LUAN Haoxiang, WU Jin*, PAN Jiayu

(1. Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; 2. College of Civil and Architecture, Guangxi University of Science and Technology, Liuzhou 545006, China; 3. Jiangsu Huangjiang Construction Group Co., Ltd.,Yangzhou 225200, China)

Abstract: The characteristics of surface appearances, mass loss, relative dynamic modulus of elasticity and strength loss of different recycled aggregate concretes(RAC) exposed to freeze-thaw cycles were analyzed.It was found that the freeze-thaw resistance of RAC could be determined by the recycled aggregate compositions and admixtures. Both the saturation degree and the air void structure were the key factors influencing the freezethaw damage on concrete. Some major proposed freeze-thaw deterioration mechanisms were utilized to interpret the freeze-thaw damage on recycled aggregate concrete. Meanwhile, some potential measures to enhance the freeze-thaw resistance of concrete were summarized and discussed.

Key words: recycled aggregate concrete; freeze-thaw durability; aggregate composition; deterioration mechanism; improvement measures

1 Introduction

With the fast urbanization expansion progress all over the world, a bulk of building wastes were generated during the rapid development period of construction industry. The subsequent treatment cost of the construction wastes increased sharply daily,and environment pollution caused by the construction wastes has become more and more serious. The current major solutions to the demolitions were transported to suburban areas, and almost all of them were taken as the landfill materials. That would lead to the occupation of suburban lands and the contamination of the ground there consequently. However, the waste concrete could be crushed, washed and graded, to produce the recycled aggregate, that can be utilized as one of the raw materials of recycled aggregate concrete (RAC).So that, the RAC technology can realize the recycled utilization of large quantities construction demolition wastes, and can make full use of the limited natural resource and alleviate the environment pollution caused by the building wastes meanwhile. It has been the main trend to develop environment-friendly concrete and actualize the ecological architecture products. With the guidance of national policies and enhancement on the residents’ awareness of environment protection,many scholars have paid more and more attention to the promotion and utilization of the RAC technology. It has become a great trend to recycle the demolition and achieve the environment protection effect.

The mechanical and durability properties of the RAC containing recycled coarse aggregate (RCA) were different from that of natural aggregate concrete (NAC),especially their freeze-thaw durability performance attributed to the porous aggregates and the complicated microstructure of interfacial transition zones (ITZs).Thus, the mechanical and durability properties must be investigated and tested prior to any applications in practice of the RAC. Many researches and experiments had been conducted on the RAC freeze-thaw durability performance. The main purpose of this paper is to make comparisons based upon the analyses on the summary of RAC properties, characterized by surface profiles,mass loss, relative dynamic modulus of elasticity(RDME) reduction and the strength loss after freezethaw cycles. The influencing factors on the freeze-thaw durability of RAC, and some improvement measures could be reflected, which could do some favor to the promotion on the RAC application at cold climate areas.

2 The freeze-thaw durability performance of RAC

Many researches were mainly conducted on the RAC incorporated RCA. Liuet altook the RCA from parent concrete with different strengths as one of the main raw materials. After being immersed in water for 24 h, the RCA could be fully saturated. The RAC specimen were poured adopting normal mixing method. Then the freeze-thaw durability of these RAC could be explored[1]. At present, few studies were conducted on the RAC with its fine aggregate replaced by recycled aggregate. There was not an agreement on that whether the freeze-thaw resistance of RAC incorporated fine recycled concrete aggregate(FRCA) would be improved or weakened. J A Bogaset alexplored the freeze-thaw resistance of RAC containing FRCA[2]. Both of the RACs with recycled coarse aggregate and recycled fine aggregate exhibited good freeze-thaw durability performance. It was so common that recycled aggregates containing waste ceramic components. Since the mechanical properties of the ceramic aggregates were quite different with that of sand and gravel aggregates, the mechanical and durability performances of RAC containing ceramic components were various with RAC consisted of recycled gravel aggregates. César Met alconcluded that the RAC containing ceramic aggregate may be apt for the structure concrete. RAC was more freezethaw resistance than NAC because of its lower scaling rate and narrower cracks[3]. Guet alconducted some experiments on mechanical and durability performance of the RAC containing recycled brick aggregate.Their results revealed that RAC of good freeze-thaw resistance attributed to its small air void spacing factor and average pore diameter, after its inner pore structure was tested and analyzed[4]. Wanget alexplored the freeze-thaw durability of RAC mixed in steel fiber, and its volumetric dosage was 0, 1%, 2%,3%, respectively[5]. After the specimen subjected to freeze-thaw cycles, it was found that the freeze-thaw durability of RAC could be improved with the fiber mixed in. That was consistent with the conclusions drawn by Piotr Berkowskiet al[6].

Hudaet alconcluded that the freeze-thaw resistance performance of RAC would decreased with the RCA replacement ratio increased[7]. Tuyanet alstudied the freeze-thaw resistance, mechanical and transport properties of self-consolidated concrete(SCC) containing RCA. It indicated that the RCA would cause considerable reduction on mechanical and transport properties of SCC. Meanwhile, the mass loss increased obviously with the increment of the freezethaw cycles and the RCA content[8]. It was found that at the certain hydration degree, higher water to cement ratio (w/c) would increase the amount of liquid water available within the pores of RAC. If thew/cratio was determined, the lower hydration degree would decrease the freeze-thaw resistance durability of RAC[9]. S T Yildirimet alstated that the internal curing process after the water ingress into the concrete could enhance the freeze-thaw resistance performance of RAC.Concrete with 50% RCA of semi-saturation could obtain satisfactory freeze-thaw resistance performance,relatively[10].

The saturation degree would be increased with the moisture uptake process during the freeze-thaw cycles due to the high porosity of RAC[11]. If the critical degree of the pore saturation was reached, the surface scaling and internal damage would commence[12,13]. An amount of fly ash and air-entrainment agent admixture would improve the freeze-thaw resistance of RAC[14,15].The freeze-thaw resistance durability of RAC could be improved if the recycled aggregate was pre-wetted before mixture process[16]. It was accepted that if the aggregate replacement ratio was less than 50%,RAC could be suggested to be applied at cold climate areas[17].

2.1 The surface degradation profiles of RAC after exposed to freeze-thaw cycles

The durability of different RACs would be various with each other attributed to the recycled aggregate compositions. In general, frost induced deterioration on concrete was resulted from the internal liquid solid phase transformation under freeze-thaw load. In the superficial of concrete, the progressive removal of small chips or flakes on the paste, and the detachment of small aggregate just beneath within the surface regions, took place continually with the proceeds of freeze-thaw cycles.

The surface profiles of these RACs subjected to freeze-thaw cycles are depicted in the following figures.Even though the cycle numbers were different, it could be distinguished out that the various surface scaling resistance properties of different RACs. All of the surfaces on RAC specimens became increasingly rough with the continuous cycles. Until the end of 100 cycles,many pits appeared on the surfaces of the RAC and pieces of aggregates were scaled from the edges of the specimens, as shown in the Fig.1. Meanwhile, serious cracks occurred and scattered on the surface, penetrated till the another sides of the surface. For RAC with its fine aggregate fully replaced by FRCA, the surface profile in the Fig.2 indicated its better surface scaling resistance, relatively. Some small chips were scaled off,but its specimen edges were almost kept integrated in spite of exposure to 300 freeze-thaw cycles.

Fig.1 The surface appearances of RAC containing RCA after 100 cycles[18]

Fig.2 The surface appearances of HCAE100R after 300 freezethaw cycles[2]

The surface appearances of RAC incorporated recycled brick aggregates were given in Fig.3. After 125 freeze-thaw cycles, the surface scaling developed in the same way as the RAC with RCA. Many cracks appeared, much mortar were scaled off and led to the brick aggregates exposure to ambient directly, which indicated its poor freeze-thaw resistance performance.The surface profiles of RAC containing recycled porcelain or ceramic aggregates were depicted in Fig.4, the surface profiles were comparable with that of RAC with RCA, and many pits appeared after the flakes of mortar scaled off, some large holes indicating coarse aggregates at that position were spalled during freeze-thaw cycles. It can be found that the surface appearances of RAC with fiber mixed in were much more seriously damaged compared with that of other RACs. The mortars and coarse aggregates at the superficial area were detached and scaled off from the deeper zone of the concrete specimen, leaving the fiber admixtures visible directly, as given in Fig.5.

Fig.3 The surface appearances of RAC with the utilization of waste brick aggregates after 125 freeze-thaw cycles[19]

Fig.4 The surface appearances of RAC with the recycled porcelain or ceramic aggregate after 50 freeze-thaw cycles[20]

Fig.5 The surface appearances of the RAC with fiber mixed in[21]

From the above results, it can be accepted that RAC with recycled fine aggregate can keep its surface appearance better than RAC incorporated recycled coarse aggregate after subjected to freezethaw cycles. The surface profiles of RAC containing recycled brick aggregate would be significantly depended on the water-cement ratio and the recycled brick aggregate content. While the surfaces of RAC incorporated porcelain aggregate or ceramic aggregate were comparable with that of RAC containing recycled coarse aggregate, and the surface condition of RAC mixed in fiber was the worst after freeze-thaw cycles.

2.2 The mass loss of RAC after exposed to freeze-thaw cycles

The phenomena such as surface spalling and aggregate removal were often evaluated to be recognized as one of the typical frost damages, which is always characterized by the mass loss ratios of the specimens[22,23]:

where ΔWn(%) is the mass loss of the specimen after freeze-thaw cycles,Wnis the mass of the specimen after freeze-thaw cycles (kg),W0is the mass of the specimen before the freeze-thaw cycles (kg).

Fig.6 The mass loss of the specimens during freeze-thaw cycles(RC-HC-N[1]; HCAE100R[2]; B-30%[4]; R3[5])

As illustrated in Fig.6, the mass loss of all the specimens increased with the progressive freeze-thaw cycles. It can be seen that since the commence to the initial 125 freeze-thaw cycles, the ascend ratios of all these curves were kept at a relative low level, which indicated that these RACs can stay integrated within this period. Until the end of the 125 cycles, the mass loss of RACs were 0.26%, 0.3%, 0.52% and 0.19%,respectively. Once exposed to freeze-thaw cycles, the mass loss on the R3 was more than the other RACs,revealing its relative poor freeze-thaw resistance performance.

With the cyclic load proceeds, a transition point can be observed on the mass loss curve of the RAC R3. The slope of the curve increased sharply after then. While the curve of the RAC RC-HC-N and RAC HCAE100R ascended gently, the curve of RAC B-30%ascended a little bit more obviously than that of RAC RC-HC-N and HCAE100R. As depicted in Fig.6,the RAC R3 lost its mass severely compared with other RACs, especially after 125 freeze-thaw cycles.Until the end of 200 cycles, the mass loss of RAC R3 was 3.01%, much more than that of other RACs.The mass loss ratios were 0.37%,0.24% and 0.7% for RAC RC-HC-N, RAC HCAE100R and RAC B-30%,respectively. It can be seen that the mass loss of RAC HCAE100R kept the smallest until the end of 300 cycles.

The mass loss of RACs containing ceramic aggregates or porcelain coarse aggregate are summarized in Fig.7. For the reason that the specimens were exposed to much less freeze-thaw cycles, much more sever mass loss was observed compared with the results of previous RACs, according to the experiment results available from the references.

Fig.7 The mass loss of the specimens during freeze-thaw cycles(CA-2: 50% recycled ceramic coarse aggregate;FA-2: 50% recycled ceramic fine aggregate; CFA-1: 30%recycled ceramic coarse aggregate and 30% recycled ceramic fine aggregate; C4: 100% recycled ceramic aggregate; RCC-30%: 30% recycled porcelain aggregate;RCC-50%: 50% recycled porcelain aggregate; RCT-30%:30% recycled ceramic aggregate; RCT-50%: 50% recycled ceramic aggregate, mixture proportions data from the references[20,24,25])

As demonstrated in Fig.7, mass loss occurred on almost all RACs during the continuous freeze-thaw cycles. From the commence of freeze-thaw cycles,RACs lost their mass sharply, except the RAC RCT-30% and RAC RCT-50%. While the mass of RAC RCT-50% increased during the initial 25 cycles, and the mass of RAC RCT-30% was nearly kept unchanged within this period. The mass loss curves for these RACs ascended slightly after 25 cycles, except RAC RCT-30%, with its curve ascended much more obviously than the others. After then, all of the mass loss curves were lifted up with a dramatic slope. Therefore, it could be accepted that all of these RACs with recycled coarse ceramic or porcelain aggregates incorporated,would suffer surface scaling or aggregate detachment when subjected to freeze-thaw cycles. The maximum and minimum mass loss were 3.35% and 1.71%, on the RCC-50% and RCT-50%, respectively.

2.3 The RDME reduction of RACs after exposed to freeze-thaw cycles

For the occurrence of internal crack and the disintegration between mortar and aggregate within the concrete, which would cause the decay on mechanical performance such as compressive and split strength, the direct laboratory indicator is the reduction on relative dynamic modulus of elasticity (RDME)[23,26,27], which can be determined based on the fundamental transverse frequency change and figured out according to the following equation.

wherePiis the RDME of the specimens after freezethaw cycles,f0iis the fundamental transverse frequency before the freeze-thaw cycles,fniis the fundamental transverse frequency after freeze-thaw cycles,respectively.

Fig.8 The RDME of the specimens during freeze-thaw cycles(RC-HC-N[1]; HCAE100R[2]; B-30%[4]; R3[5])

It can be found in Fig.8 that the RDME variation curve of RAC RC-HC-N, R3 and HCAE100R,decreased during the whole process until the end of freeze-thaw cycles. Since the commence to the initial 50 freeze-thaw cycles, the downtrend of the curve to RAC RC-HC-N, R3 and HCAE100R were almost at the same step. Subsequently, the RDME of RAC RCHC-N was kept higher than other RACs. While the RDME of RAC HCAE100R and R3 decreased slightly and that of RAC B-30% decreased more obviously meanwhile. A short upward trend occurred on the RDME of RAC B-30% during 75-100 cycles, but a dramatic decline on the curve indicated the serious cracks within the concrete, and the RDME dropped down to 74.8% until the end of 125 cycles. Even short fluctuates appeared during 125-150 cycles and 175-225 cycles, its RDME was 71.1% at the end of 250 freezethaw cycles, much less than the initial value before the freeze-thaw test, and also much less than that of the RAC RC-HC-N, R3 and HCAE100R.

The RDME variations of the RACs containing recycled ceramic aggregates or porcelain coarse aggregate were shown in Fig.9, attributed to the much more dramatic decline on the RDME and the less freeze-thaw cycles that the specimens exposed to. As depicted in Fig.9, the RDME of these RACs declined from the beginning to the end of the freezethaw cycles. A slight decline process occurred on the RDME of RAC FA-2 and RCC-30%, during an short period between the 25-50 cycles. While the RDME curves of other RACs declined sharply. Until the end of each individual experiment, it can be discovered that only RAC RCC-30% and RCC-50% were satisfactory according to the specification limitation, and their RDME were 67.8% and 73.1%, respectively. And their mass losses were less than the specification limitation, so that it can be accepted that only these two RACs were of freeze-thaw resistance referring to the specification requirements[28,29].

Fig.9 The RDME of the specimens during freeze-thaw cycles[20,24,25]

2.4 The strength loss of RACs after freezethaw cycles

With the proceeds of freeze-thaw cycles, the deteriorations on concrete surfaces and cracks within the internal of concrete would develop progressively.The surface deterioration would damage the integrity of the specimen, and the internal crack would loosen the entire structure. The RAC would be proned to the freeze-thaw attack, which would lead to the strength loss of the concrete. The strength loss can be calculated according to the following equation:

where Δfc(%) is the strength loss of the specimens,fcnis the cubic compressive strength of the specimens after freeze-thaw cycles (MPa),fc0is the cubic compressive strength of the reference specimens under standard conditions after 28 curing days (MPa).

As shown in Fig.10, RACs would suffer strength loss after freeze-thaw cycles. Among the column height difference in Fig.10, the diversity of various RACs can be figured out. Also, the slopes of lines in Fig.10 can be the direct indicator revealing the strength loss resistance of RACs subjected to freeze-thaw cycles. The strength loss of RAC containing fiber was 47.23% after 50 freeze-thaw cycles, while that of RAC containing ceramic aggregate was 15.4% at the end of 50 cycles. Only 7.14% of the strength reduced on RAC with recycled clay brick aggregates after 150 cycles,and that was much smaller that of 23.9% on RAC with ordinary recycled coarse aggregates. It is so obvious to distinguish out that, the RAC containing recycled clay brick aggregates would have more strength loss resistance than that of RAC with ordinary recycled coarse aggregates, referring to the slopes of lines in Fig.10.

Fig.10 The strength loss of RAC specimens after freeze-thaw cycles

If RAC was subjected to 250 or more freezethaw cycles, the results can be a reliable reference for its field engineering application[28]. After 250 cycles,the strength loss of RAC containing recycled highstrength coarse aggregate was 8%, which could be the preference choice for RAC engineering structure utilization, combining its mass loss and RDME results.Even though the mass loss of RAC with recycled fine aggregate was the smallest and its RDME entirely conformed to the specification requirements. However,its strength suffered serious loss after exposed to 300 freeze-thaw cycles. And the slope of the line suggested its weaker strength loss resistance performance,compared with that of RAC incorporated recycled coarse aggregate.

2.5 The result discussions and analyses

It is so common that the recycled aggregate contain coarse and fine aggregates, ceramic and porcelain aggregates, waste brick aggregates,et al. According to the methods available from the references, the recycled fine, coarse aggregates and recycled brick aggregates were mainly obtained after jaw crusher and screening machine treatments on the construction demolition waste concretes. The main sources of the waste concrete were concrete buildings,roads or pavements. Recycled ceramic or porcelain aggregates were acquired from wastes of ceramic or porcelain products, after crush and screen processes.The aggregate compositions could be the significant account for the different freeze-thaw resistance of recycled aggregate concretes. There were noticeable differences between the majority properties due to the different sources of the aggregates. The basic mechanical properties of these aggregates were given in Table 1, which can be the references for the further analysis and discussion[3,4,19,20,21].

Table 1 Physical properties of these recycled aggregates

As it can be found in Table 1, the size distribution of these recycled aggregates conformed to the requirements in the specification[31]. It can be found that the water absorption ratio of the recycled brick aggregate was 15.2%, much more than that of the other aggregates. RAC mixed in waste brick aggregate would absorb much more liquid into the specimens during the freeze-thaw cycles. According to the closed container mechanism, liquid would freeze as the temperature decreased, if there was no space for the 9% ice formation expansion. The concrete matrix would crack for that the expansion pressure was much more than the concrete strength. The expansion pressure could increase up to or more than 200 MPa during the liquidsolid transformation process[32,33]. Lots of micro cracks occurred within the RAC incorporated recycled brick aggregates, which was reflected on the dramatic drop on the RDME curve, as depicted in Figs.8-9.

It is clear that the recycled aggregates were different with each other. An old paste layer was attached on the surfaces of the recycled gravel aggregates, while there were bulges on the outer shape of ceramic and porcelain aggregates. The surfaces of ceramic and porcelain aggregates were much smoother than that of the recycled brick aggregates and gravel aggregates. That could be one of the main factors weakening the strength of RAC containing ceramic aggregates or porcelain aggregates during the cement hydraulic and strength formation periods[3,30]. Referring to the microstructure analysis results, cracks on the ITZ in RAC containing ceramic aggregates were denser and wider than other RACs[1,3,19].

Fig.11 The damage in the ITZ of RAC containing ceramic aggregate[3]

As depicted in Fig.11, most cracks occurred at the outer edge of the ceramic aggregates, and propagated perpendicularly to the outer edge. The deteriorations occurred there damaged the internal integrity and reduced the bond effects between mortar and coarse aggregates within the concrete. That was consistent with the results of the mass loss, RDME reduction and strength loss curves given in Figs.6-10. The slopes of the curves to the RAC containing ceramic aggregates or porcelain aggregates were much greater than that of RAC containing gravel and brick aggregates.

As the ambient temperature decreased, the liquidsolid phase transition of water within concrete took place progressively. The volume of solid ice increased gradually firstly, then the ice formation process would stop as the temperature dropped continually. It was reported that there would be no ice formation when the temperature was lower than -50 ℃[35]. The volume expansion of water was close to 9.17%, while that of for deicer salt solution with 0.5% NaCl by mass could reach up to 11.2% at -17.5℃[36]. For fully saturated concrete specimens, there was not enough space within the pore structure of concrete to accommodate the liquid volumetric expansion, and the generated expansion pressure can be the cause of freeze-thaw deterioration.That could crack the paste area within concrete and lead to the deteriorations after freeze-thaw cycles. For the old mortar layer attached on the recycled aggregates and the weak part at ITZ within RAC, the crack and deterioration would be much serious after subjected to freeze-thaw cycles. However, it is seldom that concrete become fully saturated in practial engineering situations. Hence, the freeze-thaw deterioration can be determined by the liquid content after moisture uptake process of concrete[8,33,37-39]. Referring to the critical degree of saturation hypothesis, the freeze-thaw deterioration initiated once the critical saturation degree was reached[40]. The water absorption behavior and the critical saturation degree under freeze-thaw cycles have been investigated and explored. Two-linear model was found on the liquid absorption experiment results[38,41].The experiment results indicated that the critical saturation degree for NAC was approximately 86%-88% under freeze-thaw cycles[42,43]. The liquid ingress within RAC would be much more obvious because of the high liquid absorption behavior of recycled aggregate. Frost deteriorations would commence and develop on RAC when its critical saturation degree was reached during freeze-thaw cycles.

Fig.12 The freeze-thaw deterioration on RAC with recycled coarse aggregate

For partly saturated concrete, as the temperature decreased down to the freezing point, the liquid within the pores froze gradually. The unfrozen liquid would be expelled to flow away from the frozen area.According to the hydraulic pressure mechanism, the average distance between adjacent pores, so-called the spacing factor, was of great importance to the freezethaw resistance of concrete. When the unfrozen liquid was pushed to flow away during freezing process, the hydraulic pressure would be built-up. The pressure increased with the ice formation and the liquid flow processes. If the air-pores were too far from adjacent ones, the hydraulic pressure would increase and exceed the tensile strength of concrete matrix[22]. The hydraulic pressure mechanism revealed the importance of the pore structure within the concrete to its freeze-thaw resistance performance.

