<b>ABSTRACT</b></br>

A substantial fraction of the existing housing stock is built with low-quality reinforced concrete that shows poor mechanical properties. Those concretes, labelled low-grade concretes, present certain drawbacks when common strengthening techniques are used for their rehabilitation. Over recent decades, a number of investigations have added to our knowledge of strengthening materials in the form of inorganic-based composites. Amongst those materials, Steel Reinforced Grout (SRG) presents optimum characteristics for flexural strengthening in situations where the use of other retrofitting techniques is not recommendable.

Previous applications of SRG include the reinforcement of constructive components that include masonry walls, arches, and even slabs, in positions where the adherence of externally bonded organic composites such as FRP can present difficulties. The adherence of organic binders is not appropriate for low-performance concrete substrates and can cause FRP laminate debonding and the detachment of the concrete substrate.

The central theme of this study is the strengthening of low-performance RC beams with SRG to resist flexural forces. This innovative material forms a cement-based matrix, rather than an organic binder, which is a partial solution for the above-mentioned lack of full compatibility between ancient concrete and externally bonded strengthening solutions. In addition, SRG presents additional advantages such as: fire resistance, durability, and some reversibility. Tests are performed on eighteen reinforced concrete (RC) beams (17 MPa): two reference specimens and sixteen specimens to study particular aspects of the SRG strengthening solution: the strengthening ratio and the performance of two anchorage systems. The results achieved in this research work lead us to conclude that SRG is an effective solution for the retrofitting of low-grade reinforced concrete to increase its load-flexural and deformation capacity.

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<b>ACKNOWLEDGEMENTS</b></br>

The authors wish to express their gratitude for funding through the Basque Government (ADVICE project - Emaitek 2015 - contract 049639), CDTI (Spanish Ministry MICINN - TERREME project - contracts IDI-20101592 and IDI-20101594), the Spanish Government (DESCLIMA project - contract RTI2018-097079-B-C31 - MCIU/AEI/FEDER, UE), as well as UPV/EHU (contract PPGA19/61), and finally, our thanks to research groups: IT1314-19 (Basque Government) and GIU19/029 (UPV/EHU).