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Article

The Mechanical Properties and Failure Mechanisms of Steel-Fiber- and Nano-Silica-Modified Crumb Rubber Concrete Subjected to Elevated Temperatures

by
Yihong Wang
1 and
Jiawei Chen
2,*
1
School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China
2
College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(23), 16340; https://doi.org/10.3390/su152316340
Submission received: 27 September 2023 / Revised: 1 November 2023 / Accepted: 2 November 2023 / Published: 27 November 2023
(This article belongs to the Special Issue Sustainable Structures and Construction in Civil Engineering)
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Abstract

:
Steel-fiber- and nano-silica-modified crumb rubber concrete (SFNS-CRC), a new kind of environmentally friendly concrete, is characterized by its high performance. It achieves the recycling and reuse of waste rubber and promotes sustainable development in the rubber industry. This study used 12 groups of 288 specimens to study its mechanical properties and failure mechanisms when subjected to elevated temperatures. In the experiments, a heating and loading apparatus invented in our laboratory was used. The chosen crumb rubber concrete contained 5% rubber by volume. Through specimen analysis, the failure modes, mass loss, and compressive and splitting strengths of the specimens, as well as their failure mechanisms, were tested and are discussed while taking into account three variables, namely steel fiber volume ratio (0%, 0.5%, 1.0%, and 1.5%), nano-silica content (0%, 1%, and 2%), and temperature (20 °C, 200 °C, 400 °C, and 600 °C). The test results indicate that the typical damage shapes of CRC subjected to elevated temperatures can be significantly ameliorated through the addition of steel fibers and nano-silica. This can lead to evident improvements in brittle failure and render CRC ductile. Essentially, it improves the integrity of SFNS-CRC specimens. The compressive and splitting tensile strengths of concrete mixtures subjected to elevated temperatures increase with an increase in the steel fiber content. There is an obvious improvement in the compressive strength when subjected to elevated temperatures and after adding nano-silica. The CRC with a content of 1.0% steel fiber is optimal, and the optimal content of nano-silica is 1.0%. In addition, SFNS-CRC performs better in terms of mechanical properties when subjected to elevated temperatures. The splitting tensile strength of SFNS-CRC is improved using steel fibers, and nano-silica plays a crucial role in improving compressive performance. SEM and XRD analyses helped verify the test results.

1. Introduction

Against the backdrop of the development and prosperity of the automotive industry, a multitude of scrap tires have been produced. Every year, more than half of waste tires are discarded with no treatment [1]. Discarding scrap tires without processing has become a global social and environmental problem that does severe harm to people’s lives and belongings, pollutes the environment, and causes massive breeding of flies and mosquitoes. Also, these non-biodegradable waste tires can easily trigger fires [2]. Therefore, it is necessary to raise concerns worldwide to solve this unavoidable black pollution issue.
As an environmentally friendly material, crumb rubber concrete (CRC), also called rubberized concrete, is a kind of cement concrete that contains crumb rubber. Crumb rubber is obtained from discarded waste tires after they are mechanically crushed, ground, treated via dust removal, and cleaned. Previous studies have shown that adding crumb rubber can greatly improve many of concrete’s properties. Khaloo et al. found that an increase in rubber content is conducive to improving concrete’s toughness [3]. Consistently with other scholars’ findings [3,4], Zheng et al.’s study proved that the crack growth rate of CRC is obviously slower than that of plain concrete [5]. Li et al. found that adding crumb rubber can enhance concrete’s energy-absorbing capacity [6]. In addition, CRC has been proven to have several excellent characteristics, such as cracking resistance, improved dynamics and fatigue behavior, outstanding acoustic performance, and electrical resistivity [7,8,9,10]. Therefore, CRC is very beneficial for environmental protection and has significant economic value.
As discussed before, compared with ordinary concrete, CRC possesses multiple advantages. However, the hydrophobic qualities of crumb rubber may contribute to weak contact between crumb rubber and the surrounding mortar. Rubber aggregates possess lower strength and elasticity modulus, resulting in the lower strength of CRC [11], which greatly reduces the use frequency of CRC in structural components. Most applications of CRC are pavement layers, parking lots, tennis courts, etc.
To extend its practical ability and universal applicability in the field of construction engineering, it is vital to explore an approach that improves the performance of CRC. The mechanical properties of CRC have been the subject of recent studies of a number of researchers. Physical and chemical modifications are the two types of methods that can be distinguished. In the field of physical modifications, Hesami et al. used polypropylene fibers in the process of CRC modification. It was found that this substance could greatly improve the compressive strength, flexibility, tensile strength, and abrasion resistance of CRC [12]. Turatsinze et al. discovered that the addition of steel fibers could enhance the performance of CRC, for example, via enhancing its cracking resistance [13]. Using non-destructive evaluation techniques, specifically ultrasonic pulse velocity (UPV) and acoustic emission testing (AE), it was demonstrated that the sensitivity of acoustic emissions to the fracture events that ultimately lead to the destruction of the material can provide an accurate indication of the additional characterization capabilities of the mechanical properties of concrete after it has been subjected to fire [14]. Voutetaki et al. used a wireless structural health inspection (SEM) system to effectively detect fiber concrete cracks, providing a reliable quantitative assessment of damage using statistical index values measured via piezoelectric lead zirconate titanate (PZT) transducers [15]. Xie et al. presented a flexural and compression test of crumb rubber- and steel fiber-reinforced concrete with recycled aggregates [16]. Noaman et al. discovered that steel fibers are an effective additive that can improve compression toughness and prevent brittleness failure through improving concrete’s ductility [17]. Steel fibers have become an increasingly popular additive that is confirmed to be effective in the modification of CRC. It has been established by previous studies that steel fibers are crucial in improving the durability of concrete and preventing crack propagation [18,19]. Other researchers have chemically treated rubber surfaces or enhanced the interface strength between crumb rubber and mortar. Mohamed K. et al. decided to choose metakaolin as an additive to CRC [20], while Thomoglou et al.’s research on adding optimized amounts of carbon microfibers (CFs), polypropylene microfibers (PPs), and carbon microfibers (SWCNTs) and studying them at the nano-, micro- and multi-scale levels showed that with these additions, the flexural strength, toughness, and electrical conductivity of cement-based specimens can be improved [21,22]. Meanwhile, for Fakhri and Saberi, silica fume was a better choice [23]. Nano-silica was also the choice of many researchers, who showed it had an obviously positive effect on improving the strength of CRC [24,25,26,27].
On the other hand, as the economy and society develop quickly, population density also increases and high-rise buildings develop rapidly and become more and more dense. All these factors make it particularly important to pay attention to building fires. Many scholars have pointed out that the mechanical strength of concrete is seriously weakened after exposure to elevated temperatures and during high temperatures [28,29,30,31,32]. Concrete with a dense internal structure can even burst when exposed to high temperatures caused by the fire. This not only threatens structural safety, but also causes incalculable damage to human life and the safety of buildings. Adding crumb rubber to concrete can eliminate the spalling of concrete at high temperatures. Lyu et al. investigated spalling behavior and the microstructural and mechanical properties of UHPC at high temperatures using CR and steel fibers and showed that adding crumb rubber to UHPC can eliminate the spalling of concrete storms at high temperatures. The spalling and mechanical behavior of mixes with or without a CR admixture was also illustrated via microstructural analysis [33]. The concrete structure is the main structural form of contemporary buildings; therefore, its resistance to high temperatures is particularly important for buildings in fire conditions. These conditions objectively require that the concrete material not only has higher strength but also has good fire resistance.
However, there has been no intensive research into the combined modified effects of steel fibers and nano-silica on CRC subjected to elevated temperatures. For this purpose, research on the mechanical properties of steel-fibers- and nano-silica-modified CRC (SFNS-CRC) subjected to elevated temperatures is presented in this paper. The failure mode, mass loss, compressive and splitting tensile strength, and influencing factors analysis were analyzed through experimental research on 12 groups of 288 specimens. In addition, the failure mechanism was analyzed microscopically using XRD and SEM.

2. Experimental Program

2.1. Raw Materials

The test’s raw materials consisted of cement, fine aggregate, coarse aggregate, steel fibers, nano-silica, water, and crumb rubbers with diameters of 1–2 mm. The use of naphthalene formaldehyde water reducer ensured the workability of the concrete mixture.
In this experiment, concrete specimens were cast using ordinary Portland cement (P.O. 42.5). The coarse aggregate (crushed stone) was continuously graded with a particle size of 5 mm–20 mm and a specific gravity of 2.66. In relation to the fine aggregate (river sand), its fineness modulus was 3.40 and its gravity was 2.65. The tensile strength of the steel fibers (hooked at both ends) was 1345 MPa, with a length of 35 mm and an aspect ratio of 64. The nano-silica was presented in the form of a white powder with an average particle size of 30 nm and an apparent density of 30–60 g/L. The rubber particles originated from waste rubber tires. After cleaning, bead stripping, crushing, and vibration sieving, the rubber particles were obtained. The naphthalene formaldehyde water reducer with a 25% water-reducing ratio was used to ensure that the slump of most concrete mixtures was 40 to 80 mm. The raw materials mentioned above are shown in Figure 1.

2.2. Experimental Design and Mixture Proportion

The effects of elevated temperatures, steel fibers, and nano-silica on the compressive and splitting tensile strengths of CRC were the main purpose of these tests. The concrete mixture proportion was based on our previous research [34]. Taking into account the impact of steel fibers on the workability of concrete, an increase of 0.5% by volume percentage of steel fibers per cubic meter of concrete mixture added 8 kg of water, but the water-to-cement ratio was still kept constant. The volume percentage method was used to mix crumb rubbers and steel fibers, while the mixing method for the nano-silica was equivalent to the amount of cement, that is, the replacement rate of cement.
Therefore, CRC with a C35 strength grade and 5% crumb rubber (which is equivalent to adding 50 kg of crumb rubber to 1 m3 of concrete) was chosen as the basic group, which was called C35CR5. In this paper, C35 means the compressive strength of concrete is between 35 and 45 MPa. Four different steel fiber volume ratios, 0%, 0.5% (39 kg/m3), 1.0% (78 kg/m3), and 1.5% (117 kg/m3), were considered. Based on existing research into the properties of concrete under high temperatures, taking everything into account, four temperature cases of 20 °C (room temperature, as the control group), 200 °C, 400 °C, and 600 °C were involved. In the case where the volume percentage of the steel fiber was 1.0%, two types of nano-silica with 1% and 2% content were selected. Additionally, the CRC with a high strength of C45 with 5% crumb rubber was specifically designed to investigate the effects of strength grades. C45CR5 was the name given to the second basic group.
Thus, C35 and C45 were the two strength grades, and C35CR5 and C45CR5 were the two basic groups. Different steel fibers and nano-silica contents were allowed in these two basic groups, in a similar manner. Through 26 groups of tests, the optimum concrete mix proportion was determined. Table 1 displays the details of the mixture proportion for the twelve concrete mixtures. A total of 24 specimens was poured for each concrete mixture proportion. Twelve of the specimens were used for the compressive test, and the average value of the three specimens was used as its compressive strength at a specific temperature. Another 12 specimens were used for the splitting tensile tests.

2.3. Specimen Preparation and Test Procedures

A shaft mixer played a vital part in the whole process of preparation for the mixture of the specimens. First, the aggregate materials were mixed and then, after 90 s, steel fibers were gradually added into the former. After that, stirring was continued for 180 s to ensure the uniform dispersion of steel fibers. If the steel fibers agglomerated, it was evident that the mixing could not be stopped. Then, the crumb rubbers and cement were added sequentially (if this mix proportion included nano-silica, the nano-silica was added with the cement) to ensure the uniformity of the mixture. The final step was to add water and then to reduce the water. Before the end of the whole process, the stirring continued for an extra 180 s. After that, the slump test was immediately conducted. In the standard curing room, the specimens were cured at a temperature of 20 ± 2 °C and a relative humidity of above 95%. The compressive and splitting tensile strengths of 28 d (d refers to days) were tested by casting 24 standard concrete test cubes (150 × 150 × 150 mm3) at 20 °C, 200 °C, 400 °C, and 600 °C.
Compression and splitting tensile tests were performed on the high-temperature mechanical property testing machines invented by our laboratory [35,36]. The testing machines (Figure 2.) included a loading device for applying pressure, a heating device, and a displacement acquisition device for measuring the deformation to the concrete test specimens. These three devices can work synergistically to realize the synergy of heating and loading and the synchronization of loading and measuring. All specimens subjected to elevated temperature were allowed to stand in a naturally ventilated environment for 24 h to reduce excessive moisture on the specimen surface before being heated. The test heating rate was 5 °C/min, and a constant temperature was maintained for 2 h. The heating procedure shown in Figure 3 was based on some scholars’ previous studies [28,29,30,31,32,37]. The load procedure followed the standard for test methods of mechanical properties on ordinary concrete (GB/T 50081-2002) [38] and the standard test methods for fiber-reinforced concrete (CECS 13:2009) [39]. The loading rate of compression and splitting tensile tests were fixed at 0.5 MPa/s and 0.05 MPa/s, respectively. The specimen was placed in the center of the upper and lower pressure plates of the testing machine. Specifically, the specimen used for the splitting tensile strength test was aligned with the center line of the pads of the upper and lower pressure plates.
Studies of the microstructure of concrete components, their inter-relationships, and their relationships with macroscopic properties for understanding and controlling the macroscopic properties of concrete are also discussed in this paper. After the specimens cooled down, micro-samples with relatively complete surface morphology were fabricated and selected for XRD phase analysis and SEM microstructure observation. The XRD phase analysis samples were ground into powder in an agate mortar and sieved with a 200-mesh sieve, then placed in a fluted glass slide for phase analysis operated in an X-ray diffraction analyzer. The 10 × 10 × 3 mm3 samples cut from the specimens were used for SEM microstructure observation. The bottom of the samples was smoothed with sandpaper, and then the samples were placed into the metal spraying equipment to spray metal. Carbon conductive double-sided tape was used to adhere the sample surface to the sample table. Then, a ZEISS EVO HD15 scanning electron microscope (SEM) was used for microscopic observation and photographing.

3. Test Results and Discussion of Mechanical Properties

3.1. Modes of Failure

The specimens selected in this paper were six concrete mixtures from the second basic group. Due to the similarity of modes of failure and the quantity of specimens subjected to elevated temperatures, the typical damage shapes of compressive and splitting tensile test cubes at 20 °C and 600 °C are the only ones shown in Figure 4 and Figure 5, respectively. These were selected at random from each set of three cubes. Through the test of material properties, it was found that the increase in steel fiber and nano-silica content leads to the presence of certain definite regularities of the failure mode.
Figure 4a–f show the typical damage morphology of concrete specimens after being compressed at room temperature (20 °C). It is obvious that there are penetrating cracks in the CRC, which was penetratingly cracked to varying degrees, and that the specimen was seriously damaged. As the bearing capacity was reached, fragments from the CRC specimen ruptured, while fragments from the modified specimens were ruptured to a lesser degree. The crack generation and expansion were proportional to the amount of steel fiber added to the CRC during the loading process. The SFNS-CRC sample is relatively complete, as shown in Figure 4e. Compared with others, the crack width is thinner and more evenly distributed, and almost no large cracks were found. The damage to the SFNS-CRC sample is better with the content of nano-silica maintained at 2%, as shown in Figure 4f.
Compared with the compressive damage shapes of specimens at room temperature (20 °C), each set of three specimens was damaged more severely at 600 °C. In particular, the surface of CRC was partially exfoliated under compressive loading at 600 °C, as shown in Figure 4g. The more steel fibers mixed into the CRC specimens, the better the cubes can maintain their integrity when damaged by horizontal comparison (seen from Figure 4a–d,g–j). The integrity of specimens basically remained, and the width of the microcracks gradually became smaller (as shown in Figure 4e,f,k,l). This is in agreement with the results of Lyu et al., that the participation of steel fibers and rubber particles can inhibit the spalling of concrete subjected to high temperatures [33]. Hence, steel fibers and nano-silica can greatly ameliorate the typical damaged shapes of CRC subjected to elevated temperatures.
Figure 5 depicts the typical damage morphology of concrete specimens after splitting tensile tests at 20 °C and 600 °C. Cracks appeared initially at the bottom and top of each test cube, then they gradually progressed. The entire height of the specimen was cracked vertically, as shown in Figure 5a. The incorporation of steel fibers led to a significant improvement in this situation. The tension was borne by the steel fibers after the concrete was invalidated. The addition of steel fibers can greatly improve the ability of failed concrete to withstand tensile force, and the steel fibers make a cracking sound when breaking in the later stages of loading. In addition, with the increase in steel fiber content, the number of microcracks rises, as well as the time required for loading and the damage process. In the last two samples containing nano-silica, the number of microcracks decreased, and the fatal cracks throughout the samples were replaced by several shorter ones.
Typical damage shapes of the specimen splitting tensile test at 600 °C are shown in Figure 5g–l. The CRC specimen split in half when the damage occurred. The fracture damage took place without any prior indication and was accompanied by a loud noise, which clearly illustrates brittle failure. Meanwhile, with the bond action of steel fibers, all the residual specimens maintained their integrity at high temperatures when damage occurred. Simultaneously, many microcracks can be seen near the main cracks, and the cracks gradually decreased with the increasing steel fibers. As shown in Figure 5k,l, the samples with nano-silica basically had fewer microcracks and the cracks were more uniform. Hence, the bond action of steel fibers is more obvious in the tensile tests and nano-silica can make the concrete denser, significantly reducing the appearance of microcracks.

3.2. The Loss of Mass

All samples subjected to high temperatures were placed in a naturally ventilated environment for 24 h to remove moisture from the surface of the samples, were weighed, and were then heated. The mass of the sample before the test was recorded as m 1 . After the high-temperature test was completed, the temperature controller was turned off, allowing the high-temperature furnace cavity to naturally cool down to room temperature. Then, the test block was removed and weighed. The test mass after the test was recorded as m 2 . The mass loss rate of the specimen can be calculated by the following formula: γ = m 1 − m 2 / m 1 .
Table 2 shows the mass of concrete specimens subjected to different temperatures. The mass loss ratio of the concrete specimens can be observed in Figure 6. The average mass loss rates of concrete specimens in the second group were 2.867%, 6.807%, and 9.360% at 200 °C, 400 °C, and 600 °C, respectively. Those in the first group were 1.730%, 5.826%, and 8.502%. It can be seen from Table 1 that the average water consumption of the second group of concrete mixture proportions is greater than that of the first group. After the concrete hydration reaction, more capillary water, physically absorbed water (gel water), and chemically bounded water in calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2) are produced. The loss of this water at high temperatures causes the mass loss rate in the second group of concrete specimens at each temperature to be slightly higher than that of the first group.
The mass loss of concrete subjected to elevated temperatures is primarily attributed to the loss of water content in the concrete [34]. The capillary water and physically absorbed water mainly evaporate within 200 °C, while chemically bonded water can only be released at higher temperatures as the calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2) decompose, generally recognizing above 350 °C [35]. In addition, the viscous fluidization of crumb rubber above 170 °C also results in the mass loss of concrete. The release of chemically bound water, and the rubber viscous fluidization all started at 200~400 °C; thus, the mass loss is the biggest during this temperature section in our experiment.

3.3. Compressive, Splitting Tensile Strengths and Influence Factors Analysis

3.3.1. Compressive and Splitting Tensile Strengths

The arithmetic average values of the test results of the three specimens for the compressive and splitting tensile strengths at the same temperature in each set of specimens were taken as the 28-day effective strengths. As shown in Table 3, the strengths of each specimen group subjected to elevated temperatures are lower than those at room temperature. The bond action of steel fibers greatly increases the strength of concrete at the same temperature. Due to the incorporation of nano-silica, the compressive strengths of SFNS-CRC at each temperature are greatly higher than that of the 1.0% steel fiber-modified CRC. However, the splitting tensile strength of SFNS was firstly enhanced and then decreased with the increasing nano-silica contents. Hence, the mechanical properties of CRC, with a certain content of steel fibers and nano-silica added, are improved to a significant degree when subjected to elevated temperatures. The specific details of the of the influencing factors analysis are shown in Table 3 below.

3.3.2. Influencing Factors Analysis

  • The effect of steel fiber content
The effects of steel fiber content on the strengths of C35CR5 and C45CR5 subjected to different temperatures are shown in Figure 7. The test results show that at 200 °C, 400 °C, and 600 °C, the second modified CRC group (C45CR5) contained 1.5% steel fiber, which resulted in increases in compressive strengths by 30.26%, 26.77%, and 22.50%. Meanwhile, the splitting tensile strengths experienced growth rates of 94.74%, 174.57%, and 137.33%. Both strengths gradually increased with the increasing steel fiber contents. In addition, the splitting tensile strength subjected to elevated temperatures increased continuously as the amount of steel fiber content increased. The dispersed steel fibers have the advantage of improving the uniformity of the internal stress of the concrete and decreasing the stress concentration, thereby increasing the strength of the concrete. It is worth mentioning that the effect of the steel fiber contents on the strength of CRCs subjected to elevated temperatures was more significant than those subjected to room temperature. The concrete damage at high temperatures highlights the function of the steel fibers. Moreover, since the steel fibers could continue to bear the tensile force at the fracture cross-section after the failure of the concrete, the increasing amplitude of splitting tensile strength subjected to elevated temperatures was more pronounced than that of compressive strength. This is similar to the possible role of steel fibers in controlling the thermal cracking process [40]. When the content of steel fibers reached 1.5% at room temperature, the individual strengths of the first group (C35CR5) showed a downward trend (see Figure 7). This is due to the fact that highly dosed steel fibers tend to ball up during concrete mixing, which reduces the quantity of uniformly distributed steel fibers. Simultaneously, there were defects between the cement matrix and the aggregates because of high dosage steel fibers, which rendered the test results unsatisfactory. However, at 600 °C, the high temperature caused severe damage to the concrete matrix and reduced the enhancement of concrete strength by high-dose steel fibers. At 200 °C and 400 °C, according to the microscopic analysis provided later in this article, it can also be concluded that the concrete matrix was not seriously fractured at this temperature. Therefore, the splitting tensile strength of high-dose steel fiber concrete was not reduced. Hence, due to the aforementioned mechanical properties and workability, CRC with a steel fiber content of 1.0% is the optimal choice.
2.
The effect of nano-silica contents
The test results show that at 200 °C, 400 °C, and 600 °C, the compressive strengths of the first modified CRC group (C35CR5), with 2% nano-silica content, increased by 9.31%, 25.18%, and 33.48%, respectively, and the growing rates of the second group were 8.81%, 19.32%, and 29.77%, respectively. Abreu et al. also pointed out that adding stable nano-silica and reducing the cement content can significantly improve the strength of concrete [41]. Figure 8 also indicates that the compressive strength of SFNS-CRC increased with the increasing nano-silica contents and the strength of SFNS-CRC at the same elevated temperature were significantly higher than those of CRC. In addition, the two groups’ splitting tensile strength showed a tendency to increase first and then decrease with the increase in nano-silica contents at the same temperatures. At 200 °C, 400 °C, and 600 °C, the splitting tensile strengths of the first modified CRC group (C35CR5) with 1% nano-silica content increased by 16.18%, 12.23%, and 6.40%, respectively, and the second group had a similar result. At 2% nano-silica, the decrease in splitting tensile strength is mainly due to the excessive nano-silica that creates a lower slump in SFNC-CRC. This affects the uniform distribution of the steel fibers in the concrete, reducing the distribution of steel fibers in the splitting tensile cross section, resulting in a decrease in splitting tensile strength. Thus, nano-silica exerts a positive effect on the strengths of SFNS-CRC at elevated temperatures. Furthermore, the optimal content of nano-silica is 1.0%.
3.
The effect of different temperatures
Figure 9 shows that the strengths of the six groups of concrete specimens generally presented the trends of descending → increasing → descending with the increasing temperature. At 200 °C, the strength decreases because the vapor pressure causes the matrix to shrink, further expanding the microcracks. At 400 °C, the change in ambient temperature creates new substances that fill the gaps in the aggregate, and the decomposition of the chemically bound water enhances the cementation of the cement slurry, thereby increasing the strength of the concrete at this temperature. At 600 °C, the cement hydration product is severely broken by the high temperature, the structure is loose, and the bonding force between particles is weak, resulting in a decrease in strength. Further analysis will be discussed in detail later in this article. The strength of the concrete with six concrete proportions subjected to elevated temperatures was lower than those at room temperature. From the polylines in each sub-figure in Figure 9, it can also be seen that the strengths of SFNS-CRC were significantly higher than those of other concrete samples at each temperature, and the unmodified CRC was the lowest. From these results, we can draw the conclusion that the modified effects of steel fibers and nano-silica on CRC strength subjected to elevated temperatures are ideal.

4. Failure Mechanism Discussion Based on Micro-Analysis

4.1. Micro-Analysis Based on XRD and SEM

The relationship between microstructural and macroscopic properties is the core of modern materials science. Compared with other engineering materials, the microstructure of concrete is more complicated. The change in composition further affects the microstructure and mineral composition of the concrete, changes the state of the interface transition zone, and subsequently affects the mechanical properties of the concrete. Simultaneously, elevated temperatures have a significant influence on the microstructure and mineral composition of SFNS-CRC. In this paper, the large-sized steel fibers could not change the chemical composition of the test specimens, and mainly played a role at the macroscopic and physical levels. Therefore, in the micro-analysis of XRD and SEM, the effects of nano-silica and different elevated temperatures are mainly discussed. The three different nano-silica contents of the first group were selected, and the 1% nano-silica content of the first group was chosen to study the effects of different elevated temperatures on the concrete.

4.1.1. Influence of Nano-Silica

Figure 10 depicts the microtopographies of SFNS-CRC with various nano-silica contents. From Figure 10a, plate-like calcium hydroxide Ca(OH)2 crystals existed in the concrete matrix of CRC, and the overall morphology of the matrix was loose. When 1% nano-silica was added, the structure of the matrix became dense; however, there was still crystallized Ca(OH)2, as shown in Figure 10b. From Figure 10c, it can be seen that when the nano-silica content was 2%, the microstructure of the matrix was significantly improved, Ca(OH)2 crystals could not be found, and the C-S-H gels spatially overlapped with each other to form a dense continuous phase. Because of the small-size effect of nano-silica, the surface had more atoms and unsaturated bonds which could give it a higher surface energy and chemical activity. This allowed the C-S-H gel to form a network with a core of nano-silica. Moreover, nano-silica had a micro-aggregating effect, which could fill the gaps in the interfacial transition zone and the voids in the cement matrix. Both these effects reduced the number and size of the voids and increased the density of the concrete matrix and the strength of the SFNS-CRC.
Through the use of scanning electron microscopy (SEM) analysis, Abreu et al. found improvements in the microstructure of concrete with stabilized nano-silica, which contributed to gains in its mechanical properties [41]. It can be seen from the XRD pattern of Figure 11 that the diffraction peaks of Ca(OH)2 at 2θ = 18° and 2θ = 34° decreased significantly with the increasing nano-silica contents. It was confirmed that the Ca(OH)2 was significantly reduced during the formation of C-S-H gel. However, the maximum diffraction peak of quartz (SiO2) crystal at 2θ = 26.5° was not closely related to the nano-silica content. This was because the sand in the matrix mortar was inevitably ground together in the sample during grinding. Since the C-S-H gel was not crystal, there was no distinct diffraction peak in the XRD pattern. In conclusion, the voids and micro-defects of SFNS-CRC were reduced and the compactness of the matrix was significantly improved after the addition of nano-silica. Simultaneously, nano-silica further enhanced the crack resistance of steel fibers and also improved the mechanical properties of SFNS-CRC.

4.1.2. Influence of Elevated Temperatures

The SEM microtopography and XRD at different temperatures are presented in Figure 12. As shown in Figure 12b, the structure of the C-S-H gel at 200 °C was still relatively intact, but the density was lower than that at room temperature. This was mainly because the microcracks were magnified via the evaporation of free water, adsorbed water, and C-S-H interlayer water in the SFNS-CRC cement slurry at 20–200 °C. In addition, the vapor pressure could generate compressive stress on the capillary channel, causing the matrix to shrink and further magnifying the microcracks in the cement slurry. Therefore, the strengths of the concrete mixture were significantly reduced at 200 °C. It can be observed from Figure 12c and Figure 13 that the cement hydration product of SFNS-CRC started to decompose at 400 °C. However, this temperature condition promoted the formation of hydrogarnet to fill the voids between aggregates. Simultaneously, the chemically bound water of C-S-H and hydrated sulfoaluminates decomposed to enhance the cementation of the cement slurry. Thus, the stress concentration of the cracks in the cement slurry was mitigated, and the adhesion between the steel fibers and the matrix increased to some extent. Therefore, the strength of the concrete mixture at 400 °C was higher than that at 200 °C. At 600 °C, the volume of the cement matrix expanded, further destroying the matrix structure. The cement hydration products were decomposed by heat, the cement slurry structure loosened, and the inter-particle adhesive force was reduced. In addition, the network structure of the C-S-H gel was destroyed, and no continuous or large gels were present in the matrix, which decomposed to form granular crystals. The cracks in the matrix further expanded and extended. All of the above phenomena resulted in a decrease in the strength of the concrete mixtures.
The XRD patterns of SFNS-CRC are shown in Figure 13. It can be observed that the diffraction peaks of Ca(OH)2 at 2θ = 18° and 2θ = 34° in the XRD pattern were significantly reduced with the increasing temperatures. The diffraction peak of Ca(OH)2 was hardly observed at 600 °C, which confirmed that Ca(OH)2 crystal was decomposed at 600 °C [36]. The C2S and C3S were formed from thermal decomposition and original anhydrous cement phases [37,38]. At 2θ = 30.8°, the diffraction peak of hydrogarnet at 400 °C was significantly higher than those at other temperatures, which proved that 400 °C could promote hydrogarnet’s formation. Since the hydrogarnet crystals were adequately crystallized, they could fill voids between the aggregates and form a relatively dense structure with the gel. Therefore, the strength of SFNS-CRC can be increased at 400 °C. Meanwhile, the diffraction peaks of hydrogarnet at 600 °C were significantly reduced, indicating that hydrogarnet was decomposed at 600 °C. The decomposition of hydrogarnet, gel, and other hydrates resulted in a decrease in the strength of SFNS-CRC at 600 °C. All this verifies our test results.

4.2. Failure Mechanism Discussion

Our previous studies examined the role of crumbs in CRC [29]. In this paper, steel fiber is regarded as both a load-bearing element and a power transmission element with uniform distribution in CRC that redistributes the stress and ensures the stress uniformity of CRC. The behavior of the deformation of concrete is greatly enhanced due to the bond between concrete and steel fiber, which relieves the brittle failure mode and lends it ductility. Therefore, as the steel fiber content rises, the ductility of the CRC increases significantly.
In addition, the hydrophobicity of crumb rubber leads to a large number of pores in CRC. Nano-silica can improve this situation. As a result of the positive effects of the functional filling or refining of the pore size, thus rendering CRC more physically dense, and through the interface of the transition zone between the aggregate and cement matrix via the use of the chemical reaction products (C-S-H gel), due to the pozzolanic activity of nano-silica, it can effectively improve the non-compactness of CRC and the utilization rate of steel fibers. In addition, the composite of crumb rubber, steel fiber, and nano-silica can prevent cracks from further propagating and consolidating, thus preserving the integrity of the specimens. Therefore, adding steel fiber and nano-silica into CRC can improve its strength. Steel fiber has a major impact on the improvements in the splitting tensile properties of modified CRC, while nano-silica has a major impact on its compression properties. When these two additives are used together in optimal amounts, the modification effect can itself be optimal.

5. Conclusions

Mixing waste rubber particles into concrete has greatly promoted the sustainable development of the waste rubber industry. In this study, considering the influences of different steel fiber contents, nano-silica contents, and temperature changes, the failure modes, mass loss, compressive strength, splitting tensile strength, and failure mechanisms based on micro-analysis of 12 SFNS-CRC groups were investigated and discussed. Several conclusions can be drawn:
(1)
The integrity of SFNS-CRC specimens could be fundamentally preserved. The width of the microcracks gradually became smaller. Steel fibers and nano-silica can greatly ameliorate the typical damaged shapes of CRC when subjected to elevated temperatures. Brittle failure can be greatly improved, as can the ductility of the concrete mixtures.
(2)
SFNS-CRC has superior mechanical properties compared with CRC when subjected to elevated temperatures:
  • The compressive and splitting tensile strengths of concrete mixtures subjected to elevated temperatures increase with the increasing steel fiber contents. The increasing amplitude of splitting tensile strength was more pronounced than that of compressive strength. The effects of steel fiber contents on the strength of CRC subjected to elevated temperatures are more significant than those subjected to room temperature.
  • The improvements in the compressive strength after adding nano-silica subjected to elevated temperatures is obvious. The voids and micro-defects of SFNS-CRC are reduced. The compactness of the matrix is significantly improved after the addition of nano-silica. Simultaneously, nano-silica further enhances the crack resistance of steel fibers and improves the mechanical properties of SFNS-CRC as well. Furthermore, the optimal content of nano-silica is 1.0%.
(3)
Through micro-analysis of SEM microtopographies and substances from XRD, the addition of nano-silica can not only result in the superior compactness of concrete, but also provide a higher surface energy and chemical activity helping generate C-S-H gel. Furthermore, it is proven that 400 °C can promote the formation of hydrogarnet from XRD; thus, the test results can be verified.

Author Contributions

Conceptualization, Y.W. and J.C.; methodology, Y.W. and J.C.; validation, J.C.; formal analysis, J.C.; investigation, J.C.; resources, Y.W.; data curation, Y.W. and J.C.; writing—original draft preparation, J.C.; writing—review and editing, J.C.; visualization, Y.W. and J.C.; supervision, Y.W.; project administration, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This experimental work was supported by the Key Scientific Research Projects of Henan Province (18A560022).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data from the research will be available upon request to the authors.

