Full-Scale Model Experimental Study of the Flexural Behavior of Hollow Slabs Strengthened by UHPC

The hollow slabs strengthened by ultrahigh performance concrete (UHPC) composite beam show many advantages over tra-ditional reinforcement methods. In this paper, full-scale model load tests were carried out on an nonstrengthened prestressed concrete hollow slab and an UHPC-strengthened prestressed concrete hollow slab, comparing the load deﬂection, crack width, bearing capacity, deformation resistance, and self-vibration frequency of the two. Static loading experimental results indicate that UHPC enhances the overall performance of prestressed concrete hollow slabs by decreasing deﬂection and crack width and improving bearing capacity. The strengthening eﬀects of UHPC on a prestressed concrete hollow slab’s ﬂexural behavior are also discussed, such as deﬂection, crack width, bearing capacity, deformation resistance, self-vibration frequency, ﬂexural behavior, and cracking load. Deﬂection and crack width under a load of 800kN decreased by 45.8% and 56.3%, respectively, and the initial self-vibration frequency, ultimate bearing capacity, and cracking load increased 19.2%, 21.4%, and 50%, respectively. The plane assumption can be made generally throughout the overall test process while using UHPC strengthening, which signiﬁcantly constrains crack width and improves stiﬀness and deformation capacity. The UHPC layer and the prestressed concrete hollow slab were connected by shear studs to produce a good composite action between them, and the bending performance and bearing capacity of the whole structure were clearly improved. In addition to experiments, a validated numerical model is developed to verify the ﬂexural performance of hollow slab strengthened by UHPC.


