Dynamic Single-Fiber Pull-Out of Polypropylene Fibers Produced with Different Mechanical and Surface Properties for Concrete Reinforcement

Abstract

In strain-hardening cement-based composites (SHCC), polypropylene (PP) fibers are often used to provide ductility through micro crack-bridging, in particular when subjected to high loading rates. For the purposeful material design of SHCC, fundamental research is required to understand the failure mechanisms depending on the mechanical properties of the fibers and the fiber–matrix interaction. Hence, PP fibers with diameters between 10 and 30 µm, differing tensile strength levels and Young’s moduli, but also circular and trilobal cross-sections were produced using melt-spinning equipment. The structural changes induced by the drawing parameters during the spinning process and surface modification by sizing were assessed in single-fiber tensile experiments and differential scanning calorimetry (DSC) of the fiber material. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements were applied to determine the topographical and wetting properties of the fiber surface. The fiber–matrix interaction under quasi-static and dynamic loading was studied in single-fiber pull-out experiments (SFPO). The main findings of microscale characterization showed that increased fiber tensile strength in combination with enhanced mechanical interlocking caused by high surface roughness led to improved energy absorption under dynamic loading. Further enhancement could be observed in the change from a circular to a trilobal fiber cross-section.

Fiber Spinning of Ultrahigh Molecular Weight Isotactic Polypropylene: Melt Spinning and Melt Drawing

“…Commercial iPP‐1 bearing a reduced melt viscosity and crystallinity enabled a very smooth and even fiber surface, whereas our UHMW‐iPP with its ultrahigh molecular weight and increased crystallinity led to a high surface roughness induced by fibrillation. This high surface roughness is well known for UHMW‐PE fibers, especially for high draw ratios [11] . Surprisingly, annealing of the polymer fibers melted the fiber surface for all investigated fibers (Figure 2).…”

“…This high surface roughness is well known for UHMW-PE fibers, especially for high draw ratios. [11] Surprisingly, annealing of the polymer fibers melted the fiber surface for all investigated fibers (Figure 2). Underneath that melted surface, the fibrillated structure was preserved for UHMW fibers (Figure 2, b).…”

“…The degree of crystallinity was determined by using the melt enthalpy for polypropylene ΔH lit , 100 % = 207 J/g. [11] Table 2 illustrates the influence of the crystallite orientation induced by fiber spinning and increased lamellae thickness by the annealing process.…”

Mineral-bonded composites for enhanced structural impact safety: The vision of the DFG GRK 2250

“…adopted from Curosu et al [5] SHCC micromechanics under impact is hence a crucial aspect to be considered towards the design of structural retrofitting components. To this aim, in-depth studies investigate the best engineered fibres to foster the overall dissipative capability of SHCC subjected to impact [6].…”

Development of Concrete Mixture for Spun-Cast Full-Scale Precast Concrete Pipes Incorporating Bundled Steel and Polypropylene Fibers

“…The workability reduced with increasing fiber content, although it was still in the permissible range. Wolfel et al [ 32 ] concluded from their study on various types of polypropylene fibers that the efficiency of the fibers could be enhanced by use of higher strength polypropylene fibers with increased surface roughness. Wang et al [ 33 ] studied the effect of various types of fibers on the crack resistance of concrete and reported improved resistance of the polypropylene fiber-reinforced concrete to cracking induced by restrained shrinkage.…”