Seismic performance of masonry walls retrofitted with steel reinforced grout Stefano De Santis 1, * ,† , Paolo Casadei 2 , Gerardo De Canio 3 , Gianmarco de Felice 1 , Marialaura Malena 1 , Marialuisa Mongelli 2 and Ivan Roselli 2 1 Roma Tre University, Department of Engineering, Via Vito Volterra 62, 00146 Rome, Italy 2 Kerakoll S.p.A. Headquarter, Via dell’Artigianato 9, 41049 Sassuolo (MO), Italy 3 ENEA, Casaccia Research Centre, UTT MAT-QUAL Via Anguillarese 301, 00123 Rome, Italy SUMMARY An innovative solution for the seismic protection of existing masonry structures is proposed and investigated through shake table tests on a natural scale wall assemblage. After a former test series carried out without re- inforcement, the specimen was retrofitted using Steel Reinforced Grout. The strengthening system comprises horizontal strips of ultra-high strength steel cords, externally bonded to the masonry with hydraulic lime mor- tar, and connectors to transversal walls, applied within the thickness of the plaster layer. In order to assess the seismic performance of the retrofitted wall, natural accelerograms were applied with increasing intensity up to failure. Test results provide a deep understanding of the effectiveness of mortar-based composites for improv- ing the out-of-plane seismic capacity of masonry walls, in comparison with traditional reinforcements with steel tie-bars. The structural implications of the proposed solution in terms of dynamic properties and damage development under earthquake loads are also discussed. Copyright © 2015 John Wiley & Sons, Ltd. Received 15 December 2014; Revised 18 May 2015; Accepted 10 August 2015 KEY WORDS: masonry; seismic retrofitting; composites; out-of-plane behaviour; strengthening; shake table 1. INTRODUCTION Past earthquakes have shown that the out-of-plane capacity of front walls is one of the crucial issues in the vulnerability of masonry structures, especially in historical centres [1]. Given the discontinuous nature of masonry, the external walls tend to separate from the internal structures, such as transverse walls and floors, and either fail by overturning (Figure 1(a)) or disgregate (Figure 1(b)), depending on constrain conditions, presence of openings [2], and section morphology [3]. Out-of-plane failures have been observed during recent earthquakes on masonry buildings [4], aggregates [5], and churches [6, 7]. Therefore, improved retrofitting solutions are needed that combine structural effectiveness, economic and environmental sustainability, and possibility of being integrated with maintenance works. Traditional steel tie-bars, which have been being installed since centuries, proved to effectively avoid the out-of-plane overturning [8]. However, the localized constrain provided by the end-plates cannot always prevent the failure by out-of-plane bending, especially for slender masonry walls (Figure 1(c)). As an alternative to steel bars, reinforced concrete bond beams have been used in the last decades to provide a box-type behaviour and a higher bending capacity to the walls, but in several cases the resulting stiffness *Correspondence to: Stefano De Santis, Roma Tre University, Department of Engineering, Via Vito Volterra 62, 00146 Rome, Italy. † E-mail: stefano.desantis@uniroma3.it Copyright © 2015 John Wiley & Sons, Ltd. EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2016; 45:229–251 Published online 16 September 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/eqe.2625