# Mechanical-Damage Behavior of Mortars Reinforced with Recycled Polypropylene Fibers

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

_{2}emissions [4], cement-based materials also generate significant environmental impacts derived from their production. Production of mortar and concrete faces new challenges nowadays, in addition to the requirement of high strength of the past decades, which are focused on the extension of life span by reducing its porosity and controlling crack growth. The latter challenges can be tackled valorizing waste from other industries following the concepts of sustainability and circular economy [5].

## 2. Materials and Methods

#### 2.1. Mortar Materials

#### 2.2. Mortar Mix Proportions

^{3}of mortar and the RPFs’ lengths were 0 (plain mortar), 20 and 50 mm. Table 3 provides the material proportions used in each mortar mix as well as the mortar mix identification (ID) code, where the first number indicates the dosage and the second number indicates the length of the RPFs (e.g., M0-0, indicates 0 kg/m

^{3}and 0 mm length of RPFs, therefore, plain mortar). Dosages of commercially available polypropylene fibers usually vary between 4 and 9 kg/m

^{3}in weight and between 0.4% and 1% in volume [27]. This study selected dosages ranging from 2 to 8 kg/m

^{3}, which is similar to the range suggested for commercially available polypropylene fibers and equivalent to the range suggested in a previous study [18] addressing the effectiveness of a new natural fiber that morphologically was similar to the RPFs presented in this study.

#### 2.3. Mortar Specimen Preparation

#### 2.4. Morphological Properties of RPFs

^{®}Force PP-48” [27] macro synthetic polypropylene fibers, which satisfied the standard ASTM C1116 [29]) and compared that value to the surface roughness obtained for RPFs. Figure 2 shows a photograph of the RPFs obtained in this study and the SikaFiber

^{®}Force PP-48 fibers used as comparison in terms of surface roughness. Regarding diameter measurements, these measurements were taken at the mid-section of the RPFs.

#### 2.5. Compressive Strength

#### 2.6. Flexural Strength and Flexural Toughness Indices

_{5}, I

_{10}, and I

_{20}were calculated using the midspan-displacement corresponding to 3δ, 5.5δ and 10.5δ, respectively. It is important to note that the minimum value of any toughness index is 1.0, which indicates a brittle failure-mode where the specimen collapses immediately after the formation of the first crack. Therefore, the behavior of the material can change from brittle to quasi-brittle as the values of toughness indices increase due to the addition of fibers. For each mortar mix, values of AV and SD of toughness indices were calculated from each individual bending load versus midspan-displacement curve of each mortar mix.

#### 2.7. Impact Strength

^{2}) and h represents the varying height at which the projectile was thrown (from 5 to 90 cm every 5 cm). For each mortar mix, values of AV, SD, and COV of the cumulative impact energy at first crack and at collapse were calculated.

#### 2.8. Analysis of Variance of Experimental Results

_{0}, which states that the AV performance of all mortar mixes are equivalent (i.e., the addition of RPFs did not have an effect over the specific experimental performance under study), against the alternative hypothesis, H

_{A}, which indicates that at least one AV performance is different, under a specific significance level, typically 5% as implemented in this study and in several previous experimental studies (e.g., [33,39]).

_{0}, pair-wise single-factor ANOVA tests were applied to identify individually if each fiber-reinforced mortar performed, on average, different from the plain mortar mix under a specific test.

_{cr}) that is a function of the number of groups under study (e.g., five mortars for one-way ANOVA tests), the significance level (5% in this paper) and the number of values obtained for each group under study (six specimens were tested for each mortar mix at each specific test).

_{st}) was calculated based on the experimental results and compared to F

_{cr}. If f

_{st}was less than F

_{cr}, this meant that the differences among the AV performance of the five mortars under a specific experimental performance were not statistically significant for the one-way ANOVA test or that the differences among the AV performance of two specific mortars were not statistically significant for the pair-wise ANOVA test.

_{0}is true) of each ANOVA test are also provided. For more information about the ANOVA tests see [40].

## 3. Results and Discussion

#### 3.1. Morphological Properties of RPFs

^{®}Force PP-48 fiber (1.34 mm diameter), its microscopy and its 3D evaluation of its surface implemented at the mid-section. As expected, the AV roughness value of the Sika Fiber

^{®}Force PP-48 fibers was 0.2 mm, a value exceeding the maximum roughness value of 50 μm (N12) established by the standard DIN4766. This was a significant difference in roughness since the roughness of the commercial polypropylene fiber is 250 times the roughness of the RPF.

^{®}Force PP-48) that present larger roughness values since they are designed specifically as reinforcement fibers. As the roughness of the RPFs was approximately 250 times smaller than the roughness of the Sika Fiber

^{®}Force PP-48 used in this study as a roughness control, this provided initial insights that the required aspect ratio to generate an adequate load transfer between the matrix and the fiber should be larger. Therefore, this study selected one short length (e.g., 20 mm) and one long length (e.g., 50 mm), leading to a small and large aspect ratio, respectively, in order to capture two extreme behaviors of the fiber addition due to its inclusion.

#### 3.2. Compressive Strength

#### 3.3. Flexural Strength and Toughness Indices

_{5}, I

_{10}and I

_{20}were approximately 1.0. In the case of mortar mixes M8-20 and M8-50, the incorporation of fibers had a positive impact on the flexural toughness indices, I

_{5}, I

_{10}and I

_{20,}where the increment in length (from 20 to 50 mm) showed the larger impact increasing toughness for fiber dosage of 8 kg/m

^{3}. Regardless of the latter, the values of flexural toughness indices for mortar mixes M8-20 and M8-50 represent only a residual toughness considering that the flexural post peak load reaches only up to 20% of the peak load (M8-50).

#### 3.4. Impact Strength

^{3}of mortar by Araya Letelier et al. [18]. The pig fibers where 30 mm long and the same order of magnitude fiber diameter and of AV roughness as RPF [18].

^{®}Force PP-48 fibers are more than two orders of magnitude of roughness of RPFs), it is possible that if roughness of RPFs could be improved less fiber content and more effect of fiber length could be expected to achieve similar performance to the one reported in this work.

## 4. Comments and Conclusions

^{3}of mortar) and different fiber lengths (20 and 50 mm) in terms of compressive strength, flexural strength and toughness, and impact strength. The following comments and conclusions can be drawn.

- Morphologically, RPFs presented low surface roughness values that were, on average, 250 times smaller than the average surface roughness value of a commercial polypropylene macro fiber specially designed to reinforce cement-based materials. Consequently, the interface strength between mortar and RPFs is expected to be lower than commercial polypropylene fibers specifically designed as fiber reinforcement. Increasing roughness of RPFs could be a path to achieve improved toughness and crack control using these fibers.
- Plain and fiber-reinforced mortar mixes presented a brittle failure mode of the matrix (sudden drop of load) under flexural testing. After peak load, plain mortar mixes as well as mortar mixes with small fiber dosages (2 kg/m
^{3}) exhibited a similar behavior where the specimens were no longer able to sustain any residual load and they collapsed immediately. Only in the case of mortar mixes with larger fiber dosages (8 kg/m^{3}) the specimens were able to sustain some residual load and larger displacements before collapse. The latter was reflected in their values of flexural toughness indices, and those residual loads and increments in displacements were larger for specimens with 50 mm fiber length. - In terms of impact strength, there was no statistically significant difference in the average cumulative impact energy value at the occurrence of the first crack among the mortar mixes, which confirms that the occurrence of the first crack depends mostly on the brittle behavior of the mortar matrix. In terms of average cumulative impact energy values at collapse, there was a statistically significant difference between the average performances of 8 kg/m
^{3}fiber-reinforced mortar mixes compared to plain mortar. More work needs to be done in terms of fiber length sensitivity as changes of impact strength were observed as result of fiber length changes in mortars with 8 kg/m^{3}of RPF. The incorporation of these RPFs were able to distribute the damage across the specimens allowing increments of average impact strength up to 204%.

