*Article* **Peat Fibers and Finely Ground Peat Powder for Application in Asphalt**

**Patricia Kara De Maeijer 1,\* , Hilde Soenen <sup>2</sup> , Wim Van den bergh <sup>1</sup> , Johan Blom <sup>1</sup> , Geert Jacobs <sup>1</sup> and Jan Stoop <sup>1</sup>**


Received: 20 November 2018; Accepted: 28 December 2018; Published: 4 January 2019

**Abstract:** In this study, the feasibility of a natural peat fiber and finely ground peat powder as a modifier for bitumen was investigated. Initially, the as-received peat material was characterized in detail: the material was ground to various degrees, separated into fiber and powder fractions, and the gradation of the powder fraction as well as the size of the fibers were determined. A possible solubility in bitumen, the moisture content, and the density of both fractions were evaluated, and a limited chemical characterization of the fibers was conducted. Secondly, the rheological behavior of the powder and the fibers when blended with bitumen was evaluated. Additionally, a limited asphalt study was conducted. The rheological data showed the stiffening effects of the powder fraction and the presence of a fiber network, which were obvious as a plateau modulus towards lower frequencies. The fiber network was strain-dependent and showed elastic effects. This was further confirmed by the multiple stress creep recovery (MSCRT) tests. These tests also indicated that the fibers should improve the rutting resistance, although it was not possible to confirm this in asphalt rutting tests. Asphalt drainage tests demonstrated that adding dry peat, whether this is ground or not, is effective in reducing the binder drainage. However, the data also revealed that the amount of added peat fibers and powder should be limited to avoid difficulties in the compaction of these asphalt mixes.

**Keywords:** peat; asphalt; rheology; drainage; wet process; rutting

#### **1. Introduction**

Peat is an accumulation of partially decayed vegetation or organic matter formed in wetlands: *fens* with typical plants, such as bushes and trees, which are fed by ground water rich in nutrients; and *bogs* with typical plants, such as mosses, cotton grass, and heather, which are fed by rain water poor in nutrients [1]. The most-used material is Sphagnum moss peat, which is the main material building up in bogs in the Northern hemisphere. In some countries, peat is regarded as a slow-renewable material, although the rate of extraction and usage of peat far exceeds the rate of reforming. In Finland in 2016, bogland usage was 9.39 million ha with peat usage of 3 million m<sup>3</sup> and 3% of the annual energy production was provided by peat [2]. Peat, apart from usage as an energy provider, has agricultural applications, such as increasing the water-holding capacity of sands, and industrial applications, such as an oil absorbent or as an efficient filtration medium for mine waste streams, municipal storm drainage, and septic systems [3].

The major distresses that occur in asphalt pavement are related to crack formation, permanent deformation, and water damage. Moreover, the properties of asphalt change with time, mainly due to ageing effects occurring in the binder phase. Additives, such as polymers, crumb rubber, waxes, and surface-active components, have been used to prevent distresses and improve the durability of

the pavement. Among the additives, fibers have also been used. Fibers, in particular cellulose fibers, are added to avoid binder drainage during transportation from the asphalt plant to the construction site, typically in open mixes that contain a high binder percentage. Fibers have also been added for other reasons, such as increasing the viscosity, and, related to this, the rutting resistance. Glass fibers, for example, have a potential to improve fatigue life and deformation characteristics by increasing the rutting resistance [4]. The application of natural fibers, such as banana [5], bamboo [6], cellulose [7], coconut [8,9], hemp [10], jute [11], kapok [12], peat [13,14] and sisal [15], has so far been used for improving the drainage, water sensitivity, and stability, and increasing the tensile strength, of the asphalt pavement. Typically, the optimum added fiber content into the asphalt mix is 0.3–0.5% by weight of the asphalt mix [16]. In terms of workability, mixes with fibers showed a slight increase in the optimum binder content compared to the control mix. This is comparable to the addition of very fine aggregates. The proper quantity of bitumen to coat the fibers is dependent on the absorption and the surface area of the fibers. Therefore, this content is affected not only by the fiber concentration but also by the fiber type [17]. In addition, the degree of homogeneity or dispersion of the fibers within the mix is also important and determines the strength of the resulting mixes [4]. If the fibers are longer, typically more than 40 mm, a so called "balling" problem may occur, i.e., some of the fibers may lump together, and other fibers may not blend well with bitumen. Short fibers may not provide any reinforcement effect and can serve just as a filler in the mix [17]. The inclusion of fibers during the mixing process as a stabilizing agent has several advantages, including the possibility of using an increased binder content, creating an increased film thickness around the aggregate, an increased mix stability, and interlocking between the fibers and the aggregates, which improves the strength and reduces the possibility of drain down during transport and paving. Peat has already been applied in asphalt as a stabilizing additive for peat-based asphalt–concrete mixes, providing high performance to the road surface at a low cost [13].

Peat has perhaps a unique trait. Wettability, or the hydrophilic property of the peat, is observed as long as the peat contains a minimum moisture level (depending on the peat type, this is around 70%). Below this level, the hydrophilic character of peat weakens, and it becomes hydrophobic, meaning it will expel water [18]. For the application of fibers in bitumen or asphalt, hydrophobic characteristics are preferred.

This study will discuss the results of an investigation on peat itself and on the effects of adding peat powder and peat fibers into bituminous binders and the asphalt mix, where peat is seen as an example of a natural additive, such as cellulose and many other fiber types.

#### **2. Materials and Methods**

#### *2.1. Materials*

The Finnish company *VAPO Fibers* provided different sizes of peat material: medium (PM), long (PL), and extra-long (PEL) peat fibers with approximate lengths of <8 mm, <16 mm, and >16 mm, respectively. Extra-long peat fibers (PEL) with a moisture content of 20 wt% and 55 wt% were chosen for the present study, and denoted as PEL20 and PEL55, respectively. Peat was dried for at least 2 h in an oven at 110 ◦C before applying it in the experiments. Peat was ground in a blender "Philips ProBlend 6", which resulted in a fine powder mixed with a fiber fraction (10–30 mm). Ground peat was subsequently sieved to separate the fiber from the powder fraction (see Figure 1a,b).

Three different powders: PEL20, PEL20(3), and fine FPEL20, with a grinding time of 1, 3, and 10 minutes, respectively, were obtained (see Figure 2a). The granulometric composition of these three powders in comparison to Duras II filler is shown in Figure 2b. It can be seen in Figure 3a that powders after grinding for 3 min still may contain very fine fiber fractions (below 1 mm). Grinding for a longer time excluded this possibility. During the sieving of peat and the separation of fibers and powder, it was observed that in the as-received peat the fiber content is around 3%, and the powder content around 97%.

**Figure 1.** The preparation of peat material for testing: (**a**) grinding and (**b**) sieving.

**Figure 2.** (**a**) The obtained peat powders and (**b**) the granulometric composition of the peat powders and filler.

**Figure 3.** An example of fluorescence images of PEL20: (**a**) powder and (**b**) fibers (scale 50×).

The base bitumen was a standard 70/100 unmodified binder, with a penetration of 70 dmm and a softening point of 47.2 ◦C.

#### *2.2. Test Methods*

The density of peat fibers and powders was defined according to the standard NBN EN 15326+A1 [19]. In this paper, toluene was used as the test liquid.

Fourier Transform Infrared Spectroscopy (FT-IR) combined with attenuated total reflection (ATR) was used. The instrument was a Nicolet IS 1, with a diamond cell (smart-orbit).

Fluorescence microscopy was performed with a Carl Zeiss Axioskop 40Fl microscope equipped with a digital camera DeltaPix DP200. In fluorescence mode, a high-pressure mercury arc lamp HBO50, which transmits intense light with a wavelength between 450 and 490 nm, was used. Microscopy images of peat powder FPEL20 and fibers are shown in Figure 3a,b. These images give an indication of the variation in the particle and fiber shape and size.

Scanning Electron Microscopy (SEM) images were scanned using a Coxem EM 30 P. A high vacuum and Tungsten element with an accelerating voltage of 20 KV and a magnification of 236 times were used. In Figure 4, a SEM image of the fiber fraction is shown, indicating the thickness and thickness variability within this fraction, and a minor porosity effect on the fibers.

**Figure 4.** An SEM image of peat fibers (scale 236×) with a fiber thickness of 50 μm (the appearance of bubbles in the background is the result of evaporation of the glue on the heated surface).

The rheological properties were determined by an Anton Paar MCR 500 rheometer. Measurements were conducted at 50 ◦C using the 25 mm plate geometry to relate with the rutting test. The base binder was investigated at a gap setting of 1 mm, while samples modified with peat were investigated with a gap setting of 1.5 mm. Stress sweeps, frequency sweeps, and repeated creep measurements were performed. For some samples, frequency sweeps were also recorded at 40 ◦C, 60 ◦C, and 70 ◦C to observe the effect of temperature. The specimens were prepared in silicon moulds and afterwards transferred to the rheometer plate. Special precautions to obtain repeatable results were taken, which will be discussed in the results section.

The binder drainage test was performed in accordance with NBN EN 12697-18 [20] using drainage baskets constructed from 3.15 mm perforated stainless-steel sheets, in accordance with ISO 3310-2, on the side and base, to form 100-mm cubes with feet at each corner of the base. The asphalt mixes consisted of 1100 g batches of (loose) stone mastic asphalt (SMA) mix with the following composition: 70.7% crushed porphyry aggregates with the maximum size of 10 mm, 20.4% crushed porphyry sand, 8.90% Duras II filler, and 6.9% bitumen of standard penetration grade 70/100.

The rutting resistance of asphalt mixes was evaluated using the wheel tracking test in accordance with standard NBN EN 12697-22 [21]. The same mix composition as for the drainage tests was used for all of the measurements. Six slabs (see Figure 5a) with dimensions 18 × 50 × 5 cm were produced. The asphalt mixing temperature was 150 ◦C.

Wheel tracking tests were performed with the LCP rut tester at 50 ◦C. The slabs were conditioned at 50 ◦C for a period of 12 h prior to the testing. The rut depth in the slabs was measured manually, using a specifically designed setup (see Figure 5b) at 15 predetermined locations. Rut depths were measured after 1 000, 3 000, 5 000, 10 000, 20 000, and 30 000 load cycles.

The volumetric properties of the asphalt slabs were determined according to NBN EN 12697 [22,23].

**Figure 5.** The rutting test: (**a**) the prepared slabs for testing and (**b**) the rut depth measuring procedure.
