Development of Reinforced Concrete Slab Bridge System for Immediate Traffic Opening During Curing Period of Cement Concrete Pavement

Abstract

A temporary traffic bridge system (TTBS) is proposed for immediate traffic opening during cast-in-place cement concrete pavement construction in high-traffic urban areas. The basic behaviors such as strains, stresses, and deflections of the TTBS fabricated using reinforced concrete slabs were analyzed numerically and verified through experiments. The concept of the TTBS was explained first, and a detailed description of its components was provided. Afterwards, a TTBS using reinforced concrete slabs was designed and a numerical analysis model for it was created. Using the numerical analysis model of the TTBS, the basic behaviors such as stresses and deflections of the slabs were analyzed when the loads of heavy vehicles such as buses were applied to the interior, joint, and edge of the reinforced concrete slabs. In addition, the behavioral characteristics according to the configuration of the joint between slabs of the TTBS were also analyzed. It was confirmed that the strength of the TTBS can be secured by designing with appropriate shear keys. These shear keys simply create a physical interlocking at the slab joints without applying special elements to the slab joints. They also enable the rapid assembly and disassembly of slabs suitable for the TTBS. To verify these numerical analysis results, small-scale reinforced concrete slabs were manufactured and a TTBS was constructed to conduct experiments. The behaviors obtained through experiments of the reinforced concrete slabs were compared with the behaviors obtained through numerical analyses, and it was confirmed that they were very similar, thus verifying the appropriateness of the numerical analysis model. This study eventually demonstrated that the TTBS can effectively be applied to convert existing asphalt pavements in congested urban areas to more durable cement concrete pavements while minimizing the public inconvenience caused by traffic control. Furthermore, the TTBS constructed using reinforced concrete slabs was evaluated as structurally safe and thus suitable for field application.

1. Introduction

Almost all urban roads in Korea are constructed with asphalt pavements, and, due to the rapid increase in traffic volume in urban areas, the pavement sections subjected to loads exceeding the traffic volume predicted during the design phase are increasing. The design service life of urban asphalt pavements is generally set to 10 to 20 years [1,2,3,4,5,6,7]. However, the actual service life is much shorter due to intensive damage such as rutting, cracking, and potholes caused by repeated deceleration and acceleration at bus stops and intersections [8,9,10,11]. Such recurring damage results in not only increased maintenance costs but also social costs. The social costs include logistics delays due to traffic control during repair work, the increased risk of traffic accidents, and increased public complaints [12,13,14,15]. On the other hand, cement concrete pavements are evaluated as a suitable alternative for sections where asphalt pavements frequently fail, as they not only have a design service life of over 30 years but also do not experience damage such as rutting or potholes [16,17,18,19]. Consequently, many countries are applying cement concrete pavement to sections in urban areas where asphalt pavement damage is frequent [20,21,22,23,24,25,26,27,28,29,30,31]. In many U.S. cities, cement concrete pavement is commonly used for bus stop sections and is also actively applied to major roads in downtown areas [24,25,26,27,28,29,30,31]. In the case of Nagoya, which has the most widespread application of cement concrete pavement in urban areas in Japan, cement concrete pavement is actively being incorporated into city roads, intersections, and bus stops [32,33,34]. In the case of Singapore, cement concrete pavement is applied only to certain sections where vehicles are stopped waiting for traffic signals at intersections [35,36,37,38]. Taiwan is also applying cement concrete pavement to bus stops and bus lanes [39]. Therefore, converting frequently damaged sections of existing asphalt pavement to cement concrete pavement can improve the performance of urban pavement. However, traffic closures during the curing period are unavoidable for cast-in-place cement concrete pavements, and the widespread application of precast concrete pavements and ultra-rapid hardening cement concrete pavements is limited due to high construction costs [40,41]. Therefore, there is a need to develop technology that can minimize traffic disruptions during the curing period while applying low-cost cast-in-place cement concrete pavement.

Meanwhile, in urban areas, during the construction of underground transportation facilities such as subways, underground utilities, and new underground roads, temporary structures such as deck plates or temporary bridges made of various materials and structural elements are installed to replace the functions of existing roads [42,43,44,45,46,47]. These temporary structures are used to maintain traffic functions in limited urban spaces by keeping upper-level traffic open. They also protect lower-level workspaces during the construction period [42,43,44,45,46,47]. It is believed that applying the structural principles of such temporary structures to cast-in-place cement concrete pavement construction will enable the simultaneous realization of early traffic opening and protection of the underlying pavement during the curing period. Accordingly, our research team proposed the concept of the temporarily protected concrete pavement (TPCP) method, which applies the structural principles of temporary structures to enable the application of cast-in-place cement concrete pavement while minimizing the impact on the existing road traffic flow [48]. The TPCP method consists of a temporary traffic bridge system (TTBS) that can protect cement concrete pavements while they are curing and cast-in-place cement concrete pavement. Therefore, it is necessary to evaluate the behavior and structural safety of the TTBS and to review the design of the joints between the traffic plates and the load transfer characteristics.

