Short Fibre Reinforced Cementitious Composites and Ceramics

This paper demonstrates the widely accepted hypothesis that the compressive testing is a particular case of a cyclic test where failure occurs during the first cycle. To perform this, a test on 32 fiber-reinforced high-performance concrete specimens have been carried out. Sixteen of them have been tested under low-cycle fatigue compressive loading up to failure. Eight of them have been tested under monotonic compressive loading, until failure too. And the last eight specimens have remained intact. All of them have been scanned using a Computed Tomography (CT) Scan in order to define the pattern of their damage, which includes voids and cracks. The results show that the average damage maps of monotonic and fatigue series are statistically identical, which confirms the hypothesis previously described. In addition, both series are different to the intact series, which means that not a random damage distribution occurs when specimens collapse.

Investigations on ordinary fibre-reinforced concrete showed that the time-dependent deformations under tensile load in cracked concrete are larger than the deformations in uncracked concrete. The so-called tensile creep in the cracked cross section depends on some different factors like type of fibres, fibre content, load level, concrete mix, environmental condition, etc. Given the lack of sufficient data about tensile creep in ultra-high performance fibre-reinforced concrete (UHPFRC), a large experimental program financed by the DFG (Deutsche Forschungsgemeinschaft) was started at the University of Kaiserslautern.

This paper presents a report about work in progress of research on the influence of the fiber orientations on the tensile strength of steel fiber concrete. Different fiber orientations in different parts of a structural element are caused by the casting process. Here, as an example, a small plate was cast of self-compacting concrete containing hooked-end steel fibers. The plate was cut into three beams, which in turn have been subjected to X-ray Computed Tomography scanning to obtain fiber orientations and to three-point bending test, to assess the tensile strength and fracture behaviour.

Although concrete itself is not a combustible material, concrete mixtures with high density, such has high-performance concretes (HPCs), are susceptible to significant damage during fires due to explosive spalling. Past research has shown that the inclusion of polymer fibres in high density concrete can significantly mitigate this fire damage. The exact mechanisms causing this increased spalling resistance are not yet fully understood, but it is thought that the fibres facilitate moisture transport during fire exposure, which in turn contributes to relief of internal stresses in the spalling-susceptible region. In this study, X-ray Computed Tomography (CT) was applied to observe the interaction between polymer fibres and cracking during thermal exposure. For this purpose, two concrete samples containing different polymer fibre types were subjected to incremental application of a defined thermal exposure. CT images were acquired before and after each thermal exposure and powerful image processing tools were used to segment the various material components. This enabled a detailed analysis of crack formation and propagation as well as the visualization and quantification of polymer fibre characteristics within the concrete. The results demonstrated that the orientation of both fibres and cracks in polymer-fibre reinforced concrete tend to be anisotropic. The results also indicated that crack geometry characteristics may be correlated with fibre orientation, with cracks tending to run parallel to fibre beds. Clear quantitative relationships were also observed between heating and increasing cracking levels, expressed in terms of both crack surface area and crack volume.

The last five decades have seen a large research effort focused on fibre reinforced concrete. Most of the research studies have been devoted to the use of steel fibre and mechanical characteristics of fibre reinforced concrete. Only a limited number of research programmes dealt with properties (both geometrical and mechanical) and development of steel fibre. In this paper a new type of engineered steel fibre will be analysed in comparison to commercially available fibre. Properties of concretes reinforced by a small volume of fibre were of special interest. Conducted tests covered both fresh mix behaviour and strength characteristics of hardened concrete.

Short composite fibres are a relatively new product for concrete disperse reinforcement. In this experimental research, 14 different composite fibres were developed and single fibre pull-out micromechanics was investigated. Three main groups of composite fibre were—composite glass fibres (GF), composite carbon fibres (CF) and hybrid fibres (HF). Composite fibre manufacturing consisted of glass, carbon or combined fibre filament preparation, impregnation with epoxy resin, epoxy curing, quality control and cutting in short discrete macro- fibres. All three composite fibre groups were manufactured with straight, uneven and undulated geometries. Fibre surface finish was smooth and rough. Uneven fibre geometry was achieved by not aligning all fibre filaments in fibre tow. Undulated geometry was a result of interlaced fibres. The rough fibre outer surface finish was achieved by adding an extra layer of epoxy resin containing fine quartz grains. All macro-fibres were cut in 50 mm length. Single fibre pull-out samples with a pre-defined crack between two concrete parts were prepared to investigate fibre pull-out behaviour. Fibre pull-out laws were obtained and analysed. Composite fibre improvement geometry and surface with roughening outer surface made a huge impact on fibre pull-out resistance.

This paper presents a report about work in progress of research on the influence of the flow of SCFRC on the fiber orientations. Mechanical properties of the short steel fiber reinforced cementitious materials mostly depend on the fiber orientation and spatial dispersion. Many studies have shown that it is possible to achieve the desired fiber orientation by optimizing the parameters of rheological properties or the casting process. In order to improve the key mechanical properties, multiple statistical experiments with various factors are needed. This paper analyzes the influence of casting velocity and formwork surface quality on the fiber distribution and orientation. A suitable technique for our method was to replace Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC) by a transparent polymer with similar rheological properties as SFRSCC. Preliminary analysis of the experimental results shows that the fibers tend to orient mostly perpendicular to the flow direction and turn their orientation longitudinally near the walls. Experiments showed that the fiber spatial distribution was affected by the casting velocity. Faster casting velocities provided more preferable homogeneous distribution. Moreover, the roughness of the bottom of the formwork demonstrated some influence on the fiber orientations but no significant impact on the spatial dispersion. In addition, we used the image analysis method to estimate fiber orientation and distribution.

Computed tomography (CT) technique is of increasing interest in research related to concrete technology. This technology provides the possibility of visualize the internal structure of concrete, including pores, cracks, aggregates and fibres. In this paper, the CT scan is used to determine the position and orientation of the fibres in case of steel fibre reinforced high strength concrete elements (SFRHSC). This paper shows a home-made numerical procedure, automated through a MATLAB routine, which enables, fast and reliable, get the orientation of each and every one of the fibres and their center of gravity. The procedure shown can be used with any type of fibre reinforced material, with the only restriction that a wide difference between density of fibres and density of matrix is needed. The algorithm is simple and robust. The result is a fast algorithm and a routine easy to use. In addition, the validation tests show that the error is almost zero.

A direct single-step catalyst-free CVD technique has been used for producing alumina nano-whiskers covered by a few layers of defective graphene. The hybrid whiskers have been then exploited as electroconductive fillers to oxide ceramics. The electrically conductive additives do not substantially change the mechanical properties. However, the resistivity of the composites undergoes a considerable drop turning the dielectric oxides into conductive composites by addition of 2 vol% of fillers. Three-dimensional Monte Carlo simulation of systems of polydisperse prolate ellipsoids, using the critical path based tunneling-percolation model, has been exploiting for estimation of a tunnelling length-scale. The value of percolation threshold is found to be 2.23 nm for the materials under consideration, with is in a good agreement with experimental data.