doi:10.1016/j.cscm.2020.e00334

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Case studyImplementation of the use of hydration stabilizer admixturesat a ready-mix concrete plantLuiz de Brito Prado Vieira*, Antonio Domingues FigueiredoUniversidade de São Paulo Escola Politécnica, São Paulo, SP, BrazilA R T I C L E I N F OArticle history:Received 22 September 2019Received in revised form 19 December 2019Accepted 30 December 2019Keywords:Ready-mixed concreteWaste managementWaste reuseReturned concreteHydration stabilizer admixtureA B S T R A C TWaste generated during concrete production is approximately 3 % of the total volume ofconcrete produced in a ready-mixed concrete plant. The economic and environmental costsof such waste are high, which means that more solutions are required to eliminate thisproblem. This paper presents a case study that focuses on the implementation of a processfor reusing the adhered and leftover concrete using a hydration stabilizer admixture (HSA)in a ready-mix concrete (RMC) plant and it aims to present the operational procedures andthe results obtained during this implementation. The methodology adopted was based onpreliminary laboratory studies followed by real-scale tests in order to evaluate thetechnical and financial feasibility of implementing the process under real RMC conditionsfocusing on the impact of using HSA on setting time and compressive strength of concrete.In sequence, the results of 1500 tests carried out for reuse leftover concrete and adheredconcrete with an HSA admixture were presented and analyzed showing that thecharacteristic compressive strength of the reference concrete was similar to that ofstabilized concrete.. This study contributes to the advance of knowledge in the area of RMCconcrete waste reuse, since it proves that the use of HSA is feasible as long as parameterizedin previous laboratory tests considering the influence of temperature and the setting timefor concrete stabilization. The study also concludes that the use of an HSA does notnegatively affect the compressive strength of concrete. Finally, in the case studied, themethod of reusing concrete with an HSA is economically advantageous for RMC since therevenue generated by the reuse of reused raw materials is higher than the cost of the HSAused in the stabilization of the concrete leftovers and the adhered concrete.© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).1. IntroductionAccording to Vieira et al. [1], the amount of concrete residue generated in a ready-mixed concrete (RMC) plant in Brazilcorresponds to approximately 3 % of the total volume of concrete produced. Although this percentage is quite small,considering that approximately 25 billion tons of concrete are produced each year worldwide, the concrete waste generatedcould be approximately 750 million tons. A considerable part of this is produced in RMC plants. Therefore, it appears quiteclear that the problem cannot be overlooked.Concrete production in RMC always generates some amount of concrete waste [2]. There are two different types ofconcrete waste that can be generated in the production of RMC from returned concrete: adhered concrete and leftoverconcrete. Adhered concrete is the material impregnated inside the drum of the concrete mixer truck after its total unloading.* Corresponding author.E-mail address: luiz.vieira@vcimentos.com (L. de Brito Prado Vieira).https://doi.org/10.1016/j.cscm.2020.e003342214-5095/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Case Studies in Construction Materials 12 (2020) e00334Contents lists available at ScienceDirectCase Studies in Construction Materialsjournal homepa ge: www.elsevier.com/locate/cscm

