Characteristics of Recycled Polypropylene Fibers as an Addition to Concrete Fabrication Based on Portland Cement

High-performance concrete has low tensile strength and brittle failure. In order to improve these properties of unreinforced concrete, the effects of adding recycled polypropylene fibers on the mechanical properties of concrete were investigated. The polypropylene fibers used were made from recycled plastic packaging for environmental reasons (long degradation time). The compressive, flexural and split tensile strengths after 1, 7, 14 and 28 days were tested. Moreover, the initial and final binding times were determined. This experimental work has included three different contents (0.5, 1.0 and 1.5 wt.% of cement) for two types of recycled polypropylene fibers. The addition of fibers improves the properties of concrete. The highest values of mechanical properties were obtained for concrete with 1.0% of polypropylene fibers for each type of fiber. The obtained effect of an increase in mechanical properties with the addition of recycled fibers compared to unreinforced concrete is unexpected and unparalleled for polypropylene fiber-reinforced concrete (69.7% and 39.4% increase in compressive strength for green polypropylene fiber (PPG) and white polypropylene fiber (PPW) respectively, 276.0% and 162.4% increase in flexural strength for PPG and PPW respectively, and 269.4% and 254.2% increase in split tensile strength for PPG and PPW respectively).

1. Introduction

Due to high demand, the global production of fossil-based plastics has grown from 1.5 million tons in 1950 to 288 million tons in 2012 and 322 million tons in 2015. It can be assumed that the upwards trend will continue [1,2,3,4,5]. There are several factors that contribute to the rapid growth of plastics consumption, such as low density, fabrication capabilities, long life, lightness and low cost of production [6]. In addition, plastic is non-corrosive, soft, flexible, not easily damaged, and with high heat and electrical insulation features [7]. For these reasons, plastic has been used widely in the packaging, preservation and distribution of food, and in the housing industry among other uses. For example, the packaging of food accounts for 40% of global plastic consumption ( ) [8]. Taking also into account that plastic is resistant to acids and detergents makes it the safest, and at the same time easiest to produce and cheapest solution for packaging in the food industry.

The wide application of plastics in most daily activities increases the volume of plastic waste [9]. Approximately half of plastic products are single-use, which causes the generation of different sorts of plastic waste, needing hundreds of years to degrade [8]. This leads to severe environmental concerns, such as human health hazards, effects on animal life, water (between 8 and 24 tons of plastic waste enter the oceans each minute [10,11]) and air pollution (about 400 million tons of CO2 [12]) [13], soil impurities [14] and other concerns [15,16,17,18]. Therefore, plastic waste is one of the five priority areas in EU action plan for a circular economy [19].

An effective way to improve this condition is the management of polymer materials in various branches of the economy. Nowadays, large amounts of plastic waste are used in the construction industry. Concrete plays an important role in the process of recycling waste materials [21,22], especially plastic waste, in construction. Plastic waste can be used in concrete as an aggregate (aggregate in concrete or asphalt concrete, synthetic aggregate or as a binder in concrete by melting), resin in polymer concrete, synthetic agent or powder and as synthetic fibers [23,24,25]. According to Czarnecki [26], replacing virgin polymer fibers with recycled waste plastic fibers seems to be becoming a standard development. The properties of recycled fibers can be higher than the properties of virgin fiber and they depend on the manufacturing method, percentage composition and other factors [23].

Traditionally, fibers are used in concrete to improve its post-cracking performance by bridging cracks and preventing the initiation and propagation of shrinkage cracks [27,28,29]. The mechanism of energy absorption and crack control of fiber in concrete was presented in [30,31,32]. By applying polypropylene fibers to the mix, higher tensile strength of the concrete is obtained [33,34,35,36,37]. The effect of polypropylene fibers on the compressive strength of concrete has been discussed in many works. For example, the influence of the addition of polypropylene fibers on the compressive strength of concrete compared to the compressive strength of steel fiber-reinforced concrete was presented in [38,39]. These issues for lightweight concrete were considered in [40], and for high-performance concrete in [41,42,43]. The decrease or increase in compressive strength with the addition of polypropylene fibers was observed. Eidan et al. [44] obtained an almost 8% lower compressive strength for fiber-reinforced concrete compared to samples without fibers. The authors used only one type of polypropylene fibers with a short length (6 mm and 12 mm) and low tensile strength (300–400 MPa). A few studies [38], however, present the influence of the applied addition of recycled polymer fibers on the increase of compressive strength [45]. Akand et al. [46] present a 1–2% increase in compressive strength obtained by adding polypropylene fibers to the concrete mixture. Around a 2% increase in compressive strength for samples with 0.1 wt.% addition of fibers (length 19 mm, diameter 30 μm and tensile strength around 270 MPa) was achieved by Fu et al. [47]. Serrano et al. [39] increased compressive strength by about 74.3% (for 1.0 wt.% fibers) and 73.4% (for 2.0 wt.%) compared to samples without fibers. The slight reinforcement of concrete (around 4.6%) by adding 1.0 wt.% of polypropylene fibers was indicated by Matar and Assaad [48]. They obtained compressive strength equal to around 62 MPa for w/c ratio 0.38, using polypropylene fibers with 80 μm diameter, 12 mm length and tensile strength of 520 MPa. Matar and Assaad have shown that the percentage increase of compressive strength independently from the used w/c ratio (in the article we tested w/c ratio equal to 0.38 and 0.5). Similar increase in compressive strength was observed by Singh [49]: 4.56% with the addition of 0.15% (by volume) of polypropylene fibers (fc = 38.10 MPa). Tough fibers with 18 μm diameter, 9 mm length, and aspect ratio 500, of 0.91 g/cm3 and with low tensile strength (the author does not give the exact value) were used. Moreover, ordinary Portland cement of Grade 43 was used. A percentage increase in compressive strength ranging from 4 to 12% with the addition of 0 to 3% of fibers constructed of plastic waste was determined by Alsadey and Salem [50].

Li et al. [41] obtained a 6.8% higher compressive strength for modified samples with the addition of 3 kg/m3 polypropylene fibers than unmodified samples. Similar results were obtained by Qin et al. [45] (6.2% increase compared to samples without fibers). The authors proved that even the small addition of fibers can have higher compressive strength (0.37 wt.% by mass of cement). Fallah and Nematzadeh [51] used different percentage compositions of concrete mix and smaller amounts of polypropylene fibers (0.1, 0.2, 0.3, 0.4 and 0.5 wt.%). They obtained compressive strength equal to 65.6 MPa for the mix with 0.1% fiber 12 mm length, 20 μm diameter and tensile strength around 350 MPa. This value is around 11.5% higher than in samples without fibers. Hiremath and Yaragal obtained much higher compressive strength in concrete with 0.1 wt.% polypropylene fibers, equal to 105 MPa. Such high value is probably the result of a large quantity of cement (900 kg/m3) and the lower grain of aggregates: quartz powder (grain size 10–45 μm) and silica sand (grain size 150 ÷ 600 mm). The maximum value of compressive strength obtained by Li et al. [41] is 159.7 MPa. They used high-class cement (CEM I 52.5 N) and a higher ratio of cement (almost the same amount of cement as aggregates), which indicated higher mechanical properties. Li et al. [42] obtained compressive strength equal to 147.2 MPa, as they used an almost double quantity of cement, which highly influences compressive strength. The effect of the type of fibers, their properties, such as length, diameter aspect ratio, and their effect on the properties of concrete changes with the dosage [52].

This study aims to contribute to this growing area of research by exploring the influence of the content of polypropylene fiber on the compressive, flexural and split tensile strength of fiber-reinforced concrete. However, unlike other studies, the subject of this research was the fibers from byproducts of recycling plastic packaging. Two types of recycled polypropylene fibers (green polypropylene fiber (PPG) and white polypropylene fiber (PPW)) were investigated. The fibers were made at the same process of recycling, but PPW were produced using an additional extruder applying a structure to their surface.

Whereby, the findings of the study make an important contribution towards an effective and ecological solution of utilizing plastic waste (packaging waste) in concrete.