Contents lists available at ScienceDirect Structures journal homepage: www.elsevier.com/locate/structures Microstructural investigation of recycled aggregate concrete produced by adopting equal mortar volume method along with two stage mixing approach Nishikant Kisku a , Puja Rajhans c , S.K. Panda a, ⁎ , Vikrama Pandey b , Sanket Nayak a a Department of Civil Engineering, Indian Institute of Technology (ISM), Dhanbad, India b Department of Civil Engineering, BIT Sindri, Dhanbad, India c Department of Civil and Environmental Engineering, BIT Meshra, Ranchi, India ARTICLEINFO Keywords: Recycled aggregate concrete Equivalent mortar volume Two stage mixing approach BSE image analysis WDS analysis ABSTRACT This article discusses about equal mortar volume (EMV) mix design method and two stage mixing approach for preparing recycled aggregate concrete (RAC) by considering the residual mortar content (RMC) on the surface of the recycled concrete aggregate (RCA). In this method of mix design, the suitable percentage of RCA component depends on the RMC value of respective RCA. For the present source of RCA, the suitable percentage of RCA component for the proposed method of mix design is found to be 61.01%. Eleven different concrete mixes are prepared with proposed mix design method in conjunction with ten different mixing approaches suggested in published research articles and one proposed two stage mixing approach (TSMA sic ). The mechanical properties of the concrete prepared from the above mixes are compared among all and with the control concrete mix which is produced by following conventional mix design method from natural coarse aggregate (NCA). The best me- chanical properties are found to be for the concrete prepared with proposed EMV mix design method and two stage mixing approach, and the properties are nearly similar to that of control mix. The microstructural analysis reveals the reason for the improvement of interfacial transition zone (ITZ) and the properties due to the adoption of the EMV mix design method and the mixing approach. Quantitative analysis and elemental mapping are also carried out to know the presence of strength contributing elements. 1. Introduction The increase in population, rapid urbanization and the requirement of infrastructural development has led to the substantial growth of the construction industry. In the necessity of making new structures, the old buildings are being demolished. Due to the demolishing of old build- ings, a large amount of debris is generated, which leads to various environmental pollutions. The environment is experiencing massive impact due to increased construction practices resulting emission of CO 2 in the atmosphere, depletion of natural resources, generation of a large number of waste materials and difficulties in the disposal of the same. The assessment of the impact of the current problem on the en- vironment is an important agenda for present day scenario and there- fore, several initiatives have taken to overcome this problem. One of the best solutions for the above problem is using C&D waste as an aggregate in preparation of new concrete. Recycling of C&D waste has enormous potential and using C&D waste as an aggregate reduces the use of virgin concrete aggregate (VCA). Dimitrious et al. [46] in Cyprus carried out cost analysis of RAC and presented that RAC are cheaper than NAC. This is because the cost of mineral admixture in Cyprus is much more expansive than Portland cement and use of this mineral admixture re- sults higher total cost. Many researchers [1–11] have observed that the concrete prepared with RCA may exhibit inferior mechanical and dur- ability properties than that of concrete prepared with natural coarse aggregate (NCA). One of the major reasons may be due to presence of two interfacial transition zones (ITZs): old ITZ (region between old mortar and virgin aggregate) and new ITZ (region between new mortar and recycled aggregate/virgin aggregate). The old ITZ has lots of pores and cracks which negatively affects the mechanical properties of RAC. Many published research articles discusses about the reduction of mechanical properties of recycled aggregate concrete (RAC) due to percentage component of RCA. Table 1 presents percentage reduction of mechanical properties of concrete prepared with different replace- ment percentage of RCA by following conventional mix design method https://doi.org/10.1016/j.istruc.2020.01.044 Received 6 November 2019; Received in revised form 11 January 2020; Accepted 28 January 2020 ⁎ Corresponding author. E-mail address: sarat@iitism.ac.in (S.K. Panda). Structures 24 (2020) 742–753 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved. T