Assessing the potential of recycled fine aggregate from freeze-thawed parent concrete for structural concrete

Recycled fine aggregate (RFA) generated from waste concrete, especially in harsh environment, can be considered as an alternative to natural sand. The yield rate, gradation and properties of RFA from natural aggregate concrete with the target strength of C40 as parent concrete (PC) every 200 freeze–thaw (FT) cycles are investigated. To more accurately evaluate the application potential of RFA, the mechanical properties and durability of recycled fine aggregate concrete (RFAC) is further studied. The results showed that as the FT cycles of PC increased, the yield rate of RFA decreases and the grading curve of RFA meets the requirements of Class II aggregate. The limit FT cycles of PC in Class II and III RFA are 148 and 450, respectively. For the compressive strength of RFAC that meets the design requirements, the FT cycles of PC are no more than 530. Based on 50 years of RFAC in Class D and Class E environments, the limit FT cycles of PC are 663 and 200, respectively. The limit FT cycles of PC are 221 based on 50 years of RFAC service in cold regions. Through the FT cycles of PC, the Class of RFA and the mechanical and durability of RFAC can be directly predicted. This provides a theoretical and data support for improving the utilization rate of waste concrete in FT environment.


Introduction
Concrete has attracted extensive attention due to its wide applications [1].In recent years, the rapid development of urbanization and industrialization in developing countries has increased the consumption of concrete, resulting in the consumption of non-renewable resources such as natural sand and cementitious materials.This, in turn, causes serious environmental damage and a large amount of construction waste.Over the past few years, the annual production of construction waste in China has exceeded 2 billion tons [2], with some cities even having a phenomenon of 'construction waste siege'.Consequently, the disposal of construction waste has become an urgent issue.The proportion of waste concrete in construction waste is high [3], particularly in developing countries like China.The recycle of waste concrete is beneficial to reduce the consumption of natural resources and promote the resource utilization of construction waste, which is an optimal approach to achieve sustainable development of the construction industry [4,5].
Recently, the utilization of recycled aggregate (RA) to replace natural aggregate in structural concrete has become a hotspot.The suitability of RA has attracted much attention.However, loose, porous attached mortar (AM) and interfacial transition zone (ITZ) in RA will lead to low density, high water absorption and high crushing value [6], thereby adversely impacting the mechanical properties and durability of recycled aggregate concrete (RAC) [7].The application of low-quality RA is strictly limited in engineering construction [8].The strength of parent concrete (PC) affects the quality of recycled coarse aggregate (RCA), resulting in differences in its physical properties.Chen et al [9] reported that high-strength PC can generate RCAs with good physical properties.Findings from previous studies [10] revealed that the quality of RCA with the same particle size decreased as the PC compressive strength decreased.The service environment not only caused damage to concrete structure, but also affected the performance of RCA [2,11].As a component of crushed waste concrete, the physical properties of RFA generated from PC in actual environments such as freeze-thaw (FT) cycles should be investigated to explore its potential applications.
In cold areas, more and more concrete structures, such as dams and bridges are subjected to FT cycles.The ability to resist FT cycles of concrete has become a critical indicator to estimate the durability of structures.The average number of annual FT cycles varied indifferent regions [12], indicating that concrete suffered different FT cycles within its designed service life.The FT cycles of concrete would lose its ability to protect steel bars, or lead to rapid structural degradation to failure [13].In addition, improper treatment of waste concrete suffered by FT cycles would cause serious economic losses and heavy environmental burdens [14].The RCA originated from freeze-thawed concrete has been reported, highlighting its considerable potential for recycling [15].The RFA generated in the crushing process of waste concrete is often discarded with low utilization rate, causing a huge waste of renewable resources.It is, thus, urgent to study the recycling potential of RFA from PC in FT environment, to provide a theoretical support for the utilization of waste concrete in cold regions.
We aim to assess the application potential of RFA from PC in FT environment.The NAC with a strength grade of C40 was prepared as PC.After every 200 FT cycles, PC was crushed to generate RFA until PC failure and recycled fine aggregate concrete (RFAC) was being prepared for RFA.The effects of FT cycles of PC on the particle size and properties of RFA are analyzed.According to the requirements of structural concrete aggregate, the FT cycle limit of PC based on RFA Class is explored.Then the effect of FT cycles of PC on the physical properties, mechanical properties and durability of RFAC are investigated.The results could fill the gap of RFA from freeze-thawed PC and provide theoretical support for the utilization of waste concrete in cold regions.

Materials
The natural coarse aggregate (NCA) was supplied by Changzhou China Railway City Construction Component Co., Ltd with a particle size of 5-20 mm.The grading curve is shown in figure 1(a), and the physical properties are shown in table 1. Natural fine aggregate (NFA) is river sand with a grading curve shown in figure 1(b), and its fineness modulus is 2.8.Cement was an ordinary silicate cement (OSC) of P.O 42.5 supplied by Jiangsu Yangzi Cement Factory.The addition of fly ash (FA), silica fume (SF) and slag (SL) was used to enhance the workability and durability of the fresh concrete mixture.The chemical composition of the cementitious materials is shown in table 2. The JK-PCA polycarboxylate superplasticizer (SP) was used to increase the fluidity of the mixture, provided by Changzhou Institute of Architectural Science.Triterpenoid saponins air-entraining agent (AEA) was provided by Jiangsu Nigao Science & Technology Co., Ltd, which was used to improve the frost resistance  durability of concrete.The preparation of RFA is shown in figure 2. The RFAs produced by PC after 0, 200, 400, 600 and 800 FT cycles were labeled as RFA0, RFA200, RFA400, RFA600 and RFA800, respectively.

Specimen preparation
The target strength grade of concrete in this test is C40.The concrete proportion in this experiment was designed using the overall calculation method [16], which is an innovative design methodology that has been extensively adopted to improve the performance of concrete.Due to the high water absorption rate of RFA, the additional water consumption [17] was added with the water absorption rate of RFA at 30 min.FA, SF and SL accounted for 20%, 5% and 10% of the cementitious materials, respectively, while SP and AEA were 0.8% and 0.01% of the cementitious materials, respectively, as shown in table 3. AEA was only added to the concrete tested for FT cycles.The RFAC containing 100% RFA0, RFA200, RFA400, RFA600 and RFA800 were labeled as RFAC0, RFAC200, RFAC400, RFAC600 and RFAC800, respectively.The mixtures were prepared by the two-stage mixing method [18] , and the specimens were cured for 28 d after forming and demolding.The experimental process is shown in figure 2.  The PC sample was continuously dried at 60 ± 3 °C for 24 h, and then cooled to room temperature after every 200 FT cycle until FT failure.The PC was crushed and sieved into RFA by a jaw crusher through a two-stage crushing process (figure 3).The yield rate of RFA was calculated as follows: where, Y n is the yield rate of RFA after n FT cycles of PC, %; G n is the mass of RFA obtained from PC after n FT cycles, g; M n is the mass of PC after n FT cycles, g.

Physical and mechanical properties of RFAC
The porosity of RFAC was tested by ASTM C642-21 [22].The water absorption and strength were determined in accordance with Chinese code GB/T 50081-2019 [23].

Durability performance of RFAC
The Cl −1 penetration resistance of RFAC (Φ100 mm × 50 mm) were evaluated using the rapid chloride migration coefficient method [24].
The rapid FT test was carried out based on Chinese criteria GB/T 50082-2009 [24] to measure the relative dynamic modulus of elasticity (RDEM) and mass loss rate (MLR).The temperature range of the central specimen was −18 ± 2 °C to 5 ± 2 °C during FT cycles process and the duration of each FT cycle was controlled to 2 ∼ 4 h.Durability factor (DF) is an indicator to assess the frost resistance of RFAC, calculated on the basis of the specification GB/T 50476-2019 [25].

Microscopic analysis
The micro-hardness of ITZ and AM in RFA was measured by a digital Vickers micro-hardness tester (HVS-1000SS).RFA specimens with a size of about 4 mm × 4 mm × 4 mm were prepared and then polished to identify diamond indentations.The prepared specimens were placed on the cap to find ITZ.The load and loading time were 10 g and 10 s, respectively.The micro-hardness of ITZ and AM in RFA was the average of three measurements.
The micro-morphology of RFA was studied by scanning electron microscopy (SEM).RFA samples with a size of 5 mm × 5 mm × 3 mm were made and cleaned with compressed gas to remove surface impurities.The sample was dried at 60 °C, and then its surface was sprayed with gold.The microstructure of RFA produced by PC under different FT cycles was observed.

Compressive and splitting tensile strength
The compressive strength and splitting tensile strength of PC at different FT cycles is shown in figure 4. The compressive strength of PC at 28 d was greater than 40 MPa, which met the target compressive strength requirements.The accumulation of FT cycles resulted in a decrease in the compressive strength and splitting tensile strength of PC.After 800 FT cycles, the compressive strength loss of PC was as high as 32.3%, but still greater than 35 MPa; the splitting tensile strength loss of PC was up to 29.4%, still more than 3.5 MPa.This is because the internal cracks may be compressed to close when the sample was extruded, and this part of the defect cannot be accurately reflected in the number of FT cycles [2].Therefore, the strength loss cannot accurately represent the FT damage of PC.The deterioration of ITZ was an important reason for the loss of PC strength [26].Under the action of FT cycles, micro-cracks were gradually generated in ITZ.The stress concentration appeared at the cracks when the specimen was compressed, and then the cracks expanded and reduced the strength of PC [27].

MLR and RDEM
The MLR and RDEM of PC during FT cycles are shown in figure 5.As the number of FT cycles increases, the MLR of PC gradually increases and RDEM gradually decreases, indicating that the apparent spalling and damage to the internal structure of PC gradually increases.From figure 5, MLR and RDEM of PC changes significantly when the FT cycles are more than 600.This is because the damage accumulation caused by FT cycles on PC  structures leads to the connection and penetration of cracks [28].The number of FT cycles can reach up to 800, the MLR and the RDEM of PC were 4.52% and 55.9% respectively.The MLR of PC is less than 5% and RDEM is less than 60%, indicating that the frost resistance failure based on RDEM loss occurs in PC [24].

RFA yield rate
As shown in figure 6(a), the yield rate of RFA decreases from 31.9% to 30.9% as the FT cycles of PC increase.FT cycles can significantly weaken the mortar and interface of concrete [27], leading to its internal cracking and apparent spalling and the reduction of RFA yield rate; on the other hand, due to the internal damage of PC in FT cycles, a large number of cracks are generated, and the powders less than 0.15 mm are easily generated during the crushing.The RFA yield of PC is similar under different FT cycles.To better reflect the effect of FT cycles on the particle distribution of RFA, the RFA obtained from crushing was sieved, and the gradation curves are shown in figure 3(b).It is clear from figure 3(b) that the gradation of RFA generated after the FT cycles of PC of meet the requirements of Class II aggregate and moves downwards, indicating that the proportion of finer particles in the RFA gradually increases.This indicates that the FT cycle has a cumulative effect on the damage of the PC, causing the PC as a whole to become loose or even disintegrate, which may have an adverse effect on the quality of generated RFA.

Physical Properties of RFA
The physical properties of RFA are shown in figure 7. The performance of RFA generated by PC without FT cycles is optimal, and the physical properties of RFA gradually deteriorates as the FT cycles of PC increase.The physical properties of RFA and the FT cycles follow the Boltzmann function relationship.According to the requirements of Chinese code on recycled fine aggregate for concrete and mortar [21], the apparent and bulk density of RFA can meet the requirements of Class III aggregate.The crushing value, soundness and water requirement ratio of recycled mortar cannot meet the requirements of Class I aggregate.When the FT cycles are 600 and 800, the crushing value, soundness of RFA and the water requirement ratio of recycled mortar no longer meet the requirements of Class III aggregate.Similarly, when the FT cycles are 800, the strength ratio of recycled mortar no longer meet the requirements of Class III aggregate.The limit number of PC FT cycles based on RFA Class is shown in table 4. The crushing value and soundness of RFA, as well as the strength ratio of recycled mortar have a significant effect on the RFA Class, especially the soundness, which is similar to previous results on RCA.The PC limit FT cycles based on RFA Class is consistent with the PC FT cycles based on soundness differentiation of RFA Class.The corresponding PC limit FT cycles for RFA Class II and III are 148 and 450 respectively.

Micro-hardness
Figure 8 illustrates the micro-hardness of ITZ and AM of RFA.As shown in figure 8(a), as the FT cycles increase, micro-hardness of RFA decreases.The micro-hardness of RFA0 generated by PC without FT cycles is the highest.When the FT cycles reach 800 times (PC fails), the micro-hardness of ITZ and AM is 12.6 HV and 36.7 HV, respectively, which is consistent with the results of Liu et al [29].Compared to that in RFA0, the microhardness of ITZ and AM in RFA800 decreases by 49.5% and 68.2%, respectively.It is noteworthy that the micro- hardness of AM is always higher than that of ITZ in PCFT cycles.Compared with AM, ITZ is more vulnerable and prone to damage.The findings are consistent with previous results, indicating that the ITZ is a weak region of concrete [30].The results of micro-hardness indicate that FT cycles can decrease the interfacial bonding properties between AM and NCA, as well as the looseness of AM.This maybe that during repeated FT cycles, free water enters the PC and generates expansion stress, leading to cracks or even spalling of the concrete, further weakening the bonding between the concrete components.From figure 8(b), it can be seen that as the PC FT cycles increase, the micro-hardness of ITZ and AM of RF decreases linearly.This may be due to the gradual destruction of the surface and internal structure of PC as the FT cycles gradually increase, and its FT damage gradually accumulate sand is irreversible.

Microscopic observation
The SEM photographs of the RFA generated by PC after different FT cycles are shown in figure 9.The microscopic morphology of RFA0 is regular, with a well-preserved microstructure and dense internal mortar, as shown in figure 9(a).The structural integrity of RFA0 is the highest because its PC did not undergo FT cycles, and the internal cracks and pores remains stable.Micro-cracks emerged in the RFA due to the FT-induced damage to PC, as shown in figures 9(b) to (e).Apparently, the width of micro-cracks in RFA gradually increased as the FT cycles of PC and the degree of mortar looseness increase.The growth of micro-cracks facilitated the infiltration of free water, which further accelerates the expansion and continuity of the cracks.This is because the FT damage is irreversible and gradually accumulated.The SEM images intuitively demonstrate that the microstructure of RFA generated by PC after FT cycles gradually deteriorate.The deterioration of the microstructure of RFA is macroscopically manifested as a decrease in RFA performance.

Water absorption and porosity
Figure 10 shows the water absorption and porosity of RFAC.As shown in figure 10(a), the water absorption of RFAC specimens is stable after 48 h.At 72-h, the water absorption rates for RFAC0, RFAC200, RFAC400, RFAC600 and RFAC800 are 3.76%, 3.97%, 4.18%, 4.36% and 4.58%, respectively.It can be seen that as the FT cycles of PC increase, the water absorption of RFAC prepared by manufactured RFA also increase.Based on [29], the increase in concrete porosity leads to higher water absorption.To better characterize the internal pore structure of RFAC, the porosity of RFAC is shown in figure 10(b).The porosity of RFAC0 is 10.2% in figure 10(b).As predicted, the RFA derived from FT damaged PC further increased the porosity of the prepared RFAC.Thus, the porosity of RFAC containing RFA800 is as high as 15.2%.The development and penetration of micro-cracks in RFA directly led to the deterioration of the pore structure of RFAC, which increases the porosity and the water absorption of RFAC.When PC is 800 FT cycles, the compressive strength of RFAC containing RFA800 decreases the most (14.7%).Both RFAC600 and RFAC800 do not meet the compressive strength requirements of 40 MPa.According to the fitting function, the maximum FT cycles of PC for RFAC meeting the compressive strength requirements of 40 MPa is 530, as shown in figure 12(b).It can be reported that adding low RFA is not conducive to preparing RFAC with satisfactory strength.This is because there are old and new interfaces as weak links in RFAC.There are microcracks at the old interface, which weakens the interface bonding strength and is prone to fracture during extrusion.Therefore, it is necessary to limit the FT cycles of PC to obtain RFA with good performance.[31] reported that the porosity of RFAC would directly affect the permeability coefficient of Cl −1 .From the experimental results of RFAC porosity, it can be seen that the porosities of RFA and RFAC are closely related to the FT cycles of PC.Loose and porous RFA can provide a convenient channel for Cl −1 penetration into RFAC.Thus, the greater the porosity, the worse the resistance to Cl −1 erosion of RFAC.

Frost resistance
The MLR of RFAC is shown in figure 14(a).As the FT cycles progresses, the MLR of RFAC gradually increases.When the FT cycles reach 300 times, the MLR of RFAC is less than 5%, indicating there is no concrete failure based on MLR.The RDEM of RFAC is shown in figure 14(b), showing that the damage degree of the internal structure of RFAC increases as the FT cycles increase.After 300 FT cycles, the RDEM of RFAC200, RFAC400, RFAC600 and RFAC800 is 59.8%, 58.6%, 54.6% and 50.6%, respectively, which are lower than 60%.At this time, these concrete specimens fail.In order to better measure the service life of RFAC in actual cold regions, the DF of RFAC is calculated, as shown in figure 14(c).With the FT cycles of PC that generates RFA increasing from 0 to 800, the DF of RFAC gradually decreases from 63.2% to 52.6%.This is because the increase in FT cycles leads to the development of pores and micro-cracks in RFA, which further leads to an increase in the deterioration of the internal structure of RFAC, followed by a sharp decrease in dynamic elastic modulus.To this end, RFAC800 has the lowest DF of 52.6%.As shown in the fitting curve in figure 14(c), the DF of RFAC decreases linearly as the FT cycles of PC increases.Under high saturation environmental conditions in cold regions, the corresponding DF for 50 years of use of RFAC is 60% [25], therefore the ultimate FT cycles for PC are 221.

Conclusions
In this paper, the effects of FT cycles of PC on the yield rate, gradation and physical properties of RFA were analyzed.The micro-hardness of RFA was measured and the microstructure was observed.According to the requirements of structural concrete aggregate, the FT cycle limit of PC based on RFA Class is obtained.In addition, the effects of RFA generated by PC after different FT cycles on the physical properties, compressive strength and durability of RFAC are further studied.Based on the experimental studies, the conclusions are as follows: The yield rate of RFA decreases from 31.9% to 30.9% as the FT cycles of PC increases, and the grading curve of RFA still meets the requirements of Class II aggregate.As the FT cycles of PC increase, the physical properties of generated RFA gradually deteriorate and follows the Boltzmann function, and the micro-hardness of ITZ and AM decreases linearly.The limit FT cycles of PC for Class II and III RFA are 148 and 450 respectively.
The water absorption and porosity of RFAC increase linearly as the FT cycles of PC that generates RFA increase, resulting in a linear increase in drying shrinkage of RFAC.The drying shrinkage of RFAC800 is up to 28.1% greater than that of RFAC0.The compressive strength of RFAC prepared from RFA decreases gradually quadratic ally with the FT cycles of PC.The limit FT cycles of PC for RFAC meeting the requirements of compressive strength is 530.
The Cl −1 permeability coefficient of RFAC increases gradually in a quadratic function as the FT cycles of PC increase.The FT cycles of PC based RFAC used for 50 years in Class D and Class E environments are 663 and 200, respectively.As the FT cycles of PC increases, the DF of RFAC decreases linearly.The limit FT cycles of PC for 50 years of RFAC service are 221 in cold and highly saturated environments.

Figure 3 .
Figure 3.The preparation process of RFA.

Figure 4 .
Figure 4. (a) Compressive strength and (b) splitting tensile strength of PC during FT cycles.

3. 3 . 2 .
Drying shrinkage Figure 11 is the development of drying shrinkage in RFAC specimens.From the results in figure 11(a), RFAC0shows the lowest drying shrinkage regardless of curing age.The drying shrinkage of RFAC200 is only 8.2% greater than that of RFAC0, indicating a minor degree of deterioration.However, compared with RFAC0, the drying shrinkage of RFAC containing 100% RFA800 from FT-damaged PC increases by up to 28.1%, thereby increasing the risk of concrete cracking.The drying shrinkage of RFA-prepared specimens increases linearly when the FT cycles of PC increase to 800, as shown in figure11(b).As the FT cycles of PC increases, the porosity of RFA increases, which further leads to the increase of porosity and water absorption of RFAC, as well as the shrinkage rate of RFAC after water evaporation.The results of drying shrinkage indicate that excessive FT cycles of PC would restrict the reuse of prepared RFA and weaken the engineering application potential of RFA.3.3.3.Compressive strengthThe compressive strength of RFAC is shown in figure12.As the FT cycles of PC increase, the compressive strength of RFAC decreases gradually, showing a quadratic function with the FT cycles of PC (figure12(b)).

Table 2 .
Chemical composition of cementitious materials.

Table 4 .
Limit FT cycles of PC based on RFA class.