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Article

Mechanical Properties of Polypropylene Fiber Recycled Brick Aggregate Concrete and Its Influencing Factors by Gray Correlation Analysis

by
Shangwei Gong
1,
Lichao Bai
1,
Zhenyu Tan
2,
Lina Xu
3,
Xiaohong Bai
4 and
Zhanfang Huang
1,*
1
School of Civil Engineering and Geomatics, Shang Dong University of Technology, Zibo 255049, China
2
School of Engineering Management, Krirk University, Bangkok 10220, Thailand
3
School of Transportation Science and Engineering, Jilin Jianzhu University, Changchun 130118, China
4
College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(14), 11135; https://doi.org/10.3390/su151411135
Submission received: 1 June 2023 / Revised: 3 July 2023 / Accepted: 16 July 2023 / Published: 17 July 2023
(This article belongs to the Special Issue Climate Change and Enviromental Disaster)
Browse Figures
Versions Notes

Abstract

:
Making construction waste into raw materials for recycled concrete is beneficial for resource conservation and environmental protection. This paper investigated the effects of different recycled brick aggregate (RBA) replacement rates (30%, 50%, 70%, and 100%) and different contents of polypropylene fibers (PPFs) (0.08%, 0.10%, 0.12%, 0.16%, and 0.2%) on the mechanical properties of recycled brick concrete. Gray correlation was also used to analyze the degree of effect factors on the mechanical properties of concrete. The results showed that the mechanical properties decreased when the natural coarse aggregate (NCA) was replaced with RBA, while PPFs could better improve the mechanical properties of RBA concrete. The improvement of compressive and flexural properties was optimal when the PPF content was 0.12%; the improvement of tensile properties was optimal when the PPF content was 0.2%. In addition, PPFs significantly improved the toughness of RBA concrete. The gray correlation degrees between compressive strength (tensile strength, flexural strength) and NCA, RBA, and PPFs were 0.8964 (0.8691, 0.8935), 0.7301 (0.6530, 0.7074), and 0.5873 (0.5870, 0.5840), respectively.

