Copyright © 2021 Te Author(s). Published by Vilnius Gediminas Technical University Tis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unre- stricted use, distribution, and reproduction in any medium, provided the original author and source are credited. COMPARATIVE ANALYSIS OF FLEXURAL STIFFNESS OF CONCRETE ELEMENTS WITH DIFFERENT TYPES OF COMPOSITE REINFORCEMENT SYSTEMS Haji Akbar SULTANI 1* , Viktor GRIBNIAK 1 , Arvydas RIMKUS 1 , Aleksandr SOKOLOV 1 , Lluis TORRES 2 1 Vilnius Gediminas Technical University, Vilnius, Lithuania 2 University of Girona, Girona, Spain Received 05 June 2020; accepted 09 October 2020 Abstract. Various materials and reinforcement technologies have been created for concrete structures. However, there is no uniform methodology to compare the mechanical characteristics of diferent reinforcement systems. In structural systems, residual stifness can estimate the efciency of the reinforcement. Tis study introduces a simplifed approach for the fex- ural stifness analysis. It employs a new testing layout designed with the purpose to form multiple cracks in a small labora- tory specimen. Te achieved solution requires neither iterative calculations nor a description of the loading history. Several composite reinforcement schemes, including internal glass fbre reinforced polymer (GFRP) bars, carbon fbre reinforced polymer (CFRP) sheets and near-surface mounted (NSM) strips are considered. Te analysis of the test results reveals a substantial efciency of the external CFRP reinforcement systems. Keywords: concrete composite, reinforcement, residual stifness, analytical model, fexural tests. Introduction Various materials and reinforcement technologies have been created for concrete structures, but there is no uni- form methodology to compare the mechanical character- istics of diferent reinforcement systems. Residual stifness of fexural elements is the focus of the research. Numer- ous studies investigated this issue. However, only several works addressed fexural efects. Fundamental studies by Kaklauskas and Ghaboussi (2001) and Torres et al. (2004) could be mentioned in this context. Elaborate numerical procedures are an intrinsic attribute of the “exact” ap- proaches (Gribniak et al., 2017). Iterative nature of the analysis procedures ofen complicates applicability of the exact techniques: the calculation errors are accumulated following the load history (Gribniak et al., 2017). Te de- velopment of more reliable algorithms employed the rein- forcement-related tension-stifening concept was the con- sequence of the further improvements (Kaklauskas et al., 2011; Torres et al., 2015; Kaklauskas & Gribniak, 2016). Such models, however, are not useful for the analysis of the elements reinforced with a combination of diferent types of composite reinforcement. Residual stifness can determine the efciency of the reinforcement system (Gribniak et al., 2019). Tat is the object of this research. A new testing procedure was devel- oped to estimate residual fexural stifness of the concrete elements with composite reinforcement systems. Several composite reinforcement schemes including internal glass fbre reinforced polymer (GFRP) bars, external bond re- inforcement (EBR) system using carbon fbre reinforced polymer (CFRP) sheets, and near-surface mounted (NSM) strips are considered. Te proposed geometry of the test specimens is suitable for application of the tension-stif- ening modelling concept (related to average deformations of the concrete). Te corresponding analytical model can represent stress-strain behaviour of tensile concrete (in- dependently on the reinforcement system applied). Te equivalent tensile stress of the concrete is the parameter proposed to quantify the residual stifness of the fexural element. Representing a closed-form solution of the fex- ural stifness problem, the proposed analytical model re- quires neither iterative calculations nor a description of the loading history. Te application of this technique is illustrated experimentally. Civil engineering Statybos inžinerija Mokslas – Lietuvos ateitis / Science – Future of Lithuania ISSN 2029-2341 / eISSN 2029-2252 2021 Volume 13, Article ID: mla.2021.13713, 1–5 https://doi.org/10.3846/mla.2021.13713 *Corresponding author. E-mail: haji-akbar.sulatani@vgtu.lt