Experimental Study on Mechanics and Pressure Sensitivity of Carbon Fiber RPC

To study the mechanical properties and conductivity enhancement effect of carbon fiber (CF) on reactive powder concrete (RPC), this work systematically investigated the changes in the compressive strength, elastic modulus, Poisson’s ratio, and pressure sensitivity of RPC under standard curing system. With age, CF content, and pressure sensitivity as parameters, the micro-method for scanning mechanism was used to study the effect mechanism of CF content on RPC. The experimental results showed that with the increase of carbon fiber content, poisson’s ratio and compressive strength of carbon fiber RPC tended to increase first and then decrease. The elastic modulus decreased first and then increased, and 0.75% was the turning point of fiber content. With the increase of carbon fiber content, the resistivity decreases gradually, while the resistivity change rate (FCR) shows a trend of increasing and decreasing first, and there is a strong corresponding relationship between strain and FCR.When the CF content is 0.75%, the maximum FCR is 38.8%, which is increased by 33%. the CF RPC with a CF content of 0.75% is the highest stress and strain sensitivity group, reaching 2.05%/ Mpa and 513.7, respectively.


Introduction
Damage detection and evaluation of concrete structures have been a hot topic in recent years 1 . Smart concrete is a relatively new civil engineering material. It can better sense the changes of stress and strain by adding an appropriate amount of conductor or semiconductor material to the cementbased material 2 . However, the shortcomings of smart concrete are also obvious, such as low mechanical properties, poor durability, short cycles, and easy damage 3,4 . The addition of fiber can significantly improve the mechanical properties and corrosion resistance of cement mortar [5][6][7] .To address the aforementioned disadvantages, carbon fiber (CF) is added to reactive powder concrete (RPC). RPC is a new cementbased composite material first developed by Bouygues in France in 1993 with Portland cement and various mineral admixtures as the cementing system 8,9 . The basic principle of RPC formulation is to increase the activity and fineness of each part of the material. Ultrafine powder such as silica fume is used for achieving a low water-to-gel ratio (W/B) to reduce porosity and obtain better strength 10 . RPC has the comprehensive advantages of light weight, high strength, high toughness, and high durability, making it applicable to petroleum, nuclear power, municipal, marine, and military facilities. With these advantages, RPC offers high social benefits 11 .
At present, local and international research have been conducted on the mechanical properties and electrical properties of CF cement-based composites. CF is a kind of microcrystalline graphite material. CF has similar properties to carbon, such as stable structure, high temperature resistance, high tensile strength, large elastic modulus, corrosion resistance, small specific gravity, and strong toughness. CF is often used in the reinforcement and toughening of concrete materials, maintenance, and reinforcement [12][13][14] . Shu et al. 15 studied the effect of CFs of different sizes on the mechanical properties of cement mortar and concluded that micro CFs can help large CFs to disperse and significantly improve the crack resistance of cement mortar. The addition of CF can greatly improve the compressive strength and reduce the resistivity of the cement matrix 16 . Rangelov et al. 17 studied CFs of different sizes and incorporated them into the cement mortar mixture. The 28d compressive strength increases by 11%, and the 7d tensile strength increases by 56%. Park et al. 18 incorporated CF content of 1%, 2%, and 3% into cement mortar. They concluded that when the CF content is 1%, 2%, and 3%, the compressive strength of CFRCC is higher than that of ordinary cement mortar and increases respectively by 62%, 58.1%, and 55.2%. The tensile strength of CFRCC is improved compared to that of ordinary cement mortar by 103%, 172%, and 255%. Li et al. 19  With the development of construction and information technology, continuous innovation and reform of new materials, new processes and new technologies, real-time monitoring of safety performance of major civil engineering buildings, and self-diagnosis of building health have received increasing attention. The incorporation of CF not only improves the mechanical properties of concrete and cement mortar but also promotes the development of smart concrete. CF is considered to be the most suitable part for conductive concrete. Through uniaxial compression tests, Azhari and Banthia 20 concluded that CFs and carbon nanotubes significantly increase the conductivity of cement-based materials. In addition, conductive concrete can sense microcracks and failures. For conductive concrete, many factors that affect electrical performance exist. Sassani et al. 21 experimentally investigated the five influencing factors of conductive concrete (CF content, length, volume ratio of coarse and fine aggregate, expansion agent, and water content). They concluded that the CF content is the factor that exerts the greatest effect on resistivity, followed by the thickness aggregate ratio, fiber length and expansion agent, and moisture content. Han et al. 22 used a direct current two-electrode method to measure the resistance and studied the effect of different CF content and length on the pressure sensitivity of CF concrete. The results show that the addition of CF significantly improves the mechanical properties of cement mortar. As the amount of fiber increases, the resistivity becomes lower, and the increase in fiber length is conducive to the establishment of an excellent conductive network 19 . Chiarello and Zinno 23 studied the effect of CF content and length on the electrical conductivity of cement mortar. The results reveal that when the CF content is too high and exceeds the threshold current value, it has little effect on the electrical performance. Baeza et al. 24 studied the effect of the amount of CF and the aspect ratio of the concrete on the electrical resistivity of the concrete. The experimental results indicate that the electrical conductivity is proportional to the length of the CF. As the aspect ratio increases, the penetration threshold will be generated in advance. Hambach et al. 25 studied the effect of the amount of CF on the resistivity of conductive concrete. The experiments demonstrate that when the amount of CF increases from 0% to 1%, the resistivity increases from 10-6 to 10-2. When the amount of CF exceeded 1%, resistivity change is reduced significantly. As a non-metallic conductive material, carbon has good electrical conductivity.
Adding chopped CF to the cement matrix not only has good pressure sensitivity but also has a beneficial effect on the mechanical properties of RPC.
To date, research on intelligent cement-based composite materials at home and abroad is focused mainly on CF cement mortar with high water-to-binder ratio and CF concrete containing coarse aggregate. However, few reports exist on the pressure sensitivity and mechanical properties of CF RPC. The advantage of CF RPC is the combination of high strength and high sensitivity to meet the actual needs of engineering. In this test, the mineral admixture is a silica fume-ore powder-fly ash cementing material system. Due to the incorporation of mineral admixtures, on the one hand, can make full use of industrial waste, promote sustainable development; On the other hand, the internal structure of RPC could be optimized to effectively reduce the internal temperature rise of concrete and improve the anti-corrosion and anti-crack performance of RPC. The test explores the mechanical properties and pressure sensitivity of CF RPC with different amounts of CF under standard curing methods. The corresponding characteristics between the resistivity change rate (FCR) and external load can promote the further development of smart concrete.

Test materials
This study adopted P.II-grade 52.5 cement produced by Yatai Cement Industry of Jilin Province, China. The cement inspection report, which conforms to the general Portland Cement Inspection Standard (GB175-2007) 26 , is shown in Table 1. Silica fume produced produced by Shandong Boken Silicon Material Co., Ltd. was used, and the activity index was 116%. Table 2 summarizes the silica fume composition. S95-grade mineral powder produced by Gongyi Longze Water Purification Materials of Henan Province, China were used. Table 3 lists the performance indicators. fly ash produced by Changchun Datang Second Thermal Power of Jilin Province, China. All of these were in accordance with the technical specifications of the Application of Mineral Admixtures (GB/T51003/2014) 27 . HSC polycarboxylic acid high-performance water reducing agent produced by Qingdao Hongxia Polymer Material Co., Ltd. was used, ≤0.06 0.007 and the water-reducing rate was 28.5%. The selected quartz sand was natural river sand. Such sand was obtained in Jilin Province. The sand with the particle size below 1.18 mm was selected as the test sand. The labor cost of such sand was reduced in comparison with that of the sieved fine sand. The fineness modulus was 2.158, which is a fine sand value. The CF produced by a company in Shandong was used. Table 4 presents the performance indexes.

