Effect of fiber dosage on water permeability using a newly designed apparatus and crack monitoring of steel fiber–reinforced concrete under direct tensile loading

Abstract

Cracks in concrete structures have always been the main reason to allow the aggressive and harmful agents to infringe the concrete resulting in its deterioration and decreasing lifespan. In the present study, the water permeability of the cracked concrete has been investigated. The consequences of cracking on the durability and endurance of concrete were also studied. A state-of-the-art permeability setup was designed to measure the water flow in normal and fiber-reinforced concrete under direct tensile loading. The setup was convenient for determining the average stress applied to the concrete specimens and simultaneously the maximum crack opening. Furthermore, the effect of fiber content on the cracking geometry (tortuosity and roughness) was evaluated by incorporating the coordinate data of the cracked surface using a 3D sensor-based laser scanning data acquisition system. To understand the effect of fiber content on the cracking geometry (tortuosity and roughness), the acquired data were then analyzed. Test results show that the designed setup is suitable to measure the water permeability under direct tensile loading. Water permeability decreased upon increasing the steel fiber dosage. Besides, the results show that tortuosity decreased while surface roughness increased with the fiber dosage increment. Promising preliminary results indicated that there is an inverse relationship between surface roughness and water permeability. The crack sensing setup successfully monitored the crack.

Keywords

Water permeability direct tensile loading fiber-reinforced concrete surface roughness crack tortuosity crack monitoring

Highlights

A new device to evaluate water permeability is designed.

A special direct tensile loading setup is designed.

A 3D scanning system is used for the evaluation of surface topographical data of cracked specimens.

The influence of fibers on tensile loading capacity, water permeability, and surface roughness and tortuosity were studied.

Crack was monitored during the test.

Introduction

The durability of concrete has always been one of the important concerns and research areas for structural engineers and researchers. The durability of concrete is mainly affected by the inception of hazardous fluids into the concrete; for instance, the ingress of water, acids, alkalis, chlorides, sulfates, and gases such as O₂ and CO₂. Water Permeability is one of the most important aspects of the durability of concrete.^1,2 Fiber-reinforced concrete performs better than plain concrete regarding its durability and mechanical properties.³ Cracks on the structural concrete of different sizes (minor to major) play a vital role in the ingression of harmful and hazardous substances in concrete, which ultimately causes the deterioration of concrete.^4–6 The service life of any structure is mostly based on the durability factors like chemical, thermal, and mechanical properties.⁷ To make the concrete less porous and control the cracks, high-performance concrete and ultra-high performance concrete with and without fibers are used.^8,9 Different aspects like water–cement (w/c) ratio, type of cement, aggregate size, type, fraction, and geometry of fiber play an important role in the durability of concrete.^10–12 Concrete undergoes different loads and stresses during its service life like compression, flexural, and tensile loadings. Concrete is globally considered a weak material under tensile and flexural loading. To enhance the loading capability, different types of fibers such as steel fibers and synthetic fibers are used.^13,14 Some researchers have also used natural fibers like coconut fibers and human hair to improve the strength shortcomings.^15,16 Macro fibers like steel, polyvinyl alcohol, cellulose fiber, and polypropylene fibers are used to control and minimize the formation and proliferation of the macro crack, while microfibers used are to restrict the small cracks. Some cracks may also appear due to environmental factors like extreme weather conditions. In this regard, extended research has been carried out.^17,18

Permeation of water and other liquids in concrete depends on the loading type, material, and environmental conditions.¹⁹ However, by considering the micromechanical properties of materials used in concrete, a low permeable composite is possible by maximizing the crack bridging capability of fibers. On the contrary, a concrete matrix that can minimize water pervasion even under a cracked state is inevitable for a better structural performance of infrastructures.²⁰ Several studies were carried out to conduct the water permeability tests under different loading conditions like compression, tension, or bending.^21–23 An indirect loading system was used in most cases, which could be different from real-time cracking scenarios.^24,25 Cracks in concrete are the reasons for water impregnation; nevertheless, a better fiber distribution along the cracked region can minimize and narrow the crack width. It could facilitate creating multiple micro-cracks, which ultimately reduces water permeability.²⁶ Steel fiber–reinforced concrete (SFRC) performs better post-cracking behavior in bending and direct tensile loading; however, for the better effectiveness of SFRC, better fiber orientation and distribution are important.²⁷ Fiber-reinforced matrix decreases crack tortuosity and increases surface roughness that reduces the crack permeability.²⁴

Through an extensive review of the existing literature, the present study aims to design an innovative device to monitor the real-time water permeability of concrete under direct tensile loading conditions. The research on this important domain is still in the primary stages. This investigation will assist in bridging the previous research gap. The study is a significant contribution to the civil engineering research community and the structural engineers to understand this important aspect of concrete structures since cracks due to tensile loading have always been a matter of concern regarding the durability of concrete. Therefore, it is inevitable to broaden our research approaches to understand concrete behavior under direct tensile loading.