^{3}) to effectively achieve a load transfer between the RPFs and the matrix and, consequently, to enhance the fracture performance of mortars in terms of flexural toughness and impact strength, without affecting the mechanical performance of mortars. In the case of short RPFs (e.g., 20 mm), their incorporation still generates environmental benefits since these waste-based short fibers can be encapsulated, reducing the disposal of this waste in landfill, without impacting the overall mechanical-damage performance of mortar mixes.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**(

**a**) Used and discarded domestic sweep; and (

**b**) 50 and 20 mm long recycled polypropylene fibers (RPFs) obtained.

**Figure 5.**Morphological analysis including photography, microscopy and 3D surface elevation of: (

**a**) RPFs; and (

**b**) Sika Fiber

^{®}Force PP-48.

**Figure 6.**Compressive strength average (AV), standard deviation (SD), and coefficient of variation (COV) values for each mortar mix.

**Figure 8.**Digital image correlation (DIC) strain evaluation of flexural performance: (

**a**) M0-0 at cracking; (

**b**) M8-50 at cracking; (

**c**) M0-0 immediately after cracking; and (

**d**) M8-50 immediately after cracking.

**Figure 10.**Impact strength: (

**a**) mortar mix M8-50 slab specimen showing damage distribution; and (

**b**) cumulative impact energy for each mortar mix.

Materials | Properties | ||
---|---|---|---|

Cement | Portland Cement (High Strength) | ||

Blaine fineness: 355 (m^{2}/kg) Density: 3.2 (g/cm^{3}) | |||

Sand | Type (a) | Type (b) | |

Density (g/cm^{3}): | 2.63 | 2.53 | |

Water absorption (%): | 2.55 | 2.37 | |

Fineness modulus (dimensionless): | 2.65 | 1.17 | |

Water | Potable | ||

Plasticizer | Type: Lignosulphonate based. Density: 1.15 (g/cm^{3}) |

Diameter (mm) | Density (g/cm^{3}) | Water Absorption (%) | Tensile Strength (MPa) | Young’s Modulus (GPa) | Elongation to Break (%) |
---|---|---|---|---|---|

0.38 to 0.51 | 0.99 | 0 | 250 | 5 | 55 |

Mortar Mix ID# | W/C ^{1} | Cement (kg/m^{3}) | Water (kg/m^{3}) | Sand ^{2} (kg/m^{3}) | P ^{3} (kg/m^{3}) | Fiber (kg/m^{3}) | Fiber (mm) | |
---|---|---|---|---|---|---|---|---|

(a) | (b) | |||||||

M0-0 | 0.55 | 450 | 247.5 | 168.5 | 1330.5 | 3.15 | 0 | 0 |

M2-20 | 2 | 20 | ||||||

M2-50 | 2 | 50 | ||||||

M8-20 | 8 | 20 | ||||||

M8-50 | 8 | 50 |

^{1}Water-cement ratio;

^{2}In saturated surface dry condition;

^{3}Plasticizer (0.7% in weight of cement).

Name | Dimensions (mm) | Test | Specimens for Each Mortar Mix |
---|---|---|---|

Cube | 150 × 150 × 150 | Compressive strength (28 days) | 6 |

Prism | 150 × 150 × 500 | Flexural strength and toughness (28 days) | 6 |

Slab | 100 × 300 × 400 | Impact strength (28 days) | 6 |

**Table 5.**Analysis of variance (ANOVA) test (5% significance level) for compressive strength at 28 days.

Test | Source | DF | SS | MS | f_{st} | F_{cr} | p-Value | Statistically Significant? |
---|---|---|---|---|---|---|---|---|

All mortar mixes (one way ANOVA) | Treatment (RPF) | 4 | 72.80 | 18.20 | 2.66 | 2.76 | 0.056 | Not |

Error | 25 | 171.07 | 6.84 |

Test | Source | DF | SS | MS | f_{st} | F_{cr} | p-Value | Statistically Significant? |
---|---|---|---|---|---|---|---|---|

