Use of neutron radiography and tomography to visualize the autonomous crack sealing efficiency in cementitious materials

Penetration of moisture into building materials is at the origin of several damage mechanisms. In the case of cement-based materials crack formation is a common problem and highly accelerates the ingress of water and aggressive substances. Crack repair may be needed, however, repair works are expensive and in some cases cracks are even not accessible. Therefore, in this research we aim at autonomous crack sealing. Upon crack appearance, damage is sealed autonomously by the release of encapsulated agents. Visualization of the water uptake by means of neutron radiography for samples with manually and autonomously sealed cracks showed that in both cases ingress of water into the crack can be prevented depending on the type of agent. The efficiency of three different agents was examined and it was shown that the use of polyurethane or a water repellent agent were most promising. Neutron tomography scans demonstrated that poor results were obtained when encapsulated methyl methacrylate was used, since one component of the agent hardened inside the capsules before crack appearance. From the results we can conclude that autonomous sealing of cracks is feasible and that neutron radiography and tomography are suitable non-destructive test techniques to visualize the autonomous crack sealing efficiency.     

An analytical method to determine the bridging stress transferred within the fracture process zone: I, general theory

A new method has been developed to determine the cohesive stress distribution within the fracture process zone (FPZ). This new method is based on the determination of the compliance curve after successive sawcutting. The extension of FPZ can also be predicted by the theory if parameters of the strain-softening are available. Analytical results evidence that a K_r-curve is closely connected with FPZ and the strain-softening.     

Role of disjoining pressure in cement based materials

During the hydration process of Portland cement a nanoporous gel is formed. The high internal surface of the hydration products, mainly calcium-silicate hydrates, and the numerous adsorbed cations interact with water. At an equilibrium moisture content below 50% adsorbed water molecules reduce the surface tension of the solid particles. Water adsorbed at higher relative humidity is at the origin of disjoining pressure acting in small gaps between particles. Both processes are major mechanisms of shrinkage and swelling of hardened cement paste. Results of investigations into the disjoining pressure are summarised. The relevance of disjoining pressure for the behaviour of cement-based materials will be discussed.     

C-kit receptor in the human vas deferens: distinction of mast cells, interstitial cells and interepithelial cells

The molecular mechanisms underlying the regulation of vas deferens (VD) motility and semen emission are still poorly understood. Interstitial cells of Cajal (ICC), which harbour the c-kit receptor (CD117), provide the basis of coordinated gut motility. We investigated whether c-kit receptor-positive cells also exist in the normal human VD. Enzyme and fluorescence immunohistochemical techniques were applied on serial sections of human proximal, middle, and distal VD segments (n=49) employing 13 different monoclonal and polyclonal antibodies recognizing the c-kit receptor. The c-kit receptor was detected in either round- or spindle-shaped cells. On account of their antigenic profile, the round- and oval-shaped c-kit receptor-positive cells were identified as mast cells (MC) occurring in all layers of the VD except the epithelium. In contrast, two distinct populations of exclusively c-kit receptor-positive spindle-shaped cells were found within the lamina propria and, rarely, in the inner and outer smooth muscle layers, as well as within the epithelium. Different shaped c-kit receptor-positive MC and IC were present in all layers of the human VD. Our findings demonstrate the presence of different c-kit receptor-positive cells also in the human VD. Their rather ubiquitous distribution within the lamina propria and muscle layers suggests that IC and MC may modulate the neuromuscular transmission and the propagation of electrical signals in multiple systems involved in the draining of fluids. The importance of the c-kit receptor-positive interepithelial cells remains unclear.     

Self-organized formation of layered carbon–copper nanocomposite films by ion deposition

The quasi-simultaneous deposition of low energy-mass-selected C⁺ and metal⁺ ions leads to the formation of metal–carbon nanocomposites. In the case of C⁺ and Cu⁺ deposition, a homogeneous distribution of small copper clusters in an amorphous carbon matrix is expected. However, at a certain C⁺/Cu⁺ fluence ratio and energy range, alternately metal-rich and metal-deficient layers in an amorphous carbon matrix with periods in the nm range develop have been observed. The metal-rich layers consist of densely distributed crystalline Cu particles while the metal-deficient layers are amorphous and contain only few and small Cu clusters. The formation of multilayers can be described by an interplay of sputtering, surface segregation, ion induced diffusion, and the stability of small clusters against ion bombardment. This formation has been investigated for the a-C:Cu system with respect to the ion energy and the C⁺/Cu⁺ fluence ratio. The sputter coefficient S_M = r_f S_CCu + S_CuCu is the parameter to switch between layer growth (S_M < 1) and homogeneous cluster distribution (S_M > 1).     