1. Introduction

With the economic development and urbanization of developing countries, many old buildings need to be demolished and rebuilt globally every year [1,2], generating a large amount of construction waste. Red bricks and concrete waste account for approximately 85% of construction waste [3,4]. The disposal of construction waste is mostly carried out by simple landfill treatment, which occupies a large amount of land resources, pollutes the environment, and indirectly accelerates the consumption of natural resources [5,6]. Therefore, it is important to improve the utilization of construction waste to protect the ecological environment [7]. For the sake of clarity, the abbreviations used in this paper and their specific meanings are shown in Table 1.
Recycling construction waste as recycled aggregates and partially or completely replacing natural aggregates to produce recycled aggregate concrete are effective ways to solve the shortage of natural aggregates, waste concrete treatment, and related environmental problems [8,9]. The preparation of recycled concrete conforms to the “low-carbon, green” environmental protection concept. Moreover, the development of new technologies, such as ultrasonic pulse velocity, also provides new methods for testing the strength of recycled concrete [10]. Therefore, many scholars have conducted extensive research on the mechanical properties of recycled concrete. Zheng et al. [11] investigated the mechanical properties of recycled stone aggregate concrete and recycled brick aggregate (RBA) concrete with different strength classes. The results showed that the compressive strengths of RBA concrete and recycled stone aggregate concrete were not significantly different when the concrete strength was C25. However, the compressive strength of RBA concrete was significantly lower than that of recycled stone aggregate concrete when the concrete strength was C50. Xu et al. [12] found that the compressive and tensile strengths of RBA concrete were lower than those of natural coarse aggregate (NCA) concrete. The RBA had a small effect on the tensile strength of the concrete, but it had a greater effect on the compressive strength. As the replacement rate of RBA increased, the compressive strength showed a trend of first decreasing and then increasing. Liang et al. [13] reported the relationship between the compressive strength and the water–cement ratio of 100% RBA concrete and established a formula for the water–cement ratio and compressive strength of RBA concrete. It was shown that the compressive strength of RBA concrete decreased with the increase in the water–cement ratio. In concrete structural applications, it has also been discovered that the shear capacity of channels in light aggregate concrete was less than that in NCA concrete [14,15,16].
RBA has disadvantages, such as having larger porosity, higher water absorption, lower apparent density, and a higher crushing index than NCA [17,18]. Thus, the recycled concrete application rate in practical projects is not widespread [19,20]. Therefore, it is particularly important to research the improvement of the properties of RBA concrete. Many scholars have provided a new way of thinking about research on concrete property improvement. Raydan et al. [21] revealed that processing waste glass into glass particles added to concrete could improve the mechanical properties. Shariati et al. [22] showed that concrete permeability could be improved by making waste tires into aggregates. Toghroli et al. [23] found that the coupling of composite polymer, silica fume, and rubber aggregate could improve the mechanical properties and water permeability of concrete.
Currently, the most common way to improve the performance of concrete is to add fibers. The addition of fibers in recycled concrete can inhibit the generation and development of cracks, improve the pore structure, reduce water absorption, and improve toughness. Different types of fibers have different improvement effects on recycled concrete. Yuan et al. [24] added 1.35% glass fibers to recycled concrete specimens with a water–cement ratio of 0.3. It was found that glass fibers could effectively reduce the water absorption of the recycled concrete, and the water absorption of the specimens with glass fibers decreased from 3.49% to 1.99%. Rishav et al. [25] reported that polypropylene fibers (PPFs) could effectively reduce the pore space inside recycled concrete and improve its mechanical properties. Omidinasab et al. [26] showed that steel fibers could reduce the negative effects of recycled aggregates on mechanical and impact properties. Mehrabi et al. [27] and Toghroli et al. [28] discovered that steel fibers were the most effective at improving the flexural strength of recycled concrete by comparing steel fibers, waste plastic fibers, and macrofibers.
Compared with other fibers, PPFs have the advantages of being lightweight and easy to handle and having a low cost, low thermal conductivity, acid and alkali resistance, etc. These characteristics make PPFs more suitable for incorporation into concrete [29]. The high tensile strength of PPFs could better compensate for the shortcomings of the high brittleness of concrete. Zhang et al. [30] proposed an examination of the effect patterns of different PPF admixtures on the mechanical properties of RBA concrete. It was concluded that the compressive strength of the recycled concrete was most improved by 3.7% with 1.2% PPF contents, and the tensile strength of the recycled concrete was most improved by 47.5% with 0.9% PPF contents. Li et al. [31] conducted experiments by adding different percentages of PPFs to recycled concrete. It was observed that the effect of PPFs on the cubic compressive strength and axial compressive strength of the recycled concrete was not significant. The optimum admixture of PPFs at 0.9% yielded the best improvement in the tensile strength of the recycled concrete, with 71.1% improvement.
PPFs do not participate in the hydration reaction of the recycled concrete, which improved the mechanical properties of the concrete through physical connections that inhibited the generation and development of pores and improved the pore structure [32,33]. Xu et al. [34] performed a scanning electron microscopy (SEM) analysis of recycled concrete. The results showed that the PPFs transferred the load to the surrounding matrix through bridging cracks, which effectively improved the mechanical properties of the recycled concrete. Xue et al. [35] reported that PPFs could improve the inhomogeneity of the internal voids of concrete. Wang et al. [36] showed that PPFs had a good inhibitory effect on the crack initiation and expansion stages of recycled concrete. PPFs could increase the loading capacity of the specimen after the peak load, put the specimen in the damage mode of “cracking but not breaking”, and improve the toughness and crack resistance of the recycled concrete. However, the mechanical properties of recycled concrete do not grow indefinitely with the addition of fibers [37,38]. Li et al. [39] analyzed the microfabrication structure of recycled concrete. They found that fiber incorporation increased the void volume of concrete. Excessive fibers could introduce mass air bubbles, resulting in weak fiber dispersion and the macroporous deterioration of recycled concrete. The optimum PPF content obtained in experiments differs due to different factors, such as aggregates, gradations, and mixture proportions [40,41]. Małek et al. [42] investigated the mechanical properties of two different types of PPFs by testing them in recycled concrete. The results showed that a PPF dosing of 1% provided the best mechanical property improvement for recycled concrete. Jorbat et al. [29] showed that a PPF dosing of 0.35% was the best for improving the mechanical properties of two different types of plain concrete. Therefore, adding the right number of fibers could effectively improve the mechanical properties of recycled concrete [43,44].
Research on existing PPF concrete has revealed that the data on PPF admixtures varies greatly among studies [30,45,46]. The fluidity of the concrete and the PPF agglomeration phenomenon are the main factors that determine the optimal PPF admixture. At the same ratio, the fluidity of concrete gradually decreases with an increase in the PPF mixture. A decrease in concrete fluidity often makes it difficult to mix the concrete mixture evenly, which in turn affects the concrete’s mechanical properties. A large number of PPFs requires the addition of a water-reducing agent to maintain the fluidity of the concrete. Because the length-to-diameter ratio of PPFs is very large, they can easily agglomerate with each other. More often than not, the larger the PPF admixture, the more agglomeration occurs. In fact, it is very difficult to control the agglomeration of PPFs in concrete. In this test, it was also found in the preliminary adaptation stage that a high PPF admixture could greatly reduce the fluidity of the concrete. Without considering water-reducing agents and the need to maintain good fluidity of concrete, the maximum admixture of PPFs was not high [45]. Under repeated tests, the highest PPF amount was determined to be 0.2%.
Previous studies have mainly focused on the effects of PPFs on the properties of recycled stone aggregate concrete, while there have been relatively few studies on the mechanical properties of PPF-strengthened RBA concrete. The objective of this study was to investigate the effect of the addition of different RBA replacement rates (30%, 50%, 70%, and 100% replacement rates) and different amounts of PPF (0.08%, 0.10%, 0.12%, 0.16%, and 0.20% by volume fraction) on the mechanical properties (compressive strength, tensile strength, and flexural strength) of concrete. Moreover, the study explored the effects of NCA, RBA, and PPFs on recycled concrete through gray correlation model analysis.

2. Experimental Details

2.1. Materials

P·O 42.5 ordinary Portland cement of the Conch brand was adopted for this experiment, and its main physical properties and chemical composition are shown in Table 2 and Table 3, respectively. The performance of the cement met the requirements of GB 175-2007 [47]. The fine aggregate adopted natural river sand with a grain size of not more than 4.75 mm, and the fineness modulus was 2.9. The gradation curve of fine aggregate is shown in Figure 1. The red waste bricks came from a brick demolition house in a rural area of Zibo City and were crushed, sieved, and cleaned to remove surface impurities to make RBA. Both NCA and RBA were continuously graded with a grain size of not more than 26.5 mm, and their gradation curves and appearance are shown in Figure 2 and Figure 3, respectively. According to the grading standards of aggregates in GB/T 25177-2010 [48], the NCA and RBA conformed to the requirements of zone I aggregate and zone III aggregate, respectively. The properties of the coarse aggregates are shown in Table 4. The concrete mixture was made with city tap water in accordance with the JGJ 63-2006 water use standard [49]. PPFs were purchased from a private company in China. The properties of the PPFs are shown in Table 5, and their appearance is shown in Figure 4.

2.2. Mixture Proportions and Specimen Preparation

A total of 10 concrete mixture proportions were designed according to JGJ 55-2011 [50]. The concrete mixture slump requirements met the reference proportions. The concrete mixture proportions are shown in Table 6. The specimen numeration is illustrated in Figure 2. The strength of the concrete decreased significantly when the replacement rate of the recycled aggregates exceeded 50% [45,46]. Zheng et al. [11] found that PPFs provided the highest mechanical strength to concrete at a 25% replacement rate by adding PPFs to recycled concrete with different replacement rates. Zhang et al. [30] found that PPFs provided the highest mechanical strength to concrete at a 30% replacement rate. To make the RBA concrete with high strength, PPFs were selected to modify the concrete at a 30% replacement rate for this experiment.
RBA has many internal cracks, resulting in a high water absorption rate [51,52]. RBA needs to be saturated surface dry before the specimens are poured [53,54]. Otherwise, the RBA may absorb part of the water, reduce the water–cement ratio of the test, and produce certain shrinkage cracks, which adversely affect the properties of recycled concrete [55]. Therefore, the RBA was soaked in water for 24 h and then removed to dry for 2 h. The treated RBA was considered to be in a saturated surface dry state [56].
To explore the water percentage required for the saturated surface dry state of the RBA after treatment, the following tests were performed. Twelve kilograms of RBA were taken for soaking and air-drying treatment. The treated RBA was divided into three parts and measured separately, and the average value was taken. The water percentage required to achieve the saturated surface dry state of the RBA was calculated following the formula in (1):
W = m 2 − m 1 m 1
where W is the water percentage required to saturate the surface dry state of RBA after treatment (%), m1 is the quality of RBA in the laboratory (kg), and m2 is the quality of RBA after treatment (kg). The water percentage required for the saturated surface state of the RBA after the treatment was calculated to be 9.12%. The difference between the water percentage required to saturate the surface and the water absorption rate of the RBA was not significant. Basically, it could be considered that the RBA reached the saturated surface dry state after soaking for 24 h and drying for 2 h.
According to JGJ/T 221-2010 [57], the feeding method of concrete was to place PPFs, NCA, and sand into a mixer for 50 s of dry mixing, then add cement, prewetted treated RBA, and water for 130 s. The compressive strength and tensile strength tests adopted a specimen size of 100 mm × 100 mm × 100 mm, and the flexural strength test adopted a specimen size of 100 mm × 100 mm × 400 mm. After the pouring was completed, the surface needed to be covered with plastic film to prevent the concrete moisture from evaporating. The molds were removed after two days and sent to the standard curing room for 28 days.

2.3. Test Procedure

The compressive strength test was loaded at a rate of 0.5 MPa/s, the tensile strength test was loaded at a rate of 0.07 MPa/s, and the flexural strength test was loaded at a rate of 0.07 MPa/s [58].
The conversion coefficient of compressive strength is 0.95, and the strength formula is as follows:
f c u = 0.95 F A
where fcu is the compressive strength (MPa); F is the failure load (N); and A is the bearing area (mm2). The conversion coefficient of tensile strength is 0.95, and the strength formula is as follows:
f t s = 0.85 2 F π A
where fts is the tensile strength (MPa). The conversion coefficient of flexural strength is 0.95, and the strength formula is as follows:
f f = 0.85 F L b h 2
where ff is the flexural strength (MPa); L is the span between supports (mm); b is the section width (mm); and h is the section height (mm).

