Concrete made with hybrid blends of crumb rubber and metakaolin: Optimization using Response Surface Method Omid Rezaifar ⇑ , Mohsen Hasanzadeh, Majid Gholhaki Department of Civil Engineering, Semnan University, Semnan, Iran highlights  Hybrid blends of crumb rubber and metakaolin in concrete production is used.  Response Surface Method is utilized to design the experiments.  The mathematical equations are obtained by regression analysis and then verified.  An optimization is made to maximize the strength minimizing water absorption.  The correlation between the responses is illustrated. article info Article history: Received 9 March 2016 Received in revised form 11 June 2016 Accepted 14 June 2016 Keywords: Concrete Crumb rubber Metakaolin Optimization Response Surface Method Environment abstract The purpose of this study was to optimize the concrete made with crumb rubber (CR) as waste material in conjunction with metakaolin (MK). The study seeks to use CR as partial replacement for fine aggregates and use MK as partial replacement for cement. To this end, prior to conducting the tests, the experiments were designed by Response Surface Method (RSM). Upon execution of the experimental program, regres- sion analysis was used to obtain the equations. Then, optimization was achieved when as the compres- sive strength maximized, the water absorption of the mixture was minimized. Compressive strength of the concrete decreased with increased rubber content. It can be seen that the negative effect of CR on compressive strength of concrete can be relatively mitigated by incorporating a certain amount of MK. The water absorption decreased with increased CR content up to about 9 vol.% but increased when CR content exceeded this ratio. Based on the results obtained, an optimum mixture can be achieved with a 3.3 vol.% replacement of sand with CR and 19.5 vol.% replacement of cement with MK to have a struc- tural clean concrete. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Disposal of waste materials is a world-wide critical problem. One such waste material is tire rubber known for decomposability. In most countries, a large number of waste tires are stockpiled annually. For example, the quantity of tires discarded annually is over 10 million tons in Iran and Turkey [63], 200–300 million tons in the United States [2] and 3.4 million tons in the European Com- munity [53]. The huge stockpiles of waste tires are the result of the ever-increasing automobile industry and extended use of tire- wheel vehicles in the transportation system. Generally, the first way to dispose of waste tires is to landfill them which over time become homes to animals and insects spreading diseases. Another alternative to dispose of waste tires is burning which contributes to global warming and serious environmental pollution due to emission of toxic gases [17]. Therefore, some countries have for- bidden it and created an effective legislation for converting them to other products. Ground waste tires can be utilized in asphalt [52,60] and many cementitious products such as culverts [62], bricks [61], paving blocks [32,54], slabs [28], pre-cast concrete panels [56], acoustic panels [27], side-walks, running tracks [10], roller-compacted concrete [34], self-compacting concrete [39,46,64], high strength concrete [37], and masonry walls [49]. In this case, a good solution is to use them in concrete and cemen- titious products to both dispose of waste materials and improve particular concrete properties. Researchers have indicated that using CR in concrete and cementitious products improves freeze-thaw cycles [48], fatigue performance [33], brittleness index, and kinetics of fracture http://dx.doi.org/10.1016/j.conbuildmat.2016.06.047 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Civil Engineering, Semnan University, Campus #1, Semnan, P.O. Box 35131-19111, Iran. E-mail addresses: orezayfar@semnan.ac.ir (O. Rezaifar), m_hasanzadeh@sem- nan.ac.ir (M. Hasanzadeh), mgholhaki@semnan.ac.ir (M. Gholhaki). Construction and Building Materials 123 (2016) 59–68 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat