# Non-Destructive Multi-Method Assessment of Steel Fiber Orientation in Concrete +
## 4 Discussion  
The X-ray CT results provide both a qualitative and a quantitative demonstration of the variations in fiber orientation characteristics amongst the specimens. In Figure 7, one can visually observe that nearly all fibers within specimen Q40--Cylinder are well aligned with the x-axis of the sample (i.e., perpendicular to the cylindrical axis). This result is confirmed by the concentrations of fiber orientations measured at (\(\Phi,\Theta\)) = (0\({}^{\circ}\), 90\({}^{\circ}\)) and (\(\Phi,\Theta\)) = (180\({}^{\circ}\), 90\({}^{\circ}\)), which are the spherical coordinate points corresponding to alignment along the x-axis. Similarly, Figure 7 indicates that the fibers in specimen Q40 \(|\mid\mid\) Cylinder are well aligned with the z-axis of the specimen (i.e., the cylindrical axis). For specimen Q40* Cylinder, a relatively random scattering of different fiber orientations can be observed (Figure 7). These analysis results, thus, confirm the successfulness of the casting techniques in producing the desired controlled fiber orientation characteristics and can be used as a baseline of fiber orientation information, against which the SIP and ultrasonic measurement results can be evaluated. In principle, it is conceivable that the measurement results of the non-destructive testing methods are influenced by systematically different (air) pore contents in the specimens due to the manufacturing process and that the observed properties could be distorted as a result. Therefore, the pore radius distributions and the air void contents of the five specimens were determined and compared. Air voids were considered to be those with pore diameters greater than 160 \(\upmu\)m, twice the voxel size of the X-ray CT studies performed here. Mercury intrusion porosimetry (MIP) was used as a reference method (compare also [27, 28]). For this purpose, 20-30 g of sample material was manually chipped off each of the remnants of the prisms (after the cylinder samples had been taken), dried at 105 \({}^{\circ}\)C, and examined by MIP. MIP is used to detect pore sizes down to approximately 3.6 nm. Due to its underlying physics, however, only pore throat diameters can be determined and the volume of wide pore areas (which can only be reached via narrow pore throats) is attributed to the pore throats. The pore throat diameter distributions are shown in Figure 10, the corresponding total porosities in Table 2. The total porosities of all samples range between 12.14% and 15.19%, but no systematic differences are observed: neither directly between the three samples with 40 kg/m\({}^{3}\) fibre content, where the fibres were manually placed in different orientations, nor with respect to the samples with 0 or 80 kg/m\({}^{3}\) fibre content. Thus, it can be concluded that the casting process had neither a systematic influence on the overall porosity nor on the pore size distribution of the samples. The MIP method is not suited, though, for the detection of larger pores, as they are usually accessed via narrow pore throats. However, these might be assessed using X-ray CT. To determine the pore content within the volume examined with CT, a low image grey value threshold for pores is used. The boundary between pore (air) and material (cement) is defined by the determined material surface. The ISO threshold value air/cement,  
Figure 9: Probe polarization and fibre alignment (schematic), spectrum of the corresponding received signals for the samples Q40—Prism (**top**) and Q40 \(|\mid\mid\) Prism (**bottom**). automatically determined by the software from the histogram analysis, serves as the starting value for the automatic surface detection with local threshold values. In the CT analysis, however, only objects consisting of two or more adjacent voxels are recognized as pores in order to exclude noise (\"false pores\"). Consequently, only pores with a diameter larger than 160 \(\upmu\)m can be detected.