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The impact resistance of functionally layered two-stage fibrous concrete (FLTSFC) prepared using the cement grout injection technique was examined in this study. The impact resistance of turtle shells served as the inspiration for the development of FLTSFC. Steel and polypropylene fibres are used in more significant quantities than usual in the outer layers of FLTSFC, resulting in significantly improved impact resistance. An experiment was carried out simultaneously to assess the efficacy of one-layered and two-layered concrete to assess the effectiveness of three-layered FLTSFC. When performing the drop-mass test ACI 544, a modified version of the impact test was suggested to reduce the scattered results. Instead of a solid cylindrical specimen with no notch, a line-notched specimen was used instead. This improvement allows for the pre-definition of a fracture route and the reduction of the scattering of results. The testing criteria used in the experiments were impact numbers associated to first crack and failure, mode of failure, and ductility index. The coefficient of variation of the ACI impact test was lowered due to the proposed change, indicating that the scattering of results was substantially reduced. This research contributes to the idea of developing enhanced, more impact-resistant fibre composites for use in possible protective structures in the future.

Protective structures subjected to intensive loads that may benefit from the use of multilayer composite structures with excellent hardness and impact resistance represent an emerging research field in recent times. In this study, low-velocity projectile impact tests were performed on Functionally-graded Preplaced Aggregate Fibrous Concrete (FPAFC) mixtures to evaluate their performance. The effects of projectile needle type, fibre type and hybridization in addition to the number of layers in the composites on projectile impact were investigated. The bioinspiration of the excellent impact strength of turtle shells was used to design an FPAFC comprising a higher amount of steel and polypropylene fibres at the outer layers. In parallel, one and two-layered concretes were also cast to assess the effectiveness of three-layered FPAFC. The tests were performed on disc specimens using non-deformable compound bevel, convex edge and hollow edge projectiles. The damage severity was quantified by the top damage area, bottom damage area and depth of penetration. In addition, a simple analytical model for predicting the composite mass expulsion was developed and implemented. Findings indicated that regardless of fiber type and distribution, the compound bevel projectile needle produced the lowest impact numbers for all single, double and triple-layer specimens compared to the convex edge and hollow edge projectiles. Repeated projectile impacts increased the penetration depth and damaged area at the top and bottom surfaces of all targets. Targets were more resistant to convex edge and hollow edge projectile penetration than the compound bevel. The experimental and analytical model results for mass expelled from the top surface are reasonably acceptable. This research gives an idea of developing advanced fibrous composite with superior impact resistance for the promising protective structures.

This research examined the performance of functionally graded two-stage fibrous concrete (FTSFC) against modified repeated falling-mass impacts. This study led to the concept of creating improved multiphysics model of fibre composites with better impact resistance for potential protective constructions. FTSFC was developed based on the bio-inspiring strength of turtle shells. The excellent impact resistance of FTSFC was accomplished by including a larger quantity of steel and polypropylene fibres in the outer layers. At the same time, one- and two-layered concrete were cast and compared to evaluate the efficiency of three-layered FTSFC. To minimize the dispersed test results, a modified form of the 544 drop-mass impact test was recommended by the American Concrete Institute (ACI). The modification was a knife-edge notched specimen instead of a solid cylindrical specimen without a notch. This modification predefined a crack path and reduced the dispersion of results. Cracking and failure impact numbers, ductility index, and failure mode were the testing criteria. The suggested modification to the ACI impact test decreased the coefficient of variance, showing that the dispersion of test results was reduced significantly. This study led to the concept of creating improved, fibre composites with better impact resistance for potential protective constructions.