Depth profiles of carbonates formed during natural carbonation

The depth profiling of carbonates formed during a natural carbonation of mortars with one face exposed directly to rain and the opposite face sheltered have been measured. The amount of carbonates formed on mortars sheltered from rain is generally higher. The depth of carbonation evaluated by the phenolphthalein test cannot be directly correlated with the carbonate profile. The CO₂ diffusion coefficients calculated on the basis of the distribution of carbonates are of the same order of magnitude as those measured by a direct method. A threshold value of diffusivity appears when the sand concentration increases. This point is discussed and compared with other characteristics of mortars and concretes that exhibit similar behavior.     

Evaluating GPS sampling rates for pedestrian assistant systems

In recent years, assistant systems have come to widespread use and support people in various situations e.g. in getting from A to B. For quite a time also assistant systems with special attention to older people have been developed. For example, in case of cognitive impairments where autonomous living indoors as well as outdoors is affected, assistant systems can be valuable aids. First attempts for outdoor assistance with GPS-based location systems offering the possibility to define geo-fences for raising an alert if a known area is left have been made. The quality of these systems is largely dependent on the precision of localisation which among others is influenced by the sampling rate. This paper reports on an empirical study under real world conditions to determine a suitable GPS sampling rate for movement analysis of (cognitively impaired) pedestrians. The work considers GPS measurement and interpolation errors as well as track losses as a result of low sampling rates. For the study, GPS data for different environmental settings and movement scenarios for sampling rates of 1, 2, 3, 4, 5, 10, 15, 20 and 25 s has been collected. The impact of sampling rates on movement parameters like track length and speed has been empirically measured. Additionally, the influence of smoothing approaches on data quality and whether downsampling of data has the same effect as recording with corresponding lower sampling rate has been studied. Results show that across all tested scenarios a sampling rate of 3–5 s seems to be appropriate with respect to speed and track length. Additionally, it can be argued that smoothing improves data quality of highly sampled data (up to 4 s). With downsampling, outliers are less in comparison to data sampled at the corresponding sampling rate.     

Durability of strain-hardening cement-based composites (SHCC)

Strain-hardening cement-based composites were named after their ability to resist increased tensile force after crack formation, over a significant tensile deformation range. The increased resistance is achieved through effective crack bridging by fibres, across multiple cracks of widths in the micro-range. Whether these small crack widths are maintained under sustained, cyclic or other load paths, and whether the crack width limitation translates into durability through retardation of moisture, gas and other deleterious matter ingress, are scrutinised in this paper by evaluation of test results from several laboratories internationally. This contribution is a short version of the State-of-the-Art report by RILEM TC 208-HFC, Subcommittee 2: Durability, developed during the committee life 2005?C2009.     

Composing the feature map retrieval process for robust and ready-to-use monocular tracking

This paper focuses on the preparative process of natural feature map retrieval for a mobile camera-based tracking system. We cover the most important aspects of a general purpose tracking system including the acquisition of the scene's geometry, tracking initialization and fast and accurate frame-by-frame tracking. To this end, several state-of-the-art techniques - each targeted at one particular subproblem - are fused together, whereby their interplay and complementary benefits form the core of the system and the thread of our discussion. The choice of the individual sub-algorithms in our system reflects the scarcity of computational resources on mobile devices. In order to allow a more accurate, more robust and faster tracking during run-time, we therefore transfer the computational load into the preparative customization step wherever possible.From the viewpoint of the user, the preparative stage is kept very simple. It only involves recording the scene from various viewpoints and defining a transformation into a target coordinate frame via manual definition of only a few 3D to 3D point correspondences. Technically, the image sequence is used to (1) capture the scene's geometry by a SLAM-Method and subsequent refinement via constrained Bundle Adjustment, (2) to train a Randomized-Trees classifier for wide-baseline tracking initialization, and (3) to analyze the view-point dependent visibility of each feature. During run-time, robustness and performance of the frame-to-frame tracking are further increased by fusing inertial measurements within a combined pose estimation.