Fiber length effect on strength properties of polypropylene ﬁber reinforced cemented tailings backﬁll specimens with different sizes Gaili Xue a,b , Erol Yilmaz c,d,⇑ , Weidong Song a,b , Shuai Cao a,b a State Key Laboratory of High-Efﬁcient Mining and Safety of Metal Mines of Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China b School of Civil and Resources Engineering, University of Science and Technology Beijing, Beijing 100083, China c First Quantum Minerals Ltd., Cayeli Bakir Isletmeleri A.S., PO Box 42, Madenli, Cayeli, Rize TR53200, Turkey d Department of Civil Engineering, Recep Tayyip Erdogan University, Rize TR53100, Turkey highlights  The structural integrity of ﬁber reinforced cemented tailings backﬁll (CTB) is limited.  Fiber length effect on strength behavior of CTB specimens with different sizes was investigated.  The peak and end strains of cubic CTB specimens are larger than those of cylindrical CTB ones.  The longer the ﬁber length does not necessarily lead to the higher UCS of CTB specimens. article info Article history: Received 19 April 2019 Received in revised form 19 December 2019 Accepted 5 January 2020 Keywords: Fiber reinforcement Cemented tailings backﬁll Size effect Fiber length Geometric shape abstract The strength properties of cemented tailings backﬁll (CTB: an engineered mix of processing tailings, hydraulic binder and mixing water) is strongly affected by its structural integrity. However, our under- standing of the structural integrity of ﬁber reinforced CTB samples as well as its evolution with time is limited. Thus, a comprehensive laboratory investigation is conducted to study the effect of ﬁber length on strength properties of CTB prepared with different specimen sizes. The unconﬁned compressive strength (UCS) tests are conducted on CTB samples prepared with four different lengths (0, 6, 12 and 18 mm) and sizes (cubic mold sizes: 40, 70.7 and 100 mm, accompanied with cylindrical mold sizes: 50  100 mm) of polypropylene ﬁber to study their strengths. Experimental results have shown that the peak and end strains of cubic CTB samples are larger than those of cylindrical CTB ones, and UCS of C-40 CTB is higher than that of the cylinder U50. At this time, the size effect and geometry are the main reasons for the differences observed in the strength properties of the above two backﬁlling bodies. The UCS of CTB samples decreases with increasing the volume ratio when the ﬁber length varies from 0 mm to 6 mm. When the ﬁber length varies from 12 mm to 18 mm, the size effect of ﬁber-reinforced CTB samples is not obvious, and the ﬁber length becomes the key to inﬂuence the coupling curve of two factors. In addition, the longer the ﬁber length does not necessarily lead to the higher UCS of CTB samples, but the integrity of the sample under the same external load is better. The ﬁndings of this study will help to better evaluate and forecast the UCS behavior of CTB samples under various ﬁber conditions. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction The underground mining methods with backﬁll have been widely used worldwide because of their advantages in controlling ground pressure, realizing waste-free mining, and capably utilizing mineral resources [1,2] . However, cemented tailings backﬁll (CTB) is mechanically similar to concrete, having problems of high brit- tleness, bending, poor tensile strength, and catastrophic failure [3,4] . CTB, a mix of tailings, binder and water, has become a popu- lar technique of optimizing underground mine support and waste management systems to augment the mine revenue [5–9]. Indeed, CTB is an engineered product that can be pumped or directed underground by gravity but does not release liquid after placement [10–12]. To effectively control the crack propagation and better improve the strength, toughness, ductility and crack resistance, different types of ﬁbers have been effectively used in cement- based composite materials, such as CTB, rock-tailings, mortars https://doi.org/10.1016/j.conbuildmat.2020.118113 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: yilmazer@fqml.com (E. Yilmaz), sandy_cao@ustb.edu.cn (S. Cao). Construction and Building Materials 241 (2020) 118113 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat