Assessment of fracture process in forta and polypropylene fiber-reinforced concrete using experimental analysis and digital image correlation

Abstract

This paper aims to characterize the evolution of the fracture process and the cracking behavior in forta-ferro (FF) and polypropylene (PP) fiber-reinforced concrete under the uniaxial compressive loading using experimental analysis and digital image correlation (DIC) on the surface displacement. For this purpose, 6 mix designs, including two FF volume fractions of 0.10%, and 0.20% and three PP volume fractions of 0.20%, 0.30%, and 0.40%, in addition to a control mix were evaluated according to compressive strength, modulus of elasticity, toughness index, and stress–strain curves. The influence of fibers on the microstructural texture of specimens was analyzed by scanning electron microscope (SEM) imaging. Results show that FF fiber-reinforced concrete specimens demonstrated increased ductility and strength compared to PP fiber. DIC results revealed that the major crack and fracture appeared at the peak load of the control specimen due to brittleness and sudden gain of large lateral strain, while a gradual increase in micro-crack quantity at 75% of peak load was observed in the fiber specimens, which thenbegan to connect with each other up to the final fracture. The accuracy of the results supports DIC as a reliable alternative for the characterization of the fracture process in fiber-reinforced concrete.

Graphical abstract

Keywords

fiber-reinforced concrete / forta-ferro and polypropylene fiber / fracture process / cracking behavior / digital image correlation

1 Introduction

1.1 Background

A major defect of normal concrete is its quasi-brittle characteristic, which can result in fractures with the smallest plastic deformation due to its relatively low energy absorption. This weakness can be counteracted by the inclusion of fibers through the formation of stress-transfer bridges for additional reinforcement [1]. The inclusion of fibers into the cement paste matrix can help retain some degree of structural integrity and ameliorate the post-cracking properties of concrete, thereby enhancing crack resistance. In recent decades, the application of fiber reinforced concretes (FRCs) have been widely increased in various fields, such as bridge decks [2], shotcrete tunnel linings [3], pavement construction [4], repair and rehabilitation work [5], and ground slabs [6]. For instance, recent studies investigated new applications for short fibres in composite railway sleepers [7] and mortar containing oil-contaminated sand [8] and concluded that the use of these fibres improves flexural and shear performance in railway sleepers, as well as mechanical properties of the geopolymer mortar. ACI Committee 544 [9] and ASTM C1116 [10] classified fibers into 4 categories (steel, glass, synthetic and natural) based on the type of material. Among the commonly used fibers, forta-ferro (FF) and polypropylene (PP) fibers are two of the most widely used types, both in research and practical applications, due to their relatively low price, lightweight characteristic, and the obvious improvement of engineered properties [11–16]. A study conducted by Dashti and Nematzadeh [13] found that the addition of a low volume content (0.4%) FF fiber enhanced the F_c, but a higher volume fraction content caused the opposite effect. Bagheri et al. [17] reported that a volume fraction of FF fiber up to 0.5% had no significant effect on strength properties, while a volume of 1% FF fiber led to a reduction in strength properties. Hasan-Nattaj and Nematzadeh [18] reported the F_c of FF- FRC with various volume fractions of FF fiber, specifically 0.2%, 0.35%, 0.5%, 0.65%, and 0.8%. The results showed a maximum F_c improvement of 16.9% compared to plain concrete with the addition of 0.35% FF fiber. For PP fibers, Song et al. [19] reported the F_c of PP-FRC at a fiber content of 0.6 kg/m³ (0.067% volume fraction), and results showed a 5% increase compared to the plain concrete. Zhang et al. [11] studied the influence of various PP fiber contents of 0, 0.8, 1.2, 1.6, 2.0 and 5.0 kg/m³ on the F_c. Results showed that incorporating PP fibers in concrete can have an increasing effect on strength, though this trend weakens past a certain fiber content. A PP fiber content of 0.8 kg/m³ (0.089% volume fraction) showed the greatest improvement in F_c compared to others. Alsadey and Salem [20] reported a 12% increase in F_c at 1.82 kg/m³ (2% volume fraction) PP fiber content. Other studies have introduced a wide range of about 0.5 to 2 kg/m³ as the appropriate PP fiber content [21–26]. From the above, we can see that many previous studies have investigated the relationship between the volume fraction of FF or PP fibers and the F_c, but obtained contradictory results. Therefore, a comprehensive study is needed to specify the optimal volume fraction of FF and PP fibers on F_c, focusing on all aspects of the fiber-reinforced specimen when subjected to load (stress-strain), failure process, and microstructure.

It is essential to access qualitative and quantitative data on surface strain field and cracking of FRC. Digital image correlation (DIC) is utilized to continuously monitor strain development and the fracture process. This technique has received much attention [27–30] owing to its various advantages, such as non-destructive and easy-to-use design, full-field and continuous analysis up to failure. In contrast, traditional measurement techniques, such as strain gauges extensometers and linear variable differential transformer sensors (LVDTs), can rarely provide accurate assessments of strain fields or for early crack-tracking. Besides, strain gauges only measure at the embedded points and their direction, thus do not allow for a full-field response. Therefore, DIC can be used to quantitatively measure and monitor strain development and the fracture process in FRC.

1.2 Research gaps and questions

Investigating the literature background reveals that, despite the copious studies conducted in this field, there are still gaps that require further investigation to better assist researchers in the future. These gaps include determining the proper ratio of PP and FF fibers by comprehensive examination in both macro- and micro-structure aspects, as well as investigating failure behavior and the use of the DIC technique to detect and monitor crack development. Accordingly, this study seeks to investigate the following questions.

• What is the ideal ratio of PP and FF fibers that should be utilized in FRC?

• How do PP and FF fibers influence cracking behavior?

• Is the DIC technique able to control full field strain and detect cracks with high accuracy as an alternative to traditional measurement techniques?

1.3 Research significance

This study provides a comprehensive investigation to fulfill the research gaps and address the questions listed above using both macro- and micro-structure analysis, as well as the DIC technique. In this regard, the effect of PP and FF fibers on compressive strength, stress-strain curve, crack development, and failure process of FRC concrete are analyzed. The aim of this research is to i) quantify and discuss the effect of FF and PP fibers on the mechanical properties of FRC; ii) use the DIC technique to monitor strain development, measure the surface strain field, and detect crack development; iii) assess the role of FF and PP fibers from a microstructural perspective on the texture of FRC and fracture behavior; and iv) provide a comprehensive comparison with previous studies to determine the optimal content of FF and PP fibers.

2 The digital image correlation technique

DIC is a powerful, non-contact optical technique used to determine displacement and surface strain of an object based on acquisition and comparison of digitized images taken at different loading steps. Therefore, DIC is of great interest to many research fields, such as civil engineering and structural health monitoring [31–33], solid mechanics, materials science and mechanical engineering [34,35], biomechanics [36], and medicine [37]. It was originally developed by researchers at the University of South Carolina in the early 1980s [38,39]. DIC is based on the correlation of digital images taken from the surface of specimens during test execution and could be done with a digital camera (2D mode or in-plane displacement) or two cameras at different angles (3D mode or out-of-plane displacement). It uses the greyscale light intensity to compare the track points, thus each image is transformed from RGB color to greyscale color (black/white) and stored as a matrix. Each array of the matrix has a value between 0 (black) and 255 (white) depending on the intensity of its gray light [40], thus each representing an unique point of specimen surface. To determine the in-plane displacement of the specimens’ surface, a virtual grid is assigned to the image or on a specific region of interest (ROI). In order to obtain the specimens’ deformation, the image captured before and after loading are correlated with each other to matched points on the grid of the ROI, in other words, the deformed location of the image is identified as the maximum of the correlation coefficient. There are various correlation criteria that differ in the computation required in calculations and reliability. The zero-normalized sum of squared differences (ZNSSD) was recently suggested as the most robust and authentic due to its insensitivity to illumination lighting noise [41]. Moreover, the reliability of the correlation could be enhanced by considering two subsets of pixels sized \left[\left(2m+1\right)\ast \left(2m+1\right)\right], instead of two single pixels, since it covers a wider range of gray level.

The Newton-Raphson, coarse-fine, and Levenberg-Marquardt methods are the iterative approaches used for nonlinear optimization to maximize the 2D correlation coefficient [42]. Fig.1 schematically shows the subset position with center P in the undeformed and deformed image. A typical function for determining the correlation coefficient is defined as following equation:

(1)S\left(x,y,u,v,\frac{\mathrm{\partial }u}{\mathrm{\partial }x}\text{,}\frac{\mathrm{\partial }u}{\mathrm{\partial }y}\text{,}\frac{\mathrm{\partial }v}{\mathrm{\partial }x}\text{,}\frac{\mathrm{\partial }v}{\mathrm{\partial }y}\right)=1-{\text{C}}_i\left(f\left(x,y\right),g\left(x^{\prime },y^{\prime }\right)\right),

where C_i is a correlation function that obtains a value of one if a complete correlation is performed, f\left(x,y\right)is the gray level value of the reference image at coordinates \left(x,y\right), and g\left(x^{\prime },y^{\prime }\right) is the gray level pixel-value of the deformed specimen (second image) at points \left(x^{\prime },y^{\prime }\right). By minimizing the correlation coefficient S, the values of coordinates (x, y), displacement (u, v), and the derivatives of the displacements {\displaystyle \frac{\mathrm{\partial }u}{\mathrm{\partial }x}}, {\displaystyle \frac{\mathrm{\partial }u}{\mathrm{\partial }y}}, {\displaystyle \frac{\mathrm{\partial }v}{\mathrm{\partial }x}}, and {\displaystyle \frac{\mathrm{\partial }v}{\mathrm{\partial }y}} can be specified [43]. These are, in turn, utilized for subsequent analysis, for instance, the computation of fracture parameters.

To identify the small sets of pixel position, the surface of specimens must have the random speckle pattern (random gray intensity distribution) that result in deformation on the specimen surface [44]. According to the above mentioned, the 2D DIC technique consists of the subsequent steps: (i) specimen and imaging equipment preparations (experimental preparations); (ii) recording the specimens’ images during the loading test; (iii) computational analysis of the acquired images to obtain the displacement and strain information.

3 Experimental plan

3.1 Materials

Cement: According to ASTM C150 [45] requirements, ordinary Portland cement type II (CEM-II 42.5 N) was used, whose physical and chemical characteristics are reported in Tab.1.

Aggregate: River aggregates were used for both fine and coarse aggregate sections. The fineness modulus and specific gravity of fine aggregates were 2.54 and 2.68 respectively, while the specific gravity of coarse aggregates was 2.69. According to ASTM C33 [46] requirements, the grading curves of the fine and coarse aggregates are depicted in Fig.2.

Fibers: The fibers used in this study were FF fibers, a hybrid synthetic fiber made up of 100% virgin copolymer/PP that consists of twisted bundle non-fibrillated monofilament along with fibrillated network and monofilament based on PP, whose physical and geometric properties are presented in Tab.2 and Fig.3, respectively.

High range water reducer (HRWR): HRWR agent is a high-performance superplasticizer based on polycarboxylate ether used to form the unique performance in improving workability performance.

Water: Ordinary, clean, potable water with a neutral pH value scale (\cong 7) and free of harmful suspended particles, bacteria, and chemical substances was utilized for both mixing and curing hardened concrete cubes.

3.2 Mix design and specimen preparation

In this study, a total of 30 specimens of 150 mm cubic were made in 6 mix designs (6 × 5 = 30). One mix design was used as the control, while the other five were used to investigate the effect of FF and PP fibers on the physico-mechanical characteristics of concrete specimens at 28 days. To reduce error, the average value of five identical specimens was used for each mix design. Tab.3 presents the mixture proportions of plain and fiber concrete specimens with their respective mix ID and volume fraction of used fibers.

To prepare the concrete mix with uniform texture, the following steps were carried out: i) aggregates were combined and mixed for 30s; ii) cement was combined with fiber and mixed with the aggregate mixture for 2 min; iii) a blend of water and superplasticizer was poured into the mixture and mixed for 5 min. The mixture was cast in the 150 mm cubic molds in three-layers, each layer compacted 30 times with a tamping rod. The cubic specimens were demolded after 24h and immersed in the limewater tank at 25 ± 1ºC for at least 28 days in accordance with the requirements of ASTM C511 [47].

3.3 Experimental setup

Loading condition: To evaluate the uniaxial F_c of hardened concrete specimens, a servo-hydraulic loading machine with a total capacity of 2000 kN was used (see Fig.4). The loading rate of the compression testing in the stress-controlled apparatus was equal to 0.5 MPa/s, in accordance with British standard BS EN 12390-3 [48]. It is worth noting that, in order to avoid out-of-plane errors in DIC analysis, cubic specimens were used instead of cylindrical specimens. For this purpose, 30 cubic specimens with dimensions of 150 mm were made by 6 mix designs (5 specimens per mix design) and exposed to a compression test in accordance with British standard BS EN 12390-3 [48]. Finally, stress-strain curves of the specimens were obtained up to failure using a data acquisition system.

DIC: To specify the influence of fibers on the fracture process of concrete specimens, DIC was applied to the stacked images at different stages of loading. A digital single lens reflex (DSLR) camera mounted on a tripod and two surface mounted device (SMD) ring lights were utilized on both sides of the camera to record the specimens’ images under loading. Although out-of-plane movement was not considered as a serious problem, the camera was set at a distance of 1000 mm from specimens’ surface as recommended in previous scholars’ studies [49] to reduce any influence of out-of-plane movement. To reduce probable vibration of the testing machine, all video imaging was recorded using a digital camera with a pixel resolution and lens with a 6.1−30.5 mm focal length (Fig.4). The images of the specimens’ surface were shot continuously every 2s and the desired image was extradited from the video. DIC analysis of the acquired images at ROI was executed by the VIC-2D software package to measure deformation, detect cracking, and determine the development of the fiber concrete cross-section.

Scanning electron microscope (SEM): SEM imaging was used to specify the effect of FF and PP fibers on the microstructural texture of specimens. For this aim, small pieces were taken from the virgin specimens and soaked in ethanol for over a week to inhibit any chemical reactions. It is worth noting that one of the major impacts on the feasibility and quality of SEM imaging is the nature of the specimens, especially their conductivity. Thus, a gold coating was applied to all specimens prior to imaging to ensure a better signal. Image scanning was performed by apparatus (Phenom ProX, Netherland) in 20kV voltage and magnifications of 250X and 500X.

4 Results and discussion

This section will provide an in-depth analysis of the effect of FF and PP fiber content on dominant characteristic parameters of FRC. Subsection 4.1 discusses the ultimate compressive stress, strain at peak stress, ultimate strain, modulus of elasticity, toughness index, and density. The average value of the three specimens for each mix design was reported. The mentioned FRC parameters were computed and are reported in Tab.4. Subsections 4.2 and 4.3 assess analyzes the computed DIC vs. measured stress-strain curve and surface strain field using DIC, respectively. Subsection 4.4 evaluates the failure and cracking mechanisms of fiber-reinforced specimens compared to the plain specimen. Finally, the effect of fibers on microstructural texture is discussed in further depth in subsection 4.5.

4.1 Stress-strain curve from displacement sensor

Fig.5 describes the general failure process of plain and FRC and presents the stress-strain curves obtained for the FF and PP fiber specimens. As shown in Fig.5(a), the entire fracture process during the compression test (irrespective of the loading condition) considering the different filling intervals was divided into four stages. Firstly, the Consolidation Stage (OA or O1A1) is when the structure of specimens changes with the addition of fibers since the plain specimen contains pores and cracks (i.e., under the influence of initial loading, the tiny, internal pores and the layered surface is compacted; the stress-strain curve is concave). Secondly, the Linear Elastic Stage (AB or A1B1) is when the stress concentration occurs in cracks. It is generally assumed that damage is less than threshold values, thus the material behaves in a linear-elastic manner. The third stage is the Crack Propagation Stage (BC or B1C1) when the deformation of the specimens transforms from elastic to plastic deformation (i.e., the starting point of the failure and crack propagation; the curve becomes convex). During this stage, the structure of the fiber-reinforced specimens prevents and reduces crack propagation, reducing the stress concentration at the crack tip. The cracks are essentially bridged by the fibers, which help to gradually transmit the stress into un-cracked zones, thus decreasing the number of cracks and improving the overall strength of the fiber-reinforced specimens [49]. Finally, the forth Strain Softening Stage (CD or C1D1) occurs when the cracks quantities are developed till reach coalescence, so specimen transfers to the stage of yield failure and finally their badly damaged. Moreover, bridging and frictional restraint depend on material composition. For instance, the maximum size of aggregates, as well as the difference in stiffness and strength between the matrix and aggregates form the descending part of the softening branch in the post-peak regime. Fig.5(b) shows how the addition of FF and PP fibers had a significant effect on the softening branch of the stress-strain curve, as seen from the reduced slope. For reinforced specimens, as the volume of fiber increased, the softening branch of the stress-strain curve became more horizontal. Thus, under the same strain values, the specimen with higher fiber volume undergoes greater stress. This is attributed to fiber bridging and the bonding between fiber and matrix, resulting in stress distribution and reduced crack development.

4.1.1 Ultimate stress

The ultimate F_c values of the fiber-reinforced specimens with different volume fractions of FF and PP fibers are presented in Fig.6 and Tab.4. Results indicate that the addition of FF and PP fibers has different effects on the F_c of the concrete. The results show that the addition of FF fibers into the cement matrix of the concrete increases the F_c due to the higher bond strength between the cement matrix and aggregates. The maximum improvement in ultimate F_c for the FF fiber-reinforced specimens is seen at 0.20% volume fraction with a 30% increase in effectiveness compared to the plain specimen. In contrary, the presence of PP fibers in the mixture generally reduces the ultimate F_c due to the lower bond strength formed in the C-S-H gel, thereby weakening the cement matrix and surrounding aggregates [50]. Nonetheless, it is worth noting that the alteration of 0.20% volume fraction of PP improves the F_c (prevents a reduction in F_c). A downward trend is then observed. While the F_c of the PP 0.40 specimen is approximately the same as that of the plain specimen, it prevents crack propagation due to higher ductility.

To determine the optimal fiber volume fractions, results from this study were compared with that of previous studies [17,18,51,52]. Fig.7 illustrates how the use of FF fiber within the specified range has a significant effect on the F_c of FF fiber-reinforced specimens compared to the plain specimen. A comparison of the results indicates that a volume fraction of 1.5 to 2.5 kg/m³ is the most appropriate FF fiber content. However, a study shows that the addition of 75% fiber (1.8 to 3.15 kg/m³) only slightly increases the strength by 2%, which due to the high cost of fiber denies its justifiability.

For PP fiber, many studies have reported that the addition of PP fiber can only slightly increase or, in most cases, decrease F_c [11,20,53–55]. Fig.8 provides a comprehensive comparison of the results from this study with eight previous studies [11,20–26] to validate the effect of PP fiber on F_c. The figure suggests that the incorporation of PP fiber will likely reduce the F_c compared to the plain specimen. However, through comparison,it can be concluded that the addition of PP fiber within a specified range of 1.35 to 2 kg/m³ may be a suitable volume fraction. A previous study reported a similar result that PP fiber fractions that are too low or too high may cause internal defects, which poses a challenge to improving strength [55]. However, it should not be overlooked that the greatest contribution of PP fiber effectiveness to increasing ductility, which is mentioned in the next subsection.

4.1.2 Strain at peak stress and ultimate strain

Fig.9 illustrates the changes in strain at peak stress versus the different volume fractions of FF and PP fibers in the concrete specimens. Results show that FF and PP fibers have variable effects on concrete specimens. While FF fibers reduce the strain of FF fiber-reinforced specimens at peak stress compared to the plain specimen, PP fibers increase the strain of PP fiber-reinforced specimens at peak stress compared to the plain specimen. The maximum strain at peak stress values for fiber-reinforced specimens are approximately 0.0017 and 0.0024 for 0.10% volume fraction of FF fiber and 0.30% volume fraction of PP fiber, respectively. Specifically, their changes are approximately −4% and +33% compared to the peak strain of the plain specimen (0.0018). Although results show that the FF fiber-reinforced specimens experience lower strain at peak stress compared to the PC and PP fiber specimens, they experience the most stress overall according to results from the previous subsection. In other words, the FF fiber-reinforced specimens demonstrate the best performance in maintaining the integrity of concrete components among all mixtures, which is in agreement with a previous study [56]. On the other hand, PP fiber-reinforced specimens demonstrate a higher ductility with approximately +30% more strain at peak stress than the plain specimen. It can be seen from Fig.9 that increasing the volume fraction of FF and PP fibers significantly improves the ultimate strain of FRC specimens relative to plain concrete. The maximum ultimate strain values of concrete containing FF and PP fibers are 0.0037 (FF 0.10) and 0.0044 (PP 0.30), respectively, which is approximately 37% and 63% greater than the ultimate strain of plain concrete (0.0027).

4.1.3 Modulus of elasticity

Modulus of elasticity is an influential parameter and useful indicator for evaluating the material properties of concrete and elastic deformation during the structural analysis [16,49]. In this study, the modulus of elasticity, also known as the secant modulus, is the slope of the chord that starts from the origin and ends at 40% of the maximum strain on the stress-strain curve [50]. Fig.10 shows the role of FF and PP fibers on the modulus of elasticity of the specimens. Fig.10 and Tab.4 indicate that the elastic modulus of all FF fiber-reinforced specimens increased compared to the plain specimen, but reduced for all PP fiber-reinforced specimens. The attendance of FF fibers in the specimens covers the micro-cracks and improves the rigidity of concrete due to their cohesion and bonding. On the other hand, the addition of PP fiber leads to a reduction in elasticity modulus values. Similar results have been reported by researchers in previous studies [57–60], suggesting that the use of PP fiber in concrete has a negative effect on the elasticity modulus.

4.1.4 Toughness index

Toughness index is an essential parameter that indicates a specimen’s ability to absorb energy from start loading to its failure, which reflects the deformation capacity of the specimen under loading. This characteristic depends not only on its carrying capacity but also on its deformability. Given the role of fibers in reinforced concrete, evaluation of this parameter is preferred. The toughness index was calculated for all tested specimens from the area below the corresponding stress-strain curve up to the ultimate strain (Fig.11). Results show that the toughness index generally increased with the addition of increasing volume fractions of fibers into the mix designs, with the highest increase of 52% in toughness index for the FF 0.20 specimen compared to plain concrete, which agrees with previous findings regarding the role of fibers in increasing toughness [61].

4.1.5 Density

Fig.12 shows the density values of all tested specimens. It can be seen that the concrete density reduces for both FF and PP fiber-reinforced specimens as the fiber volume content increases. The maximum reduction in density values were approximately 4.7% and 7.3% compared to plain concrete for FF and PP fiber specimens, respectively. This is likely due to the low specific gravity of fibers.

4.2 Comparison of computed digital image correlation vs. measured stress-strain curve

Fig.13 depicts the strain-stress curves for all specimens. A comparative analysis was performed on the stress-strain curves obtained from the displacement sensor and DIC technique to determine their effectiveness. The strain of the specimens on the front surface was measured using DIC through analysis of the collected images during different stages of loading. To determine the axial strain, three are located in the front specimen surface which are respectively on the top, middle and bottom side of the surface with a height of 75 mm. It can be seen from Fig.13 that the results from DIC and displacement sensor follow a similar trend, supporting the idea that DIC can be developed as a technique to replace the displacement sensor. Furthermore, the computed DIC curves achieved excellent performance in terms of accordance and utility with a very slight difference compared to the displacement sensor results. The slight difference may be due to surface texture of the specimen and/or the accuracy of DIC implementation, which is an issue that has also been reported in other studies [62,63].

The relation between the lateral and axial strains of plain and FRC specimens obtained using the DIC technique is shown in Fig.14. The axial and lateral strains at the beginning stage of loading induce the vertical deformation and lateral dilation of the concrete specimens, respectively. Almost uniform deformation is observed in the plain specimen before the peak load. However, the lateral deformations of FF and PP fiber specimens from the growth of micro-cracks and fracture of interfacial zones continuously increase before peak load. Similar results have been reported by previous studies [63–65]. Moreover, the fracture process of the fiber specimens began at an early loading stage and gradually increased until the whole pre-peak range. As shown in Fig.14, the lateral deformation of the PP 0.20 specimen was more explicit during the pre-peak loading stage, indicating more micro-cracks compared to the plain specimen. On the other hand, the PP 0.40 specimen indicates worse failure conditions compared to the plain specimen due to low deformation. This finding supports the conclusion that volume fractions of PP fiber that are too high may lead to internal defects, which is a challenge to strength improvement, as mentioned in subsection 4.1.1.

4.3 Surface strain field using digital image correlation

Fig.15 and Fig.16 depict the lateral strain and lateral displacement contour maps for plain and FRC specimens at different loading stages, respectively. The following contour maps are the recorded images at four stages of the peak load for each specimen. As seen in Fig.15, the specimens that reach the red color on the surface strain fields tolerate higher strain. It is observed that the strain fields of all specimens gradually increased during the loading process. In this regard, the results indicate that a significant amount of visible micro-cracks appeared at approximately 75% of the peak load. For the PC specimen, a single crack was observed at the peak load due to brittleness and sudden gain of large lateral strain. In other words, the plain concrete experienced a sudden fracture because of the absence of a reinforcing material since it would help hold the concrete components together during the loading process. On the other hand, the micro-cracks in fiber-reinforced specimens gradually appeared during the third stage (75% of peak load) and began to connect with each other during the fourth stage (100% of peak load), which results in some extensive cracks in the fiber specimens. The FF 0.2 and PP 0.2 specimens experience more lateral strain compared to the PC specimen. It is well known that the mechanical properties of concrete specimens, such as ductility, can be improved with the addition of fibers [66]. It can be seen from Fig.16 that the lateral displacement contour maps show more displacement in the final stages of loading for all specimens. This is due to the loss of specimen integrity as the load continuously increased throughout the process. Almost all displacements of the PC specimen occurred suddenly due to the lack of reinforcing material, while the displacements of the reinforced specimens began earlier since the fibers prevent large single displacements through the bridging effect by

4.4 Comparison of the fracture mechanism

A comparison of the fracture mechanism was conducted by investigating the behavior and characteristics of the cracks formed on each of the plain and FRC specimens. To better understand this process, it is essential to first elucidate the role of fibers on fracture zone cracking. Fig.17 schematically examines the differences in the interactions that occur in the fracture zone of plain and fiber-reinforced specimens. As shown in Fig.17(a), the only components that counteract crack growth in the plain specimen are the strength of the cement matrix, aggregates, and bonding between them (i.e., Interfacial Transition Zone). Fiber-reinforced specimens not only have the same inhibitors mentioned above, but also increased adhesion between the fiber and concrete through bridging and reinforcement (Fig.17(b)). The failure mechanism of the specimen is such that micro-cracks gradually appear on the surface following the start of loading. For the plain specimen, the micro-cracks are looking for a way to propagate and may encounter three regions: cement matrix, aggregates, and ITZ. In the face of each of these regions, the micro-cracks will either change their path, forming a branched micro-crack, or pass through the region, forming a main crack. Finally, due to the weakness in the transfer zone, we quickly observe the formation of the main crack in the plain specimen. For the fiber-reinforced specimen, fibers across the fracture zone increase its load carrying capacity and crack resistance by bridging between two regions of the cement mortar matrix. This, in turn, reduces the extent of the stress concentration and allows the cement matrix to extend in uniform stress. In other words, it increases the fracture toughness. Hence, the fibers play a significant role in the cracking mechanism through this phenomenon. However, a high fiber volume content will cause poor dispersion and conglobation, leading to weak ITZs and internal flaws that will eventually decrease the fiber reinforcement effect on the concrete specimens.

Fig.18 shows the fracture pattern of concrete specimens and indicates the mentioned mechanism of failure. From the fracture pattern of the PC specimen we can see that the failure occurred during the two main cracks. However, fiber-reinforced specimens show the formation of micro-cracks and had more cracks compared to the plain specimen. In other words, the PC concrete specimens chipped off at maximum loading, but the FRC specimens did not. This may be due to the bridging-phenomenon’ss presence of PP and FF fiber in the concrete specimens. Moreover, the multiple tiny cracks in the PP and FF fiber specimens are likely due to the fiber bridging phenomenon that help to control the sequential expansion of a single crack, which is similar to results reported in previous studies [67–69]. The direction of the main crack in the PC specimens was parallel to the direction of the load. We can also see that the propagation of the crack was approximately straight and developed from top to bottom on the PC specimen. The FRC specimens demonstrated a different load pattern; the main cracks appeared sinuate, oblique, and not completely spread out in the direction of the load. The straight-through crack patterns do not be realized. Moreover, it is worth noting that the fracture pattern of the specimens confirm the surface strain field results in subsection 4.3.

4.5 Microstructural analysis

The microstructural texture and mechanical properties of concrete are closely related due to its heterogeneous structure, which consists of three phases of cement matrix, aggregates, and ITZ between them. In FRC, the ITZ formed between fibers and the cement matrix influences the weak regions that can affect its mechanical properties [70–75]. In this section, SEM imaging was utilized to investigate the microstructure morphology of the three group specimens, including PC (without fibers), FF 0.20 (with 0.20% FF fibers), and PP 0.30 (with 0.30% PP fibers).

Fig.19(a)–Fig.19(c) shows the SEM images of PC, FF, and PP specimens, respectively. As shown in Fig.19(a), certain regions of the cement matrix are relatively dense in the plain specimen, while others are rougher and porous in texture. Also, depressions and irregular micro-cracks caused an incomplete and sporadic block, which could reduce the stability of the cement matrix. Meanwhile, pores and micro-cracks at the ITZ could result in a loose interfacial zone, which is why the main fracture of PC specimen occurred in the bond border regions between the cement matrix and aggregate (i.e., ITZ). It is worth noting that the ITZ is the weakest zone of the PC specimen, thus the main cracks developed in almost a straight line.

From Fig.19(b) and Fig.19(c), it can be observed that the presence of FF or PP fibers led an overall improvement in the texture density and uniformity of the cement matrix. The cement matrix surfaces of FF and PP specimens appeared much smoother and homogeneous compared to the plain specimen. Due to the better hydration response and subsequent filling of the porous texture, the pore size and number of micro-cracks decreased remarkably. The main difference between the fiber reinforced and plain specimens is visible when looking at the bridging effect of fibers. As seen in Fig.19(b), a twisted bundle of FF fibers prevent crack propagation and provide greater contact surface of the cement matrix. As shown in Fig.19(c), loose ITZ bonding between the PP fiber and cement matrix is clearly visible and degrades the strength of the specimen. The same phenomenon was also seen by Smarzewski [71]. Conversely, it was observed that the micro bumps of cement hydration on the surface of the PP fibers induced a physical interlocking effect due to the formation of an uneven surface, thus improving the strength of the specimen. From this perspective, Yuan and Jia [76] reported a similar result that the addition of PP fiber improves the strength of the cement specimen. Indeed, these findings support the mentioned results in subsection 4.1 , which have been reported in numerous previous studies [11,20,53,54].

5 Conclusions

In this study, the effect of FF and PP fiber on compressive behavior and fracture behavior of reinforced concrete specimens containing different fiber volume fractions was investigated using the DIC technique. The stress-strain curves obtained from the displacement sensor were compared to those computed by DIC. The contour maps of the surface strain field were obtained for all specimens. In addition, SEM imaging analysis was conducted to examine microstructure morphology. Finally, the following main conclusions can be drawn.

1) Both FF and PP fibers could increase the compressive strength of concrete, but FF fiber demonstrated higher effectiveness. Approximately 30% and 22% improvement on the ultimate compressive stress were found for FF-FRC and PP-FRC, respectively. The inclusion of fibers changed the fracture process of specimen from brittle to ductile, increasing the toughness index of the fiber specimens by a maximum of 50%.

2) Comprehensive comparison with previous studies indicated the mentioned volumes of FF (1.5 to 2.5 kg/m³) and PP (1.35 to 2 kg/m³) in this study can be considered as optimal volumes. It showed that lower fiber volumes also indicated the possibility of increasing compressive stregth in order to improve the properties. Higher fiber volumes not only did not result in significant strength increase, but caused internal defects, posing a great challenge to strength improvement.

3) The stress-strain curves obtained using the DIC technique were highly similar to those obtained using the displacement sensor. The contour maps of the surface strain field showed that the micro-cracks gradually appeared at 75% of the peak load and subsequently began to connect to each other.

4) Microstructural analysis showed that the FF and PP fibers prevent crack development through the bridging effect and the integrity of the FRC specimens were compared to the plain specimen.

5) As a result, the DIC technique proved to be an efficient method to characterize the behavior of materials with notable advantages, such as simple crack detection, precise consideration of crack development, and significant estimation of displacement and strain fields.

6) The findings of this study will help to i) reassure researchers that DIC is a reliable technique for crack detection and displacement and strain field estimation; ii) clarify the effect of FF and PP fibers on fracture behavior from both macro- and micro-structural perspectives; iii) facilitate future research surrounding FF and PP fibers with suitable content to benefit the best performance.

To conclude, this study assessed the fracture process of FF and PP fiber-reinforced concrete using experimental analysis and DIC. Further research can be conducted to target and expand on this investigation, such as examining several factors simultaneously or various fiber types (3D, 4D and 5D hooked end, crimped and hybrid) at different volume fractions or with different concrete types (recycled or green materials), all of which could be the subject of future studies.

References

[1] Shi F, Pham T M, Hao H, Hao Y. Post-cracking behaviour of basalt and macro polypropylene hybrid fibre reinforced concrete with different compressive strengths. Construction & Building Materials, 2020, 262: 120108

[2] Khan M, Ali M. Use of glass and nylon fibers in concrete for controlling early age micro cracking in bridge decks. Construction & Building Materials, 2016, 125: 800–808

[3] Behfarnia K, Behravan A. Application of high performance polypropylene fibers in concrete lining of water tunnels. Materials & Design, 2014, 55: 274–279

[4] Hesami S, Salehi Hikouei I, Emadi S A A. Mechanical behavior of self-compacting concrete pavements incorporating recycled tire rubber crumb and reinforced with polypropylene fiber. Journal of Cleaner Production, 2016, 133: 228–234

[5] Kassimi F, El-Sayed A K, Khayat K H. Performance of fiber-reinforced self-consolidating concrete for repair of reinforced concrete beams. ACI Structural Journal, 2014, 111(6): 1277–1286

[6] Alani A M, Beckett D. Mechanical properties of a large scale synthetic fibre reinforced concrete ground slab. Construction & Building Materials, 2013, 41: 335–344

[7] Yu P, Manalo A, Ferdous W, Abousnina R, Salih C, Heyer T, Schubel P. Investigation on the physical, mechanical and microstructural properties of epoxy polymer matrix with crumb rubber and short fibres for composite railway sleepers. Construction & Building Materials, 2021, 295: 123700

[8] Abousnina R, Alsalmi H I, Manalo A, Allister R L, Alajarmeh O, Ferdous W, Jlassi K. Effect of short fibres in the mechanical properties of geopolymer mortar containing oil-Contaminated sand. Polymers, 2021, 13(17): 3008

[9] ACICommittee 544. Report on Fiber Reinforced Concrete. Farmington Hills: ACI, 1996

[10] ASTMC1116M-10a. Standard specification for fiber-reinforced concrete. West Conshohocken: ASTM, 2015

[11] Zhang H, Wang L, Zheng K, Bakura T J, Totakhil P G. Research on compressive impact dynamic behavior and constitutive model of polypropylene fiber reinforced concrete. Construction and Building Materials, 2018, 187: 584–595

[12] Nematzadeh M, Fallah-Valukolaee S. Erosion resistance of high-strength concrete containing forta-ferro fibers against sulfuric acid attack with an optimum design. Construction & Building Materials, 2017, 154: 675–686

[13] Dashti J, Nematzadeh M. Compressive and direct tensile behavior of concrete containing Forta-Ferro fiber and calcium aluminate cement subjected to sulfuric acid attack with optimized design. Construction & Building Materials, 2020, 253: 118999

[14] Broda J. Application of polypropylene fibrillated fibres for reinforcement of concrete and cement mortars. In: Yilmaz S, Ozmen H B, eds. High Performance Concrete Technology and Applications. London: IntechOpen, 2016, 189–204

[15] Kooshkaki A, Eskandari-Naddaf H. Effect of porosity on predicting compressive and flexural strength of cement mortar containing micro and nano-silica by multi-objective ANN modeling. Construction & Building Materials, 2019, 212: 176–191

[16] Lezgy-Nazargah M, Emamian S A, Aghasizadeh E, Khani M. Predicting the mechanical properties of ordinary concrete and nano-silica concrete using micromechanical methods. Sadhana, 2018, 43(12): 196

[17] Bagheri A, Jamali A, Gorgani-Firoozjah M, Zanganeh H. Comparison of the performance of macro-polymeric fibers and steel fibers in controlling drying shrinkage cracks of concrete. Int. J. Struct. Civ. Eng. Res, 2018, 7(4): 302–308

[18] Hasan-Nattaj F, Nematzadeh M. The effect of forta-ferro and steel fibers on mechanical properties of high-strength concrete with and without silica fume and nano-silica. Construction & Building Materials, 2017, 137: 557–572

[19] Song P S, Hwang S, Sheu B C. Strength properties of nylon-and polypropylene-fiber-reinforced concretes. Cement and Concrete Research, 2005, 35(8): 1546–1550

[20] Alsadey S, Salem M. Influence of polypropylene fiber on strength of concrete. American Journal of Engineering Research, 2016, 5(7): 223–226

[21] Mohamed R A S. Effect of polypropylene fibers on the mechanical properties of normal concrete. Journal of Engineering Sciences, 2006, 34(4): 1049–1059

[22] Patel P, Desai A K, Desai J A. Evaluation of engineering properties for polypropylene fibre reinforced concrete. International Journal of Advanced Engineering Technology, 2012, 3(1): 42–45

[23] Ramezanianpour A, Esmaeili M, Ghahari S A, Najafi M H. Laboratory study on the effect of polypropylene fiber on durability, and physical and mechanical characteristic of concrete for application in sleepers. Construction & Building Materials, 2013, 44: 411–418

[24] Ramujee K. Strength properties of polypropylene fiber reinforced concrete. International Journal of Innovative Research in Science, Engineering and Technology, 2013, 2(8): 3409–3413

[25] Ahsana F K M, Varghese S. Behavioral study of steel fiber and polypropylene fiber reinforced concrete. International Journal of Research in Engineering and Technology, 2014, 2(10): 17–24

[26] KhanSKhanR AKhanA RIslamMNayalS. Mechanical properties of Polypropylene Fibre reinforced concrete for M 25 & M 30 mixes: A Comparative study. International Journal of Scientific Engineering and Applied Sciences, 2015, 1(327–340)

[27] Wu Z, Rong H, Zheng J J, Xu F, Dong W. An experimental investigation on the FPZ properties in concrete using digital image correlation technique. Engineering Fracture Mechanics, 2011, 78(17): 2978–2990

[28] Hamrat M, Boulekbache B, Chemrouk M, Amziane S. Flexural cracking behavior of normal strength, high strength and high strength fiber concrete beams, using Digital Image Correlation technique. Construction & Building Materials, 2016, 106: 678–692

[29] Quiceno Pérez V, Cotes Prieto D, Zapata Orduz L E. Mechanical characterization of self-compacting steel fiber reinforced concrete using digital image correlation. Engineering Fracture Mechanics, 2021, 246: 107618

[30] Fallah Pour A, Nguyen G D, Vincent T, Ozbakkaloglu T. Investigation of the compressive behavior and failure modes of unconfined and FRP-confined concrete using digital image correlation. Composite Structures, 2020, 252: 112642

[31] Gajewski T, Garbowski T. Calibration of concrete parameters based on digital image correlation and inverse analysis. Archives of Civil and Mechanical Engineering, 2014, 14(1): 170–180

[32] Omondi B, Aggelis D G, Sol H, Sitters C. Improved crack monitoring in structural concrete by combined acoustic emission and digital image correlation techniques. Structural Health Monitoring, 2016, 15(3): 359–378

[33] Peters W, Ranson W. Digital imaging techniques in experimental stress analysis. Optical Engineering (Redondo Beach, Calif.), 1982, 21(3): 213427

[34] MalesaMSzczepanekDKujawińskaMŚwierczAKołakowskiP. Monitoring of civil engineering structures using digital image correlation technique. In: EPJ Web of Conferences. Les Ulis: EDP Sciences, 2010

[35] Sebastiani M, Eberl C, Bemporad E, Pharr G M. Depth-resolved residual stress analysis of thin coatings by a new FIB–DIC method. Materials Science and Engineering A, 2011, 528(27): 7901–7908

[36] Sadowski T, Kneć M. Application of DIC technique for monitoring of deformation process of SPR hybrid joints. Archives of Metallurgy and Materials, 2013, 58(1): 119–125

[37] Sztefek P, Vanleene M, Olsson R, Collinson R, Pitsillides A A, Shefelbine S. Using digital image correlation to determine bone surface strains during loading and after adaptation of the mouse tibia. Journal of Biomechanics, 2010, 43(4): 599–605

[38] Sutton MA, Wolters W, Peters W, Ranson W, McNeill S. Determination of displacements using an improved digital correlation method. Image and vision computing, 1983, 1(3): 133–139

[39] Chu T, Ranson W, Sutton M A. Applications of digital-image-correlation techniques to experimental mechanics. Experimental Mechanics, 1985, 25(3): 232–244

[40] Karimi A, Navidbakhsh M, Kudo S. A comparative study on the mechanical properties of the healthy and varicose human saphenous vein under uniaxial loading. Journal of Medical Engineering & Technology, 2015, 39(8): 490–497

[41] Tekieli M, De Santis S, de Felice G, Kwiecień A, Roscini F. Application of Digital Image Correlation to composite reinforcements testing. Composite Structures, 2017, 160: 670–688

[42] Pan B, Qian K, Xie H, Asundi A. Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review. Measurement science and technology, 2009, 20(6): 062001

[43] Bruck H, McNeill S R, Sutton M A, Peters W H III. Digital image correlation using Newton-Raphson method of partial differential correction. Experimental Mechanics, 1989, 29(3): 261–267

[44] Sutton M A, McNeill S R, Helm J D, Chao Y J. Advances in two-dimensional and three-dimensional computer vision. In: Photomechanics. Berlin: Springer, 2000, 323–372

[45] ASTMC150/C150M-12. Standard Specification for Portland Cement. West Conshohocken: ASTM International, 2012

[46] ASTMC33-74a. American Society For Testing and Materials. Concrete and Material Agregates (including Manual of Agregates and Consrete Testing). West Conshohocken: ASTM International, 1918

[47] ASTMC511–21. Standard Specification for Mixing Rooms. Moist Cabinets, Moist Rooms, and Water Storage Tanks Used in the Testing of Hydraulic Cements and Concretes. West Conshohocken: ASTM International, 2021

[48] BSIBS EN 12390-3. Testing hardened concrete Part 3: Compressive strength of test specimens. London: British Standards Institution, 2002

[49] Hoult N A, Take W A, Lee C, Dutton M. Experimental accuracy of two dimensional strain measurements using digital image correlation. Engineering Structures, 2013, 46: 718–726

[50] Shan L. Experimental study on mechanical properties of steel and polypropylene fiber-reinforced concrete. Applied Mechanics and Materials, 2014, 584: 1355–1361

[51] Son D H, Bae B I, Lee M S, Lee M S, Choi C S. Flexural strength of composite deck slab with macro synthetic fiber reinforced concrete. Applied Sciences (Basel, Switzerland), 2021, 11(4): 1662

[52] Dashti J, Nematzadeh M. Flexural behavior of GFRP bar-reinforced calcium aluminate cement concrete beams containing forta-ferro fibers in acidic environment. Construction & Building Materials, 2020, 265: 120602

[53] Bei-Xing L, Ming-xiang C, Fang C, Lu-ping L. The mechanical properties of polypropylene fiber reinforced concrete. Journal of Wuhan University of Technology-Mater. Sci. Ed., 2004, 19(3): 68–71

[54] Qin Y, Zhang X, Chai J, Xu Z, Li S. Experimental study of compressive behavior of polypropylene-fiber-reinforced and polypropylene-fiber-fabric-reinforced concrete. Construction & Building Materials, 2019, 194: 216–225

[55] Wang D, Ju Y, Shen H, Xu L. Mechanical properties of high performance concrete reinforced with basalt fiber and polypropylene fiber. Construction & Building Materials, 2019, 197: 464–473

[56] Richardson A. Compressive strength of concrete with polypropylene fibre additions. Structural Survey, 2006, 24(2): 138–153

[57] Choi Y, Yuan R L. Experimental relationship between splitting tensile strength and compressive strength of GFRC and PFRC. Cement and Concrete Research, 2005, 35(8): 1587–1591

[58] Puertas F, Amat T, Fernández-Jiménez A, Vázquez T. Mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres. Cement and Concrete Research, 2003, 33(12): 2031–2036

[59] Alhozaimy A, Soroushian P, Mirza F. Mechanical properties of polypropylene fiber reinforced concrete and the effects of pozzolanic materials. Cement and Concrete Composites, 1996, 18(2): 85–92

[60] Zhang M, Mirza J, Malhotra V. Mechanical properties and freezing and thawing durability of polypropylene fiber-reinforced shotcrete incorporating silica fume and high volumes of fly ash. Cement, Concrete and Aggregates, 1999, 21(2): 117–125

[61] Madadi A, Eskandari-Naddaf H, Shadnia R, Zhang L. Characterization of ferrocement slab panels containing lightweight expanded clay aggregate using digital image correlation technique. Construction and Building Materials, 2018, 180: 464–476

[62] Fallah S, Nematzadeh M. Mechanical properties and durability of high-strength concrete containing macro-polymeric and polypropylene fibers with nano-silica and silica fume. Construction and building materials, 2017, 132: 170–187

[63] Li W, Xiao J, Sun Z, Shah S P. Failure processes of modeled recycled aggregate concrete under uniaxial compression. Cement and Concrete Composites, 2012, 34(10): 1149–1158

[64] Du T, Wang W, Liu Z, Lin H, Guo T. The complete stress-strain curve of recycled aggregate concrete under uniaxial compression loading. Journal of Wuhan University of Technology-Mater. Sci. Ed., 2010, 25(5): 862–865

[65] Choi S, Shah S P. Fracture mechanism in cement-based materials subjected to compression. Journal of Engineering Mechanics, 1998, 124(1): 94–102

[66] FayyadT MLeesJ M. Experimental investigation of crack propagation and crack branching in lightly reinforced concrete beams using digital image correlation. Fracture Mechanics, 2017, 182: 487–505

[67] Nematzadeh M, Maghferat A, Zadeh Herozi M R. Mechanical properties and durability of compressed nylon aggregate concrete reinforced with forta-ferro fiber: experiments and optimization. Journal of Building Engineering, 2021, 41: 102771

[68] Bakthavatchalam K, Rajendran M. An experimental investigation on potassium activator based geopolymer concrete incorporated with hybrid fibers. Materials Today: Proceedings, 2021, 46: 8494–8501

[69] Khan M Z N, Hao Y, Hao H, Shaikh F U A. Mechanical properties of ambient cured high strength hybrid steel and synthetic fibers reinforced geopolymer composites. Cement and Concrete Composites, 2018, 85: 133–152

[70] Li J, Niu J, Wan C, Jin B, Yin Y. Investigation on mechanical properties and microstructure of high performance polypropylene fiber reinforced lightweight aggregate concrete. Construction and Building Materials, 2016, 118: 27–35

[71] Smarzewski P, Barnat-Hunek D. Property assessment of hybrid fiber-reinforced ultra-high-performance concrete. International Journal of Civil Engineering, 2018, 16(6): 593–606

[72] Korouzhdeh T, Eskandari-Naddaf H, Kazemi R. The ITZ microstructure, thickness, porosity and its relation with compressive and flexural strength of cement mortar; influence of cement fineness and water/cement ratio. Frontiers of Structural and Civil Engineering, 2022, 16(2): 191–201

[73] Emamian S A, Eskandari-Naddaf H. Effect of porosity on predicting compressive and flexural strength of cement mortar containing micro and nano-silica by ANN and GEP. Construction & Building Materials, 2019, 218: 8–27

[74] Emamian S A, Eskandari-Naddaf H. Genetic programming based formulation for compressive and flexural strength of cement mortar containing nano and micro silica after freeze and thaw cycles. Construction & Building Materials, 2020, 241: 118027

[75] Kargari A, Eskandari‐Naddaf H, Kazemi R. Effect of cement strength class on the generalization of Abrams’ law. Structural Concrete, 2019, 20(1): 493–505

[76] Yuan Z, Jia Y. Mechanical properties and microstructure of glass fiber and polypropylene fiber reinforced concrete: an experimental study. Construction & Building Materials, 2021, 266: 121048

RIGHTS & PERMISSIONS

Higher Education Press