All mortar mixes (one way ANOVA) | Treatment (RPF) | 4 | 1.16 | 0.29 | 2.27 | 2.76 | 0.090 | Not |

Error | 25 | 3.19 | 0.13 |

ID | I_{5} | I_{10} | I_{20} | |||
---|---|---|---|---|---|---|

AV | SD | AV | SD | AV | SD | |

M0-0 | 1.00 | 0.00 | 1.00 | 0.00 | 1.00 | 0.00 |

M2-20 | 1.00 | 0.00 | 1.00 | 0.00 | 1.00 | 0.00 |

M2-50 | 1.03 | 0.03 | 1.03 | 0.03 | 1.03 | 0.03 |

M8-20 | 1.23 | 0.11 | 1.62 | 0.21 | 1.76 | 0.25 |

M8-50 | 1.86 | 0.19 | 2.63 | 0.38 | 2.96 | 0.44 |

Test | Source | DF | SS | MS | f_{st} | F_{cr} | p-Value | Statistically Significant? |
---|---|---|---|---|---|---|---|---|

All mortar mixes (first crack-one way ANOVA) | Treatment (RPF) | 4 | 1.1 × 10^{5} | 2.8 × 10^{4} | 2.53 | 2.76 | 0.0658 | Not |

Error | 25 | 2.8 × 10^{5} | 1.1 × 10^{4} | |||||

All mortar mixes (collapse-one way ANOVA) | Treatment (RPF) | 4 | 4.7 × 10^{6} | 1.2 × 10^{6} | 30.23 | 2.76 | <1 × 10^{−5} | Yes |

Error | 25 | 9.8 × 10^{5} | 3.9 × 10^{4} | |||||

M0-0 versus M2-20 (collapse-pair wise ANOVA) | Treatment (RPF) | 1 | 3.5 × 10^{3} | 3.5 × 10^{3} | 0.36 | 4.96 | 0.564 | Not |

Error | 10 | 1.0 × 10^{5} | 1.0 × 10^{4} | |||||

M0-0 versus M2-50 (collapse-pair wise ANOVA) | Treatment (RPF) | 1 | 7.0 × 10^{4} | 7.0 × 10^{4} | 5.94 | 4.96 | 0.035 | Yes |

Error | 10 | 1.2 × 10^{5} | 1.2 × 10^{4} | |||||

M0-0 versus M8-20 (collapse-pair wise ANOVA) | Treatment (RPF) | 1 | 8.5 × 10^{5} | 8.5 × 10^{5} | 26.37 | 4.96 | <4 × 10^{−4} | Yes |

Error | 10 | 3.2 × 10^{5} | 3.2 × 10^{4} | |||||

M0-0 versus M8-50 (collapse-pair wise ANOVA) | Treatment (RPF) | 1 | 3.3 × 10^{6} | 3.3 × 10^{6} | 59.14 | 4.96 | <2 × 10^{−5} | Yes |

Error | 10 | 5.6 × 10^{5} | 5.6 × 10^{4} |

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## Share and Cite

**MDPI and ACS Style**

Araya-Letelier, G.; Maturana, P.; Carrasco, M.; Antico, F.C.; Gómez, M.S. Mechanical-Damage Behavior of Mortars Reinforced with Recycled Polypropylene Fibers. *Sustainability* **2019**, *11*, 2200.
https://doi.org/10.3390/su11082200

**AMA Style**

Araya-Letelier G, Maturana P, Carrasco M, Antico FC, Gómez MS. Mechanical-Damage Behavior of Mortars Reinforced with Recycled Polypropylene Fibers. *Sustainability*. 2019; 11(8):2200.
https://doi.org/10.3390/su11082200

**Chicago/Turabian Style**

Araya-Letelier, Gerardo, Pablo Maturana, Miguel Carrasco, Federico Carlos Antico, and María Soledad Gómez. 2019. "Mechanical-Damage Behavior of Mortars Reinforced with Recycled Polypropylene Fibers" *Sustainability* 11, no. 8: 2200.
https://doi.org/10.3390/su11082200