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Leftovers concrete is the remaining concrete that is not discharged at the construction site. While leftovers concrete isgenerated occasionally, adhered concrete is a constant residue because it is always present every time the ready-mixedconcrete truck returns to the plant [1]. One of the strategies applied by RMC companies to prevent returned concrete fromturning into concrete waste and thereby reducing the volume of waste disposal in landfills is to use admixtures such ascement hydration stabilizers [3]. The use of hydration stabilizer admixtures (HSAs) generates favorable conditions ofreutilization for all types of returned concrete, ie, leftover concrete and adhered concrete. This strategy provides conditionsfor the total elimination of concrete waste using the returned concrete as a whole and avoids the generation of slurry andhigh consumption of water, which is the case of the concrete recycling equipment used to reuse the aggregates separately[4]. It is interesting to note that some research uses the term reuse to name the technique of using the HSA to avoid thegeneration of concrete waste [5,6] but others use the term recycle [7–9].According to Gebremichael et al. [10], HSAs allow the leftovers concrete to remain fresh for up to 72 h longer, permittingits reuse. This research also indicates that the use of stabilized concrete with an HSA as a raw material for new concretes canincrease the compressive strength of the final product in some cases.Cheung et al. [11] indicated that the use of HSAs was a good strategy to improve the sustainability in RMC productionbecause these admixtures avoid losses of entire truckloads owing to problems that occur during transport. A study byHaddad et al. [12] confirmed the tendency of overall compressive strength reduction of concrete in a laboratory study.Despite its effect on the final compressive strength, the use of HSAs causes a reduction in the compressive strength at an earlyage [13].According to [9] the HSA acts like a high-range retarder; it differs from conventional retarders because it prevents thehydration of tricalcium aluminates (C3A). In addition, the physical properties of stabilized concrete are as good as those ofuntreated control concrete. An HSA affects the induction period of the hydration reactions of the cement in such a mannerthat once its effect has waned, the hydration reactions return to a normal rate [14,15]. Thus, the higher the amount of HSAadded, the longer is the initial setting time, with a slight change between the initial and final concrete setting times.However, above a certain amount of HSA, the increase in the dose of HSA has no effect [7,6].According to Paolini & Khurana [8], HSA is an active surface agent which inhibits the nucleation of CSH and CH whenadded at the beginning of concrete mixing providing a reduction on the rate of calcium and sulfate dissolution and theprecipitation of the hydrated compounds as a consequence. When the HSA is added later (when the nucleation of hydratedcement products already had begun) this kind of admixtures acts retarding the growth of hydrates crystals.The stabilization period varies according to the type of cement and the type of mineral admixtures used. Naturally, theuncertainties produced by the variation in the results causes difficulties in the implementation of the technology underindustrial operation conditions. Therefore, it is important to ensure an adequate mapping of the behavior of the interactionbetween returned concrete and the HSA to enable its implementation at RMC plants. The higher the amount of clinker in thecement, the higher is the admixture content required to stabilize the concrete for a given period [9].The time of stabilization of the stabilized concrete is reduced when the new concrete is added in order to complete thevolume of the truck load [8]. Previous studies by Benini [16] demonstrated that it is possible to activate the stabilizedconcrete with 1 % HSA in the weight of the cement with the addition of incremental cement in a proportion of 1:1. Thispositive horizon of admixture application has been emphasized in previous laboratory studies [5,7]. However, doubts aroseregarding the risk of real-scale production behavior variations for the effective implementation of this technology.Much of the current literature on the use of HSA is focused on small-scale laboratory studies, and experimental researchusing field study results is rare [10,12,9,14]. Moreover, even these last ones are about the use in specific cases or in a singleplant and not in the simultaneous implantation of the procedure of reuse of residues with HSA in multiple unitsconcomitantly, as is the case of the study here related.To facilitate the introduction of the use of HSA in the RMC production process, it is necessary to define which concrete canbe reused. This selection is particularly important to better understand the impact of different amounts of HSA on the settingtime of the returned concrete, which is a critical aspect for RMC operation in urban centers with traffic difficulties, such asthe city of São Paulo. In addition, it is necessary to understand if it is possible to apply the results obtained in laboratorystudies in the day-to-day operation of an RMC plant, thereby ensuring the reduction of long-term concrete waste generation.The objective of this paper is to present the operational procedures and results obtained during implementation of theprocess of reusing concrete returned with an HSA in one of the largest RMC companies in Brazil. In this sense, the studyemphasizes the method used for the industrial implementation of concrete waste reuse technology through the use of HSA,based on available information about the behavioral analysis of the effect of this admixture on the setting time and thecompressive strength of the concrete.It is important to highlight the innovative character of this paper because besides presenting a successful case study in fullscale, this paper also describes in detail the procedures used to conduct the implementation of a concrete reuse processreturned with HSA that encompasses the reuse of leftovers and adhered concretes, since the phase of the initial studies. Thisallows process replication in other RMCs regardless of geographic region, commercial product mix or local practice2. MethodologyDifferent tests were carried out to verify the feasibility of reuse the returned concrete with an HSA. The HSA used in thetests was classified by ASTM C 494 as type B and D (retarding admixtures and water-reducing and retarding admixtures). The2 L. de Brito Prado Vieira, A. Domingues Figueiredo / Case Studies in Construction Materials 12 (2020) e00334

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