Abstract

A lot of money is spent every year on the restoration of the concrete on structures and manufactured elements deteriorated by different types of degradation. Degradation of concretes is attributable to inadequate mix-design; anyway some environments are so aggressive that requiring additional care in order to increase durability. One of the most aggressive environments for concrete is acidic one, such as in the inner walls of sewer pipes. Deterioration of sewer pipes, made in concrete, is a great problem when service life is lower than 30 or less years, and maintenance or even replacement of damaged concrete sewer pipes are requested. In the case of a sewer pipe, the exposure class is the XA3, which concerns concrete structures exposed to chemical attacks by sulphate ions and aggressive chemical agents. To achieve a 30+ design life, materials exhibiting long-term durability have to be selected. However an extensive characterization of the most common concrete used for this application lacks due to the complexity of the chemical attack during service life. The processes leading to corrosion of the concrete sewer pipes are really complex. As previously described by some authors, hydrogen sulphide produced by anaerobic wastewater is released into the pipe, where micro-organisms convert aerobically hydrogen sulphite to sulphuric acid, which reacts with the concrete. The acid attacks first the calcium hydroxide and even tobermorite gel. Accordingly, under attack, calcium hydroxide forms gypsum and the calcium silicate hydrate (C-S-H) forms both anhydrous gypsum and an incoherent mass. However, different types of sulphate ions can be found in the wastewater. In the presence of a sodium sulphate source, calcium hydroxide reacts to form secondary gypsum. It is important to find out how to control these processes in order to increase the life of the facilities. The literature results lacking in prediction of the concrete resistance, because the experiments neglected both the presence of aggregate and the real conditions in the sewage pipes. Actually, it has been observed that the rate of degradation is higher just above the waterline at the sides of the pipes, where the fluctuating water level continually wash away the sulphate deposits, reducing the degradation resistance. Given that formation of hydrogen sulphide and sulphuric acid is the controlling factor, it is likely that pH is not continuously below 4, value for a fast concrete degradation, but shows a not well predictable wavy trend. It is worth noting that alternate wetting and drying of the concrete is even more detrimental than a continuous exposure to chemical attack. Thus the knowledge of the degradation rate is preliminary to the choice of a protection method or to design a suitable formulation of the mixes. Aim of this thesis is to develop procedures for evaluating the resistance of concrete mixes for sewage pipes. At this regard it seems that the concrete should be formulated for resisting cyclically to acid and sulphate attack. Indeed, the previous theories, which were developed so far upon testing standard mortars in acidic or sulphate solution, have to be critically revised, taking into account that the real attack involves cyclically both acid and sulphates. In this thesis new data, which were recorded on various concretes with different composition through a new chemical test, alternating acid and sulphate attack are described and discussed. The aim is to discriminate the effect and the behaviour of concrete compositions in an aggressive environment, as close as possible to the real attack condition, and in the meantime sufficiently fast. The concretes under investigation contain portland-limestone cement, blast furnace slag cement, sulphates resistance pozzolanic cement with or without silica fume, respectively, at constant water/cement ratio (w/c 0.39), and two type of aggregates, limestone and silicates. This thesis is divided in two main parts: the first part regards the studies on concrete degradation in aggressive environment and the second one describes the development of protective system. After an introduction of the degradation problem in chapter one, chapter two describes the studies on degradation of different concretes prepared with various cements and limestone aggregate, using a cyclic acid/sulphate test with sulphuric acid and sodium sulphate. Chapter three shows the results regarding the degradation of different cements prepared with two type of aggregates, limestone and silicates, using a cyclic test with acid sulphuric and a complex (mixed) sulphate solution (containing sulphates). The second part of the thesis describes the development of a pozzolanic coating and the results of the cyclic acid/complex sulphate test on a concrete with this protective coating. The last chapter shows preliminary results regarding the development of a new hybrid coating, which contains also zirconium oxide and could have a protective effect without the drawbacks evidenced upon using pozzolanic addition or coating.