At the same time, due to the restriction of the expansion of MgO by the steel bars, the expansion of the bridge deck in the later stage gradually stabilized, and no harmful expansion was produced. When steel fiber and MgO were used at the same time, the three-dimensional distribution of steel fiber further limited the expansion of MgO. The hydration expansion of MgO in confined space reduced the porosity of concrete, optimized the pore structure, and improved the strength and durability of concrete. The research on the performance of concrete in the in-situ test section showed that MgO and steel fiber were safe for the bridge deck, which not only solved the problem of shrinkage cracking of the bridge deck but also further improved the mechanical properties of the bridge deck.To prevent cracks caused by shrinkage of the deck of the Xiaoqing River Bridge, MgO concrete (MC) and steel fiber reinforced MgO concrete (SMC) were used. The deformation and strength of the deck were measured in the field, the resistance to chloride penetration of the concrete was measured in the laboratory, and the pore structure of the concrete was analyzed by a mercury intrusion porosimeter (MIP). The results showed that the expansion caused by the hydration of MgO could suppress the shrinkage of the bridge deck, and the deformation of the deck changed from −88.3 × 10 Cracks are the direct cause of concrete penetration, which seriously affect the durability of concrete. Due to shrinkage and temperature changes, concrete volume will be deformed, which is the main reason for early cracks. From the perspective of structural calculation, deformation under free conditions will not produce tensile stress, and the concrete will not crack. However, once the deformation is restrained, tensile stress will be generated, and since the tensile strength of concrete is very low, it is easy to crack. In thin-walled structures such as bridge decks, due to the large surface area, the volume deformation caused by shrinkage and temperature is greater, and the cracking caused by the constraints of steel bars and bearings is also more serious. ,2. EarlyThe cracks of cement-based materials are closely related to the non-load deformation of the materials. American academic Bazant pointed out that the early deformation of concrete can be divided into drying shrinkage, chemical shrinkage, and thermal shrinkage . Early nThe bridge deck is an important part of bridge. It not only needs to have high strength to bear automobile loads but also needs to have good crack resistance and waterproof performance to ensure that the rainwater will not penetrate into concrete and lead to corrosion of steel bars ,12,13. FThe Changshen Expressway is one of the “three vertical” lines of China’s highway network, connecting the Northeast, the Yellow River Delta, the Yangtze River Delta, and the Pearl River Delta and is a major north–south corridor through eastern China. As shown in In general, the precast concrete slab has been stacked in the construction site for about one month, the shrinkage has been almost completed, and there will be no obvious shrinkage in the later stage. However, the cast-in-place concrete in the upper part will shrink, and the shrinkage is constrained by the precast slabs below, resulting in tensile stress ,19,20,21−6, preventing shrinkage cracks in airport pavement. However, at the same time, MgO reduced the compressive strength of concrete by 27% [Shrinkage of concrete, as a material property, has been extensively studied in the past decade, and researchers have proposed many measures to reduce shrinkage cracks. Huang added 8% MgO to the concrete, which caused the concrete to expand by 60 × 10e by 27% . Yousefie by 27% . From thMgO has primarily been used in dam and airport pavement to reduce shrinkage cracks. Many researchers have proved that MgO concrete is superior to ordinary concrete in terms of carbonization resistance, chloride ion penetration resistance, sulfate corrosion resistance, and freeze-thaw cycle, which means that MgO can also be used in bridges to achieve the purpose of inhibiting shrinkage cracks ,29,30,313, its bulk density was 1.48 g/cm3, its porosity was 46.7%, and its water absorption was 0.45%. The fine aggregate was river sand, with a fineness modulus of 2.94, apparent density of 2.63 g/cm3, bulk density of 1.54 g/cm3, porosity of 41.7%, and water absorption of 0.40%. The MgO expansion agent was produced by Wuhan Sanyuan Special Building Materials Co. LTD in China, and its activity was 115 s. The steel fiber was wave shaped steel fiber , and its detailed parameters are shown in In this research, the cement was Class 52.5 ordinary Portland cement . Fly ash was Class I fly ash . The chemical composition of cement and fly ash was tested according to GBT 176, 2017 “Chemical Analysis Method of Cement” , and theF was changed with the change of concrete length, and the deformation of concrete could be calculated by calculating the change value of F [ε is expansion or contraction of concrete, in 10−6; k is the constant coefficient of the strain gauge, in 10−6/F; ΔF is the change of F; b is the temperature correction factor, in 10−6/°C; a is the thermal expansion coefficient of concrete, in 10−6/°C; and ΔT is the change of the temperature of concrete, in °C.As shown in lue of F . The str2. The material used was SMC, and S3 was used to measure its temperature and deformation. The area of the second part was 519 m2. The material used was MC, and S4 was used to measure the temperature and deformation of MC.As shown in The strength of concrete was tested by a concrete rebound instrument . As shown in Considering that in winter, in order to prevent the deck from freezing, deicing salt is often sprinkled on the deck. When chloride ions diffuse into the bridge deck, they cause corrosion and expansion of steel bars, which lead to cracking and affect the safety of the structure. In order to analyze the chloride permeability of SMC, a rapid chloride permeability test was carried out according to the procedure described in ASTM C1202 ,37,38,39Q is total flux through the specimen within 6 h (C); 0I is initial flux (A), to 0.001A; tI is flux (A) at specified time t (min), to 0.001A.A cylindrical concrete specimen with a height of 50 mm and a length of 100 mm was used to test the chloride diffusion coefficient. Two different solutions, namely 3% NaCl solution and 0.3 mol/L NaOH solution, were selected in the experiment. A concrete chloride flow meter was used to measure the flux through the sample within the specified time. The total electric flux of the concrete test block for 6 h was calculated as follows according to Formula (2):According to the Nernst–Planck equation, the chloride diffusion coefficient (CDC) can be calculated by Q according to Formula (3) as follows:The influence of MgO and steel fiber on the pore structure of concrete was analyzed by mercury intrusion porosimeter (MIP). First, the concrete was broken into small pieces with a length of 2 mm, and the coarse aggregate was removed. Then in order to stop the hydration of cement and MgO, the samples were immersed in anhydrous ethanol for 24 h. After that, the samples were vacuum dried for another 12 h. By comparing the changes of the pore structure before and after adding MgO and steel fiber into the concrete, the reasons why the deformation, strength, and durability of bridge deck changed can be explained from the microscopic point of view.T is the temperature difference between the inside and the surface of the bridge deck (°C); E is the elastic modulus of concrete (MPa); α is the thermal expansion coefficient of concrete (°C−1); μ is Poisson’s ratio of concrete; and xC and yC are the stress coefficients.The internal temperature variation of deck could be divided into three stages. The first stage was the rapid temperature rise stage. Due to the exothermic hydration of cement and MgO, the temperature increased rapidly to 40.9 °C at 17 h and reached 46.9 °C at 41 h. On the other hand, the heat on the concrete surface dissipated rapidly, and the temperature dropped rapidly to 37.9 °C, resulting in a temperature difference of 13 °C between the inside and outside of the bridge deck. According to the stress calculation formula (Formula (4)) proposed by Bradbury, the tensile stress increases with the increase of temperature difference . TherefoThe second stage was the cooling stage. In this stage, the hydration rate slowed down noticeably. As the heat diffused into the air, the internal temperature decreased gradually, and it decreased to 28.1 °C at 106 h. The third stage was the natural temperature stage. After 106 h, most of the hydration of cement and MgO was completed, and the temperature changed with the external environment temperature. Compared with the three different stages, the following conclusions can be reached: (1) In the temperature rise stage, the temperature difference between the inside and outside of the deck was very small, and the concrete was not fully hardened during this time, so it was not easy to produce cracks. (2) In the temperature drop stage, the surface temperature was higher than the internal temperature in the daytime. However at night, the surface temperature was less than the internal temperature. The alternating action of plus–minus temperature stress made it easy to produce temperature cracks in the deck. (3) What is more, in the early stage, the strength of the concrete was low, and the large temperature stress made it easy to produce through-cracks in the bridge deck. Therefore, the maintenance of concrete needed to be strengthened in the temperature drop stage. As shown in −6, which was only 34.9% and 45.2% of MC and SMC, respectively. The second stage was the shrinkage stage, lasting for 100 h. The third stage was the stable stage, at which point the shrinkage or expansion no longer changed dramatically.