Issue 46

L. Sorrentino et alii, Frattura ed Integrità Strutturale, 46 (2018) 285-294; DOI: 10.3221/IGF-ESIS.46.26 286 The durability and the strength of stone are affected by mechanical and chemical weathering processes. The atmosphere, water, dissolved salts, acid rain and temperature fluctuations act as agents of decay, inflicting visible damage to stone as a decay, as demonstrated by Winkler [1]. Cohen and Monteiro [2] showed that limestone and granite are affected by weathering agents and are particularly susceptible to superficial dissolution caused by carbon dioxide (CO2), sulfur dioxide (SO2) and nitric acid (NOx) dissolved in water as acid rain. It is evident that the weathering results in a loss of strength in natural stone. Reinforcement provides stone members with flexural strength required for use in long spans, columns and slabs. However, in some cases, reinforced stone may fail to perform to its desired capacity because of faulty design, use of inferior materials, poor construction practices and insufficient maintenance. If such a problem occurs early in a structure's service life, repair and strengthening of the concrete sections may be more favorable than replacement or reconstruction of the failing members. At the beginning of the second half of the last century, the use of external steel reinforcing to strengthen existing concrete bridges and buildings was investigated by researchers in South Africa and France. Thin steel plates were bonded with epoxy to the tension face of concrete beams to provide additional local stiffness. Subsequently, Mays [3] applied this technique to reinforce concrete members in Europe, the United Kingdom, Japan, New Zeland, South Africa and the United States. Since steel plates are readily available and relatively inexpensive, repair of structures by externally bonded reinforcement is an attractive alternative to replacement. However, corrosion of the external metal plate remains a problem. A wide variety of civil engineering applications sees the introduction of fibre-reinforced polymers (FRPs). Neale [4] found these materials to be particularly attractive for applications involving the strengthening and rehabilitation of existing structures. Composite materials were proposed as a corrosion-resistant alternative to external steel reinforcement of concrete members. Iyer et al. [5] used sheets of graphite fibers in an epoxy matrix to strengthen cracked concrete beams in an existing bridge. Also, Saadatmanesh and Ehsani [6] showed that glass fiber composites, well bonded with epoxy to concrete beams, double the ultimate capacity of the beams also employed to increase strength and ductility with encouraging results in terms of mechanical behavior and cost effectiveness. Recently it was investigated by Sisti et al. [7] and Aiello et al. [8] a new type of reinforcement for historic masonry buildings made by recycled old stone or bricks with GFRP grits, that demonstrated to improve the bending capacity of the structure. External composite reinforcement was infrequently applied to natural stone, as demonstrated by Kurtis and Dharan [9]. To determine the effect of external reinforcement on the load-carrying capacity of two types of stone, 3-point bend tests were performed on marble and Travertine marble (actually a limestone) reinforced with HS carbon fibers in an epoxy matrix. The results show how the load capacity of the stone may be increased of about 5-10 times. In a previous work, Polini et al. [10] investigated the use of external composite reinforcement on natural stone. It involved the production of a hybrid structure “natural stone/composite” very thin, the use of less expensive composite materials, such as glass fiber, and the comparison between two natural stones that are largely used for decorative application, marble and granite. The results demonstrated that the load capacity of the stone can be increased by a factor of 7 and 6 for granite and marble respectively. In another work, Bellini et al. [11] created a new hybrid structure, in which the thin laminate made of composite material was substituted with a sandwich structure. In this work, sandwich structural laminates based on composite materials are used as external reinforcement both to increase the mechanical resistance and to decrease weight of natural stone. High strength glass/epoxy laminates were bonded to the lower surfaces of marble and granite beams, and 3-point bend tests were performed on both reinforced and unreinforced specimens. Such reinforcement is useful to increase low initial tensile strength or to restore strength lost by weathering. An increase in strength can result in the use of longer spans and thinner sections, decreasing dead load. Therefore, the use of external composite reinforcement of natural stone in application such as exterior cladding, flooring, countertops, and desktops can result in weight saving and possible cost saving. The materials and the methods to produce the specimens are deeply described in the next sections; then, the test to mechanically characterize the stone-composite sandwich specimens are deeply discussed and the obtained results are presented and analyzed. M ATERIALS AND METHODS n this work, the new hybrid material was constituted by a sandwich structural laminate in composite materials glued to a stone tile. Two kinds of sandwiches were considered: the first one was self-produced by gluing two composite skins to a DIAB Divinycell P60 core, the second one was a commercial sandwich of Hexcel Corporation, that is I