International Concrete Abstracts Portal

In the frame of a European sponsored Brite-Euram research project program, long-term exposure tests are conducted. Notched cubes are used to test the effect of exposure to different environments for different periods of time. The different environments are: outside exposure; alternatively drying and wetting for one week in lab environment; wetting in demineralised or chloride water and drying in CO2 environment. Reference testing is done after 6, 12 and I8 months. To assess the mechanical behaviour, the wedge splitting test is performed. The electrochemical testing is done on the same types of cubes in which fibres are embedded. The comparison is made between fibres bridging the crack and fibres fully embedded in the concrete. Afterwards fibres are taken out and analysed with the microscope. This is done for the cubes mechanically tested and for the cubes electrochemically tested. No decrease of strength could be found after the 18 months of exposure. The electrochemical measurements on the sensors show negligible corrosion. This is confirmed by the microscopic analysis. Maximum measured corrosion depth is 16um.

The parameters that are relevant for prediction of service lifetime with respect to chloride ingress, are associated with large uncertainties. The present paper summarizes statistical distributions for relevant parameters based on full-scale measurements from the Gimsoystraumen bridge in Norway. A large. number of chloride profiles are available, and for each of these the diffusion coefficient and surface concentration are estimated. Extensive measurements of concrete cover are also performed. These probability distributions are subsequently employed as input to a prediction model for chloride concentration at the steel reinforcement. Since the input parameters are represented in probabilistic terms, the chloride concentration is also a stochastic quantity. The probability of exceeding the critical threshold can accordingly be determined as a function of time. A design format which reflects the statistical properties of the relevant parameters is considered.

The deterioration concerns the outside walls of private houses made with hollow concrete blocks and covered with one-layer of white cement based coating. Six years after construction, the walls presented a random local ejection of the coating leaving in the bottom of the hole a rust coloured deteriorated material. Rust-coloured stains on the surface of the wall was also formed. The field study showed that the bottom of the hole reached systematically the concrete block and that the intensity of the deterioration was panially related with the climatic exposure of the walls. Scanning Electron Microscopy, X-Ray Energy Dispersion Spectroscopy and X-Ray Diffraction investigations on sound and deteriorated samples showed that the deterioration results from the oxidation of pyrite nodules of some millimeters of diameter included in the course aggregate of the concrete mixture of the blocks. The oxidation process of the pyrite occurred in wet conditions and in contact with cement compounds producing expansive gypsum and colloidal ferrous hydroxides. The random cracking and the low permeability of the coating were contributing factors to the deterioration process.

Measurements of relative humidity (RH) by means of wooden sticks of the species Ramin (Gonystylus macrophyllum) have been used in Norway since 1995. The method is especially applicable to concrete with high internal humidity as in structures damaged by Alkali Aggregate Reaction (AAR) or freezing and thawing. Results up to now show that the relative humidity is stable 5 cm from the surface and that ingress of rainwater is an important Source of water in most of the outdoor exposed concrete structures. Correlation tests have shown that wooden sticks still are in good condition and reliable after 7 years continuous use. Measurements on surface treated columns and beams in two structures show that the relative humidity 5-cm from the surface decreased significantly during four years monitoring, which was not the case in untreated concrete. It is concluded that surface treatment is able to reduce the relative humidity in concrete structures exposed to rainwater.

Recently, a number of cases of damaged concrete structures due to alkali-silica reaction (ASR) and/or chloride-induced steel corrosion have been reported. Some ASR-affected concrete structures have been found to be in a severe condition when ASR expansion has continued for a long period. An inspection for the footing of concrete piers was carried out and fractures at bends in steel bars on the edge of the footing was found. It is considered that the fractures were due to excessive ASR expansion in the concrete. However, mechanism of fracture of steel bar due to excessive ASR expansion is not clear. And as results, a model footing made with reactive aggregate was made and tested. In this study, the relationship between the ASR expansion and the stress of steel bar was investigated. From the experimental results, it was found that thestress at the bends in steel bar due to ASR expansion was attained at the yield point of steel bar or beyond it.