In accordance with Figures

In accordance with the analysis above, we proposed a relationship between the proportion of polypropylene fiber under the static loading strain rate

Peak stress is an important mechanical parameter for examining the compressive dynamic performance of concrete. We extracted the peak stress points from the compressive stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions in order to comprehensively investigate the influence of polypropylene fiber content and loading strain rate on the compressive performance. The influence of the strain rate effect on the peak stress of concrete can be described quantitatively by the dynamic improvement coefficient

Based on the compressive stress-strain curves of polypropylene fiber reinforced concrete under different loading conditions and equation (

The compressive peak stress of polypropylene fiber reinforced concrete under different loading conditions. (a) Peak compressive stress. (b) Peak compressive stress dynamic improvement coefficient.

In accordance with Figure ^{−5}/s is equal to 25.72 MPa, while the peak comprehensive stress under the static loading strain rate of 5 × 10^{−3}/s is equal to 31.63 MPa, suggesting a maximum increase of 22.98% due to the influence of loading strain rate. When the polypropylene fiber content is equal to 0.2%, 0.4%, and 0.6%, the peak comprehensive stress of concrete under the static loading strain rate is equal to 37.19 MPa, 33.61 MPa, and 32.16 MPa, respectively, suggesting a maximum increase of 28.99%, 33.01%, and 35.98%. By analyzing the general trend, it can be seen that the peak comprehensive stress of concrete with different polypropylene fiber contents gradually increases under the influence of the loading strain rate. With the increase of polypropylene fiber content, the increasing amplitude of the peak compressive stress gradually increases under the influence of the loading strain rate. The existing literature [^{−5}/s∼5 × 10^{−3}/s), and it is found that the peak compressive stress dynamic improvement amplitude is ranged from 20% to 25%. In our study, the experimental results of the peak compressive stress dynamic improvement amplitude of ordinary concrete are basically consistent with that of the existing literature (difference within ±5%). Thus, it can be concluded that the increasing amplitude of the peak compressive stress of polypropylene fiber reinforced concrete under the influence of loading strain rate is higher than that of ordinary concrete. This is because the increase in the strain rate produces a certain level of hysteresis effect on the concrete damage so that the stress of concrete cannot be evenly distributed, resulting in rapid development of cracks. More specifically, damage of a part of coarse aggregate can be observed from the failure mode, which eventually leads to the gradual increase of peak compressive stress. Meanwhile, the increase of polypropylene fiber content will intensify the impact on the lateral deformation of concrete under the compressive action; the higher the strain rate, the greater the impact on lateral deformation. At last, the increase in polypropylene fiber content will increase the increasing amplitude of the peak compressive stress under the influence of the loading strain rate.

The quantitative relationship between the peak compressive stress dynamic improvement coefficient and the loading strain rate for ordinary concrete under medium-low strain rates can be generally described by the equation as shown in equation (

In accordance with our mathematical regression analysis based on the experimental data and equation (

In accordance with our analysis from equations (

In order to put forward the equation for describing the coupling effect of polypropylene fiber content and loading strain rate on the peak compression stress dynamic improvement coefficient, we established a linear equation between the polypropylene fiber content and the slope of equation (

Relationship between the polypropylene fiber content and the slope of dynamic improvement coefficient equation a.

Then, we expanded parameter

The coupling effect of loading strain rate and polypropylene fiber content not only affects the peak stress parameters but also significantly affects the deformation parameters of concrete. Both the elastic modulus and peak strain reflect the deformation parameters of concrete. In order to quantitatively describe the influence of the loading strain rate and polypropylene fiber content on the elastic modulus, this paper defines elastic modulus by equation (

In accordance with the compressive stress-strain curves of polypropylene fiber reinforced concrete and equation (

Elastic modulus of polypropylene fiber reinforced concrete under different loading conditions. (a) Elastic Modulus. (b) Elastic Modulus dynamic improvement coefficient.

From Figure ^{−5}/s and is increased to 16.19 × 10^{3} MPa under the loading strain rate of 5 × 10^{−3}/s, suggesting a maximum increase of 19.57% due to the influence of loading strain rate. When the polypropylene fiber content is 0.2%, 0.4%, and 0.6%, the corresponding elastic modulus is equal to 9.94 × 10^{3} MPa, 5.95 × 10^{3} MPa, and 4.27 × 10^{3}, respectively, under static strain rate, and is equal to 12.89 × 10^{3} MPa, 8.23 × 10^{3} MPa, and 5.91 × 10^{3} MPa, respectively, under the loading strain rate of 5 × 10^{−2}/s; thus, the elastic modulus achieves a maximum increase of 29.72%, 38.25%, and 38.50%, respectively, due to the influence of the loading strain rate. Considering the range of strain rate from 1 × 10^{−5}/s to 5 × 10^{−3}/s, the dynamic performance of ordinary concrete under compression was studied in the literature [

A similar expression as equation (

In accordance with equations (

For the changing pattern of the peak strain of ordinary concrete under the influence of the loading strain rate, the findings of existing literature [

In accordance with Figure ^{−5}/s to 5 × 10^{−3}/s falls in the range of 2039

Peak compressive strain of polypropylene fiber reinforced concrete under different loading conditions. (a) Compressive peak strain. (b) Peak compressive strain variation coefficient.

In accordance with our analysis on the influence of the polypropylene fiber content and loading strain rate on the compressive dynamic performance of concrete, the following conclusions are drawn:

For the same loading strain rate, the concrete with higher polypropylene fiber content remains a better integrity upon compressive failure. For the same polypropylene fiber content, the concrete remains a relatively better integrity upon compressive failure under a higher strain rate, accompanied by the damage of a part of coarse aggregate. When the polypropylene fiber content is high, the static and dynamic failure modes of concrete are similar.

For the static loading strain rate, the peak compressive stress of concrete increases first followed by a gradual decrease, with the increase of polypropylene fiber content. For the same polypropylene fiber content, the peak compressive stress of concrete gradually increases with the increase of loading strain rate. As the polypropylene fiber content increases, the increasing amplitude of the peak compressive stress dynamic improvement coefficient is gradually increased under the influence of the loading strain rate.

For the same loading strain rate, the elastic modulus and peak strain of concrete gradually increase with the increase of polypropylene fiber content. For the same polypropylene fiber content, the elastic modulus of concrete gradually increases with the increase of loading strain rate, while the peak strain shows a discrete varying pattern.

Based on the coupling effect of the polypropylene fiber content and loading strain rate, we proposed the relationship equation for the peak stress dynamic improvement coefficient of concrete and established the equation describing the relationship between the elastic modulus dynamic improvement coefficient and the loading strain rate of concrete for the same polypropylene fiber content. Meanwhile, we also analyzed the stress mechanism for the effect of polypropylene fiber content and loading strain rate on the compressive dynamic performance of concrete in detail.

The data used to support the findings of this study are available from the corresponding author upon request (e-mail:

There are no conflicts of interest.

This work was supported by the Key Laboratory of Failure Mechanism and Safety Control Techniques of Earth-rock Dam of the Ministry of Water Resources (YK319010) and National Natural Science Foundation of China (51708273). The authors gratefully acknowledge the financial support.