Key Engineering Materials Vol. 919

Abstract: Structural strengthening using composite materials is nowadays one of the most interesting techniques to overcome weaknesses of masonry structures constituting large part of the building heritage. The use of FRCM composites is becoming more and more widespread due to some limitations of FRP retrofitting systems. In this framework, the presented experimental study is aimed at evaluating the in-plane and the out-of-plane behaviour of masonry walls strengthened with different types of FRCMs, analyzing in detail failure modes, capacity increments and efficiency of the strengthening systems when tested using two different configurations. To this purpose, bidirectional basalt grids and unidirectional steel fiber sheets, coupled with a lime based mortar, were used for retrofitting clay brick masonry walls subjected to diagonal compression tests and out-of-plane flexural tests. Experimental outcomes, when considering the in-plane or the out-of-plane direction, show that the adopted different layout strictly influences the flexural and shear strengthening efficiency of the reinforcement.

Abstract: Nowadays, FRCM (Fibre Reinforced Cementitious Matrix) systems are highly attractive for the building materials market; thus, their optimization and development cover an essential role. This work points out the chemical and physical parameters influencing the carbon-FRCM mechanical behaviour. Three different FRCMs composed of commercially available carbon fabric and different inorganic matrices are involved. Matrices are specifically developed to enhance the adhesion with the fabric and differ in organic additive used. Moreover, different fabric geometry (twisted and untwisted) and fibre coatings are considered: micro-silica, fine silica aggregate and medium-size silica aggregate. A new shear test setup is designed to obtain an inexpensive characterization method and employs traditional mechanical tests. Morphological and compositional analyses were performed on the surface fractures. On equal reinforcement typology, significant improvements in shear strength are promoted by organic additives and fabric coatings. Also, pull-out test displays that the twisted bundle promoted the fibre-to-matrix adhesion and remarkably modified the sample failure mechanism compared to the untwisted one. Finally, the FRCM mechanical performance is primarily influenced by mechanical adhesion contribution that might be increased by adopting simple geometrical choices or fabric surface treatments.

Abstract: Over the last few years, the effectiveness of textile-reinforced mortar (TRM) composite systems for structural retrofitting has led to the widespread adoption of these materials in the practice and to the issue of up-to-date design guidelines. Nonetheless, the weak interfacial bonding that is frequently observed between matrix and fibres is likely to cause inconsistent failure modes and, generally speaking, to severely limit the reinforcing potential of the textile. A promising solution to tackle this issue consists in treating the surface of the reinforcing fibres with a functional coating to improve the adhesion at the interphase. In this paper, a pilot study is presented to assess the effectiveness of a fully sustainable polymer coating, consisting in polyvinyl alcohol (PVA) loaded with with rice husk ash (RHA) or with a 50/50 mixture of RHA and silica fume (SF). The coating was applied on basalt fabrics to reinforce TRM coupons that were mechanically tested under uni-axial tensile loads. The mechanical properties of the TRM samples were significantly increased by up to 20%, and the peak load was attained at a higher deformability level, which is a clue of the enhanced ductility of the reinforced elements.

Abstract: Architectural heritage nowadays includes concrete structures constructed in the 20^th century. These buildings are usually under-detailed, since the actual behavior of reinforced concrete at the time of their construction was not clearly understood, whilst building codes incorporating seismic resistance design, especially in seismic prone areas, did not exist. This inevitably led to inefficient design and consequently to severe damages in many historic concrete buildings during past seismic events. This paper explores the use of novel Engineered (Fiber Reinforced) Cementitious Composites (ECCs), with strain hardening abilities in tension, for the repair and strengthening of old sub-standard reinforced concrete columns, focusing on their confining and shear strengthening potentials. The experimental results show that, when replacing the reinforcement cover with fiber reinforced ECCs, the fibers bridge tensile cracks, limiting their opening and increasing their resistance against volumetric expansion, ultimately leading to increased amounts of energy dissipation. ECCs may thus by used in the repair of historic concrete structural elements.

Abstract: The paper investigated the effect of locally sourced fly ash and ground granulated blast-furnace slag (GGBS) on the compressive strength and chloride resistance of concrete. The mix proportion was cementitious material (total of ordinary Portland cement (OPC), fly ash and GGBS): sand: coarse aggregate: water of 1:2:3:0.6 in which 20% by mass of total cementitious materials was replaced by class F fly ash and GGBS. Compressive strength and rapid chloride penetration tests were conducted at 28, 56 and 120 days. The results shows that fly ash and GGBS reduce slightly the compressive strength but improve significantly the choloride resistance of concrete. Within the range of investigation, 10% of fly ash and 10% of GGBS are recommended to replace OPC as they improve the chloride resistance and maintain the compressive strength of concrete.

