# Repair and Reinforcement of Concrete Bridges Based on Cement-Based Composite Materials

Authors: Ning Huang; Zhigang Ruan; Li Zhang; Wei Wang; Hongyin Wang
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

At present, epoxy resin is almost used as a binder in bridge reinforcement, but there are some drawbacks due to the huge difference in mechanical properties and temperature incompatibility between it and concrete. Cement-based composites show obvious multi-crack cracking characteristics and strain hardening behavior in the process of stress and have better mechanical properties and durability than traditional cement materials, and have been successfully used in many practical projects. In this paper, the cement-based composite material is used as the concrete beam member of the bridge deck continuous material, and its mechanical properties are characterized and compared with the traditional cement slurry material properties. The results show that the ductility of cement-based composites is significantly affected by the fiber content. When the volume ratio is less than or equal to the critical volume rate, the ductility increases gradually with the increase of fiber content. The maximum value of the plastic strain at the initial crack is greater than that of the concrete beam member using cement mortar as the bridge deck continuous material, and the crack resistance of the bridge deck continuous structure is significantly improved. The addition of cement-based composites can significantly increase the cracking load of the test beams. Under the same load conditions, the crack resistance of the continuous structure of the cement-based composite bridge deck is better, and local cracking is less likely to occur.


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## Body

## 1. Introduction

With the rapid economic development brought about by the reform and opening up, great achievements have been made in the construction of road and railway bridges in China. The number of bridges has increased year by year, and the spans have also been continuously improved. China’s bridge industry is entering a stage of rapid development. At the same time, many bridges built in the 1980s and 1990s experienced various diseases after a long period of use. The diseases of bridge structures (such as concrete cracks, water seepage, surface weathering, spalling, exposed reinforcement and reinforcement corrosion, carbonization of concrete protective layer, and foundation erosion) seriously affect the safety and driving comfort of bridge structures [1–3]. In view of the occurrence of diseases, it is necessary to take effective measures for reinforcement and maintenance to ensure the normal use of the bridge structure.Concrete is one of the traditional building materials used in bridge construction in various forms. In recent years, great progress has been made in concrete composition, admixtures, and construction techniques. Many new materials, new methods, and new processes have also appeared in the maintenance, repair, and reinforcement of concrete. Timely repair and reinforcement prolongs the service life of the concrete structure and saves a lot of investment. Due to design mistakes, rough construction, and other reasons, the reinforcement and upgrade of the existing concrete structure has become a top priority [4].The failure of the continuous section of the ordinary concrete bridge deck is mainly due to the fact that the bridge deck is continuously located at the connection of the adjacent bridge spans, which is the most significant part of the bridge deformation. Compared with the bridge span structure, the overall stiffness and strength of the bridge deck continuous structure are relatively weak. In the past, traditional concrete materials have defects such as poor ductility, low tensile strength, large shrinkage deformation, and brittle failure. Under the combined action of dislocation and deformation, the continuous structure of the bridge deck is prone to damage, water seepage, and other diseases. Therefore, it is necessary to find a reinforcement method suitable for resisting these unfavorable factors [5–7].There are many methods available for strengthening concrete structures, and the “fiber-reinforced polymer” (FRP) bonding method is one of the most popular. It is a composite material with fiber as a reinforcing agent, and the most commonly used materials are carbon fiber and glass fiber. These composite materials can be made into sheets, panels, rods, or fabrics. Sticking these reinforcing materials on the concrete surface or embedded in the concrete will have a good reinforcement effect. (1) Epoxy resin may cause suspected “eczema” poisoning to concrete; (2) Due to its low permeability, it can cause freezing and thawing problems of concrete; (3) It has poor temperature compatibility with the reinforced concrete material. Unfavorable restraint effects may occur; (4) The mechanical properties of the reinforced concrete are different [8–10].The use of cement-based composite materials for reinforcement is a new type of reinforcement technology that has emerged in recent decades. It has the advantages of light weight, high strength, good durability, and easy construction. The cement-based composite material is a fiber-reinforced cement-based composite material formed by incorporating polyvinyl alcohol (PVA) fibers with a volume content of 2% into the cement base in a certain way. It has the advantages of crack resistance, impermeability, high deformation, and self-healing. Concrete structures are reinforced with cement-based composite materials [11–13]. The composite material is made of fiber and modified cement, which can avoid the disadvantages of epoxy resin mentioned above. In addition to the preparation of cement-based composites, domestic and foreign scholars have carried out a series of research work on the mechanical properties (tensile properties and bending properties) of cement-based composites. Some scholars established the similarity relationship between the direct tensile test and the three-section four-point bending test, and deduced the formula for calculating the equivalent tensile strain under the direct tensile test; some scholars tried to use the “zero-thickness interface model” to simulate the stress failure process of cement-based composites, and the calculated results are in good agreement with the experimental results. Based on the existing research results and relevant concrete structural specifications, some scholars have proposed a calculation method for the reinforcement of concrete structural members with cement-based composite materials [14–16]. In this paper, the cement-based composite material is used as the concrete beam member of the bridge deck continuous material, and its mechanical properties are characterized and compared with the traditional cement slurry material properties [17–20].


## 2. Methodology

### 2.1. Experimental Raw Materials

#### 2.1.1. Cement

The cement selected in this project is ordinary Portland cement with a density of 3125 kg/m3, the cement strength grade is 42.5, and its chemical composition is shown in Table 1.Table 1
Chemical composition of cement.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3LOIContent/%59.3021.916.273.781.642.414.69


#### 2.1.2. Fly Ash

The fly ash is grade I fly ash with a density of 2548 kg/m3. The addition of fly ash is not only economical but also improves the workability of the mixture and helps to achieve self-compacting of cement-based composites, improving the durability of hardened cement composites, whose chemical compositions are shown in Table 2.Table 2
Chemical composition of fly ash.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3K2OLOIContent/%3.9548.233.315.571.270.303.14.3


#### 2.1.3. Quartz Sand

The fine aggregate is 70-140-mesh high-quality quartz sand with an apparent density of 2628 kg/m3.


#### 2.1.4. Additives

The water-reducing agent is a high-efficiency water-reducing agent of polycarboxylate, with a solid content of 20%; the plasticizer is propyl methyl cellulose (HPMC). The fiber is polyvinyl alcohol (PVA) fiber.


### 2.2. Experimental Design

In order to carry out the flexural performance test of the concrete continuous beam, the size and reinforcement of the concrete beam member are shown in Figure1, and the single-span section size of the member is 1200 mm × 100 mm × 120 mm. The concrete strength grade adopted in the experiment is C40, and the mix ratio is shown in Table 3. Longitudinal stress steel adopts HRB335 hot-rolled steel bar; frame erection bar adopts HPB235 hot-rolled smooth round steel bar, stirrup adopts HPB235 hot-rolled smooth round steel bar, stirrup spacing is 100 mm, and the thickness of concrete protective layer is 30 mm. The bridge deck continuously adopts ordinary cement mortar, ordinary concrete, and cement-based composite materials.Figure 1
Section and reinforcement of single-span concrete simply supported beams.Table 3
Design of concrete mix ratio.
Cement, (kg/m3)Sand, (kg/m3)Stone (kg/m3)Water, (kg/m3)w/cw/bFly ash (kg)Water reducer (kg)45067810401590.3530.30460.012.8The mix ratio of the continuous structure of the cement-based composite bridge deck is shown in Table4.Table 4
Cement-based composite material mix ratio (kg/m3).
Water cement ratioCementFly ashSandWaterFiberWater reducerw/b = 0.35498.52749.78388.28434.9719.53By controlling the load size, according to the different percentages of the ultimate load, the connecting sections are ordinary cement mortar, cement-based composite materials, and ordinary concrete beams. By detecting the strain and displacement, the bending moment of the concrete continuous beam under different load levels is calculated, and the reinforcement effect of the cement-based composite material is evaluated. The experimental groupings are shown in Table5.Table 5
Grouping of flexural tests of concrete continuous beams.
GroupMaterials used in the connection sectionECementitious compositesMCement mortarCOrdinary concreteThe test beams are divided into three groups: the connecting section of group C is ordinary concrete beam; the connecting section of group M is ordinary cement mortar; the connecting section of group E is cement-based composite material. The results of improving the flexural performance of continuous concrete beams with different connecting section materials are discussed.After curing for 28 days, the damage depth and compressive strength of concrete were first determined by the ultrasonic rebound comprehensive method. Since the damage will lead to the change of the frequency characteristics of the structure itself, the dynamic characteristics of the beam were tested by the hammer method, and the beam was excited by the force hammer. An acceleration sensor is installed in the middle of the beam span, and the JM3840 dynamic acquisition system is used to record the acceleration time-history curve PH. The four-point bending test of the reinforced concrete beam was carried out according to the test method of the “Concrete Structure Test Method Standard” (GB50152-92). The load-displacement relationship during the stress process is used to determine the failure mode.


## 2.1. Experimental Raw Materials

### 2.1.1. Cement

The cement selected in this project is ordinary Portland cement with a density of 3125 kg/m3, the cement strength grade is 42.5, and its chemical composition is shown in Table 1.Table 1
Chemical composition of cement.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3LOIContent/%59.3021.916.273.781.642.414.69


### 2.1.2. Fly Ash

The fly ash is grade I fly ash with a density of 2548 kg/m3. The addition of fly ash is not only economical but also improves the workability of the mixture and helps to achieve self-compacting of cement-based composites, improving the durability of hardened cement composites, whose chemical compositions are shown in Table 2.Table 2
Chemical composition of fly ash.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3K2OLOIContent/%3.9548.233.315.571.270.303.14.3


### 2.1.3. Quartz Sand

The fine aggregate is 70-140-mesh high-quality quartz sand with an apparent density of 2628 kg/m3.


### 2.1.4. Additives

The water-reducing agent is a high-efficiency water-reducing agent of polycarboxylate, with a solid content of 20%; the plasticizer is propyl methyl cellulose (HPMC). The fiber is polyvinyl alcohol (PVA) fiber.


## 2.1.1. Cement

The cement selected in this project is ordinary Portland cement with a density of 3125 kg/m3, the cement strength grade is 42.5, and its chemical composition is shown in Table 1.Table 1
Chemical composition of cement.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3LOIContent/%59.3021.916.273.781.642.414.69


## 2.1.2. Fly Ash

The fly ash is grade I fly ash with a density of 2548 kg/m3. The addition of fly ash is not only economical but also improves the workability of the mixture and helps to achieve self-compacting of cement-based composites, improving the durability of hardened cement composites, whose chemical compositions are shown in Table 2.Table 2
Chemical composition of fly ash.
Chemical compositionCaOSiO2Al2O3Fe2O3MgOSO3K2OLOIContent/%3.9548.233.315.571.270.303.14.3


## 2.1.3. Quartz Sand

The fine aggregate is 70-140-mesh high-quality quartz sand with an apparent density of 2628 kg/m3.


## 2.1.4. Additives

The water-reducing agent is a high-efficiency water-reducing agent of polycarboxylate, with a solid content of 20%; the plasticizer is propyl methyl cellulose (HPMC). The fiber is polyvinyl alcohol (PVA) fiber.


## 2.2. Experimental Design

In order to carry out the flexural performance test of the concrete continuous beam, the size and reinforcement of the concrete beam member are shown in Figure1, and the single-span section size of the member is 1200 mm × 100 mm × 120 mm. The concrete strength grade adopted in the experiment is C40, and the mix ratio is shown in Table 3. Longitudinal stress steel adopts HRB335 hot-rolled steel bar; frame erection bar adopts HPB235 hot-rolled smooth round steel bar, stirrup adopts HPB235 hot-rolled smooth round steel bar, stirrup spacing is 100 mm, and the thickness of concrete protective layer is 30 mm. The bridge deck continuously adopts ordinary cement mortar, ordinary concrete, and cement-based composite materials.Figure 1
Section and reinforcement of single-span concrete simply supported beams.Table 3
Design of concrete mix ratio.
Cement, (kg/m3)Sand, (kg/m3)Stone (kg/m3)Water, (kg/m3)w/cw/bFly ash (kg)Water reducer (kg)45067810401590.3530.30460.012.8The mix ratio of the continuous structure of the cement-based composite bridge deck is shown in Table4.Table 4
Cement-based composite material mix ratio (kg/m3).
Water cement ratioCementFly ashSandWaterFiberWater reducerw/b = 0.35498.52749.78388.28434.9719.53By controlling the load size, according to the different percentages of the ultimate load, the connecting sections are ordinary cement mortar, cement-based composite materials, and ordinary concrete beams. By detecting the strain and displacement, the bending moment of the concrete continuous beam under different load levels is calculated, and the reinforcement effect of the cement-based composite material is evaluated. The experimental groupings are shown in Table5.Table 5
Grouping of flexural tests of concrete continuous beams.
GroupMaterials used in the connection sectionECementitious compositesMCement mortarCOrdinary concreteThe test beams are divided into three groups: the connecting section of group C is ordinary concrete beam; the connecting section of group M is ordinary cement mortar; the connecting section of group E is cement-based composite material. The results of improving the flexural performance of continuous concrete beams with different connecting section materials are discussed.After curing for 28 days, the damage depth and compressive strength of concrete were first determined by the ultrasonic rebound comprehensive method. Since the damage will lead to the change of the frequency characteristics of the structure itself, the dynamic characteristics of the beam were tested by the hammer method, and the beam was excited by the force hammer. An acceleration sensor is installed in the middle of the beam span, and the JM3840 dynamic acquisition system is used to record the acceleration time-history curve PH. The four-point bending test of the reinforced concrete beam was carried out according to the test method of the “Concrete Structure Test Method Standard” (GB50152-92). The load-displacement relationship during the stress process is used to determine the failure mode.


## 3. Results and Discussion

### 3.1. Effect of Fiber Addition on Mechanical Properties

Different fiber contents (volume fraction 0.5%, 1.0%, 1.5%, and 2.0%) were selected to make corresponding specimens, and the mechanical properties were tested to compare and analyze the effect of fiber addition on mechanical properties.


#### 3.1.1. Deflection Test

With the increase of fiber volume ratio, the deflection of the midspan section of the specimen gradually increases, indicating that the fiber volume ratio has a great influence on the ductility of cement-based composites. When the fiber volume ratio of PVAF specimens is less than 1%, the deflection value of the midspan section is very small, showing the characteristics of brittle failure. Except for the specimens with brittle failure, the ultimate load gradually increased with the increase of fiber content. For example, the average ultimate load of PVAF-1.5 group was 236 N, and that of PVAF-2 group increased to 258 N. For specimens with brittle failure, the distribution of ultimate load test values is uneven and the dispersion is large, which is due to the low fiber content, which is affected by the dispersion of cement-based tensile strength (Figure2).Figure 2
Effect of fiber volume ratio on deflection.


#### 3.1.2. Equivalent Ultimate Tensile Stress and Equivalent Ultimate Tensile Strain

Figures3 and 4reflect the effect of the fiber content of the PVAF group on the equivalent ultimate tensile stress and equivalent ultimate tensile strain.Figure 3
Equivalent ultimate tensile stress of different dosages.Figure 4
Equivalent ultimate tensile strain of different dosages.It can be seen from Figure3 that, except for the brittle failure specimen, the equivalent ultimate tensile stress and equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content are also large. When the fiber content is 2%, the average equivalent ultimate tensile stress is 3.3 MPa, and when the fiber content is 1.5%, the average equivalent ultimate tensile stress is 2.7 MPa. It can be seen from Figure 4 that the equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content is also large. When the fiber content is 2%, the average equivalent ultimate tensile strain is 3.1%, when the fiber content is 1.5%, the average equivalent ultimate tensile strain is 1.9%, and the specimen has no ductility when the fiber content is 1%.


#### 3.1.3. Toughness Index

It can be seen from Figure5 that with the decrease of fiber volume content, the toughness index of PVAF decreases. When the fiber content is 2%, the average is (I5 = 4.78, I10 = 10.53, I20 = 22.65), when the fiber content is 1.5%, the average is (15 = 4.59, I10 = 9.23, I20 = 11.5), the fiber shows no ductility at 1% dosage.Figure 5
Toughness index of different dosages.


#### 3.1.4. Compressive Strength

It can be seen from Figure6 that the fiber content has a certain effect on the compressive strength of cement-based composites. The compressive strength of the high fiber content is low. When the fiber volume content is 2%, the average compressive strength is 38.9 MPa. When the fiber volume content is 1.5%, the average compressive strength is 41.2 MPa, and when the fiber volume content is 1%, the average compressive strength is increased to 49.5 MPa. The main reason why the strength of cement-based composites decreases with the increase of fiber content is that the number and size of air bubbles in cement-based composites increase with the increase of fiber content.Figure 6
Compressive strength of different dosages.


### 3.2. Test Results and Analysis of the Continuous Structure of the Cement-Based Composite Bridge Deck

#### 3.2.1. Load-Displacement Curve

The displacement curves of concrete beams recorded in the test under graded loading conditions are shown in Figure7. The displacement curves of concrete beams with three different bridge deck continuous materials are similar. The concrete beam using concrete as the bridge deck continuous construction material has a relatively obvious yield load (45.23 kN), and the concrete beam using cement mortar as the bridge deck continuous construction material has a yield load of 42.78 kN. However, no obvious yield load was measured for concrete beams using cement-based composite materials as the continuous construction material of the bridge deck.Figure 7
Load-displacement curve of concrete beam. (a) C. (b) M. (c) E.
(a)(b)(c)


#### 3.2.2. Cracking Load and Ultimate Load

The cracking loads and ultimate loads of concrete beams with three different bridge deck continuous materials are shown in Table6.Table 6
Cracking load and ultimate load.
No.Bridge deck continuous materialCracking load (kN)Ultimate load (kN)C-lConcrete2170.3M-lCement mortar18.571.2E-lCementitious composites1774.9It can be seen from Table6 that the cracking load and ultimate load of concrete beams with different bridge deck continuous materials are not much different. The concrete beam of the bridge deck continuous construction material and the concrete beam using the cement-based composite soil as the bridge deck continuous construction material have the largest ultimate load.


#### 3.2.3. Crack Development and Failure Mode

According to the hierarchical loading, the crack development process of the beam member in the midspan pure bending section is described. The failure mode of the C-1 beam whose the bridge deck continuous structure material is ordinary concrete is that the tensile steel bars yield, the concrete in the compression area is crushed, and transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joint of the two spans, and the bridge deck continuous structure is under tension. The failure mode of the M-1 beam with ordinary cement mortar as the continuous structure material of the bridge deck is that the tensile steel bars yield, the concrete in the compression zone is crushed, the continuous structure of the bridge deck is partially peeled off from the top surface of the beam, and the continuous structure of the bridge deck at the joint between the two spans. Irregular transverse cracks are generated on the upper surface, and the continuous structure of the bridge deck is damaged in tension. The failure mode of the E-1 beam whose bridge deck continuous structural material is cement-based composite material is that the tensile steel yields, the concrete in the compression zone is crushed, and many small transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joints of the two spans. The continuous structure of the bridge deck is damaged in tension.Table 7
Overview of continuous top cracks on different bridge decks at joints.
No.Bridge deck continuous materialNumber of cracksAverage crack width/mmAverage crack spacing/mmC-1Concrete20.359-M-1Cement mortar60.156-E-1Cementitious composites80.0971.69