Preparation and curing of test pieces
1. The RPC mix ratio is shown in Table 5. To be close to the actual project, the test did not add dispersant but changed the stirring process to make the CF uniformly dispersed. First, the weighed sand, cement, silica fume, fly ash and mineral powder are put into the HJW-60 single-horizontal shaft forced concrete mixer with the rotating speed of 45 RPM for 1 minute. The fiber is then added and stirred for 2 min. Following, 90% of the water-reducing agent is put in the weighed water and stirred for 1 min. After that, the remaining 10% of the waterreducing agent is added, and stirring is continued for 3 min. The test uses 100 mm×100 mm×100 mm concrete compressive strength standard test pieces, 100 mm×100 mm×300 mm concrete prism compressive test pieces, elastic modulus test pieces, and pressuresensitive test pieces. The test piece is made and vibrated in accordance with the specifications in "Reactive Powder Concrete" (GB/T 31387-2015) 28 .
After the artificial vibration is completed, the test piece is placed on the vibration     Figure 1. The resistance test uses the VC8900 multi-function meter and the direct current two-electrode method for testing. The strain is a DH-3817 static strain gauge, the collection frequency is 20 HZ, and the loading speed is 5 KN/s. During loading, load, resistance, and strain values are collected simultaneously. The resistivity is shown in Formula 3, and the resistance change rate is expressed as Formula 4. The formulas for stress and strain sensitivity are given in (5) and (6).
Where R-Resistance of test piece A-Section electrode area L-Length between two electrodes of test piece ρ-Resistivity under corresponding load during test ρ 0 -Resistivity of the test piece before loading FCR-resistivity change rate σ-stress along the loading direction ɛ-radial strain

Test results
A large number of tests were performed on the basic mechanical properties of CF RPC, including experimental studies on the compressive strength of cube and prism under standard curing regimes at 7d and 28d. Table 6 summarizes the results of various tests of CF RPC.

Analysis of compressive strength test results and failure morphology
When CF is not added to RPC, the concrete will continue to drop slag during the load increase, and then brittle failure will occur directly after the loud noise. After the incorporation of CF, the concrete gradually develops cracks from both sides along with the increasing load. At the moment when the ultimate load is reached, the crack will also be loud, but the integrity of the specimen is better than that of the unincorporated fiber, as shown in Figures 2b and 2d. The test results show that the brittleness characteristics of RPC can be significantly alleviated by the incorporation of CF. With the increase of the amount of CF, the brittle failure is continuously slowed down. The bond between CF and cement matrix is relatively strong, which increases the energy consumption when the fiber is pulled out. With the increase of the load, the phenomenon of slag dropping and peeling appears, but the CF RPC can maintain better integrity, improve brittle failure, and have certain plastic failure characteristics.
When the CF content does not reach the agglomeration limit, the combined effect of CF in the cement collective to   CF RPC increases by 0.38%. When the CF content is 1.25, the RPC matrix agglomeration is serious, the defects are further increased, and the compactness is decreased. The compressive strength is increased by 2.6%.

Stress-strain curve analysis of CF RPC
At present, most structures work in the elastic range. Thus, the test adopts the rising section of the CF RPC stress-strain curve. As shown in Figure 4, the axial stress-strain curves of RPCs with different CF contents under a single-cycle compressive stress are similar in shape. When the axial stress is 0-0.1σ p , it is the first stage, that is, compact stage. In this stage, the microcracks inside the RPC are gradually closed, and the shape of the axial stress-strain curve is a straight line. When the stress is 0.1-0.6σ p , this stage is called the elastic stage, and the axial and radial stress-strain curves are approximately straight lines. Owing to the different amounts of CF, the time at which the CF RPC reaches the end of the elastic phase varies. When the stress is 0.6-0.9σ p , CF RPC enters the third stage, which is the plastic stage. At this stage, the CF and the cement matrix are co-stressed. As the stress increases, the CF is continuously pulled out and cut. CF gives full play to the role of reinforcement and crack resistance, resulting in a strain growth rate much greater than the strain. Taking the CF RPC content of 0.75% as an example for analysis, in the 0.6-0.9σ p stage, the stress increases from 55.8 Mpa to 83.7 Mpa, which is a 50% increase, and the radial strain increases from 1611 to 2753, an increase of 70.88%. Afterσ> 0.9σ p , the fourth stageunstable development stage-is where plastic deformation increases sharply, slope decreases significantly, and axial and radial strain increases significantly. When the stress reaches σ p , the CF RPC increases with the increase of microcracks, the CF stops the work, and then the CF RPC test piece is destroyed with a loud noise.

Analysis of RPC elastic modulus and
Poisson's ratio of CF As shown in Figure 5, the elastic modulus of RPC under different CF content changes in a broken line. The elastic modulus of plain RPC is 41.7Gpa. When the CF content is 0.5%, 0.75%, 1.0%, and 1.25%, the elastic modulus of CF RPC is 38.2 Gpa, 37.5 Gpa, 37.9 Gpa, and 39.4 Gpa.
As can be seen from Figure 4, considering the elastic stage of the axial stress-strain curve, the slopes of the CF RPC are all smaller than those of the plain RPC. When the CF content is 0.5%, 0.75%, 1.0%, and 1.25%, the elastic modulus of CF RPC is reduced by 7.2%, 8.9%, 8.0%, and 4.3%, respectively. Owing to the incorporation of CF, the elastic modulus tends to decrease. When the CF content is in the range of 0% to 0.75%, the elastic modulus gradually decreases. When the amount of CF is in the range of 0.75% to 1.25%, the elastic modulus of CF RPC rises to a different extent than when the amount of fiber is 0.75%. The incorporation of CF has a certain effect on the Poisson's ratio of RPC. In this test, the compressive stress is only 1/3σ p . At this lower stress level, RPC is basically in the elastic stage. CF plays a role in the deformation of RPC. The Poisson's ratio of prime RPC is 0.194. When the CF content is 0.5%, 0.75%, 1.0%, and 1.25%, the Poisson's ratio of CF RPC is higher than that of plain RPC, that is, 24.7%, 30.4%, 28.8%, and 22.6%, respectively. When the CF content is in the range of 0% to 0.75%, the Poisson's ratio of carbon fiber RPC is gradually higher than that of plain RPC. In the range of 0.75% to 1.25%, the Poisson's ratio of CF RPC decreased to different degrees when the content of CF was 0.75%. This is mainly due to the high elastic modulus of CF that restrains the lateral deformation of the RPC specimen and reduces the lateral strain. When the CF content is low, the incorporation of CF presents a positive effect on the RPC matrix, whereas when the CF content exceeds the optimal content, the internal defects of the cement matrix will gradually emerge, resulting in a negative effect on the strength and toughness of RPC.
In order to study the interfacial bonding property and crack resistance of carbon fiber in cement matrix. As shown in Figure 6a, carbon fiber is closely bonded to RPC matrix, and the collective adhesion between carbon fiber and cement is relatively ideal. The carbon fiber is broken in the cement collective instead of being pulled out. This is because carbon fiber was dispersed in the RPC matrix and formed a fiber network, which prevented the development and connection of cracks inside RPC, thus achieving the effect of improving strength and cracking resistance. As shown in Figure 6b, under the action of axial stress, the CF bears part of the stress and absorbs part of the energy. The cement matrix crack gradually develops. When the crack develops to the fiber, the driving force of the crack is blocked and the crack stops developing. Meanwhile, the crack penetration can be effectively prevented, so that the mechanical properties of carbon fiber RPC can generate gain effect. Table 7 is the summary of the results of carbon fiber RPC resistivity test under different content at 28d. Owing to the high density of CF RPC, the overall conductivity of the structure is low, which results in a higher resistivity of RPC. The relationship between resistivity and CF content is shown in Figure 7. When the CF content in RPC is from 0% to 0.5%, the resistivity of CF RPC is significantly reduced by 75.3%. When the CF content was increased from 0.5% to 0.75%, the resistivity of the CF RPC decreased by 97.7%, and the resistivity decreased with the increase of the CF content. As shown in Figure 7, the percolation threshold of CF RPC is 0.5%-0.75%. As RPC has a very low waterto-binder ratio, with the further increase of the CF content, CF agglomeration is serious and internal defects increase, which is not conducive to the overlap between CF. When the CF content is 1%, the resistivity is increased by 114.2% compared with 0.75%. When the CF content is from 1.0% to 1.25%, the RPC resistivity increases by 19.0%.

FCR law under monotonic loading
(1) Change law of stress, strain, and FCR under different CF content Figure 8a indicates that within 0-40 Mpa, FCR gradually increases as the load gradually increases. When the CF content is 0.5%, the maximum FCR is 29.1%. When the CF content is 0.75%, the maximum FCR is 38.8%. When the CF content is 1.0%, the maximum FCR is 37.1%. When the CF content is 1.25%, the maximum FCR is 23.5. When the CF content is 0%, only the cement particles in the RPC matrix are conductive, and no obvious pressure sensitivity is noted under the action of the classification load. After adding CF, its conductive principle is contact and non-contact conductivity between carbon fibers. With the continuous increase of the load, the CF is continuously overlapped. At the same time, the electronic transition is gradually performed. When the CF content is 0.5%, the probability of overlap between the two CFs is very low, the barrier width increases, and the electronic transition is difficult. When the CF content is 0.75%, the FCR is the maximum. However, as the CF content is gradually increased, the overlap probability and the degree of overlap between the CF are significantly increased, and the electronic transition is relatively easy, causing the FCR to gradually decrease. Figure 8b is a graph of the strain-FCR effect of CF RPC under different CF content under the stress of 0-40 Mpa.   23.5%. The strain and FCR increase with the increase of the load. When the CF content is 0.75%, the FCR changes the most. The FCR and strain of CF RPC have a strong corresponding relationship. Under the same load, the FCR and strain relationship increases when the CF content is within the 0.5%-0.75% and gradually decreases when the CF content is within the range of 1.0%-1.25%. As the degree of change in FCR increases, so does the strain. In the range of 0 Mpa to 40 Mpa, the strain and FCR gradually increase as the load increases. CF RPC has a good correspondence between strain and FCR under monotonic loading and has stable and regular sensing performance.

Correlation between strain and resistivity change rate
To verify the correlation between the change rate of RPC resistivity and strain, this experiment performed a quadratic function fitting on the CF at 0.5%, 0.75%, 1.0%, and 1.25%, as shown in Figure 9.  Table 8. During the uniaxial compression test of RPC, the change rate of resistivity is positively related to the strain. When the CF content is 0.5%, 0.75%, 1.0, and 1.25%, the R2 values are 0.985, 0.998, 0.968, and 0.999, respectively. The fitting equations are y=50x+24.2, y=20x-5.1, y=28.8x+45.4 and y=43x+2.8, respectively. When the CF content is 0.75% and 1.25%, the correlation between FCR and strain is high, and R2 is greater than 0.99. In the range of 0 Mpa to 20 Mpa, when the CF content is 0.5%, 0.75%, 1.0%, and 1.25%, the degree of change in FCR increased by 14.3%, 32.5%, 26.9%, and 18% respectively compared to that of plain RPC. When the CF content is 0.75%, the FCR of CF RPC changes the most and sensitivity is at the highest. When the CF content is 1.25%, the compressive strength is lower than the 0.5%, 0.75%, and 1.0% compressive strength, but the fitting degree is the best. For CF RPC, no correlation is found between compressive strength and degree of fit. The values of R2 under different CF content are all greater than 0.96, indicating that the resistivity characteristics and strain can be expressed by linear equations.

Stress and strain sensitivity
(1) Perceptual performance under monotonic loading As shown in Figures 10a and 10b, the stress and strain sensitivity of CF RPC under different stresses are different, and the change in RPC test piece is reflected from the sensitivity value. When σ = 10 Mpa, as the amount of CF  As the load increases, the magnitude of the change in stress and strain sensitivity gradually decreases. Under the action of σ = 10Mpa, compared with the benchmark group of RPC with 0.5% CF content, when the CF content is 0.75%, 1.0%, 1.25%, the increase in stress sensitivity of CF RPC compared with the benchmark group is 169.7%, 52.6%, and 44.7%. The increase in strain sensitivity is 152.3%, 58.1%, and 80.2%. Under the action of σ = 20 Mpa, the increase in stress sensitivity is 126.4%, 87.5%, and 25% and the increase in strain sensitivity is 101.1%, 98.3%, and 51.6%. The increase of stress sensitivity under σ= 30 Mpa is 38.5%, 40.9%, and -16.8%. The increase in strain sensitivity is 25.4%, 47.5%, and -4.8%. Under the action of σ = 40Mpa, the stress sensitivity increases are 32.8%, 26.1%, and -20.1%; the strain sensitivity increases are 33.6%, 34.6%, and -7.2%. When the mixing amount is 0.75%, the carbon fiber bond in the cement group is relatively tight, the electronic transition is relatively simple, and there is no internal defect caused by the agglomeration phenomenon. A test piece with a CF content of 0.75% is more advantageous as a sensing material for detecting the health structure of cement-based materials. Under the action of large loads, the phenomenon of "bluntness" in pressure sensitivity appears. When the CF RPC is pressed, the distance between two adjacent CFs is reduced, the ability of electrons to transition is enhanced, the resistance is reduced, and the current is increased. With  the increase of CF content and load, the overlap rate of CF is getting higher and higher, the electronic transition is relatively easy, and the degree of change in stress sensitivity and strain sensitivity gradually decreases. When the load amplitude increases to a critical value, a large number of conductive networks are formed inside the specimen. As the load gradually increases, the internal conductive network changes are not as obvious as initially, and the corresponding FCR is also less sensitive to changes in stress and strain. In a comprehensive way, specimens with fiber content of 0.75% have more advantages as cement-based materials for health structure detection and sensing.

Conclusion
When the CF content is in the range of 0% to 0.75%, the compressive strength of the CF RPC increases with the increase of the CF content. When the CF content exceeds a critical value (0.75%), The increasing rate of compressive strength of carbon fiber RPC decreased, but the strength was still better than that of raw RPC. With the increase of carbon fiber content, Poisson's ratio first increases and then decreases. The elastic modulus decreased first and then increased.
CF can effectively improve the conductivity of RPC. When the amount of CF in RPC is from 0% to 0.5%, the resistivity of CF RPC is significantly reduced by 75.3%. When the CF content is increased from 0.5% to 0.75%, the resistivity of the CF RPC decreases by 97.7%, As the carbon fiber content continues to increase, the resistivity will be flat and slightly increased.
In the range of 0-40 Mpa , as the load gradually increases, the FCR shows a gradually increasing change.When the CF content is 0.75%, the maximum strain and FCR are 1.22×10-2 and 38.8%, respectively, and the maximum FCR is increased by 33%. When the fiber content exceeds 0.75%, the content continues to increase, and FCR shows a gradually decreasing trend.
With the increase of carbon fiber content, the stress and strain sensitivity of carbon fiber RPC reached the best value at 0.75% content, reaching 2.05%/Mpa and 386.1 respectively. CF RPC has a stable sensing function, and the