Deterioration Mechanisms Of Reinforced Concrete Structures Under Combined Mechanical And Environmental Action

In practice it may be frequently observed that reinforced concrete structures need substantial repair or even need to be replaced a long time before the desired or designed service life is reached. That means the service life of many reinforced concrete structures is not sufficient. Early failure is in most cases not due to insufficient strength or stiffness but due to inadequate durability under the given environmental conditions. In this context it must be pointed out that durability under one dominant deteriorating mechanism as for instance chloride penetration or carbonation is taken into consideration for important structures nowadays. But in reality reinforced concrete structures are nearly always exposed to combined mechanical and environmental actions. It may be anticipated that service life can be shortened significantly by simultaneous or subsequent action of combined mechanical and environmental loads. In order to better understand the combined action it is necessary to investigate changes in the nano and micro structure and the migration of pore solution as well as of dissolved ions in the pore solution.The micro structure of the cement based matrix has been measured by means of mercury intrusion porosimetry. The composition of the pore solution has been analyzed by ion chromatography. CaCO3 and Ca(OH)2 profiles as well as chloride profiles have been determined. Most important results and the conclusions, which may be drawn are summarized in the following:1. The rate of carbonation increases with increasing applied tensile load. Under compressive load the rate of carbonation first decreases up to a load level of approximately 30% of the ultimate load. If, however, the compressive load is further increased the rate of carbonation increases with increasing compressive load. Surface impregnation with silane slows down the rate of carbonation. But an applied load increases the rate of carbonation even after surface treatment with silane. Based on these results a model has been developed which allows predicting the carbonation depth under an applied tensile or compressive load as function of time.2. Capillary absorption of water and of salt solutions has been determined. The influence of an applied load has been studied in particular. The chloride diffusion coefficient has been determined by applying Fick?s second law. Concrete has been exposed to salt solutions by immersion, by drying wetting cycles, and in an exposure site in the tidal zone. Under compressive load the chloride diffusion coefficient decreases until about one third of the ultimate load. Further increase of the compressive load increases the chloride diffusion coefficient again. It may be assumed that the micro structure becomes denser first and then formation of micro cracks through which chloride can migrate dominates. Under increasing tensile load the coefficient of diffusion of chlorides increases steadily. Little chloride penetrates specimens with silane impregnated surfaces. Based on these results a model to predict penetration of chloride into mechanically loaded structural elements has been developed.3. Capillary absorption of water, chloride absorption and chloride diffusion of carbonated concrete has been determined. After accelerated carbonation, capillary absorption of liquids by concrete is enhanced, and the chloride diffusion coefficient increases. The pore size distribution, total porosity, average pore size and the most probable pore size of carbonated concrete was analyzed. Changes of the microstructure induced by carbonation and their influence on chloride penetration were studied. It could be shown that the total porosity of hardened cement paste decreases by 30% to 40% during carbonation of concrete. The effect of carbonation on pore size distribution depends on the type of concrete. For concrete with a low water-binder ratio and high content of binder, the proportion of pores with a size lager than 200 nm increases in particular. A model describing the relationship between microstructure and chloride transportation in concrete was established based on the experimental results.4. After complete carbonation, the amount of free chloride ions in the pore solution increases by a factor ranging between 1 and 11. The amount of chemically bound chloride is lowered by 27% and up to 54%, and the pH value decreases from values in the range between 13.19 and 13.47 to values in the range between 7.6 and 8.1. Results of X-ray diffraction crystallography also show that in the diffraction spectrum of carbonated concrete strong peaks of CaCO₃ can be observed, while the peaks of Ca(OH)₂ and Friedel's salt nearly disappeared. In addition to lowering the pH value of the pore solution, carbonation will release more chloride from the hydration products into pore solution and the entire chemical environment of concrete is changed considerably. It can be concluded that carbonation facilitates chloride penetration into concrete.5. Based on the models which have been developed to describe carbonation and chloride penetration of reinforced concrete structures under the influence of mechanical load, the reliability index of concrete structural elements under combined mechanical and environmental action has been determined. Based on probabilistic reliability theory main factors which influence reliability of concrete structures were analyzed. It turns out that mechanical load has a significant influence on reliability and service life of reinforced concrete structures. Finally it is recommended that in the future, more attention is to be paid on the influence of mechanical load on the long term performance and on service life of reinforced concrete structures.