Abstract: Prevention of rebar corrosion is achieved in the design and construction phases, by means of suitable mix design, casting and curing, and adequate cover depth; this approach have been introduced in international standards (EN 206) and design codes (Eurocode 2). Additional protection methods (cathodic protection, stainless steel or galvanised rebars, corrosion inhibitors, concrete coatings) can be used in very aggressive environment, especially in presence of chlorides, or when increased service life is required. In this work a simplified performance-based approach, based on Monte Carlo simulation, has been used to evaluate the service life (initiation time of corrosion) in chloride containing environments. The results confirmed that cathodic prevention and stainless steels are the most effective protection methods to guarantee a safe working condition in a severe environment. The use of pozzolanic or slag cement is confirmed as an effective way to slow chloride transport and by this way to increase the service life.

Abstract: Alkali activated materials and geopolymers have attracted a lot of attention in the last 20 years thanks to their excellent mechanical performances, durability and sustainability properties, especially for civil applications. These materials also exhibit promising properties as fire- and corrosion-resistant protection systems. In a previous study, a 20-mm coating based on light-weight alkali activated mortar (LWAAM) suitable for the protection of steel structures against fire was successfully developed. To understand if the same coating is also able to ensure corrosion protection to steel structures, this study reports the results obtained in two different chloride-rich environments. The corrosion performance of the new system based on steel coated by LWAAM (using expanded perlite and hydrogen peroxide in the mix) was compared with a steel coated by a traditional alkali activated mortar (NWAAM). Electrochemical tests on steel samples immersed in an alkaline solution simulating the pore environment of the binder or embedded in the two different types of mortars were carried out in presence of different chloride concentrations. It was found that the alkaline environment is able to passivate the steel surface, however, the increasing of chloride ions concentration, affects passive film stability and promotes steel corrosion. In presence of low chloride concentration (i.e., 0.2M NaCl), the increased porosity of the LWAAM did not impair the steel corrosion protection, when compared with NWAAM.

Abstract: Calcium chloride is one of the main de-icing salts for removing snow and ice from roads, infrastructures and service areas. It is well known that reinforced concrete structures, if exposed to calcium chloride, can suffer from severe damages due to both corrosion of steel reinforcement and chemical attack of the cement paste. This paper aims at evaluating the resistance to chemical attack of mortars manufactured with different low-carbon binders (alkali activated slag cements, calcium sulphoaluminate cement-based blends, high volume ultrafine fly ashes cements) in presence of CaCl₂-based de-icing salts in cold weather (temperature about 4°C). Results indicated that alkali activated slag-based mortars are quasi-immune to calcium chloride attack due to their mineralogical composition. On the contrary, calcium sulphoaluminate-based blends show the total loss of binding capacity, especially when calcium sulphoaluminate cement is used with gypsum and Portland cement. Finally, the partial substitution of Portland cement with ultrafine fly ash strongly reduces the mass change and the strength loss of mortars submerged in 30 wt.% CaCl₂ solutions due to the strong reduction of calcium hydroxide responsible for the calcium oxychloride formation in the cement paste.

Abstract: Concrete industry produces a great environmental impact. The total, or partial, substitution of ordinary Portland cement (OPC) with Calcium sulfoaluminate (CSA) cement could be a possible solution, due to its lower production temperature and thus lower CO₂ emission. Therefore, there is an essential need to assess the durability properties of concrete produced with CSA cement. In this work a preliminary study on durability of high performance fiber reinforced concretes produced with CSA cement in total or partial substitution of OPC, also with ground granulated blast-furnace slag (GGBS), was performed. Compressive strength and electrical resistivity of the different concrete mixes and electrochemical tests to evaluate corrosion condition of the embedded steel fibers, were assessed. The results show that substitution of OPC with CSA cement improves the mechanical properties of concrete but promotes corrosion of the steel fibers, affecting the durability of this material.

Abstract: Stainless steel reinforcing bars show excellent corrosion resistance in concrete exposed to harsh environments. In this combined electrochemical and surface analytical work, an explanation for this behavior is proposed. XPS surface analytical results (thickness, composition of the passive film and of the interface beneath the film) obtained on black steel, FeCr alloys, and a series of stainless steels after exposure to alkaline solutions simulating concrete are reported. Pitting potentials were determined in the same solutions with electrochemical experiments. It is shown that the pitting potentials of the steels can be related to the Cr (III) oxy-hydroxide and Mo (VI) content in the passive film. It is proposed to calculate a Cr and Mo oxide equivalent similar to the well-known pitting resistance equivalent number (PREN). A correlation between the critical chloride content in concrete (reported in literature for CEM II A/LL and CEM I) and the pitting potential for carbon steel, Fe12%Cr alloy, DIN 1.4301 and DIN 1.4571 stainless steels is proposed to link results of solution analysis and performance in concrete.