Journal of Engineering Science and Technology Review 10 (5) (2017) Research Article

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1 Jestr Journal of Engineering Siene and Tehnology Review 1 (5) (17) Researh Artile JOURNAL OF Engineering Siene and Tehnology Review Mehanial Properties and Durability of Fiber-reinfored onrete Jingjing Zhang 1,*, Jianwei heng, Yuanming Dou 1 and Qin Xin 1,3 1 Shool of ivil and Transportation Engineering, Hebei University of Tehnology, Tianjin 341, hina hina Fortune Land Development, Langfang 6511, hina 3 Department of Eletrial Engineering, Wayne State University, Detroit, MI 48, United States Reeived 9 Marh 17; Aepted 1 Otober 17 Abstrat Traditional methods of pasting arbon and aramid fiber sheets or boards on onrete are usually adopted for onrete reinforement due to the high brittleness and poor toughness of onrete. However, many fators affet the use of these methods, and the harateristis of fiber materials are not maximized. This study proposed a test model in whih hopped arbon, aramid, and hybrid arbon-aramid fibers (1:1) were diretly added to onrete to fully utilize the high strength and toughness of fibers and improve the properties of onrete. The influene of different types and mixing ratios of fibers on the mehanial properties and durability of onrete was analyzed through axial ompressive strength, stati elasti modulus, and arbonation tests. A alulation model based on the axial ompressive strength, arbon dioxide onentration, and arbonization time of onrete was developed with existing arbonation depth alulation models and verified using experimental data. Results demonstrate that the ompressive strength, elasti modulus, and antiarbonization apaity of the fiber-reinfored onrete initially inrease and then derease with an inrease in fiber ontent and are better than those of ordinary onrete. The ompressive strength and elasti modulus of the hybrid fiberreinfored onrete with a mixing ratio of 1% inrease the most among all of the studied onrete samples, and the antiarbonation apaity of the arbon fiber-reinfored onrete with a mixing ratio of 1% is the best. The proposed method provides a referene for pratial property and durability evaluation of fiber-reinfored onrete in pratial engineering. Keywords: arbon Fiber, Aramid Fiber, ompressive Strength, Elasti Modulus, arbonization 1. Introdution The large weight, brittleness, and poor durability of onrete have been well investigated due to the extensive appliation of onrete as a building material in the building industry. Researhers have applied fiber boards or sheets for struture reinforement and repair to improve the inherent inferior properties of onrete by maximizing the light weight and high frature toughness of fibers. Performane has been verified to be exellent. However, traditional fiber sheets or boards exert a good reinforement effet in the parallel diretion of the fiber but exhibit poor performane in the vertial diretion. The stress state of omponents subjeted to stresses in multiple diretions simultaneously is often unpreditable. ontinuous fiber exerts a limited reinforement effet in this ase, and its utilization ratio is redued. Aurately determining the reinforement degree is also diffiult. The stiffness of omponents an inrease signifiantly after reinforement, and suh an inrease ould result in the stripping of the reinforement material and ause sudden rupture of the omponents. The reinforement effet also depends on the performane of the adhesive material. onstrution requires manpower, material resoures, and money, whih are neither eonomi nor environment friendly. Thus, the bearing apaity and durability of onrete should be improved by hanging the * address: @163.om ISSN: Eastern Maedonia and Thrae Institute of Tehnology. All rights reserved. doi:1.513/jestr.15.8 properties of onrete. Domesti and foreign sholars have presented a method of diretly mixing hopped fibers with onrete to maximize the advantages of fiber materials, redue the weight of the struture, and improve the mehanial properties of onrete. Fibers are distributed randomly in large numbers within a sample to share stresses originating from different diretions and thus improve omponent performane, avoid brittle frature, and inrease the utilization ratio of fibers[1]. arbon fiber in fiber-reinfored onrete has been extensively studied beause of its high strength, high modulus, small speifi gravity, and high toughness[-3]. Aramid fiber, a new type of fiber for onrete reinforement, has eliited muh attention from domesti and foreign sholars due to its high strength, high modulus, light weight, and high resistane to orrosion and fatigue[4-5]. Reent studies on aramid fiber-reinfored onrete have foused on pasting aramid fiber sheets on the surfaes of building omponents to improve the bearing apaity of strutures[6-7]. However, only a few studies have examined the diret appliation of aramid fiber in onrete in the form of hopped fiber. Therefore, hopped arbon, aramid, and hybrid arbon-aramid fibers were diretly added to onrete in this study to analyze the basi mehanial properties and durability of the added materials.. State of the art Researhers have onduted numerous studies on fiber sheet or board reinforement. Dong et al.[8] tested the bending

2 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) performane of five reinfored onrete beams with retangular setions and found that externally pasting arbon fiber sheets effetively inrease the limit-bearing apaity and deformation resistane of beams, delay the development of onrete raks, and signifiantly improve the overall rigidness of reinfored beams. However, when the researhers reinfored pre-raked beams through the same method, the bending apaity and dutility of the beams were redued. heng et al.[9] also pasted arbon fiber sheets on ontinuous onrete beams for reinforement. The method did not inrease the bearing apaity of omponents in the elasti state but signifiantly inreased it in the plasti state. The reinfored ontinuous beams showed that internal fore was redistributed. Shang et al.[1] found that the interfaial bond between a arbon fiber board and onrete is a key reinforement property and exerts an important effet on the mehanial behavior and failure modes of reinfored strutures. Wen [11] measured the damage resistane of a arbon fiber omposite material and alulated the degree of damage on the basis of the hanges in resistane. Deng et al.[1] performed a bending test on 11 reinfored onrete beams and found that when stress transfer is unstable, the hybrid fiber loth is partially removed from the onrete, and a poor reinforement effet is obtained. Aording to these studies, reinforement is not guaranteed when fiber sheets or boards are used to reinfore omponents. Instead, the bearing apaity of omponents inreases and their dutility dereases; this senario an even ause the stripping damage of reinforement materials. As a result, fiber performane is not maximized. The method of mixing fiber with onrete has been proposed to avoid dutility redution and maximize the advantages of fiber materials. Song and Yin[13] onduted ompression and impat resistane tests on hybrid fiberreinfored onrete. They found that although the ompressive strengths of steel and arbon fiber-reinfored onrete (FR) are improved, the improvement in the strength of the hybrid fiber-reinfored onrete is more signifiant. Impat resistane improves with an inrease in fiber volume. The researhers also studied the positive effet of steel and arbon fiber-reinfored onrete on the basis of the experimental results. He et al.[14] investigated the dynami harateristis of hybrid arbon-aramid fiberreinfored onrete (/AFR) and disovered that flexible fiber-reinfored onrete an bear serious deformation; this method of onrete reinforement inreases the energy dissipation of materials, improves dynami performane, and exerts an evident damping effet. Zhang et al.[15] found that inorporating aramid fiber into ement mortar improves the flexural strength and shok resistane of the material. Performane is maximized when the volume fration of doped fiber is 1.%. Qiao et al.[16] disovered that aramid fiber-reinfored onrete (AFR) possesses better rak resistane than polypropylene fiber-reinfored onrete. Moreover, aramid demonstrates good strength and toughening performane at low water/ement ratios and in standard uring onditions. These studies foused on the effet of aramid fiber on enhaning the dynami performane, impat resistane, and rak resistane of onrete, and limited researh has been onduted on the influene of aramid fiber on enhaning the basi mehanial properties of onrete and the effet of hybrid arbon aramid fibers on arbonation performane. To address the defiienies of existing studies, we mixed arbon, aramid, and hybrid arbon aramid fibers with onrete and performed axial ompressive, stati elasti modulus, and arbonation tests to analyze the mehanial properties and durability of fiber-reinfored onrete. This work provides a referene for the appliation of fiberreinfored onrete in engineering. The remainder of this study is organized as follows. Setion 3 introdues the test sheme and test system. Setion 4 provides an analysis of the basi mehanial properties of fiber-reinfored onrete on the basis of experimental data and presents a arbonation depth alulation model established aording to influening fators. Setion 5 summarizes the onlusions. 3. Methodology 3.1 Test Introdution Sixty prism samples with dimensions of 15 mm 15 mm 3 mm and a onrete grade of 4 were designed for axial ompressive and stati elasti modulus tests. Thirty prism samples with dimensions of 1 mm 1 mm 3 mm and with the same onrete grade were designed for the arbonation test. Two fators, namely, fiber type and fiber mixing rate (ontaining three levels eah), were used. The samples were divided into 1 groups aording to the priniple of orthogonal experiments, as shown in Table 1. Table 1. Orthogonal test sheme Test group Fiber type Fiber doping rate Number (ontrol group).5% 3 3 1% % 3 5 A.5% 3 6 A 1% 3 7 A 1.5% 3 8 /A.5% 3 9 /A 1% 3 1 /A 1.5% 3 Note: stands for arbon fiber, A stands for aramid fiber, and /A represents hybrid fiber. 3. Sample Preparation Ordinary Portland ement with a strength grade of 4.5 was used. Ordinary sand was utilized as the fine aggregate, and gravel with a maximum diameter of 3 mm was adopted as the oarse aggregate. Tap water was also used. The mixing proportion of onrete was as follows: water: sand: gravel: water-reduing agent = 1:.39:1.9:.88:1%. The onrete was artifiially mixed. The post-fiber mixing method was applied in preparing onrete to disperse the fiber well and maximize its toughness. The samples were removed from the mold after being poured for 4 h and maintained for 8 d in a standard uring room with 9% humidity and temperature to allow the samples strength to reah the maximum value. The speifi parameters of the fiber in this test are shown in Table. 3.3 Test Equipment and Method The axial ompressive test was onduted on a 5 kn pressure testing mahine, whih is shown in Fig. 1. The loading proess was ontrolled by stress. The samples were plaed on the platform of the testing mahine, and load was applied on the samples along the axial diretion at a rate of.5 MPa/s until the samples failed. The speifi axial ompressive strength was then obtained. The test data were automatially olleted by the test system. 69

3 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) Table. Fiber parameters Fiber type arbon Aramid ( kg / m ) Tensile strength (MPa) Tensile modulus (GPa) Elongation (%) Fiber diameter (µm) Length (mm) Density 3 E = Fa F L A Δn (1) where E is the elasti modulus of onrete (MPa); Fa is the load when the stress is one-third of the axial ompressive strength (N); F is the initial load when the stress is.5 MPa; A is the pressure area of the speimen ( mm ); L is the standard measuring distane (mm); and Δn is the average value of the deformations on both sides of the sample when F is loaded to Fa for the last time (mm). Prior to the arbonation test, the samples that had been ured for 8 d were plaed in a drying box for ontinuous drying at 6 for 48 h. The surfaes of the dried samples were evenly oated with heated wax for sealing, exept for the two opposite sides. Parallel lines were drawn with a penil on the reserved sides at an interval of 1 mm along the length diretion to predetermine the measuring points of arbonation depth. Then, the samples were plaed in an NJW-TH-9 omputer automati onrete arbonation test hamber manufatured by Beijing Naiheng Tehnology Development o., Ltd., for arbonation at a humidity of 7 ± 5%, temperature of ± 5, and O onentration of ± 3%, as shown in Fig. 3. The samples were subsequently removed from the arbonation text hamber on the 3rd, 7th, 14th, and 8th day and split by the pressure testing mahine. The powder on the surfae was leaned, and 1% phenolphthalein alohol solution was dropped on the surfae. After 3 s, the arbonation depths of the setions were measured at predetermined measuring points. The mean value of the data was used as the arbonation depth of the onrete sample. Fig. 1. Pressure testing mahine at 5 kn The stati elasti modulus test was onduted on a kn pressure testing mahine built by Shanghai Siene Zongheng Mahine Manufaturing o., Ltd. The amount of deformation was measured by a strain gauge with a pasting range of 15, on both sides of the prism and at a standard distane of 15 mm. Data were olleted with a WKD3811A multifuntional stati strain gauge produed by Tianjin Weekend (Fig. ). Fig.. Pressure testing mahine at kn and the data olletion system Load was applied to the initial load value at a benhmark stress of.5 MPa, whih was 11,5 N in the test. The onstant load was maintained for 6 s, and the deformation reading of eah measuring point within 3 s was reorded. The load was ontinuously and uniformly applied to the other loads immediately, with the stress being one-third of the axial ompressive strength. The onstant load was maintained for 6 s, and the deformation reading of eah measuring point within 3 s was reorded. Thereafter, the load on was unloaded. This loading proess was repeated four times, and the strain values were reorded. Then, the load was applied at the same speed until the samples failed. The failure loads were reorded. The differene with the measured axial ompressive strength should be less than %. The stati modulus of elastiity was alulated with the following equation. Fig. 3. arbonation test hamber 7

4 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) Result Analysis and Disussion 4.1 Axial ompressive Test The axial ompressive strength of onrete is an index that represents the mehanial properties of onrete and an important fator in the alulation of bearing apaity. Standard values of the axial ompressive strength of prisms were determined aording to the designed test in Setion 3.3 to analyze the speifi effets of arbon, aramid, and arbon-aramid hybrid fibers on the mehanial properties of onrete and determine the optimal dosage and best performane. The results are shown in Table 3. Table 3. Axial ompressive strength Fiber ontent Number.5% 1% 1.5% A /A A /A A /A Average Improvement(%) The ompressive strength(n/mm ) Fiber ontent(%) Fig. 4. ompressive strength with different fiber ontents A /A Fig. 4 shows that with the inrease in fiber ontent, the ompressive strength of onrete initially inreased and then dereased. The ompressive strength of the hybrid fiberreinfored onrete sample with a mixing ratio of 1% inreased the most (approximately 1% inrement). When the fiber ontent was small, the fibers were uniformly dispersed to maximize the advantage of high tensile strength. Energy was dissipated in the loading proess due to the pullout and frature of fibers, and this ondition inreased the overall ompressive strength of the sample. When the mixing ratio of fibers was 1.5%, the fibers were too plenty to be dispersed. They easily agglomerated in the mixing proess, whih made the onrete weak and led to a slight deline in ompressive strength. Different fibers exerted different influenes on the ompressive strength of onrete. arbon fiber performed better than aramid fiber in inreasing ompressive strength beause the density of aramid fiber was lower than that of arbon fiber. At the same volume fration fiber ontent, the amount of aramid fiber inreased, whih affeted the ompatness of onrete and redued the ompressive strength of onrete. However, when aramid and arbon fibers were mixed into the onrete samples, the synergy maximized the advantages of these fibers and thus greatly improved the ompressive strength of onrete. A signifiant differene in the failure proess of the fiber-reinfored and ordinary onrete was observed during the test. X-shape raks ourred in the setion of the ordinary onrete prism sample under loading. When the ultimate load was reahed, the onrete broke down, and its strength suddenly dereased, indiating typial brittle failure. During the loading proess of the fiber-reinfored onrete samples, ompressive toughness inreased signifiantly due to fiber rak resistane and toughening. When 4% of the ultimate load was applied, vertial miro raks ourred in the middle of the samples. The raks extended to both ends as the load inreased, and new vertial raks ourred. The raks developed obliquely at the ends with new bifurated raks. The pull-out and frature sounds of the fiber inside the samples were heard. Upward and downward raks developed when the ultimate load was applied. The bearing apaity began to deline, whih indiated sample failure. onrete spalling, whih is a harateristi of dutile frature, was not observed in the entire proess. 4. Stati Elasti Modulus Test 4..1 Test Results arbon and aramid fibers possess high modulus, but estimating their effets on improving the elasti modulus of onrete after mixing is impossible. Thus, a stati elasti modulus test was onduted to obtain the parameter values. The orresponding elasti modulus of the fiber-reinfored onrete was alulated with Formula (1) and is shown in Table 4. Table 4. Elasti modulus ( 1 4 ) Fiber ontent Number.5% 1% 1.5% A /A A /A A /A Average Improvement(%)

5 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) onrete speimen FR speimen AFR speimen /AFR speimen Fig. 5. Failure diagram of speimens 13 arbonation depth(mm) than 1%. Thus, the elasti modulus of ordinary onrete was diretly used in the alulation proess. A /A Seletion of Elasti Modulus alulation Methods Many empirial formulas an be utilized to alulate the elasti modulus of onrete. These formulas an be lassified into three main types aording to ompressive strength. The alulation formula proposed by the European onrete Assoiation (EA) is as follows: E = 9.5( fu + 8)1/ () 1.6 Fiber ontent(%) where E refers to elasti modulus ( N / mm ) and f u refers to ompressive strength ( N / mm ). The alulation formula of onrete under 6 aording to hina s onrete struture design ode (D) is as follows: Fig. 6. Elasti modulus of onrete with different fiber ontents Analysis of the data in Table 4 indiated that the elasti modulus and ompressive strength of onrete presented the same hange trend. The elasti modulus of the fiberreinfored onrete inreased with the inrease in axial ompressive strength. The elasti modulus of the 1% hybrid fiber-reinfored onrete inreased the most (approximately 1% inrement). Fig. 6 presents the hange in the elasti modulus of onrete with different fiber ontents. The elasti modulus of the onrete mixed with arbon and hybrid fibers was higher than that of ordinary onrete. The hybrid fiber exerted the best enhanement effet among all the studied fibers. When the aramid fiber ontent was high, the elasti modulus of aramid fiber was slightly smaller than that of ordinary onrete. The addition of fiber in onrete generally inreased the elasti modulus, but the improvement was less E = f u (3) The alulation formula provided by hen Zhaoyuan et al. (ZY) is as follows: ( ) E =.61 fu (4) The reliability of these formulas was verified using the measured data. The results are shown in Table 5. 7

6 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) Table 5. alulated values of elasti modulus by different formulas Fiber ontent Formula.5% 1% 1.5% A /A A /A A /A EA D ZY Experimental data The figures alulated with Formulas () and (4) are lose, but they are larger than the experimental value. The figure alulated with Formula (3) fits the experimental value well. Thus, Formula (3) should be used to alulate the elasti modulus of fiber-reinfored onrete. 4.3 arbonation Test During the maintenane of onrete strutures, alium hydroxide and a saturated solution are produed by the hydration of ement filled into the pores of onrete and reat with steel rods to form a layer of dense and insoluble Fe O 3, Fe 3 O 4, and Fe ( OH ) 3 on the surfae known as a passive film. In this state, steel rods with miro raks annot be orroded by air and water, but when the alkalinity of onrete dereases due to a hange in the external environment, the steel rods hange from a passive state to an ative state and begin to be orroded under the o-existene of water and arbon dioxide or hloride ions in the air. In atual omponents, steel orrosion is the main fator that degrades the durability of onrete strutures. The O ontent in the air inreases with the development of industries and inrease in automobile usage, leading to onrete arbonation. onrete arbonation is auses steel de-passivation and orrosion, whih affet the durability of reinfored onrete strutures. Therefore, analyzing the inrease in the anti-arbonation apaity of onrete is ruial. Fibers an improve the arbonation resistane of onrete beause fibers are evenly distributed in ement mortar after being added to onrete. They onnet with one another to form a network that inhibits aggregate submersion, prevents the segregation of onrete mixture, and dereases the bleeding of onrete, thereby reduing the number of pore hannels in the onrete. Fibers that are largely distributed in the mortar also redue and even blok pores. The addition of fibers also redues or prevents the formation, growth, and expansion of raks in onrete and hinders rak onnetivity. In other words, in fiber-reinfored onrete, fibers weaken the diffusion path of O. As a result, the anti-arbonation apaity of fiber-reinfored onrete is higher than that of ordinary onrete. In this study, the influene of fibers on the anti-arbonation apaity of onrete was further analyzed through an experiment Test Results The arbonization depths of different fiber-reinfored onrete sample were obtained through a arbonation test, the design of whih has been explained in Setion.3. Given that the number of samples was large, the average value of three samples in a group was obtained. The arbonation depths of different groups of samples on the 3rd, 7th, 14th, and 8th day are listed in Table 6. The arbonation depth of onrete inreased with age and inreased rapidly in the first 7 d to nearly half of the final arbonation depth. Table 6. Average arbonation depth Fiber ontent arbonation time.5% 1% 1.5% A /A A /A A /A 3d d d d Modulus of elastiity( 1 4 N/mm ) Fiber ontent(%) A /A Fig. 7. arbonation depths on the 8th day with different fiber ontents Fig. 7 shows that the arbonation depths of the onrete samples mixed with fiber were all smaller than those of ordinary onrete. Ordinary onrete exhibited poor antiarbonation performane beause onneted bleeding hannels existed inside the onrete. These hannels were the pore hannels that remained after exess water evaporation and bleeding from the slurry. Water gaps aumulated at the edge of the oarse aggregate. The addition of fibers prevented the development of raks. The fibers filled up the pores, thus enhaning the antiarbonation apaity of onrete. The enhanement by anti-arbonation apaity was exellent when the fiber ontent was small. The effets of arbon and arbon - aramid hybrid fibers were better than that of the aramid fiber beause a large amount of fiber led to uneven dispersion and agglomeration, thus resulting in poor ompatness of onrete. 73

7 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) arbonation Depth alulation Model Given the importane of the arbonation resistane of onrete to the durability of omponents, loal and international sholars have proposed various arbonation depth predition models[17-18]. All studies have agreed that arbonation depth is proportional to the square root of arbonization time. The expression is as follows: x = k t (5) where x refers to the arbonation depth (mm), k refers to the oeffiient that influenes arbonation, and t refers to arbonization time. To determine the value of k and its influening parameters, researhers have onduted many experimental studies on the O diffusion oeffiient, water ement ratio, and ompressive strength of onrete and have presented different alulation models. The representative models are as follows: Theoretial model based on Fik s Law: A mathematial model based on the first law of diffusion and the absorption harateristis of O in porous media was given by the former Soviet sholar Alexeyev[19] and his olleagues. x = where D e o, m e, o t (6) D refers to the effetive diffusion oeffiient of O in onrete, refers to the arbon dioxide onentration on the surfae of onrete, and m refers to the volume of O absorbed by onrete per unit volume when being fully arbonized. Given that the mathematial physial formula based on the O diffusion oeffiient is omplex, the model is rarely applied. Model based on diffusion theory and experiments: A mathematial model of arbonation depth was proposed by Zhang Yu et al.[] from Tongji University. W γ x k k k k ( RH) = RH O T s O t (7) γ γ where HD W refers to the water ement ratio; O refers to the onentration of O ; RH refers to environmental humidity; k refer to the parameters related to humidity, O onentration, temperature, and stress state of onrete; γ refers to the orretion oeffiient of the hydration HD degree of ement, with a value of 1. when maintained for 9 d and.85 when maintained for 8 d; and γ refers to the orretion oeffiient of ement variety, with a value of 1. for Portland ement and 1. minus the ontent of admixtures for other ements. Empirial model based on the ompressive strength of onrete: A alulation model was proposed by Di Xiaotan of hinese Aademy of Arhitetural Sienes[1]. In the model, onrete strength is the main parameter, and the environment, maintenane onditions, and ement variety are the influening fators. 6 x t f = α 1α α3 1. (8) u where f u refers to the ompressive strength of onrete and α, α α refer to the parameters of maintenane onditions, 1, 3 ement variety, and environment, respetively. The speifi values were provided by Referene [1]. Based on these alulation theories and models and onsidering the main influential onditions in the experimental proess, this study proposed a alulation formula of the arbonation depth of onrete with axial ompressive strength maintained for 8 d and O onentration and arbonization time of onrete as the main fators. x = β t (9) f u The onversion of the ube and axial ompressive strengths of a prism uses =.88α α f. Inorporation into Formula (9) yields..88α α β x = 1 f t f 1 u (1) where α 1 refers to the orretion oeffiient of strength with a value of.76 for onrete 5 and below,.8 for highstrength onrete 8, and linear interpolation in the middle; α refers to the brittle redution fator with a value of 1. for onrete 4 and below,.87 for high-strength onrete 8, and linear interpolation in the middle; β refers to the oeffiient related to fiber type determined by experimental data; and f refers to the axial ompressive strength of onrete ( N / mm ) Model Verifiation Based on Experimental Data Aording to the priniple of mathematial statistis, the experimental data and formula were fitted with arbonation depth as the funtion and arbonization time as the independent variable to draw the arbonation depth urves of different types of fiber-reinfored onrete and test the reliability of the formula. The experimental data of the.5% mixing ratio was regarded as an example. The alulation models of different fibers were as follows: 73.51α.5%, 1α x = t f The fitting degree was.98188, and the mean square error was α.5% A, 1α x = t f The fitting degree was.9693, and the mean square error was α.5% /A, 1 α x = t f The fitting degree was.9697, and the mean square error was The auray of the model was good. 74

8 Jingjing Zhang, Jianwei heng, Yuanming Dou and Qin Xin/Journal of Engineering Siene and Tehnology Review 1 (5) (17) onlusions To resolve the high brittleness and poor toughness of onrete, a novel method of adding arbon and aramid fibers into onrete was developed. Through experiments on onrete samples mixed with different types and ratios of fibers, the study analyzed the mehanial properties and durability of the samples. The following onlusions ould be drawn: (1) The ompressive strength of onrete samples mixed with arbon and hybrid fibers initially inreased and then dereased with the inrease in the fiber mixing ratio. The strength of onrete mixed with aramid fiber is inversely proportional to the mixing ratio of fiber and is higher than that of ordinary onrete. Many vertial raks are observed on the surfae of the fiber-reinfored onrete samples, indiating good toughness. () The elasti modulus and ompressive strength of the fiber-reinfored onrete show the same hange trend. The addition of fiber slightly influenes the elasti modulus of onrete. Therefore, during alulation, the elasti modulus of ordinary onrete an be used diretly. (3) The arbonation resistane of different types of fiberreinfored onrete is enhaned ompared with that of ordinary onrete. The effet on the arbon fiber-reinfored onrete is the most signifiant, followed by that on the hybrid fiber-reinfored onrete and aramid fiber-reinfored onrete. The mixing ratio of fiber is inversely proportional to anti-arbonation apaity. arbonation depth alulation formulas based on ompressive strength, O onentration, and arbonization time are proposed through an experiment and verified by the test data. The alulation value shows a good agreement with the test value. This study ombined tests and theoretial analysis to obtain the improvement degrees of onrete reinfored by different types of fibers and mixing rates. A arbonation depth alulation formula of fiber-reinfored onrete was proposed based on the test data. The formula provides a referene to atual engineering. The parameter seletion of the model exerts a ertain influene on appliable onditions due to the multiple fators that affeted the present arbonation experiment. In future studies, environmental humidity and other fators should be introdued to broaden the appliation sope of the model. Aknowledgements The study was supported by the Siene and Tehnology Projet of Hebei Provinial Transportation Department (Grant No. Y-146). Aess artile distributed under the terms of the reative ommons Attribution Liene Referenes 1. Hu Haitao, Li Ni, Xiong Jie, Researh on Quasi-stati mehanial properties of short ut aramid fiber-reinfored ement mortar. 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