#### 3.2.4. Continuous Structural Fracture Model of Bridge Deck

(a)
Development of cracks on the top surface of the continuous structure of the bridge deck at the joint.Under the same load condition (70 kN), the concrete beams of three different bridge deck continuous materials all produce a wide main transverse crack on the top surface of the bridge deck continuous structure at the joint. The continuous structure of the concrete bridge deck does not produce other secondary cracks, the continuous structure of the cement mortar bridge deck produces a number of secondary cracks with irregular directions and different directions, and the continuous structure of the cement-based composite bridge deck produces multiple cracks in the same direction around the main crack. The widths of the main cracks on the top surface of the bridge deck continuous structure at the joints of three different bridge deck continuous materials under the same load condition (70 kN) were measured, and the results are shown in Table7.Since the M-l beam, that is, the continuous multiple cracks of the cement mortar bridge deck, develop in different directions, the crack spacing is not counted. It can be seen that the E-1 beam, that is, the continuous joint of the cement-based composite bridge deck, has the largest number of tensile cracks, but no matter the width of the main crack or the average crack width is the smallest. In line with the characteristics of cement-based composite materials, the tensile crack width is small, the number of strips is large, and the direction is consistent. It shows that the use of cement-based composite material as the continuous material of the bridge deck can effectively improve the cracking mode at the joints. The crack width is smaller. It is not easy to distinguish with the naked eye, and the impact on the structural integrity is smaller.(b)
Bridge deck continuous structural strain.In order to obtain the continuous strain distribution of the bridge deck at the joints, according to the “Code for Design of Concrete Structures” (GB50010-2010), the continuous top and side surfaces of the bridge deck at the joints are pasted with strain sensors to measure the continuous strain development of the bridge deck during loading. The results are shown in Figure8. In the initial stage of loading, the bending moment is small, and the fiber strain along the beam height is also small and varies linearly. The working condition of the beam is similar to that of a homogeneous elastic beam; when the bending moment increases to the cracking moment Mcr, the fibers at the edge of the tension zone. When the strain reaches the ultimate tensile strain, the section cracks immediately and the neutral axis moves down. The measured strain indicates that the deformation of the steel bar and the concrete is coordinated.Figure 8
Continuous top surface strain of bridge deck at joints.When using concrete as the continuous construction material of the bridge deck and using cement mortar as the continuous construction material of the bridge deck, the continuous maximum tensile strain of the bridge deck at the joint is smaller, which are 0.726% and 1.091%, respectively, and the cement-based composite material is used as the bridge deck. The maximum tensile strain that the continuous construction material can withstand before cracking is 9.5434%, which is 13.13 times and 8.74 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively. The experimental results show that the continuous cement-based composite bridge deck can effectively improve the crack resistance of the continuous structure of the bridge deck, and can withstand greater tensile strain before cracking, that is, it can withstand greater variable loads. When concrete is used as the continuous construction material of the bridge deck and cement mortar is used as the continuous construction material of the bridge deck, the loading grades when the bridge deck is continuously cracked at the joints are 15.41 kN and 17.38 kN, respectively. The loading level of cement-based composite material as the continuous bridge deck structural material is 38.23 kN, which is 2.48 times and 2.20 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively (Figure8). It shows that the use of cement-based composite materials as the bridge deck continuous structural material can resist the development of cracks better than the concrete bridge deck continuous structure and the cement mortar bridge deck continuous structural material when receiving the same variable load.


## 3.1. Effect of Fiber Addition on Mechanical Properties

Different fiber contents (volume fraction 0.5%, 1.0%, 1.5%, and 2.0%) were selected to make corresponding specimens, and the mechanical properties were tested to compare and analyze the effect of fiber addition on mechanical properties.


### 3.1.1. Deflection Test

With the increase of fiber volume ratio, the deflection of the midspan section of the specimen gradually increases, indicating that the fiber volume ratio has a great influence on the ductility of cement-based composites. When the fiber volume ratio of PVAF specimens is less than 1%, the deflection value of the midspan section is very small, showing the characteristics of brittle failure. Except for the specimens with brittle failure, the ultimate load gradually increased with the increase of fiber content. For example, the average ultimate load of PVAF-1.5 group was 236 N, and that of PVAF-2 group increased to 258 N. For specimens with brittle failure, the distribution of ultimate load test values is uneven and the dispersion is large, which is due to the low fiber content, which is affected by the dispersion of cement-based tensile strength (Figure2).Figure 2
Effect of fiber volume ratio on deflection.


### 3.1.2. Equivalent Ultimate Tensile Stress and Equivalent Ultimate Tensile Strain

Figures3 and 4reflect the effect of the fiber content of the PVAF group on the equivalent ultimate tensile stress and equivalent ultimate tensile strain.Figure 3
Equivalent ultimate tensile stress of different dosages.Figure 4
Equivalent ultimate tensile strain of different dosages.It can be seen from Figure3 that, except for the brittle failure specimen, the equivalent ultimate tensile stress and equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content are also large. When the fiber content is 2%, the average equivalent ultimate tensile stress is 3.3 MPa, and when the fiber content is 1.5%, the average equivalent ultimate tensile stress is 2.7 MPa. It can be seen from Figure 4 that the equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content is also large. When the fiber content is 2%, the average equivalent ultimate tensile strain is 3.1%, when the fiber content is 1.5%, the average equivalent ultimate tensile strain is 1.9%, and the specimen has no ductility when the fiber content is 1%.


### 3.1.3. Toughness Index

It can be seen from Figure5 that with the decrease of fiber volume content, the toughness index of PVAF decreases. When the fiber content is 2%, the average is (I5 = 4.78, I10 = 10.53, I20 = 22.65), when the fiber content is 1.5%, the average is (15 = 4.59, I10 = 9.23, I20 = 11.5), the fiber shows no ductility at 1% dosage.Figure 5
Toughness index of different dosages.


### 3.1.4. Compressive Strength

It can be seen from Figure6 that the fiber content has a certain effect on the compressive strength of cement-based composites. The compressive strength of the high fiber content is low. When the fiber volume content is 2%, the average compressive strength is 38.9 MPa. When the fiber volume content is 1.5%, the average compressive strength is 41.2 MPa, and when the fiber volume content is 1%, the average compressive strength is increased to 49.5 MPa. The main reason why the strength of cement-based composites decreases with the increase of fiber content is that the number and size of air bubbles in cement-based composites increase with the increase of fiber content.Figure 6
Compressive strength of different dosages.