Experimental program

Materials and mix proportioning

The experimental study used ordinary Portland cement; fly ash and steel fibers were purchased locally. Table 1 shows the concrete matrix proportion. The targeted compressive strength was 30 MPa. Two types of crushed sands were used with different fineness modulus of 1.77 and 1.75, respectively. Coarse aggregates having a fineness modulus of 5.62 were used. A maximum ɸmax = 10 mm size coarse aggregates were used in this experimental campaign.

Table 1.

Concrete mix design (kg/m³).

Cement	Fly ash	Fine aggregate	Coarse aggregate	Water	Superplasticizer (SP)
390	155	848	822	220	0.5% by binder

Type 1 cement and class “F” fly ash were employed as cementitious materials. Polycarboxylate-based superplasticizer (SP) is used as a water retarder. Figure 1 shows the macro steel fibers incorporated in the experimental setup. Table 2 shows the important mechanical properties of fibers used in the experimental program. The water-binder ratio used in this campaign is 0.50. A laboratory concrete mixer and a vibrating table with a frequency of 350 Hz were used to mix and compact the concrete. The mixing order of the materials was as follows: first, the coarse and fine aggregates along with cement and fly ash were added gradually and dry mixed for 3 min to ensure the homogenous mixing. Steel fibers were added and mixed for another 3 min. Finally, water with SP was added and mixed for another 3 min for homogeneity of the concrete matrix. All the materials were added manually.

Table 2.

Properties of steel fibers as provided by the manufacturer.

Length	Diameter (mm)	Aspect ratio (mm)	Tensile strength (MPa)	E-modulus (GPa)
35	0.54	65	1345	200

Prismatic beams and cubes were cast, having a varying quantity of steel fibers. The specimens were tested after the curing time of 28 ± 2 days.

Loading arrangement and specimens

This work done is inspired from a previous study carried out by Charron et al.²⁸ The significance and novelty of this study is the application of tensile stresses on concrete specimens. In the previous studies,^28–30 cracks were induced using an embedded steel rebar in the concrete specimen. Once the cracks were generated, water permeability was evaluated, while in this study a direct tensile loading system was used to study the water permeability of cracked specimens under direct tensile loading. Instead of an embedded steel rebar, a high strength glue was used to generate the tensile stresses in the concrete specimens which helped to evaluate the real-time water permeability and to monitor the cracking. The overall loading setup has been elaborated in the following section. The specimens tested consist of two steel plates bonded with concrete using a high strength glue on top and bottom of beams submitted to tensile loading. According to the test report provided by the manufacturer, the seventh day tensile strength provided by glue between steel and C-45 concrete is 4.6 MPa. The seventh day compressive strength provided by the glue was 91.2. The most important advantage of this bonding system was it had minimum eccentricity effects. A uniform uniaxial loading was ensured throughout the test. The glue was provided by a local manufacturer. The length of the specimen is 300 mm, having a prismatic cross section of 100×100 mm². A universal testing machine (UTM), as shown in Figure 2, having a loading capacity of 2.5 MN, was used. A steel bar embedded in the center of a plate similar to the dimensions of the glued steel plate was fixed in the jaws of the universal testing machine (UTM), which directly transfers the load to the specimen. This formation ensures the yielding of the specimen. A water sealing tape on the longitudinal faces of the specimen was used to ensure the sealing of water. Silicon rubber tape was used beneath the outer edges of aluminum boxes to prevent water leakage and unilateral water flow. A water sealing tape is an excellent option having a minimum interference on the mechanical behavior of concrete.

Concept and design of permeability setup

The concept of water permeability setup was to design a system that can measure permeability in normal and fiber-reinforced concrete under direct tensile loading and to monitor the concrete cracking. Four Linear Variable Differential Transformer (LVDT)s were installed on the longitudinal sides of the specimen and crack width was monitored throughout the test. In this way, the water permeability setup and the tensile strain are evaluated simultaneously. The water permeability assembly consists of two aluminum boxes and a vacuum pump–based water pressure system that maintains a constant vacuum in the outlet tank. The aluminum boxes, along with the silicon sealing, are placed on two longitudinal faces of specimen. The boxes are held together using two adjustable clamp system. Compared to the specimen area (280 mm x 80 mm) mm used during permeability test, a smaller area (20 mm × 20 mm) mm is used in cladding system. Therefore, the lateral constraint and its effect on crack pattern and crack growth was ignored. One of the boxes is connected to the inlet water tank, while the other face box is connected to the vacuum container, which is used as an outlet water tank. The inlet water tank is kept full before the test to ensure a uniform flow of water. The outlet water tank is connected with the sensor-based vacuum container that stores the water passed through the cracked region of specimen throughout the test. As the crack appears on the specimen, water starts to flow through the crack raising the water level in the outlet tank. A vacuum pump–based setup is used to circulate the water through the crack according to the crack width. The vacuum-based setup was very efficient measuring the water seeping through the crack. This vacuum-based setup helps to stop the possible water leakage, which is very evident in most of the water permeability setups. A high precision weight scale was used to measure the water passed through the crack and reach the outlet tank. It helped to maintain sufficient water flow throughout the testing. The data acquisition system also monitored this weight scale during the testing. The cracked permeability coefficient K_f (m²) is calculated using Darcy’s equation (1).²¹ The concrete is considered as a homogenous material here assuming laminar flow

k_{f} = \frac{Q μ}{A_{f} ρ} {(\frac{Δ p}{Δ x})}^{- 1}

(1)

where

Q = water flow in m³/s,

A_f = cross-sectional area of water flowing area,

$\frac{Δ_{p}}{Δ_{x}}$ is assumed uniform pressure gradient in Pa/m,

μ = 0.001 Pa s is the dynamic viscosity of water, and

ρ = 998 kg/m³ is water density at 20˚C.

Only that part of the specimen’s cross section (280 mm × 80 mm) was considered with direct water contact. At the same time, the remaining sealing area was deducted. This permeability setup can measure the flowed water through the specimen during the entire test. Therefore, the water permeability of both plain and fiber-reinforced concrete was calculated considering a uniform pressure gradient of 0.05 MPa. The detailed water permeability setup along with the loading system is shown in Figure 2

Crack geometry and crack monitoring

The cracked surfaces were later used to study the surface roughness (R_N) and tortuosity (τ). A 3D laser-based data acquisition system was used, as shown in Figure 3.³¹ The accuracy of laser sensors was 0.01 mm. The proposed method is significant to demonstrate the effect of fibers on the topography of cracked concrete surfaces.³² The path of specimen movement was controlled according to the alignment of the laser sensors as shown in Figure 3. Surface roughness (R_N) and crack tortuosity (τ) can be characterized by the following equation^33,34

R s = \frac{S_{t}}{S_{0}}

(2)

τ = {(\frac{L_{O}}{L_{t}})}^{2}

(3)

Here, L_t and L_o are the total crack length and projected crack length, while S_t and S_O ²⁴ are the total surface crack and projected surface crack, respectively.

The approach used to find the total length and projected length is depicted in Figure 4.

The total length can be calculated using equation 4

L_{t} = \sum_{i = 1}^{n} \sqrt{{[z (i) - z (i - 1)]}^{2} + δ^{2}}

(4)

where z (1) and z (i-1) are the corresponding height to the point (i) and (i-1) δ is element size. Projective covering technique^35,36 is used to find the total surface area A_t; the illustrative view of the method is shown in Figure 4(b).

The area A₁ and A₂ of the triangles can be calculated using Heron’s formula

A_{1} = \sqrt{l 1 (l 1 - a 1) (l 1 - b 1) (l 1 - c 1)}

(5)

The total surface area evaluated using

A_{t} (δ) = \sum_{i, j = 1}^{N (δ)} A_{i, j}

(6)

The crack tortuosity (τ) and surface roughness (R_N) were assessed using MATLAB programming. The crack monitoring was carried out using the four-probe method.

Test results and discussion

American Society of Testing and Materials (ASTM) standards C192/C192 M was used to perform the compressive strength tests after the curing period of 28 days. Table 3 shows the average results. A gradual improvement is noticed in the compressive strength upon the addition of fiber quantity.^37–39 It is a common understanding among the research community that fibers perform little to enhance compressive strength. From nil to 25% of strength increase in compressive strength is possible.⁴⁰ The compressive strength results meet the recommendations of the Fib model 2010.⁴¹ Jawad et al.⁴² postulated that the improvement in compressive strength of fiber-reinforced concrete is due to the confinement of fiber reinforcement. Compression produced a lateral expansion along with tension and shear. The tension and shear are resisted by fibers, which cause an increase in compression strength. For higher fiber dosages, this confinement can decrease the concrete specimen transversal deformation and improve its compressive strength. However, to ensure the workability of concrete, a fiber dosage of 1% is recommended.

Table 3.

Comparison of fresh and hardened properties of the specimens with different fiber contents.

Nomenclature	Specimen (ID)	V_(sf) (%)	V_(sf) (kg)	Air content (%)	Slump (mm)	f _(cu)
						MPa
						Mean	SD.
Plain concrete	PC	0	0	2	125	44.36	2.52
Steel fiber 39	SF-39	0.5	39	2.3	110	45.83	0.97
Steel fiber 58.5	SF-58.5	0.75	58.5	2.5	100	50.13	0.81
Steel fiber 78	SF-78	1	78	2.8	90	38.9	2.94
Steel fiber 97.5	SF-97.5	1.25	97.5	3	75	43.07	1.82

Note: f _(cu) and SD is compressive strength and standard deviation, V_(sf) is the volume content of the steel.

Tensile tests

The main concept of this project was to study the water permeability under the direct tensile loading behavior of normal and fiber-reinforced concrete. Therefore, tests were carried out to understand this aspect. Figure 5 illustrates the effect of loading on the crack opening displacement (COD), where displacement is monitored using four LVDTs fixed on the longitudinal faces of the specimen. When the first micro-crack appears at the end of the elastic region, it further propagates until the localization of minor cracks to a macro crack. The localized crack is not always at the center of the specimen, which appears at different lengths according to the weakest zone in a specimen. The COD against the maximum load measured during the test for different fiber dosages is shown in Figure 5. The cracking scenario can be defined in three phases: elastic zone, first cracking point, and post-cracking zone. In the elastic zone, the concrete shows maximum strength under the elastic region. At the first cracking point, the concrete cracks, while at the post-cracking zone, the fibers show their significance and provide extra strength until the fibers fail. Tensile test results of fiber-reinforced concrete show the global trend of improved post-crack behavior. Increasing the fiber dosage showed a positive influence on load-carrying capability.

However, most of the specimens showed a strain-softening behavior after the first crack. The steel fiber dosage higher than 1% showed multiple cracking phenomena, which has been illustrated in Figure 6.

Nevertheless, in all the cases, the first crack strength and post-cracking strength improved upon fiber dosage increment. Plain concrete showed an elastic phase of 0–28 kN, while in cases of fiber-reinforced concrete, it had an elastic phase of 0–32 kN and 0–39 kN, respectively. The results explicate that the tensile strength improves upon fiber increment; especially, the post-cracking behavior considerably enhanced, which is in accordance with the results simulated by Ref. [43].

Fiber dosage of more than 1% (by volume) increases the cracking stress substantially.⁴⁴ The better post-cracking behavior for higher fiber dosages 1% and 1.25% is because of the intrinsic property of steel fiber–reinforced concrete specimens by controlling initial crack spacing, which ultimately helps to control crack widths. Therefore, the ability of fiber-reinforced concrete to transfer the tension across cracks helps to increase the tension stiffening and reduce crack spacing. The post-cracking tensile strength exhibited by steel fiber–reinforced concrete mainly depends on the type and dosage of fiber used.⁴⁵

The better post-cracking behavior shown by specimens containing higher fiber dosage is due to the more number of fibers along the cracked region, which helped in the post-cracking scenario.⁴⁵ Therefore, it is important that all fibers are oriented along the tensile load direction which will ultimately play a significant role in post-cracking behaviors. The load drops when the crack opens explicitly, which is consistent with previous research findings. The first cracking load for different fiber dosages has been postulated in Table 4.

Table 4.

First crack strength, surface roughness (Rn) evaluation, and crack tortuosity (τ) evaluation for different fiber content.

Nomenclature	Specimen (ID)	Volume of steel fiber V_(sf) (%)	Volume of steel fiber V_(sf) (kg)	First crack load (kN)	Surface roughness (Rn)	Crack tortuosity (τ)
Plain concrete	PC	0	0	28.00	1.356	0.863
Steel fiber 39	SF-39	0.50	39.0	32.15	1.425	0.865
Steel fiber 58.5	SF-58.5	0.75	58.5	28.17	1.503	0.82
Steel fiber 78	SF-78	1.00	78.0	28.83	1.531	0.681
Steel fiber 97.5	SF-97.5	1.25	97.5	34.83	1.859	0.613

Table 4 explicates that increasing fiber dosage resulted in a higher first crack load. Compared to plain concrete, increasing the steel fiber dosage to 1.25% induced 21% increase in first crack load. This can be ascribed to the fact at upper fiber dosage, fibers are closely spaced. Thus, they can better prevent the initiation of micro-cracks into macro cracks. This deferral of micro-cracks propagation increased the load-carrying capacity and the corresponding ultimate strain.^46,47 Similar results were previously confirmed by Kwan et al. and Shu et al.⁴⁸ They contemplated that higher fiber volume helps to improve both the first crack strength and post-cracking strength. Similarly, Abdallah et al.⁴⁹ established a correlation between the number of fibers and post-cracking behavior. They argued that more fibers along the fractured zone delivered better post-peak and post-cracking strength. Furthermore, it is observed in this experiment that multiple cracking is possible for a fiber dosage higher than 1.5%.^50,51 Moreover, the equal distribution of fibers and fiber orientation are important features to perceive the multiple cracking. Similar conclusions were presented by Manish Roy et al.⁵² Nevertheless, research should be conducted to validate the exact fiber dosage to achieve the state of multiple cracking irrespective of the shape and dimension of the specimen.

Surface roughness (R_N) and tortuosity (τ)

Crack roughness and tortuosity are important aspects of a better bond, larger surface area, and crack length. Many researchers have emphasized this important feature. Different methods have been used by different researchers.^53–58 The addition of fibers helps to increase the surface roughness (R_N) and a better tortuous surface area. As it is witnessed from Figure 7 that the three-dimensional view shows that increasing the fiber dosage helps to increase the height of the surface peaks that confirms the increased surface roughness upon the increasing fiber dosage. The change in surface roughness is gradual for different fiber dosages; the surface roughness increased gradually for steel fiber dosage 0.5% which is 1.425 to steel fiber 1.25% which is 1.859. Whereas the tortuosity decreased for steel fiber dosage 0.5%, which is 0.863 to steel fiber dosage 1.25%, which is 0.613. In this analysis, surface roughness and tortuosity showed an inverse relationship for different fiber dosages. Water permeability also decreases when the surface roughness increases and tortuosity decreases. This analysis shows that surface roughness and water permeability are directly related; rough surfaces provided by higher fiber dosages provide a rough surface which creates an obstruction to the flow of water which ultimately decreases water permeability. Similar results were formerly confirmed by Akhavan et al.²⁴ Another study suggests the patent relationship between crack width, permeability, surface roughness, and crack tortuosity.⁵⁹ They suggested that although surface roughness and tortuosity exhibit fractal behavior, the results could be used to estimate the crack permeability. Ansari et al.⁵⁵ studied the significance of the tortuosity factor as the relation with permeability. However, steel fibers perform better to improve surface roughness (R_N) and tortuosity (τ). This is because of the better post-cracking behavior of steel fibers. Another likely reason could be the better bond between the concrete matrix and steel fibers. The calculated results of tortuosity (τ) and roughness (R_N) are represented in Table 4. Results show that higher fiber dosage results in enhanced surface geometry. Surface roughness (R_N) can be evaluated using a 3D presentation.^60,61 Figure 7 shows the 3D view of the normal and fiber-reinforced concrete specimens. It can be observed from the figure that the increase of fiber results in more height differences among the peaks. Table 4 shows the statistical analysis of surface roughness (R_N). The table shows that the surface roughness increases effectively in all cases upon raising the fiber dosage. More ever, the standard deviation showed a little difference. The analysis further clarifies that crack surface area firmly affects upon higher content of fiber.

The results of Table 4 further demonstrate that the surface roughness factor can be adopted to assess the crack surface geometry. The value of surface roughness increased with the increasing of fiber dosage. It means the actual area of crack surface increases upon the addition of steel fiber. This roughness provides an uneven and tortuous path to water flow, which ultimately slows down the water ingression resulting in lower permeability. Li et al.⁶² also suggested that roughness and permeability are closely correlated to the cracked composites. Similarly, the crack tortuosity factor (τ) also decreased upon the increment of fiber resulting in a larger crack length. Table 4 shows the tortuosity factor (τ) of all specimens. It can be noticed that the fiber content rise showed a declining trend in the tortuosity factor (τ) of all specimens. The overall variation coefficient is less than 5% which shows the regularity among (τ) results.

Water permeability

Water permeability test results are demonstrated in Figure 8. Permeability results clearly show better performance of steel fibers compared to normal concrete performed. The results show that the upper fiber dosage, 1.5% showed the least water permeability, indicates the advantage of steel fibers. The higher fiber dosage helped to provide a more tortuous and rougher path for the water flow, which affects the water permeability. Results of surface roughness show a patent relationship between fiber dosage and permeability; surface roughness increases for higher dosages, which ultimately decreases the water permeability. Besides, more steel fibers across the cracked region showed obstruction to the flow of water, which compensates to decrease the water permeability. Steel fiber–reinforced concrete offers a rough cracked surface which helps to minimize the water permeability.⁶³ There was a patent correlation between incoming water and outgoing water. This relation was varying according to the fiber content. The vacuum pump–based pressure system helped to maintain a uniform pressure gradient throughout the test. This concept helped to minimize the possible influence of pressure difference, which could change the permeability results. These test results show a global trend, which is a validation of this new setup. When the first crack appears, water starts to penetrate the cracked region and gradually increases as the crack widens until the crack localization happens. Fibers across the cracked region help minimize the crack opening and widening phenomenon, which hinders decreasing water permeability. Crack length and width depend on the type of concrete. For normal concrete, the crack is sudden, thus providing easy access to incoming water.⁶⁴ Steel fibers showed a better post-crack behavior, which minimizes the water permeability by narrowing the crack and increasing the surface roughness.⁶⁵ The overall crack length increases as the fiber content increases; this is because crack width varies along the crack length. Cracks are tortuous; therefore, actual crack length is more than nominal crack length. Such features are more helpful in reducing the overall permeability of fiber-reinforced concrete for higher fiber dosages because the cracks will become more tortuous and rough to hinder the flow of water. It is important to have an even fiber distribution to fully utilize the advantage of fibers across the cracked region. Another important parameter is the loading rate. To have a minimum effect on the results and minimize the scatter on test results, the loading rate must be as lower as possible, especially after the first crack.

Crack sensing and monitoring system

Structural health monitoring has been a concern and an emerging area of research for structural engineers and researchers. The capability to monitor the reliability of civil infrastructure in real time offers the opportunity to minimize the maintenance and inspection costs while providing better public safety.^66,67 The crack sensing and monitoring is basically a resistance change–based sensing technique. When the crack initiates, the volumetric resistance is starting to increase as the conductive path is disconnected and continuously increased as the crack localized. In the same way, the resistance measurement method is applied to sense the water permeability. It has an inverse relationship between Figure 9 water permeability and frictional change of resistance (FCR). The FCR dropped abruptly once the water reached the cracked zone; this effect is because the water acts as an electrolyte and helps to improve the conductivity and reduce the overall resistance. Similarly, if the water level decreases, the resistance is increased as the overall moisture content decreases. These phenomena can be witnessed in water-FCR graph.

To measure the FCR of the specimens, a four-probe electrode method was used during this experimental program. Conductive adhesive tapes used as electrodes and named A, B, C, and D. The distance between the electrodes A and D through which 100 (Hertz) alternating current passes is 260 mm. Similarly, the distance between B and C, which helps measure the voltage, is 180 mm. Voltmeter (V1) controls the inner electrode voltage, while the voltage of the fixed resistor (R) is measured using (V2). The current was determined using Ohm’s law. The external electrode consists of a 12V direct current power supply, while the inner electrode consists of a 5 K fixed resistor (R). Figure 8 shows the schematic diagram of the setup. During the permeability test (Water-FCR), the relationship between load-FCR and the effect of water penetration has been studied. Frictional change in resistance is evaluated using equation 7

F C R (%) = \frac{R s (t) - R s (t o)}{R s (t o)}

(7)

The results show a successful crack sensibility by the designed setup. The FCR remained constant until the specimen reaches its peak load. Once the crack appears, the FCR increases dramatically, as shown in Figure 10(a). An increase in resistance is attributed to the fact that the conductive path is established by the steel fiber is disconnected due to crack and the increment in resistance is linear as the crack localization has occurred. Crack sensing sensitivity would be decreased, as the conductive path is more stable and stronger by direct contact with the neighboring fiber. The highest sensitivity of the conductive concrete is at the percolation zone, and it decreases above the percolation threshold because of lower resistance change. The fractional change in resistance across the cracked region increases because the matrix-to-matrix contact increases, fibers breakage, and fiber pullout. The results show conformity with the previous research studies.^68–70

Similarly, the relationship between water penetrated in the concrete during the water permeability and FCR showed an inverse relationship. The FCR dropped abruptly once the water reached the cracked zone; this effect is because the water acts as an electrolyte⁷¹ and helps to improve the conductivity and reduce the overall resistance. Goldfeld et al.⁷² studied a similar study for AR-glass-based textile-reinforced concrete to detect water penetration in the concrete within the cracked region. If the crack is around the electrodes zone, this water-FCR effect could be witnessed. Figure 10(b) shows the test results. This concept could be helpful in the structural health monitoring of similar water structures like offshore structures, dams, and water-submerged piers of bridges. To monitor the crack and water permeability, a self-sensing setup throughout the member is needed to make the whole member conductive by adding other conductive admixtures in the concrete mix.

Conclusions

In this research program, a novel permeability device was successfully designed and tested to understand the behavior of plain and fiber-reinforced concrete specimens under direct tensile loading conditions. Based on the test results, the following conclusions are obtained.

1. The designed device measured the water permeability of normal and fiber-reinforced concrete specimens under direct tensile loading. The designed method could be a better contribution to the researchers and structural engineers.

2. Steel fiber dosage greater than 1% showed better post-cracking behavior.

3. It was noticed that the higher fiber dosage helped in reducing the water permeability of cracked concrete specimens.

4. Higher fiber dosage helped to improve the first crack strength and showed a better post-cracking behavior by bridging the crack opening along the crack region.

5. A laser-based data acquisition was used to analyze the crack surface geometry of the specimens. The results were unequivocal; a significant influence of higher fiber content was noticed, influencing the surface roughness and crack tortuosity.

6. The factors for surface roughness number (Rn) and crack tortuosity (τ) can be used to understand the effect of fibers on the crack geometry of concrete specimens.

7. Surface roughness (RN) increased as the fiber dosage increased while the tortuosity factor decreased upon enhancing fiber dosage.

8. Based on load-FCR and water-FCR relationships, cracking point and water permeability were successfully monitored. This method could be helpful in the structural health monitoring of concrete members under direct tensile loading in offshore marine constructions.

Footnotes

Acknowledgments

The contribution of the National Natural Science Foundation of China is highly acknowledged. Besides, the opportunity provided by the Chinese Government Scholarship is highly appreciated. The author also wants to thank his parents for their unwavering support throughout the journey.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study is funded by the financial support from the National Natural Science Foundation of China (U1904177), the training plan for youth backbone teachers in colleges and universities in Henan province (2019GGJS019), and the Henan province transportation science and technology plan project (2019J-2-10). The authors also want to acknowledge the financial support of Zhengzhou University Presidential Scholarship for providing best research facilities for international students.

ORCID iD

Zahoor Hussain

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Effect of fiber dosage on water permeability using a newly designed apparatus and crack monitoring of steel fiber–reinforced concrete under direct tensile loading

Abstract

Keywords

Highlights

Introduction

Experimental program

Materials and mix proportioning

Loading arrangement and specimens

Concept and design of permeability setup

Crack geometry and crack monitoring

Test results and discussion

Tensile tests

Surface roughness (RN) and tortuosity (τ)

Water permeability

Crack sensing and monitoring system

Conclusions

Footnotes

Acknowledgments

Funding

ORCID iD

References

Surface roughness (R_N) and tortuosity (τ)