Introduction
Under the pressure of population explosion and land limitation, high-rise buildings and underground construction will be widely applied in civil engineering. These buildings are faced with tremendous challenge of fire damage in the process of construction and service. Building fires are frequently reported world-wide in recent years, seriously threatening the safety of lives and properties. Concrete, a good fire-resistant material due to its inherent non-combustibility and poor thermal conductivity, has been widely utilized in civil engineering.
However, when concrete is exposed to heat, chemical and physical reactions will occur at elevated temperatures, such as loss of moisture, dehydration of cement paste and decomposition of aggregate. These changes bring serious damage to micro- and meso-structure of concrete, which follows a mechanical deterioration in concrete and even detrimental effects at the structural level, due to concrete spalling causing steel bars exposure to the flames. From 40s of the last century, studies in the field of residual mechanical properties for concrete have been taken and great developments have been achieved. It is found by Chang et al. [
] that the compressive stress-strain curves become flatter with the temperature increases, meanwhile the corresponding compressive strength and elastic modulus decrease gradually. Furthermore, Rostasy et al. [
] reported that the appearance of micro cracks related to strain incompatibilities between the aggregates (which expand upon heating) and the cement paste (which shrinks upon heating) may also deteriorate the mechanical properties of concrete after heating. Due to the initial cracks that most probably occur at the interface between mortar and aggregate, they might be one of the negative factors to deteriorate the mechanical properties of concrete after elevated temperatures. However, intensive study on mechanical behaviors of mortar-aggregate interface after elevated temperatures is still very limited by now.
At mesoscopic scale, concrete is generally regarded as a heterogeneous composite material consisting of mortar matrix, aggregate and interfacial transition zone (ITZ) between mortar and aggregate [
]. For recycled aggregate concrete (RAC), it is a more complicated case that five phases are included, namely, aggregate, old hardened mortar, new hardened mortar, old ITZ and new ITZ [
]. Differing from the mortar matrix away from aggregate, the mortar around ITZ has the characteristics of high porosity, low density, high water absorption and low strength. Besides, micro cracks, which may result from capillary porosity extension and strain concentration led by dry shrinkage during cement paste hardening, are also highly deficient to cement particles. Some investigations about mechanical behaviors of concrete ITZ have been carried out. Mehta and Monteiro [
] proved that the meso-structure of ITZ weakens concrete. Rao et al. [
] reported that the bond strength between aggregate and mortar is related to additions, the type of mortar and roughness of the surface of aggregate. Akcaoglu et al. [
] investigated the effects of coarse aggregate size and water to cement (
w/
c) ratio of the matrix on the formation of ITZ and on the failure process of concrete under uniaxial compression. Guinea et al. [
] concluded that the compressive and tensile strength and the modulus of elasticity are strongly affected by the quality of the interface, but the fracture energy depends on the shape of the particles. For RAC, Poon et al. [
] reported that both old ITZ and new ITZ play a role in mechanical properties of RAC. Xiao et al. [
] investigated the properties of ITZ in RAC by nanoindentation and concluded that the old ITZ and new ITZ are 40-50mm and 55-65mm respectively and varied with the type of aggregate. However, the effect of ITZ on the mechanical properties of concrete still remains controversy. For instance, Diamond et al. [
] executed SEM technology to observe the interfacial properties of normal concrete and the results indicated that there is little physical basis to conclude that the ITZ weakens concrete and renders it more permeable. However, it is confident to say that conducting the research on the properties of ITZ occupies an important position in the field of meso-scale mechanical behavior of concrete.