## 3.1.1. Deflection Test

With the increase of fiber volume ratio, the deflection of the midspan section of the specimen gradually increases, indicating that the fiber volume ratio has a great influence on the ductility of cement-based composites. When the fiber volume ratio of PVAF specimens is less than 1%, the deflection value of the midspan section is very small, showing the characteristics of brittle failure. Except for the specimens with brittle failure, the ultimate load gradually increased with the increase of fiber content. For example, the average ultimate load of PVAF-1.5 group was 236 N, and that of PVAF-2 group increased to 258 N. For specimens with brittle failure, the distribution of ultimate load test values is uneven and the dispersion is large, which is due to the low fiber content, which is affected by the dispersion of cement-based tensile strength (Figure2).Figure 2
Effect of fiber volume ratio on deflection.


## 3.1.2. Equivalent Ultimate Tensile Stress and Equivalent Ultimate Tensile Strain

Figures3 and 4reflect the effect of the fiber content of the PVAF group on the equivalent ultimate tensile stress and equivalent ultimate tensile strain.Figure 3
Equivalent ultimate tensile stress of different dosages.Figure 4
Equivalent ultimate tensile strain of different dosages.It can be seen from Figure3 that, except for the brittle failure specimen, the equivalent ultimate tensile stress and equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content are also large. When the fiber content is 2%, the average equivalent ultimate tensile stress is 3.3 MPa, and when the fiber content is 1.5%, the average equivalent ultimate tensile stress is 2.7 MPa. It can be seen from Figure 4 that the equivalent ultimate tensile strain of the cement-based composite specimen with high fiber content is also large. When the fiber content is 2%, the average equivalent ultimate tensile strain is 3.1%, when the fiber content is 1.5%, the average equivalent ultimate tensile strain is 1.9%, and the specimen has no ductility when the fiber content is 1%.


## 3.1.3. Toughness Index

It can be seen from Figure5 that with the decrease of fiber volume content, the toughness index of PVAF decreases. When the fiber content is 2%, the average is (I5 = 4.78, I10 = 10.53, I20 = 22.65), when the fiber content is 1.5%, the average is (15 = 4.59, I10 = 9.23, I20 = 11.5), the fiber shows no ductility at 1% dosage.Figure 5
Toughness index of different dosages.


## 3.1.4. Compressive Strength

It can be seen from Figure6 that the fiber content has a certain effect on the compressive strength of cement-based composites. The compressive strength of the high fiber content is low. When the fiber volume content is 2%, the average compressive strength is 38.9 MPa. When the fiber volume content is 1.5%, the average compressive strength is 41.2 MPa, and when the fiber volume content is 1%, the average compressive strength is increased to 49.5 MPa. The main reason why the strength of cement-based composites decreases with the increase of fiber content is that the number and size of air bubbles in cement-based composites increase with the increase of fiber content.Figure 6
Compressive strength of different dosages.


## 3.2. Test Results and Analysis of the Continuous Structure of the Cement-Based Composite Bridge Deck

### 3.2.1. Load-Displacement Curve

The displacement curves of concrete beams recorded in the test under graded loading conditions are shown in Figure7. The displacement curves of concrete beams with three different bridge deck continuous materials are similar. The concrete beam using concrete as the bridge deck continuous construction material has a relatively obvious yield load (45.23 kN), and the concrete beam using cement mortar as the bridge deck continuous construction material has a yield load of 42.78 kN. However, no obvious yield load was measured for concrete beams using cement-based composite materials as the continuous construction material of the bridge deck.Figure 7
Load-displacement curve of concrete beam. (a) C. (b) M. (c) E.
(a)(b)(c)


### 3.2.2. Cracking Load and Ultimate Load

The cracking loads and ultimate loads of concrete beams with three different bridge deck continuous materials are shown in Table6.Table 6
Cracking load and ultimate load.
No.Bridge deck continuous materialCracking load (kN)Ultimate load (kN)C-lConcrete2170.3M-lCement mortar18.571.2E-lCementitious composites1774.9It can be seen from Table6 that the cracking load and ultimate load of concrete beams with different bridge deck continuous materials are not much different. The concrete beam of the bridge deck continuous construction material and the concrete beam using the cement-based composite soil as the bridge deck continuous construction material have the largest ultimate load.


### 3.2.3. Crack Development and Failure Mode

According to the hierarchical loading, the crack development process of the beam member in the midspan pure bending section is described. The failure mode of the C-1 beam whose the bridge deck continuous structure material is ordinary concrete is that the tensile steel bars yield, the concrete in the compression area is crushed, and transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joint of the two spans, and the bridge deck continuous structure is under tension. The failure mode of the M-1 beam with ordinary cement mortar as the continuous structure material of the bridge deck is that the tensile steel bars yield, the concrete in the compression zone is crushed, the continuous structure of the bridge deck is partially peeled off from the top surface of the beam, and the continuous structure of the bridge deck at the joint between the two spans. Irregular transverse cracks are generated on the upper surface, and the continuous structure of the bridge deck is damaged in tension. The failure mode of the E-1 beam whose bridge deck continuous structural material is cement-based composite material is that the tensile steel yields, the concrete in the compression zone is crushed, and many small transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joints of the two spans. The continuous structure of the bridge deck is damaged in tension.Table 7
Overview of continuous top cracks on different bridge decks at joints.
No.Bridge deck continuous materialNumber of cracksAverage crack width/mmAverage crack spacing/mmC-1Concrete20.359-M-1Cement mortar60.156-E-1Cementitious composites80.0971.69


### 3.2.4. Continuous Structural Fracture Model of Bridge Deck

(a)
Development of cracks on the top surface of the continuous structure of the bridge deck at the joint.Under the same load condition (70 kN), the concrete beams of three different bridge deck continuous materials all produce a wide main transverse crack on the top surface of the bridge deck continuous structure at the joint. The continuous structure of the concrete bridge deck does not produce other secondary cracks, the continuous structure of the cement mortar bridge deck produces a number of secondary cracks with irregular directions and different directions, and the continuous structure of the cement-based composite bridge deck produces multiple cracks in the same direction around the main crack. The widths of the main cracks on the top surface of the bridge deck continuous structure at the joints of three different bridge deck continuous materials under the same load condition (70 kN) were measured, and the results are shown in Table7.Since the M-l beam, that is, the continuous multiple cracks of the cement mortar bridge deck, develop in different directions, the crack spacing is not counted. It can be seen that the E-1 beam, that is, the continuous joint of the cement-based composite bridge deck, has the largest number of tensile cracks, but no matter the width of the main crack or the average crack width is the smallest. In line with the characteristics of cement-based composite materials, the tensile crack width is small, the number of strips is large, and the direction is consistent. It shows that the use of cement-based composite material as the continuous material of the bridge deck can effectively improve the cracking mode at the joints. The crack width is smaller. It is not easy to distinguish with the naked eye, and the impact on the structural integrity is smaller.(b)
Bridge deck continuous structural strain.In order to obtain the continuous strain distribution of the bridge deck at the joints, according to the “Code for Design of Concrete Structures” (GB50010-2010), the continuous top and side surfaces of the bridge deck at the joints are pasted with strain sensors to measure the continuous strain development of the bridge deck during loading. The results are shown in Figure8. In the initial stage of loading, the bending moment is small, and the fiber strain along the beam height is also small and varies linearly. The working condition of the beam is similar to that of a homogeneous elastic beam; when the bending moment increases to the cracking moment Mcr, the fibers at the edge of the tension zone. When the strain reaches the ultimate tensile strain, the section cracks immediately and the neutral axis moves down. The measured strain indicates that the deformation of the steel bar and the concrete is coordinated.Figure 8
Continuous top surface strain of bridge deck at joints.When using concrete as the continuous construction material of the bridge deck and using cement mortar as the continuous construction material of the bridge deck, the continuous maximum tensile strain of the bridge deck at the joint is smaller, which are 0.726% and 1.091%, respectively, and the cement-based composite material is used as the bridge deck. The maximum tensile strain that the continuous construction material can withstand before cracking is 9.5434%, which is 13.13 times and 8.74 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively. The experimental results show that the continuous cement-based composite bridge deck can effectively improve the crack resistance of the continuous structure of the bridge deck, and can withstand greater tensile strain before cracking, that is, it can withstand greater variable loads. When concrete is used as the continuous construction material of the bridge deck and cement mortar is used as the continuous construction material of the bridge deck, the loading grades when the bridge deck is continuously cracked at the joints are 15.41 kN and 17.38 kN, respectively. The loading level of cement-based composite material as the continuous bridge deck structural material is 38.23 kN, which is 2.48 times and 2.20 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively (Figure8). It shows that the use of cement-based composite materials as the bridge deck continuous structural material can resist the development of cracks better than the concrete bridge deck continuous structure and the cement mortar bridge deck continuous structural material when receiving the same variable load.


## 3.2.1. Load-Displacement Curve

The displacement curves of concrete beams recorded in the test under graded loading conditions are shown in Figure7. The displacement curves of concrete beams with three different bridge deck continuous materials are similar. The concrete beam using concrete as the bridge deck continuous construction material has a relatively obvious yield load (45.23 kN), and the concrete beam using cement mortar as the bridge deck continuous construction material has a yield load of 42.78 kN. However, no obvious yield load was measured for concrete beams using cement-based composite materials as the continuous construction material of the bridge deck.Figure 7
Load-displacement curve of concrete beam. (a) C. (b) M. (c) E.
(a)(b)(c)


## 3.2.2. Cracking Load and Ultimate Load

The cracking loads and ultimate loads of concrete beams with three different bridge deck continuous materials are shown in Table6.Table 6
Cracking load and ultimate load.
No.Bridge deck continuous materialCracking load (kN)Ultimate load (kN)C-lConcrete2170.3M-lCement mortar18.571.2E-lCementitious composites1774.9It can be seen from Table6 that the cracking load and ultimate load of concrete beams with different bridge deck continuous materials are not much different. The concrete beam of the bridge deck continuous construction material and the concrete beam using the cement-based composite soil as the bridge deck continuous construction material have the largest ultimate load.


## 3.2.3. Crack Development and Failure Mode

According to the hierarchical loading, the crack development process of the beam member in the midspan pure bending section is described. The failure mode of the C-1 beam whose the bridge deck continuous structure material is ordinary concrete is that the tensile steel bars yield, the concrete in the compression area is crushed, and transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joint of the two spans, and the bridge deck continuous structure is under tension. The failure mode of the M-1 beam with ordinary cement mortar as the continuous structure material of the bridge deck is that the tensile steel bars yield, the concrete in the compression zone is crushed, the continuous structure of the bridge deck is partially peeled off from the top surface of the beam, and the continuous structure of the bridge deck at the joint between the two spans. Irregular transverse cracks are generated on the upper surface, and the continuous structure of the bridge deck is damaged in tension. The failure mode of the E-1 beam whose bridge deck continuous structural material is cement-based composite material is that the tensile steel yields, the concrete in the compression zone is crushed, and many small transverse cracks are formed on the upper surface of the bridge deck continuous structure at the joints of the two spans. The continuous structure of the bridge deck is damaged in tension.Table 7
Overview of continuous top cracks on different bridge decks at joints.
No.Bridge deck continuous materialNumber of cracksAverage crack width/mmAverage crack spacing/mmC-1Concrete20.359-M-1Cement mortar60.156-E-1Cementitious composites80.0971.69


## 3.2.4. Continuous Structural Fracture Model of Bridge Deck

(a)
Development of cracks on the top surface of the continuous structure of the bridge deck at the joint.Under the same load condition (70 kN), the concrete beams of three different bridge deck continuous materials all produce a wide main transverse crack on the top surface of the bridge deck continuous structure at the joint. The continuous structure of the concrete bridge deck does not produce other secondary cracks, the continuous structure of the cement mortar bridge deck produces a number of secondary cracks with irregular directions and different directions, and the continuous structure of the cement-based composite bridge deck produces multiple cracks in the same direction around the main crack. The widths of the main cracks on the top surface of the bridge deck continuous structure at the joints of three different bridge deck continuous materials under the same load condition (70 kN) were measured, and the results are shown in Table7.Since the M-l beam, that is, the continuous multiple cracks of the cement mortar bridge deck, develop in different directions, the crack spacing is not counted. It can be seen that the E-1 beam, that is, the continuous joint of the cement-based composite bridge deck, has the largest number of tensile cracks, but no matter the width of the main crack or the average crack width is the smallest. In line with the characteristics of cement-based composite materials, the tensile crack width is small, the number of strips is large, and the direction is consistent. It shows that the use of cement-based composite material as the continuous material of the bridge deck can effectively improve the cracking mode at the joints. The crack width is smaller. It is not easy to distinguish with the naked eye, and the impact on the structural integrity is smaller.(b)
Bridge deck continuous structural strain.In order to obtain the continuous strain distribution of the bridge deck at the joints, according to the “Code for Design of Concrete Structures” (GB50010-2010), the continuous top and side surfaces of the bridge deck at the joints are pasted with strain sensors to measure the continuous strain development of the bridge deck during loading. The results are shown in Figure8. In the initial stage of loading, the bending moment is small, and the fiber strain along the beam height is also small and varies linearly. The working condition of the beam is similar to that of a homogeneous elastic beam; when the bending moment increases to the cracking moment Mcr, the fibers at the edge of the tension zone. When the strain reaches the ultimate tensile strain, the section cracks immediately and the neutral axis moves down. The measured strain indicates that the deformation of the steel bar and the concrete is coordinated.Figure 8
Continuous top surface strain of bridge deck at joints.When using concrete as the continuous construction material of the bridge deck and using cement mortar as the continuous construction material of the bridge deck, the continuous maximum tensile strain of the bridge deck at the joint is smaller, which are 0.726% and 1.091%, respectively, and the cement-based composite material is used as the bridge deck. The maximum tensile strain that the continuous construction material can withstand before cracking is 9.5434%, which is 13.13 times and 8.74 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively. The experimental results show that the continuous cement-based composite bridge deck can effectively improve the crack resistance of the continuous structure of the bridge deck, and can withstand greater tensile strain before cracking, that is, it can withstand greater variable loads. When concrete is used as the continuous construction material of the bridge deck and cement mortar is used as the continuous construction material of the bridge deck, the loading grades when the bridge deck is continuously cracked at the joints are 15.41 kN and 17.38 kN, respectively. The loading level of cement-based composite material as the continuous bridge deck structural material is 38.23 kN, which is 2.48 times and 2.20 times that of the continuous concrete bridge deck and the continuous cement mortar bridge deck, respectively (Figure8). It shows that the use of cement-based composite materials as the bridge deck continuous structural material can resist the development of cracks better than the concrete bridge deck continuous structure and the cement mortar bridge deck continuous structural material when receiving the same variable load.


## 4. Conclusion

(1)
The ductility of cement-based composites is significantly affected by the fiber content. When the volume ratio is less than or equal to the critical volume ratio, the ductility increases gradually with the increase of fiber content.(2)
By designing the laboratory model of the bridge deck continuous structure, concrete, cement mortar, and cement-based composite materials were used as the bridge deck continuous structure materials, and four-point bending tests were carried out on each group of beam members. Through the analysis of the test results, it is shown that the use of cement-based composite materials as the continuous material of the bridge deck can delay the stress and strain concentration phenomenon, that is, under the same load conditions, the continuous crack resistance of the cement-based composite bridge deck is better than that of the cement mortar bridge deck. Concrete bridge deck continuous structure is more resistant to the development of cracks than cement mortar bridge deck continuous structure materials.



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*Source: 7441144-2022-05-12.xml*