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.     

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).     

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.     

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.     

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.     

Determination of relevant parameters for the use of electronic dosemeters in pulsed fields of ionising radiation

Active electronic dosemeters using counting techniques are used for radioprotection purposes in pulsed radiation fields in X-ray diagnostics or therapy. The disadvantage of the limited maximum measurable dose rate becomes significant in these radiation fields and leads to some negative effects. In this study, a set of relevant parameters for a dosemeter is described, which can be used to decide whether it is applicable in a given radiation field or not. The determination of these relevant parameters-maximum measurable dose rate in the radiation pulse, dead time of the dosemeter, indication per counting event and measurement cycle time-is specified. The results of the first measurements on the determination of these parameters for an electronic personal dosemeter of the type Thermo Fisher Scientific EPD Mk2 are shown.     

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.