Discussion of the article 'Prediction of creep of recycled aggregate concrete using back-propagation neural network and support vector machine'

In a recent study, Rong et al.1 investigate the prediction of recycled aggregate concrete (RAC) creep using back-propagation neural network and support vector machine. For this purpose, the authors compiled a database of experimental results on the creep of RAC on which they first tested five analytical RAC creep prediction models2-6 and concluded that the performance of all five models is inadequate, thereby justifying the use of a back-propagation neural network and a support vector machine. The main argument for declaring the performance of the five analytical models inadequate is the analysis of “performance indices” of the correlation coefficient (R), mean absolute error (MAE), mean square error (MSE), and integral absolute error (IAE). The found ranges of values were 0.45–0.55 for R, 0.41–0.64 for MAE, 0.33–0.70 for MSE, and 0.33–0.53 for IAE. Nonetheless, there are errors and uncertainties regarding the study that are pointed out herein, some methodological and some formal.Postprint (published version

Parametric study of long-term deflections of reinforced recycled aggregate concrete members according to the fib model code 2010

In this study, a parametric analysis of the deflection calculations of reinforced RAC members is presented. Within the parametric analysis, member support conditions, ambient conditions (influencing shrinkage and creep), reinforcement ratios, and quasi-permanent–to–design load ratios are varied. Through the analysis, the change in deflections is observed against RCA percentage (from NAC to RAC with 100% of coarse RCA) and span-to-effective depth ratio. The results of the analysis enable a clear overview of the variability of the deflections of RAC members relative to NAC.This work was supported by the Ministry for Education, Science and Technology, Republic of Serbia [grant number TR36017] and the SAES project [BIA2016-78742-C2-1-R] of the Spanish Ministerio de Economía, Industria y Competitividad. This support is gratefully acknowledged.Postprint (published version

Model Code 2010 creep and shrinkage models extension to recycled aggregate concrete

Recycled aggregate concrete (RAC) produced with recycled concrete aggregate (RCA) is one of the most promising ways of eliminating concrete waste and saving natural resources. However, shrinkage and creep behaviour of RAC, important for serviceability design of reinforced and prestressed concrete structures, are still insufficiently studied and guidelines for RAC serviceability design are still not incorporated into design codes and standards. This study aims to systematize the knowledge gained on RAC shrinkage and creep behaviour thus far and to offer analytic expressions for predicting RAC shrinkage strain and creep coefficient. For this purpose, databases of previously published results on RAC shrinkage and creep were compiled and the results on RAC were analysed relative to companion natural aggregate concrete (NAC). The results showed a systematically higher shrinkage and creep of RAC relative to NAC. Finally, analytic expressions for correction coefficients, dependent on RAC compressive strength and RCA replacement ratio, were formulated for predicting RAC shrinkage strain and creep coefficient using the fib Model Code 2010.This work was supported by the Ministry for Education, Science and Technology, Republic of Serbia under grant number TR 36017 and Spansh Ministerio de Economía, Industra y Competividad under the SAES project BIA2016-78742-C2-1-R. This support is gratefully acknowledgedPostprint (published version

Sustainability assessment of recycled aggregate concrete structures: a critical view on the current state-of-knowledge and practice

The environmental impacts of activities such as raw material extraction, construction of infrastructure, and demolition, place construction as one of the sectors that exert the highest pressures on the environment, society, and economy. Some of the major environmental impacts for which the construction industry is responsible are mineral resource depletion, greenhouse gas emissions, and waste generation. Among the different strategies that exist to decrease such impacts, recycling demolition waste into recycled concrete aggregates has been considered a promising alternative. As such, at present, the literature dealing with the impact assessment of recycled aggregate concrete structures is very extensive. Therefore, the objective of this article is to present a critical view of the state-of-the-art in terms of sustainability assessment of recycled aggregate concrete structures, taking a holistic perspective by considering environmental, social, and economic impacts.Peer ReviewedPostprint (published version

ECO2 framework assessment of limestone powder concrete slabs and columns

Producing limestone powder requires comparably far less energy than the production of ordinary Portland cement (OPC), making it a promising sustainable solution for partial replacement of OPC in concrete. Lower production energy could be translated into lower environmental impact and lower cost, which are two pillars of the sustainability of the resulting concrete. However, the question remains if replacing OPC with larger percentages of limestone powder would compromise the performance of the resulting concrete to a level that surpasses the environmental and economic gains. In order to assess the collective impact of these concretes, a performance-based multi-criteria decision analysis framework, ECO2, is used. For that purpose, 26 experimentally verified, concrete mixtures with and without limestone powder were evaluated through potential application in two types of reinforced concrete (RC) structural elements (slabs and columns) under identical environmental condition. The main results of the research showed a clear environmental advantage of concrete with a reduced OPC content, but the relatively higher superplasticizer amount in some cases could affect the final sustainability performance of the resulting mix. In the case of RC slabs, the best ECO2 score was obtained for concrete containing limestone powder. Mixtures with 200–250 kg of cement per unit volume of concrete had the highest ECO2 score for all the considered criteria. In the second case, due to the nature of the structural performance requirements in columns, the crucial influence of the concrete compressive strength is clear. The obtained results have shown approximately equal sustainability potential of OPC and limestone concretes in vertical elements such as columns. However, it seems that a certain improvement in the design of concrete mixtures with a high limestone powder content could make these competitive in all fields.This work was supported by the Ministry for Education, Science and Technology, Republic of Serbia [grant number TR36017].Peer ReviewedPostprint (published version

Improved serviceability and environmental performance of one-way slabs through the use of layered natural and recycled aggregate concrete

The following are available online at http://www.mdpi.com/2071-1050/12/24/10278/s1: Excel file with input data for the parametric numerical studyRecycled aggregate concrete (RAC), i.e., concrete produced with recycled concrete aggregate (RCA) has been heavily investigated recently, and the structural design of RAC is entering into design codes. Nonetheless, the service load deflection behavior of RAC remains a challenge due to its larger shrinkage and creep, and lower modulus of elasticity. A novel solution to this challenge is the use of layered concrete, i.e., casting of horizontal layers of different concretes. To investigate the potential benefits and limits of layered concrete, this study contains a numerical parametric assessment of the time-dependent sustained service load deflections and environmental impacts of homogeneous and layered NAC and RAC one-way slabs. Four types of reinforced concrete slabs were considered: homogeneous slabs with 0%, 50% and 100% of coarse RCA (NAC, RAC50 and RAC100, respectively) and layered L-RAC100 slabs with the bottom and top halves consisting of RAC100 and NAC, respectively. In the deflection study, different statical systems, concrete strength classes and relative humidity conditions were investigated. The results showed that the layered L-RAC100 slabs performed as well as, or even better than, the NAC slabs due to the differential shrinkage between the layers. In terms of environmental performance, evaluated using a “cradle-to-gate” Life Cycle Assessment approach, the L-RAC100 slabs also performed as well as, or slightly better than, the NAC slabs. Therefore, layered NAC and RAC slabs can be a potentially advantageous solution from both structural and environmental perspectives.This study has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 836270 and from the United States Department of State through the Fulbright Visiting Scholar Grant “Optimization of Stratified Recycled Concrete Structures Based on Numerical Analyses and Life Cycle Assessment.” Any opinions, findings, conclusions, and/or recommendations in the paper are those of the authors and do not necessarily represent the views of the funding organizations.Peer ReviewedPostprint (published version

Creep of recycled aggregate concrete: experimental database and creep prediction model according to the fib Model Code 2010

Recycled aggregate concrete (RAC) is a promising solution to addressing the sustainability issues raised by concrete production. However, its long-term properties are not fully characterised, especially creep behaviour, which is significant for the design of RAC structures. This study presents the results of a meta-analysis of previously published research on the creep behaviour of recycled aggregate concrete (RAC). The main goal of the study is to formulate an analytic expression for the creep coefficient of RAC as an extension of the fib Model Code 2010 creep prediction model. A database of experimental results on the creep of RAC and companion natural aggregate concrete (NAC), produced with the same effective water-cement ratio, was compiled from available literature and using strict selection criteria. The database is comprised of results from 10 studies, 46 creep curves (14 NAC and 32 RAC) and 233 data points. Compared with companion NAC, RAC displays a larger creep coefficient; the difference between RAC and NAC increases with increasing recycled concrete aggregate (RCA) content and decreases with increasing RAC compressive strength. When predicting the creep coefficient of RAC using the fib Model Code 2010’s creep prediction model, relative to the model’s performance on companion NAC, the creep coefficient of RAC is underestimated. A correction coefficient for the creep coefficient of RAC, ξcc,RAC, is proposed for use with the fib Model Code 2010 model, dependent on RAC compressive strength and RCA replacement ratio.Peer ReviewedPostprint (author's final draft