The objectives of this study are to propose the concept of the TTBS for immediate traffic opening during the curing period of cast-in-place cement concrete pavement, to analyze the structural behavior of the TTBS composed of reinforced concrete slabs, and to verify its structural safety through experiments. The main factors considered in this study are the loading position on the slab, the configuration of the slab joint, and the protrusion height of the shear key. Based on these factors, the strains, stresses, deflections, and load transfer characteristics of the TTBS slabs were systematically investigated through numerical analysis and small-scale experiments. To achieve these objectives, the concept and components of the TTBS were established first, and a TTBS was designed using reinforced concrete slabs. The basic behavior of the TTBS slabs was then analyzed through numerical analysis, and the load transfer characteristics according to the shape of the joint between slabs were also analyzed. To verify the appropriateness of the numerical analysis results, small-scale reinforced concrete slabs of the TTBS were fabricated and experiments were conducted. This paper describes the content and results of this research in detail.

2. Concept of TTBS

The concept of the temporarily protected concrete pavement (TPCP) method is explained first. As shown in Figure 1a, the existing pavement is removed, formwork is installed, and a cast-in-place cement concrete pavement is constructed as shown in Figure 1b. Then, as shown in Figure 1c, the TTBS is immediately installed to allow traffic to pass over the TTBS during the curing period of the cement concrete pavement. After the cement concrete pavement has cured, the TTBS is removed, and traffic is allowed to pass through the cement concrete pavement, as shown in Figure 1d. Therefore, the TTBS is the most important structure for applying the TPCP method.

The main structural elements of the TTBS are the traffic plates, the slope plates, and the support blocks, as shown in Figure 2. The traffic plates serve as a bridge to allow vehicles to pass while the cement concrete pavement cures. The support blocks are structural elements that support the weight of traffic plates and the vehicle loads acting on them. In practice, the support blocks are isolated from the formwork to prevent the transmission of traffic-induced vibrations and to minimize local disturbance to the uncured cement concrete pavement. The slope plates are structural elements connected to each end of the bridge. The bridge consists of multiple traffic plates. The slope plates allow vehicles to enter or exit the bridge. These structural elements can be designed in various shapes using various materials. In this study, the traffic plates made of reinforced concrete slabs, which are widely used as structural members due to their excellent economic efficiency and durability, are considered.

The reinforced concrete slabs used as traffic plates are prefabricated at the factory, and the width of each slab is selected as 3.48 m to accommodate the general road lane width of 3 m and the widths of formworks and support blocks. The length of each slab is selected as 1.5 m, considering transport efficiency and constructability for easy assembly and disassembly. The joints between reinforced concrete slabs can either incorporate elements to enhance the load transfer efficiency, or simply allow contact without any load transfer elements. In this study, both of these cases are considered, and, in the case of load transfer, the slab is designed so that the joint geometry can function as a shear key.

3. Design of TTBS Using Reinforced Concrete Slabs

The design details of a reinforced concrete slab, a main component of the TTBS for immediate traffic opening during cement concrete pavement construction, are described. In this study, the design of a reinforced concrete slab applicable to the TTBS was conducted using the specifications of a reinforced concrete slab verified through a previous study [48], and the design of the method using a shear key is mainly explained among the various methods for inducing efficient load transfer between adjacent slabs.

3.1. Design of Reinforced Concrete Slab

The basic dimensions and reinforcement and material designs used for the single reinforced concrete slab that constitutes the TTBS were based on a previous study [48] that verified the structural safety of the reinforced concrete slab under heavy vehicle loads. As previously mentioned, the width of the reinforced concrete slab was selected as 3.48 m and the length of each slab was selected as 1.5 m. The thickness of the slab was selected as 0.3 m based on the structural safety analysis results [48].

In relation to the concrete mix design, the water–cement ratio was set to 40%, the fine aggregate ratio was set to 49%, and the maximum sizes of coarse and fine aggregates were set as 20 mm and 10 mm, respectively, following the Korean cement concrete pavement construction guidelines [49]. In this case, the compressive strength of concrete on the 28th day was 34 MPa and the elastic modulus was 27.5 GPa [48]. The diameter of the reinforcement was selected as 16 mm (D16) and the concrete cover thickness was set to 75 mm. The spacings of the transverse and longitudinal reinforcements are 225 mm and 330 mm, respectively [48]. The design details of the reinforced concrete slab are summarized in

Table 1. Design details of reinforced concrete slab.

Category	Design Parameter	Value
Slab dimension	Length (m)	1.5
	Width (m)	3.48
	Thickness (m)	0.3
Concrete	Maximum size of coarse aggregate (mm)	20
	Maximum size of fine aggregate (mm)	10
	Water–cement ratio (%)	40
	Fine aggregate ratio (%)	49
	Compressive strength (MPa)	34
	Elastic modulus (GPa)	27.5
Reinforcement	Diameter of reinforcement (mm)	16
	Concrete cover (mm)	75
	Transverse reinforcement spacing (mm)	225
	Longitudinal reinforcement spacing (mm)	330

.

3.2. Design of Slab Joint

Since the TTBS is a system that connects multiple precast reinforced concrete slabs longitudinally, it is essential to ensure the integrity between adjacent slabs and suppress faulting. To achieve this, various methods can be applied to the joints between slabs, but only those that facilitate rapid assembly and disassembly are suitable for the TTBS. Therefore, in this study, shear keys were applied to simply create physical interlocking at the slab joints without applying any special elements to the slab joints.

According to the Technical Advisory—Concrete Pavement Joints [50] of the Federal Highway Administration (FHWA), shear key joints are suggested to prevent uneven settlement between adjacent slabs. As shown in Figure 3, it is recommended that the protrusion height of the shear key applied with the tie bar be designed as 0.2d of the total slab thickness d, the protrusion depth as 0.1d, and the flat section at the top and bottom of the slab as 0.4d. However, in the absence of the tie bar, the volume of the shear key is insufficient. This causes the failure of the shear key protrusion and local damage to the upper and lower parts of the shear key. As a result, it is impossible for the shear key to function properly.