3. Results and Discussion

The average, standard deviation, and coefficient of variation for each group of tests are shown in Table 7.

3.1. Compression Resistance

Figure 3 shows the compressive properties of recycled concrete with the RBA replacement rate. The results showed that the compressive strength decreased as the RBA replacement rate increased. The compressive strength was reduced by 21%, 20%, 41%, and 46% when the RBA replacement rate was 30%, 50%, 70%, and 100%, respectively. The reason was that RBA suffered secondary damage after being crushed by the jaw crusher, causing more microcracks inside them [59] and resulting in a less robust RBA than NCA. In addition, the compressive strength increased slightly when the specimens had a 50% RBA replacement rate. The reason for this was probably that the elastic modulus of the RBA was closer to that of the mortar, which could have reduced the stress concentration and made better use of the strength of the RBA when subjected to external loads [60,61].
Figure 4 shows the compressive properties of recycled concrete with the PPF admixture when the RBA replacement rate was 30%. The compressive strength showed a trend of increasing and then decreasing with increasing PPF admixture content. Compared with recycled concrete without PPFs, the compressive strength was increased by 0.07%, 0.79%, 6.94%, 6.44%, and 1.79% when the PPF admixture was 0.08%, 0.10%, 0.12%, 0.16%, and 0.20%, respectively. Akid et al. [45] found that the compressive strength of NCA concrete was increased by up to 16% by PPFs. Zhang et al. [30] showed that the compressive strength of concrete with a 30% replacement rate was increased by up to 3.7% by PPFs. Das et al. [46] observed that the compressive strength of concrete with a 100% replacement rate was increased by up to 3% by PPFs. The compressive strength improvement of RBA recycled concrete provided by PPFs was slightly higher than that of recycled stone aggregate but much less than that of NCA concrete. The reason for the increased compressive strength was that the PPFs inside the recycled concrete could form a three-dimensional mesh structure [62], which could better prevent the generation of cracks and reduce the stress concentration and tip effect inside the recycled concrete. The reason for the decreased compressive strength was that the excessive PPFs were not uniformly dispersed, knotted, and caked during the mixing process, as described by Ref. [63], which led to an increase in the weak ITZ between the PPFs and the cement paste [64,65].
The fit regression model between the compressive strength and the PPFs was as follows:
y = 27.97531 + 0.1456 0.0567 π 2 e − 2 ( x − 0.14619 ) 2 0.0567 2 R 2 :   0.8626

3.2. Tensile Properties

Figure 5 shows the tensile properties of recycled concrete versus the RBA replacement rate. The results showed that the tensile strength decreased with increased RBA replacement. The tensile strength was reduced by 30%, 27%, 32%, and 54% when the RBA replacement rate was 30%, 50%, 70%, and 100%, respectively. This may be explained by the complex ITZ and the low-pressure crushing index of the RBA. The interface transition zone within the recycled concrete was roughly divided into NCA mortar and RBA mortar, in which a complex interface transition zone could reduce the tensile strength [66]. Moreover, the RBA crushing index was much greater than that of NCA, which was destroyed before NCA when the recycled concrete was loaded.
Figure 6 shows the tensile properties of recycled concrete for the PPF admixture. The tensile strength increased with an increasing PPF admixture content. Compared with recycled concrete without PPFs, the tensile strength of the recycled concrete was increased by 37.73%, 38.77%, 49.43%, 50.05%, and 50.26% when the amount of PPF admixture was 0.08%, 0.10%, 0.12%, 0.16%, and 0.20%, respectively. Das et al. [46] found that the tensile strength of NCA concrete was increased by up to 17% by PPFs. Zhang et al. [30] showed that the tensile strength of concrete with a 30% replacement rate was increased by up to 39.9% by PPFs. Ahmed et al. [38] observed a tensile strength of concrete with a 100% replacement rate was increased by up to 23% by PPFs. The tensile strength improvement of RBA concrete provided by PPFs was much higher than that of NCA concrete and recycled stone concrete. The significant increase in tensile strength may be explained by the fact that PPFs could strengthen the friction and mechanical bite between the coarse aggregate and the cement paste matrix and act as a “bridge” in the cracked cross-section [24,36].
The fit regression model between the tensile strength and PPFs was as follows:
y = 5.94949 x + 1.97729 R 2 :   0.94

