Issue 50

A.G. Lekatou et alii, Frattura ed Integrità Strutturale, 50 (2019) 423-437; DOI: 10.3221/IGF-ESIS.50.36 424 I NTRODUCTION he employment of modern materials in restoration works of ancient and modern monuments has been a common practice in the last decades. The application of AISI 316L stainless steel as a reinforcement of architectural members in the ancient theater of Dodona in the region of Epirus, Greece, is a typical example. With regard to the conservation of the monuments, the need for cost-effective combined with earthquake-resistant solutions is a critical factor to consider. Therefore, the replacement of an expensive steel reinforcement, like 316L steel, with a less expensive steel, like 304L, combined with low-cost corrosion inhibitors could become a more profitable alternative, as long as it proves an equally safe alternative. 304L austenitic stainless steel finds a wide variety of applications in many industrial fields as it combines a satisfactory corrosion performance, good mechanical properties and formability. Its good corrosion resistance is due to the Cr 2 O 3 -based passive film, the passivity of which is improved by the presence of nickel [1,2]. Reinforced concrete is the most widely used construction material due to its cost effectiveness, versatility and environ- mental compatibility, as well as its exceptional mechanical characteristics, longevity and corrosion resistance [3,4]. The great resistance to corrosion is owing to the chemical stability of the hydrated Portland cement and the passivity of steel in the highly alkaline pore solution of the concrete, its pH ranging from ~12.5 to ~13.5 [5-7]. Many historical buildings and monuments are located in urban and coastal regions and often in the vicinity of industrial areas. The consequent environmental-due deterioration of the reinforced concrete has been a serious problem in the last decades causing severe aesthetic, structural and economy issues. Corrosion of steel reinforcement is the most significant factor responsible for the premature deterioration of the serviceability, durability and seismic resistance of reinforced concrete structures. The two commonest types of atmospheric attack to the concrete, are: a) chloride infiltration into the concrete through its porous structure, when it is exposed to marine environments and deicing solutions and b) concrete carbonation, as a result of the reaction between the atmospheric CO 2 (mostly in urban areas) and Ca(OH) 2 of concrete [5]. Besides these two modes, concrete is also subjected to acid rain attack, as a consequence of the super-intensive urban and industrial activity during the last decades. In the presence of Cl - , austenitic stainless steels embedded in carbonated or alkaline concrete, although still passive, can suffer localized (pitting or crevice) corrosion [8]. The interaction of Cl - with the concrete chemical constituents, results in the deposition of voluminous products in the concrete pores, subsequent stresses, cracking and Cl - access to the steel surface through the cracks [5]. Regarding the mechanical behavior of reinforcing steels due to chloride corrosion, a modest loss of strength but a marked reduction of the ductility of carbon steels have been shown [9,10]. Carbonation-induced breakdown of passivity on stainless steel rebars is not an issue, since Cr-containing steels are expected to be passive in intermediate pH (pH~9) concrete pore solutions [11]. In the last decades, the degradation by acid rain has notably been accelerated, particularly in the aggressive environments of big cities, industrial areas, airports etc., causing serious damage in the durability and aesthetical value of architectures [12]. Acid rain (AR) attack and carbonation are interrelated since they both occur more frequently in urban areas with high concentrations of sulfur oxides (SO x ) and nitrogen oxides (NO x ), which in turn combine with the atmospheric water, to form sulfurous/sulfuric and nitrous/nitric acids [13]. Both, carbonation and AR attack of concrete lead to a decrease in basicity owing to acidic pollution. However, acid rain, a strongly corrosive medium, does not only contain H + , but also NH 4 + , Mg 2+ , SO 4 2- , NO 3 - , Cl - , etc.; hence, the mechanism of concrete degradation by AR is more complex than the mechanism of pure acid attack [14]. Moreover, the various sources of AR (vehicle exhausts, internal combustion engines, power stations, smelters, pulp mills etc. that burn fossil fuels and natural sources like volcano eruptions, lightning strikes, pulp mills etc. [15]) cause a wide range of AR pH values and compositions in different regions of world. The corrosion of steel reinforced concrete structures is accelerated by acid rain. More specifically, it is generally accepted that the acidic constituents of AR rapidly react with Ca(OH) 2 to form hydrated salts soluble in rainwater. (It is recognized that Ordinary Portland Cement (OPC) has minimal resistance to acid (pH ≤ 3) attacks [16]). These solutions penetrate the interior of the concrete through its pores. After the evaporation of the rainwater, the salts redeposit causing stresses to the concrete pores. The resulting cracking and spalling of the concrete allows further inward diffusion of aggressive species and, eventually, corrosion and mechanical degradation of the steel reinforcement [14,17-19]. The significant influence of H + (dissolving erosion) and SO 4 2- (expanding erosion) contained in AR on the erosion, appearance and strength of cementitious materials has previously been manifested [14,20]. The degradation of the mech- anical properties of the concrete (compressive strength, fracture toughness, modulus of elasticity, tensile properties) by AR has also been demonstrated [21-23]. Amongst the various methods applied to protect reinforced concrete against corrosion (corrosion inhibitors, epoxy coatings, steel galvanizing, industrial by-products and waste), the partial replacement of OPC with fly ash (FA) is a relatively inexpensive and ecological method [24,25], whilst causing a reduction of the unit cost of concrete [26]. The T