Contents lists available at ScienceDirect Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp Strain rate-dependent compressive behavior and failure mechanism of cementitious syntactic foams Halim Kerim Bas a , Weihua Jin a,* , Nikhil Gupta b , Dung D. Luong b a Department of Civil and Urban Engineering, Tandon School of Engineering, New York University, 6 MetroTech Center, Brooklyn, NY, 11201, USA b Department of Mechanical and Aerospace Engineering, Tandon School of Engineering, New York University, Brooklyn, 6 MetroTech Center, NY, 11201, USA ARTICLE INFO Keywords: Cement Hollow glass microsphere High strain rate Syntactic foam Micro-CT analysis ABSTRACT The present work was focused on studying the strain rate sensitivity of cementitious syntactic foams (CSF), which are particulate composites reinforced with hollow glass microspheres (HMG). Different density CSFs (1.31–1.74 g/cm 3 ) with different volume fractions (20–40%) of HGMs were tested with a split-Hopkinson pressure bar setup. The true particle densities of the HGMs were in the range of 0.38–0.60 g/cm 3 . In addition, the macro- and micro-scale failure mechanisms were investigated with high-speed camera imaging, micro-CT scanning, and electron microscopy. The results showed that both the CSFs and the baseline material (control sample), which is the cement paste matrix of the CSFs, showed strain rate sensitivity in mechanical properties in the 10 2 –10 3 s -1 strain rate range. CSFs had relatively lower strain rate sensitivity in comparison to the matrix material. In the same range of strain rate, both the CSFs and the control sample showed significant changes in their macro and micro failure mechanisms depending on their age, composition and loading rate. The level of damage at the peak load for the high strain rate was higher in comparison to the same materials tested under quasi-static loading conditions for CSFs and the cement matrix. 1. Introduction Structures encounter different types of loads leading to different strain rates of strains during their service life, as illustrated in Fig. 1.A structural material must be tested under a wide range of strain rates that match with these loading conditions. Certain structures such as nuclear power plants and tall towers are now designed with con- sideration of dynamic loads such as impact or blast [1–3]. Most structural materials show a strain rate sensitive behavior when tested over a wide range of strain rates. Cementitious composite ma- terials, such as concrete, are also known to have strain rate sensitive properties (e.g. tensile and compressive strength, stiffness, and ducti- lity) [4–8]. Specifically, the compressive strength of cementitious composites is established to be strain rate sensitive. However, the source of this sensitivity is not well established and factors such as in- ertial effects, the viscoelastic characteristic of the hardened cement paste, and the time-dependent micro-crack growth are thought to contribute to the strain rate sensitivity. Although some studies argue that the inertial confinement effect is minimal and cannot be the un- derlying phenomenon behind the strain rate sensitivity of the ce- mentitious materials [9], available numerical simulation studies performed only considering the inertial effects have affirmed the role of this parameter on strain rate sensitivity [10]. A numerical study by Li et al. also reported the strong effect of inertial confinement on the strain rate sensitivity of cementitious materials at greater than 10 2 s -1 strain rates and call this effect a pseudo-strain rate sensitivity [11,12]. There are additional factors that are related to the methods of testing but still lead to an apparent strain rate sensitivity such as specimen geometry, uniformity of the strain along the specimen length, effects related to the stress-wave propagation, and the limitations of the one-dimensional wave theory [2]. In addition, cementitious materials tend to show dif- ferent amounts of strain rate sensitivity based on the material quality, age, saturation, the type of aggregate used, and the curing conditions [12,13]. Addition of reinforcements also affects the strain rate sensi- tivity of the cementitious materials [3]. This work is a continuation of our earlier work [14] and focuses on the high strain rate (HSR) compressive behavior of a lightweight par- ticulate composite called cementitious syntactic foam (CSF). The lightweight of the CSFs is due to their hollow glass microsphere (HGM) fillers. Polymer matrix syntactic foams have already found numerous structural applications [15,16] and cementitious syntactic foams, based on the same concept, are also promising in lightweight structures. CSFs https://doi.org/10.1016/j.cemconcomp.2018.10.009 Received 18 March 2018; Received in revised form 20 September 2018; Accepted 11 October 2018 * Corresponding author. Department of Civil and Urban Engineering, Tandon School of Engineering, New York University, 15 MetroTech Center, Brooklyn, NY, 11201, USA. E-mail address: wjin@nyu.edu (W. Jin). Cement and Concrete Composites 95 (2019) 70–80 Available online 13 October 2018 0958-9465/ © 2018 Elsevier Ltd. All rights reserved. T