3.3. Flexural Performance

Figure 7 shows the flexural properties of recycled concrete with the RBA replacement ratio. The results showed that the flexural strength decreased with an increasing RBA replacement rate. The flexural strength was reduced by 6%, 10%, 34%, and 35% when the RBA replacement rate was 30%, 50%, 70%, and 100%, respectively. The reason may be that the RBA made from waste bricks had disadvantages, such as a high crushing index, high water absorption, and high porosity [17,18], which made the properties of RBA much weaker than those of NCA.
Figure 8 shows the flexural properties of recycled concrete with the PPF admixture. As with the case of compressive strength, the flexural strength increased and then decreased with an increasing PPF admixture. Compared with recycled concrete without PPFs, the flexural strength was increased by 0.15%, 2.3%, 5.22%, 5.19%, and 4.27% when the PPF admixture was 0.08%, 0.10%, 0.12%, 0.16%, and 0.20%, respectively. Yuan et al. [24] found that the flexural strength of NCA concrete was increased by up to 9% by PPFs. Zhang et al. [30] showed that the flexural strength of concrete with a 30% replacement rate was increased by up to 12% by PPFs. Das et al. [46] observed that the flexural strength of concrete with a 100% replacement rate was increased by up to 17% by PPFs. Compared with other aggregates, it had the lowest increase in the flexural strength of RBA concrete by PPFs. The reason for the increased flexural strength was that the appropriate amount of PPF admixture could better fill the pores, improve the pore structure, and make better use of the bridging effect of PPFs to redistribute the stress and reduce the stress concentration [67]. The reason for the decreased flexural strength was that the excessive PPFs required a large amount of cement paste to cover the fibers, which reduced the fluidity of the concrete mix and affected the denseness of the recycled concrete [68].
The fit regression model between the flexural strength and PPFs was as follows:
y = { 3.4956 + 0.1824 e − 0.5 ( x − 0.12 0.01557 ) 2 x < 0.12 3.4956 + 0.1824 e − 0.5 ( x − 0.12 0.05574 ) 2 x ≥ 0.12 R 2 :   0.75165

3.4. Tension–Compression Ratio

The tension–compression ratio refers to the ratio of tensile strength to the compressive strength of the concrete and is an important index to evaluate the brittleness of materials [63,69]. In this experiment, the tensile–compression ratio of recycled concrete with a 0.45 water–cement ratio and 19 mm PPF length is shown in Figure 9. Compared with recycled concrete without PPF, the tension–compression ratio of recycled concrete was increased by 37.65%, 37.79%, 39.83%, 40.99%, and 47.67% when the PPF admixture was 0.08%, 0.10%, 0.12%, 0.16%, and 0.20%, respectively. The reason for the increasing tension–compression ratio was that the admixture of PPFs reduced the brittleness and improved the deformability of the RBA concrete, resulting in the increase in tensile strength being much greater than the increase in compressive strength.

3.5. Scanning Electron Microscopy Analysis

The SEM images of the specimens with different PPF admixtures are shown in Figure 10. The ITZ of the PPFs and the matrix was a weak link inside the concrete, and this is an important structure affecting the properties of the concrete [62]. Figure 10a shows that the ITZ of the PPFs and the matrix was tighter. These dispersed PPFs could effectively play a positive role, such as connecting the substrate and transferring the load. Figure 10b shows the crisscrossing and disorderly distribution of PPFs in the matrix, which eventually formed a three-dimensional distribution network within the concrete [34]. This structure could effectively reduce the stress concentration and the generation and development of cracks in the concrete, which had a positive impact on the macromechanical properties [64]. Figure 10c,d show that the PPFs agglomerated together, and the ITZ of the PPF and matrix was loosely bound. The agglomeration of PPFs led to an increase in the weak ITZ between the PPFs and the matrix, which negatively affected the macromechanical properties [65].

