Concurrent effects of recycled aggregates and polypropylene fibers on workability and key strength properties of self-consolidating concrete Pierre Matar a , Joseph J. Assaad b,⇑ a Lebanese University, Faculty of Engineering, Department of Civil Engineering, Roumieh, Lebanon b University of Balamand, Faculty of Engineering, Department of Civil & Environmental Engineering, Balamand Al Kurah, Lebanon highlights  Concurrent effects of recycled aggregates and polypropylene fibers on internal friction.  Quantifying the magnitude of internal friction on workability of SCC.  Changes in SCC rheological properties and passing ability due to RAC and PPF.  Assessing key strength properties due to RAC and PPF. article info Article history: Received 30 January 2018 Received in revised form 4 July 2018 Accepted 15 December 2018 Keywords: Self-consolidating concrete Workability Polypropylene fibers Recycled aggregates Rheology abstract The concurrent effects of recycled concrete aggregate (RCA) and polypropylene fibers (PPF) on workability of self-consolidating concrete (SCC) are not quantified in literature. Different SCC series containing 25% to 100% RCA and 0.25% to 1.75% PPF are investigated in this study; the water-to-binder ratio varied from 0.38 to 0.5, while high-range water reducer adjusted to maintain slump flow at 700 ± 25 mm. Flowability, cohesiveness, passing ability, and segregation resistance were evaluated and compared with the limitations specified by the European Guidelines for SCC. Test results have shown that SCC rheological properties and passing ability dramatically curtailed with PPF inclusions, especially when mixtures are proportioned with higher RCA replacement rates. This was attributed to combined effects of fiber additions that hinder the cement matrix deformation together with increased RCA surface rough- ness that accentuates internal friction and resistance to flow. Mixtures containing increased RCA and PPF concentrations exhibited better stability, reflected by reduced aggregate segregation. Series of regres- sion models were proposed to predict the coupled effects of RCA and PPF additions on SCC workability responses. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction The effect of recycled concrete aggregate (RCA) on workability of highly flowable self-consolidating concrete (SCC) requires par- ticular attention during casting sustainable structural-grade con- crete members. In fact, RCA obtained by crushing and grading concrete rubble collected from demolition sites is composed of natural coarse aggregate (NCA) and adhered mortar that give the aggregate skeleton specific characteristics such as reduced density, higher water absorption, greater angularity, and rougher surface texture [1–3]. This consequently alters internal resistance to flow with direct influence on SCC deformability and ease of placement in complex forms with congested reinforcement. Tang et al. [4] found that flow rate and passing ability of SCC made with 0.35 to 0.5 water-to-binder ratio (w/b) curtailed when RCA replacement rates exceeded 75% of total coarse aggregate volume. Faleschini et al. [5] found that yield stress (s 0 ) and plastic viscosity (g) of fresh concrete increased with RCA additions; on average, these were 1.5 to 2 folds higher than equivalent rheological properties obtained using NCA concrete. Similar conclusions were made by Assaad [6] when testing rheology and workability of SCC prepared with 0.38 and 0.5 w/b; the flow rate and passing ability worsened due to increased internal friction when RCA replacement rates exceeded 50% of total coarse aggregate volume. Although workability can be improved with high-range water reducer (HRWR), the rate of slump loss over time has often found to increase with RCA additions, given the higher absorption that reduces free mixing water after concrete batching [1,7]. Poon https://doi.org/10.1016/j.conbuildmat.2018.12.091 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: pmatar@ul.edu.lb (P. Matar), Joseph.Assaad@balamand.edu.lb (J.J. Assaad). Construction and Building Materials 199 (2019) 492–500 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat