J. Cent. South Univ. (2021) 28: 1459-1475

DOI: https://doi.org/10.1007/s11771-021-4710-1

Mechanical properties of polyvinyl alcohol-basalt hybrid fiber engineered cementitious composites with impact of elevated temperatures

WANG Zhen-bo(王振波)¹, HAN Shuo(韩硕)¹, SUN Peng(孙鹏)¹,LIU Wei-kang(刘伟康)¹, WANG Qing(王庆)²

1. School of Mechanics and Civil Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China;

2. School of Civil Engineering, Guangzhou University, Guangzhou 510006, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: In the present study, the mechanical properties of polyvinyl alcohol (PVA)-basalt hybrid fiber reinforced engineered cementitious composites (ECC) after exposure to elevated temperatures were experimentally investigated. Five temperatures of 20, 50, 100, 200 and 400 °C were set to evaluate the residual compressive, tensile and flexural behaviors of hybrid and mono fiber ECC. It was shown that partial replacement of PVA fibers with basalt fibers endowed ECC with improved residual compressive toughness, compared to brittle failure of mono fiber ECC heated to 400 °C. The tension tests indicated that the presence of basalt fibers benefited the tensile strength up to 200 °C, and delayed the sharp reduction of strength to 400 °C. Under flexural load, the peak deflections corresponding to flexural strengths of hybrid fiber ECC were found to be less vulnerable ranging from 20 to 100 °C. Further, the scanning electron microscopy (SEM) results uncovered that the rupture of basalt fiber at moderate temperature and its pullout mechanism at high temperature was responsible for the mechanical evolution of hybrid fiber ECC. This work develops a better understanding of elevated temperature and basalt fiber impact on the residual mechanical properties and further provides guideline for tailoring ECC for improved fire resistance.

Key words: engineered cementitious composites; hybrid fiber; basalt fiber; mechanical properties; elevated temperature

Cite this article as: WANG Zhen-bo, HAN Shuo, SUN Peng, LIU Wei-kang, WANG Qing. Mechanical properties of polyvinyl alcohol-basalt hybrid fiber engineered cementitious composites with impact of elevated temperatures [J]. Journal of Central South University, 2021, 28(5): 1459-1475. DOI: https://doi.org/10.1007/s11771-021-4710-1.

1 Introduction

Engineered cementitious composites (ECC) is a particular type of high-performance fiber reinforced cementitious composite, which features strain-hardening behavior accompanied by build-up of numerous tight cracks [1]. The tensile strain capacity of ECC is generally two orders of magnitude higher than that of conventional concretes. Of special interest is the individual crack designed to first widen steadily to about 100 mm and then maintain up to ultimate tensile strain [2]. More importantly, the crack width of ECC is independent of the deformation of structural member or the ratio of reinforcement. Up to now, the advantages of ECC in terms of impact resistance, energy-dissipating capacity and long-term durability have been verified by numerous investigations [3-5]. Various novel characteristics like low shrinkage and combination between high-strength and high-ductility were sequentially added into ECC materials [6-12], which laid foundation for its wide applications. During the past decades, ECC has received extensive application research such as bridge deck link slabs [13], blast resistant panels [14], jointless concrete pavement [15], artificial dam of underground reservoir [16] and ECC composite beams [17, 18].

With the expanded field applications of ECC materials, its risk of exposure to elevated temperatures and fires during service is increasing. Although the level of early warning and protection against disasters has been improved, the fire and high temperature accidents are still among the most frequent and devastating disasters. This holds even more serious for ECC structures due to the non-fire-resistant attributes of polymeric fibers, which are commonly used as reinforcement in ECC mixtures. Therefore, the fire resistance of ECC should be evaluated to understand its mechanical properties and explosive spalling under high temperatures. For quite some time now, many efforts have been devoted to the high temperature resistance of ECC with polyvinyl alcohol (PVA) fibers only. SAHMARAN et al [19] reported the residual compressive properties of ECC at temperatures up to 800 °C. The residual compressive strength of ECC did not change significantly before 400 °C, but reduced to about 50% and 30% of the initial ambient compressive strength at 600 and 800 °C, respectively. The findings are similar to that of conventional concretes. Next, SAHMARAN et al [20] further evaluated the effects of fly ash and PVA fibers on the high temperature resistance of ECC. It was found that increasing the fly ash content can effectively improve the residual compressive properties in the temperature range of 200-600 °C. The melting mechanism of PVA fibers prevented the occurrence of explosive spalling of ECC at high temperature. YU et al [21] and YU et al [22] investigated the impact of heating-cooling regimes and curing ages on the compressive properties of ECC after exposure to elevated temperature. The test results indicated that extended heating duration enhanced the compressive strength of ECC at 200 °C, but the compressive strength was continuously reduced beyond 200 °C, which was related to the complex physical and chemical changes inside the material. MECHTCHERINE et al [23] explored the in-situ and residual tensile properties of ECC at temperatures of 22, 60, 100 and 150 °C. According　to the test results, both of the in-situ and residual tensile strength encountered a gradual decline with increasing temperature. The residual tensile strain capacity of ECC decayed linearly with high temperatures, whereas the in-situ strain capacity was improved within temperature range of 22-100 °C. Unfortunately, the test temperatures applied by MECHTCHERINE et al [23] were lower than the melting point of PVA fibers (240 °C), so fiber melting mechanism was not included in this research. BHAT et al [24] further raised the test temperature up to 600 °C, and examined the tensile performance of ECC at high temperatures. The measured tensile stress-strain curves revealed that the strain-hardening characteristics of ECC were maintained up to 200 °C, with reduced strain capacity. At 200-600 °C, the strain-hardening features of ECC disappeared.

Blending different types of fibers into ECC matrix turn out to be an effective solution to obtain an optimal balance between distinct material properties [25-27]. For instance, it has been recognized that fiber reinforced cementitious composites with high modulus fiber has high strength but low strain capacity, such as steel and carbon fibers, whereas those containing relatively low modulus fibers, such as PVA and PE fibers, behave in the opposite manner [25, 26]. Regarding the deterioration of high-temperature behavior induced by polymetric fiber melting, blending fibers with high melting point (such as steel fiber and basalt fiber) into ECC were proposed and tested [30-32]. LIU et al [30, 31] investigated the fire resistance of ECC with hybrid PVA and steel fibers. It is indicated that hybrid fiber ECC outperformed conventional concrete in terms of compressive strength reduction with elevated temperature. The tensile curves of ECC exposed to 300 °C did not show brittle failure due to the presence of steel fibers. POURFALAH [32] conducted compression and flexural tests on hybrid ECC using PVA fibers and steel fibers at temperatures ranging from 20 to 600 °C. The hybrid fiber ECC was found to produce less debris under compression from 200 to 600 °C, whereas the flexural tests disclosed that the strength and deflection were less vulnerable up to 100 °C compared to ECC counterparts. However, to the best knowledge of the authors, no work has been done on the fire resistance of ECC with hybrid PVA and discontinuous basalt fiber up till now, which is not in　line with the increasing interest in using mineral basalt fibers. The basalt fibers are manufactured from volcanic rocks originating in high depth underneath the earth surface or on the earth surface as a molten magma with high alkali resistance and melting point between 1500 and 1700 °C [33, 34]. It is characterized by ease to process, high temperature resistance, superior stability, reduced thermal conductivity, and good cost-effectiveness [35, 36], which is expected to be a promising alternative to polymeric fiber. The benefits of the hybrid PVA-basalt fiber system have been evaluated by a few studies at ambient temperature. WANG et al [37] found that hybridization with basalt fiber and PVA fiber equipped ECC with improved tensile strength at small crack width. Besides, partly replacing PVA fiber with basalt fiber may lead to lower material cost, as basalt fiber is cheaper than PVA fiber. The experimental results of LOH et al [38] indicated that inclusion of basalt fiber had better enhancement in the flexural strength than that of PVA fiber, except a compromise in ductility. These findings further strengthen the motivation to explore the elevated temperature performance of ECC made with hybrid PVA-basalt fiber.

In this paper, the residual mechanical properties and microstructural properties of PVA-basalt hybrid fiber reinforced engineered cementitious composites (HyECC) at elevated temperature up to 400 °C were thoroughly investigated. The temperature range was selected based on the consideration that substantial deterioration of fiber or fiber-matrix interface may occur under temperature lower than 400 °C. And the change of fiber or fiber-matrix interfacial properties was of principal interest in the evaluation of mechanical properties at elevated temperatures. Particular attention is given to the basalt fiber content and elevated temperature impact on the compressive, tensile and flexural properties of HyECC. Three basalt fiber contents of 0, 0.5% and 1.0% were set while keeping the total fiber content as 2.0%, and five temperatures of 20 (ambient temperature), 50, 100, 200 and 400 °C were imposed in the experimental program. In addition, scanning electron microscope (SEM) was employed to assess the microstructure evolution associated with mechanical properties. The results are essential in order to develop a better understanding of elevated temperature and basalt fiber impact on the residual mechanical properties and to further provide guidelines for tailoring ECC for fire resistance.

2 Experimental program

2.1 Materials and specimen preparation

The cementitious materials used in ECC and HyECCs comprised of ordinary Portland cement (OPC) type 42.5, Class I fly ash (FA) and expansion agent (EA). The chemical compositions of cementitious materials (OPC, FA and EA) are given in Table 1. As fine aggregate, silica sand with grain size ranging from 75 to 150 μm was used. Amongst, fly ash was included to promote the secondary hydration and modify the PVA fiber pullout behavior. The expansion agent was used to compensate the relatively large shrinkage of ECC mixtures related to the exclusion of coarse aggregates and higher binder content. Typically, the ultimate drying shrinkage strain of ECC is approximately three times that of concrete [6, 7], which tended to cause seriously early-age cracking of structures made or strengthened with ECC [39, 40]. In our earlier studies, ECC with characteristics of low shrinkage was achieved primarily through tailoring binder composition with the use of expansion agent [41]. The present study followed the developed mix proportion containing expansion agent, but with a focus on the fiber hybridization impact on the mechanical properties at elevated temperature.

Table 2 demonstrates the mix proportions of ECC and HyECCs, in which fiber content is expressed as volume fraction of the mixtures and the other ingredients are expressed as weight proportion of cementitious materials. In mix design, the total volume fraction of fibers was maintained as 2.0%, while the PVA fibers were partially replaced with basalt fibers at volume fractions of 0.0%, 0.5%, and 1.0%, respectively. Thus, three test series were produced as summarized in Table 2, which were designated as ECC, HyECC1 and HyECC2,　respectively. The PVA fibers and basalt fibers were manufactured by Kuarary Company (Tokyo, Japan) and Dengdian CBF Company (Zhengzhou, China), respectively. The dimensional information and mechanical properties of the fibers are presented in Table 3.

Table 1 Chemical composition of ordinary Portland cement, fly ash and expansion agent (wt%)

To produce ECC and HyECC specimens, the cementitious materials and silica sand were first dry-mixed together for 1 min in a concrete mixer of 30 L capacity. Water was slowly added with superplasticizer into the dry mixture and wet-mixed for another 3-5 min which results in an appropriate fluidity. During this period, the bottom of the mixer should be checked and scraped manually. After that, the fibers were slowly dispersed into the slurries and stirred at a higher speed for another 3 min to obtain uniform distribution.

The fresh ECC mixtures were cast into greased molds by layers and moderately vibrated on a vibration table. After finishing the surface, the specimens were sealed with polyethylene sheets to prevent moisture loss and stored for 24 h at room temperature prior to demolding. Then all the specimens were conditioned in a standard curing environment ((20±2) °C and >95% humidity) up to 28 d.

For each mixture, compression, tension and flexure tests were conducted to obtain the corresponding residual mechanical properties after various temperature treatment. Cube specimens with a dimension of 70.7 mm×70.7 mm×70.7 mm were used for compression testing, dog-bone specimens recommended by the Japan Society of Civil Engineers (JSCE) [42] were prepared for tension testing, and prism specimens with a dimension of 40 mm×40 mm×160 mm were fabricated for flexure testing, respectively.

2.2 Test procedure

In this study, the compression, tension and flexure specimens were exposed to the designed temperatures of 50, 100, 200 and 400 °C, respectively. To do so, a series of cured specimens were directly placed into the electric furnace with a chamber of 16 L capacity. The heating regime adopted for the specimens is shown in Figure 1. A constant heating rate of 10 °C/min was implemented until the target temperature was attained. The heating rate was selected to build moderate temperature increase within ECC specimen, as excessive rate may create temperature shock. Subsequently, the specimens were held at the designed temperatures for 1 h to ensure a uniform temperature. After that, all the specimens were allowed to naturally cool in the air to ambient temperature. For comparison purpose, control specimens not exposed to elevated temperatures were also prepared and tested under ambient temperature (20 °C).

The cube specimens were compressed at displacement control on a servo-hydraulic testing machine with 3000 kN capacity. The displacement increment at a rate of 0.5 mm/min was adopted for all compression tests. The compression test set-up is shown in Figure 2. During the loading process, the stress and vertical displacement of the crosshead were recorded simultaneously using a data acquisition system connected to the control system.

The tensile specimens were tested in uniaxial tension on a 300 kN capacity MTS material testing system with hydraulic wedge grips. Prior to loading, aluminum plates were epoxy glued onto the ends of the specimens to strengthen the ends for gripping. The actuator displacement rate used for controlling the test was set to be 0.15 mm/min. Two extensometers with a gage length of 80 mm were positioned on both sides of the specimen to measure the axial strain. The tensile test set-up and the geometry of dog-bone specimen are shown in Figure 3. The stress and strain were recorded using a data acquisition system connected to the control system.

Table 2 Mix proportions of ECC and HyECCs

Table 3 Dimensional information and mechanical properties of PVA and basalt fibers

Figure 1 Heating regime adopted for specimens

Figure 2 Compression test set-up

Three-point bending tests were carried out on the prism specimens by using a MTS material testing machine with a capacity of 300 kN. The actuator displacement at a rate of 1.5 mm/min was used for controlling the test. Each beam was tested with a clear span of 100 mm. The geometry of the prism specimen and the experimental setup are presented in Figure 4. Both the load applied and the mid-point deflection were measured and recorded during the test to evaluate the residual flexural behavior.

The scanning electron microscope (SEM) of Hitachi SU8020 was used to investigate the microstructure of ECC and HyECC mixtures after exposure to elevated temperatures. All the samples were taken from the dog-bone specimens after exposure to various temperatures. Then they were pretreated with metal coating to get a conductive surface. The morphology of the mixtures was observed with focus centered on the fiber-matrix interface and fiber surface characteristics. SEM tests were carried out at 20.0 kV accelerated voltage.

Figure 3 Tension test set-up (a) and geometry diagram (b) of dog-bone specimen (Unit: mm)

Figure 4 Flexure test set-up (Unit: mm)

3 Results and discussion

3.1 Influence of elevated temperatures on compressive properties

Typical compressive stress-strain curves of ECC cubes before and after exposure to 50, 100, 200 and 400 °C are presented simultaneously in Figure 5. Amongst, the strain is derived from the vertical displacement of crosshead. Herein, the compressive　 stress-strain curves serve as qualitative representation of ECC’s deformation properties, which allow for evaluating the role of elevated temperature and basalt fiber content.

Figure 5 Effect of elevated temperatures on compressive stress-strain curves of ECC

As seen, the elevated temperature has a significant impact on the compressive properties of ECC cubes. At ambient temperature, the compressive curve of ECC specimen exhibited a gradual stress drop after peak stress, which is consistent with the previous investigation [9]. After moderate temperature treatment (exposure to 50 and 100 °C), increased peak stress and steeper stress drop were resulted. Up to 200 °C, the ECC specimens approached a reduced peak stress close to the ambient compressive strength but still a steep stress drop at the descending stage. By contrast, the specimens heated to 400 °C suffered a sharper stress drop roughly half of the peak stress. It can be seen that elevated temperatures appear to impair the role of PVA fibers in bridging cracks during stress drop stage. Beyond the melting point of PVA fibers, the bridging effect tended to be eliminated gradually. Therefore, the failure mode of ECC cubes changed from ductile failure to brittle failure.

The compressive stress-strain curves of HyECC cubes before and after exposure to 50, 100, 200 and 400 °C are shown in Figure 6. Compared to ECC mixtures with PVA fibers only, the HyECC mixtures displayed lower peak stress. This is probably attributed to the larger number of voids and bubbles incorporated by basalt fiber during mixing process, see Figure 7. As its diameter (17 mm) is much thinner than that of PVA fiber (39 mm), the quantity of basalt fiber is much larger than that of PVA fiber at the same volume fraction. Moreover,　this event is detrimental to the fiber dispersion. However, partial replacement of PVA fibers with basalt fibers equipped ECC with improved residual compressive properties in terms of toughness. It is noted that the descending stage after peak stress became more stable due to incorporation of basalt fibers. Even if the treated temperature exceeded the melting point of PVA fibers, the HyECC cubes still exhibited ductile failure, and this case became more pronounced when the basalt fiber content increased. It can be evidenced by the crack patterns of different specimens after subjected to compression tests (Figure 7(b)), since multiple finer cracks and more tortuous crack paths were observed on HyECC cubes exposed to 400 °C.

Figure 6 Effect of elevated temperatures on compressive stress-strain curves of HyECCs:

The residual compressive strengths of three mixtures after exposure to various temperatures are determined and listed in Table 4. Herein, each data presents an average value from three specimens. Figure 8 displays the influence of elevated temperatures on the residual compressive strength of ECC and HyECCs, where the error bars represent the standard deviation of each test data. As seen,　the residual compressive strength of ECC, HyECC1 and HyECC2 mixtures experienced a similar development trend, i.e. initial increase up to 50 °C, sequential decrease till 200 °C and slight increase again up to 400 °C. At 50 °C, the residual compressive strengths of ECC, HyECC1 and HyECC2 were increased by 28.8%, 19.3% and 25.2% compared to their irrespective ambient strength. The significant improvement might be related to the accelerated hydration of cementitious materials due to moderate temperature treatment, which results in a denser microstructure [43]. This observation also hints that a moderate temperature treatment at 50 °C is beneficial for enhancing the compressive strength of ECC and HyECCs [42]. After exposure to higher temperatures of 100 and 200 °C, build-up of vapor pressure created internal cracks, which contributed to the decrease in compressive strength [44]. It is interesting to find that the residual compressive strength of ECC, HyECC1 and HyECC2 after exposure to 400 °C was respectively increased by 17.7%, 14.4% and 14.8% compared to that of 200 °C. This can be explained by the fact that when the heated temperature goes beyond melting point of PVA fibers, the resulted channels provided escape paths for water vapor and alleviated the internal pressure induced damage [44]. Accordingly, the HyECC mixtures with lower PVA fiber content exhibited less increment in compressive strength. Besides, it should be pointed out that the residual compressive strength of the mixtures over all range of temperature exposures is higher than their ambient compressive strength. Neither visible cracks nor spalling of cubes were found after heating treatment.

Figure 7 ECC and HyECC cubes after compression test:

Table 4 Residual compressive strength of different mixtures

Figure 8 Influence of temperature on compressive strength

3.2 Temperature impact on tensile performance and microstructural properties

Typical tensile stress-strain curves of ECC dog-bone specimens before and after exposure to 50, 100, 200 and 400 °C are shown in Figure 9. In addition, the SEM images of ECC after exposure to elevated temperatures are shown in Figure 10, to evaluate the microstructure evolution of tensile specimens.

As expected, ECC without heat treatment exhibited typical strain-hardening and multiple cracking features under tensile load. It is in accord with the failure patterns that a multitude of closely spaced micro-cracks form in ECC composites, see Figure 11(a). The ultimate tensile strain in excess of 3.0% was achieved by ECC at ambient temperature. SEM image in Figure 10(a) shows that, PVA fibers were mainly pulled out from the matrix, with moderately abraded ends. The tensile properties of ECC appeared to be more sensitive to the elevated temperature than compressive properties. After heated, the ECC specimens suffered an obvious reduction in tensile strain capacity, attributing to the deterioration of fibers and interfacial properties [43]. However, the strain-hardening characteristics of tensile stress-strain relation were retained up to 200 °C. At 400 °C, the melting of PVA fibers rendered ECC specimens absent of reinforcement (Figure 10(d)), thus resulting in brittle rupture like normal mortar behaved. The ECC specimen in Figure 11(b) illustrated singe crack with smooth fracture surface. Comparison of stress-strain curves shown in Figure 9 provides a more distinct variation of their tensile properties.

Figure 9 Effect of temperature on tensile stress-strain curves of ECC with PVA 2.0%

Typical tensile stress-strain curves of HyECC specimens before and after exposure to 50, 100, 200 and 400 °C are shown in Figures 12 and 13. The SEM images of HyECC1 after exposure to elevated temperatures are shown in Figure 14.

Figure 10 SEM images of ECC mixtures after exposure to elevated temperatures:

Figure 11 ECC and HyECC specimens after tensile test:

It can be seen that partial replacement of PVA fibers with basalt fibers attains to reduced ductility but increased tensile stress of ECC at ambient temperature. The crack patterns shown in Figure 11(a) indicated weakened multiple cracking behaviors with increasing content of basalt fibers. But increasing basalt fiber content created higher tensile stress. As mentioned above, incorporation of basalt fibers led to a much larger quantity of fibers than using only PVA fibers at the same volume fraction. Preliminary studies on basalt-ECC [33, 45] indicated that basalt fiber possesses a lower elongation but a higher chemical bond composites compared to PVA fiber. As a result, basalt fiber tended to be ruptured directly instead of pulled-out from the matrix, which was clearly illustrated in Figure 14(b). This mechanism helps to provide enhanced fiber bridging stress. However, it might lead to reduced complementary energy in crack bridging law and decreased strain-hardening potential in stress-strain relation [46].

Figure 12 Effect of temperature on tensile stress-strain curves of ECC with 1.5% PVA-0.5% basalt

Figure 13 Effect of temperature on tensile stress-strain curves of ECC with 1.0% PVA-1.0% basalt

Figure 14 SEM images of HyECC1 mixtures after exposure to elevated temperatures:

For HyECC1 mixtures, the tensile stress-strain curves generally exhibited “double-peak” characteristics up to 200 °C. The basalt fibers mainly contributed to the first peak which governed the tensile strength. And the second peak is controlled by the PVA fibers which possessed more superior fiber-matrix interfacial properties. At 50 °C, the second peak of stress-strain curve gained a slight enhancement compared to that of ambient temperature. It is the fiber-matrix interaction rather than the fiber strength that governs the bridging properties of PVA fibers. Thus, the bridging stress of PVA fibers increased, as a response to the matrix enhancement discussed in Section 3.1. This mechanism is believed to be correlated with the peeling end of PVA fiber shown in Figure 10(b). After exposure to 100 and 200 °C, the second peak experienced an obvious decline, mainly attributed to the thermal damage induced by internal vapor pressure [44]. As evidence, loose interfaces caused by enlarged space between PVA fiber and matrix were observed in Figure 10(c). Up to 400 °C, the tensile stress-strain curves of HyECC1 demonstrated brittle feature similar to that of ECC. The basalt fibers with relatively smooth surface were pulled out from loose matrix, as seen from Figure 14. Although not melted, the basalts fibers only appeared to provide limited bridging effect.

Compared to HyECC1, the “double-peak” feature of tensile stress-strain curves of HyECC2 was weakened. It is reasonable because the content of PVA fibers decreased, which is closely related to the magnitude of the second peak stress. After exposure to elevated temperature of 50 and 100 °C, the bridging stress provided by both basalt fibers and PVA fibers suffered a continuous reduction. As the temperature heated to beyond 200 °C, tensile brittle rupture occurred. This is probably due to the reduced content of PVA fibers, the internal cracks induced by vapor pressure and the looseness of fiber-matrix interfacial transition zone [43, 44].

The residual tensile strength and peak strain corresponding to tensile strength of different mixtures are determined and summarized in Table 5. Figure 15 displays the variation of residual tensile strength and peak strain of ECC and HyECCs with elevated temperatures. From Figure 15(a), it can be seen that the presence of basalt fibers equipped ECC with enhanced tensile strength at ambient temperature. And the ambient tensile strength was enhanced with increasing content of basalt fibers. In particular, the HyECC1 and HyECC2 achieved 19% and 28% increase in tensile strength relative to that of ECC. As treated temperature raised, the residual tensile strength of all the three mixtures encountered a decline trend. The mono PVA fiber ECC only received a significant reduction in tensile strength until 200 °C, attributing to the damaged interface (Figure 10(c)). The incorporation of basalt fibers seemed to delay the occurrence of sudden strength reduction to 400 °C, as the basalt fibers helped to bridge the cracks at 200 °C. It is noted that the residual tensile strength of HyECCs prevailed that of ECC within temperature range 20-200 °C.

Table 5 Residual tensile strength and strain capacity of different mixtures

Figure 15 Influence of temperature on tensile strength and peak strain:

As seen from Figure 15(b), the peak strains of ECC and HyECCs exhibited totally different variation characters with elevated temperature. Heat treatment played a decisive role in reducing the tensile ductility of ECC. At 50 and 100 °C, the peak strains were decreased by 57% and 76% compared to that of ambient temperature. Up to 200 °C, the peak strain was maintained as 0.44%, which was still about 40 times that of normal concrete. Prior to melting of PVA fibers, the peak strains of ECCs were almost one order of magnitude higher than that of HyECCs. It indicates that partial replacement of PVA fibers with basalt fibers tended to eliminate the tensile ductility of ECC. Similar results of mixture containing 1.5% PVA and 0.5% basalt fibers were observed in Ref. [37]. On the one hand, the strong bonding strength between basalt fiber and matrix and high strength of basalt fiber endowed ECC with enhanced tensile strength. On the other hand, the rupture of basalt fibers and the associated early exhaustion of strength capacity of PVA fibers contributed to catastrophic tensile strain capacity [37]. Orienting the high-temperature-performance based ECC design, an optimizing strategy towards balance between enhancement of high-temperature resistance and compromise of ambient temperature ductility might be needed. As discussed above, the peak strains of HyECCs were mainly governed by the bridging effect of basalt fibers. Therefore, tailoring was recommended on basalt fiber surface to optimize the fiber bridging properties associated with tensile strain capacity.

3.3 Influence of elevated temperatures on flexural performance

The flexural stress-deflection curves of ECC prisms before and after exposure to 50, 100, 200 and 400 °C are displayed in Figure 16. As seen, ECC specimens at ambient temperature demonstrated distinct deflection-hardening and multiple cracking properties (Figure 17) after first crack occurred, which was in line with the tension testing results.

Figure 16 Effect of temperature on flexural stress-deflection curves of ECC with PVA 2.0%

Similarly, the elevated temperatures imposed an important change to the flexural properties of ECC. Within temperature range from 50 to 200 °C, the deflection-hardening and multiple cracking features were continuously weakened, mainly attributing to the abovementioned deterioration mechanism. The deflection-hardening behaviors were still retained in some flexural curves at 200 °C. And the gradual stress drop after second peak suggested that the PVA fibers were pulled out from damaged interfaces. Up to 400 °C, the melting of PVA fibers caused an obvious brittle failure.