4. Gray Correlation Analysis

Gray correlation analysis is a multifactor analysis method using mathematical statistics [70,71,72]. Gray correlation analysis treats the influencing factors and properties as subsequences and parent sequences, respectively. The gray correlation degree is calculated to judge the influence relationship between the subsequence and the parent sequence. The higher the value of the gray correlation degree, the closer the relationship between the subsequence and the parent sequence. The specific method steps are as follows:
(1)
This test uses impact factors as subsequences: Xi(k) = {Xi(1), Xi(2), Xi(3), … Xi(n)}, where i indicates NCA, RBA, or PPF, and n indicates the type of test performed. The test uses mechanical properties as the parent sequences: Yj(k) = {Yj(1), Yj(2), Yj(3), … Yj(n)}, where j represents the compressive, tensile, and flexural strength.
(2)
The original data matrix was set as Zi(k) = {Xi(1), Xi(2), Xi(3), … Xi(n), Yj(1), Yj(2), Yj(3), … Yj(n)}. The mean value method was used to make the data of each different unit index dimensionless for the sake of equivalence and homoscedasticity of the experimental data. The processed matrix was set to Zi(k) = {Xi(1), Xi(2), Xi(3), … Xi(n), Yj(1), Yj(2), Yj(3), … Yj(n)}.
Z ′ i ( k ) = Z i ( k ) 1 n ∑ k = 1 n Z i ( k )
(3)
The absolute difference series for each sequence were determined to form the matrix Δm:
Δ m = | Y j ( k ) − X i ( k ) |
(4)
The maximum difference between the two levels was calculated as M, and the minimum difference between the two levels was calculated as m:
M = max i max k Δ m ( k ) m = min i min k Δ m ( k )
(5)
The correlation coefficient was calculated, where ρ was 0.5, with the following formula:
ε 0 i ( k ) = m + ρ M Δ m ( k ) + ρ M
(6)
The gray correlation degree was calculated:
γ 0 i = 1 n ∑ k = 1 n ε 0 i
The specific calculation of the gray correlation was as follows.
(1)
The original matrix is Zi(k).
Z i ( k ) = ( 100 0 0 35.27 2.7446 3.7106 70 30 0 27.95 1.924 3.4956 50 50 0 28.1 2.013 3.356 30 70 0 20.78 1.8574 2.4587 0 100 0 19.04 1.268 2.3985 70 30 0.8 27.97 2.65 3.501 70 30 0.1 28.17 2.67 3.576 70 30 0.12 29.89 2.875 3.678 70 30 0.16 29.75 2.887 3.667 70 30 0.2 28.45 2.891 3.645 )
(2)
Dimensionless processing.
Z ′ i ( k ) = ( 1.6667 0.0000 0.0000 1.2808 1.1549 1.1078 1.1667 0.7500 0.0000 1.0150 0.8090 1.0436 0.8333 1.2500 0.0000 1.0204 0.8464 1.0019 0.5000 1.7500 0.0000 0.7546 0.7810 0.7340 0.0000 2.5000 0.0000 0.6914 0.5332 0.7160 1.1667 0.7500 1.2121 1.0157 1.1143 1.0452 1.1667 0.7500 1.5152 1.0230 1.1227 1.0676 1.1667 0.7500 1.8182 1.0854 1.2089 1.0980 1.1667 0.7500 2.4242 1.0804 1.2139 1.0977 1.1667 0.7500 3.0303 1.0332 1.2156 1.0882 )
(3)
The absolute difference series for each sequence were determined.
Δ 1 = ( 0.3858 1.2808 1.2808 0.1517 0.2650 1.0150 0.1871 0.2296 1.0204 0.2546 0.9954 0.7546 0.6914 1.8086 0.6914 0.1509 0.2657 0.1964 0.1437 0.2730 0.4922 0.0812 0.3354 0.7327 0.0863 0.3304 1.3439 0.1335 0.2832 1.9971 )
Δ 2 = ( 0.5118 1.1549 1.1549 0.3577 0.0590 0.8090 0.0131 0.4036 0.8464 0.2810 0.9690 0.7810 0.5332 1.9668 0.5332 0.0524 0.3643 0.0978 0.0440 0.3727 0.3925 0.0422 0.4589   0.6093 0.0473 0.4639 1.2103 0.0490 0.4656 1.8147 )
Δ 3 = ( 0.5589 1.1078 1.1078 0.1231 0.2936 1.0436 0.1686 0.2481 1.0019 0.2340 1.0160 0.7340 0.7160 1.7840 0.7160 0.1215 0.2952 0.1669 0.0991 0.3176 0.4476 0.0686 0.3480 0.7202 0.0689 0.3477 1.3265 0.0785 0.3382 1.9421 )
(4)
The values of M and m are shown in Table 8.
(5)
The correlation coefficient was calculated.
ε 01 ( k ) = ( 0.7800 0.4737 0.4737 0.9388 0.8546 0.5363 0.9107 0.8792 0.5348 0.8616 0.5415 0.6159 0.6389 0.3847 0.6389 0.9393 0.8541 0.9036 0.9453 0.8492 0.7243 1.0000 0.8094 0.6237 0.9953 0.8125 0.4610 0.9538 0.8424 0.3604 )
ε 02 ( k ) = ( 0.6665 0.4660 0.4660 0.7431 0.9560 0.5560 1.0000 0.7185 0.5446 0.7881 0.5104 0.5648 0.6571 0.3378 0.6571 0.9621 0.7394 0.9216 0.9700 0.7348 0.7243 0.9716 0.6909 0.6257 0.9668 0.6885 0.4543 0.9653 0.6877 0.3561 )
ε 03 ( k ) = ( 0.6796 0.5001 0.5001 0.9502 0.8221 0.5161 0.9123 0.8528 0.5270 0.8628 0.5232 0.6098 0.6163 0.3774 0.6163 0.9516 0.8211 0.9136 0.9715 0.8068 0.7329 1.0000 0.7882 0.6148 0.9997 0.7884 0.4525 0.9906 0.7941 0.3569 )
(6)
The results of the correlation values are shown in Table 9.
According to the gray correlation model analysis, under the influence of NCA, RBA, and PPFs, the correlation indexes of compressive, tensile, and flexural strength changed significantly. Among them, the largest correlation degree for compressive strength was between compressive strength and NCA, which was 0.8964; the second largest was between compressive strength and RBA, which was 0.7301; and the smallest correlation degree, between compressive strength and PPFs, was 0.5873. The largest correlation degree for tensile strength was between tensile strength and NCA, which was 0.8691; the second largest was between tensile strength and RBA, which was 0.6530; and the smallest correlation degree, between tensile strength and PPFs, was 0.5870. The largest correlation degree for flexural strength was between flexural strength and NCA, which was 0.8935; the second largest was between flexural strength and RBA, which was 0.7074; and the smallest correlation degree, between flexural strength and PPFs, was 0.5840.