Therefore, in this study, a shear key with a stronger shape than the existing design was used at the joint between the TTBS slabs. The protrusion depth of the shear key was maintained at 0.1d, and the protrusion height of the shear key was designed to be 0.4d, which is twice the existing protrusion height of 0.2d, to suppress the shear failure at the shear key protrusion. In addition, the flat sections at the top and bottom of the slab were each configured as 0.3d to alleviate the stress concentration phenomenon in the shear key area. In order to ensure the ease of assembly and disassembly of the slabs and the physical interlocking effect, the inclination of the shear key was applied at 1:4, the same as before. The suitability of this type of shear key is explained in detail in the following section on the numerical analysis of the TTBS, and the design drawing of the reinforced concrete slab with this shear key applied is shown in Figure 4.

4. Numerical Analysis of TTBS

4.1. Analysis Overview

In this study, numerical analysis models of TTBS slabs were developed using the finite element analysis program ABAQUS 2024 [51] to systematically analyze the structural behavior of TTBS slabs and the load transfer characteristics between slabs. Consistent with the previously presented design specifications, the dimensions of the analysis model of the reinforced concrete slab were established as 1.5 m in length, 3.48 m in width, and 0.3 m in thickness. To increase the analysis efficiency and reduce the computation time, a symmetrical model was implemented with a half-width of 1.74 m by considering the geometric symmetry of the slab.

Since reinforced concrete slabs used in the TTBS must be assembled, disassembled, and reused, they must behave within a linear elastic range. Therefore, the concrete slab and steel bars in the numerical analysis model were assumed to be linear elastic materials to account for this. The concrete slab and steel bars were modeled using eight-node reduced integration solid elements and two-node linear truss elements, respectively. This approach was used to analyze the accurate stress distribution of the slab and the complex contact behavior of the slab joints effectively. For the material properties used in the analysis, an elastic modulus of 27.5 GPa and a Poisson’s ratio of 0.15 were applied for concrete slabs, and an elastic modulus of 200 GPa and a Poisson’s ratio of 0.3 were applied for steel bars. In addition, by applying embedded region constraints based on the assumption of perfect bonding between the concrete and steel bar, excellent numerical convergence and reliability were secured even in complex steel bar layouts.

The TTBS is a system where multiple precast slabs are assembled longitudinally. Therefore, the continuous connectivity and load transfer characteristics observed in actual field conditions must be properly considered. To achieve this, the numerical analysis model was configured by interconnecting three slabs to simulate the assembled slab behavior under load application. Slab A is a slab with a shear key groove at one end, slab B is a slab that serves as an intermediate connection with a shear key and a groove configured at each end, respectively, and slab C is a slab with a shear key at one end, as shown in Figure 5. The protrusion height of the shear key was divided into two cases: the protrusion height of 0.2d recommended by FHWA [50] for a slab thickness d, and the modified protrusion height of 0.4d to suppress shear key failure and increase the effective resistance area. Consequently, a total of six numerical analysis model slabs configured by combining slab joint shapes and shear key protrusion heights are developed, and the load transfer performance according to the joint geometry was systematically analyzed and compared. In the numerical analysis, the basic structural behavior of a single slab was first evaluated, and then the load transfer characteristics of assembled slabs according to the connectivity between slabs were analyzed in detail.

4.2. Behavior of Single TTBS Slab

To evaluate the fundamental structural behavior of a single TTBS slab, numerical analysis was performed on a total of six analysis models considering the shear key arrangement and protrusion height. The boundary and loading conditions of the numerical analysis models were configured as shown in Figure 6 to simulate actual support conditions and vehicle load characteristics. As a boundary condition, vertical displacements were constrained at an inner position of the support block equal to the length of the support block at each corner of the slab where the support block is installed. In addition, transverse displacements were constrained at the symmetric cross-section to apply the symmetry boundary condition, thereby simulating the same behavior as the entire slab.

The loading conditions were determined based on the load characteristics of metropolitan buses operating in urban areas of Korea. The magnitude of the load was set by applying a rear axle weight ratio of 62.5% to the gross vehicle weight of 161.7 kN. Considering the symmetry of the analysis model, a wheel load of 50.532 kN, which is half of the rear axle load, was applied [48]. Two representative loading cases were established, as shown in Figure 6: a center loading to evaluate the general bending behavior and an edge loading to investigate the behavior under the most structurally unfavorable loading condition. Since the slab edge is a critical location where the stress concentration is maximized due to discontinuities, the structural safety at this location was rigorously evaluated. The load application position was selected as the wheel path, where buses predominantly travel and repeated traffic loading accumulates, and the load was applied as a uniformly distributed load over an area of 0.6 m by 0.24 m to simulate the tire contact area.

Examples of the stress distribution and deflected shape of a slab are shown in Figure 7 when a load is applied to the center and edge of the slab, and the maximum stress and deflection were analyzed and shown in

Table 2. Numerical analysis results for single slab.

Protrusion Height	Case	Load Position	Max. Stress (MPa)	Max. Deflection (mm)
0.2d	Slab A	Center	1.094	0.341
		Edge	1.610	0.446
	Slab B	Center	1.095	0.341
		Edge	1.610	0.446
	Slab C	Center	1.095	0.341
		Edge	1.611	0.446
0.4d	Slab A	Center	1.093	0.340
		Edge	1.610	0.446
	Slab B	Center	1.094	0.341
		Edge	1.610	0.446
	Slab C	Center	1.094	0.341
		Edge	1.611	0.446

. The analysis results showed that the principal stress and deflection of the slab under the same loading condition were nearly identical, regardless of the shear key protrusion height or the shape of the slab joint. This indicates that the effect of the shear key geometry on the overall rigidity of the slab is negligible, and the behavior of the slab is dominated by the bending behavior of the slab itself. In addition, the maximum tensile stress occurring under the most unfavorable loading conditions, such as edge loading, was analyzed to be significantly lower than the concrete flexural strength of 5 MPa [52], confirming the sufficient structural safety of the proposed TTBS slab.

4.3. Behavior of Assembled TTBS Slabs

The numerical analysis was performed to compare and analyze the behavioral characteristics of assembled TTBS slabs according to the shear key protrusion heights of 0.2d and 0.4d defined previously. A hard contact condition was assigned to the contact surfaces between adjacent slabs. The friction coefficient for the inter-slab contact surfaces was set by referencing the standard value of 0.6 specified in KDS 14 20 22 (Shear and Torsion Design Standard for Concrete Structures) and ACI 318-19 [53,54]. Based on these standards, a friction coefficient of 0.6, which complies with the design criteria, was adopted to simulate the load transfer behavior of the assembled slabs.

Regarding the boundary conditions, the same support condition as in the single slab analysis was applied to each slab to replicate the actual connection state, and three loading cases were established based on the load application positions. In the first case, to simulate a critical condition where a vehicle passes over the joint, a load was applied to the edge of slab B, where the shear key is configured, within the joint section between slabs A and B. In the second case, to analyze the stress characteristics according to the shear key arrangement under the same situation, a load was applied to the edge of slab B, where the shear key groove is configured, within the joint section between slabs B and C. In the third case, a load was applied to the center of slab B to simulate a vehicle passing through the center of the slab. The load application position and area were set identically to the single slab analysis. Figure 8 illustrates the detailed support conditions and load application positions of the assembled slab analysis model.

The numerical analysis results confirmed that the maximum principal stress occurred at the joints between slabs A and B and between slabs B and C under the loading conditions where the loads were applied to the edge of slab B. The detailed analysis results are summarized in

Table 3. Numerical analysis results under edge loading for assembled slabs.

Category	Protrusion Height	Between Slabs A and B		Between Slabs B and C
		Slab A	Slab B (Loaded)	Slab B (Loaded)	Slab C
Max. deflection (mm)	0.2d	0.149	0.149	0.150	0.149
	0.4d	0.149	0.149	0.150	0.149
Max. principal stress (MPa)	0.2d	0.638	0.727	0.720	0.642
	0.4d	0.627	0.704	0.706	0.640

. Regarding the maximum deflection, identical values were obtained for both the joints between slabs A and B and between slabs B and C regardless of the shear key protrusion height. This is attributed to the fact that the overall deformation is dominated by the flexural rigidity of the slab itself rather than the local geometry of the shear key. Meanwhile, compared with the single-slab analysis results, the maximum deflection at the joints between slabs A and B and between slabs B and C decreased by 66.6% and 66.4%, respectively. This indicates that the loads were effectively distributed to the adjacent slab through the joint of the assembled slabs.

Meanwhile, a slight difference in the maximum principal stress was confirmed depending on the shear key protrusion height. The maximum stress at the joint between slabs A and B decreased from 0.727 MPa for the protrusion height of 0.2d to 0.704 MPa for the protrusion height of 0.4d, while the stress at the joint between slabs B and C also decreased from 0.720 MPa to 0.706 MPa. This is analyzed as a result of the slight decrease in the stress concentration at the joint region due to the expansion of the cross-sectional area of the shear key caused by the increased height of the protrusion. Furthermore, comparing the maximum principal stress with a single slab, reduction effects of 54.8% and 55.3% for the protrusion height of 0.2d, and 56.3% and 56.1% for the protrusion height of 0.4d were observed at the joints between slabs A and B and between slabs B and C, respectively. These results verify that the TTBS joints are effective in the structural stress release.

Under the center loading condition at slab B, it was confirmed that the maximum principal stress and maximum deflection occurred at the center of slab B, where the loads were applied, and the detailed analysis results are summarized in

Table 4. Numerical analysis results under center loading for assembled slabs.

Category	Protrusion Height	Slab A	Slab B (Loaded)	Slab C
Max. deflection (mm)	0.2d	0.125	0.144	0.125
	0.4d	0.125	0.143	0.125
Max. principal stress (MPa)	0.2d	0.400	0.704	0.400
	0.4d	0.399	0.703	0.399

. For the maximum deflection, the case of the protrusion height of 0.4d showed only a decrease of 0.001 mm compared to the case of the protrusion height of 0.2d, so the difference in deflection depending on the shear key protrusion height was confirmed to be negligible. This is believed to be because, as with the previous edge-loading condition, the overall deformation is determined by the flexural rigidity of the slab itself. Compared to the maximum deflection of a single slab, reductions of 57.8% for the protrusion height of 0.2d and 58.1% for the protrusion height of 0.4d were observed. The maximum principal stress also showed only a minor difference depending on the shear key protrusion height, unlike the edge-loading condition. This means that the shear key configuration has no effect when the loads are applied far from the joint. In comparison with a single slab, a maximum stress reduction of 35.7% was observed under the center loading for the assembled slabs.

As a result, the proposed protrusion height of 0.4d was found to have improved the stress distribution performance through the load transfer compared to the existing protrusion height of 0.2d, especially under joint load conditions.

4.4. Load Transfer Characteristics at TTBS Joints

The numerical analysis was performed to evaluate the load transfer characteristics by analyzing the shear stress of the TTBS slab joint. The joint load conditions described previously were applied to the analysis. The results indicated that shear stress was concentrated at the joints and the maximum shear stresses are summarized in

Table 5. Shear stress analysis results for assembled slabs.

Category	Protrusion Height	Between Slabs A and B		Between Slabs B and C
		Slab A	Slab B (Loaded)	Slab B (Loaded)	Slab C
Max. shear stress (MPa)	0.2d	0.100	0.118	0.096	0.099
	0.4d	0.093	0.102	0.086	0.095

. As shown in the table, the shear stress when the loads are applied to the joint edge of slab B between slabs A and B where the shear key protrusion exists (see Figure 8) is larger than the shear stress when the loads are applied to the joint edge of slab B between slabs B and C where the shear key groove exists. The maximum shear stress at the joint between slabs A and B decreases from 0.118 MPa for the protrusion height of 0.2d to 0.102 MPa for the protrusion height of 0.4d. Similarly, at the joint between slabs B and C, the maximum shear stress decreases from 0.099 MPa for the protrusion height of 0.2d to 0.095 MPa for the protrusion height of 0.4d. The reductions in shear stress are 13.6% and 4.0%, respectively. Therefore, it was found that the shear key protrusion height of 0.4d was more effective in reducing shear stress compared to the protrusion height of 0.2d. The results of this analysis imply that the shear key joint of the TTBS with a protrusion height of 0.4d functions effectively as a structural load transfer element.

5. Experimental Verification of TTBS Behavior

5.1. Experiment Preparation

To experimentally verify the results of the numerical analysis, a load test was performed using small-scale TTBS slabs with a 1/6 scale ratio. The 1/6 scale ratio was selected by considering the effective experimental spacing of the universal testing machine (UTM) for installing the specimens and measurement devices. In addition, the structural similarity between the 1/6 scale reinforced concrete slab and the full-scale slab had been systematically verified in the previous study [48]. Based on these considerations, the 1/6 scale ratio was adopted in this study. The dimensions of the small-scale TTBS slab used in this experiment were 250 mm in length, 580 mm in width, and 50 mm in thickness. The concrete mix design and reinforcement design were applied identically to the specifications of the relevant previous research [48]. A total of three small-scale TTBS slabs, slab A, slab B, and slab C, were fabricated with the shear key protrusion height set to 0.4d. The 0.4d configuration was selected for experimental verification as it was identified as the optimized design in the numerical analysis. While the 0.2d configuration was not experimentally tested due to the practical difficulty of accurately fabricating a 10 mm high protrusion at a 1/6 scale, the validity of the shear key with the 0.4d configuration by comparing the experimental and numerical analysis results will also provide confidence in the validity of the 0.2d configuration.

The fabrication process of the experimental slab is as follows: First, the formwork was precisely fabricated according to the design dimensions, and a mold capable of realizing the complex shape of the shear key was fabricated using gypsum and attached to the inside of the formwork. The steel bars were placed according to the longitudinal and transverse reinforcement design and were firmly fixed to the formwork to prevent positional changes during concrete pouring. Subsequently, concrete was poured, and compaction and surface finishing work were performed. The experimental slabs were demolded after 1 day of wet curing, and then cured in water at a constant temperature for 28 days to achieve the design strength. The overall fabrication process and shapes of the small-scale TTBS slabs are shown in Figure 9.

The experimental program consisted of two cases for the comparison and verification with the numerical analysis results. One is to evaluate the basic structural behavior of a single slab and the other is to investigate the load transfer characteristics of assembled slabs composed of slabs A, B, and C. By applying the same support conditions as the numerical analysis model, behavior consistent with the numerical analysis model was induced. The steel support blocks measuring 30 mm by 30 mm, fabricated by applying a 1/6 reduction ratio to the actual size of the support block (180 mm by 180 mm), were installed at the corners under the slab. High-strength H-beams were placed beneath the support blocks to maintain stable support and provide sufficient space for the installation of measuring equipment. The loading conditions were applied identically to the numerical analysis. The loading area measuring 100 mm by 40 mm, determined by applying a 1/6 reduction ratio to the actual tire contact area of a bus (600 mm by 240 mm), was used. Because the load is distributed over an area, the applied load was scaled down by a ratio of 1/36 the square of the length reduction ratio to maintain an equivalent stress to the full-scale slab. Accordingly, the actual wheel load of 50.532 kN was proportionally reduced to 1.404 kN.

Strain gauges and displacement transducers were installed to precisely measure the strain and deflection of small-scale TTBS slabs. For the single-slab experiments, strain gauges were placed across the entire slab, and displacement transducers were installed according to the load application positions, as shown in Figure 10. Additional displacement transducers were placed under the H-beams to compensate for measurement errors caused by micro-deformations of the support H-beams. For the assembled slab tests, strain gauges and displacement transducers were installed in the area near the joint between slab B, which receives the direct load, and slab C, which receives the transferred load, as shown in Figure 11.

5.2. Experimental Analysis of Single Slab

To evaluate the basic structural behavior of a single slab, experiments were conducted under center- and edge-loading conditions, and the experiment setup and the arrangement of measurement sensors are shown in Figure 12. The experiments were repeated three times for each loading condition and slab type. Detailed analysis results regarding strains are presented in

Table 6. Measured strains for single slab under center loading.

Strain at Each Gauge Location (με)
Load Location	Specimen	Test Number	SG-1	SG-2	SG-3	SG-4	SG-5	SG-6	SG-7	SG-8	SG-9
Center	Slab A	1	31	36	31	33	34	34	30	38	32
		2	31	38	32	33	35	32	31	37	31
		3	32	39	30	34	34	33	31	37	32
		Average	31	38	31	33	34	33	31	37	32
	Slab B	1	31	36	32	34	33	34	32	39	31
		2	32	38	31	31	35	32	32	37	32
		3	30	39	31	33	35	32	32	37	32
		Average	31	38	31	33	34	33	32	38	32
	Slab C	1	32	38	32	33	33	33	32	36	32
		2	32	37	32	32	34	33	30	37	32
		3	30	37	32	32	33	33	32	37	32
		Average	31	37	32	32	33	33	31	37	32
	Average strain		31	38	31	33	34	33	31	37	32

and

Table 7. Measured strains for single slab under edge loading.

Strain at Each Gauge Location (με)
Load Location	Specimen	Test Number	SG-1	SG-2	SG-3	SG-4	SG-5	SG-6	SG-7	SG-8	SG-9
Edge	Slab A	1	51	37	51	28	36	29	21	33	20
		2	50	38	52	29	35	27	21	33	19
		3	50	38	50	28	36	29	21	33	20
		Average	50	38	51	28	36	28	21	33	20
	Slab B	1	51	37	52	29	36	28	22	33	20
		2	51	39	53	28	35	29	20	33	21
		3	51	37	52	29	36	29	20	33	22
		Average	51	38	52	29	36	29	21	33	21
	Slab C	1	52	38	51	28	36	29	19	33	20
		2	51	37	51	28	36	28	21	33	20
		3	51	38	52	29	35	27	20	34	22
		Average	51	38	51	28	36	28	20	33	21
	Average strain		51	38	51	28	36	28	21	33	21

, while detailed analysis results regarding deflections are summarized in

Table 8. Measured deflections for single slab under center loading.

Deflection at Each Displacement Transducer Location (mm)
Load Location	Specimen	Test Number	Correction Value	Uncorrected			Corrected
				DT-1	DT-2	DT-3	DT-1	DT-2	DT-3
Center	Slab A	1	0.137	0.185	0.194	0.184	0.048	0.057	0.047
		2	0.136	0.186	0.194	0.184	0.050	0.058	0.048
		3	0.139	0.179	0.195	0.182	0.040	0.056	0.043
		Average	0.137	0.183	0.194	0.183	0.046	0.057	0.046
	Slab B	1	0.127	0.168	0.183	0.167	0.041	0.056	0.04
		2	0.127	0.166	0.182	0.178	0.039	0.055	0.051
		3	0.127	0.183	0.191	0.173	0.056	0.064	0.046
		Average	0.127	0.172	0.185	0.173	0.045	0.058	0.046
	Slab C	1	0.127	0.173	0.199	0.172	0.046	0.072	0.045
		2	0.126	0.172	0.183	0.171	0.046	0.057	0.045
		3	0.128	0.172	0.181	0.175	0.044	0.053	0.047
		Average	0.127	0.172	0.188	0.173	0.045	0.061	0.046
	Average deflection		0.130	0.176	0.189	0.176	0.046	0.059	0.046

and

Table 9. Measured deflections for single slab under edge loading.

Deflection at Each Displacement Transducer Location (mm)
Load Location	Specimen	Test Number	Correction Value	Uncorrected			Corrected
				DT-1	DT-2	DT-3	DT-1	DT-2	DT-3
Edge	Slab A	1	0.073	0.129	0.144	0.128	0.056	0.071	0.055
		2	0.073	0.129	0.143	0.128	0.056	0.070	0.055
		3	0.074	0.132	0.146	0.139	0.058	0.072	0.065
		Average	0.073	0.130	0.144	0.132	0.057	0.071	0.058
	Slab B	1	0.038	0.096	0.113	0.095	0.058	0.074	0.057
		2	0.037	0.094	0.108	0.095	0.057	0.071	0.058
		3	0.038	0.095	0.107	0.095	0.057	0.069	0.057
		Average	0.038	0.095	0.109	0.095	0.057	0.071	0.057
	Slab C	1	0.056	0.113	0.127	0.113	0.057	0.071	0.057
		2	0.055	0.119	0.129	0.118	0.064	0.074	0.063
		3	0.056	0.109	0.126	0.108	0.053	0.070	0.052
		Average	0.055	0.114	0.127	0.113	0.059	0.072	0.058
	Average deflection		0.055	0.113	0.127	0.113	0.059	0.072	0.058

. The analysis results showed that the deviation between the repeated experimental results was very minimal, confirming that the reliability and consistency of the experimental data were sufficiently ensured.