Due to the complexity of inner structure of concrete and random distribution of multiple phases, it is difficult to investigate the mechanical behavior of mortar-aggregate interface directly with conventional concrete specimens. Finite element analysis (FEA) combined with antecedent experiment result by SEM or nanoindentation is a common-used method to research on this problem. Xotta et al. [
] established the finite element model on the meso-scale level considering hygro-thermo- mechanical properties of different components in concrete to evaluate the internal stress state which is not visible by macro-scale methods. To investigate the interfacial properties directly, modeled concrete (MC) specimens were applied in many researches. Coarse aggregate is idealized to a round shape and is surrounded by mortar, as displayed in Fig. 1(a). Shah and Winter [
] first proposed the notion of MC with single modeled aggregate, testing on MC and proving that cracks usually initiated at ITZs. Buyukozturk et al. [
] designed the MC with nine modeled aggregates and investigated cracks development and failure process. The concept of modeled recycled aggregate concrete (MRAC) was first proposed by Xiao et al. [
,
] to simplify the real RAC, shown in Fig. 1(b). With the conception of MC and MRAC, attentions can be focused on the interested variables whereas neglecting the effect of other parameters related to geometric schemes on the mechanical behaviors of mortar-aggregate interface. The model has been proved to represent the manifold phases of RAC properly and reasonable to be applied in the study on interfacial properties of RAC [
].
Push-out test is one of meso-mechanical laboratory methods used in evaluating the interfacial properties between different materials [
,
]. For the advantage that measuring the interfacial strength from actual composites directly, the push-out test is more and more recognized by researchers. In this paper, MC and MRAC were applied to simplify normal concrete and RAC. Push-out tests were conducted after elevated temperatures to investigate the mechanical behaviors of mortar-aggregate interface. This research is instructive for further understanding on the deteriorating process of concrete after elevated high temperatures.
Experimental program
Specimen design
MC and MRAC with single aggregate were designed to represent different phases and their connections between each other in concrete. MC and MRAC were designed as square slabs, with dimensions of 120 mm × 120 mm × 30 mm, as displayed in Fig. 2. MC was a simplified model of normal concrete, consisting of a modeled aggregate and mortar with different strength, i.e., M20, M30 and M40 respectively. The modeled aggregate which located in the center of the specimen was a cylinder granite stone with a diameter of 30mm and a height of 30mm. The mortar cast around the aggregate was marked as M20, M30 and M40. For simulating the different phases in RAC and comparing with MC, MRAC was designed as a controlled group to MC. Being similar to MC, there was a single modeled aggregate which has the identical dimensions with that in MC located at the center of the specimen. The old hardened mortar was simplified as a 5mm thick cylindrical vessel surrounding the aggregate, and its strength was designed as M30, whereas the new hardened mortar was marked as M20, M30 and M40, respectively.
Materials and mix proportions
Modeled aggregates used in tests were cored from the plates of granite stone. The fine aggregates selected in the tests were river sand. Mixing water was tap water and an ordinary Portland cement with grade of 42.5 was supplied. Mix proportions of mortar are listed in Table1.
Specimens preparation
At the beginning of the test process, the modeled recycled coarse aggregates (MRCAs), which consisted of modeled aggregate and old hardened mortar, were cast to represent the recycled coarse aggregates in RAC. The modeled aggregate was fixed vertically at the bottom of molds, casting M30 mortar intended to represent old hardened mortar around the aggregate. After being placed in the laboratory for 24 h, the poured plates were cured in a curing room with 20±5°C temperature and 95% relative humidity for 28 days. After that the MRCAs were cored from the poured plate, as is designed with 5mm thick old hardened mortar adhered around it. Following that the modeled aggregates and MRCAs were used to cast MC and MRAC specimens with different mortar, namely M20, M30 and M40 accordingly. Photos of MRCA and casting process of MRAC are displayed in Figs. 3 and 4, respectively. The specimens were covered by plastic films preventing vaporing. Prior to withdrawing molds, the specimens were kept in a laboratory with 20±5°C temperature for 24 h. After removing molds, the specimens were cured in a curing room with 20±5°C temperature and 95% relative humidity for another 28 days. Nine standard mortar cubes for each strength grade with dimensions of 70.7 mm × 70.7 mm × 70.7 mm for measuring strengths and elastic modulus as a reference were cured under the same environment with MC and MRAC. The mechanical properties of mortar were measured at the same time when the MC and MRAC were undergoing the push-out tests and are given in Table 2. For analyzing and discussing smoothly, in the sequel, MC and MRAC are labeled as MC-the strength grade of mortar-elevated temperatures and MRAC-the strength grade of new hardened mortar-elevated temperatures. For example, MC whose strength grade of mortar is M30 and which exposed to 600°C is labeled as MC-M30-600. Besides, specimens with the same strength grade of mortar which exposed to all temperatures are labeled as first two positions in nomenclature mentioned above. For instance, MRAC-M40 means the assemblage of MRACs whose strength grade of new hardened mortar is M40 under all temperatures.
Pre-test under elevated temperatures
A 36 kW AI-518P high-temperature electric furnace was used in this study. The furnace is able to achieve a maximum operating temperature of 1000°C. Prior to the pre-test, the specimens were taken out of curing room and were dehydrated in a drying oven under 105°C temperature for 24 h to remove free water in the specimens completely to prevent the concrete surface spalling. Temperatures were set at 200°C, 400°C and 600°C, and the elevating temperature rate was controlled at 4°C/min. After reaching a target temperature, the temperature was maintained for 1.5 h, 2 h and 2.5 h (soaking period) responding to the target temperature 200°C, 400°C and 600°C. Under this regime, the temperature on the surface can be considered to be the same as the center considering the small dimensions of the specimens. The specimens were cooled down to the room temperature within the furnace by opening the air vent at top of the furnace. Following that the specimens were taken out of the furnace and kept in plastic bags until push-out tests.
Push-out tests
The push-out tests were undertaken with a computer control electric universal testing machine WDW-50kN in Tongji Structural Laboratory. The measurement and loading systems are displayed in Fig. 5. A YHD-30 displacement transducer was installed at the bottom of modeled aggregate with the accuracy of 0.003 mm. The load was applied on the top surface of modeled aggregate uniformly, and the loading rate was controlled at 0.7 mm/min.
Experimental results and discussions
Experimental phenomena
Experimental phenomena of MC
1) MC at ambient temperature
Failure modes of MC with different strength grade mortar at ambient temperature are displayed in Fig. 6. It should be mentioned that the pictures shown are randomly selected from the six samples with the same mortar. With the subjected increasing load, cracks in MC at ambient temperature triggered within ITZ and developed in a short time with clear sounds. The failure shown brittleness since the specimen was split into several pieces immediately after the cracks appeared on the surface of the specimen. With the decrease of the mortar’s strength grade, the number of cracks tended to increase around the mortar-aggregate interface. MC at ambient temperature with different mortar were almost split into 2-3 fractions.
2) MC after 200°C elevated temperatures
Failure modes of MC after 200°C elevated temperatures are demonstrated in Fig. 7. The process of failure process resembles that of MC at ambient temperature. The cracks still penetrated the specimens and split them into parts with clear sounds. Like MC at ambient temperature, the decreasing strength grade increase the number of cracks near ITZs. Under this temperature, most specimens generated cracks that split the specimens. However, it can be found that the widths of cracks of MC after 200°C elevated temperatures are smaller than that of MC at ambient temperature.
3) MC after 400°C elevated temperatures
Failure modes of MC after 400°C elevated temperatures are displayed in Fig. 8. Cracks appeared with the increasing load, but the widths of cracks were smaller than MC at lower temperatures. For MCs with M20 and M30 mortar, failure modes are similar to that of specimens after 200°C elevated temperatures. However, for MC with M40 mortar, new failure mode appeared that the cracks appeared on the surface and did not penetrate the specimens. MC-M40 remained a whole as it failed. Besides, there is no clear sound when the specimens failed. The bond between mortar and aggregate failed and the aggregate was pushed out.
4) MC after 600°C elevated temperatures
Failure modes of MC after 600°C elevated temperatures are shown in Fig. 9. Prior to loading, cracks could be found at the ITZs by naked eyes in the vicinity of aggregates. With the increase of vertical displacement, cracks around the interface connected with each other. For MC-M40, the aggregate was pushed out, and the mortar away from aggregate remained integrated. The bond between mortar and aggregate was completely failed. For MC-M30, the cracks occurred only on the surface and did not penetrate through the height of the specimen. The mortar stayed as a whole when the specimen failed. For MC-M20, several cracks appeared on the surface but only a few of them penetrated the specimen. This phenomenon may be explained that when samples were exposed to heat, strains concentrated in the vicinity of the aggregate since the cement paste shrank because of loss of moisture or dehydration, while the aggregate expanded caused by heat. These strain concentration results in initial damage induced by heating and micro cracks generation and propagation. It indicates that the mechanical properties of the interface between mortar and aggregate were deteriorated before loading. The mortar away from aggregate was less affected by this damage. Besides, mortar with higher strength grade has greater capacity of resisting cracks. These reasons lead to the phenomenon that the mortar of MC with a higher strength grade after a higher temperature tends to remain integrated.
Experimental phenomena of MRAC
Compared to MC, failure process of MRAC is more complicated to predict due to the complex components, that is, five phases of RAC mentioned above. The trend remains the same that with a higher strength grade mortar after a higher temperature, the mortar tends to being integrated. Unlike MC, due to the constituent difference, the initial damage caused by heat may appear at different location and may develop in different patterns with different strength grade of mortar. It should be mentioned that the old hardened mortars of all specimens in this study were all set as M30.
1）MRAC-M40
Typical failure modes of MRAC-M40 after different temperatures are given in Fig. 10. Most cracks generated near old ITZs, and this can deduce that the failure mode essentially related to the relative strength between old ITZ and new ITZ. Besides, the strain concentration due to the different behavior under elevated temperatures may further make the cracks concentrated in the vicinity of aggregate. Except the MRAC-M40-600, MRAC were split into two fractions through the old ITZ and stretch to the edges of the specimens. For MRAC-M40-600, prior to loading it can be seen that cracks at old ITZ connected with each other through old hardened mortar prior to loading.
2）MRAC-M30
Figure 11 shows the typical failure modes of MRAC-M30 after different temperatures. The failure process included three types: bond failure at old ITZ, bond failure at new ITZ and bond failure at both old ITZ and new ITZ. Under most cases failure still occurred at old ITZ, because the initial damage induced by strain difference between aggregate and mortar by heating, especially for MRAC-M30-600, the modeled aggregate was pushed out without obvious cracks propagation in mortar matrix. Another interesting fact is that the cracks of MRAC-M30-600 appeared at an early load stage in the vicinity of the aggregate, and keep connecting with each other. When it comes to fail, the interface had formed a failure face and the rest connected regions were broken with a sudden jolt.
3）MRAC-M20
From the failure modes shown in Fig. 12, it can be concluded that the majority of failures occurred at old ITZs, whereas there are several cracks initiated at new ITZs and lead to the failure. Although the strength of old hardened mortar is higher than that of new hardened mortar, initial damage induced by heat mostly concentrated at old ITZs due to the different behaviors of old hardened mortar and the aggregate under elevated temperatures. Furthermore, the MRCA of MRAC-M20-600, which included modeled aggregate and old adhered mortar, was pushed out as a whole. And the crack development is quite similar to that of MRAC-M30-600. This case indicates that the influence of different elevated temperatures on mechanical behaviors of old ITZ and new ITZ may be different.
In conclusion, for both MC and MRAC, with higher temperature and higher strength of mortar, the specimens tends to remain integrated or the width of cracks tends to be smaller after the aggregate was pushed out. For MRAC, with the decrease of strength of new hardened mortar, cracks may more likely appeared at new ITZs, but the majority of specimens failed because of the cracks initiated at the old ITZs.
Residual load-displacement curves after elevated temperatures
The residual load-displacement curves displayed in Figs. 13 and 14 are the average curves of MC and MRAC with the identical strength grade mortar and after the same elevated temperatures. Although the tests show variations in residual peak load and its corresponding displacement, the similarities of these curves seem remarkable, as demonstrated in Fig. 15. Besides, these curves show obvious nonlinear behaviors in the process of loading. Generally, these curves consist of ascending portion, descending portion and residual portion. In this paper, the deformation stiffness is represented as the secant of ascending portion of residual load-displacement curves. It is found that for MC, the deformation stiffness remained similar with the increase of the elevated temperatures. By observing the failure process of samples, cracks triggered when the load reached about 80% of residual peak load and failed suddenly with clear sound. Whereas for MRAC, the deformation stiffness decreased with the increase of temperature. Besides, the cracks appeared at a lower level of load with the increasing elevated temperatures. After 200°C heating the cracks triggered at about 80% residual peak load. However, after 600°C heating, the cracks occurred when the residual load exceeded 40% of residual peak load and mostly concentrated near the aggregate. This phenomenon may be explained by the complexity of components of MRAC that more weak ITZs involved in the failure process, and the stage where cracks appeared may be one of the reasons of the decreasing deformation stiffness with the increasing elevated temperature. For MRAC-M40, since the strength of new hardened mortar is higher than that of old hardened mortar, the failure mode resembles that of MC, which is elaborated previously, and the deformation stiffness remained similar with the increase of temperature, which also resembles that of MC.
It is widely accepted that the mechanical behaviors of mortar-aggregate interface significantly depend on adhesive force, friction and interlocking force between the mortar and aggregate [
]. Since the profiles of modeled aggregates were uniform, the interlocking force could be ignored. Thus the adhesive force and friction played a role in mechanical behaviors of MC and MRAC. In ascending portion, both the adhesive force and friction transmitted the vertical load into mortar matrix around the modeled aggregate. When adhesive force culminated and began to fail, the load declined rapidly to a relative stable residual value. Due to the similar residual load in residual portion, it can be assumed that the friction between aggregate and mortar with different strength grades remained a similar value after different temperatures heating. Thus, it is instructive to investigate the mechanical behaviors of mortar-aggregate interface, particularly adhesive force, through the residual peak load and its corresponding displacement. In this paper, residual peak load is terms as the maximum load in residual load-displacement curve, and the peak displacement is the displacement at the residual peak load.
Residual peak load and peak displacement
The variations of residual peak load and the peak displacement with the increase of elevated temperatures are demonstrated in Figs. 16 and Fig. 17, respectively. Besides, decrease percentages of residual peak load after different elevated temperatures are listed in Table 3.
From Fig. 16 and Table 3, it can be concluded that for MC and MRAC with different mortar, the general trend that peak residual peak load decreased with the increase of elevated temperatures is obvious. This trend is consistent with the experimental results that the elevated temperatures decrease the strength of concrete [
]. For MC and MRAC with M40, the residual peak load remained relative stable when the elevated temperatures were lower than 400°C, and bond behaviors were severely deteriorated when temperature excessed 400°C. An average 43.5% drop of residual peak load can be found after the samples were exposed to 600°C compared to the residual peak load at ambient temperature. This experimental result is in great agreement with the research by Ma et al. [
] that when the temperature rises to 400°C, micro-cracks in ITZ start to propagate and their intensity increases with temperature. And for MRAC-M40, it has been illustrated that its failure mode is similar to that of MC, thus the trend of residual peak load as a function of temperature is similar to that of MC. Whereas for MRAC-M20 and MRAC-M30, the decrease percentage of residual peak load stayed low when the temperatures were lower than 200°C, but it increased when the temperature exceeded 200°C. The decrease percentages are listed in Table 3. This phenomenon is decided by physical and chemical changes in mortar matrix and interface between mortar and aggregate. Compressive and tensile strength keep constant or even increases slightly below 300°C [
]. Beyond 350°C, the calcium hydroxide decomposes into lime and water or further convents into C-S-H due to the accelerated pozzolanic reactions at a high temperature [
-
]. C-S-H starts to decompose at around 560°C [
] and it decomposes into b-C
2S at 600°C -700°C [
,
], which leads to a dramatic deterioration of mechanical properties of mortar [26]. Besides, the increase of porosity and different thermal strains for mortar matrix and aggregate also result in the deterioration.
On the other hand, for MC and MRAC, at the same elevated temperatures, the residual peak load increased with the increase of strength grade for mortar. Generally, the residual peak load of MC is higher than that of MRAC with the same mix design. However, when the temperature is below 200°C the residual peak load of MC with M20 is lower than that of MRAC-M20. Since the MRAC-M20 tends to fail due to the new weak ITZ, which had been demonstrated previously, the modeled aggregate and the old hardened mortar could be treated as united. Thus the larger surficial area of interface where loads were transmitted may be the reason that the residual peak load of MRAC-M20 is higher than that of MC-M20. However, this effect is counteracted and the residual peak load of MRAC-M20 is lower than that of MC-M20 when the elevated temperature approach to 400°C.
In general, peak displacement of MC tends to decline when the elevated temperatures exceeded 400°C because the residual peak load decreased rapidly in this case. However, peak displacement of MRAC fluctuated in a small range with the increasing elevated temperatures. Peak displacement is influenced by two factors: the residual peak load and the deformation stiffness. Since these two factors are both mutable, the peak displacement is difficult to predict. In essence, the complicated constituents of MRAC is the reason for the vibration of the peak displacement with the increasing elevated temperatures.
In general, the elevated temperatures deteriorate the interfacial properties of both NAC and RAC. The interfacial properties of RAC obviously decreases at an earlier stage under elevated temperatures compared to those of NAC. Besides, after being exposed to 600°C the average residual peak load of MRAC decreases to 57.0%, more remarkably than that of MNAC, whose average decrease percentage is 41.5%. In brief, the interfacial properties of RAC after elevated temperatures are lower than those of NAC, which may indicate the fact that the mechanical properties of RAC may be more sensitive to those of NAC.
Conclusions
Based on this experimental study, some conclusions are summarized as follows:
1) The mechanical properties of interface between aggregate and mortar is significantly influenced by the elevated temperatures, in terms of the failure mode and residual load displacement curves of MC and MRAC. For both MC and MRAC, with the increase of elevated temperatures, the widths of cracks decrease. With a higher temperature and higher strength grade of mortar, the specimens tend to fail as a whole without splitting mortar matrix. For MRAC, the crack initiating evidently depends on the relative strength of old hardened mortar and new hardened mortar. The decreasing strength grade of new hardened mortar tends to generate cracks near new ITZs.
2) Residual load-displacement curves show obvious nonlinearity and consist of ascending portion, descending portion and residual portion. For MC and MRAC-M40, the deformation stiffness remained similar after different elevated temperatures. Whereas for MRAC-M20 and MRAC-M30, the deformation stiffness decreases with the rising of elevated temperatures. This difference may be directly related to the fact that for MRAC cracks appeared at a lower load stage.
3) The mechanical properties of interface between mortar and aggregate are deteriorated by elevated temperatures. For MC and MRAC-M40, the residual peak load remains constant when the temperature is lower than 400°C and decreases rapidly after exceeding 400°C. While for MRAC-M20 and MRAC-M30, the residual peak load decreases as the temperature exceeds 200°C.
4) For MC, the peak displacement declines rapidly when the elevated temperatures exceed 400°C. For MRAC, the peak displacement fluctuates in a small range with the increase of elevated temperatures.
In general, the interfacial properties of RAC after elevated temperatures are lower than those of NAC. For MC, the average residual peak load decreases by 41.7% after 600°C elevated temperature, compared to 57.0% for MRAC. In the future, studies will be carried out to investigate the influence of ITZ deterioration caused by high temperatures on fire-resistant properties of RAC based on these findings.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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