Flexural behaviour and ultimate bending capacity of high-volume fly ash reinforced concrete beams

Large number of studies analyzed physical and mechanical properties of high-volume fly ash concrete, but only a few discussed its structural behavior. Material properties are an important input parameter for structural analysis, but they are insufficient for reliable conclusions to be made. This study analyses flexural behavior of reinforced concrete beams made with 63% of low-calcium class F fly ash in total cementitious materials mass, using experimental method and analyzing current code predictions (EN 1992-1-1). The analysis was done by comparing beams made with two longitudinal reinforcement ratios, made with traditional cement concrete (OPC) and high-volume fly ash concrete (HVFAC), both corresponding to concrete class C30/37. Beams were tested in a four-point bending test measuring vertical displacement, crack development, concrete strains and longitudinal reinforcement strains. According to this research, the flexural performance of HVFAC beams is similar to flexural performance of corresponding OPC beams in terms of ultimate bending capacity. The significant difference was noticed regarding cracking extent that was higher in HVFAC beams. Available ultimate bending moments code predictions can be applied on HVFAC beams with similar precision and variation of results, like for OPC beams. However, this cannot be concluded for parameters depending on the cracking behaviour, like cracking moments or deflections. Results and analysis presented in this study indicate that HVFAC can be used in structural elements subjected dominantly to bending, like beams and slabs. More research regarding structural behavior of HVFAC using full-scale long-term tests is needed to develop larger database for reliability analysis.This work was supported by the Ministry for Education, Science and Technology, Republic of Serbia [Grant No TR36017]Peer ReviewedPostprint (author's final draft

Shrinkage of recycled aggregate concrete: experimental database and application of fib Model Code 2010

The final publication is available at Springer via http://dx.doi.org/10.1617/s11527-018-1258-0This paper describes a meta-analysis of previously published studies on the shrinkage strain of recycled aggregate concrete (RAC). The study aims at providing an analytic expression for the shrinkage strain of RAC to be used in conjunction with the existing fib Model Code 2010 shrinkage prediction model. For this purpose, a database of experimental results on the shrinkage of RAC and companion natural aggregate concrete (NAC), produced with the same water-cement ratio, was compiled using strict selection criteria. Results from 19 studies entered into the database, consisting of 125 shrinkage curves (39 NAC and 86 RAC) with a total of 424 data points. A comparison of RAC and companion NAC revealed that, on average, RAC displays a larger shrinkage strain. This difference increases with increasing recycled concrete aggregate (RCA) content and with decreasing compressive strength. Applying the fib Model Code 2010 shrinkage prediction model revealed that, relative to its performance on NAC, the shrinkage strain of RAC is underestimated. Finally, a correction coefficient for the shrinkage strain of RAC, ξcs,RAC, to be used in conjunction with the fib Model Code 2010 model, was proposed in the form of a bivariate power function with RAC compressive strength and RCA replacement ratio as variables.Peer Reviewe

Creep of recycled aggregate concrete: experimental database and creep prediction model according to the fib Model Code 2010

Recycled aggregate concrete (RAC) is a promising solution to addressing the sustainability issues raised by concrete production. However, its long-term properties are not fully characterised, especially creep behaviour, which is significant for the design of RAC structures. This study presents the results of a meta-analysis of previously published research on the creep behaviour of recycled aggregate concrete (RAC). The main goal of the study is to formulate an analytic expression for the creep coefficient of RAC as an extension of the fib Model Code 2010 creep prediction model. A database of experimental results on the creep of RAC and companion natural aggregate concrete (NAC), produced with the same effective water-cement ratio, was compiled from available literature and using strict selection criteria. The database is comprised of results from 10 studies, 46 creep curves (14 NAC and 32 RAC) and 233 data points. Compared with companion NAC, RAC displays a larger creep coefficient; the difference between RAC and NAC increases with increasing recycled concrete aggregate (RCA) content and decreases with increasing RAC compressive strength. When predicting the creep coefficient of RAC using the fib Model Code 2010’s creep prediction model, relative to the model’s performance on companion NAC, the creep coefficient of RAC is underestimated. A correction coefficient for the creep coefficient of RAC, ξcc,RAC, is proposed for use with the fib Model Code 2010 model, dependent on RAC compressive strength and RCA replacement ratio.Peer Reviewe