Introduction
Prefabricated reinforced concrete or prestressed concrete hollow slab is widely used in the construction of expressways and urban bridges, because it offers the advantages of standardized design, integrated production, assembly construction, low cost, low building height, overall beauty, clear structure force, and so on. Owing to their lower design load standards, the flexural behavior of bridges built in the 20th century reserve reduces driving safety and affects the normal operation of highway and urban road traffic. Maintenance and rehabilitation of highway bridge decks is a continual challenge for bridge owners and transportation agencies. Transportation agencies must extend the service lives of existing bridge decks using limited funds, with limited time needed for replacement or major rehabilitation. e deterioration associated with bridge diseases is progressive and irreversible. Resilient and durable repair and rehabilitation solutions are urgently needed for these aging reinforced concrete bridge decks.
Common reinforcement methods include active strengthening and passive strengthening. Passive strengthening includes pasting steel plate [1], carbon fiber board, and the like, but passive strengthening material can bear only the internal forces caused by an active load. Active strengthening, by contrast, requires that the sticking strengthening material be prestressed to form a prestressed reinforcement system, which can improve the utilization rate of strengthening materials-a method widely used in bridge strengthening. e most common prestressed reinforcement materials include prestressed steel strand, steel wire, steel wire rope, and fiber-reinforced polymer (FRP). FRP, which has the advantages of heat resistance and corrosion resistance, has been widely used to reinforce concrete bridges, but after strengthening, the beam's ductility decreases, and the beam often exhibits brittle failure [2][3][4][5][6]. Prestressed steel strand and steel wire have good ductility, are cost-efficient, and bond well with concrete, advantages that make them suitable for use in prestressed concrete bridges [7][8][9][10]. Hui Peng et al. [11] pointed out that externally bonded prestressed FRP plates and near surface-mounted (NSM) FRP strips represent two important techniques for strengthening concrete structures with FRP materials. However, they have considerable limitations. Prestressed NSM FRP strips combine the advantages of these two strengthening methods and are thus an extremely attractive approach to using FRP to retrofitting concrete structures. Although prestressed FRP strips can be anchored by epoxy through near-surface mounting, the failure modes of beams strengthened through this technique could include debonding of the epoxy-concrete interface and delamination of the concrete cover. ese failure modes can be avoided by extending the bond length of FRP strips or using U-wraps of carbon fiber reinforced polymer (CFRP) sheets. Al-osta M A et al. [12] studied the reinforcement effect of various interface treatment methods and reinforcement positions on reinforced concrete beams, finding a higher interface bonding performance for the sandblasting interface treatment method, with the U-shaped hoop reinforcement the most suitable reinforcement method for not only improving the bending capacity by 89%, but also reducing members' midspan deflection by 20% compared with that of unreinforced members'.
Ultrahigh performance concrete (UHPC) is a new type of cement-based composite material that has ultrahigh durability and mechanical properties. UHPC material should be used in places chosen to reflect its ultrahigh strength, ductility, and durability, consistent with the basic principles of UHPC reinforcement [13]. Bridge structure systems differ, whether in structure environment, main load form, or reinforcement demands. Beam and slab structures are subjected mainly to bending and shear forces, and their bearing capacity is often accompanied by midspan deflection along with other diseases. eir reinforcement thus focuses on improving their bending bearing capacity and restraining their further deflection [14]. Muhammad Safdar et al. [15,16] used UHPFRC material to replace the top and bottom parts of reinforced concrete beams, effectively enhancing NC beams' bending capacity. Because ultimate bearing capacity is still determined by longitudinal reinforcement at the bottom of a beam, H. M. Tanarslan [17] used a thin precast UHPC layer to strengthen reinforced concrete beams. Although affixing UHPC thin layer at the bottom of a beam can effectively enhance the ultimate bearing capacity of strengthened members, their deflection is also difficult to control. Hor Yin et al. [18] used UHPC material to strengthen a reinforced concrete slab and found that a reinforced concrete slab strengthened by UHPC at its bottom could effectively inhibit development of cracks in strengthened components during bending shear, effectively improving the structure's bending bearing capacity. Zmetra et al. [19] proposed a novel repair method for steel girder ends with corrosion damage, using UHPC to encase the corrosion. For this repair, UHPC panels were attached to girders with shear studs welded to the web and flange surrounding the corroded area, allowing shear and bearing forces to transfer from the girder to the UHPC panels. e UHPC panels provided a new load path, increasing the bearing capacity of the girder. Gaston Doiron et al. [20] suggested that the UHPC's durability can contribute to other types of bridge repairs and provided a brief overview of four North American pier repair/retrofit projects.
Xu et al. [21] used UHPC mixed with polymer fiber to strengthen plain concrete beams by using a cast-in-place plain UHPC layer to reinforce their underside, greatly improving their bending capacity while inhibiting the development of cracks. Wu and Lin [22] evaluated the influence of U-shaped UHPC reinforcement over the failure mode, fracture strength, ultimate bearing capacity, and deformation of RC beams. Qi [23] carried out UHPC reinforcement tests on three full-scale box girder roof local models and found that, under positive and negative bending moments, reinforcement could effectively improve the overall stiffness of the specimen, which exhibited good ductility and obvious deformation before failure. Liu et al. [24], who proposed pouring a layer of UHPC atop the bridge deck, carried out theoretical and experimental research and found that the lateral strain and vertical deflection of the bridge deck were reduced by about 60%, with the stress also being reduced under the action of vehicle load, and that the UHPC layer could effectively improve the stress of the bridge deck. Deng and Zhang [25] verified that the bottom of a reinforced concrete beam strengthened with a thin layer of UHPC exhibited greatly improved bending capacity and reduced midspan deflection. Yang and Zhi [26] studied the flexural behavior of UHPC beams and found that UHPC showed good compression performance, good ductility, and ultimate deformation capacity. Zhang et al. [27] conducted an experimental study of the flexural capacity of RC beams and found that newly fabricated integral RC beams had the same flexural capacity as cast-in-place RC beams, with good integrity and ductility. Zhu et al. [28] analyzed the different failure modes of UHPC-RC composite members under bending deformation, conducting static load tests of RC beams strengthened with UHPC, and found that UHPC can be used to improve the flexural strength of RC beams or slabs. In addition, cost analysis against other strengthening technologies (such as CFRP) is proposed by them. Finally, they suggested some examples to guide applications of reinforced concrete structure UHPC strengthening technology.
UHPC-strengthened hollow slabs have been used in engineering, but the research on the bending properties of hollow slabs, such as the strengthening effect of UHPC and improvements to bending capacity, has involved only solid beams and scale model beams, which cannot effectively guide the design and application of UHPC-strengthened hollow slabs. Accordingly, full-scale model tests of bending performance of UHPC-reinforced hollow slab bridges would 2 Advances in Materials Science and Engineering be theoretically significant while having practical value for engineering. e purpose of this paper is to study the influence of UHPC strengthening on the bearing capacity, crack inhibition, bending stiffness, flexural behavior, natural frequency, and overall consistency of prestressed concrete hollow slabs through full-scale model testing while theoretically calculating cracking load and bearing capacity, to prove that UHPC has a notable strengthening effect on prestressed concrete hollow slabs. In addition, a numerical analysis is carried out to further analyze the flexural performance of hollow slab strengthened by UHPC.

