Band 5, Heft 6

The formation of blisters is a well-known phenomenon in coating of concrete and other mineral substrates. Especially, when dealing with wet or humid substrate, osmosis may occur. A semipermeable membrane is formed between a concrete coating and the concrete surface, followed by water transport, which results in fluid-filled blisters. In this report, the conditions necessary to promote osmosis are explained in detail. The type of concrete, the condition of its surface, the presence of osmosis-promoters and the type of coating are important factors which influencethe formation of blister. Evaluation of the literature and test results lead to a set of recommendations on how to avoid the formation of osmosis-caused blisters.

Stainless steel is essentially applied in structural elements in which chemical and mechanical loads are combined and which hardly allow periodic checks. Fixations are typical examples. In case of failure dramatic consequences have to be expected. This is also valid for steel reinforcement in concrete. It is exposed to combined chemical and mechanical load, it can hardly be controled and damage leads at least to enormous costs of repair. Due to the fact that up to now the expenditures for construction are rarely considered together with the expenditures for maintenance, the use of stainless steel is still exceptional. If it is possible, however, to produce cheap steel with a corrosion resistance comparable to steel 1.4401 or even better, wide spread use of stainless steel as concrete reinforcement should be possible. New types of austenitic and duplex steel as being developed in the Laboratory for metal research of Swiss Federal Institute of Technolog (ΕΤΗ) in Zurich without Ni seem to be promising materials. In this case it is tried to replace expensive components by cheap ones such as nitrogen.

First of all it is pointed out that ten years ago little was known on deterioration and repair of exposed brick masonry or terracotta. A comprehensive research programme has been initiated and financially supported by the German Federal Ministry for Research and Technology (BMFT). One aim of this research programme was to identify deterioration mechanisms in close cooperation between representatives from services for the protection of monuments and from research institutes. In addition repair methods meeting both practical needs and requirements of the protection of monuments were to be developed. Different types of damage have been characterized at numerous buildings. As a result repair methods specific for each building have been established. In this contribution most important results of the huge research programme are briefly mentioned.

A new commercially available dielectric technique for the non-destructive determination of moisture in building materials based on the principle of 'time-domain reflectometry' (TDR) is presented. TDR measurements on samples of sandstone, brick, concrete and floor cover matched very well with results of conventional moisture measuring methods such as oven-drying or calciumcarbide-technique. The new method showed only a low influence of salt content or surface moisture of the material on the results.

The extension of cracks and pores are considered to be a theoretical basis for the prediction of durability of concrete of monuments. For this purpose a two-level structural model has been introduced: (I) matrix of hardened cement paste with fine and coarse aggregates, (2) voids and cracks created by external actions. The main parameter of this approach is the critical stress intensity factor: [xxx] The stress intensity factor of the material in the vicinity of the crack tip governs the crack growth. If all micro- and macro-defects are taken into consideration on the different levels of the hierarchic structural system, it is possible to formulate the global response. Each pore or crack creates a characteristic stress field around its­ elf in the material. If these pores are classified into periodic volume elements, the individual stress fields can be superimposed. This results in the complex state of stress under an imposed deformation.