−6 at 520 h. In this stage, if no measures are taken to reduce shrinkage, the shrinkage of the upper cast-in-place slab will be restrained by the lower precast slab, resulting in tensile stress. When the tensile stress exceeds the tensile strength of the concrete, it will lead to cracking, which was also one of the main reasons for the cracking of many bridge decks in the past. According to Formula (5), the width of the crack increases with the length of concrete slab and the shrinkage deformation [d is the width of the crack; C is the constant coefficient; L is the length of the concrete slab; ΔT is the change in temperature; α is the thermal expansion coefficient of concrete; and ε is the shrinkage deformation of concrete.It can be seen from ormation . Moreove−6 at 520 h, attributed to the generation of Mg(OH)2. MgO made the concrete from original shrinkage to expansion and fundamentally prevented the shrinkage cracking of the bridge deck. For SMC, by contrast, due to the steel fiber restricted the expansion of MgO, part of the energy generated by MgO was used to tension the steel fiber and generated self-stress, which reduced the porosity of concrete and improve the strength of concrete [−6 at 520 h. During the whole time, both MC and SMC were always in the expansion state, so there was no shrinkage cracking, which proved that MgO and steel fiber could be used to solve the early cracks of decks.For MC, the shrinkage of concrete was compensated by the expansion of MgO hydration, and the deformation was 24.9 × 10concrete ,46,47,48c) and expansion (εM). As shown in −6, which completely compensated the shrinkage of the concrete. In the later period, the expansion did not increase without limit, but gradually tended to a stable value. Therefore, the use of MgO in the bridge deck is safe and will not cause stability problems due to harmful expansion. (2) Due to the restraint of steel fiber, the expansion of SMC was smaller than that of MC. In the early stage (21 h), the expansion of SMC was 60.7 × 10−6, only 65.8% of MC. In the later stage (520 h), the expansion of SMC was 55.8 μ ε, only 48.9% of MC. In contrast, for SMC, the energy generated by the hydration of MgO was not completely used to expand the concrete, but was partially used to tension the steel fibers, which was helpful to improve the density of the concrete.The deformation measured by strain gauge was the comprehensive result of concrete shrinkage and MgO expansion, which was the sum of shrinkage 2 grew in the restricted space, which reduced the porosity of the concrete, improved the density, and significantly increased the strength.The strength test results showed that after 8% MgO was added to the bridge deck, the strength of the deck at 21 d was improved due to the increase of the total amount of cementing material. However, in the early days, the strength of the matrix was small, and the constraint on expansion of MgO was small. The hydration of MgO produced Mg(OH)The results of the rapid chloride permeability test of concrete at different ages are shown in It can be seen from The porosity of mortar with two different mix ratios is given in When steel fiber and MgO were added to concrete at the same time, MgO was used as an expansion element, and steel fiber simultaneously played a role in limiting expansion and preventing cracks. Both of them had a compound effect and produced chemical compressive stress in concrete, reduced the porosity of concrete, and optimized the pore structure, which was the key to improving the performance of SMC.(1)With the increase of bridge spans, the cement content of modern concrete is increasing, which significantly increases the early shrinkage. The shrinkage of REF measured at the Xiaoqing River Bridge in 520 h reached −88.3 μ ε. If no measures are taken to reduce shrinkage, the shrinkage under the constraint of the precast slab below will produce tensile stress, which will lead to shrinkage cracks.(2)Due to the radiation of the sun, the temperature of the bridge deck is very high during the daytime, but at night the temperature will quickly drop. In this process, the bridge deck is subjected to repeated tension and compression. Because the concrete is not completely hardened in the early stage, the strength is low, and it is easy to produce temperature cracks. Therefore, in the early stage of construction, it is necessary to strengthen the maintenance of the bridge deck.(3)−6, preventing shrinkage cracks of the deck. The later expansion of MgO tended to a stable value, and there was no backward shrinkage. In addition, the strength test showed that MgO did not reduce the strength of the deck.After adding MgO to concrete, the expansion of MgO hydration compensated the shrinkage, and the expansion of MC in 520 h was 24.9 × 10(4)2 was generated in the restricted space, resulting in pre-pressure, which improved the performance of concrete. When MgO and steel fiber were used at the same time, the expansion of SMC decreased to 9.1 × 10−6 in 520 h, and the strength increased by 17.5% in 21 days. The test results showed that the combined action not only prevented shrinkage cracking of the slab, but also improved the strength of the concrete. In this way, temperature cracks and shrinkage cracks could be effectively prevented.Due to the restraint of steel fibers, Ca(OH)(5)The use of MgO alone had little effect on the resistance to chloride penetration of concrete, with a maximum increase of only 4.6%. On the other hand, the combined effect of MgO and steel fibers at the same time significantly improved the resistance to chloride penetration of concrete, with a maximum increase of 40.7%.(6)The combined effect of MgO and steel fiber significantly improved the pore structure of concrete, which reduced the total porosity of SMC by 32.2% and the harmful pore by 50.7%, which also explained the reason for the increase of rebound strength of SMC from the micro level.(7)Through in-situ tests at the construction site, this study clearly shows that the combined application of MgO and steel fiber can still maintain good performance in outdoor environments. Therefore, SMC, a new type of cement-based material, should be widely promoted and applied to bridge decks of long-span bridges, which has great significance in improving the durability of bridges.In this research, the effect of MC and SMC on the temperature, deformation, and strength of a bridge deck is studied. Through detailed experimental study, the following conclusions can be drawn:


Fractal analysis was used to characterize the interface roughness using laser scanning data. In general, the fts increased with the increasing value of interface fractal dimension. The relationship between fts and fractal dimension value was further analyzed, considering the old concrete ages and the F-T cycles. The results show that the effect of roughness on the bonding property of new-to-old concrete is more significant than the age of old concrete, and the influence of the F-T cycles on the bonding surface is mainly reflected in the initial stage of the F-T deterioration process. The relative dynamic elasticity modulus decreased obviously under F-T cycles, especially for the specimens with low interface roughness. In combination with the results of two non-destructive methods (ultrasonic non-destructive test and relative dynamic elastic modulus test), the larger roughness and the smaller age of old concrete can improve the bond performance.The bonding surface structure generated by the repair of concrete structures has been paid more attention as a weak point. The effects of old concrete age, interface roughness, and freeze-thawing (F-T) attack on adhesive interface are comprehensively investigated. In this study, six kinds of interface roughness and five different old concrete age are designed. The interfacial bonding property is mainly evaluated by splitting tensile strength ( Identification, evaluation, reinforcement and repair of aging concrete structures has become a popular industry . With thThe interfacial bonding properties can be obtained by various tests, and the splitting test is the most widely used ,13,14. MThe concrete structure is subject to the continuous freezing-thawing cycles in the engineering scale, especially in the cold region. The consideration of durability, such as frost resistance, is essential to evaluate the reliability of repaired structures. Guo et al.  found thThe damage evolution and plasticity development of concrete are the key to improve the durability of concrete structure under F-T cycles ,20. Yi eThe above studies showed that the influence of the bond surface roughness on the splitting tensile strength, but only the qualitative variation is analyzed, no quantitative relationship is obtained. F-T deterioration, as major erosion during service, has not been sufficiently considered in the effectiveness of concrete repair structures. It is urgently needed to establish a prediction model for the splitting tensile strength of bonding interface considering the roughness and freezing-thawing action.D. The splitting tensile tests of the specimens with different roughness, different ages of old concrete and different numbers of F-T cycle were carried out to obtain the influence of the factors on the interface bonding property. The relationship between splitting tensile strength and fractal dimension value was further analyzed, considering the old concrete ages and the F-T cycles. In combination with two non-destructive methods (ultrasonic non-destructive test and relative dynamic elastic modulus test), the relationship between splitting tensile strength of new-to-old concrete and the durability was further researched.In this paper, different interface roughness of the new-to-old concrete was characterized with the fractal dimension 2 and 35 N/mm2, respectively. The type of ordinary Portland cement used was P.O42.5, with the density of 3.16 g/cm3. Coarse aggregate is 5–20 mm gravel, and fine aggregate is river sand (fineness modulus = 2.7). The specimens with different ages of the old concrete  were also prepared with 95% relative humidity curing environment and a temperature of 20 °C. After casting the new concrete on the old concrete, the new-to-old concrete was cured at 95% relative humidity curing environment and a temperature of 20 °C. The concrete composition of the old concrete and new concrete are shown in To cast the concrete specimens with same roughness and reduce the discreteness of the test results, a number of plaster models with the same roughness were prepared as follows: the selected fracture surface of concrete specimen was placed in a mold of 100 mm × 100 mm × 100 mm, then the isolation agent was applied evenly on the rough surface, and the plaster was poured on it; after the plaster generated strength, the concrete and plaster model could be separated. The plaster models with the same roughness were made repeatedly. It is necessary to point that the initial concrete specimens with different natural roughness were obtained by splitting tensile test. The standard cubes were used in the experiments with the dimensions of 100 mm × 100 mm × 100 mm. The cubic compressive strength of the old concrete and the new concrete were 30 N/mmδ cover the whole rough surface, and the height of four corner points of each small square can be obtained through a laser scanner (δ and the amount of squares N(δ) satisfy the following relation:As the concrete bond surface was fractal like the coastline, fractal dimension was used to characterize the roughness of the interface. In this study, MATLAB  was used to process the laser scanning data and calculated the fractal dimension values (D) of five different fracture surfaces ,32. For  scanner . Then thFinally, the fractal dimension is used to directly represent the roughness of interface. The fractal dimension values (D) of group T, C, Q, S, G, and B are 2.103, 2.237, 2.255, 2.304, 2.315 and 2.341, respectively. tsf is the splitting tensile strength (MPa); 0.85 is the scale conversion coefficient when a non-standard specimen (100 mm × 100 mm × 100 mm) is used; P is the failure load of the specimen (N); and A is the split surface area (mm2).Dynamic and Static Fatigue Testing Machine FLPL204  was used to obtain the splitting tensile strength. According to GB/T 50081-2002 , the splDR was proposed to characterize the defects inside new-to-old concrete structure, which was expressed as:RD is the relative defect degree, VT is the wave velocity of old concrete at T days for new-to-old concrete structure (km/s), which satisfied T > 3, V3 was the wave velocity of old concrete at 3 days for new-to-old concrete structure (km/s).The main working principle of ultrasonic nondestructive testing is to emit ultrasonic waves of different frequencies inside the material or structure. The signal is received and analyzed by the instrument to further understand the damage and defects inside the structure. Due to the complexity of the internal medium of concrete, the transmission of ultrasonic wave in concrete will have different attenuation degrees, which are mainly divided into two forms: absorption and scattering. A large number of experimental studies show that the acoustic attenuation in concrete is mainly scattering attenuation. The relative defect degree According to GB/T 50082-2009 , the rapnP is the relative dynamic modulus after n times of F-T cycles, nf is the transverse fundamental frequency of the specimen after n times of F-T cycles and is the transverse fundamental frequency of the specimen without F-T attack. With the increase of the number of freeze-thaw cycles, the dynamic modulus of each specimen decreases accordingly. The relative dynamic modulus calculation formula is shown as follows:The fracture failure surface of new-to-old concrete occurred along the bond interface basically, and the failure surfaces are shown in The splitting tensile tests were carried out on the specimens with different ages of old concrete and various surface roughness. The bonding surface for specimens in group B after splitting tensile tests is shown in The splitting tensile strength results are shown in 2) was above 0.90 for six groups. It was confirmed that the old concrete age was an important factor to evaluate the internal defects of new-to-old concrete, which was related to new-to-old concrete strength closely. Furthermore, when the age of old concrete was less than 28 days, its influence on the interface strength of new and old concrete was more obvious.In addition, there was a power function relationship between the splitting tensile strength and the age of the old concrete, as shown in According to the interfacial bonding mechanism of new-to-old concrete, interfacial bonding force mainly includes mechanical force, van der Waals force and chemical force. The mechanical forces include the mechanical bite force caused by the radiation growth of cement hydration products on the interface to the old concrete surface and the physical mechanical meshing force caused by the surface roughness. Van der Waals forces mainly refer to the forces caused by the interaction of crystal molecules in cement matrix. The chemical force is mainly formed by the chemical reaction between the cement hydration products in the old concrete and the interface agent. For the specimens in group T in this work, due to the small interface roughness, the interface adhesion force mainly came from the mechanical bite force and chemical force formed by the radiation growth of cement hydration products in the new concrete and the old concrete onto each other’s concrete surface. After 28 days, the cement hydration of the concrete was nearly completed, and the rate of the cement hydration reaction slowed down . When thD increased, the splitting tensile strength increased correspondingly, and the growth law was linear in general. The correlation coefficient (R2) was above 0.98. The mechanical force played a key role in the interface adhesion of the new-to-old concrete, and the larger the fractal dimension D, the larger the bonding area and the greater the mechanical force. It also confirmed that the fractal analysis was a feasible method to evaluate the splitting tensile strength of new-to-old concrete. Furthermore, the results had an important guiding significance for the improvement of new-to-old concrete structure.The splitting tensile strength of new-to-old concrete specimen with different interface roughness was shown in Since the roughness and the age of the old concrete both had influence on the interfacial bonding, the splitting tensile strength of the new-to-old concrete under the influence of these two factors were compared with the splitting tensile strength of the complete specimen of C30 in DR is characteristic parameter, which can be used to evaluate the strength [L is the loss rate of splitting tensile strength (%), ts-tf is the splitting tensile strength with old concrete at t days (km/s), which satisfied t > 3, ts-3f is the splitting tensile strength with 3 days of old concrete (MPa).The mechanical properties of concrete is mainly affected by internal defects, and the relative defect degree strength ,37. The strength :(5)L=ftsDR) increased with old concrete curing days, which represented the degradation of the internal density of the concrete correspondingly. The loss rate of splitting tensile strength was also increased with longer age of old concrete. It could be seen that the relative defect degree (DR) met the exponential relationship with the loss rate of splitting tensile strength, and the correlation coefficient R2 arranged from 0.9569 to 0.9969 The larger roughness and the smaller age of old concrete can improve the bond performance, and the effect of roughness on the bonding property of the new-to-old concrete structure is more significant than that of old concrete age. The influence of roughness, old concrete age and F-T cycles on the splitting tensile strength of new-to-old concrete was analyzed quantitatively.(2)D can characterize the adhesive interface roughness well, and the splitting tensile strength changes linearly with the fractal dimension D. The relationship between F-T resistance of new-to-old concrete and fractal dimension D satisfy a power function.Fractal dimension (3)The ultrasonic non-destructive test can describe the defect situation of the bonding interface, and for the new-to-old concrete specimens with different old concrete ages and interface roughness, the relative defect degree has an exponential relationship with the loss rate of splitting tensile strength.(4)Small interface roughness cannot improve the F-T resistance of the new-to-old concrete obviously, and when the fractal dimension of interface is above 2.3, the improvement becomes significant.Based on the experimental investigation and the discussion of results, the following conclusions are drawn:


Stress waves are emitted and received by a pair of piezoceramic transducers embedded in the concrete-filled GFRP tube, and the energy and damage indices at different levels of loading in the tube are obtained by wavelet packet to evaluate the damage degree of GFRP tube nano-SiO2 concrete column. Through the experimental studies, the effects of different nano-SiO2 contents, concrete grades, and superplasticizer on the damage were analyzed to gain load–displacement curves, load–energy index curves, and load–damage index curves. The results show that the wave method can be adopted to monitor the damage of GFRP tube nano-SiO2 concrete column. The specimens with 3% nano-SiO2 content have the smallest energy change rate, indicating that adding 3% nano-SiO2 content into concrete can effectively delay the development of damage. After the addition of superplasticizer, with the increase in the strength grade of concrete, the cracks in the specimen tend to develop slowly, and therefore the specimens have a stronger resistance to damage. The damage of the specimens with the nano-SiO2 content of 1% appeared the latest, while the damage without the nano-SiO2 specimen appeared the fastest. The experimental results show that this method can better monitor the damage of the Nano-SiO2 concrete in the glass fiber reinforced polymer (GFRP) tube.This paper proposes a new approach to damage detection of nano-SiO The mechanical properties and durability of concrete can be improved by adding appropriate amounts of nano-SiO2 [Nano-SiOall size ,2,3. Thez et al. . The prez et al. . Antibacz et al. . The chaz et al. . At presano-SiO2 ,10,11.Traditionally, concrete is often reinforced with steel rebars or integrated with other steel components, such as tubes ,13,14 anCurrently, many scholars have conducted extensive research on the health monitoring of civil structures using lead zirconate titanate (PZT) transducers ,39,40,412 concrete columns. When the GFRP tube nano-SiO2 concrete column is in the natural environment, the concrete in the GFRP tube will inevitably produce tiny cracks under the influence of loads and various emergencies . As time goes on, the continuous increases in such cracks will greatly reduce the strength and stiffness of the structure, causing great harm to the whole structure. Hence, this paper intended to systematically study the damage identification of GFRP tube nano-SiO2 concrete columns by the wave method, perform wavelet packet analysis to calculate the stress wave energy before and after concrete column damage, and then gain the relative damage index to reveal the damage of the concrete in GFRP tube. Moreover, the influences of concrete grade and nano-SiO2 content on the damage of GFRP tube nano-SiO2 concrete column were analyzed, and the appropriate content of nano-SiO2 in concrete was suggested, which has not only profound theoretical importance but also significant engineering application value for the application of piezoceramics in the field of structural health monitoring.Even so, few scholars have studied the damage identification of GFRP tube nano-SiOThe piezoceramic material refers to a crystalline material that generates charge transfer on the surfaces of both ends when subjected to pressure, as well as being a kind of smart material with the characteristics of self-perception, self-diagnosis, and self-repair. All piezoelectric materials have a positive piezoelectric effect and a negative piezoelectric effect. The positive piezoelectric effect refers to the fact that when a piezoelectric material deforms because of mechanical force applied, it can cause the internal positive and negative charge centers to move relative each other and cause polarization. As a result, charges with opposite signs appear on both surfaces of the material, and the charge density is proportional to the external force. The polarity of the charge changes with the direction of the external force, converting mechanical energy into electrical energy. Piezoceramic materials can be made into sensors using the positive piezoelectric effect, which have the advantages of fast response speed, wide frequency response range, and simple fabrication. The sensors can be used in health monitoring systems to detect structural damage sensitively and determine the location and size of the damage ,58. In cIn the use of piezoelectric materials for structural health monitoring and damage identification technology ,61, therTraditional signal analysis and processing methods generally use Fourier analysis, which is a fixed window function analysis method. It plays an irreplaceable role in the processing of stationary random signals. However, a large number of real signals are non-stationary time-varying signals. Traditional Fourier analysis cannot reflect the characteristics of a signal, such as non-stationary, short duration, time-domain and frequency-domain localization. The traditional Fourier transform processing is not ideal for non-stationary signals and hence it has been improved, which promotes the development of wavelet theory analysis. The developed wavelet theory analysis can perform multi-resolution analysis and solve many difficult problems in Fourier analysis, so it is an excellent tool for time domain analysis and frequency analysis of signals. Wavelet analysis is a time-frequency localization analysis method which can change time and frequency window. As it only decomposes the low-frequency signal in the process of decomposition, and does not decompose the high-frequency signal, its frequency resolution decreases with the increase in frequency . The wavn signal subsets with different frequency bands. The signal subset is Xi,j, where j is the frequency band , which can be expressed as:Firstly, the received original signal (Si) of the ith measurement was decomposed into 2Secondly, the energy of the jth sub signal in each band of the N-layer signal can be defined as:The total energy of the original signal Si after wavelet packet decomposition and reconstruction is defined as:In structural health monitoring, the damage degree of structure or component is defined as damage index, which is a quantitative measure. Generally, the parameters related to the structural characteristic parameters, such as the elastic modulus and the energy of monitoring signal, are selected to judge the damage degree quantitatively . Finally2 concrete filled GFRP tube column specimens were designed in line with different parameters, including concrete grade, nano-SiO2 content, and superplasticizer content, among which the concrete grades include C10, C20, C30, and C40, and the nano-SiO2 contents are 0%, 1%, and 3%, respectively. The content of the superplasticizer is 0.8%. The tested GFRP tube was produced by pultrusion process and made of E glass fiber and polyester resin, with a wall thickness (T) of 6 mm, a column height (H) of 250 mm, and a square cross section with side length (L) of 100 mm, as shown in 2 content. A pair of piezoceramic sensors were made into smart aggregate and embedded in the concrete of GFRP tube, of which one sensor, SS1, acted as an exciter to generate stress wave signals, and the other sensor, SS2, acted as a receiver to receive stress wave signals. The piezoceramic transducers are manufactured by Hengke Technology Co., Ltd., Qinhuangdao City, Hebei Province, China.In this experiment, a total of 13 nano-SiO3. The mechanical property test of concrete was sampled according to GB/T50080-2002 “Standard for Test Methods of Performance of Common Concrete Mixtures” [2 adopted hydrophilic gas phase nano-silica Hydrophilic-380, with a specific surface area of 380 mm2/g and a particle size of 7–40 nm. The nano-SiO2 powder and the prepared nano-SiO2 solution are shown in 2 is shown in 2 becomes viscous with a poor fluidity. The superplasticizer was appropriately added so as to reduce the influence of poor fluidity. All experimental specimens are shown in In accordance with GB/T1447-2005 “Test Method for Tensile Properties of Fiber Reinforced Plastics”  and GB/Tixtures”  and confThe cross-sections of the specimens were polished smooth prior to each test to prevent bias during loading. The 1000 kN electro-hydraulic servo universal testing machine was used for loading, and the displacement control loading mode was adopted, with a speed of 0.5 mm/min. The loading device is shown in 2 contents.At the initial stage of loading, there was no obvious change in the GFRP tube wall. With the continuous increase in the load, the top end corner of the specimen was torn, accompanied by the hissing sound of glass fiber tearing; when approaching the ultimate load, the longitudinal tearing became more and more obvious until complete tearing. In order to further observe the damage of the concrete inside the GFRP tube, the torn side of the GFRP tube was stripped. 2 had the smallest crack width and the smallest number of cracks, whereas the concrete without nano-SiO2 had the largest crack width and the largest number of cracks.It can be seen from 2 contents, and The experimental data were processed to obtain load–displacement curves, where the horizontal axis (Δ) represents displacement, the vertical axis (P) represents load, and A, B and C represent corresponding loads when the sensor receives the maximum energy. 2 content are damaged; however, when specimens with 0% and 3% nano-SiO2 contents are damaged, the loads are respectively 18.7% and 17.9% of the ultimate load. As can be seen from As can be seen from 2 contents, and The collected data were analyzed and calculated by wavelet packet to obtain the load–energy index curves of specimens with superplasticizer. 2 content have the highest initial energy, and can bear the largest load with the increase in load until being destroyed. However, when the concrete inside specimens is damaged, specimens with 1% nano-SiO2 content have the maximum energy attenuation slope value of 4.71, whereas specimens with 3% nano-SiO2 content have the minimum energy attenuation slope value of 0.53., implying that adding 3% nano-SiO2 content to concrete can effectively delay the development of damage.At the initial stage of loading, the compression and compaction of concrete in GFRP tube played a major role, and the energy of the received signal increased and gradually reached the maximum. Then, with the further increase in load, the concrete was damaged under compression, and micro-cracks began to develop, resulting in a gradual decrease in energy. As can be seen from As it can be seen from 2 contents, and The load–damage index curve of each specimen can be gained from 2 content have the highest load at the beginning of damage, indicating that the damage of specimens with 1% nano-SiO2 content occurs the latest, followed by specimens with 3% nano-SiO2 content, yet the damage without the nano-SiO2 specimen occurs the fastest. With the increase in load, the damage index of specimens without nano-SiO2 increases rapidly to over 0.8, followed by specimens with 1% nano-SiO2 content, while the damage index of specimens with 3% nano-SiO2 content changes the slowest, demonstrating that the damage of specimens without nano-SiO2 content develops the fastest. The damage index of each specimen changes relatively slowly when reaching 0.6. As can be seen from As can be seen from 2 concrete filled glass fiber reinforced polymer (GFRP) tube column is mentioned. The results show that the damage index based on wavelet packet energy can effectively monitor the damage of concrete in GFRP tube, and the damage identification method based on wave analysis is feasible and effective. Based on the analysis of the experiment results, the conclusions can be summarized as follows:In this paper, a new method of damage identification using piezoelectric transducers to monitor nano-SiOThe stress wave method using piezoceramic transducers can monitor the damage of the concrete in the glass fiber reinforced polymer (GFRP) tube well. The failure modes of specimens and the development trend of load–displacement curves are well reflected in the changes in energy and damage indices.At the initial stage of load, when the concrete is under compression and compaction, the stress wave energy increases; however, as the load increases, cracks appear in the concrete and develop, the energy decreases, and the damage index increases from 0 to 1.2 content have the largest initial energy and the maximum ultimate load. When the concrete inside specimens is damaged, the energy change rate of specimens with 3% nano-SiO2 content is the smallest, indicating that adding 3% nano-SiO2 content to concrete can effectively delay the development of damage.Specimens with 1% nano-SiOAfter the addition of superplasticizer, the higher the concrete grade of the specimen, the slower the development of cracks in concrete, and the stronger the resistance to damage. When no superplasticizer is added, the higher the concrete grade is, the faster the damage of the specimen develops.2 content develops the fastest. After the damage index reaches 0.6, the damage index of each specimen changes relatively slowly.The damage index of specimens with 0% nano-SiOThe structure health monitoring technology using piezoelectric transducers can monitor the health status of the structure in real time and produce early warnings of potential risks, which is suitable for the long-term monitoring of high-rise buildings and large-scale infrastructure. In future research, the interface peeling of GFRP tubular concrete columns using piezoelectric tranducers and the monitoring and identification of internal concrete defects will be studied. Different characteristic parameters of signals, such as signal amplitude, signal energy and signal power spectral density, will be selected as research objects in the time and frequency domain, and various damage indexes based on characteristic parameters will be proposed.


The concrete industry has long been adding discrete fibers to cementitious materials to compensate for their (relatively) low tensile strengths and control possible cracks. Extensive past studies have identified effective strategies to mix and utilize the discrete fibers, but as the fiber material properties advance, so do the properties of the cementitious composites made with them. Thus, it is critical to have a state-of-the-art understanding of not only the effects of individual fiber types on various properties of concrete, but also how those properties are influenced by changing the fiber type. For this purpose, the current study provides a detailed review of the relevant literature pertaining to different fiber types considered for fiber-reinforced concrete (FRC) applications with a focus on their capabilities, limitations, common uses, and most recent advances. To achieve this goal, the main fiber properties that are influential on the characteristics of cementitious composites in the fresh and hardened states are first investigated. The study is then extended to the stability of the identified fibers in alkaline environments and how they bond with cementitious matrices. The effects of fiber type on the workability, pre- and post-peak mechanical properties, shrinkage, and extreme temperature resistance of the FRC are explored as well. In offering holistic comparisons, the outcome of this study provides a comprehensive guide to properly choose and utilize the benefits of fibers in concrete, facilitating an informed design of various FRC products. Conventional concrete is a brittle material by nature, with a (relatively) weak performance in tension. To alter this characteristic and avoid a sudden brittle failure of concrete structures, reinforcing materials are embedded into the concrete matrix. Since ancient times, people have been putting fibers, such as straws and hairs, in mortars and bricks to improve their tensile properties. These ancient and simple methods of concrete reinforcement have now been transformed into the advanced methods that involve using discontinuous fibers distributed randomly throughout the concrete matrix . The resThe main fiber properties that govern the performance of FRC in the fresh and hardened states include fiber dimensions, elastic modulus, tensile strength, ultimate strain, and bonding and chemical compatibility with the matrix. Considering different fiber materials used in the current practice, the four main categories of metallic, glass, polymer, and natural fibers can be identified. As the name implies, metallic fibers refer to the fibers that are made from metals. The most common type of metallic fibers is steel fibers, but stainless-steel fibers have recently gained a growing attention because of high corrosion resistance. Glass fibers are broadly defined as the fibers that are derived from naturally occurring minerals or rocks. The two general types of glass materials used as fiber reinforcement in cementitious matrices are silica and basalt glass. Polymer fibers are considered as manmade fibers that are neither metallic nor glass fibers. A wide variety of synthetic polymer fibers are deemed suitable for application in FRC, including but not limited to polypropylene (PP), nylon, polyvinyl alcohol (PVA), polyolefin (PO), carbon, polyethylene (PE), polyester (PET), acrylic (PAN), and aramid. Natural fibers are the fibers that occur in nature within the organic tissue of plants. In this study, the main properties of FRC with various polymer and glass fibers are explored in detail, while an investigation of the effects of metallic and natural fibers is deferred to a separate study.The inclusion of fibers is known to affect various fresh and hardened properties of concrete, while the primary goal of including fibers in concrete is often to prevent (or control) the propagation of cracks. Cracking in concrete is a multiscale process, which starts at the micro-scale when cracks begin to form under applied stresses in the interfacial transition zones (ITZs) between the cement paste and aggregates. Such microcracks spread through the paste until they meet other microcracks, and eventually, grow into a large macrocrack . Once a There are a range of crack-inducing phenomena in concrete, including early-age plastic shrinkage cracks, which can be reduced (or even eliminated) by the inclusion of fibers at low dosages. The inclusion of fibers can also be beneficial for controlling the cracks after hardening. This is highly dependent on not only the fiber type and dosage, but also the dimensions of the fibers added to the cementitious matrix. Microfibers are low diameter, high aspect (length/diameter) ratio fibers that are most often less than 18 mm in length. Microfibers can be effective at arresting microcracks as they extend beyond the ITZ and propagate through the cement paste. This is primarily achieved by bridging the microcracks, facilitating the transfer of tensile stresses. Due to the micro-scale reinforcement action provided by microfibers, it is generally accepted that they most significantly affect the strength properties of FRC prior to full crack formation, as characterized by the stress-strain curve prior to the peak stress. Ultimate strength gains in both compression and tension have been reported for FRC with microfiber ,4,5, depMacrofibers are characterized by their increased lengths (and reduced aspect ratios) compared to microfibers. There is no set standard to define the size boundaries between micro and macrofibers, which creates some overlap in the definitions. However, macrofibers are rarely shorter than 18 mm and generally have diameters larger than 0.1 mm. Macrofibers are effective at bridging the cracks in concrete once they have grown past the micro stage. This is because macrofibers are long enough to provide stress transfer across crack openings when a single crack has formed from the growth of microcracks. If the fiber-matrix bond condition is kept unchanged, the higher the elastic modulus of the macrofiber, the smaller the crack width under the same applied load. This feature relies on the existence of sufficient bond between the fibers and the matrix to develop the strength of the individual fibers and utilize their high stiffness. Besides reported exceptions for the fibers with a high modulus of elasticity ,9, macroIn this study, the properties of FRC containing different polymer and glass fibers are investigated with necessary details and comparisons. 3H6)n [Polypropylene (PP), with a chemical formula of (C3H6)n , is a co3H6)n ,13,14,1512H22N2O2)n that can have a range of strength properties dependent on the base polymer, manufacturing technique, and additives used to make it [Nylon is a synthetic fiber with a chemical formula of (C make it . Althoug2H4O)n, is a relatively high-strength synthetic fiber originally developed for the replacement of asbestos in asbestos cement [Polyvinyl alcohol (PVA), with a chemical formula of  that encompass polypropylene and polyethylene as subgroups [Polyolefin (PO) is a polymer fiber formed by the polymerization of olefin monomer units n, can be produced with a wide range of mechanical properties. In the past, PE fibers were being characterized by low strength and elastic modulus, similar to PP and PO fibers. However, the development of ultra-high density PE has greatly increased the strength and stiffness of this type of fiber. From the performance perspective, it can be generally stated that the higher the fiber density and molecular weight, the higher the strength and stiffness potential. These fiber properties depend on the degree of molecular alignment achieved by advanced production processes, involving heat pressure and catalysts [Polyethylene (PE) fiber, with a chemical formula of  and poly (PCDT). The PET and PCDT fibers are made using different processes and have different chemical and mechanical properties. PET fibers often have a higher strength and stiffness than PCDT fibers, which are characterized as more ductile. With reference to use as concrete fibers, contrary to PCDT fibers, PET fibers have been subject to extensive research, mostly as fibers recycled from consumer products. Henceforth, the polyester fibers refer to the PET variety. It must be noted that, although PE and PET share the polyethylene name, they are chemically different, as PET is a polyester, not a type of polyethylene.3H3N)n. The name “acrylic” is short form for, and essentially interchangeable with, polyacrylonitrile (PAN). As previously mentioned, PAN fiber is also the precursor material used to manufacture PAN-based carbon fiber. Acrylic fibers with a high tensile strength and elastic modulus were employed in the form of small-diameter short-cut fibers to replace carcinogenic asbestos [Acrylic is a polymer that contains at least 85% acrylonitrile by weight , with a asbestos . As listAromatic polyamide is a polymer in which at least 85% of the amide group is bound directly to two aromatic rings . Known i2)n monomers. Glass fibers are manufactured by extruding melted parent material into a filament form. During the extrusion process, the filaments are coated with a material called sizing, which equips the fibers with the desired surface texture and interfacial properties for the matrix within which they will be used. With regards to glass fibers used in concrete, individual sizing-coated glass filaments are typically gathered into the strands of around 200 filaments and cut to desired lengths. Depending on the production process and intended use, glass strands can be made to disperse back into their filament (microfiber) form when in contact with water (water dispersible) or they can be manufactured to stay in an integral strand (macrofiber) form. A new type of macro glass fiber has been recently developed by impregnating glass strands with an alkali-resistant polymer resin. This type of resin-impregnated fiber follows the same concept as glass fiber-reinforced polymer (GFRP) rebar, only on a smaller scale.In this study, glass fibers refer to the fibers that are derived from naturally occurring minerals or rocks, consisting of , depending on the composition of the parent material. The main difference between basalt and silica glass fibers is that basalt glass fibers tend to have significant levels of iron, potassium, magnesium, and sodium oxides, while silica glass fibers typically have low dosages of the mentioned oxides, but they can contain a significant dosage of boron oxides . AlthougThe first type of glass fibers used for concrete reinforcement was E  glass. The E glass was originally developed for use in electrical applications. The material was found to have satisfactory mechanical properties and was then tested for use as fiber reinforcement in polymer matrices and eventually cementitious matrices. Due to the degradation of glass fibers in concrete, however, alkali-resistant (AR) glass fibers were later developed. AR glass fibers have a relatively high tensile strength and elastic modulus compared to most polymer fibers. The most common application of AR glass concrete fibers is thin sheet components for exterior façade panels . Such paIn recent years, basalt glass fibers have received growing attention in the fiber concrete industry. Basalt glass fibers typically have a higher elastic modulus and tensile strength than silica glass fibers, as listed in The state-of-the-art review presented through this study aims to provide a holistic guide on the capabilities, limitations, and potential applications of different types of polymer and glass fibers used in concrete categorized by their effects on various fresh and hardened properties of FRC. Building on the past efforts ,36,37,38Concrete’s matrix is known to be corrosive to certain materials due to its very high pH value, originating from highly alkaline hydration products. Therefore, it is important to ensure the chemical stability of any fibers added to FRC before investigating their properties and suggesting possible applications. Stability of fibers refers to their ability to withstand the concrete environment during their expected service life without experiencing material degradation or dimensional alteration, hence, maintaining their efficiency. In particular, when fibers undergo deterioration, they can lose a portion of their cross-section, which adversely affects their load-carrying capacity, diminishing the advantages of FRC over plain concrete.As for bond characteristics, the bond between a fiber and its surrounding concrete is quantified as the force needed for the fiber to either be pulled out of the concrete matrix or experience rupture. When a bulk of discontinuous fibers are added to concrete, a combination of the aforementioned failure modes is expected under external loads, based on the fiber-matrix bond, which is influenced by the concrete’s properties and fiber’s characteristics. As stated in Banthia , fiber pThe hydrophobic nature of PP fibers often results in an overall weak fiber-matrix bond, leading to a mode of failure governed by the fiber pull-out under external loads. However, if the concrete matrix’s strength is increased sufficiently, or an appropriate mechanical anchorage is provided to the fibers (with geometric modifications), the mode of failure can change to the rupture of individual fibers, utilizing their full capacity. Cifuentes et al.  confirmeChemical pre-treatments can also be adopted to increase the fiber-matrix bond for PP fibers. López-Buendía et al.  used alkThe pull-out behavior of nylon fibers from the concrete matrix is known to be very similar to PP fibers . HoweverPVA fibers are characterized as hydrophilic, have a non-circular cross section, and form hydrogen bonds with the concrete matrix. These characteristics give PVA fibers the ability to form a strong bond in FRC applications , which iPO fibers are very compatible with the concrete matrix and do not degrade in the concrete environment. The PO fiber-matrix bond is mechanical in nature . DependiCarbon fibers are chemically inert, and as a result, do not undergo strength deterioration in the concrete environment ,62,63,64High-strength polyethylene (HSPE) fibers are chemically inert, providing high stability and degradation resistance in the concrete environment, in addition to high resistance against acids and seawater. Recycled PE fibers also adequately withstand the alkalinity of the concrete environment . The HSPDespite an overall promise, the main concern with the use of polyester fibers in cementitious composites is the uncertainty with regards to their stability in the highly alkaline environment of concrete. Most of the available studies have reported some level of degradation after a prolonged exposure of this type of fibers to extreme environments. Kim et al.  studied Contrary to the studies that confirmed the degradation of PET fibers in alkaline environments, Ochi et al.  concludeEarly forms of acrylic fibers exhibited low strength and elastic modulus, as well as poor resistance to acids and alkalis, which limited their applications in concrete . HoweverA limitation of aramid fibers for use as a concrete fiber is the lack of clarity in the literature about the level of strength degradation of this type of fibers in the concrete environment . Uomoto Despite superior mechanical properties, the main limitation of glass fibers in cementitious composites is their chemical sensitivity to alkaline environments. The alkali degradation of non-alkali resistant glass fibers are well documented in the literature ,82,83. WTo reduce the degradation of glass fibers in alkali environments, zirconium oxides are added to the glass fiber production process to produce alkali-resistant (AR) glass fibers. The degradation prevention provided by the presence of zirconium oxides in glass fibers is because the Zr–O bonds are stable under alkali attacks, in contrast to the Si–O bonds, which break in the presence of hydroxides. This leads to a zirconium dioxide protective layer on the exposed fiber surface, serving as a barrier to prevent possible fiber breakdowns . Adding Based on the literature ,87, AR gIt is generally accepted that AR silica glass fibers mixed in cementitious matrices lose some of their reinforcing effectiveness over time because of their chemical sensitivity to the alkaline environment, as explained in the previous section ,90. In oIn addition to adding zirconium to the chemical structure of glass, applying alkali-resistant sizing to the filament surface during production, or changing the chemistry of the concrete matrix, glass fiber strands can be impregnated with alkali-resistant and surface-bonding resins, such as epoxy and vinyl ester, to improve their long-term durability. These types of polymer-impregnated glass fibers are made into the macro concrete fibers that are relatively new to the concrete industry and are essentially miniature versions of GFRP rebars. The alkali degradation of GFRP macrofibers has not been fully described in the literature, however, due to the similarities that these fibers share with GFRP rebars, the research related to the durability of GFRP rebars can be cautiously extrapolated to evaluate the long-term durability of GFRP macrofibers.The investigations that utilized accelerated aging techniques report concerns about the durability of glass-based fibers in concrete. Significant amounts of degradation and strength loss have been found, especially under high temperature and aggressive chemical environments ,93,94. TAlthough several studies have focused on the bond properties of silica GFRP bars in concrete, limited studies are available concerning the bond properties of silica glass fibers in concrete, which can be highly different from the bond properties of silica GFRP bars, due to differences in their size and shape. In a study completed by Scheffler et al. , the pulWith regards to chemical durability, basalt glass fibers show an alkali degradation similar to that of E glass fibers ,100. In Mingchao et al.  tested tAccording to Jiang et al. , the SEMConcrete is the most commonly used material in the construction industry. One of the principal reasons for such a widespread application is the concrete’s workability in the fresh state, making it possible to form several shapes in various sizes without needing any special treatments . TherefoMohod  reportedNylon fibers are hydrophilic and can absorb a small amount of water during the mixing process . SeveralWhen PVA fibers are used in concrete, the workability of concrete drops due to the PVA fiber’s water absorption. Hossain et al.  evaluateLimitations in the fresh state as a result of adding PO fibers are similar to those previously discussed for PP fibers. PO fibers with surface indentations may further decrease the workability compared to smooth PO fibers, mainly because of the increased surface area per fiber. Several studies have shown that PO fibers can be used in SCC mixtures ,111. No Macro carbon fibers are uncommon since carbon fibers tend to break into shorter lengths during the mixing process because of their brittle nature . In partZhang and Li  reportedAlthough most of the past studies incorporated 1.0% PET fiber by volume (or lower) into FRC, PET macrofibers were reported to mix well in concrete at 1.5% or even up to 3.0% ,119. It Addition of PAN fibers from a low volume up to 2.5% was reported to decrease the workability of FRC to the extent that the water-to-cement ratio has to be increased substantially. Superplasticizers can also be needed to accommodate the fiber addition, especially with using low-diameter fibers ,120,121.Nanni  investigAR glass fibers can be used in the FRC made with conventional mixing procedures. However, it has been reported that high fiber volumes are difficult to achieve when using glass fiber filaments in concrete with conventional mixing procedures, because such fibers tend to disperse into the matrix unevenly, therefore, an increase in water-to-cement ratio or additional mixing becomes required . The addAyub et al.  studied Since basalt glass fibers have a density relatively similar to that of the concrete matrix, BFRP macrofibers have been reported to mix well (at volumes up to 4.0%) in concrete using conventional mixing procedures, compared to most other fibers . In a seThe engineering properties of the FRC used for structural design are often derived from pre-peak mechanical properties, where strength and modulus of elasticity are determined. It is generally accepted that the tensile and flexural strengths of a plain concrete is approximately 10% and 15% of the concrete’s compressive strength, respectively . To impr3 is reached. The inconsistences among the available studies regarding the ability of PP fibers to increase pre-peak strength properties can be attributed to the variations in the fiber’s dosage, geometry, and mechanical properties, as well as the characteristics of the concrete matrix. Studies have found that recycled PP fibers can deliver similar mechanical properties, while averting fiber degradation in concrete [There are some inconsistencies in the literature as to whether or not PP fibers notably affect the strength parameters of FRC prior to crack propagation. Some studies have reported that the compressive strength of FRC increases or is not affected by the addition of macro PP fibers, but the tensile strength is significantly improved at volumes below 0.55% ,129. On concrete ,132.Song et al.  observedThe studies on the use of PVA fibers have reported different results regarding their effects on the mechanical properties of FRC. In particular, it has been found that even low volumes of micro PVA fibers can reduce the compressive strength of FRC significantly . On the 3 PO macrofibers. This led to a satisfactory outcome, as only small tensile stresses were anticipated in the concrete. By eliminating conventional bars, the construction cost and time of construction were both significantly reduced.Similar to other low-strength synthetic fibers, addition of low volumes of PO fibers to concrete does not have a significant effect on the mechanical properties of FRC. Alberti et al.  reportedCarbon fibers can improve the mechanical properties of cementitious composites if a sufficient volume is included. The extent of improvement is proportional to the strength and modulus of elasticity of the carbon fibers used. Stronger and stiffer carbon fibers more effectively increase the strength properties, while the weaker ones more likely contribute to enhancing the toughness. Among the limited studies available, Yao et al.  tested tDopko et al.  tested vHSPE fibers have shown adequate reinforcing effects in concrete. In the limited studies available, mixtures with HSPE fibers (as low as 0.025%) have exhibited higher flexural strengths compared to those made with 0.1% fibrillated PP fibers . In a seResearch has shown that polyester fibers are capable of improving the mechanical properties of concrete. The bulk of research that has been conducted on PET fibers involves monofilament macrofibers made from recycled plastics; however, limited studies have also investigated non-recycled PET fibers. Swamy and Barr  tested 2Fraternali et al.  studied PAN fibers with different tensile strengths, lengths, and volumes are reported to have a negligible or negative effect on the compressive strength of FRC ,120. Mo Zhang et al.  investigSeveral studies have reported that the addition of glass fibers does not have a significant effect on the compressive strength of FRC, and in some cases, they were found to only marginally increase the compressive strength ,152,153.3) does not change the flexural strength of FRC. However, by including more silica glass fibers, splitting tensile and flexural strengths of FRC improves [In another study, Arslan  observedimproves ,154,155.improves ,151. Owiimproves  utilized3) decrease the compressive strength as the fiber volume increases for both normal and high-strength concrete. These contradictory results originate from different fiber characteristics and different concrete mixtures. While lower fiber contents can improve the packing of concrete, higher fiber contents, along with lower water-to-cement ratios, can signify the negative effects of fiber addition on the compressive strength of FRC. Studies have shown that basalt glass fibers enhance the splitting tensile and flexural strengths of FRC, regardless of the fiber content and fiber length [Different results have been reported on the pre-peak mechanical properties of basalt FRC. Yang and Lian  found thr length ,157,158.r length  found thr length  investigr length ,154. Brar length  investigr length  reportedThe expected performance and service life of reinforced concrete structures can be significantly affected with the occurrence of cracks, as investigated in several studies ,167,168 Several studies have reported significant increase in the post-crack residual strength and toughness of FRC, as a result of macro PP fiber addition ,146,180.Nylon fibers are utilized to enhance the post-peak characteristics of FRC. When dispersed in low volumes, nylon microfibers have minimal effects on the pre-crack mechanical properties. However, a higher toughness and a more ductile failure mode can be achieved with the addition of nylon fibers ,183,184.PVA fibers have tensile strengths in the same range as steel fibers, however, the elastic modulus of PVA fibers is less than 25% of steel, as reflected in 3 PO macrofiber contents with the FRC made with 26 kg/m3 of steel fibers. The study stated that, regardless of fiber volume, toughness and ductility increase with the addition of PO fibers, providing improved residual strengths. From the fracture energy results, it was noted that the addition of PO fibers can increase the fracture energy of concrete up to 75% of steel FRC after a 1 mm deflection, i.e., 1/300 of the span length. However, when the deflection increases to 5 mm, i.e., 1/60 of the span length, PO fibers outperform steel fibers by 40%, proving the higher efficiency of PO fibers in high deflections. It should be noted that an admixture for improving the fiber-matrix bond had been used in the cited study for high-volume fiber mixtures.Macro PO fibers are typically used to increase the post-crack residual strength of the concrete. They can improve post-crack ductility and limit crack growth, but due to their low modulus, they are often not as effective for low deflections or small crack widths as other fibers with higher elastic moduli . RamakriFRC made with 0.5% carbon, PP, and steel fibers were tested by Yao et al.  for theiLow-strength PE fibers are effective in increasing the post-crack flexural ductility, especially at large deflections . YamagucThere are only a few studies on the post-peak properties of non-recycled polyester fibers, while there are several studies conducted on recycled PET fibers. Swamy and Barr  used 20 Ductility and post-crack residual strength are known to increase with the incorporation of PAN fibers into the concrete matrix ,77. AmonNanni  found th3 to 3.0 kg/m3 of silica glass fibers to measure the fracture energy of a set of beam samples. It was reported that adding 1.0 kg/m3 silica glass fibers has the maximum efficiency by increasing the fracture energy up to 35%, compared to the plain concrete. It was also mentioned that the specimens with silica glass fibers achieved a higher ductility and energy dissipation capacity, compared to the basalt FRC specimens.Regardless of fiber length and volume fraction, silica glass fibers are found to increase the concrete’s ductility and toughness ,154,155.Basalt glass fibers are known to be effective in enhancing the concrete’s post-peak mechanical properties. Jalasutram et al.  reportedIn a study performed on BFRP macrofibers , it was 2, into concrete, endangering the durability of reinforced concrete structures [Water plays a critical role to initiate and help with the hydration reactions required to achieve the desired fresh and hardened properties of concrete. However, the consumption or the loss of water can result in shrinkage, incurring tensile stresses that can exceed the relatively low tensile strength of the concrete matrix . The forructures ,196,197.ructures ,199,200 PP fibers can highly reduce the drying shrinkage cracking of concrete by increasing the capacity of FRC to resist shrinkage-induced strains . The avaNylon fibers have found to be effective when it comes to restricting the propagation of drying and plastic shrinkage cracks in concrete. Nam et al.  substituBoth macro and micro PVA fibers are reported to be effective in controlling drying shrinkage cracks in concrete. It has been found that PVA fibers added to the concrete at relatively low volumes (below 0.5%) decrease the shrinkage-induced crack widths by 90% for microfibers and 70% for macrofibers. The PVA fibers did not affect the restrained drying shrinkage stress development rate and time of first crack generation. However, they controlled the crack widths once cracks initiated. In a separate investigation , pre-craPO fibers of different lengths and aspect ratios are reported to be effective in controlling plastic shrinkage and thermal cracking in concrete overlays. Shorter fibers proved to be most effective for such applications at the same volume dosage . YousefiLimited studies have measured the effect of carbon fibers on the shrinkage of concrete. Carbon fibers have shown to be effective in reducing the restrained shrinkage and drying shrinkage cracking potential of carbon FRC . Dopko ePešić et al.  investigRecycled PET fibers are reported to be effective in controlling shrinkage-induced cracks. According to Borg et al. , recycleThe addition of PAN fibers minimizes the drying shrinkage cracking potential, regardless of the volume fraction of the fiber, noting that including higher volumes leads to a better performance ,120. FanZhao et al.  investig3) has a significant contribution to reducing the cracked area and the length of cracks in both regular concrete and SCC. It was also concluded that, although increasing the fiber content increases the concrete’s ability to withstand drying shrinkage, the efficiency of fibers begins to diminish beyond a certain dosage [Small additions of glass fibers have been found effective in controlling shrinkage-induced cracks. Barluenga and Hernández-Olivares  studied n dosage . Soranakn dosage  indicaten dosage  tested dn dosage  investigBranston et al.  tested tIn a hardened concrete, water can be found in two main phases: free water and bound water, while the latter is further categorized to physically bound and chemically bound water. When concrete is exposed to extreme temperatures, the water inside the concrete evaporates and if the entrapped vapor does not find a way out, it generates an internal tensile stress, which can eventually make the concrete implode and spall. One of the methods used to increase the concrete’s capacity to withstand extreme temperatures for an extended period of time is the incorporation of fibers. As discussed in PP fibers have a relatively low melting point (from 160 to 170 °C ) in compNylon fibers have a melting point between 215 and 265 °C ,182.The melting point of PVA fiber is reported to be between 220 and 230 °C ,217,218.PO fibers have a (relatively) low melting point, which is reported to be 150 °C . Hence, There are limited studies concerning the fire resistance of carbon FRC. Chen and Liu  comparedPE fibers have a low melting temperature, i.e., 130 °C for regular HDPE fibers and 150 °C for gel spinned HDPE fibers ,221. ThePET fibers have a low melting point of 160 °C, as reported by Sadrmomtazi and Tahmouresi . TherefoMo et al.  investigAramid fibers are reported to start their tensile strength loss at 200 °C and they completely lose their strength when the temperature reaches 400–500 °C, which is a relatively high tolerance compared to other fibers . TherefoMirza and Soroushian  comparedBasalt glass fibers have an outstanding thermal stability, which is reflected in a study conducted by Sim and Park . In the The addition of fibers can greatly alter many fresh and hardened properties of concrete, depending on the fiber’s chemical and physical characteristics. Based on several investigations reviewed in the current study, this section provides a synthesis of the most common trends and observations reported on the performance of fibers in each of the categories of stability and bond, workability, pre-peak and post-peak mechanical properties, shrinkage, and extreme temperature resistance. Additionally, to help the researchers and engineers in the concrete industry with the selection of fibers, the performance of each of the fiber types in each of the performance categories has been qualitatively ranked as weak (W), fair (F), good (G), or excellent (E) in Portland cement concrete provides a highly alkaline environment, with a pH value as high as 13.5, which protects steel rebars from corrosion. Such an environment is proven to cause deterioration in some fiber types. Therefore, ensuring their long-term stability is of paramount importance. Among the fibers investigated, both silica and basalt glass fibers have shown to be significantly degraded in the concrete matrix, reflecting the need to employ alternative methods to enhance the alkalinity tolerance of the glass-based fibers. Such methods range from the coating of the individual fibers with alkali-resistant materials to covering the fibers with polymer resins. In addition, aramid fibers, especially the acid spun type, are reported to undergo a notable degradation in concrete. Furthermore, PAN and PET fibers are susceptible to some levels of degradation in alkaline environments. The other reviewed fibers, however, have shown great chemical stability in concrete.When FRC is subjected to external loads, fibers tend to fail by pull-out and/or rupture. The occurrence of these two modes of failure is a function of fiber’s elastic modulus and fiber-matrix bond. As the modulus of elasticity decreases and the fiber-matrix bond increases, the majority of fibers tend to fail due to rupture. The bond between the individual fibers and the concrete matrix is often governed by the concrete’s properties, such as water-to-cement ratio, and fiber characteristics, such as material, length, and shape. One of the main reasons for the addition of fibers to a concrete mixture is their ability to enhance the concrete’s post-peak mechanical properties, in terms of ductility and residual strength. Thus, a high fiber-matrix bond that leads to a sudden fiber rupture is not favorable. On the other hand, a weak fiber-matrix bond introduces other issues, particularly with limiting the capability of fibers to bridge the cracks. Therefore, an optimum bond is desired for fibers to have the best efficiency. PVA fibers form a hydrogenic bond with the concrete matrix, while the other fibers primarily have a mechanical bond. Thus, PVA fibers tend to fail due to rupture and a high fiber-matrix bond is expected from them. Nylon, PAN, and PO fibers are reported to make a relatively strong bond with the concrete, due to their swelling and increase of friction, irregular cross-section, and damage during the mixing process, respectively. In contrary, PE fibers form a relatively weak bond with the concrete matrix, presenting a challenge to its use in FRC. Several methods have been attempted, with various degrees of success, to increase the fiber-matrix bond. Among the examples are using the fibrillated form, changing the shape of individual fibers, and adding coating materials.Regardless of the dosage and characteristics of fibers, the addition of fibers decreases the workability of concrete. This is further exacerbated by increasing the fiber content. Longer fibers can cause a higher friction with fresh concrete, further reducing the workability of FRC mixtures. However, it should be noted that, in a fixed volume fraction, shorter fibers cause a higher decrease in workability, due to their higher surface areas. Studies have shown that using proper admixtures, such as water reducers and pozzolans, in addition to employing appropriate mixing equipment, increase the maximum amount of fibers that can be included in the FRC without causing workability issues. PVA fibers can significantly decrease the workability of FRC due to their water absorption characteristics. Nylon fibers are also reported to have some water absorption. While this helps them further disperse in the concrete at low volume fractions, a significant reduction in the workability is observed at high volume fractions. Compared to other fiber types, basalt glass fibers are reported to mix well with concrete, mainly due to having a density similar to the concrete’s density.The studies available in the literature have reported sometimes contrary observations regarding the effects of fiber addition on the compressive strength of FRC. This can be attributed to different properties of concrete and fibers used in the experiments. When it comes to compressive strength, dense packing plays a key role to ensure that the entrapped air is minimum. Therefore, regardless of fiber type, using short fibers with a low dosage can lead to an increase in the compressive strength. This can be achieved well, especially in the concrete mixtures made with a high water-to-cement ratio. On the other hand, fibers have been consistently reported as a proper addition to improve the concrete’s splitting tensile and flexural strengths, where macrofibers have led to more pronounced improvements in comparison to microfibers. Increasing the fiber content can result in improved splitting tensile and flexural strengths, as long as fibers are well dispersed. In general, the higher the fiber’s modulus of elasticity, the better it can enhance the splitting tensile and flexural strengths of the concrete. Comparative studies have shown that glass fibers can improve the flexural strength of concrete more than PO fibers, while they are both more efficient than high-tenacity PP and nylon fibers. High-modulus carbon and aramid fibers also greatly help with augmenting the splitting tensile and flexural strengths of the concrete, as long as the required fiber-matrix bond is achieved.One of the main limitations of the plain concrete is its brittle behavior after reaching the ultimate strength, which is captured as a peak and then a sharp drop in the stress-strain curve. In order to overcome this drawback, fibers are incorporated into the mixture to enhance the concrete’s post-peak mechanical properties, which span ductility, toughness, and residual strength. Even low volumes of fibers have proven effective in improving all of the post-peak mechanical properties of the concrete, except for some reported cases regarding the compressive energy absorption. The reason for the overall positive contribution of fibers is their ability to bridge over cracks, facilitating the transfer of loads from one end to the other end of the crack. Therefore, higher fiber contents provide more bridging pathways, which help with conveying more stresses, thus, further increasing the post-peak mechanical properties of FRC. The past studies have shown that microfibers are more effective in controlling microcracks, whereas macrofibers deliver a better performance in limiting macrocracks. Therefore, macrofibers can be more efficient in enhancing the post-peak properties of the concrete since macrocracks are generated after the peak of the stress-strain curve is reached. High-modulus fibers, such as carbon and aramid, followed by glass fibers, have shown a great potential to improve the post-peak mechanical properties of FRC. On the other hand, the contribution of nylon fibers to the post-peak mechanical properties is expected to be the least compared to PP, PVA, and HSPE fibers.Since concrete has a low tensile strength, shrinkage-induced cracks caused due to water consumption and/or loss can be a great concern for long-term durability. This is because when the concrete loses water as a result of excessive evaporation, internal tensile stresses are generated with the possibility of exceeding the concrete’s tensile strength, leading to the formation and propagation of cracks. To address the issue of shrinkage-induced cracks, adding fibers to the concrete has been investigated in several studies. Regardless of their material characteristics, fibers can reduce the crack width and number of cracks, while delaying the time of the first crack. Additionally, it has been found that the higher the fiber content, the lower the shrinkage-induced cracks to the extent that such cracks can be entirely eliminated. The fiber aspect ratio is one of the important factors, affecting the performance of FRC subjected to shrinkage. In general, fibers with a high aspect ratio have a better performance than those with a low aspect ratio. This can be confirmed with the great potential of fibrillated PP fibers to limit shrinkage-induced cracks.When concrete is exposed to extreme temperatures or fire, the water inside the concrete matrix evaporates, generating an inside vapor pressure in the form of tensile stresses that can lead to the explosion and spalling of concrete. However, the addition of fibers is proven to be an efficient way to reinforce the concrete against extreme temperatures. Fibers enable FRC to withstand high temperatures with two methods: firstly, fibers with a high melting temperature  can increase the tensile strength of concrete. Therefore, they can delay the explosion time and help concrete retain a high residual strength after the fire. Secondly, fibers with a low melting point  melt during the fire, forming internal channels that serve as evacuation pathways for water vapors. If a proper combination of fiber dosage and length is employed, the concrete’s explosion and spalling can be minimized or even eliminated. The available studies suggest that the FRC products with the fibers that fall into the second category have a better performance overall, when exposed to elongated extreme temperatures.PP fibers are one of the most cost-effective concrete fibers. This advantage, paired with an excellent chemical stability in the concrete environment, satisfactory mechanical properties, and wide availability, has made the PP fibers one of the popular concrete fibers. The fibrillated PP microfibers are primarily used for plastic shrinkage crack control, while the monofilament PP macrofibers are employed for controlling the cracks caused by external loads, temperature gradients, or drying shrinkage.Nylon fibers can absorb the mixing water, and in turn, reduce the workability more than other concrete fibers. These aspects limit the application of nylon fibers to relatively low fiber volumes, especially if microfibers are used. Another limitation of nylon fibers is that, while they provide advantages similar to PP fibers in concrete, they are, in general, more expensive. Increasing attention to recycled nylon fibers can help decrease the unit cost of nylon fibers with the possible use for thermal and plastic shrinkage crack control purposes.PVA fibers form a strong chemical bond with the concrete matrix, increasing the possibility of fiber rupture under external loads, which is not a favorable feature where an increase in post-peak mechanical properties is needed. This feature, along with the relatively high cost of PVA fibers and their significant water absorption, which decreases the mixture’s workability more than other fibers, can limit the application of PVA fibers in FRC products. Thus, they are not as readily available as other less expensive synthetic fibers.PO fibers have a relatively low elastic modulus, similar to PP fibers, causing a relatively low residual strength at small crack widths. Most concrete fiber suppliers provide some forms of PO fibers, as they work well for crack controlling purposes. PO fibers are relatively inexpensive and fall in the same price range as PP fibers, making them one of the least expensive concrete fibers. However, they are not well represented in the literature, most likely due to the absence of a widespread need to them in practice because of the popularity and abundance of PP microfibers.Carbon fibers can withstand the alkaline environment of concrete better than glass fibers. They also have a (relatively) high strength-to-weight ratio. However, due to the issues observed during the mixing process of carbon macrofibers with conventional methods, they are not commonly used, especially in the mixtures that contain coarse aggregates. Further to the high price of most carbon fibers, they are often less effective than other synthetic fibers for several concrete applications. Thus, carbon fibers are considered an expensive specialty fiber in the concrete industry.As a result of low strength and stiffness, the FRC products made with conventional PE fibers can suffer from poor mechanical properties. However, high-strength and high-stiffness PE fibers have shown satisfactory mechanical properties with a potential to be used in cementitious composites. However, HSPE fibers are not very practical in conventional concrete applications, mainly because of their (relatively) weak bond with concrete.The use of PET fibers in FRC has been limited to laboratory tests and research investigations at the time of this review. It is expected that, as the production technology and product quality of recycled PET fibers improve in the future, these fibers gain traction in the concrete industry, owing to their economic and environmental benefits over traditional synthetic fibers.In the category of acrylic fibers, PAN microfibers have been found to offer effective solutions, as they can provide benefits similar to other low-strength/modulus fibers. However, compared to other common synthetic fibers, the literature suggests that acrylic fibers more adversely affect the workability of the concrete mixture, while they can provide an increased fiber-matrix bond strength, along with a significant residual strength at small crack openings. The limited general use of PAN fibers in the concrete mixtures that contain coarse aggregates is likely because other less expensive synthetic fibers can provide similar benefits, especially in the absence of acrylic macrofiber production.The main drawback of aramid fibers for FRC applications is their cost. Since aramid fibers are relatively expensive and may not provide enough additional benefits over other common concrete fibers, their use has been limited, which can justify the limited number of relevant studies available in the literature. However, recent works on Kevlar and Technora fibers have shown promising reinforcing potential, which can be further utilized, especially if the cost drops.AR silica glass fibers are able to significantly improve the strength and ductility of concrete, owing to their relatively high strength and stiffness. There are inconsistencies in the workability reported for the FRC mixtures made with AR silica glass fibers, but this can be attributed to the fact that the fibers can come in a wide range of sizes and surface areas. Degradation of silica glass fibers in concrete is often a concern, which can be addressed with an adequate zirconia content, proper sizing application, concrete binder adjustment, or even polymer impregnation. The AR silica glass fibers are effectively used as concrete fibers in various applications with a wide availability and relatively low price.Basalt glass fibers have shown degradation issues in the concrete’s alkaline environment. Similar to silica glass fibers, various actions have been taken to increase their long-term stability in concrete. Basalt microfibers are known to be effective for increasing the splitting tensile and flexural strengths of FRC, further to reducing the plastic shrinkage cracks. BFRP macrofibers have shown a great potential as an effective solution to improve the post-crack performance of FRC and control the propagation of cracks. The basalt glass fibers are anticipated to gain popularity in the concrete industry, as the production increases and new applications are identified.In this review study, a variety of fibers have been investigated for their contribution to different fresh and hardened properties of concrete. Additionally, they have been compared with each other in order to signify their potential in altering each of the properties of interest. From this holistic investigation, the capabilities and limitations of each fiber type can be summarized as:FRC products are becoming more popular in the concrete industry and are anticipated to continue to grow as researchers, engineers, and contractors become more familiar with designing, mixing, and placing FRC, while the new codes and standards for structural concrete accept the strength and service life benefits that can be gained from adding randomly dispersed discrete fibers to the concrete. Holistic investigations, similar to the investigation presented in the current review study, are expected to pave the way to make informed decisions regarding the fiber types of choice, depending on not only their cost and availability, but also the fresh and hardened properties of FRC desired for a wide variety of concrete applications.