INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 9, ISSUE 04, APRIL 2020 ISSN 2277-8616 576 IJSTR©2020 www.ijstr.org Mechanical Performance Of Sustainable Concrete Including Recycled Fine And Coarse Aggregate Improved By Silica Fume Hemin G. Ahmed, Khaleel H. Younis, Shelan M. Maruf Abstract: In this investigational research, the performance of recycled aggregate concrete (RAC) has been tackled. The research studies the impact of the use of recycled fine and coarse aggregates on fresh and hardened properties of recycled aggregate concrete (RAC). Workability, compressive strength and flexural strength are examined in this paper. The effect of the use of silica fume (SF) as a method to enhance the behavior of RAC was studied. SF was used to replace the cement at contents (by mass) of 5%, 10% and 15%. Six mixtures were investigated. The mixtures made with recycled fine (RF) aggregate (at content of 50%) and natural coarse aggregate were four; these mixes made with 0%, 5%, 10%, 15% of silica fume. One mix was made with 50% RF aggregates and 100% recycled coarse aggregate. Also, a reference mix which includes natural fine and coarse aggregates was prepared for comparison purpose. It was concluded that using RF aggregates diminish the workability and strength of RAC. The use of SF decreases the workability of RAC though it enhances the compressive and flexural strength of RAC. If SF is used at contents within 10% to 15% of cement mass in RAC, its strength can be significantly enhanced. Index Terms: Silica fume, Recycled fine aggregates, recycled concrete, compressive strength, flexural strength. ——————————  —————————— 1. INTRODUCTION There is no doubt that concrete is the most common used constructional material in the construction industry [1]. This can be attributed to the fact that concrete is versatile, cost- effective and can be formed in different shapes. Nonetheless, the large quantities of CO2 that emit during manufacturing of cement and extracting the raw materials, cause serious environmental concerns and sustainability concerns [1][2]. Beside the emission of CO2, the industry of construction is also accused of the accrual of vast amounts of construction and demolition waste (CDW) which also causes real environmental issues. Issues and concerns such as the depletion of landfills and diminution of the natural resources of aggregates extraction and cement manufacturing raw materials, are among the main environmental issues. The harmful effect of such issues can be mitigated through different methodologies[3][4]. One method is the use of cementitous supplementary materials such as Silica fume (SF) in the manufacture of concrete. SF is a by-product material that can be used to replace OPC [5]. SF can significantly contribute in minimizing the use of cement which in turn may greatly help in mitigating the aforementioned environmental issues[6]. Reusing/recycling of CDW (as aggregates in concrete) is another approach that can be used to reduce the environmental-related issue of concrete [7]. Recycled aggregate concrete (RAC) usually contains recycled fine (RF) and coarse (RC) aggregates. It is obvious to say that utilizing RF and RC aggregates in the production of RAC is useful in mitigating the impacts of the environmental issues though its performance cannot reach that of the natural aggregate concrete (NAC)[4]. Previous studies[4][6][8] have stated that the performance of RAC is inferior to that of NAC. The use of RF and RC aggregates affects the behavior of RCA in both states of concrete (fresh and hardened). Both RF and RC aggregates diminish the workability of concrete reduce the mechanical characteristics of concrete [9][10]. Reduction up to 45%, 30% and 25% in compressive strength, splitting tensile strength and flexural strength, respectively, compared to NAC have been reported in the previous studies [2][4]. The lower quality of RAC has been attributed to the properties of the RF and RC aggregates as stated in the previous studies. RF and RC aggregates are composed of natural aggregate and attached paste. The attached paste is known for its porous nature and contains micro-cracks caused by the crushing process during the production of recycled aggregates [6][11]. Such properties lead to a poor structure of aggregates with in the concrete. It results in high porosity and water absorption. Indeed, such properties cause low workability of RAC when compared to NAC[8][12]. It is observed, in the previous studies conducted to examine the effect of using SF in NAC, SF can improve the performance of NCA through its pozzolanic characteristic. Due to such reaction, SF can enhance both mechanical properties and improve the durability behavior of NAC through filling ability and enhancing the micro-structure of NAC [12]. Thus, this research aims to investigate the influence of employing SF as cement replacement on the behavior of RAC. SF was used at various ratios to replace cement in this research. The RAC mixtures include both RF and RC aggregates. The sand is partially (50%) replaced with RF aggregate and in one mix the natural gravel was fully replaced with RC aggregates obtained from old concrete structures. Workability and mechanical properties such as compressive strength and flexural strength are examined in this research. 2 EXPERIMENTAL WORK 2.1 Materials The first binder employed in this research is cement (OPC) type CEM I as per BS EN 197. In Table 1, the chemical constituents and some physical characteristics of the OPC, as provided by the supplier, are illustrated. As mineral ————————————————  Hemin G. Ahmed is Assist. Lecturer at:  -Erbil Polytechnic University, ETI, Dept. of Surveying -. Erbil-Iraq.  Email: hemin.ahmed@epu.edu.iq  Khaleel H. Younis is Assist. Prof. at:  -Erbil Polytechnic University, ETI, Dept of Road Construction -. Erbil-Iraq.  -Tishk International University, Civil Engineering Dept. Erbil –Iraq  E-mail: khaleel.younis@epu.edu.iq  Shelan M. Maruf is lecturer at: - Erbil Polytechnic University, Dept of Road Construction -. Erbil-Iraq.  E-mail: shelan.maruf@epu.edu.iq