materials Article Renovation Effect of Flax FRP-Reinforced Cracked Concrete Slabs under Impact Loadings Wenjie Wang 1, *, Zonglai Mo 2 , Nawawi Chouw 3 and Krishnan Jayaraman 4   Citation: Wang, W.; Mo,Z.; Chouw, N.; Jayaraman, K. Renovation Effect of Flax FRP-Reinforced Cracked Concrete Slabs under Impact Loadings. Materials 2021, 14, 6212. https://doi.org/10.3390/ ma14206212 Academic Editors: Nadezda Stevulova and Adriana Estokova Received: 22 September 2021 Accepted: 12 October 2021 Published: 19 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Jiangsu Key Laboratory of Engineering Mechanics, School of Civil Engineering, Southeast University, Nanjing 211189, China 2 School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; mozonglai2000@163.com 3 Department of Civil and Environmental Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand; n.chouw@auckland.ac.nz 4 Department of Mechanical Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand; k.jayaraman@auckland.ac.nz * Correspondence: wenjie.wang@seu.edu.cn Abstract: The impact behaviour of flax fibre-reinforced polymer (FFRP) renovated coconut fibre- reinforced concrete (CFRC) slabs was investigated through two series of experiments and theoretical analysis. The first experiment was carried out to find out the effectiveness of FFRP retrofitted method for the partly damaged concrete structure and its performance under impact loadings. The renovation process was applied on the pre-cracked rectangular CFRC slabs of 600 mm × 300 mm × 50 mm with FFRP laminates, before the repeated impact tests. Then, the parameters of these slabs, i.e., impact force history, deflection history and damage pattern, were discussed in detail. Another experiment was conducted on the FFRP-CFRC square slabs with a dimension of 600 mm × 600 mm × 50 mm. Based on test results, the effect of different FFRP configurations was discussed to find out the effective reinforcement method. In addition, the two-degree-of-freedom spring-mass model was applied to predict the impact force. Results demonstrate that FFRP composites have a good potential to be utilised as renovated construction materials under dynamic load conditions. Keywords: drop weight impact; fibre-reinforced concrete slab; flax fibre-reinforced polymer; renova- tion method 1. Introduction Fibre-reinforced polymers (FRP) were first presented in the 1940s, followed by various FRP reinforcing products invented in Europe and Asia in the 1970s and 1980s [1]. In the past two decades, fibre-reinforced polymer (FRP) composite materials have been widely industrialized as they are economical and structurally workable, serving as load-bearing elements in buildings and bridges. Retrofitting is an art of structural modification, which becomes a more cost-effective superior alternative to the traditional techniques. In the field of concrete constructions, the FRP retrofitting method is able to provide improvement of the static and dynamic performance of infrastructures [2–5]. Mousavi and Shafei [2] investigated the impact resistance of hybrid FRP-steel-reinforced concrete slabs, results of which showed that FRP material reinforcement can minimize the damage level and dissipate the imposed energy. Abdel-Kader and Fouda [3] experimentally compared the performance between plain concrete and glass fibre-reinforced polymer (GFRP) sheet-strengthened concrete plates in terms of impact resistance under various compressive strengths. The results indicated that the GFRP reinforcement had a better performance, with a nonlinear ratio of improvement under different compressive strengths. Blast response of one-way reinforced concrete slabs retrofitted with fibre-reinforced plastic was studied by Ahmad and Azar [6], where the influence of geometrical parameters, Materials 2021, 14, 6212. https://doi.org/10.3390/ma14206212 https://www.mdpi.com/journal/materials