This work is devoted to the study of fresh and hardened properties of concrete containing recycled gravel. Four formulations were studied, the concrete of reference and three concretes containing recycled gravel with 30, 65 and 100% replacement ratios. All materials were formulated on the basis of S4 class of flowability and a target C35 class of compressive strength according to the standard EN 206-1. The paper first presents the mix design method which was based on the optimization of cementitious paste and granular skeleton, then discusses experimental results. The results show that the elastic modulus and the tensile strength decrease while the peak strain in compression increases. Correlation with the water porosity is also established. The validity of analytical expressions proposed by Eurocode 2 is also discussed. The obtained results, together with results from the literature, show that these relationships do not predict adequately the mechanical properties as well as the stressstrain curve of tested materials. New expressions were established to predict the elastic modulus and the peak strain from the compressive strength of natural concrete. It was found that the proposed relationship Efc is applicable for any type of concrete while the effect of substitution has to be introduced into the stressstrain (c1fc) relationship for recycled aggregate concrete. For the full stressstrain curve, the model of Carreira and Chu seems more adequate.
Aggregates consumption does not cease to grow in France. According to the UNPG (French national union of aggregates producers) and the UNICEM (French national union of industries of careers and building materials) aggregate production is estimated at 431 million tons in 2008, of which 79% is used in civil engineering field and 21% for building industry. In addition, 5% of this amount is produced by recycling demolition wastes. Although this percentage remains low, recycling helps to limit the environmental impact by limiting the exploitation of natural resources. These socio-economic issues are the driving forces promoting the recycled aggregates in concrete.
The valorization of recycled aggregates in concrete is not recent and many studies have shown that material made with recycled aggregates may have mechanical properties similar to those of a conventional concrete mixed with natural aggregates (Etxeberria et al. 2007; Evangelista and de Brito 2007; Li 2008; McNeil and Kang 2013). However, recycled aggregates are characterized by a high water absorption capacity related to the presence of old mortar attached to the surface of aggregates which hinders their wide use (Gomez-Soberon 2002; de Juan and Gutirrez 2009). The water absorption capacity affects both fresh and hardened states properties. At fresh state, the mix design of concrete with recycled aggregates requires an additional quantity of water to obtain a similar workability as a concrete formulated with natural aggregates (Hansen and Boegh 1986). Such a modification may obviously affect the mechanical characteristics of recycled aggregates concrete. Several studies have investigated the microstructure of recycled aggregates concrete and showed that the porosity is modified and increases with the replacement ratio (Gomez-Soberon 2002). It is also acknowledged that the high porosity of recycled concrete leads a reduction of the mechanical strengths (Gomez-Soberon 2002; Kou et al. 2011). Furthermore, several studies have shown however that mechanical properties of concrete made with recycled aggregates depend on other parameters such as the quality of concrete from which recycled aggregates are obtained (Xiao et al. 2005; Casuccio et al. 2008) as well as the replacement ratio (Beln et al. 2011).
The main goal of this work is to determine the properties of recycled aggregate concretes (RAC) at fresh and hardened states depending on replacement ratio. A concrete made with natural aggregate (NAC), designed for control operations, and three RAC with a S4 class of workability and compressive strength levels near to 35MPa were formulated and tested. The present study also examines the applicability of relationships of Eurocode 2 (EC2) to concretes made from recycled aggregates. These relationships estimate the modulus of elasticity, the peak strain and stressstrain relationship from the simple knowledge of the compressive strength.
CEM I CALCIA 52.5N CE CP2 NF cement in conformity with the standard EN 197-2 was used in all concrete mixes. The chemical and mineralogical compositions calculated by the method of Bogue are presented in Table1. The density of this cement is 3.11, its Blaine surface is 395 (m2/kg) and its compressive strength after 2days is 31.7 (MPa).
The fine aggregates are 0/4mm silico-calcareos rolled sand. For all mix designs, two size fractions of coarse natural crushed silico-calcareos aggregates were used. The particles size of the first fraction, called G1, is comprised between 5 and 10mm while it is comprised between 10 and 20mm for the second type G2. Recycled aggregates were delivered in big bags from a retreatment platform of demolition materials. They were sieved in the laboratory into three fractions GR1 (4/10mm), GR2 (10/20mm) and sand (0/5mm). In this study only the coarse recycled aggregates were used. They were dried in an oven at 1105C and then stocked in closed containers until the moment of concrete manufacture. The main properties of the natural and recycled aggregates are presented in Table2 and the grading curves are plotted in Fig.1. It is seen that the size grading of the coarse natural and recycled gravel was similar with a larger amount of small particles for GR1.
Water absorption has been characterized first, according to the standard NF EN 1097-6 at the atmospheric pressure. Dried aggregates were immersed in water during 24h then dried again in an oven at a temperature of 1105C. It can be noticed in Table2 that recycled aggregates have a significant higher water absorption capacity and a lower density than natural ones. In spite of the high water absorption capacity of the used RCA, it remains within the range recommended by the design standards (McNeil and Kang 2013; Kang et al. 2014).
The kinetic of water absorption of recycled aggregates GR1 and GR2 was also followed and the water uptake was measured by hydrostatic weighting. Aggregates samples were washed first on the sieve of 4mm and dried in an oven at a temperature of 1105C until mass stabilization. After drying the tested sample was placed between two stainless sieves (of diameter 15cm and height 7cm) for hydro-static weighing. The system was hung to a balance of 0.01g accuracy with a non-elastic wire and the mass variation is continuously recorded. The room temperature is equal to 22C. Figure2 illustrates the measurement system.
where Mwa is the mass of saturated aggregates in water at time t, Mws is the mass of dried aggregates in water at t0, and Ms is the mass of dried aggregates in air. The t0 indicates the beginning of the test.
Figure3 shows the evolution of water absorption versus time. At t=24h, water absorption of GR1 and GR2 are respectively equal to 8.3 and 6.5%. These results correspond to the values obtained following NF EN 1097-6 standard (cf. Table2). Results show also that recycled aggregates are saturated after a long time more than 24h, according to several studies (Tam et al. 2008; Djerbi Tegguer 2012). At t=24h, water absorption represents only 82 and 85% of total degree of saturation for GR1 and GR2, respectively. Finally, aggregates reach a water absorption of 10 and 7.8% for GR1 and GR2, respectively. At the opposite, for short time relative to mixing time (5min), the kinetic of absorption is fast.
The used Superplasticizer is Cimfluid 3002 produced by Axim Italcementi group with a solid content of 30%. It is a new generation product based on chains of modified polycarboxylate certified in conformity with the standard EN 934-2 and considered as a water reducing admixture.
at fresh state, all concretes are of S4 workability class where the target slump with the Abramss cone is 181cm. According to the standard NF EN 206-1 the slump for a S4 flowability is comprised between 16 and 21cm;
concretes are designated for XF2 class of environmental exposure according to the standard NF EN 206-1, where water to cement ratio (W/C) is lower than or equal 0.5 and the minimum cement content is higher than 300kg/m3.
A total of four concretes were then produced, a mix with natural aggregates called (NAC) and three concretes with recycled aggregates named RAC30, RAC65 and RAC100. The numbers indicate the rate of substitution. For NAC the cement content is taken equal to 360kg/m3 according to the standard NF EN 206-1 while for the other mixes this content was modified as will be explained below.
The quantity of water for the cement was determined based on the flowability requirement by means of spread tests with the mini flow cone for which dimensions are 8cm for lower diameter, 7cm for upper diameter and 4cm for the height (Fig.4).
where d0 is the lower diameter of the cone, dm is the average of two wafer diameters d1 and d2. In Fig.4 the relative slump m is depicted against the water to cement ratio, where a straight line fits the experimental results with a correlation ratio R2=0.99. The relative slump m=5 yields the water content necessary for a flowable paste (El-Hilali 2009).
Granular skeleton was optimized by the method of compaction using vibration. The study started by measuring the packing density of each component, i.e. sand, natural gravels G1, G2 and recycled gravels GR1, GR2. Binary mixtures of gravel were then tested to determine optimal dosages which give the densest packing. Finally ternary mixtures were tested to optimize the solid skeleton for all mixes.
Packing tests were conducted according to the test method No. 61 of LCPC (Lde et al. 2004). To determine the packing density of a given granular fraction, a sample is poured in 1632cm2 cylindrical mold then vibrated while applying a pressure of 10kPa. The packing density is calculated by the equation:
where H f is the sample height after vibration and H i the height before vibration. The results for components are summarized in Table3 where each value is the average of three measurements. From this table it can be noticed that the packing density of recycled aggregates is lower than that of natural aggregates due to the presence of old cement paste.
Optimal proportions of granular mixtures are reported in Table4 where an increase in the sand content can be observed for recycled aggregates to assess the highest packing. This increase is due mainly to the lower density and lower packing ability of recycled aggregates.
Values of Table4 were obtained by conducting packing tests on each mixture with several proportions. The curve representing the variation of packing density as a function of proportions was then plotted. The chosen optimal dosage is the point which corresponds to the maximum of the curve as shown in Fig.5 for the ternary mixture of sand with recycled aggregates. For concretes RAC30, RAC65 the solid skeleton was first optimized where 30 and 65% coarse natural aggregates were replaced by recycled aggregates GR1 and GR2 and the packing tests were conducted. The results showed that proportions sand to aggregates remain the same as for the mixture sand with recycled aggregates.
In order to corroborate the obtained experimental results, the software RENE LCPC was used (Sedran 1999). The software is able, from packing density and the size distribution curves of aggregates, to predict the packing density of a mixture. The results, plotted on Fig.4, show that theoretical results are in tune with the experimental results.
Recycled aggregates have not been pre-saturated and the amount of absorbed water was added to the mixing water. Moreover, since the amount of water is important, an additional quantity of cement was added such that the ratio of total water to cement remains constant. For NAC, the dosage of superplasticizer was gradually increased until the target slump was obtained. This dosage has not been modified for the other formulations because water initially added to mixes had allowed to obtain the slump 181cm.
Table5, containing the mixes, shows that the adopted approach leads to an increase in paste volume with a slight decrease in density. In this table, the effective water, Weff, is defined according to the standard EN 206-1 as the total water quantity, Wtot, minus the water absorbed by aggregates W eff = W tot - a M g where Mg is the weight of dry aggregates.
Cylindrical 1632cm2 specimens were prepared to determine the compressive strength, elastic modulus and splitting tensile strength. Furthermore plain and prenotched 101040cm3 prismatic specimens were cast to determine the flexural strength of studied concretes. After being removed from the mold, they were cured in a water tank at room temperature for 28days.
Uniaxial compression and tensile splitting tests were performed using a servo-hydraulic INSTRON machine with a capacity of 3,500kN by imposing a stress increment rate of 0.5MPa/s. Each test was repeated at least three times and results shown below are the averages of obtained values. In addition one cylinder of each material was instrumented with two strain gauges in order to determine the elastic modulus, and test were performed by imposing a strain rate of 1mm/min. Bending tests were performed using a 250kN closed loop INSTRON machine with a strain rate of 1mm/min. Finally, splitting strength was measured using the Brazilian test and dynamic modulus of elasticity was determined using E-Meter MK II device.
Water porosity was determined using the vacuum saturation method. The test includes two stages, the first consists on submitting 10 10 10cm specimens, dried at 605C, to vacuum (about 80mbar) during 3h. After this period and in a second stage, specimens are immersed in water during 5days. The water absorption, called water porosity, is determined as follows:
The results of workability tests and air content are given in Table6 where it can be seen that all mixtures compile with the required workability. It can be concluded that the air content of RAC is higher than concrete made with NA when the replacement ratio exceeds 30%.
The slump loss during 2h is plotted in Fig.6 for all studied materials where the slump values are averages of two measurements. During the first 20min, the loss is not significant for RAC65 and RAC100 and this is explained by the excess of available water in the mix. After 20min the loss is more pronounced when the replacement ratio is higher. This trend was also observed by Poon et al. (2011).
In order to verify if the accentuation is due to the continuous water absorption by recycled aggregates and not to the increase in paste volume, the loss in workability of two cement pastes corresponding to NAC and RAC100 was followed during 2h. Figure7 shows the changes of paste slumps with time where it can be seen that the two pastes undergo the same kinetic of loss.
Figure8 presents the water porosities, measured at atmospheric pressure and under vacuum conditions, for all concrete mixes. It can be pointed out that both porosities increase with replacement ratio. These results are in agreement with the results of the literature where the porosity increases with substitution rate (Gomez-Soberon 2002; Belin et al. 2013). It can be also shown that the ratio between the porosity measured under vacuum and the porosity measured at atmospheric pressure is constant and equal to 1.16.
The increase of porosity with replacement ratio is mainly due to the high porosity of recycled aggregates, to the increase in the paste volume and to the poor interface paste-aggregates as well as to the increase in air content (cf. Fig.9).
Figure10 shows compressive strength results at 7, 14, 21 and 28days for the four concretes produced in this work. As illustrated in this figure, comparable strengths were obtained for all concretes with a decrease of 13% for the concrete RAC30. This strength loss is due to recycled aggregates and to the increase in the total water quantity without correcting the cement content. For substitution ratio higher than 30%, two phenomena are in competition: increasing the cement content and the replacement ratio of recycled aggregates which contribute to the increase of strength in the first case and the decrease in the second. However, the materials satisfy correctly the imposed specifications given in part 3 i.e. S4 flowability and 35MPa compressive strength concretes.
Compressive strength results were compared to the Fret strength equation with 54 results found in references (Xiao et al. 2006; Etxeberria et al. 2007; Evangelista and de Brito 2007; Gomes and Brito 2009; Beln et al. 2011; Martinez-Lage et al. 2012; Pereira et al. 2012; Manzi et al. 2013). The Fret strength equation is:
where K is the Fret coefficient which depends on mix design and age, f c is the compressive strength of concrete (MPa), f cm is the normal compressive strength of cement, v c is cement content in concrete (m3/m3), v w is the water content in concrete (m3/m3) and v a is the air content in concrete (m3/m3).
The K value was evaluated based on the both natural and recycled aggregates concretes compressive strength (cf. Figure11). It is found that a value of 5.27 fits adequately the experimental results with a correlation factor R2=0.7. The obtained value is close to the theoretical value of K=5 found in the literature (Julio et al. 2006; Hacene et al. 2009).
Frets equation can therefore help to explain the obtained compressive strengths for RAC65 and RAC100. Indeed, at a constant W/C ratio, when the concentration of cement increases in the paste volume (i.e. the reduction of effective water to cement ratio), the compressive strength is maintained constant despite the increase in air content (Table5).
The variation of both static and dynamic modulus of elasticity, denoted E, at the age of 28days is plotted in Fig.12. It can be seen that the recycled aggregates have a significant effect on the elastic modulus where it decreases with the increase of replacement ratio. These results are in good agreement with the literature results which indicate a decrease in the elastic modulus (Xiao et al. 2006; Casuccio et al. 2008). This reduction is the consequence of the application of recycled aggregates with a higher porosity and a lower elastic modulus than those of the natural coarse aggregates.
The flexural strength obtained for all concretes is shown in Fig.13 with the tensile splitting strength. It is possible to conclude that the tensile strength with recycled aggregates is negatively affected when replacement ratio increases. The loss of both flexural and tensile strengths is about 6% for RAC30, 11% for RAC65 and reaches 20% for RAC100.
The correlation between tensile strength and the porosity is illustrated on Fig.14. The results, as might be expected, show a decrease in tensile strength when the porosity increases. Figure15 shows the normalized static modulus of elasticity versus the normalized tensile strength. The linear correlation indicates that the higher porosity of recycled aggregates affects both characteristics. These results are in agreement with those established by Evangelista and de Brito (2007).
Stressstrain curves were obtained by uniaxial compressive tests for all materials developed in the present work. The analysis of these curves shows that the peak strain corresponding to the maximum stress increases when increasing the replacement ratio. The evolution of this strain, normalized by the strain of NAC, is shown in Fig.16 as a function of the rate of substitution with the results of Beln et al. (2011) and Martnez-Lage et al. (2012). The results show a linear increase of the normalized peak strain with the replacement ratio. However, this increase is more significant in the context of our work and it is probably attributed to the adopted experimental conditions.
The influence of the recycled aggregates content on the complete stressstrain curves was also investigated. The results shown on Fig.17 indicate that the shape of the post-peak curve is more spread when the replacement ratio is important. This observation highlights a more dissipative behavior when recycled aggregates are used, and may be explained by a more diffuse damage related to the nature of recycled aggregates.
The validity of the previous expression was verified using more than 230 concretes formulated with natural aggregates (see Table7 in Appendix section). The results are plotted in Fig.18 and the data analysis shows that the expression of EC2 does not allow a satisfactory prediction of elastic modulus. Equation(6) fits experimental results with correlation factor R2=0.68 while the proposed Eq.(7) provides a better description of the experimental results.
The elastic modulus of RAC developed in this work together with the results found in the literature (Gomez-Soberon 2002; Etxeberria et al. 2007; Evangelista and de Brito 2007; Casuccio et al. 2008; Domingo-Cabo et al. 2009; Beln et al. 2011) are shown in Fig.19. From this figure it can be seen that Eq.(7) allows a better prediction of elastic modulus than those proposed in EC2 (Eq.(6)) for RAC. It can be pointed out also that the majority of experimental values are within the interval of 10% bounded by the two dotted red lines. Finally, it can be concluded that the relationship between E and f c does not seem affected by the nature of gravels.
The validity of this expression was verified for 66 concretes of the literature with natural aggregates (see Table8 in Appendix section). The results presented in Fig.20 show that the previous expression of EC2 does not predicted well the peak strain. However, it can be shown that the proposed expression given by Eq.9 is more suitable for the prediction of peak stain.
with = c / c1 , k = 1.05 E cm ( c 1 / f c ) and Ecm the secant modulus of elasticity. The model requires the knowledge of the static modulus of elasticity, the compressive strength and the peak strain c1. The application of this model to recycled aggregates concrete shows that it does not reproduce suitably the post-peak behavior (Fig.21). Another simple expression was proposed by Carreira and Chu (1985).
Equations(7), (9) and (11) may therefore be used for the modeling of full stressstrain relationship of recycled aggregates concrete with the modification of peak strain with the replacement ratio. Figure21 presents a comparison between the curves calculated using the modified model of Carreira and Chu (Eq.(9)) and the model of EC2 (Eq.(8)). It can be seen that this modified model is more adequate for the modeling of post-peak behavior as the model of EC2.
In this paper, a natural aggregates concrete, NAC, and three recycled concrete aggregates RAC30, RAC65 and RAC100 were prepared on the basis of an imposed constant flowability at fresh state and a target compressive strength of about 35MPa at 28days. Moreover only coarse aggregates were replaced by recycled ones with three volumetric replacement ratios being respectively 30, 65 and 100%. Based on the experimental results the following conclusions can be drawn:
The use of recycled aggregates up to 30% does not affect the demand of water of concrete, but generates a reduction of 14% of the compressive strength. By increasing the replacement ratio, the cement content increases to maintain constant W/C ratio causing an increase in the compressive strength which counterbalances the negative effect of recycled aggregates.
The strainstress curves under uniaxial compression show that the post-cracking branch is more spread out compared to NAC. In addition, the peak-strain increases by increasing the replacement ratio. These phenomena are explained by the more progressive and diffuse damage of concrete due to the presence of recycled aggregates.
New relationships for prediction of concretes elastic modulus, and a peak strain from compressive strength were proposed. The predicted results for RAC were closer to experimental results than values predicted by equations proposed in EC2. For the complete strainstress curve, a model based on the Carreira and Chus model was proposed. The modified model is more adequate for the modeling of post-peak behavior than the model of EC2.
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Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
Wardeh, G., Ghorbel, E. & Gomart, H. Mix Design and Properties of Recycled Aggregate Concretes: Applicability of Eurocode 2. Int J Concr Struct Mater 9, 120 (2015). https://doi.org/10.1007/s40069-014-0087-y
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Application of response surface methodology for mix design of recycled aggregate pervious concrete.Explanation of Ideal paste thickness (IPT), actual coating thickness (ACT) and the void content (TVC).Establishment of IPT and ACT response models to admixture doses by Box-Behnken Design.Establishment of TVC response model to aggregate gradation by simplex centroid design.Optimization of models for practical mix design.
This paper demonstrates that RSM can be used to design the mix proportion of recycled aggregate pervious concrete. From the perspective of the internal structure of recycled permeable concrete, the factors affecting its strength and porosity are the paste properties, the coating thickness of paste on the surface of aggregate and the void content of aggregate. Ideal paste thickness (IPT), actual coating thickness (ACT) and the void content (TVC) are respectively introduced to quantify and optimize these three factors through response surface methodology (RSM). Box-Behnken Design (BBD) is used to obtain the response of IPT and ACT to dosage combinations of three admixtures, and simplex centroid design is utilized to establish the response of TVC to different recycled aggregate gradations. Finally, three response surface models are optimized for practical application to find the suitable aggregate gradation and dosage combination of admixtures, and the verified experiment confirms that the model is effective.
Recycled Concrete Aggregates (RCAs) are more porous because of Attached Mortar (AM).AM influences the key properties of RCAs, among which water absorption capacity.AM of RCAs influences the strength of Recycled Aggregate Concretes (RACs).The proposed methodology predicts the RAC strength by considering the key properties of RCAs.
This paper proposes a conceptual formulation for predicting and controlling the compressive strength of Recycled Aggregate Concrete (RAC) mixtures, by explicitly taking into account the specific features of Recycled Concrete Aggregates (RCAs). In fact, since RCAs are significantly more porous than Natural Aggregates (NAs), the mix design rules commonly employed for ordinary structural concrete cannot be applied as such for RACs. Therefore, the formulation proposed herein is intended at generalising the aforementioned rules with the aim to take into account the higher porosity of RCAs. Although being a mainly conceptual methodology, the proposed formulation is supported by a wide set of experimental results: they unveil the influence of several aspects and parameters (such as source and processing procedures of RCAs, aggregate replacement ratio, water-to-cement ratio, water absorption capacity and initial moisture condition of aggregates) on the resulting compressive strength of RAC. Finally, the proposed mix design methodology demonstrates that the resulting compressive strength of RACs can be predicted by taking into account only one parameter (i.e., water absorption capacity) identifying the quality of RCAs. Further generalisations intended at controlling other physical and mechanical parameters of RAC are among the future development of this research.
This study proposes a new mix design methodfor C30 recycled concrete using recycled aggregate crushed from different strength source concrete (SC). The dosages of cement, water, sand, and coarse recycled concrete aggregate (CRCA) are the same as the corresponding components in natural concrete, but the CRCA should be pre-wetted before casting. Its weighted mean strength has a value of 43.1MPa for two strengths of SC. Compressive and splitting tensile strength tests on recycled concrete with CRCA (100% substituting limestone aggregate) made from two mixed SC with varying strengths, have been implemented to acquire the mix design method. For investigating the influence of CRCA from two strengths of SC on recycled concrete performance, test parameters of mechanical behavior including compressive strength of SC, strength difference between two mixes of SC, and proportion of two mixed CRCA were taken into account. Test results indicate that when one of the two mixed SC strengths is similar to the target strength, two requirements have to be satisfied to achieve the target compressive strength: one of the SC has to be more than 10MPa higher than the target strength, and the proportion of the higher strength SC is larger than 75% in weight of the total CRCA. The results provide useful information for recycling multiple strength SC.
Pitarch, A.M., Reig, L., Toms, A.E., Lpez, F.J.: Effect of tiles, bricks and ceramic sanitary-ware recycled aggregates on structural concrete properties. Waste Biomass Valoriz. 10, 17791793 (2019). https://doi.org/10.1007/s12649-017-0154-0
Brs, I., Silva, P.C., Almeida, R.M.S.F., Silva, M.E.: Recycling wastes in concrete production: performance and eco-toxicity assessment. Waste Biomass Valoriz. (2018). https://doi.org/10.1007/s12649-018-0382-y
Ding, X., Qi, J., Fang, W., Chen, M., Chen, Z.: Improvement on properties of recycled concrete with coarse ceramic vase aggregates using kh-550 surface treating technology. Eur. J. Environ. Civ. Eng. 24, 116 (2017). https://doi.org/10.1080/19648189.2017.1363664
Fathifazl, G., Abbas, A., Razaqpur, A.G., Isgor, O.B., Fournier, B., Foo, S.: New mixture proportioning method for concrete made with coarse recycled concrete aggregate. J. Mater. Civ. Eng. 21, 601611 (2009)
Sagoe-Crentsil, K.K., Brown, T., Taylor, A.H.: Performance of concrete made with commercially produced coarse recycled concrete aggregate. Cem. Concr. Res. 31, 707712 (2001). https://doi.org/10.1016/S0008-8846(00)00476-2
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This study was supported by the financial support from the National Intensive Research Project of China (No. 2018YFD1100402-05)andthe National Natural Science Foundation of China(No. 51908122). The financial support is gratefully appreciated.
Ding, X., Hao, J., Chen, Z. et al. New Mix Design Method for Recycled Concrete Using Mixed Source Concrete Coarse Aggregate. Waste Biomass Valor 11, 54315443 (2020). https://doi.org/10.1007/s12649-020-01073-7
Conservation of natural resources and preservation of environment is the essence of any development. Use of recycled aggregate concrete (RAC) is one such attempt and is an answer to some of the problems in constructional engineering. The concept of using RAC is now gaining popularity and research in this direction has gained momentum. In this paper, the authors have identified the most suitable method of mix design for RAC, from amongst the available conventional methods of mix design. An influencing parameter is identified and an empirical relation is suggested to modify the influencing parameter. Mix design parameters thus obtained, enable RAC to attain the desired and designed target strength without attempting any trial mixes. The suggested modified procedure, however, demands 10% more cement which is considered quite reasonable and acceptable in view of the inferior quality of recycled aggregate.
A mix design method for RAC was explored by modifying an empirical formula.Compressive strengths were measured with a wide range of parameters.Calculation method of water consumption for different workability is given.The mix design method is consistent with that of NAC.
This paper puts forward a strength-based mix design method for recycled aggregate concrete using a modified empirical formula. A total of 30 mixes, the amount of cement varies from 300kg/m3 to 500kg/m3 and the proportion of recycled coarse aggregate ranges from 0% to 100%, were prepared for the regression analysis of relationships between multiple parameters. Then for a defined workability and specified value of recycled coarse aggregate replacement ratio, and known natural water absorption of recycled coarse aggregate to be used in a mixture, two mixture design parameters, cementwater ratio and cement content, are generated from the proposed method after determining the water consumption. Moreover, the confirmatory experiments show that the test results of 26 validation mixes, designed strength ranges from 30MPa to 50MPa and recycled coarse aggregate replacement ratio varied from 20% to 100%, were in satisfactory agreement with the target compressive strength. Recycled aggregate concrete durability performances corresponding to this method were also evaluated, and results show that the increasing of recycled coarse aggregate, degrades the freezethaw resistance, drying shrinkage, chloride penetration and carbonization.