The flexural stress-deflection curves of HyECC specimens before and after exposure to 50, 100, 200 and 400 °C are shown in Figures 18 and 19. As seen, incorporation of basalt fibers altered the flexural performance of ECC significantly. Increasing the content of basalt fibers induced higher first cracking stress but weakened deflection-hardening features. As abovementioned, basalt fibers helped to provide stronger fiber bridging effect across the crack, which might create enhanced crack bridging strength but lower strain capacity of ECC.

From Figure 18, moderate deflection-hardening properties were still maintained for HyECC1 mixtures at ambient temperature. It is suggested that the first cracking stress was mainly governed by basalt fibers, and the deflection-hardening peak was controlled by PVA fibers. This can be deduced from the tensile test results that basalt fiber only took effect within a small crack opening, but PVA fiber still behaved effectively in a relatively wider crack. After exposure to elevated temperatures, the peak stress and deflection-hardening properties suffered a pronounced decline. Up to 200 °C, the deflection-hardening behaviors of flexural curves were diminished. At 400 °C, the flexural stress-deflection curves of HyECC1 exhibit brittle feature similar to that of ECC.

For HyECC2 mixtures, the deflection-hardening feature of flexural curves was further weakened, compared to that of HyECC1. To be specific, the first peak governed by basalt fibers prevailed the subsequent peak stress contributed by PVA fibers. The decreased content of PVA fibers reduced the magnitude of the second peak. After exposure to elevated temperature of 50, 100 and 200 °C, the bridging stress provided by both basalt fibers and PVA fibers suffered a continuous reduction, thus resulting in lower flexural stress. At 400 °C, the flexural stress-deflection curve encountered a sharp stress drop after peak, followed by a marginal stress tail as a sign of toughness. Combined with the observation in Figure 14(d), the pulled-out basalt fibers surrounding the channels left by melted PVA fibers, might be responsible for the moderate improvement.

The residual flexural strength and peak deflection corresponding to the flexural strength of different mixtures are determined and listed in Table 6. Figure 20 shows the variation of residual flexural strength and peak deflection of ECC and HyECCs along with elevated temperatures. As seen from Figure 20(a), ECC possessed the highest ambient flexural strength amongst all the three mixtures. Flexure load is found to be more favourable for the occurrence of multiple cracking and strain-hardening behaviors than tension load [47]. The build-up of multiple fine cracks allowed more PVA fibers to participate in the loading process, and enhanced fiber bridging effect was motivated, thus creating higher flexural strength. For HyECC1 specimens, the decreased content of PVA fibers resulted in weakened deflection-hardening properties and then lower flexural strength. Comparing to HyECC1 specimens, the flexural strength of HyECC2 specimens obtained a slight increase. Based on the above analysis, the flexural strength of HyECC2 was derived from the first peak stress. Hence, increased content of basalt fibers contributed to increased flexural strength of HyECC2. After heated, the flexural strength of all the three mixtures experienced a decrease trend. Up to 200 °C, the flexural strength of ECC encountered a sudden drop, owing to the alternation from deflection-hardening to double peak characteristics.

Figure 17 ECC and HyECC specimens after flexure test:

Figure 18 Effect of temperature on flexural stress-deflection curves of ECC with 1.5% PVA-0.5% basalt

Figure 19 Effect of temperature on flexural stress-deflection curves of ECC with 1.0% PVA-1.0% basalt

As seen from Figure 20(b), the peak deflection of ECC and HyECCs demonstrated totally different responses to elevated temperatures. Heat treatment induced a substantial reduction to the peak deflection of ECC. At 50 and 100 °C, the peak deflections were decreased by 60% and 62%, respectively, relative to that of ambient temperature. After heated up to beyond 200 °C, the peak deflection approached about 0.20 mm, as a result of deflection-softening properties. By contrast, the peak deflections of HyECC1 displayed a stable variation trend with temperature ranging from 20 to 100 °C. It means that heat treatment plays a negligible role in the flexural ductility of HyECC1 within 20 to 100 °C. In other words, partial replacement of PVA fibers with moderate content of basalt fibers (HyECC1) equipped ECC with appropriate flexural ductility till 100 °C. Comparable flexural results of mixture with identical fiber content were also observed in Ref. [37]. For HyECC2 specimens, the heat treatment only induced marginal change of peak deflections ranging in 0.16-0.28 mm. The underlying mechanism behind was similar to that of tensile performance, which had been discussed in Section 3.2.