5. Conclusions

(1)
The mechanical properties decreased with an increasing replacement rate of RBA. The compressive and tensile strengths increased slightly when RBA was replaced at a rate of 30–50%. The mechanical properties of recycled concrete were improved to different degrees by PPFs, and the effect on the tensile strength of recycled concrete was more significant. The optimum PPF content for compressive and flexural strength was 0.12%, and the optimum PPF content for tensile strength was 0.2%.
(2)
The tension–compression ratio increased with an increasing PPF admixture. PPFs could improve the toughness of recycled concrete. The role of PPFs in RBA concrete was initially analyzed by SEM images.
(3)
The gray correlation could analyze the degree of influence of each material indicator by different variables in different environments. Through the gray correlation analysis of this test, the correlation degrees of compressive strength (tensile strength, flexural strength) with NCA, RBA, and PPFs were 0.8964 (0.8691, 0.8935), 0.7301 (0.6530, 0.7074), and 0.5873 (0.5870, 0.5840), respectively.
(4)
In this study, the effects of RBA and PPFs on the mechanical properties of concrete were initially analyzed, but more in-depth research is needed to analyze their mechanisms. The agglomeration phenomenon of PPFs made it difficult to stir uniformly in the mixing process, which would greatly affect the mechanical properties of concrete and limit its application in the production process. The agglomeration of PPFs in the concrete was reduced by improving the process, such as using a high-speed mixer. Additionally, the coupling effect through PPFs and other materials (nanomaterials, slag, fly ash, and others) may be more effective than a single PPF in improving the mechanical properties of RBA concrete. This needs to be further explored in follow-up studies.