Compared with recycled aggregates, the strengths of old mortar layer at the outer of recycled aggregate particles and mortar at the ITZs were much weaker.Crack would appear at the ITZ firstly within RAC during freeze-thaw cycles. As illustrated in Fig.12(a),crack at the paste area indicated the effect from the ITZ on the good cohesiveness between mortar and high-strength recycled aggregate. Probably attributed to the weak strength of the mortar at the ITZ area, the pressure generated during the freeze-thaw cycles lead to the damage at the ITZ within RAC incorporated ordinary recycled coarse aggregate, as shown in Fig.12(b). From the results of RAC containing recycled brick aggregates after freeze-thaw cycles, it could also be observed in Fig.13 that the freeze-thaw deterioration initiated and developed at the cement paste and ITZ area.

The strong cohesiveness between the recycled coarse and fine aggregates during the cement hydration process, attributed to the small fineness modulus of the recycled fine aggregates. The mass loss of the RAC containing recycled fine aggregate was the smallest among these RACs. And its internal micro crack resistance was also excellent, indicated by the slight reduction of RDME after freeze-thaw cycles. That was consistent with the analyses and discussions on the surface profiles of RAC specimens. The strength loss of RAC containing recycled fine aggregate was 42.1%after 300 freeze-thaw cycles, as depicted in Fig.10. Its strength loss line slope was steep relatively compared with that of other RACs, that may be resulted from the higher water absorption and more porosity of the recycled fine aggregate. Moreover, its low mechanical property would also lead to the obvious strength loss on RAC.

Fig.13 The damage in the RAC containing recycled brick aggregate[19]

With the progressive freeze-thaw cycles,the surface mortars were scaled gradually, and the cohesiveness originated from the fibers would be weaken continually. The mass loss of RAC containing fiber increased obviously, especially after 125 cycles,as depicted in Fig.6. Internal crack occurred and developed within the concrete specimen, which could loosen the inner structure and exacerbate the internal deterioration. Even though the RDME reduced slightly, the strength loss was much more serious than other RACs, and its strength loss line slope was the maximum, revealing its poor freeze-thaw resistance performance. The analysis and discussion are consistent with the conclusions drawn by other scholars[6,34].

Many researches have been conducted to study the damage process and the influence factors on the freeze-thaw resistance of concrete. Some mechanisms have been proposed to interpret the freeze-thaw deterioration phenomenon. However, there is not an agreement has been reached to make a fully explanation on this issue. It was accepted that the hydraulic pressure attributed to liquid flow could be the origin of freeze-thaw deterioration. While some believed that the osmotic pressure could account for the freezethaw deterioration. The liquid within the air void froze at a relative higher temperature, while some liquid froze later, due to the pore size or the liquid of deicer salt concentration differences. An energy gradient was built up between the adjacent pores, because the frozen solid in the pores was at a lower energy state, and the unfrozen liquid in other pores was at a higher energy level relatively. Then the liquid would be transported to frozen solid from the pores at adjacent areas motivated by the energy gradient. The balance can be established after the energy gradient was eliminated[2].

This point of view was consistent with ice nucleation effect hypothesis. Ice crystal would attract the liquid moisture in the pores located nearby to migrate towards the ice. The liquid movement also took place within the concrete matrix body[26,55]. However,the liquid migration would be hindered due to the semipermeable concrete matrix, that result in the generation of osmotic pressure. Further cooling would increase the generated osmotic pressure gradually, and lead to the concrete deterioration resultantly[44]. It would be more prone to the frost damage for RAC attributed to the reduced permeability of the RCA, which would restrict the liquid migration between the pores, and brought about the rise on osmotic pressure, that was harmful to its freeze-thaw resistance. Furthermore,as the lightweight aggregate (LWA) or the RCA was introduced in the concrete, the more compact structure can be formed by the internal curing process during the freeze-thaw cycles. It was the moisture transported in the porous aggregates, that was utilized to promote further hydration on the new and old mortars at interfacial areas[10,45,46,47], which could also result in the frost damage due to the limited liquid migration through denser concrete matrix.

Crystallization pressure mechanism has also been proposed to clarify the freeze-thaw deteriorations on concrete. The crystallization pressure induced by ice formation or salt crystallization within the confined air void space can result in pressure establishment,which would damage the concrete matrix during the freeze-thaw cycles eventually. The crystallization pressure generated is associated with the saturation degree, and the interfacial condition between crystalliquid and the geometry of pore distribution within concrete[48]. Under freeze-thaw exposure with the presence of deicer salt, when the temperature dropped gradually, the liquid of low salt concentration and liquid within larger pores would freeze earlier[44,49].The spontaneous crystallization of salt component would occur definitively at the supersaturated system. Correns proposed the crystallization pressure mechanism firstly.It was discovered that the formation and dissolution process of crystal under pressure, and the crystallization pressure could be determined[50]. If the continuous crystallization was restricted within the confined space,the crystallization pressure generated could be high enough to damage the concrete. The crystallization pressure potential can reached up to 16 MPa, which was much more than the tensile strength of concrete[51].Even without the presence of deicer salt, the ice crystallization process would exert crystallization pressure on the concrete matrix, as illustrated in Fig.14. Freeze-thaw cycles would motivate the liquid migration within the pores to establish thermodynamic equilibrium[47]. It was impossible for the crystallization process at under-saturated condition. Such that it was accepted that the pore saturation state could determine the freeze-thaw resistance of RAC.

Fig.14 The schematic sketch of crystallization pressure within unsaturated porous material during freezing[48]

Recently proposed mechanism “glue spalling hypothesis” could also be the mechanism interpretation for freeze-thaw deterioration. After liquid absorption process, liquid would freeze gradually as the temperature decreased during the cooling period.The ice layer would attach on the inner surface of pores and the outer surfaces of concrete. Meanwhile,thermal constriction would occur on the ice layer and concrete matrix. However, the thermal properties of ice mismatch with that of the concrete matrix. Then,the tensile stress generated by inconsistent thermal constriction behaviors of ice layer and concrete matrix,could lead to the removal of mortar layers on concrete.That may be the reason for the occurrence of surface mortar spalling during the freeze-thaw cycles[44].

3 Measures to improve the freeze-thaw durability of RAC

3.1 The air entrainment admixture

According to the characteristics of RAC after freeze-thaw cycles and the proposed deterioration mechanisms, the pore structure of concrete is greatly related to its freeze-thaw resistance performance. The proper air-void system with the high specific surface area, low air-void spacing factor and high-amount of micro-pores within concrete can be achieved with the air-entrainment effect. The liquid absorbed into pores would be more rational during the freezing and thawing period[52-54]. When the temperature dropped down to the freezing point, liquid solid phase transformation process initiated.

The hydraulic pressure generated by the liquid flow process can be released by the larger pores, which can function as chambers attributed to the different thermal expansion coefficient of the ice and the concrete matrix, and the large pores could also mitigate the generated stress. And the liquid permeated from the adjacent pores can also be stored in the larger voids,which could alleviate the degradations on the concrete as the result[26,55].

It can be proved that the air-entrainment admixture have a significant effect on the air voids within the concrete, and also the type of the airentrainment admixture would influence the air content and pore structure[54]. For RAC with RCA, it can be found that the relative cubic compressive strength of air-entrained RAC was comparable with that of NAC,and their stress strain curves at ascend branch were very close to each other, referring to the experiment results[18]. If the fine aggregate was replaced by FRCA,from the characteristics of the RAC specimens after freeze-thaw cycles, it can be indentified that RAC with FRCA would perform better if air-entrainment admixture was mixed in[2].

3.2 The method of surface treatment on concrete

The freeze-thaw deterioration on concrete was determined by the liquid transportation process and moisture content during the freeze-thaw cycles.Whether the surface treatment on the concrete could improve the freeze-thaw resistance require deep investigations[33,39]. Capillary moisture suction is a thermodynamic spontaneous transport process between the outer liquid and the unsaturated concrete matrix through the capillary pores, when the contact angleθwas less than 90°, such that there would be molecular attraction between the liquid and concrete matrix, and the capillary rise would occur simultaneously[39].

The hydrophobic effect from silane lead to the water repelling on the pore walls, and could increase the contact angle up above 90°, such that the hydraulic pressure can be alleviated, and lead to the moisture uptake process being hindered and delayed substantially. Its schematic is demonstrated in Figs.15-16. From practical applications on the concrete surfaces, the hydrophobic effect can be directly observed, as shown in Fig.17.

Fig.15 The illustration of silane pore lining effect[39]

Fig.16 The illustration of silane pore lining effect on concrete surface[39]

When the RAC containing RCA was utilized at practical projects, the moisture uptake process would be much more obvious due to its porous structure,which could result in severe deteriorations on the surface. Referring to the hydrophobic effect after silane treatment on concrete surfaces, the water repelling effect from the hydrophobic barrier on the surface of RAC could also prevent the external water ingress process. According to the proposed mechanisms governing the freeze-thaw damage on concrete, the water uptake process and the liquid content within the concrete were the key factors influencing the freezethaw damage. Such that it can be accepted that the silane treatment can be the measure to enhance the freeze-thaw resistance durability performance of RAC.

Fig.17 Hydrophobic effect from 40% silane treatment on concrete surface[39]

3.3 The internal curing process on the RAC

The density of RCA was lower and the water absorption was higher than natural coarse aggregate because of the old cement mortar adhered on the outer of aggregate particles. The ITZ around RCA could replenish the absorbed water to the old cement layer during the internal curing process. From this point,it could be believed that the RCA was better than natural coarse aggregate due to improved interfacial bond between the aggregates and cement from the old cement layer.

The 50% saturation and 100% saturation of RCA can be achieved after soaked in water for two days and seven days, respectively. The freeze-thaw cycles experiment results indicated that the performances of RAC specimens containing fully saturated RCA were comparable with that of the reference group specimens NAC with NCA. Additionally, it was remarkable to find that the specimens with 50% RCA and 50% saturated performed excellently. While satisfactory results were observed on RAC with 100% saturated RCA after freeze-thaw cycles. It could be the reason that the RAC with 50% or 100% saturated RCA promoted the internal curing process and led to the build-up of more dense and solid ITZ, which improved the freeze-thaw durability resultantly[10].

The salt frost durability and shrinkage performance of the RAC were investigated, with half of its fine aggregate replaced by the LWA[46]. The results revealed that the internal curing process by the incorporation of pre-saturated LWA offer extra water to sustain the cement hydration process. As the result, the cryogenic suction was restricted due to the less permeable cement paste, and brought about the enhancement on the salt frost durability and mitigation on the shrinkage of concrete. Accordingly, it can be accepted that the internal curing process induced by the pre-saturated recycled aggregates could be the method to improve the freeze-thaw durability of RAC.

3.4 The mineral admixtures

The freeze-thaw durability of different concretes were explored with the binding material partly replaced by silica fume. It was demonstrated that airentrained concrete with silica based binding material mixed in could exhibit excellent frost durability[54].That probably attributed by that the air void system of concrete characterized by air-voids, spacing factor and specific surface area was optimized with the combination effects from the air-entrainment and silica based materials. The experiment result of durability factor (DF) could be the proof for the analysis. When exposed to the solution of the same sodium chloride concentration, it can be discovered that the surface scaling from concrete with its cememtitious material consisted of silica fume and cement was much less than that from concrete with cement only[44]. It was reported that concrete with silica fume mixed in could result in a more compacted and denser internal structure,which was beneficial to withstand the attack from the surrounding environment. Silica fume mixed in would prevent and restrict the expansion of concrete effectively, such that the mass loss of concrete could be reduced obviously[56].

With the properties of high water absorption and old mortar layer at the ITZ, dense internal structure could be formed and established attributed to the facilitation effect when the silica based materials was mixed in. Moreover, the expansion inhibition effect from the silica fume would also be favorable to improve the freeze-thaw resistance of RAC.

4 Conclusions

a) The recycled aggregate types and compositions could have a significant influence on the freeze-thaw durability of RAC. After freeze-thaw cycles, the RAC containing fiber mixture would suffer most dramatic mass loss and strength loss, and much reduction on the RDME occurred on the RAC containing waste brick aggregate. And RAC incorporated ceramic type aggregate performed poorer compared with other RACs, relatively.

b) The aggregate properties and outer surface conditions of the recycled aggregates could be the reasons for the different freeze-thaw resistance of RACs, and RAC with recycled aggregates of less water absorption and better surface mortar warp condition would exhibit excellent freeze-thaw resistance performance.

c) According to the proposed mechanisms, the freeze-thaw deterioration was determined by the saturation degree, the pore structure of concrete and the liquid migration within the concrete. The concrete saturation degree was influenced by the moisture uptake during the freeze-thaw cycles, and the pressure from liquid migration process or the liquid solid transition process could be the origin of the freeze-thaw deterioration on concrete.

d) Some improvement measures presented could be references for the RAC practical engineering application. If the silica fume was mixed in, the well air-entrained RAC with proper internal curing process would be of excellent freeze-thaw resistance performance after the hydrophobic treatment on concrete surfaces.

The application of recycled aggregate concrete showed great advantages based on the current research findings and feedbacks from the practical engineering.Considering the limited data available, more effective and feasible improvements on the freeze-thaw resistance of concrete should be explored. Meanwhile,the freeze-thaw resistance performance of concrete,and the deterioration mechanism require further deep investigations and explorations.