Band 6, Heft 1

The Preussian Royal Directorate for Finance, Military and Property has published the following prize question in 1775: in which way depends durability of Gothic and Roman masonry on time, on specific local conditions, on the stones and mortars used, and on the design, the execution and curing conditions? And how is new masonry to be built in order to achieve the same durability? The text which has been selected for this award is presented in full length in this contribution. First some observations made in Roman buildings are described. The observations are explained based on Roman literature on architecture and buildings. The durability of frequently used mortar and stones is discussed. The preparation of durable mortars is treated in detail. Different types of masonry are described. In conclusion it is stated that for external renderings and for mortars in masonry exposed to humid conditions cement mortar, i.e. mortar made with lime mixed with puzzolanas as a mineral binder, are to be applied exclusively. Internal renderings can be made with plaster of Paris or lime mortar. In an appendix it is pointed out that the thickness of a wall has a major influence on durability. In Holland durable and thin masonry walls are being built, however. This is possible because high quality cement mortars and bricks burnt at high temperature (clinkers) are used

After the collapse of some heavy buildings and towers in Italy and elsewhere, the hypothesis of possible continuous damage to these structures due to the dead weight and cyclic loads (wind, temperature variations, etc.) has been formulated by the authors. The long term behaviour of historic masonry has also been detected by laboratory creep and cyclic tests. The results indicate that a laboratory and on site investigation together with analytical modelling are important particularly for tall bell-towers. The application of these procedures on the Bell-Tower of the Cathedral of Monza is here reported and discussed

On two limestone monuments in Coimbra, which are located near each other, two different kinds of "black crusts" were observed: "fossil" and modern. The crusts were analyzed and the data compared. The very thin "fossil" black crusts of Santa Clara-a-Velha Monastery are formed by pellicules of gypsum, anhydrite and syngenite rich in Fe and Mn and depleted of Pb,Ti, Zn, Cu and Ba. The anthropogenic, modern crusts of the Santa Cruz Church are thicker, richer in gypsum and trace elements such as Ti, Pb, Zn, Cu and Ba. Calcium oxalates, weddellite and whewellite, appear in areas where lichens are growing. While the "fossil" black crusts have resulted from burial for at least 300 years, representing a geogenic weathering product in a rural area, the black crusts of the Santa Cruz Church represent an anthropogenic product generated from limestone attack in an urban modern environment

The origin of the susceptiblility of prestressing steel reinforcement with respect to stress corrosion and the observed failure mechanisms can be described 'in detail. So far, however, no precise method exists to determine the sensitivity of prestressing reinforcement or to distinguish different types of steel with respect to their susceptiblility for stress corrosion. This fact leads not only to prejudices in practice but also to additional safety measures (f.e. robust reinforcement), which implies economical consequences. The aim of a cooperation project between the Institute of Concrete Structures and Building Materials Technology at Leipzig University and the Institute for Materials Technology and Testing at Otto-von-Guericke- University Magdeburrg is to develop a general acceptable test method which provides us quickly with meaningful results. It turned out that the observation of electrochemical noise is a promising method to study stress corrosion of prestressing steel reinforcement. This method can be applied both in laboratory testing and under site conditions and can therefore also serve as a basis for corrosion monitoring.

Mean electrolyte concentration C m in narrow charged pores decreases as a result of repulsion of co-ions from charged walls. The theory of electrical double layers (EDL) was used for calculation of this effect in dependence on surface potential values Φ 1 and concentration C 0 of bulk electrolyte solutions. The calculations were limited to the case of 1-1 electrolytes and low values of surface potentials Φ 1 when simple analytical solutions may be used. Mean concentration of chloride ions and neutral solution in narrow negatively charged slit pores of different thickness h was calculated in dependence on Φ 1 and Debye radius values. It was shown that the effect is more pronounced in narrow pores at relatively high absolute values of surface potentials and at low concentration of a bulk solution. In narrow hydrophilic pores, a decrease in mean electrolyte concentration results from the effect of dielectric exclusion caused by decreasing values of dielectric constant of water ef. The calculations were performed using Born equation that relates the self-energy of ions W with the 8f values and ion radii. Comparing the results obtained with measured values of rejection coefficients of electrolyte solutions in reverse osmosis membranes, the dependence of £ f values on pore sizes was calculated. Repulsive forces arising in narrow pores as a result of overlapping of EDL are calculated in dependence on slit thickness, bulk solution concentration and surface potentials in the framework of the theory of electrostatic component of disjoining pressure. In the case of hydrophilic silica systems molecular attraction forces and repulsive structural forces were also taken into account. The arising structural forces are caused by overlapping of boundary layers of water with modified structure