Author Contributions

Conceptualization, L.B.; Formal analysis, L.B.; Investigation, S.G. and Z.T.; Resources, L.X.; Data curation, Z.T.; Writing—original draft, S.G.; Writing—review & editing, X.B. and Z.H.; Supervision, X.B.; Project administration, Z.H.; Funding acquisition, L.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Jilin Provincial Science and Technology Department (grant number 20220203063SF).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Sustainability 15 11135 g001 550
Figure 1. Fine aggregate grading curve.
Figure 1. Fine aggregate grading curve.
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Figure 2. Specimen number representation method.
Figure 2. Specimen number representation method.
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Figure 3. Compressive properties of specimens with RBA.
Figure 3. Compressive properties of specimens with RBA.
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Figure 4. Compressive properties of specimens with PPFs.
Figure 4. Compressive properties of specimens with PPFs.
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Figure 5. Tensile properties of specimens with RBA.
Figure 5. Tensile properties of specimens with RBA.
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Figure 6. Tensile properties of specimens with PPFs.
Figure 6. Tensile properties of specimens with PPFs.
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Figure 7. Flexural properties of specimens with RBA.
Figure 7. Flexural properties of specimens with RBA.
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Figure 8. Flexural properties of specimens with PPFs.
Figure 8. Flexural properties of specimens with PPFs.
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Figure 9. Relationship between PPF replacement rate and tension–compression ratio.
Figure 9. Relationship between PPF replacement rate and tension–compression ratio.
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Figure 10. SEM images: (a) SEM image of RBA-30-0.08; (b) SEM image of RBA-30-0.12; (c) SEM image of RBA-30-0.16; (d) SEM image of RBA-30-0.2.
Figure 10. SEM images: (a) SEM image of RBA-30-0.08; (b) SEM image of RBA-30-0.12; (c) SEM image of RBA-30-0.16; (d) SEM image of RBA-30-0.2.
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Table
Table 1. Abbreviations and their meanings.
Table 1. Abbreviations and their meanings.
AbbreviationConcrete Meaning
RBARecycled brick aggregate
NCANatural coarse aggregate
PPFPolypropylene fibers
SEMScanning electron microscopy
ITZInterfacial transition zone
Table
Table 2. Properties of P·O 42.5 ordinary Portland cement.
Table 2. Properties of P·O 42.5 ordinary Portland cement.
CementSpecific Surface Area (m2/kg)Loss on Ignition (%)Initial/Final Setting Time (min)3d/28d Flexural Strength (MPa)3d/28d Compressive Strength (MPa)
P·O 42.53122.3195/2595.7/9.124.6/45.2
Table
Table 3. Chemical composition of P·O 42.5 ordinary Portland cement.
Table 3. Chemical composition of P·O 42.5 ordinary Portland cement.
Chemical Composition (%)SiO2Al2O3Fe2O3CaOMgOSO3K2O
P·O 42.522.25.973.5760.411.632.930.98
Table
Table 4. Coarse aggregate performance.
Table 4. Coarse aggregate performance.
Coarse Aggregate TypeWater Absorption (%)Apparent Density (kg/m3)Crush Index (%)Flat Elongated Particles Content (%)Mud Content (%)
NCA1.427256.85.70.4
RBA10.29239528.58.112.1
Table
Table 5. Properties of PPFs.
Table 5. Properties of PPFs.
Type of FibersLength (mm)Equivalent Diameter (um)Density (g/cm3)Melting Point (°C)Breaking Elongation (%)Tensile Strength (MPa)Elastic Modulus (GPa)
PPF1920–500.91160–17015–30350–5005
Table
Table 6. Mix proportions of the concrete mixtures.
Table 6. Mix proportions of the concrete mixtures.
Specimen TypesCement (kg/m3)Sand (kg/m3)NCA
(kg/m3)		PPF
(%)		RBA
(kg/m3)		Water
(kg/m3)
RBC-0411615109300185
RBC-30411615765.10327.9185
RBC-50411615546.50546.5185
RBC-70411615327.90765.1185
RBC-100411615001093185
RBC-30-0.08411615765.10.08327.9185
RBC-30-0.10411615765.10.1327.9185
RBC-30-0.12411615765.10.12327.9185
RBC-30-0.16411615765.10.16327.9185
RBC-30-0.20411615765.10.2327.9185
Table
Table 7. Statistical analysis of mechanical properties.
Table 7. Statistical analysis of mechanical properties.
Specimen TypesCompressive Strength (MPa)Tensile Strength (MPa)Flexural Strength (MPa)
Ave (MPa)SD (MPa)COV (%)Ave (MPa)SD (MPa)COV (%)Ave (MPa)SD (MPa)COV (%)
RBC-035.273.63910.10322.74660.23940.08723.71060.1280.0345
RBC-3027.953.08060.11021.9240.04430.02303.49560.16740.0479
RBC-5028.11.7490.06222.0130.02710.01353.3560.01130.0034
RBC-7020.781.84130.08861.85740.05170.02782.45870.07570.0308
RBC-10019.040.29460.01551.2680.05560.04382.39850.01980.0083
RBC-30-0.0827.971.91660.06852.650.08660.03273.5010.01110.0032
RBC-30-0.1028.171.47150.05222.670.07210.02703.5760.05720.0160
RBC-30-0.1229.892.00910.06722.8750.08550.02973.6780.05920.0161
RBC-30-0.1629.750.84110.02832.8870.03240.01123.6770.08330.0227
RBC-30-0.2028.451.05730.03722.8910.31350.10843.6450.03500.0096
Ave = average value; SD = standard deviation; COV = coefficient of variation.
Table
Table 8. The values of M and m.
Table 8. The values of M and m.
Mm
Δ11.99710.0812
Δ21.96680.0131
Δ31.94210.0686
Table
Table 9. Gray correlation data analysis results.
Table 9. Gray correlation data analysis results.
NCARBAPPF
Compressive strength0.89640.73010.5873
Tensile strength0.86910.65300.5870
Flexural strength0.89350.70740.5840
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Gong, S.; Bai, L.; Tan, Z.; Xu, L.; Bai, X.; Huang, Z. Mechanical Properties of Polypropylene Fiber Recycled Brick Aggregate Concrete and Its Influencing Factors by Gray Correlation Analysis. Sustainability 2023, 15, 11135. https://doi.org/10.3390/su151411135

AMA Style

Gong S, Bai L, Tan Z, Xu L, Bai X, Huang Z. Mechanical Properties of Polypropylene Fiber Recycled Brick Aggregate Concrete and Its Influencing Factors by Gray Correlation Analysis. Sustainability. 2023; 15(14):11135. https://doi.org/10.3390/su151411135

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Gong, Shangwei, Lichao Bai, Zhenyu Tan, Lina Xu, Xiaohong Bai, and Zhanfang Huang. 2023. "Mechanical Properties of Polypropylene Fiber Recycled Brick Aggregate Concrete and Its Influencing Factors by Gray Correlation Analysis" Sustainability 15, no. 14: 11135. https://doi.org/10.3390/su151411135

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