Axial Tensile Test.
For axial tensile test, the embedded axial tensile specimen, UHPC with the size of 100mm × 100mm × 500mm, is selected. 12.9-grade ∅16 screw is used for embedded rod. By setting the screw to full length, the UHPC is indirectly stretched by the screw. When tensile microcracks appear in UHPC, the axial force gradually transfers to the screw, which can effectively avoid the sudden release of elastic potential energy. e axial force of UHPC can be obtained indirectly by measuring the tensile force of the testing machine and the axial force of the screw (measured by the strain gauge attached to the screw), so as to determine the relationship between the nominal stress and the nominal strain in the whole process. Before the test, three screws of the same batch are stretched (for calibration). e tensile strain should be controlled at 4000 με, and the screw load F L -strain ε L corresponding relation is measured; the actual length and width values of the two sections of the axial tensile section of the specimen are measured to calculate the average value a of the section area; the strain gauge is pasted longitudinally at the center of four sides of the specimen, the strain tester is connected, and the steel bracket and dial indicator are arranged at the two opposite tension sections of the face axis; the pretension force is set to 10 kN, 15 kN, and 20 kN. After the pretension force is loaded to the value, the alignment of the specimen is checked. During the formal loading, the whole process is controlled by displacement. Before 60 kN, the speed is 0.005mm/s. After 60 kN, the tensile speed is 0.002mm/s until the screw is broken. Based on the measured screw strain ε L , the screw axial force F L can be converted, and then the UHPC axial force F U can be calculated from the difference between the tensile force F of the testing machine and the screw axial force F L , from which the nominal stress σ and nominal strain ε relationship can be calculated in the whole process of UHPC tension.

Sample of Hollow Slab.
ree prestressed concrete hollow slabs used in the test were prefabricated in a factory. e material list is shown in Table 1. e detailed dimensions of the tested prestressed concrete hollow slab are shown in Figure 1. e three hollow slabs are denoted as S1, S2, and S3: Specimen S1 is not strengthened, Specimen S2 has UHPC strengthening at the top, and Specimen S3 has UHPC strengthening at the top and CFRP strengthening at the bottom. e prestressed concrete hollow slab and the UHPC panel were connected by the shear studs 19 mm in diameter and 150 mm long. e geometric dimensions, reinforcement, and shear stud arrangements of Specimens S2 and S3 are shown in Figure 2. Due to the placement of steel bars and arrangement of vertical and horizontal steel mesh in the inner layer, the thickness of one layer of steel mesh is 10 mm, plus a 15 mm thick protective layer, and the thickness of UHPC is 50 mm, with two layers of steel mesh and two protective layers. e shear stud length, UHPC hierarchical structure, and sticking position of CFRP are shown in Figure 3.

Four-Point Bending Test.
e four-point bending tests of Specimens S1, S2, and S3 were carried out on the testing machine, with an MTS hydraulic loading system being used for load. e positions of strain gauge measuring points on the specimen and the arrangement of electronic displacement meters are shown in Figure 4. e strain gauges were glued on the concrete. During the test, the deflection and strain were monitored in midspan, at the loading point, and near the support. e length of pure bending deformation is 3 m in the middle section of the specimen, and the shear span ratio is 5.3. e four-point bending tests are shown in Figure 5.
Before loading, the specimen must be preloaded to confirm that the measuring device and loading device are working normally. In the test, graded loading is adopted, with the force controlled first and the loading carried out at 0.2 kN/s. Before the loading reaches the estimated cracking load of the specimen, it is changed to graded loading, with an increment not more than 10 kN. When the specimen cracks, the increment of graded loading is changed to 5% of the predicted ultimate load. When approaching the ultimate load, the force control loading is changed to displacement control loading, with loading proceeding gradually until the specimen is damaged. Table 2, the tensile test results of UHPC are shown in the figure, and the tensile strength is 7.4 MPa. As shown in Figure 6, uniaxial tensile of UHPC material has strain hardening characteristics, and the stress-strain curve of the whole uniaxial tensile process can be simplified into three stages: (1) linear elastic stage: the initial crack strain ε cr is regarded as the termination mark there, and it is obtained from the ratio of tensile strength f tk to elastic modulus E c ; (2) the formation stage of matrix material cracking and dispersive microcracks in UHPC, marked by yield ultimate tensile strain ε tu ; (3) formation and development of macro main fracture and the decline stage of steel fiber being pulled out, marked by ultimate tensile strain ε u .

Load-Deflection
Response. Four-point bending tests of S1, S2, and S3 were carried out. A comparison of the maximum deflection of Specimens S1, S2, and S3 in the Advances in Materials Science and Engineering middle span during testing is shown in Figure 7. e loaddeflection relationship in the midspan span of three samples S1, S2, and S3 during the test is shown in Figure 8. e failure phenomena of S1, S2, and S3 at the end of the test are shown in Figure 9. Figures 7 and 8(a) show obviously smaller midspan deflections of Specimens S2 and S3 than of Specimen S1 at the same load level, a phenomenon more obvious with increased load, which indicates that the midspan deflection of prestressed concrete hollow slabs can be controlled by UHPC strengthening. Under 800 kN load, the midspan deflection of UHPC-strengthened prestressed concrete hollow slab is about 45.8% lower than that of without strengthened prestressed concrete hollow slab. e stiffness of UHPC-strengthened prestressed concrete hollow slab is greater than that without strengthened prestressed concrete hollow slab, indicating that UHPC strengthening can effectively improve the stiffness of prestressed concrete hollow   Cheese head studs for stud working e maximum deflection of Specimens S2 and S3 exceeded that of Specimen S1 by 119% and 81.9%, respectively, with the ductility of Specimen S2 exceeding that of Specimen S3, indicating that the strengthening effect of CFRP mainly improves the tensile capacity of steel bars. From Figure 7(a), it can be seen that the ultimate bearing capacities of Specimens S1, S2, and S3 were 838 kN, 1,017 kN, and 1,060 kN, respectively. Compared with S1, the increments of S2 and S3 were 21.4% and 26.5%, respectively.
From the load-deflection curve of the prestressed concrete hollow slab (Figure 8(a)), it can be seen that, from the beginning of loading to failure, the prestressed concrete hollow slab successively experiences three different stress processes: the linear elastic stage, crack stage, and failure stage.

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In the linear elastic stage, from the beginning of loading to the initial cracking of the prestressed concrete hollow slab, the ratio of loading to deformation increases linearly. At the beginning, there is no cracking in the prestressed concrete hollow slab. As load increases, discontinuous cracks appear in its interior. According to data measured by the strain gauge affixed to the surface of the prestressed concrete hollow slab, the neutral axis moves up continuously. At this stage, the deformation of the prestressed concrete hollow slab is caused mainly by the elastic deformation of aggregate and cement crystal in concrete, whereas the viscous flow of cement colloid and the change of initial crack have little influence, and the load-deflection curve is approximately linear. When the tensile strain of the tensile edge reaches the ultimate tensile strain of concrete (ε t � ε ut ), the prestressed concrete hollow slab is in the critical state of cracking (I a state); the load at this time is called "cracking load." en, the specimen enters the cracking stage. In this stage, with the increase of load, the concrete in the tensile stress zone essentially does not take effect, with the tensile stress borne mainly by the longitudinal tensile steel bar, which does not yield. e relationship load-midspan deflection is a curve, gently sloped until the tangent of the curve is horizontal, and the tensile steel bar yields.
When the tensile reinforcement yields, the tensile stress remains constant, and the bearing capacity of the UHPCstrengthened prestressed concrete hollow slab increases very slowly. During this period, the action point of the combined pressure on the concrete in the compression zone moves outward, and the internal force arm and bending moment increase. e compressive strain of the concrete at the edge of the compression zone reaches the ultimate compressive strain, the concrete is crushed, and the members enter the failure stage. e load-deflection curve is close to being horizontal. Figure 8(b) shows that the cracking loads of Specimens S1, S2, and S3 are 200 kN, 300 kN, and 300 kN, respectively. e increasing amplitudes of Specimens S2 and S3 are identical, at 50%. UHPC strengthening can improve not only the bearing capacity of a prestressed concrete hollow slab, but also its plasticity, so that UHPC can be used to strengthen a prestressed concrete hollow slab.
According to the analysis in Figure 9, after the failures of Specimens S2 and S3, no extrusion was evident on the surface of UHPC panel, whereas the top concrete surface of Specimen S1 was damaged. Specimens S1, S2, and S3 all showed the failure of bending capacity: the tensile steel bars entered the yield state, and the concrete in the compression zone was crushed.

Section Strain before Cracking.
During the test, the strain at the middle sections of Specimens S1, S2, and S3 along the section height was measured before the concrete tensile cracking occurred, as shown in Figure 10. From Figure 10, the distribution of strain along the section height of the three groups of specimens satisfies the plane section assumption, with the measured value of neutral axis height being essentially consistent with the theoretical value (see    Table 3 neutral axis height), but the neutral axis of sample S1 is lower than that in samples S2 and S3. e neutral axes of samples S2 and S3 are quite close in Figure 10(b), with notably good bonding between the UHPC parts of samples S2 and S3 and the prestressed concrete hollow slab, showing good composite effect, and exceptionally good performance of the composite part.

Comparison of Crack Width.
e relationship between the maximum crack and load of Specimens S1, S2, and S3 is shown in Figure 11. During testing, when studying the cracking process of Specimen S2, the strain gauge data of Specimen S2 changed when load F � 300 kN, with cracks seen in the front corner of the sample bottom plate, 0.02 mm wide. When load F � 320 kN, the cracks   Advances in Materials Science and Engineering expanded rapidly, with 2∼3 through cracks on the bottom surface that then expanded into a net shape, with a maximum crack on the bottom surface of 0.13 mm and a maximum crack on the side of 0.05 mm. When load F � 500 kN, some cracks penetrated the roof but had not extended into UHPC, in the compression zone; obvious crack openings occurred but did not extend to the UHPC layer. When the load reached the limit load, the strain gauge between the UHPC and the concrete was damaged, and a certain strain deviation was seen. e cracking processes of Specimens S1 and S3 are like those of Specimen S2, but the cracking load of Specimen S1 is smaller than that of Specimen S2. Figure 11 shows that the crack inhibition of Specimens S2 and S3 is improved to a certain extent and that crack propagation is slow. CFRP has little effect on the initial cracking load but can prevent crack

Effect of Crack on Natural
Frequency. UHPC strengthening has a certain influence on the natural frequency of prestressed concrete hollow slabs, as shown by the test results in Figure 12. e initial natural frequency of Specimen S3 increased 19.2%, with little change in natural frequency before the maximum crack width reached 0.2 mm. However, when the maximum crack width reached 0.2 mm, the natural frequency changed greatly.

Ultimate Bearing Capacity.
Taking Specimen S1 as an example to deduce the formula for calculating bearing capacity, we assume that it conforms to the plane section assumption before reaching ultimate load, the concrete in tensile zone does not bear tensile force, the relationship curve of compressive stress and compressive strain of concrete is as shown in Figure 13(a), the stress and strain curve of tensile reinforcement is as shown in Figure 13(b), and the ultimate tensile strain is 0.01. By solving the equilibrium equation, the bearing capacity of Specimen S1 can be calculated.
Because the compression area of the section is reinforced based only on the structural requirements, the prestressed reinforcement in the compression area and the influence of ordinary reinforcement are not considered in the calculation. A s � 1, 357.2 mm 2 , a s � 50 mm, A p � 1, 120 mm 2 , and a sp � 85 mm. A s is 12 longitudinal tensile bars' area, a s is the distance between the steel bar and the concrete edge of the prestressed concrete hollow slab, A p is two prestressed steel strands' area, and a s is the distance between prestressed steel strands and the concrete edge of the prestressed concrete hollow slab. When Specimen S1 is damaged, the steel bars and prestressed steel strands in the tensile stress zone reach the yield strength, and the environment is class I.
According to the principle of equal area and equal moment of inertia, Specimen S1's normal section in the midspan is converted into an equivalent I-section, as shown in Figure 14: e distribution of bending internal force on the normal section is shown in Figure 15, where x is the height of the compression zone on the section x < h f ′ .
Based on the static equilibrium equation F x � 0, it can be shown that where c 0 is the structural importance coefficient, taking c 0 � 1; M d is the ultimate bending moment; and F is the ultimate force.

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From equations (1) and (2), it follows that by which the bearing capacities of samples S1, S2, and S3 can be calculated. Comparisons of calculation and test results of ultimate bearing capacity are shown in Table 4: According to Table 4, compared with Specimen S1, the ultimate bearing capacities of Specimens S2 and S3 are increased 18.5% and 33.9%, respectively, with the ultimate bearing capacity measured in the test being larger than that calculated theoretically when not considering the effect of structural reinforcement in the compression area and the tensile capacity of concrete in the tensile stress area; as a result, the theoretically calculated bearing capacity is smaller than the test result. However, the trend of theoretical calculation is consistent with the experimental results, showing that UHPC strengthening can improve the bearing capacity, bending stiffness, and deformation resistance of a prestressed concrete hollow slab, helping strengthen it. At the  same time, the stiffness and deformation capacity of Specimens S2 and S3 are significantly improved.

Calculation of Cracking Load.
Calculation of cracking load is based on stress distribution diagrams, as shown in Figure 16. e cracking bending moment is M cr , X cr is the height of compression zone when the crack is about to appear, ε tu is the ultimate tensile strain of concrete, E t ′ is the tensile elastic modulus of concrete, and E t ′ � 0.5E c , where E c is the compressive elastic modulus of concrete. F x � 0, it is obtained that after which where α E � (E s /E c ) is the ratio of the elastic modulus of reinforcement to the elastic modulus of concrete, and α P � (E p /E c ) is the ratio of the elastic modulus of steel strand to the elastic modulus of concrete. e neutral axis position of Specimens S1, S2, and S3 and cracking load o can be calculated by formulas (5) and (6), as shown in Table 3: Because the carbon fiber is only 0.167 mm, its contribution to section stiffness is not considered. At the same time, the section roof and webs are reinforced only according to the structural requirements, with this part of the steel bar's contribution to the section stiffness not considered for the converted section area and the moment of inertia. e calculation results show that the section stiffness of a UHPC-strengthened prestressed concrete hollow slab is better than that in unreinforced specimens, with cracking load being increased 58.9%.

The Finite Element Analysis
e numerical analysis was conducted using ABAQUS to better understand the flexural behaviour of hollow slab strengthened by UHPC. e numerical model was established and validated on the basis of test results previously presented.

e Numerical
Calculation. e C45 concrete was modeled using element C3D8R and plastic damage model. Stress-strain curve of concrete under compression is shown in Figure 13(a). e UHPC was modeled using element C3D8R. With respect to the constitutive law of UHPC, the stress-strain relationship in tension and compression had been chosen according to experiment, as shown in Table 5.
ere were prestressed steel strand and the steel bars using element T3D2. e carbon fiber was modeled using S4R shell element, and the tensile strength is 3500 MPa. e stressstrain relationship of the prestressed steel and the carbon fiber is shown in Figure 17, and the stress-strain curve of steel bar is shown in Figure 13(b). e finite element model of hollow slab strengthened by UHPC is shown in Figure 18. e reinforcement mesh is built into concrete using embedded commands, and the equivalent lowering temperature method was studied to simulate prestress. For the calculation model, the slip of UHPC and C45 concrete is not considered, the constraint is binding, the displacement loading is adopted, and the calculation shows that the prestress of 1395 MPa corresponds to a temperature of − 596°C. e failure mode is shown in Figure 19. With the increase of load, the mid-span deflection gradually increases. e tensile concrete does not bear the load. Finally, the tensile reinforcement in the pure bending section of UHPC-reinforced hollow slab reaches the yield strength, the steel strand reaches the tensile strength, and the compressed concrete is crushed. e calculation model is at the ultimate flexural bearing capacity.

Analysis of Calculation Results.
e comparison between the load-mid-span deflection results obtained from the numerical analysis and experiment test is shown in Figure 20, and the bearing capacity of hollow slab strengthened by UHPC obtained through numerical calculation, theoretical calculation, and test is shown in Table 6.
From Figure 20, it can be seen that, in the elastic stage, the numerical results are very consistent with the test results. After cracking, the specimen enters the elastic-plastic stage.
ere is a certain error between the numerical results and the test results, and the discreteness of concrete after cracking has a great influence. e difference between the finite element calculation results and the test results is small, and ABAQUS finite element simulation is relatively reliable. Table 6 shows that the error between the numerical value of bearing capacity and the formula calculation value is within 3%, and the error between the numerical value of bearing capacity and the test value is within 8%, indicating that the theoretical calculation results are in good agreement with the test results. e coincidence of bearing capacity values shows that the scheme of strengthening hollow slab with shear studs connecting UHPC and       Advances in Materials Science and Engineering 13 prestressed concrete hollow slab is completely feasible. e numerical simulation are reliable and accurate.

Conclusions
is paper introduced a method of using UHPC to strengthen a prestressed concrete hollow slab. Full-scale model load tests without strengthened prestressed concrete hollow slab, UHPC-strengthened hollow slab, and UHPCand CFRP-strengthened hollow slab were carried out, with the following notable results: (1) In four-point bending full-scale model tests of Specimens S1, S2, and S3, two strengthening methods (UHPC strengthening and UHPC plus

Data Availability
All the data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.