Next Article in Journal
Planning Regulations and Modelled Constraints in BIM: A Dutch Case Study
Previous Article in Journal
Evaluation of Moisture-Induced Stresses in Wood Cross-Sections Determined with a Time-Dependent, Plastic Material Model during Long-Time Exposure
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Buildings
Volume 14
Issue 4
10.3390/buildings14040938
buildings-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

The Mechanical and Self-Sensing Properties of Carbon Fiber- and Polypropylene Fiber-Reinforced Engineered Cementitious Composites Utilizing Environmentally Friendly Glass Aggregate

by
Lijun Ma
,
Meng Sun
and
Yunlong Zhang
*
School of Transportation Science and Engineering, Jilin Jianzhu University, Changchun 130119, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(4), 938; https://doi.org/10.3390/buildings14040938
Submission received: 25 December 2023 / Revised: 2 March 2024 / Accepted: 26 March 2024 / Published: 28 March 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

:
In order to facilitate waste glass recycling and enable the monitoring of concrete structures, this study prepares a new type of self-sensing engineered cementitious composite (ECC) via the use of glass sand instead of silica sand. The health monitoring of a concrete structure is achieved through the addition of polypropylene (PP) fibers to enhance the flexural toughness of concrete, and adding carbon fibers (CFs) to make the concrete self aware, enabling it to sense the load changes and structural damage. The fiber dosage of ECC is optimized to analyze the effects of different fiber types and dosages on the mechanical and self-sensing properties of concrete. The results show that the hybrid fibers produce a good synergistic effect on mechanical properties, and the presence of excess fibers causes the mechanical properties of concrete to deteriorate. The critical fiber volume fraction required for the strain hardening of PP ranges from 0.75% vol to 1% vol. At different PP dosages, the CF dosage shows a positive correlation with the initial crack strength. By analyzing the effect of varied curing times and CF doping on the initial resistivity, it is found that the threshold value of CF conductivity is 0.7% vol. The role of CFs in the flexural sensitivity and pressure sensitivity tests is explained from the perspective of fiber distribution, and the fiber distribution theory is verified with scanning electron microscopy (SEM). The optimal level of CF doping for flexural sensitivity and pressure sensitivity is determined to be 1.1% vol and 0.7% vol via the use of self-sensing performance tests, respectively. An increase in PP fiber doping leads to a decrease in the initial resistivity and self-sensing properties of the material. The results of this research provide guidance regarding how to determine the optimal fiber dosage flexibly for different engineering works.

1. Introduction

The development of construction engineering is inseparable from the advancement of materials science. With urbanization and infrastructure development, the demand for high-performance building materials grows constantly. Plain concrete is brittle, and plain concrete structures tend to lose their load-bearing capacity suddenly upon cracking, which is a problem that can be solved through the use of engineered cementitious composite (ECC). Unlike conventional concrete, ECC is capable of a metal-like stress–strain response and is prone to strain hardening beyond the yield strength [1,2,3]. The mainstream varieties of fibers used in ECC are polyethylene (PE) fibers, polyvinyl alcohol (PVA) fibers, and polypropylene (PP) fibers. In the 1960s, Maalej et al. performed an experiment to determine the critical fiber volume fraction of PE fibers (0.4% vol to 0.8% vol), within which no false strain hardening occurs in the composite [4]. However, its practical application is hindered by the high cost of PE fibers and the severe cracking caused by the hydrophobicity of PE. At the beginning of the 21st century, more affordable PVA-ECC emerged. Due to its better performance in relation to the cementitious matrix, PVA fibers can easily combine with the hydration products of concrete to form a strong chemical bond. When a load is applied, the fractured fibers far outnumber the fibers pulled out due to slipping, which then affects the ductility of the ECC [5,6]. Modifying the fiber surface can effectively reduce the formation of chemical bonds between PVA fibers and the cementitious body, thus improving the tensile properties. As revealed by some studies, the adhesion of modified fibers to the matrix is significantly weaker when compared to the original oiled and oil-free fibers [7]. However, this method plays a limited role in reducing the chemical adhesion of fibers, and modifying the fibers is a complex process [8,9]. Compared with the above two types of fibers, PP fibers are less costly, making it easier to popularize the use of ECC. From the perspective of microstructures, the difference in surface structure between PP and PVA fibers leads to the variation in their ability to bridge cracks. For instance, PP fibers deteriorate at a slower rate in body–fiber bond strength, and the fiber surface of the body is more conducive to sliding in the presence of tension. Moreover, PVA fibers are less resistant to fractures, while PP fibers perform better in terms of elongation than PVA fibers [10]. The ECC concrete containing PP fibers is more capable of deflection compared to PVA fibers, which is attributed to the relatively low elasticity modulus and tensile strength of PP fibers [11].
The high ductility of ECC enables it to withstand severe deformation, which makes it suitable for the design of sensors. With conductive material integrated into the ECC, the strain-induced damage suffered by the material can be monitored via the changes in electrical signals. When the ECC sensor is subject to an external force or deformation, the resistance of the conductive material is altered. By measuring the fractional change in resistivity (FCR), the extent to which the material is deformed can be inferred, which facilitates the real-time monitoring of the whole structure [12,13]. Compared to embedded sensors, ECC sensors have a wider detection range, a longer service life, do not compromise structural integrity, and do not negatively affect mechanical properties. In order to endow conventional concrete with high electrical conductivity for self-sensing, various conductive materials have been commonly used, such as CF, graphite powder, carbon nanotubes (CNTs), and carbon black (CB). While ensuring the mechanical properties of the structure, they also significantly improve the electrical conductivity of concrete [14,15,16,17,18,19]. Despite their limited effect on the compressive strength of ECC materials, carbon nanotubes can increase the tensile strength by 54% due to their excellent bridging ability. Additionally, the conductivity of CNTs is also very high, with resistivity being reduced by 78% after the addition of only 0.04 wt% [20]. CNT-containing ECC performs well in self-sensing whether either a bending load or cyclic load is applied [21]. CB also performs well in reducing resistivity, and the addition of carbon black to ECC is effective in increasing the tensile strain. However, there is a significant reduction in the ultimate tensile strength and initial crack strength when compared with CB-free ECC [22]. As the most used electrically conductive material, CF is inexpensive and effective to a certain degree in enhancing the bending strength, bending toughness, tensile strength, and ductility. It has also been used to suppress the drying shrinkage of concrete [16]. Although both CNT and CF perform well in the detection of self-sensed damage, CF significantly improves ductility and reduces bending-induced damage. Moreover, CF far outperforms CNT in energy dissipation and load-bearing capacity. Given the same width of cracking, the loading value of a specimen containing carbon fibers is less than half of that of a specimen containing carbon nanotubes [14]. This suggests that CF has a better crack arresting effect.
In general, the use of large-sized aggregates should be avoided during the preparation of ECC. This is because the accumulation of large aggregates reduces the interfacial friction, thus suppressing strain hardening and affecting the mechanical properties of ECC [23]. Therefore, ultrafine silica sand is commonly used as the fine aggregate in ECC. However, the production of silica sand is costly and complex, which increases carbon emissions [24]. Glass sand has emerged as a solution to this problem. Waste glass takes a million years to decompose naturally, which not only causes the waste of resources, but also pollutes the environment. Therefore, producing glass sand with waste glass is an effective solution to both the recycling of waste glass and reducing the price of silica sand used in ECC. Moreover, replacing ultrafine silica sand with waste glass sand contributes to reducing carbon emissions. However, the alkali–silica reaction (ASR) is the main problem that requires solving to promote the use of glass aggregates. In alkaline pore solutions, alkali–silica gel leads to the expansion and cracking of concrete, thus reducing its strength. Commonly used methods to avoid the alkali–silica reaction (ASR) include reducing the particle size of glass aggregates and adding an appropriate amount of fly ash [25,26,27,28]. The expansion damage caused by ASR gel depends on the size of the glass particles, and the expansion caused by the alkali–silica reaction is no longer detected when the particle size of the glass aggregate is less than 1 mm [29]. Moreover, the use of fly ash in the range of 10% to 50% can effectively mitigate the formation of ASR [30].
In summary, ECC has the potential to become self-sensing concrete due to its good bending toughness, but there are few studies investigating self-sensing ECC. In order to promote the application of self-sensing engineered cementitious composites in practical engineering while solving the problem of waste glass recycling, this study, based on results in the literature, adopts glass sand in place of silica sand, and comprehensively considers the mechanical properties of fibers and electrical properties. It selects polypropylene fibers and carbon fibers to enhance the flexural toughness and self-sensing of engineered cementitious composites in order to prepare a new type of self-sensing ECC that can conduct the real-time monitoring of the structure of coagulation. By studying the effects of mixed fibers on the mechanical and self-sensing properties of self-sensing ECC, the optimal fiber dosage for different engineering needs is determined to provide a valuable reference for the application of self-sensing ECC in actual engineering.

2. Materials and Methods

2.1. Test Materials

The test was carried out with PII grade 42.5 silicate cement produced by Jilin Yatai Cement Co. (Changchun, China), and the indexes were in accordance with the Standard for Testing of General Silicate Cement (GB175-2007) [31] standards. The specific parameters are shown in Table 1. The silica fume and fly ash were produced by Hebei Shengyi Miner-al Products Trading Co. (Hebei, China), and the indicators were in line with the “Technical Specification for the Application of Mineral Admixtures” (GB/T51003/2014) [32] requirements; the specific parameters are shown in Table 2 and Table 3. In order to prevent the alkali–silica reaction from affecting the strength of the concrete, the test used 100 mesh (diameter of 0.15 mm) glass sand produced by Hongyang Cleaning Equipment Company Limited (Ningbo, China). The chemical composition of glass sand is shown in Table 4. The morphology of its appearance under scanning electron microscope (SEM) is shown in Figure 1. Polypropylene fiber was produced by Beijing Zhongfang Fiber Construction Technology Co., Ltd. (Beijing, China), with a fiber length of 12 mm. Carbon fiber was produced by Shanghai Lishuo Composite Material Technology Co., Ltd. (Shanghai, China), with a fiber length of 6 mm. Polypropylene fibers and carbon fiber specifications and properties are shown in Table 5 and Table 6, and the fiber appearance is shown in Figure 2. The water reducer utilized in the experiment is a polycarboxylate-based high-efficiency water reducer produced by Shanxi Feike New Materials Technology Co., Ltd. (Shanxi, China). The quality inspection report can be found in Table 7. The dispersant is carboxymethyl cellulose sodium produced by Fuchen Chemical Reagent Co., Ltd. (Tianjin, China), with a viscosity of 300 mPa·s. The defoamer is tri-n-butyl phosphate produced by Wuxi Asia-Pacific United Chemical Co., Ltd. (Wuxi, China), with an acidity (H+ count) of mmol/10 g ≤ 0.2%.

2.2. Mixing Ratio Design

Experiments were conducted to investigate the effect of PP fibers and CFs at different dosages on the mechanical and self-sensing properties of engineered cementitious composites. The PP fibers were designed as 0.25%, 0.5%, 0.75%, 1%, and 1.5% by volume dosage, and the CF dosage was designed as 0.5%, 0.7%, 0.9%, and 1.1% by volume dosage. In total, 20 groups of mixtures were designed, and the specific mixes are shown in Table 8. Glass sand was used to completely replace the use of silica sand in the test. To ensure the uniformity of fiber dispersion in concrete, carboxymethyl cellulose was used as a dispersant, and its dosage was 0.035% of the mass of the cementitious material. At the same time, in order to eliminate the effect of internal pores on the strength of the concrete, a defoamer was used to reduce their formation, the dosage of which was 0.015% of the mass of the cementitious material. The dosage of the tested water-reducing agent was 1.2% of the mass of the cementitious material. The experimental mixing proportions are shown in Table 8.

2.3. Test Methods

2.3.1. Mechanical Performance Testing

The compressive strength, split tensile strength, and flexural strength were deter-mined according to the Standard for Test Methods of Physical and Mechanical Properties of Concrete (GB/T50081-2019) [33]. The loading rate used to test compressive strength was set to 0.5 MPa/s, and the sample size was 100 mm × 100 mm × 100 mm. The loading rate used to test split tensile strength was set to 0.05 MPa/s, and the sample size was 100 mm × 100 mm × 100 mm. The loading rate used to test flexural strength was set to 0.05 MPa/s, and the sample size was 400 mm × 100 mm × 100 mm. Flexural toughness was determined according to “Test Methods for the Properties of Glassfibre Reinforced Cement (GB/T15231-2008)” [34], and the dimensions of the test block were 400 mm × 100 mm × 15 mm. The flexural toughness was determined through displacement loading, the rate of which was 0.1 mm/min.

2.3.2. Self-Sensing Performance Testing

The self-sensing performance was evaluated mainly by testing the pattern of changes to the FCR in the presence of external forces. Specifically, the test was conducted through the use of the two-electrode method, where the electrode was deployed to measure the electrical properties of the concrete with a DC power supply. The presence of a DC electric field leads to the aggregation and directional movement of solution ions in the concrete micropores, resulting in a polarization reaction, as manifested in the repeated fluctuations in the reading of the universal meter at a constant voltage and a continuous decrease in the current over time. Since the occurrence of polarization can have a significant impact on the accuracy of the measurement, the test was conducted at a lower voltage than 2 v as a regulated power supply for the pre-power treatment. In this way, it can be completely polarized in order to reduce the impact. The sample used to determine concrete pressure sensitivity has a size of 300 mm × 100 mm × 100 mm, the loading rate is 1 kN/s, and the loading mode is cyclic loading. In order to ensure the accuracy of pressure sensitivity results, the specimen was pre-loaded, which effectively prevents the test results being impacted by fine cracks developing in the concrete during the elastic phase. The sample used to determine bending sensitivity has a size of 400 mm × 100 mm × 15 mm, the loading rate is 0.1 mm/min, and the loading mode is monotonic loading. The electrode material used to determine pressure sensitivity is copper mesh, with a size of 60 mm × 80 mm. During the pouring process, the copper mesh was pre-buried into the concrete at a depth of 40 mm, with a distance of 200 mm between the two electrodes deployed symmetrically. The deployment of electrodes and the loading method are shown in Figure 3a and Figure 3b, respectively. To determine the bending sensitivity, the copper mesh used for the electrodes has a size of 40 mm × 15 mm, and the distance between the symmetrically deployed electrodes is 300 mm. During the test, the upper part of the test block carried the load cell, while the lower part carried the displacement sensor in order to record the load displacement curve. The deployment of electrodes and the loading method are shown in Figure 4a and Figure 4b, respectively.
The change in current was recorded at 5 s intervals through the use of a multimeter (VICTOR 890D). The volume resistivity and fractional change in resistivity (FCR) were calculated as shown in Equations (1) and (2):
ρ = U S I L
where ρ —Resistivity, Ω·cm;
U—Specimen two electrode terminal voltage, V;
S—Specimen cross-sectional area, cm2;
I—Electric current, A;
L—Electrode distance, cm.
F C R = ρ − ρ 0 ρ 0 × 100 %
where FCR—fractional change in resistivity, %;
ρ —Resistivity, Ω·cm;
ρ 0 —Initial resistivity, Ω·cm.

3. Results and Discussion

3.1. Mechanical Properties

3.1.1. Compressive Strength

The 28 d compressive strength of ECC with different fiber dosages is shown in Figure 5, and the test results show that there is a good synergy between the effects of two kinds of fibers on the compressive strength, and that the optimal fiber dosage is 0.5% PP and 0.9% CF.
The compressive strength of ECCs with different polypropylene dosages but the same CF dosage first increases and then decreases with the gradual increase in PP fiber content, with the maximum compressive strength being reached at a PP fiber dosage of 0.5%. This is consistent with the results obtained by Prinya [35] and Maedeh Orouji [36], who demonstrated that the introduction of an appropriate amount of PP fibers can improve the compressive strength of the concrete. Compared to 1.5% PP, the compressive strength of 0.5% PP was improved by 18.2%, 22.6%, 32.6%, and 22.9% when CF doping increased from 0.5% to 1.1%. When the amount of PP fiber doping is fixed, the effect of CF on compressive strength shows a similar trend, first increasing and then decreasing. An appropriate amount of CF fibers can increase the compressive strength of concrete [37]. Meanwhile, the compressive strength following 0.9% CF doping is enhanced by 14.4%, 17.6%, 8.1%, 9.4%, and 4.8% compared to 0.5% CF doping when the PP fiber content increases from 0.25% to 1.5%.
The variation in the effect of fiber dosage on compressive strength, increasing initially and then decreasing, can be attributed to the ability of an appropriate amount of fibers to fill the micropores and microcracks in concrete, acting as bridges [38], thus effectively preventing crack expansion and thereby enhancing compressive strength [39,40]. However, when the fiber admixture surpasses the critical value, the fiber dispersion becomes poor, and hydration products fail to completely encapsulate the fibers, resulting in excessive fiber aggregation and agglomerate formation in the concrete [41]. This uneven dispersion leads to localized stress concentration during compression, promoting premature crack formation and subsequently reducing the overall compressive strength of the concrete.

3.1.2. Split Tensile Strength

The test results of split tensile strength are presented in Figure 6 below, which shows that the effects of both fibers on split tensile strength first increase and then decrease. Among them, the 0.75% PP fiber doping enhances split tensile strength most significantly. When CF increases from 0.5% to 1.1%, the split tensile strength is increased by 15.6%, 16.6%, 17.2%, and 16.2% following 0.75% PP fiber doping, compared to 0.25% PP fiber doping. When the PP fiber dosage increased from 0.25% to 1.5%, the splitting strength at a 0.9% CF fiber dosage exhibited improvements of 3.9%, 9.1%, 5.1%, 8.7%, and 8.4%, compared to 0.5% CF. The test results indicate a more significant enhancement in splitting strength with PP fibers. This phenomenon can be attributed to two forms of fiber damage in concrete: fiber pullout and fiber breakage [42,43]. Under split tensile stress, the low modulus of elasticity of polypropylene fibers allows them to withstand considerable deformation without fracturing, thereby increasing the concrete’s load-bearing capacity. However, the relatively smooth surface of PP fibers weakens their bond with the hydration products within the concrete matrix, facilitating fiber pullout and further enhancing split tensile strength. Conversely, carbon fibers, with diameters in the nanometer scale, disperse more uniformly in concrete, forming tighter bonds with the cementitious material. This renders carbon fibers more susceptible to fracturing under external loads due to their high modulus of elasticity [44], resulting in an instantaneous loss of load-bearing capacity following fracturing.

3.1.3. Flexural Strength

Figure 7 shows the measured flexural strength. According to the test results, the addition of CF and PP fibers in appropriate amounts contributes to the flexural strength. With the PP dosage of 0.75% and the CF dosage of 0.9%, the flexural strength is enhanced to the most significant extent. When compared with the lowest value of PP, which is 0.25%, a CF dosage of 0.5% improves flexural strength by 31.2%. When the fiber type is fixed, the maximum and minimum flexural strengths are compared, showing that the effect of PP fibers on flexural strength is slightly more significant than CF, which contradicts the conclusion reached by Wei [45]. This is because when the overall amount of fiber doping is too high, the outcome of CF dispersion is inferior to that of PP fibers. Even if the elasticity modulus of CFs is comparably higher than PP fibers, the poorly dispersed CFs are still not tightly bonded to the cementitious material, thus exacerbating fiber pullout. Meanwhile, the length and diameter of PP fibers are larger than those of CF. PP fibers of appropriate length and diameter can form a mesh structure in concrete, similar to steel reinforcement, which limits the development of cracks [46], thereby increasing the toughness of concrete and improving its flexural strength.

3.1.4. Flexural Toughness

Figure 8 shows the results of the flexural toughness test. The ECC material shows a linear increase with the rise in stress before the initial cracking. At this point, the maximum value is defined as the first inflection point. With the gradual increase in load applied, more cracks develop on the surface of the ECC material. Meanwhile, the load deflection curve shows slight fluctuations, which indicates that the material enters into the plastic deformation stage.
When the amount of PP fiber doping falls below 1%, the stress–strain curve of ECC is close to the point of brittle damage, i.e., none of the composites exhibit a large number of significant cracks caused by strain hardening (Figure 8). When the PP fiber content is raised from 0.75% to 1%, the ECC panels show a significantly high level of bending toughness, which leads to the conclusion that the critical fiber volume fraction for strain hardening falls into this range. The mitigated effect of CF on plastic deformation can be accounted for as follows. Fiber pullout is exacerbated when the ECC slab is subjected to plastic deformation, and it is difficult to disperse the CF uniformly in the concrete, thus resulting in a weak bond with the hydration products. Moreover, fiber pullout is evident when the ECC slab is loaded continuously.
Figure 9 shows the effects of different fiber dosages on first-crack strength. This in-creases when the fiber content increases in both cases. Notably, the effect of CF on the first-crack strength is especially significant [47]. When the amount of PP doping is 1.5%, the first-crack strength is improved by about 15% at 1.1% CF, compared to that at 0.5% CF. PP fibers improve the first-crack strength by less than 5%, which is much lower compared to CF. This is because the first-crack strength is affected by the elastic modulus of the fiber, and the high elasticity modulus of CF enhances the first-crack strength significantly. The effect of CF doping on the ultimate bending strength is less significant, showing a trend of initially decreasing and then increasing. Unlike the first-crack strength, the ultimate bending strength depends more on the elastic modulus of the fiber after a combination with the matrix.

3.2. Electrical Properties

3.2.1. Resistivity

Usually, the concrete conducts electricity at room temperature in two ways: electronic and electrolytic [48]. The former is achieved by the movement of free electrons in the conductive phase of the concrete, e.g., CF and CNT. By contrast, the latter is achieved by the movement of particles in the pore liquid to form particle jumps, e.g., CB.
Figure 10 shows the variation in resistivity between different types of fibers after various years of maintenance, as measured in the test. The dimensions of the test specimen are 300 mm × 100 mm × 100 mm. The test results are as follows. With the gradual increase in curing age, the resistivity of the concrete substrate shows an overall upward trend. Basically, it experiences three stages: rapid growth, slow rise, and stabilization. The rapid growth stage lasts mainly from day 1 to day 7 of maintenance. At this stage, resistivity in-creases rapidly at any fiber dosage. The fastest rate of growth is 1.5% in the PP 0.5% CF group, with the growth rate reaching 85.35%. The primary reason for a rapid growth of resistivity in the early stage is as follows. In the early stage, an intense hydration reaction occurs, where free water is gradually converted into binding water and gelling water with low conductivity. Meanwhile, the generation of hydration products also affects the material’s structure. As a result, there is a gradual increase in the insulating barrier between the conductive materials, which decreases the pass rate of conductive particles, resulting in a rapid growth of resistivity during the early stage. With the passage of time (7 d–21 d), the hydration rate decreases gradually, free water content decreases, and insulation barrier transformation is slowed down. Thus, the growth rate of resistivity is relatively low. In the final stage of maintenance (21 d–28 d), the material is gradually molded. At this time, conductive filler has formed a more complete conductive network. Despite the continuation of hydration and the volcanic ash reaction, the intensity of the reaction is reduced significantly. In this case, the conductive network is barely affected. Therefore, substrate resistivity changes at a slow pace and stabilizes. At this time, the growth rate of resistivity is consistently less than 4%.
Figure 11 shows the resistivity of concrete with different amounts of fiber doping after 28 days of curing. Apparently, both PP fiber and CF contents affect resistivity. The resistivity decreases sharply when CF increases from 0.5% to 1.1%, which is largely attributed to the bridging of carbon fibers that increases the number of conductive pathways [39]. The highest rate of change is ascribed to CF that rises from 0.5% to 0.7%; the permeability threshold calculated to be in this range. The effect of PP fibers on resistivity shows a negative effect, which is especially obvious when using a low CF dosage. When the CF content is 0.5%, 1.5% PP fiber is 53.2% higher in resistivity than 0.25% PP, which is attributed to the fact that the increase in the content of PP fibers affects the dispersion of CFs in the concrete. Consequently, the CFs in the conductive pathway are obstructed, and the tunneling effect is suppressed. This, in turn, affects the resistivity of the concrete.

3.2.2. Stress Sensitivity Testing

Figure 12 shows the results of the pressure sensitivity test as obtained after three periods of cyclic loading. Clearly, the concrete materials perform well in terms of pressure sensitivity with different CF fiber dosages. In the presence of cyclic loading, the resistivity fluctuates significantly, and the effect of the fiber type and doping on the FCR of the concrete is significant. The effect of CF on FCR at different PP fiber dosages shows almost the same trend. The pressure sensitivity is 0.7% CF, 0.9% CF, 1.1% CF, and 0.5% CF in descending order, with the maximum FCR reaching 22.7% at 0.25% PP and 0.7% CF.
From the perspective of fiber distribution, the state of CF dispersion in the concrete can be divided into three types: fully bridged, almost bridged, and not bridged at all (Figure 13). Pressure-sensitive change is also the process of an increase or decrease in the number of conductive channels. When the concrete is pressurized, deformation occurs, which compresses the specimen, thus making the extremely close CFs (Figure 13b) come into contact with each other and creating a conductive channel. As a result, the electrical resistivity decreases. When the amount of CF in the concrete is too high, most of the fibers have formed a conductive pathway (Figure 13a), and the rate of change in resistivity is not significantly high when external pressure is applied. Conversely, when the amount of CF is too low, most of the fibers are not bridged at all (Figure 13c), and the rate of change in resistivity is also insignificant. This explains the low level of pressure sensitivity at 1.1% CF and 0.5% CF.
As revealed through horizontal comparison, PP fibers have a slight effect on the pressure-sensitive properties, and a higher amount of PP fiber doping leads to a lower level of pressure sensitivity. When CF doping reaches 0.7%, the rate of change in resistivity is 17% higher at 0.25% PP than at 1.5% PP. PP fiber is not conductive, and its electrical impact is reflected mainly in the conductive channels formed by CF bridging. The number of dispersed fibers per unit volume of concrete is infinite. When the PP fiber dosage increases, the dispersion of CF is worse, which in turn affects the electrical properties.
Horizontal comparisons found that PP fibers have a slight effect on the pressure-sensitive properties as follows: the higher the PP fiber doping, the lower the pressure-sensitive properties when doping with 0.7% CF and 0.25% PP rather than 1.5% PP, with a resistance rate of change of 17% or higher. PP fiber itself is not conductive, and its electrical impact is mainly concentrated in the impact of the CF lapping formation of conductive channels; the number of dispersed fibers per unit volume of concrete is a certain amount when the PP fiber dosage increases, and the dispersion of CFs will become worse, which in turn affects the electrical properties.
The overall trend for the change in FCR after three cycles of loading shows good reciprocity of the time–FCR curve of the specimen, but there is an irreversible relative increase in the resistivity transformation rate during the loading process. This is because the pores and microcracks inside the material matrix are compacted, and a small amount of permanent deformation occurs, which affects the fiber overlap as a conductive channel, resulting in a jagged change in the test curve.

3.2.3. Bending Sensitivity Test

In order to better observe the changes in electrical signals during the bending sensitivity test, the ECC boards with a greater bending toughness at 1.0% PP and 1.5% PP were used. Figure 14 shows the test results.
The CF doping levels that affected FCR in the bending sensitivity test are 1.1%, 0.9%, 0.7%, and 0.5% in descending order. A maximum resistivity change of 33.58% was observed when the fiber doping was 1.0% PP and 1.1% CF, which is 65.5% higher than the lowest rate of change in fiber doping of 1.0% PP and 0.5% CF. There is a positive correlation between the bending sensitivity performance and CF dosage. This is because the bending sensitivity test was conducted in the presence of monotonic loading. With a continued increase in concrete deflection during the loading process, the fibers within the concrete are gradually pulled out and fractured, resulting in increased internal damage of the material [49]. As a result, the increase in FCR is accompanied by a gradual decrease in the number of conductive channels in the bending sensitivity test. When the CF dosage is higher, the conductive channel inside the concrete is more complete, and the number of fibers pulled out is larger when the concrete is loaded. This results in a faster reduction in the number of conductive channels, which explains the highest FCR at a dosage of 1.1% CF. PP fiber doping has a certain negative effect on the performance in relation to pressure sensitivity, but not to a significant extent. When the amount of CF doping is constant, the difference in FCR between different amounts of PP fiber doping is less than 4%.

4. Scanning Electron Microscope (SEM) Investigations

The fiber was photographed under a scanning electron microscope, as shown in Figure 15. The fibers in the concrete with a small amount of fiber doping are more completely wrapped by the hydration products. When the concrete is deformed by external forces, the fibers break (Figure 15a), which reduces the bending toughness of the material. However, in the concrete with a relatively high fiber dosage, the continuous increase in load causes the slipping or pullout of the hydration products combined with the fibers in the concrete (Figure 15b). During the slipping phase, the friction between the fibers and the matrix improves the flexural toughness of the material, which explains both the higher flexural toughness reached with the high fiber dosage of ECC slabs and the more significant change in the FCR of the concrete with a high fiber dosage in the flexural sensitivity test. Figure 15c shows the distribution of three different types of fibers in the concrete, which substantiates the previous assumption. From Figure 15d, it can be seen that polypropylene fibers affect the distribution of carbon fibers, which in turn affects the formation of conductive channels.

5. Conclusions

In this study, self-sensing ECC is produced using glass sand, PP fibers, and CF. Then, the effects of different volume fractions of CFs and PP fibers on the mechanical and self-perceived properties of ECC are investigated, resulting in the following conclusions:
(1)
CFs and PP fibers have a combined effect on the mechanical properties of the concrete. The compressive, split tensile, and flexural strengths of concrete show a trend of first increasing and then decreasing with the increase in fiber doping. When the fiber doping is 0.5% PP and 0.9% CF, the compressive strength reaches its maximum of 44.1 MPa, which is an increase of 39.2% when compared to the lowest level at 1.5% PP and 0.5% CF. In terms of the split tensile strength and flexural strength, the optimal mix ratio is 0.75 PP% and 0.9% CF. In this case, the strength reaches 5.6 MPa and 6.87 MPa, respectively.
(2)
The effect of PP fiber doping on the flexural toughness is most significant. The critical value of PP fiber strain hardening falls within the range of 0.75% vol–1.0% vol. When the level of PP fiber doping exceeds this critical value, the flexural toughness of the ECC boards shows a positive correlation with the amount of PP fiber doping. The maximum mid-span deflection of ECC panels is observed for a fiber doping of 1.5 PP% and 1.1% CF. The increase in CF doping significantly enhances the first crack strength of the concrete, but its effect on flexural toughness is insignificant.
(3)
Resistivity is significantly affected by the curing time. During the period between day 1 and day 7, the resistivity increases rapidly. From day 7 to day 14, the rate of increase is slowed down. From day 14 to day 28, the resistivity stabilizes gradually. At the end of the 28-day curing, the resistivity of the specimen decreases gradually with the increase in CF content, reaching the lowest level of 225.9 Ω cm when the amount of fiber doping is 0.25% PP and 1.1%CF. It is worth noting that the rapid decrease in resistivity by 65.2% at a maximum is observed when the CF content increases from 0.5% to 0.7%, suggesting a permeability threshold in this range. The presence of PP fibers reduces the dispersion of CF, which affects the bridging of conductive channels between the fibers, thus affecting the initial resistivity.
(4)
The effects of fiber type and doping on the pressure-sensitive performance and bending sensitivity performance are analyzed, and the change in FCR in the pressure sensitivity test and the bending sensitivity test is explained from the perspective of fiber distribution. Then, the fiber distribution theory is verified by SEM. Specifically, the amount of CF doping for the pressure sensitivity performance ranked in descending order is 0.7%, 0.9%, 1.1%, and 0.5%, while that for the bending sensitivity performance ranked in descending order is 1.1%, 0.9%, 0.7%, and 0.5%, respectively. Therefore, PP fibers have a negative impact on self-sensing properties.

Author Contributions

Conceptualization, L.M. and Y.Z.; Methodology, L.M. and M.S.; Validation, M.S. and Y.Z.; Formal Analysis, L.M. and M.S.; Investigation, L.M. and M.S.; Writing—Original Draft Preparation, L.M. and M.S.; Writing—Review and Editing M.S.; Project Administration, Y.Z.; Funding Acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science Technology Department Program of Jilin Province (Grant Nos. 20230203036SF, 20220203048SF, 20220203056SF, and 20230203068SF).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the anonymous reviewers for their constructive suggestions and comments to improve the quality of the paper.

Conflicts of Interest

The authors declare no conflicts of interests.

Abbreviations

AbridgeFull name of the word
ECCEngineered cementitious composite
PP fibersPolypropylene fibers
CFCarbon fiber
SEMScanning electron microscopy
PEPolyethylene
PVAPolyvinyl alcohol
CNTCarbon nanotube
CBCarbon black
ASRAlkali–silica reaction
FCRFractional change in resistivity

References

  1. Sun, R.; Han, L.; Zhang, H.; Ge, Z.; Guan, Y.; Ling, Y.; Schlangen, E.; Šavija, B. Fatigue life and cracking characterization of engineered cementitious composites (ECC) under flexural cyclic load. Constr. Build. Mater. 2022, 335, 127465. [Google Scholar] [CrossRef]
  2. Lan, M.; Zhou, J.; Xu, M. Effect of fibre types on the tensile behaviour of engineered cementitious composites. Front. Mater. 2021, 8, 775188. [Google Scholar] [CrossRef]
  3. Adesina, A.; Das, S. Mechanical performance of engineered cementitious composite incorporating glass as aggregates. J. Clean. Prod. 2020, 260, 121113. [Google Scholar] [CrossRef]
  4. Maalej, M.; Hashida, T.; Li, V.C. Effect of fiber volume fraction on the off-crack-plane fracture energy in strain -hardening engineered cementitious composites. J. Am. Ceram. Soc. 1995, 78, 3369–3375. [Google Scholar] [CrossRef]
  5. Li, V.C.; Wang, S.; Wu, C. Tensile strain-hardening behavior of polyvinyl alcohol engineered cementitious composite (PVA-ECC). ACI Mater. J. 2001, 98, 483–492. [Google Scholar]
  6. Pakravan, H.R.; Jamshidi, M.; Latifi, M. Study on fiber hybridization effect of engineered cementitious composites with low- and high-modulus polymeric fibers. Constr. Build. Mater. 2016, 112, 739–746. [Google Scholar] [CrossRef]
  7. Curosu, I.; Liebscher, M.; Alsous, G.; Muja, E.; Li, H.; Drechsler, A.; Frenzel, R.; Synytska, A.; Mechtcherine, V. Tailoring the crack-bridging behavior of strain-hardening cement-based composites (SHCC) by chemical surface modification of poly(vinyl alcohol) (PVA) fibers. Cem. Concr. Compos. 2020, 114, 103722. [Google Scholar] [CrossRef]
  8. Arain, M.F.; Wang, M.; Chen, J.; Zhang, H. Study on PVA fiber surface modification for strain-hardening cementitious composites (PVA-SHCC). Constr. Build. Mater. 2019, 197, 107–116. [Google Scholar] [CrossRef]
  9. Ding, C.; Guo, L.; Chen, B.; Xu, Y.; Cao, Y.; Fei, C. Micromechanics theory guidelines and method exploration for surface treatment of PVA fibers used in high-ductility cementitious composites. Constr. Build. Mater. 2019, 196, 154–165. [Google Scholar] [CrossRef]
  10. Tosun-Felekoglu, K.; Felekoglu, B. Effects of fiber–matrix interaction on multiple cracking performance of polymeric fiber reinforced cementitious composites. Compos. Part B Eng. 2013, 52, 62–71. [Google Scholar] [CrossRef]
  11. Pakravan, H.R.; Latifi, M.; Jamshidi, M. Ductility improvement of cementitious composites reinforced with polyvinyl alcohol-polypropylene hybrid fibers. J. Ind. Text. 2016, 45, 637–651. [Google Scholar] [CrossRef]
  12. Chen, P.W.; Chung, D.D.L. Carbon fiber reinforced concrete for smart structures capable of non-destructive flaw detection. Smart Mater. Struct. 1993, 2, 22–30. [Google Scholar] [CrossRef]
  13. Chen, P.W.; Chung, D.D.L. Carbon-fiber-reinforced concrete as an intrinsically smart concrete for damage assessment during dynamic loading. J. Am. Ceram. Soc. 1995, 78, 816–818. [Google Scholar] [CrossRef]
  14. Yıldırım, G.; Sarwary, M.H.; Al-Dahawi, A.; Öztürk, O.; Anıl, Ö.; Şahmaran, M. Piezoresistive behavior of CF- and CNT-based reinforced concrete beams subjected to static flexural loading: Shear failure investigation. Constr. Build. Mater. 2018, 168, 266–279. [Google Scholar] [CrossRef]
  15. Hamdi, K.; Aboura, Z.; Harizi, W.; Khellil, K. Structural health monitoring of carbon fiber reinforced matrix by the resistance variation method. J. Compos. Mater. 2020, 54, 3919–3930. [Google Scholar] [CrossRef]
  16. Al-Dahawi, A.; Öztürk, O.; Emami, F.; Yıldırım, G.; Şahmaran, M. Effect of mixing methods on the electrical properties of cementitious composites incorporating different carbon-based materials. Constr. Build. Mater. 2016, 104, 160–168. [Google Scholar] [CrossRef]
  17. Han, B.; Sun, S.; Ding, S.; Zhang, L.; Yu, X.; Ou, J. Review of nanocarbon-engineered multifunctional cementitious composites. Compos. Part A Appl. Sci. Manuf. 2015, 70, 69–81. [Google Scholar] [CrossRef]
  18. Chung, D.D.L. Piezoresistive cement-based materials for strain sensing. J. Intell. Mater. Syst. Struct. 2002, 13, 599–609. [Google Scholar] [CrossRef]
  19. Ahmed, S.; Hussain, A.; Hussain, Z.; Pu, Z.; Ostrowski, K.A.; Walczak, R. Effect of carbon black and hybrid steel-polypropylene fiber on the mechanical and self-sensing characteristics of concrete considering different coarse aggregates’ sizes. Materials 2021, 14, 7455. [Google Scholar] [CrossRef] [PubMed]
  20. Yang, F.; Qian, S. Mechanical and piezoelectric properties of ECC with CNT incorporated through fiber modification. Constr. Build. Mater. 2020, 260, 119717. [Google Scholar] [CrossRef]
  21. Liu, C.; Liu, G.; Ge, Z.; Guan, Y.; Cui, Z.; Zhou, J. Mechanical and self-sensing properties of multiwalled carbon nanotube-reinforced ECCs. Adv. Mater. Sci. Eng. 2019, 2019, 2646012. [Google Scholar] [CrossRef]
  22. Deng, H.; Li, H. Assessment of self-sensing capability of carbon black engineered cementitious composites. Constr. Build. Mater. 2018, 173, 1–9. [Google Scholar] [CrossRef]
  23. Li, V.C. Integrated structures and materials design. Mater. Struct. 2007, 40, 387–396. [Google Scholar] [CrossRef]
  24. Huang, X.; Ranade, R.; Ni, W.; Li, V.C. Development of green engineered cementitious composites using iron ore tailings as aggregates. Constr. Build. Mater. 2013, 44, 757–764. [Google Scholar] [CrossRef]
  25. Bažant, Z.P.; Zi, G.; Meyer, C. Fracture mechanics of ASR in concretes with waste glass particles of different sizes. J. Eng. Mech. 2000, 126, 226–232. [Google Scholar] [CrossRef]
  26. Corinaldesi, V.; Nardinocchi, A. Influence of recycled glass addition on mortar properties. Int. J. Manuf. Ind. Eng. 2015, 2, 6–10. [Google Scholar]
  27. Adesina, A.; Das, S. Influence of glass powder on the durability properties of engineered cementitious composites. Constr. Build. Mater. 2020, 242, 118199. [Google Scholar] [CrossRef]
  28. Du, H.; Tan, K.H. Effect of particle size on alkali–silica reaction in recycled glass mortars. Constr. Build. Mater. 2014, 66, 275–285. [Google Scholar] [CrossRef]
  29. Idir, R.; Cyr, M.; Tagnit-Hamou, A. Use of fine glass as ASR inhibitor in glass aggregate mortars. Constr. Build. Mater. 2010, 24, 1309–1312. [Google Scholar] [CrossRef]
  30. Topçu, İ.B.; Boğa, A.R.; Bilir, T. Alkali–silica reactions of mortars produced by using waste glass as fine aggregate and admixtures such as fly ash and Li2CO3. Waste Manag. 2008, 28, 878–884. [Google Scholar] [CrossRef]
  31. GB175-2007; Standard for Common Portland Cement. Chinese Standard: Beijing, China, 2007. (In Chinese)
  32. GB/T51003/2014; Technical Code for Application of Mineral Admixture. Chinese Standard: Beijing, China, 2014. (In Chinese)
  33. GB/T50081-2019; Standard for Test Methods of Concrete Physical and Mechanical Properties. Chinese Standard: Beijing, China, 2019. (In Chinese)
  34. GB/T15231-2008; Test Methods for the Properties of Glassfibre Reinforced Cement. Chinese Standard: Beijing, China, 2008. (In Chinese)
  35. Chindaprasirt, P.; Boonbamrung, T.; Poolsong, A.; Kroehong, W. Effect of elevated temperature on polypropylene fiber reinforced alkali-activated high calcium fly ash paste. Case Stud. Constr. Mater. 2021, 15, e00554. [Google Scholar] [CrossRef]
  36. Orouji, M.; Zahrai, S.M.; Najaf, E. Effect of glass powder & polypropylene fibers on compressive and flexural strengths, toughness and ductility of concrete: An environmental approach. Structures 2021, 33, 4616–4628. [Google Scholar]
  37. Li, Y.F.; Yang, K.H.; Hsu, P.Y.; Syu, J.Y.; Wang, S.J.; Kuo, W.S.; Tsai, Y.K. Comparing mechanical characterization of carbon, kevlar, and hybrid-fiber-reinforced concrete under quasistatic and dynamic loadings. Buildings 2023, 13, 2044. [Google Scholar] [CrossRef]
  38. Zheng, L.; Zhou, J. Mechanical, chloride permeation, and freeze–thaw resistance of recycled micronized powder polypropylene-fiber-engineered cementitious composites. Buildings 2023, 13, 2755. [Google Scholar] [CrossRef]
  39. Muley, P.; Varpe, S.; Ralwani, R. Chopped carbon fibers innovative material for enhancement of concrete performances. Int. J. Sci. Eng. Appl. Sci. 2015, 1, 164–169. [Google Scholar]
  40. Han, B.; Zhang, L.; Zhang, C.; Wang, Y.; Yu, X.; Ou, J. Reinforcement effect and mechanism of carbon fibers to mechanical and electrically conductive properties of cement-based materials. Constr. Build. Mater. 2016, 125, 479–489. [Google Scholar] [CrossRef]
  41. Li, J.J.; Niu, J.G.; Wan, C.J.; Jin, B.; Yin, Y.L. Investigation on mechanical properties and microstructure of high performance polypropylene fiber reinforced lightweight aggregate concrete. Constr. Build. Mater. 2016, 118, 27–35. [Google Scholar] [CrossRef]
  42. Liu, B.; Guo, J.; Zhou, J.; Wen, X.; Deng, Z.; Wang, H.; Zhang, X. The mechanical properties and microstructure of carbon fibers reinforced coral concrete. Constr. Build. Mater. 2020, 249, 118771. [Google Scholar] [CrossRef]
  43. Wang, Y.; Zhang, S.; Luo, D.; Shi, X. Effect of chemically modified recycled carbon fiber composite on the mechanical properties of cementitious mortar. Compos. Part B Eng. 2019, 173, 106853. [Google Scholar] [CrossRef]
  44. Vijayan, D.S.; Sivasuriyan, A.; Devarajan, P.; Stefańska, A.; Wodzyński, Ł.; Koda, E. Carbon fibre-reinforced polymer (CFRP) composites in civil engineering application—A comprehensive review. Buildings 2023, 13, 1509. [Google Scholar] [CrossRef]
  45. Wei, H.; Wu, T.; Yang, X. Properties of lightweight aggregate concrete reinforced with carbon and/or polypropylene fibers. Materials 2020, 13, 640. [Google Scholar] [CrossRef]
  46. Sharma, U.; Gupta, N.; Bahrami, A.; Özkılıç, Y.O.; Verma, M.; Berwal, P.; Althaqafi, E.; Khan, M.A.; Islam, S. Behavior of fibers in geopolymer concrete: A comprehensive review. Buildings 2024, 14, 136. [Google Scholar] [CrossRef]
  47. Hossain, M.Z.; Awal, A.S.M.A. Flexural response of hybrid carbon fiber thin cement composites. Constr. Build. Mater. 2011, 25, 670–677. [Google Scholar] [CrossRef]
  48. Chen, B.; Wu, K.; Yao, W. Conductivity of carbon fiber reinforced cement-based composites. Cem. Concr. Compos. 2004, 26, 291–297. [Google Scholar] [CrossRef]
  49. Shahzad, S.; Toumi, A.; Balayssac, J.P.; Turatsinze, A. An experimental approach to assess the sensitivity of a smart concrete. Buildings 2023, 13, 2034. [Google Scholar] [CrossRef]
Buildings 14 00938 g001
Figure 1. Grain structure of glass sand under SEM.
Figure 1. Grain structure of glass sand under SEM.
Buildings 14 00938 g001
Buildings 14 00938 g002
Figure 2. Fiber sizes and styles.
Figure 2. Fiber sizes and styles.
Buildings 14 00938 g002
Buildings 14 00938 g003
Figure 3. Pressure sensitivity testing. (a) Pressure sensitive electrodes arrangement. (b) Loading method.
Figure 3. Pressure sensitivity testing. (a) Pressure sensitive electrodes arrangement. (b) Loading method.
Buildings 14 00938 g003
Buildings 14 00938 g004
Figure 4. Bending sensitivity measurement. (a) Bending sensitive electrode arrangement. (b) Loading method.
Figure 4. Bending sensitivity measurement. (a) Bending sensitive electrode arrangement. (b) Loading method.
Buildings 14 00938 g004
Buildings 14 00938 g005
Figure 5. Compressive strength of ECC with different fiber dosages.
Figure 5. Compressive strength of ECC with different fiber dosages.
Buildings 14 00938 g005
Buildings 14 00938 g006
Figure 6. Split tensile strength.
Figure 6. Split tensile strength.
Buildings 14 00938 g006
Buildings 14 00938 g007
Figure 7. Flexural strength.
Figure 7. Flexural strength.
Buildings 14 00938 g007
Buildings 14 00938 g008
Figure 8. Load deflection curve.
Figure 8. Load deflection curve.
Buildings 14 00938 g008
Buildings 14 00938 g009
Figure 9. Initial cracking/ultimate strength of ECC panels with the same fiber dosage.
Figure 9. Initial cracking/ultimate strength of ECC panels with the same fiber dosage.
Buildings 14 00938 g009
Buildings 14 00938 g010
Figure 10. Effect of maintenance time on resistivity.
Figure 10. Effect of maintenance time on resistivity.
Buildings 14 00938 g010
Buildings 14 00938 g011
Figure 11. Effect of fiber doping on resistivity.
Figure 11. Effect of fiber doping on resistivity.
Buildings 14 00938 g011
Buildings 14 00938 g012
Figure 12. Pressure-sensitive FCR with different fiber dosage.
Figure 12. Pressure-sensitive FCR with different fiber dosage.
Buildings 14 00938 g012
Buildings 14 00938 g013
Figure 13. CF distribution status.
Figure 13. CF distribution status.
Buildings 14 00938 g013
Buildings 14 00938 g014aBuildings 14 00938 g014b
Figure 14. Bending sensitivity of FCR with different fiber dosages.
Figure 14. Bending sensitivity of FCR with different fiber dosages.
Buildings 14 00938 g014aBuildings 14 00938 g014b
Buildings 14 00938 g015
Figure 15. Fiber distribution under SEM lens.
Figure 15. Fiber distribution under SEM lens.
Buildings 14 00938 g015
Table 1. Physical and chemical properties of cement.
Table 1. Physical and chemical properties of cement.
PropertiesStandard ValueActual Value
Physical propertiesSpecific surface area (m2/kg)≥300329
Initial set (min)≥45192
Final set (min)≤600240
Compressive strength3 days (MPa)≥17.027.7
28 days (MPa)≥42.548.7
Flexural strength3 days (MPa)≥3.55.2
28 days (MPa)≥6.57.6
Chemical propertiesLoss on ignition (%)≤5.03.64
MgO (%)≤5.01.01
CaO (%)≥66.066.54
SiO2 (%)≥20.021.03
Al2O3 (%)≥4.04.36
Fe2O3 (%)≥2.02.32
CaSO4·2H2O (%)≥2.02.14
SO3 (%)≤3.52.16
Cl− (%)≤0.060.021
Table 2. Physical and chemical properties of silica fume.
Table 2. Physical and chemical properties of silica fume.
PropertiesStandard ValueActual Value
Physical and chemical propertiesSiO2 (%)≥90.090.41
MgO (%)-0.71
Al2O3 (%)-1.04
CaO (%)-0.27
Fe2O3 (%)-0.32
Loss on ignition (%)≤2.01.37
Cl− (%)≤2.00.125
PH4.0~8.56.8
Moisture content (%)≤3.00.65
Water demand ratio (%)≤125117
Table 3. Physical and chemical properties of fly ash.
Table 3. Physical and chemical properties of fly ash.
PropertiesStandard ValueActual Value
Physical propertiesFineness (%)≤1210.6
Chemical propertiesWater demand ratio (%)≤9593
Loss on ignition (%)≤5.00.77
Moisture content (%)≤1.00.11
SiO2 (%)≥40.057.33
Al2O3 (%)≥10.018.25
Fe2O3 (%)≥2.05.65
CaO (%)≥5.06.17
SO3 (%)≤3.00.10
CaO3 (%)≤1.00.68
Strong activity index (%)≥7074
Table 4. Chemical composition of glass sand.
Table 4. Chemical composition of glass sand.
SiO2 (%)KCl (%)Na2O (%)CaO (%)MgO (%)Fe2O3 (%)Al2O3 (%)
72.810.7213.358.741.150.182.62
Table 5. Polypropylene fiber performance indicators.
Table 5. Polypropylene fiber performance indicators.
Caliber
(μm)		Lengths
(mm)		Density
(g/mm3)		Rupture Strength
(MPa)		Elongation at Break
(%)		Melting Point
(°C)
20120.9168018165
Table 6. Carbon fiber performance indicators.
Table 6. Carbon fiber performance indicators.
Caliber
(μm)		Lengths
(mm)		Density
(g/mm3)		Rupture Strength
(MPa)		Elongation at Break
(%)		Carbon ContentModulus of Elasticity (GPa)
661.7939501.4595.9%238
Table 7. Performance index of water-reducing agent.
Table 7. Performance index of water-reducing agent.
Test ItemsStandard ValueActual Value
Water reduction rate (%)≥2530
Gas content (%)≤6.03.0
Normal pressure water secretion ratio (%)≤6010
Na2SO4 (%)≤5.00.6
Cl− (%)≤0.60.03
Total alkali content (%)≤101.12
Shrinkage ratio (%)≤110102
pH5.0 ± 1.05.2
Density g/cm31.06 ± 0.021.06
Solid content (%)38 ± 1.938
Table 8. Mixing ratio design.
Table 8. Mixing ratio design.
Batch NumberCementFly AshSilica FumeGlass SandWaterCFsPP Fibers
N14805761444323600.5%0.25%
N24805761444323600.5%0.5%
N34805761444323600.5%0.75%
N44805761444323600.5%1.0%
N54805761444323600.5%1.5%
N64805761444323600.7%0.25%
N74805761444323600.7%0.5%
N84805761444323600.7%0.75%
N94805761444323600.7%1.0%
N104805761444323600.7%1.5%
N114805761444323600.9%0.25%
N124805761444323600.9%0.5%
N134805761444323600.9%0.75%
N144805761444323600.9%1.0%
N154805761444323600.9%1.5%
N164805761444323601.1%0.25%
N174805761444323601.1%0.5%
N184805761444323601.1%0.75%
N194805761444323601.1%1.0%
N204805761444323601.1%1.5%
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Ma, L.; Sun, M.; Zhang, Y. The Mechanical and Self-Sensing Properties of Carbon Fiber- and Polypropylene Fiber-Reinforced Engineered Cementitious Composites Utilizing Environmentally Friendly Glass Aggregate. Buildings 2024, 14, 938. https://doi.org/10.3390/buildings14040938

AMA Style

Ma L, Sun M, Zhang Y. The Mechanical and Self-Sensing Properties of Carbon Fiber- and Polypropylene Fiber-Reinforced Engineered Cementitious Composites Utilizing Environmentally Friendly Glass Aggregate. Buildings. 2024; 14(4):938. https://doi.org/10.3390/buildings14040938

Chicago/Turabian Style

Ma, Lijun, Meng Sun, and Yunlong Zhang. 2024. "The Mechanical and Self-Sensing Properties of Carbon Fiber- and Polypropylene Fiber-Reinforced Engineered Cementitious Composites Utilizing Environmentally Friendly Glass Aggregate" Buildings 14, no. 4: 938. https://doi.org/10.3390/buildings14040938

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop