PSI - Issue 33

Fabrizio Pittau et al. / Procedia Structural Integrity 33 (2021) 630–645 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The water permeability is effectively reduced by using crystalline hydrophilic additives Calvo et al. (2019). The application of this commercial additive is analysed in depth showing good performances with respect to water permeability. This addition seems do not have significant influence on concrete properties, probably thanks to the ability of this family of product in reducing the w/c ratio. The application of Surface Blast-cleaning Waste as a partial replacement of fine aggregate is studied by Kubissa & Jaskulski (2019) with the purpose of evaluating concrete tightness. The introduction of fine aggregates implies an increasing of the tightness and of the durability, without affecting the other concrete properties, included the shrinkage. An interesting insight on the use of bacteria insulated by agricultural soil and dry soil on the self-healing of micro cracking in concrete is discussed in Hussein et al. (2019). It is observed the effect of deposition of calcite carbonates in the voids and the closure of crack by the precipitation of calcium carbonate layer. The application of fly ash and silica fume, because of their very reduced dimension, improve the mix design quality and the concrete performances both in terms of resistance and permeability Wang et al. (2021). Rollins et al. (2018) have shown that the application of Colloidal Nano-Silica (CNS) on the external surface of concrete produces an effective barrier towards the capillary rise and consequently to colure penetration. From a technological point of view, it is necessary to consider, particularly for existing structure, the issue related to casting, segregation and local material variations De Stefano et al. (2014) Puppio et al. (2017). The variation in concrete strength can influence the service and the ultimate behaviour of the bearing elements Alecci & De Stefano (2019) Beconcini et al. (2018) Puppio, Giresini, et al. (2019) by significantly affecting degradation and permeability. The use of lining can be of great interest especially for the limitation of permeability due to cracks. No one of the research cited above refer about the possibility to delete the crack formation. The introduction of external coating to ensure vapor tightness is analyzed in this work. The main concept is to install and to test a retrofit intervention that bypass the natural property of concrete constituting an additional protective layer. Some recent applications in this field are known, particularly using polyurea or other polymer to increase the concrete durability in aggressive atmosphere, like in cooling towers and other industrial facilities. Four commercial external coatings, usually applied in different fields, are herein tested on separate concrete box with the purpose to ensure tightness. The paper presents the following structure: after this introduction, in section 2 the method of investigation is presented. Within section 3 the experimental campaign is carried out on four specimens and in section 4 the results are presented. In section 5 the results are presented and in section 6 they are discussed. Finally, the conclusions are drawn in section 7. 2. Method of investigation The aim of the experimental tests is to reproduce the serviceability conditions of concrete boxes with an internal gas pressure, to verify the effectiveness of external coatings. As discussed before, concrete is not able to guarantee gas tightness. Although, a concrete wall, if properly reinforced, is adequate to manage significant internal pressure. The tests were performed using prefabricated concrete box, typically applied in the management of withe-water. Moreover, four types of coating are chosen with different properties and a specific application cycle. Each box has been tested by injecting pressure gas into the box to investigate its tightness capacity The set up and the instruments are herein described. The geometry of the precast concrete box is shown in Fig. 1. It is characterized by an approximate cubic shape of 120 cm external edge. The wall thickness is 10 cm, with a thinner surface in the central area of the faces, essential to realize the joints (Fig. 2). The specimen is composed by three separate pieces: the box, the covering, and the manhole. These components are joined with mechanical anchoring and sealed with mastic (Fig. 3). Six threaded bars of 8 mm diameter and a steel UPN100 profiles are used to restraints the covering. The choice of these specimens guarantees a significant stress of the coatings. As a matter of fact, considering the coarse finishing of the casting and the small thickness of the walls, the seal is exclusively left to the external coatings. In addition, the internal pressure acts unfavourably on the external coating because induces tensile stress in the covering and at the interface with concrete. This point highlights the importance to test different material with different application strategies and with a primer that determines a local consolidation of concrete.