Acknowledgments

We would like to express our sincere gratitude to all the technicians at Zhengzhou University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The test’s raw materials.
Figure 1. The test’s raw materials.
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Figure 2. Concrete material performance test set-up at high temperatures: (a) testing apparatus of concrete material compressive test machine at high temperature and splitting tensile test machine at high temperature; (b) schematic diagram of concrete compression test at high temperature; (c) schematic diagram of concrete splitting tensile test at high temperature.
Figure 2. Concrete material performance test set-up at high temperatures: (a) testing apparatus of concrete material compressive test machine at high temperature and splitting tensile test machine at high temperature; (b) schematic diagram of concrete compression test at high temperature; (c) schematic diagram of concrete splitting tensile test at high temperature.
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Figure 3. Temperature–time curves on heating the specimens.
Figure 3. Temperature–time curves on heating the specimens.
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Figure 4. Typical damage shapes of concrete specimens after compressive tests.
Figure 4. Typical damage shapes of concrete specimens after compressive tests.
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Figure 5. Typical damage shapes of concrete specimens after splitting tensile tests.
Figure 5. Typical damage shapes of concrete specimens after splitting tensile tests.
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Figure 6. The mass loss ratio of specimens subjected to elevated temperatures.
Figure 6. The mass loss ratio of specimens subjected to elevated temperatures.
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Figure 7. The effects of steel fiber content on compressive and splitting tensile strengths subjected to different temperatures.
Figure 7. The effects of steel fiber content on compressive and splitting tensile strengths subjected to different temperatures.
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Figure 8. The effects of nano-silica content on compressive and splitting tensile strengths subjected to different temperatures.
Figure 8. The effects of nano-silica content on compressive and splitting tensile strengths subjected to different temperatures.
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Figure 9. The effects of different temperatures on compressive and splitting tensile strengths.
Figure 9. The effects of different temperatures on compressive and splitting tensile strengths.
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Figure 10. The SFNS-CRC microtopography of the different nano-silica contents.
Figure 10. The SFNS-CRC microtopography of the different nano-silica contents.
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Figure 11. The XRD patterns of SFNS-CRC with different nano-silica contents.
Figure 11. The XRD patterns of SFNS-CRC with different nano-silica contents.
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Figure 12. The SFNS-CRC microtopography subjected to different temperatures.
Figure 12. The SFNS-CRC microtopography subjected to different temperatures.
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Figure 13. The XRD patterns of SFNS-CRC subjected to different temperatures.
Figure 13. The XRD patterns of SFNS-CRC subjected to different temperatures.
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Table 1. Concrete mixture proportion (unit: kg/m3).
Table 1. Concrete mixture proportion (unit: kg/m3).
GroupCrumb
Rubber		CementCoarse
Aggregate		Fine
Aggregate		WaterWater
Reducing		Steel FiberNano
Silica
C35CR51SF050400703100416940-
1SF0.55041970510601774.539-
1SF1504386541045185578-
1SF1.55045760410291935117-
1SF1NS150433.6265410451855784.38
1SF1NS250429.2465410451855788.76
C45CR52SF050550703100418070-
2SF0.550574630957188539-
2SF150599572931196578-
2SF1.5506235149052045117-
2SF1NS150593.015729311965785.99
2SF1NS250587.0257293119657811.98
Note: the steel fiber type is shear end-bend hook with a tensile strength of 1350 MPa, a length of 35 mm, and an aspect ratio of 64. Nano-silica is a white powder with an average particle size of 30 nm and an apparent density of 30–60 g/L.
Table 2. Mass of specimens subjected to different temperatures.
Table 2. Mass of specimens subjected to different temperatures.
Specimen Mass of C35CR5/(kg)Specimen Mass of C45CR5/(kg)
Temperature20 °C200 °C400 °C600 °CTemperature20 °C200 °C400 °C600 °C
1SF07.7757.6847.2967.1752SF07.8257.5617.4237.012
1SF0.57.4307.2657.1036.8452SF0.57.8207.6117.3637.190
1SF1.07.6037.4157.1386.8962SF1.07.8657.7427.3527.203
1SF1.57.3207.1916.9956.7292SF1.58.2507.9957.5007.268
1SF1.0NS17.7707.6987.1907.0322SF1.0NS17.9907.7617.4757.224
1SF1.0NS27.9557.8107.4487.2752SF1.0NS27.9807.7767.3657.355
Table 3. Tests results of 28 d’ compressive and splitting tensile strengths subjected to different temperatures (unit: MPa).
Table 3. Tests results of 28 d’ compressive and splitting tensile strengths subjected to different temperatures (unit: MPa).
GroupCompressive Strengths Subjected to
Different Temperatures/(MPa)		Splitting Tensile Strengths Subjected to
Different Temperatures/(MPa)
20 °C200 °C400 °C600 °C20 °C200 °C400 °C600 °C
C35CR51SF038.732.3736.3830.282.91.991.801.93
1SF0.542.833.3838.1231.663.32.092.542.49
1SF1.044.634.5741.2635.464.92.723.273.28
1SF1.54029.3838.4630.684.43.073.673.35
1SF1.0NS147.135.7944.0039.015.13.163.673.49
1SF1.0NS247.337.7951.6547.335.22.993.603.19
C45CR52SF051.331.9241.2432.943.32.091.771.50
2SF0.552.837.9145.4736.593.62.583.172.29
2SF1.053.542.1149.4039.904.44.303.833.59
2SF1.554.341.5852.2840.355.04.074.863.56
2SF1.0NS154.946.6857.6051.705.14.864.253.73
2SF1.0NS256.145.8258.9551.784.94.054.183.42
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Wang, Y.; Chen, J. The Mechanical Properties and Failure Mechanisms of Steel-Fiber- and Nano-Silica-Modified Crumb Rubber Concrete Subjected to Elevated Temperatures. Sustainability 2023, 15, 16340. https://doi.org/10.3390/su152316340

AMA Style

Wang Y, Chen J. The Mechanical Properties and Failure Mechanisms of Steel-Fiber- and Nano-Silica-Modified Crumb Rubber Concrete Subjected to Elevated Temperatures. Sustainability. 2023; 15(23):16340. https://doi.org/10.3390/su152316340

Chicago/Turabian Style

Wang, Yihong, and Jiawei Chen. 2023. "The Mechanical Properties and Failure Mechanisms of Steel-Fiber- and Nano-Silica-Modified Crumb Rubber Concrete Subjected to Elevated Temperatures" Sustainability 15, no. 23: 16340. https://doi.org/10.3390/su152316340

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