Corresponding author: Nicu Toader E-mail: toader.nicu@gmail.com; nicu.toader@ilek.uni-stuttgart.de Journal of Bionic Engineering 14 (2017) 369–378 Energy Absorption in Functionally Graded Concrete Bioinspired by Sea Urchin Spines Nicu Toader 1 , Werner Sobek 1 , Klaus G Nickel 2 1. Institute for Lightweight Structures and Conceptual Design (ILEK), Stuttgart Universität, 70569 Stuttgart, Germany 2. Institute for Geoscience (IFG), Applied Mineralogy, Eberhard-Karls-Universität Tübingen, 72074 Tübingen, Germany Abstract Functionally Graded Concrete (FGC) is fabricated at the Institute for Lightweight Structures and Conceptual Design (ILEK) by using a layer-by-layer technique with two different technological procedures: casting and dry spraying. Functional gradations are developed from two reference mixtures with diametrically opposed characteristics in terms of density, porosity, compression strength and elasticity modulus. In this study the first mixture consists of Normal Density Concrete (NDC), with density about 2160 kg·m −3 while the second mixture helps to obtain a very lightweight concrete, with density about 830 kg·m −3 . The FGC specimens have layers with different alternating porosities and provide superior deformability capacity under bulk compression compared to NDC specimens. In addition, the FGC specimens experienced a graceful failure behaviour, absorbing high amounts of energy during extended compression paths. The porosity variation inside the layout of tested specimens is inspired by the internal structure of sea urchin spines of heterocentrotus mammillatus, a promising role model for energy ab- sorption in biomimetic engineering. Keywords: biomimetic engineering, energy absorption, sea urchin spine, functionally graded concrete, graceful failure be- haviour Copyright © 2017, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(16)60405-5 1 Introduction In our study spines of Heterocentrotus Mammilla- tus (HM, Fig. 1), were the concept generator for de- signing functionally graded concrete specimens with high-energy absorption. Under bulk compression, sea urchin spines combine desirable properties such as cas- cading graceful failure behaviour and lightweight with up to 70% pore volume. This way, sea urchin spines can withstand high impact forces during the attack of a predator while the spine remains permeable to ensure its growth and regeneration [1] . Just as concrete, the con- struction materials forming the spine are brittle. The internal structure of HM spines is a highly porous Mg-calcite network (stereom) separated by more com- pact layers. All spine samples measured showed almost the same chemical composition of Calcite (CaCO 3 ) with up to 12 wt% MgCO 3 [2] . The spines are characterised by concentric stereom layers separated by layers of higher density interpreted as former shell surface or growth layers [3] . The stereom inner structure is described as labyrinthic [4] . The porosity of the spine interior varies from 10% (almost fully dense) to 70%. Fig. 2 shows the density variation along an aboral spine of HM. Each layer has a roughly cylindrical shape and the dense ones tend to meet at the base. It is astonishing that sea urchin spines experience a graceful failure behaviour under compression forces, because the construction material is a brittle single-crystal like magnesium calcite ([Ca x Mg 1-x ]CO 3 ) [5,6] . Their behaviour results from the hierarchic structure of the porous carbonate network (stereom) [7,8] . The idea of a Functionally Graded Concrete (FGC) was first put forward by Werner Sobek [9] . The motiva- tion for developing graded building materials, with a continuous variation of the porosity in the inner structure of a construction component, is driven by the idea of adapting material characteristics precisely to the stresses that occur locally under load. The targeted positioning of lighter concrete mixtures in lower stress areas in com-