Introduction

Concrete technology has developed in a new way to overcome conventional concrete limitations such as self-compact concrete (SCC), high-performance concrete (HPC) and ultra-high performance concrete (UHPC) [1, 2]. For the structural application of conventional concrete, two primary weaknesses have caused issues: weakness under tension, and brittle behavior which results in low ductility [3,4,5,6,7]. Concrete's tensile strength is commonly known to be far lower than its compressive strength, so that, the cracks can spread more quickly [8]. High performance concrete (HPC) and high strength concrete (HSC) are being used more widely everywhere. Along with greater strength, HPC distinguishes out for its high workability, durability, resistance to a variety of outside agents, and rapid hardening rate [9, 10]. The main drawbacks are brittleness, low tensile strength, and poor resistance to crack opening and propagation. Recently, Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) has offered notable advantages over other types of concrete. Akeed et al. carried out a comprehensive review on UHPC: developments, fresh and hardened properties, microstructure and durability [12,13,14,15,16]. Fibers are required to impose the ductility in compression required for structural safety. The concrete body has many micro-cracks throughout it even before the tensile testing starts, which hampers the proper transfer of tensile stress during the test, which causes the spread of crack [17, 18]. The concept of fiber-reinforced concrete (FRC) was invented to reinforce the brittle cement-based paste with various fibers, such as steel fiber, basalt fiber, synthetic fiber, carbon fibers, and natural fibers [19]. Tahwia [20] focused on the potential of polypropylene (PP) fibers to replace steel fibers. PP fibers act as sacrificial agents that create voids in the UHPC matrix, reducing internal pressure and preventing (or delaying) the spalling of the concrete subjected to high temperatures.

The addition of fiber to concrete is one of several modifications made to the material with the aim to reduce its weaknesses by raising its tensile strength. High strength concrete with polypropylene and steel hybrid fiber produced superior flexural toughness, tensile strength, and compressive strength than plain concrete and all FRCs with single, double, or triple types of fibers [21]. Fiber hybridization is one concept that has recently received a lot of interest. The idea is to randomly include reinforcement fibers into the concrete mix to prevent early cracking caused on by loading [22]. Fiber reinforcing has been shown to increase concrete's impact resistance, fracture toughness, fatigue resistance, and capacity to absorb energy [23]. When different types of fibers are properly combined, a hybrid is produced that benefits from each fiber's special qualities and reacts synergistically [23]. In reality, the best mechanical characteristics of single fiber-reinforced concrete are nonexistent [24]. The best FRC features are provided by combining two distinct types of fibers. In contrast, flexible single fibers increase the ductility and toughness of FRC while strong and stiff discrete fiber improves initial fracture strength and ultimate capacity [25]. Polypropylene fibers, at a ratio ranging from 0.5 to 3 kg/m3, can be used to improve the concrete behavior at high temperatures [26].

The actual performance of concrete exposed to very high temperatures depends on a variety of environmental parameters, including the properties of the materials used, how the concrete was constructed, the rate at which it heated up, the highest temperature to which it was exposed, how long it was exposed, how it was cooled after reaching the maximum temperature, and its loading level at the time of cooling [27]. The risk of explosive spalling in HPC at high temperatures is reduced and fully eliminated by employing polypropylene (PP) fibers, according to previous studies [28].

It's significant to note that traditional methods for experimental design operate on the assumption that each factor may be manipulated separately from the others [29, 30]. Finding the optimal parameter becomes more time-consuming and ineffective, especially when considering the interactions between each component. Response surface methodology (RSM) can be used to solve this problem [31, 32]. The significance of the variables in terms of their influence on the response values may be assessed using these techniques to estimate experimental results using an analysis of variance (ANOVA) [33].

Research significant

The effects of combining two different types of fibers, such as carbon fibers and polypropylene, glass and polypropylene fibers, or carbon and glass fibers were the focus of earlier studies. However, there have not been many studies examining the effects of mixing the same type of fibers with different specifications on the properties of concrete. Also, there is a lack of available literature regarding the possibility of optimizing and predicting the properties of hybrid fiber-reinforced concrete (HFRC). In this research, the effect of hybrid polypropylene fibers (coarse monofilament and staple fibers) on the mechanical characteristics and resistance to elevated temperature of high-performance concrete was studied. Also, in order to optimize and predict the properties of concrete, central composite design (CCD) under response surface methodology (RSM) was used to design the concrete mixtures. The study provides insights into the optimal combinations of synthetic fibers for HFRC, which can be useful for the design of concrete structures with improved strength.

Response surface methodology


In recent years, the most widely utilized optimization technique was response surface methodology (RSM) [34, 35]. The response surface method's primary objective is to identify the variables' optimal values. RSM has the capability to estimate the relation between the independent variables and the responses, additionally to determine how each individual independent variable, or a combination of variables, affects the response [36]. As compared to the conventional method, using RSM can also reduce the number of planned experiments. The term "RSM" comes from the mathematical model's graphical approach [37, 38]. Equation (1) provides the relationship between the output and the input.

$${\text{Y}} = \beta_{0} + \Sigma \, \beta_{ij} {\text{x}}_{i} + \Sigma \, \beta_{ii} {\text{ x}}_{i} + \Sigma \, \beta_{ij} {\text{x}}_{i} {\text{x}}_{j}$$
(1)

where Y is the response to anticipated, β0 is the intercept of the model, βi points to coefficients of linear, βii signifies quadratic coefficient, βij are coefficients of variable interaction and xi, xj are independent variables. Three steps can be used to divide optimal research using RSM. The initial step is the initial work where independent parameter determination and level determination are performed out. The next step is the chosen an experimental strategy, predicting the model equation, and verifying it. The next step is to identify the optimum points and obtain the response surface plot and response contour plot as functions of the independent parameters [39,40,41,42]. Figure 1 illustrates the steps of optimization technique used in this investigation.

Fig. 1
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Steps of optimization method

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Experimental procedures

Materials

This investigation used ordinary Portland cement (OPC) CEM I 52.5 N in accordance with BS EN 197–1:2011 [43]. The cement was partially replaced by silica fume (SF), a cementitious substance. Table 1 summarizes the silica fume and cement's chemical compositions. As coarse aggregate, crushed dolomite with 12 mm nominal maximum size, 2.65 specific gravity, and water absorption of 0.98, and natural sand with 2.65 specific gravity and 0.62% water absorption rate were used. The coarse and fine aggregate met the grading requirements of BS EN 12620 [44]. Figure 2 demonstrates the grading curves for fine and coarse aggregate, respectively. A superplasticizer with a specific weight of 1.1 and compliance with ASTM C494 [45] type F was employed. The rate of water/cement was 0.25 for all concrete mixtures. The fibers are coarse monofilament (X1) and staple (X2) polypropylene fibers shown in Fig. 3. Tables 2 and 3 illustrate the physical characteristics of polypropylene (PP) fibers.

Table 1 Chemical composition of OPC and SF
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Fig. 2
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Grading curves of used aggregate, a Fine aggregate and b Coarse aggregate

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Fig.3
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Photos of polypropylene fibers. a Staple fiber, b Monofilament fibers

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Table 2 Configuration and physical properties of monofilament fiber
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Table 3 Physical and mechanical properties of staple fiber
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Mix proportion and design of experiments

During this experiment, 16 mixtures of concrete were designed, 13 mixes were designed using central composite design under response surface methodology in the Minitab program in accordance with Eq. (2), and (3) additional mixes (CO, CO-SF, CO-MF) were added as reference mixtures, where CO is a mixture without any fibers, CO-SF is a mixture with 0.75 kg/m3 staple polypropylene fiber and CO-MF is a mixture with 4 kg/m3 monofilament polypropylene fiber. The additional mixes were designed to make a good comparison between the properties of concrete without fibers and concrete using one type of polypropylene fibers to the concrete in which a hybrid of polypropylene fibers is used. The selection of control mixture was based on using the trial and error method to achieve compressive strength 65 ± 5 MPa at age 28 days and slump value in the range of 20 ± 2 cm without any segregation. Because low water-cement ratios are recommended when producing high performance concrete in order to increase strength, the ratio of water to cementitious materials (OPC + SF) was maintained at 0.25. To achieve the desired workability and to eliminate the expected negative effect of polypropylene fiber, super plasticizer was used at a rate of 2.5% of the weight of the cementitious materials.

$$N = 2^{k} ? + 2k? + n,$$
(2)

where N refers to the number of experiments; k refers to the numb of variables studied; n refers to the number of replicates [46]. In the test program, there were two independent variables. Including monofilament (X1) and staple (X2). The portion coded value is set at α = \((2^{k} )^{0.25}\)  = \(2^{20.25}\) = 1.414 with five different levels − 1.414, − 1, 0, + 1, + 1.414 refer to the final coded value. The variables' values and coded values are shown in Table 4. The proportion of HPC mixtures (kg/m3) are shown in Table 5.

Table 4 Limits and coded value of factors
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Table 5 Mixtures proportions (kg/m3)
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Samples preparations

To determine compressive strength and wet density, 100 mm cubic samples of fresh concrete were cast. The impact test was performed on cylindrical samples that measure 30 cm in height and 15 cm in diameter. Flexural tests were carried out on prismatic beams measuring 10 × 10x50 cm. In every test, three samples were examined for each curing age. The concrete resistance to elevated temperature was determined by exposing prismatic beams and concrete cubes to various temperatures at 200 °C, 400 °C, and 800 °C.

Mixing and curing

All concrete mixes were prepared in a fixed horizontal pan mixture with a capacity of 0.07 m3. The coarse and fine aggregates were dry mixed for a minute prior to mixing. After adding the cement, the dry blending continued for another minute. Following the addition of water, the mixture was continued for an additional two minutes. The required amount of fibers was then added to the wet concrete. To make sure the fibers were evenly distributed throughout the concrete, the mixture was mixed for three minutes. After 24 h, all molds were removed, and the samples were left to cure in a water cabinet.

Tests methods

For fresh concrete, the slump test was conducted in accordance with ASTM C143 [47]. The compressive strength is carried out according to ASTM C39 [48] using a cube with a side length of 10 cm after 7-, 28-, and 56-days moist curing at ambient temperature. Testing was performed at a loading rate of 0.4 MPa/s on a 1000 kN capacity electronic servo testing machine. Flexural strength was performed on 10 cm × 10 cm × 50 cm prisms specimen according to ASTM C 1018 [49] using three point loading. The clear span of the beam is 40 cm. For compressive strength and flexural strength tests, three samples were examined per mix, and the average value was reported. Excellent impact resistance is one of FRC's most important features (dynamic energy absorption as well as strength). In order to examine the resistance of hybrid polypropylene fibers-reinforced HPC against impact, the concrete disks were made according to ACI 544.2R Standard [50]. The impact test was carried out on concrete cylinders cut with a masonry saw from 15 × 30 cm cylinder to four pieces 15 cm diameter and 6 cm thickness. The average number of blows required to start the first crack and finally cause failure at age 56 days was determined for all combinations. The drop-weight impact test is performed using a standard, manually operated 13.5 kg compaction hammer and it was dropped from a height equal to 50 cm. Using a heavier mass, such as 13.5 kg, which is better at transmitting impact loads, reduces the number of blows [51]. To study the effect of elevated temperature on compressive strength and flexural strength of concrete mixtures, the specimens were subjected to 200 °C, 400 °C, and 800 °C for 1 h. The heating rate was 20 °C/min. After such a treatment, the specimens were allowed to cool with a rate of 2 °C/min. Then, the specimens were stored in dry condition at room temperature for 2 h until testing. Also, the generated mixtures were examined using scanning electron microscopy (SEM).

Results, analysis, and optimization

Slump test

In order to assure consistency of polypropylene fiber-reinforced concrete from batch to batch, the slump test can be employed as a quality control test. The uniform distribution of the fibers inside the concrete, how they interact with the cement matrix, and how well the concrete may be cast or sprayed all affect the quality of fibers. The hybrid fiber must be completely covered with cement paste in order for it to have any benefit in the concrete. The slump started changing slightly as a result of the different fiber types and content. For concrete without fibers, the slump is 20.5 cm. The addition of monofilament or staple polypropylene fibers resulted in slightly decrease in slump results. Also, the slump of polypropylene hybrid fiber-reinforced concrete mixtures slightly decreased as compared to the other specimens without or with single fiber. Combining two different fibers can create a network structure that prevents mixes from segregating and flowing concrete, hence the slump has decreased. Due to their high content and surface area, the fibers will absorb more cement paste to wrap around, and the rise in mixture viscosity results in slump loss. Figure 4 illustrates the relation between the amount of fiber and slump, it’s noticed that when the amount of hybrid polypropylene fibers increases, the slump decreases. This result agreed with A. A. Ramezanianpour et.al. [52], B. Chen and J. Liu [53], and C. Sakthipriya [54]. The fiber-reinforced concrete mixture test results are shown in Table 6.

Fig. 4
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Effect of monofilament fibers and staple fibers on concrete slump

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Table 6 Results of mixtures of the fiber-reinforced concrete
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Compressive strength

Compressive strength is one of the most important characteristics of hardened concrete, which is typically used as a characteristic material value for concrete classification. Fiber content has an effect on concrete's characteristics. It has been noted that hybrid-reinforced fiber concrete has greater compressive strength than concrete without polypropylene (PP) fibers. Compared with concrete without PP fibers, adding monofilament fiber at 4 kg/m3 can increase compressive strength by 1.2%, while using staple fiber at 0.75 kg/m3 can increase the compressive strength by 0.6%. Compressive strengths of polypropylene hybrid reinforced concrete have remarkably improvement ranged from 3.8 to 14.29%. As a result, it can be seen from the observations that the maximum percentage of increase in compressive strength can be obtained from the hybridization (5 kg monofilament and 0.75 kg staple) in the mix 6 and was equal to 14.29% compared with the control mix (CO) at age 28 days, this result is in agreement with M. Hsie et.al [55]. The reason for this is that monofilament fiber has a high elastic modulus and rigidity for pretty tough shapes. Because these mixes contained fibers, the improvement in control over the crack formation and growth under axial stress led to the rise in compressive strength. The inclusion of silica fume enhances the connection between aggregate particles and cement paste while also increasing cement paste density, which improves cement strength. The effect of independent variables on the mixes was identified when the response surface of experimental test results was performed. The model's estimate and validation depended on the P-values. P-values above 0.05 are confirmed as insignificant, whereas for 95% confidence intervals, values below 0.05 indicate that the model terms are significant. On the other hand, Figs. 5, 6, and 7 show the impact of hybrid polypropylene fibers on compressive strength at 7, 28 and 56 days. It has been found that the compressive strength improves along with the increase of hybrid fibers. The analysis of variance for compressive strength at 28 days is presented in Table 7. Equation (3) was used to set up the models' formula as follows:

$${\text{Fc 28 days }}\left( {{\text{MPa}}} \right) = 67.235 - 1.794 X_{1} + 2.459 X_{2} + 0.5026 X_{1} *X_{1} - 2.683 X_{2} *X_{2} + 1.491 X_{1} *X_{2}$$
(3)
Fig. 5
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Surface plot for compressive strength vs monofilament and staple fibers at age 7 days

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Fig. 6
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Surface plot for compressive strength vs monofilament and staple fibers at age 28 days

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Fig. 7
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Surface plot for compressive strength vs monofilament and staple fibers at age 56 days

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Table 7 The analysis of variance for 28 days compressive strength
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Figure 8 presents the compressive strength of concrete mixtures after heating to 200 oC, 400 oC, and 800 °C. For all of the mixes, a slight decrease in residual strength compared to room temperature occurred after heating to 200 °C. After being heated to 400 °C and 800 °C, all mixtures showed a significant reduction in compressive strength. This is because such a compact structure experiences extra extensive inner cracking as a result of pressure buildup brought on by the evaporation of chemically and physically bonded water. According to some experts, the breakdown of calcium hydroxide is mostly to blame for this strength reduction. The decomposition of C-S–H, Ca(OH)2, and CaCO3 was the cause of the residual compressive strength decreasing above 400 °C.

Fig. 8
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Effect of elevated temperature on compressive strength at age 56 days

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Flexural strength

Polypropylene hybrid fibers achieve an effective improvement on flexural strength. This result agreed with A. Nkem Ede and A. Oluwabambi Ige [26]. A compressive strength case is significantly less important than the positive impact of fiber content of flexural strength. The hybrid fibers can resist tensile stress in the tensile zone directly under the neutral axis when loads are applied to the beams. Coarse monofilament fibers can spread the stress of macro-cracks and bridge failures of tiny staple fibers until they are no longer able to manage the load. Increasing fiber content and bonding effect are the most important factors in enhancing flexural characteristics. Due to the completion of the reaction, the flexural strength of the polypropylene paste increased over time. The fracture faces and fibers distribution of polypropylene fiber-reinforced concrete are shown in Fig. 9. The high effect of hybrid polypropylene fiber on flexural strength is shown in Fig. 10. The analysis of variance for flexural strength at 56 days is summarized in Table 8.

Fig. 9
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Fracture faces of polypropylene fiber reinforced concrete

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Fig. 10
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Main effect plot for flexural strength at age 56-days

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Table 8 Analysis of variance for 56 days flexural strength
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The following formula was used to set up the models in Eq. (4), and the pareto chart derived from the analysis of the response surface design a.

re shown in Fig. 11 to show how the standardized effects of all variables on flexural strength are shown.

$${\text{Flexural strength 56 day}}s \, \left( {{\text{MPa}}} \right) \, = 10.423 \, - 0.7273 X_{1} + 1.693 X_{2} + 0.2146 X_{1} *X_{1} - 1.411 X_{2} *X_{2} + 0.5514 X_{1} *X_{2}$$
(4)
Fig. 11
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Pareto chart of standard effects of the input parameters in flexural strength at age 56-days

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The effect of temperature on reinforced concrete was studied on flexural strength after curing for 56 days. According to the results, which concur with D. Zeng et al. [56], H. U. Ahmed et al. [57], and Tahwia et al. [58]. The flexural strength dropped as temperature increased. Hybrid polypropylene fibers can assist avoid explosive spalling of concrete at high temperatures because they dissolve to reduce pore pressure prior to spalling. Flexural strength is mostly unchanged at 200 °C despite the dehydration of monosulfoaluminate, ettringite, and C-S–H, as well as the evaporation of free water in capillary pores [58, 59]. The results of flexural strength after exposure to different temperature are shown in Fig. 12.

Fig. 12
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Effect of elevated temperature on flexural strength at age 56 days

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Figure 13 a, b, c, and d illustrates the impact of hybrid polypropylene fiber on flexural strength after exposure to a different temperature, the results show that hybrid polypropylene fibers have a highly significant effect for resist high temperatures, these results agree with J. Xiao and H. Falkner [60]. Figure 14 shows how the optimization process could be used to determine the ideal values of factors to get the desired values of responses. The suggested mixture composed of 5 kg/m3 of monofilament fiber and 1 kg/m3 of staple fiber. The predicted results are 81 MPa compressive strength, 13.5 MPa flexural strength, and 16.37 cm slump. Figure 15a shows the electric oven that was utilized in this study, additionally; Fig. 15b illustrates the shape of the sample after the heating process.

Fig. 13
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Surface plot of flexural strength at a 20 °C and after exposed to b 200 °C, c 400 °C and d 800 °C

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Fig. 14
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Optimum content of variables to achieve the desirable values of responses

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Fig. 15
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View of the heating process of the samples and the channels produced in the samples when the polypropylene fibers melted

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Impact test

Impact resistance is one of FRC's important characteristics. The test can be utilized to compare the benefits of hybrid polypropylene fiber on concrete. The failure behavior of specimens under impact load is depicted in Fig. 16. The number of blows required at the initial crack and complete failure is shown in Table 9. It’s noticed that the number of blows increased with the addition of hybrid polypropylene fiber. This result is in agreement with O. A. Ahmad and M. Awwad [61]. The blow numbers of ultimate failure in plain concrete were found to be nearly similar to those of the initial crack, indicating that the plain concrete specimen fails immediately once the crack occurs. Hybrid fiber concrete's brittleness was successfully decreased, and its impact resistance was increased, by the addition of polypropylene fibers.

Fig. 16
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The shapes of failure on hybrid polypropylene reinforced concrete under impact load

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Table 9 The impact test results
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Microstructure

The concrete specimens' microstructure is examined using scanning electron microscopy (SEM). Materials' mechanical characteristics and their microstructure are closely related. SEM images of hybrid polypropylene fibers and non-fiber concrete are shown in Fig. 17. The results show that all mixes are cohesive, compacted, and density and the percentage of voids is low with the presence of some micro-cracks in the control mixture (without fibers) as shown in Fig. 17a. However, the microstructure of concrete is improved by the presence of staple polypropylene fibers (Fig. 17b) or monofilament polypropylene fibers (Fig. 17c). It is observed from Fig. 17d the microstructure of hybrid polypropylene fiber is the most compact, homogeneous, and the C-S–H gels also exhibit excellent development. It was difficult to notice the existence of micro-cracks, these findings agree with Z. Yuan and Y. Jia [62]. The results of SEM agreed with the results of compressive and flexural strength. SEM analysis proves that hybridization of polypropylene fibers plays an important role in improvement in all mechanical properties of concrete.

Fig. 17
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SEM images for a mix CO, b mix CO-SF, c mix CO-MF, d mix 6

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Conclusion

This study aims to evaluate, optimize, and predict the effect of hybrid polypropylene fibers (coarse monofilament and staple fibers) on the mechanical characteristics and resistance to elevated temperature of high-performance concrete. The key conclusions in this paper can be summarized as follows:

  • Concrete mixtures reinforced with hybrid polypropylene fibers have significantly improved in terms of compressive strength, flexural strength ranged, impact resistance, and resistance to elevated temperature while the workability slightly decreased.

  • The compressive strength and flexural strength of hybrid polypropylene fiber-reinforced concrete increased by (1.9 to 12%) and (14.28 to 41.9%), respectively, at age 56 days compared to control mixture without fibers.

  • The hybridization of 5 kg monofilament and 0.75 kg staple fibers achieved the highest compressive strength (84.6 MPa), flexural strength (14.9 MPa), and the optimum impact resistance at age 56 days. The increase of coarse monofilament fibers significantly improved the spalling resistance performance.

  • The residual compressive strength of mixture containing 5 kg monofilament and 0.75 kg staple fibers up to 63.8% of the initial strength after exposure to 800 0C.

  • Investigations using (SEM) technology demonstrated that adding hybrid polypropylene fibers to concrete could enhance interfacial transition zones and the matrix between aggregates and the polypropylene matrix. Subsequently, improved the mechanical properties.

  • Using RSM, the regression model offers a detailed analysis of the hybrid polypropylene fibers-reinforced concrete properties over the selected range of fiber volume fraction. The results of the lack-of-fit test along with the high values of the coefficients of multiple determinations (R2) demonstrated that the polynomial regression model was enough to forecast the necessary performance of hybrid polypropylene fiber concrete. The ANOVA results also confirmed, based on a very low P-value, that the inclusion of all model parameters is statistically significant.

The authors suggest that future research examine the impact of hybrid polypropylene fibers on the mechanical properties, durability, and resistance to elevated temperature of ultra-high-performance concrete and geopolymer concrete in order to gain a deeper understanding through the research society's collaborative efforts.