When loads were applied to the center of the slab, the maximum strain was measured at SG-2 and SG-8, the center points of the slab edges, rather than at SG-4 and SG-6, the load application locations, and the maximum deflection occurred at DT-2, the exact center point of the slab. When loads were applied to the edge of the slab, the maximum strain occurred at SG-1 and SG-3, which are the load application locations, and the maximum deflection occurred at DT-2, the center point of the edge section. This is a typical stress concentration phenomenon that occurs when loads are applied to the structurally vulnerable edge areas of a slab.

In all cases of slabs A, B, and C, the differences in strain and deflection by slab type according to the loading conditions were analyzed to be very small. Through this, it was experimentally confirmed that the presence and shape of shear keys do not significantly affect the basic structural behavior of a single slab. Furthermore, when the measured maximum strain was converted into stress by applying the elastic modulus of concrete, 27.5 GPa [48], the maximum tensile stress was analyzed to be 1.46 MPa. Since this value is much lower than the standard flexural tensile strength of concrete, 5 MPa [52], the structural safety of the proposed TTBS slab has been proven.

5.3. Experimental Analysis of Assembled Slabs

To evaluate the load transfer characteristics between TTBS slabs, loading tests were performed by assembling the slabs as shown in Figure 13. The experiments were repeated three times, and the detailed analysis results regarding strain and deflection are shown in

Table 10. Measured strains for assembled slabs.

Strain at Each Gauge Location (με)
Specimen	Test Number	SG-1	SG-2	SG-3	SG-4	SG-5
Slab B	1	23	13	24	13	13
	2	23	12	23	13	14
	3	23	12	23	14	13
	Average	23	12	23	13	13
Specimen	Test Number	SG-6	SG-7	SG-8	SG-9	SG-10
Slab C	1	20	14	22	11	11
	2	21	13	21	11	12
	3	21	13	21	12	11
	Average	21	13	21	11	11

and

Table 11. Measured deflections for assembled slabs.

Deflection at Each Displacement Transducer Location (mm)
Specimen	Test Number	Correction Value	Uncorrected	Corrected
			DT-1	DT-1
Slab B	1	0.296	0.320	0.024
	2	0.293	0.317	0.024
	3	0.299	0.322	0.023
	Average	0.296	0.320	0.024
Specimen	Test Number	Correction Value	DT-2	DT-2
Slab C	1	0.289	0.312	0.023
	2	0.288	0.311	0.023
	3	0.285	0.308	0.023
	Average	0.287	0.310	0.023

, respectively. It was confirmed that the reliability and consistency of the experimental data were sufficiently ensured, as the deviation between the results of repeated experiments was negligible.

As a result of strain analysis, the maximum strain was measured at SG-1 and SG-3, the joint section of slab B where the loads were applied. Compared to the maximum strain of 53 με measured when edge loads were applied to a single slab, the maximum strain at the edge of slab B of the assembled slabs was analyzed to be 24 με, confirming a strain reduction of approximately 55%. The reduction in strain at the slab joint implies that the loads were effectively transferred to the adjacent slab through the interlocking effect of the shear key. Even at SG-6 and SG-8, the joint section of slab C that is not subjected to loads directly, a maximum strain of 22 με was measured, which means that the loads are smoothly distributed and transferred to the adjacent slab through the shear key.

The deflection measurement data also clearly demonstrates the excellent load transfer performance of the shear key and the deflection reduction effect at the joint. The maximum deflection at DT-1 of slab B, where the loads are applied, was analyzed to be 0.024 mm, while the maximum deflection at DT-2 of slab C, where the loads are transferred through the shear key, was analyzed to be 0.023 mm. Compared to the maximum deflection of 0.075 mm that occurred when the loads were applied to the edge of a single slab, it can be seen that the deflection control performance of the assembled slabs is excellent, and the very small difference in deflection between adjacent slabs implies that the slab joint is virtually continuous. Based on the measured average deflection values, the load transfer efficiency was determined using Equation (1), and an excellent value of approximately 94% was achieved [55,56,57]. This confirms that the shear key shape with a protrusion height of 0.4d proposed in this study is very effective for the load transfer between slabs and for ensuring structural continuity.

$$Load Transfer Efficiency\left(LTE\right)={\displaystyle \frac{{\delta }_{unloaded}}{{\delta }_{loaded}}}\times 100 \left(\%\right)={\displaystyle \frac{0.023 \text{mm}}{0.024 \text{mm}}}\times 100 (\%)=94\%$$

(1)

5.4. Comparison of Numerical and Experimental Analysis Results

To verify the validity of the numerical analysis model and the structural safety and load transfer characteristics of the TTBS slabs, strains and deflections derived from the numerical analysis and experiments for a single slab and assembled slabs were compared. In the case of strain, a direct comparison was made based on geometric similarity, where the strain between the full-scale numerical analysis model and the small-scale TTBS slab remain identical. In the case of deflection, the values measured in the experiment using small-scale slabs were converted into full-scale values by multiplying by 6, which is the reciprocal of the scale ratio, and then compared with the numerical analysis results.

For a single slab, the numerical analysis and experimental results were compared for cases where loads were applied to the center and edge of the slab. Comparisons regarding strain are presented in Figure 14 and Figure 15 and

Table 12. Differences between analyzed and measured strains for single slab.

