Polypropylene fibers perform well in roughness enhancement and corrosion resistance. They can dissipate energy when cracks occur in concrete. Furthermore, they can improve the concrete tensile properties by synergistic work with it. To study the tensile properties of the multiscale polypropylene concrete, uniaxial tensile strength of 18 fiber reinforced and 3 plain concrete specimens was experimentally tested using the paste steel method. The test results indicate that both the strength and the peak strain can be substantially improved. Based on the results, a tensile damage constitutive model was proposed and implemented into FLAC3D for numerical experimentation. The numerical results are consistent with the experimental observations in general and some discrepancies are discussed.
Polypropylene fibers perform well in roughness enhancement and corrosion resistance. They can dissipate energy when cracks occur in concrete. Furthermore, they are capable of improving the concrete tensile properties by synergistic work with it. The experiments on how polypropylene fibers improve tensile strength and ductility of the concrete have been conducted by many scholars from different countries.
Ramakrishnan [
Deng et al. [
The studies on the tensile property of polypropylene fiber concrete have gotten many findings [
Raw materials of the polypropylene fiber concrete include the ordinary Portland cement of 42.4R, fine sand, crushed stones with 5~20 mm particles, and the polypropylene fiber from Ningbo Dacheng New Material Co., Ltd. The physical and mechanical properties of the three different fibers are listed in Table
Physical and mechanical properties of polypropylene fiber.
Fiber number | Diameter |
Length |
Tensile strength |
Modulus of elasticity |
Elongation |
Density |
---|---|---|---|---|---|---|
FF1 | 0.026 | 12 | 641 | 4.5 | 40 | 0.91 |
FF2 | 0.1 | 19 | 322 | 4.9 | 15 | 0.91 |
CF1 | 0.8 | 50 | 706 | 7.4 | 10 | 0.95 |
Note: FF: fine polypropylene fiber; CF: course polypropylene fiber.
The amount of polypropylene fiber (kg/m3) for different specimens.
Specimen number | Fiber table | Mix amount (kg/m3) |
---|---|---|
A0 | None | 0 |
A1 | FF1 | 0.9 |
A2 | FF2 | 0.9 |
A3 | CF1 | 6.0 |
A4 | (CF1 + FF1)▲ | 6.0 |
A5 | (CF1 + FF2)▲ | 6.0 |
A6 | (CF1 + FF1 + FF2)• | 6.0 |
Note: ▲ represents amount CF1 that is 5.1 kg/m3 and amount of FF1 or FF2 that is 0.9 kg/m3; • denotes the added amount of CF1 that is 5.1 kg/m3 and amount of FF1 or FF2 that is 0.45 kg/m3.
The dimensions of the specimens are 100 mm × 100 mm × 300 mm. 7 groups of concrete were made and each group contains three specimens made of the same kind of concrete. Using structural adhesive to glue two head faces of the specimens to the steel plate of 20 mm thickness, the tension tests were carried out through 1342 INSTRON electrohydraulic servo material testing machine, as shown in Figure
The test device and a typical test specimen.
Table
The axis-tensile test results for multiscale polypropylene fiber concrete.
Specimen |
|
|
|
|
|
|
---|---|---|---|---|---|---|
A0 | 1.362 | 1 | 119.3 | 1 | 18.30 | 1 |
A1 | 1.498 | 1.11 | 120.1 | 1.01 | 17.25 | 0.94 |
A2 | 1.602 | 1.18 | 124.4 | 1.04 | 18.40 | 1.0 |
A3 | 1.690 | 1.24 | 178.1 | 1.49 | 18.45 | 1.01 |
A4 | 1.530 | 1.12 | 133.8 | 1.12 | 18.24 | 1 |
A5 | 1.485 | 1.09 | 141.7 | 1.18 | 19.80 | 1.08 |
A6 | 1.728 | 1.27 | 145.2 | 1.22 | 18.89 | 1.03 |
The axial tensile stress-strain curve of multiscale polypropylene fiber concrete.
As shown in Figure
It can be observed from Figure
For the concrete added to crude polypropylene fiber, strain hardening of low stress occurs during the descending segment of stress-strain curve. That is, the strain increases rapidly while the attenuation of stress is not obvious. It indicates that the crude fibers can act through bridging function when the concrete cracks, so the fibers can improve the toughness of concrete under tension. However, this was not observed in A0, A1, and A2 since the fine fiber has poor tensile capacity. It cannot sustain sufficient elongation to produce the strain hardening of low stress when the concrete cracks appear.
In the process of concrete pouring, cracks or voids in concrete may be introduced due to the temperature effect or the uneven vibration. When the concrete is subjected to the external load, stress concentration occurs on the cracks, and it makes the crack expand, resulting in the damage of the structure finally. Adding polypropylene fiber to concrete can reduce the amount and scale of cracks through cementation between concrete and fiber. Thus, it substantially reduces the stress concentration in the microcracks under external load and effectively improves the peak stress of the concrete. When the macroscopic cracks occur in the concrete under tension, the polypropylene fibers can withstand the load due to the good toughness. The added fibers slow down the expansion of crack and improve the peak strain of concrete.
The crude polypropylene fiber is of higher bearing capacity than the fine type. Cracks appear in the concrete mixed with crude fiber only when the peak stress is reached. After that, the stress decreases rapidly. In addition, as the crude fibers can sustain part of the load, the low stress-strain hardening phenomenon can be observed when the stress decreases to a certain level. The fine polypropylene fiber’s bearing capacity is low. The stress decreases rapidly when the specimens are damaged, and there is no stress-strain hardening observed at low stress level.
Combining the standard tensile constitutive model [
in which
Values of control parameter
Specimen |
|
---|---|
A0 | 1.748 |
A1 | 1.695 |
A2 | 1.706 |
A3 | 0.387 |
A4 | 1.049 |
A5 | 0.51 |
A6 | 0.945 |
When
For the decreasing piece of stress-strain curve, (
Implement (
In order to verify the proposed method, the dimensions of the FLAC3D model are selected as 100 mm × 100 mm × 300 mm, the same as the test specimens. This numerical model is partitioned into 24,000 elements and 26,901 nodes, as shown in Figure
Specimen model.
Figures
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A0.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A1.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A2.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A3.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A4.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A5.
Comparison of stress-strain relationship between numerical simulation and experimental tests for specimen A6.
Comparisons show that the numerical simulation results fit the experimental tests well for specimens A0, A1, and A2. Furthermore, the numerical results match well the test results before the stress reaches the peak value. After that, there are some certain discrepancies. In addition, the numerical experimentations cannot capture the stress-strain behavior that is observed in experimental results. Generally for the same strain value, the stress value is smaller than the test result. The reason lies in the bridging effect, through which the fibers act on the concrete test specimens. However, this effect is not considered in the numerical model.
Based on the uniaxial tensile test results, this paper proposes a tensile damage constitutive model for multiscale polypropylene concrete. Numerical experimentations through FLAC3D demonstrating the reliability of the proposed model are given for a series of specimens. The main work and findings include the following: The tensile strength of multiscale polypropylene fiber concrete is higher than that of the ordinary concrete and the strength ratio ranges from 1.09 to 1.27. The strength increase of A6 is the most significant, up to 27% with three types of fiber added. The peak strain can be improved by 1%~49%. In addition, adding fiber to concrete can substantially improve the toughness of concrete. A tensile damage constitutive model is proposed. The numerical validation on this model through FLAC3D shows a good agreement with the experimental data before the strain hardening of low stress of the concrete and after that the differences between the numerical and experimental results are considerable. Therefore, this discrepancy should be further investigated in the following research.
The authors declare that they have no competing interests.
This work was financially supported by the Fundamental Research Funds for the Central Universities (Grants nos. 106112015CDJXY200007 and CDJXS12200005).