Table 6 Residual flexural strength and peak deflection of different mixtures

Figure 20 Influence of temperature on flexural strength and peak deflection corresponding to flexural strength:

4 Conclusions

In the present research, the mechanical performances and microstructural properties of PVA-basalt hybrid fiber reinforced engineered cementitious composites after exposure to elevated temperatures are thoroughly investigated through an experimental program. Special focus is centered on the basalt fiber content and temperature effect on the compressive, tensile and flexural properties. Therefore, three basalt fiber contents of 0, 0.5% and 1.0% are selected while keeping the total fiber content at 2.0%, and five temperatures of 20, 50, 100, 200 and 400 °C are imposed in the experimental program. From the results of this investigation, the following conclusions can be drawn.

1) It is noticed that the compressive properties of ECC at 400 °C are impaired by the melting of PVA fibers, resulting in compressive brittleness. However, partial replacement of PVA fibers with basalt fibers equipped ECC with improved residual compressive properties after heated, and ductile failure patterns are observed on HyECC cubes exposed to 400 °C. In addition, a moderate temperature treatment is beneficial for enhancing the compressive strength of ECC and HyECCs.

2) Comparing to the compressive behaviors, the tensile performance of ECC is more sensitive to the elevated temperature. After being heated, the ECC specimens suffer an obvious reduction in tensile strain capacity, but the strain-hardening characteristics are retained up to 200 °C. Incorporation of basalt fibers achieve reduced strain capacity but increased tensile strength of ECC up to 200 °C. And the occurrence of sharp strength reduction is delayed to 400 °C. Modification on basalt fiber surface is suggested to optimize the fiber bridging properties and tailor ECC with improved fire resistance.

3) In flexure tests, the deflection-hardening and multiple cracking features of ECC are continuously weakened within temperature range of 20-200 °C. Replacement with basalt fibers in 1.0% volume fraction renders ECC with a moderate flexural ductility, which is less vulnerable within temperature range from 20 to 100 °C. Up to 400 °C, the flexural stress-deflection curves of HyECC encounter a sharp stress drop after peak, but followed by a marginal toughness tail.

4) From microstructure observation, it is evident that the peeling of PVA fiber ends, looseness of PVA fiber-matrix interface layer and melting of PVA fibers are responsible for the deterioration of ECC and HyECC mixtures. On the other hand, the rupture of basalt fibers at moderate temperature and the smooth surface of basalt fibers surrounding the channels left by melted PVA fibers, are found to be responsible for the improved mechanical properties of ECC exposed to elevated temperatures.

Contributors

The overarching research goals were developed by WANG Zhen-bo, HAN Shuo, and WANG Qing. SUN Peng, LIU Wei-kang and HAN Shuo performed the experiments and provided the measured data. WANG Zhen-bo and HAN Shuo analyzed the measured data. The initial draft of the manuscript was written by WANG Zhen-bo. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

WANG Zhen-bo, HAN Shuo, SUN Peng, LIU Wei-kang, and WANG Qing declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

混杂聚乙烯醇-玄武岩纤维延性水泥基材料的高温后力学性能

摘要：本文试验研究了混杂聚乙烯醇(PVA)-玄武岩纤维延性水泥基材料(ECC)暴露于高温后的力学性能。测试了混杂和单掺纤维ECC分别经历20，50，100，200和400 °C温度后的残余抗压、抗拉和抗弯力学行为。结果表明：相较于单掺纤维ECC在400 °C的脆性破坏，玄武岩纤维部分取代PVA纤维可使ECC获得更高的抗压韧性；在拉伸试验中，玄武岩纤维有利于在20 ℃~200 °C范围内提高抗拉强度，并将强度急剧下降推迟到400 °C；在弯曲荷载下，混杂纤维ECC抗弯强度对应的峰值挠度在20~100 °C范围内变化较小。此外，扫描电镜(SEM)结果显示，玄武岩纤维在中温下的断裂和高温下的拔出机制是混杂纤维ECC力学性能演变的主要原因。本研究可帮助理解高温和玄武岩纤维对ECC残余力学性能的影响规律，并进一步为调整ECC获得改进的耐火性能提供指导。

关键词：高延性水泥基复合材料；混杂纤维；玄武岩纤维；力学性能；高温

Foundation item: Project(51808545) supported by the National Natural Science Foundation of China; Project(8184083) supported by the Beijing Natural Science Foundation, China; Project(2021YQLJ05) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-04-01; Accepted date: 2020-07-10

Corresponding author: WANG Zhen-bo, PhD, Associate Professor; Tel: +86-13811446552; E-mail: wangzb@cumtb.edu.cn; ORCID: https://orcid.org/0000-0002-4750-3364