Difference in Strain at Each Gauge Location (%)
Load Location	Case	SG-1	SG-2	SG-3	SG-4	SG-5	SG-6	SG-7	SG-8	SG-9
Center	Slab A	3.3	2.6	3.3	3.1	2.9	3.1	3.3	5.1	6.7
	Slab B	3.3	2.6	3.3	3.1	2.9	3.1	6.7	2.6	6.7
	Slab C	3.3	5.1	6.7	0.0	5.7	3.1	3.1	5.1	6.7
Edge	Slab A	2.0	5.0	0.0	3.7	5.3	3.7	0.0	5.7	4.8
	Slab B	0.0	5.0	2.0	7.4	5.3	7.4	0.0	5.7	0.0
	Slab C	0.0	5.0	0.0	3.7	5.3	3.7	4.8	5.7	0.0

, while comparisons regarding deflection are presented in Figure 16 and Figure 17 and

Table 13. Differences between analyzed and measured deflections for single slab.

Difference in Deflection at Each Displacement Transducer Location (%)
Load Location	Case	DT-1	DT-2	DT-3
Center	Slab A	2.8	0.0	2.8
	Slab B	4.9	1.8	2.8
	Slab C	4.9	7.0	2.8
Edge	Slab A	3.9	2.3	2.2
	Slab B	3.9	2.3	3.9
	Slab C	0.6	0.9	2.2

. As can be seen in the figures and tables, the strain and deflection obtained from the numerical analysis of the full-scale slab show very similar trends to the strain and deflection measured in the experiment with the small-scale TTBS slab. The maximum deviation of strain was analyzed to be 6.7% under the center load condition and 7.4% under the edge load condition, and the maximum deviation of deflection was calculated to be 7.0% under the center load condition and 3.9% under the edge load condition. Considering that the analyzed deviation is not significant, it was confirmed that the numerical analysis model of this study appropriately analyzes the stress distribution and deflection of a single slab.

The comparative analysis data of the numerical and experimental analysis results for the assembled slabs are summarized in Figure 18 and Figure 19 and

Table 14. Differences between analyzed and measured strains for assembled slabs.

Difference in Strain at Each Gauge Location (%)
Case	SG-1	SG-2	SG-3	SG-4	SG-5	SG-6	SG-7	SG-8	SG-9	SG-10
Slab B	4.5	0.0	4.5	0.0	0.0	-	-	-	-	-
Slab C	-	-	-	-	-	5.0	0.0	5.0	0.0	0.0

and

Table 15. Differences between analyzed and measured deflections for assembled slabs.

Difference in Deflection at Each Displacement Transducer Location (%)
Case	DT-1	DT-2
Slab B	3.4	-
Slab C	-	8.0

. The strains and deflections obtained from numerical analyses and small-scale slab experiments were found to be very similar, with the maximum deviation in the strain analyzed to be approximately 5% and the maximum deviation in deflection approximately 8%. In particular, the similarity between the numerical and experimental analysis results for slab B, where the load is applied, and the adjacent slab C implies that the load transfer characteristics through the shear key were appropriately simulated in the numerical analysis. Based on the results of this comparative analysis, the numerical analysis model developed in this study was confirmed to reliably simulate the behavior and load transfer characteristics of the TTBS slabs.

6. Summary and Conclusions

This study was conducted to develop a temporary traffic bridge system (TTBS) to be used during the curing period of the cement concrete pavement in the temporarily protected concrete pavement (TPCP) method. The TTBS made of reinforced concrete slabs was designed and its behavior was numerically analyzed when the bus loads were applied. The behavioral characteristics depending on the configuration of the slab joint for load transfer were also analyzed. The experiments were performed using small-scale reinforced concrete slabs to verify the numerical analysis results. The important findings of this study are summarized as follows:

• According to the analysis results for a single TTBS slab, the stress and deflection of the slab were found to be the same regardless of the shape of the slab joint, such as the shear key protrusion and shear key groove. This indicates that the effect of the shear key geometry on the overall rigidity of the slab is negligible, and the behavior of the slab is dominated by the bending behavior of the slab itself.
• Both the numerical and experimental analysis results for the assembled TTBS slabs showed that the shear key joints effectively transfer the load between slabs. Through the comparison with a single slab, it was confirmed that the physical interlocking of the shear key joint effectively distributes the load to adjacent slabs, significantly reducing the tensile stress and deflection. Furthermore, the shear stress distribution demonstrated that the shear key joint system possesses structural stability, with all stresses remaining well within safe design limits.
• Within this analysis framework, the protrusion height of the shear key was identified as a critical design parameter. In particular, the proposed shear key protrusion height of 0.4d has been proven to be a structurally optimized shape for the TTBS by reducing tensile and shear stresses compared to the existing shear key protrusion height of 0.2d.
• The small-scale experimental analysis results for both a single slab and assembled slabs were found to be consistent with the numerical analysis results. This confirms the reliability of the structural behavior and load transfer efficiency of the proposed TTBS.

This study confirmed that the TTBS designed using reinforced concrete slabs could be appropriately applied in the field. Since the TTBS is the most important structural element in the TPCP method, it is demonstrated that the TPCP method can effectively convert existing asphalt pavements to more durable cement concrete pavements while minimizing the public inconvenience caused by traffic control. In the future, the TPCP method will be practically validated through field experiments to evaluate the structural responses of a full-scale TTBS, such as stress distribution, deflection, and load transfer characteristics. Furthermore, field tests under actual traffic conditions should be conducted to characterize the dynamic response and fatigue of the system and to evaluate practical considerations such as the durability of the system components.