Application of Steel-Fiber-Reinforced Self-Stressing Concrete in Prefabricated Pavement Joints

Abstract

Prefabricated pavement, with its advantages of a high paving speed, low material consumption, low carbon emissions, high strength, and easy construction, has gradually been used to address issues associated with traditional cement pavement construction. However, even under the long-term combined effects of vehicle loads and environmental loads, the joints between pavement slabs remain prone to various problems. This paper proposes the use of steel-fiber-reinforced self-stressing concrete (SFRSSC) with a certain level of self-stress for joint pouring and connection to control the development of cracks in the joints and achieve seamless integration between the slabs. Additionally, the self-stress generated by SFRSSC is utilized to enhance the continuity of the prestressed design in precast slabs, thereby extending their service life. Through laboratory experiments and field tests, the self-stress magnitude, mechanical strength, and long-term applicability of SFRSSC were studied. The results indicate that SFRSSC can achieve self-stress levels of over 7 MPa under standard curing conditions, but the values decrease significantly when removed from the standard curing environment. SFRSSC exhibits a compressive strength of over 60 MPa and a flexural strength of over 9 MPa, both of which exceed the requirements of the relevant standards, making it suitable for use as a pavement joint material. During long-term monitoring in the field, SFRSSC demonstrates favorable expansion effects and high stability. The longitudinal expansion remains stable at 100 με, while the transverse expansion exhibits minor shrinkage, maintained at around 25.2 με. Therefore, the application of SFRSSC in assembly-type prestressed pavement joints shows high applicability.

1. Introduction

The technology of prefabricated prestressed concrete pavement refers to the prefabrication of the pavement structure layer in a factory, followed by transportation to the construction site for assembly, joint treatment, and other subsequent processes, thereby achieving rapid construction and repair of pavement structures [1]. This technology offers several advantages, including a fast paving speed, low material consumption, low carbon emissions, high reuse rate, and long structural lifespan. As a result, it has been widely applied in low-speed traffic roads, sidewalks, and urban roads [2,3,4,5]. The main difference between prefabricated prestressed concrete pavement and continuously cast concrete pavement lies in the composition of the pavement panels. In prefabricated pavement, the panels are primarily assembled. After assembly, joints unavoidably form, typically appearing as open cracks through the surface layer. In usual cases, some joints are preserved as expansion joints for the pavement, while others are sealed and waterproofed to form an integral connection. Currently, there are two main connection methods: dry and wet connections, with wet connections being the primary approach for prefabricated structures. However, since each pavement panel is independently installed and influenced by various construction factors, there can be height variations between adjacent panels. Additionally, with the passage of time and inadequate maintenance, factors such as cracking, water seepage, and aging often occur at these joint locations, significantly impacting the service level and driving comfort of the pavement. The vulnerability of the joint area as a weak point in the structure has been widely recognized [6,7,8]. Therefore, how to treat the joint interface, reduce the number of joints, or even achieve seamless connections has been a hot topic in research on prefabricated pavement engineering.

Currently, both domestic and international approaches to pavement panel connections mainly involve the use of transfer bars and suitable joint materials to enhance the load-carrying capacity of the joints, aiming to make the pavement structure as close to a continuous panel without joints [9,10,11]. Transfer bars transmit loads through their own shear and bending stiffness. Relevant studies have shown that the transmission efficiency between pavement panels with transfer bars is between 85% and 95% [12,13]. Although transfer bars possess relatively high transmission efficiency, there are still concerns regarding their rust prevention treatment, especially during the concrete compaction stage where loss is most evident. Moreover, if there is an excessive amount of anti-rust coating on the surface of the transfer bars, the contact stress generated by the wheel load transmission may lead to excessive damage to the concrete surrounding the transfer bars, resulting in loosening and a reduced load-carrying capacity [14,15,16,17,18]. Therefore, the selection of joint materials becomes crucial, aiming to find a material that not only improves the bonding effect between transfer bars and pavement panels but also prevents water from infiltrating and damaging the various structural layers. Based on this concept, this study uses a steel-fiber-reinforced self-stressing concrete (SFRSSC) [19,20] material to address the aforementioned issues.

2. Experiment

2.1. Experimental Materials and Ratio

The basic mix proportion used in this experiment was (cement:fly ash:water:coarse aggregate:fine aggregate:expansion agent) 1:0.28:0.42:1.87:2.81:0.11. The gel material selected was ordinary Portland cement (P.O.42.5), and the chemical composition of the Class II fly ash is shown in

Table 1. Main chemical composition of cement and fly ash (mass fraction,%).

Composite	CaO	Al2O3	MgO	SiO2	Fe2O3	SO3	LOI
Cement	65.1	3.1	2.8	23.2	2.6	1.6	1.6
Fly ash	6.2	32.6	0.7	48.5	8.7	1.2	2.1

.

The coarse aggregate consisted of natural aggregates with particle sizes of (5~10) mm and (10~20) mm, mixed in a 4:6 ratio. The fine aggregate was natural sand with a fineness modulus of 2.6. The superplasticizer used had a dosage of 1% and was a polycarboxylate-based high-efficiency water reducer with a water reduction efficiency of 25%. The steel fiber used was the milling type produced by Shanghai Harix Fiber as shown in Figure 1.

The main physical properties of the materials used are listed in

Table 2. Physical properties of materials used.

Material	Physical Properties
Cement	Ordinary Portland cement (OPC, density: 3.15 g/cm3, specific surface area: 3440 cm2/g)
Fly ash	(Density: 2.35 g/cm3, specific surface area: 4110 cm2/g)
Fine aggregate	(River sand, size: 0.35~0.45 mm, density: 2.58 g/cm3, absorption: 1%)
Coarse aggregate	(Basalt gravel, 5~20 mm, apparent density 2.933 g/cm3)
Superplasticizer	Polycarboxylic-based superplasticizer (specific gravity: 1.05 ± 0.05, pH: 5.0 ± 1.5)
Steel fiber	(Milling type, size: 32 mm ± 2, length–diameter ratio: 35–45, tensile strength ≥ 700 MPa)

. The expansion agent used in the experiment was a dual-expansion-source expansion agent produced by Wuhan Sanyuan Special Building Materials Co., Ltd. (Wuhan, China), and its chemical composition is shown in

Table 3. Main chemical composition of expansion agent (mass fraction,%).

Components	CaO	Al2O3	MgO	SiO2	Fe2O3	SO3	LOI
HCSA	65.21	4.88	1.45	1.81	1.22	22.33	3.10

.

2.2. Experiment Method

2.2.1. Stress and Mechanical Response Measurement of Steel-Fiber-Reinforced Self-Stressing Concrete

The measurement of concrete self-stress utilized a custom-designed apparatus, as shown in Figure 2. The dimensions of the specimens were primarily determined based on the steel reinforcement ratio within the concrete, with a reverse calculation performed for the cross-sectional dimensions using a 1% reinforcement ratio. Due to the smallest commercially available steel bar having a diameter of 16 mm, this study employed prismatic specimens of reinforced fiber concrete with a length of 400 mm and a cross-sectional dimension of 141 mm × 141 mm. A 16 mm diameter steel bar was centrally positioned within the specimen, equipped with a strain gauge. Both ends of the steel bar were connected to 141 mm × 141 mm × 10 mm steel plates by welding, providing constraints to the overall deformation of the specimen and transmitting feedback on changes in tensile and compressive stresses during the testing process. The strain gauge used in this study was an intelligent chord-type digital steel reinforcement stress gauge (range: ±200 MPa, sensitivity: 0.1 MPa). When the strain gauge experienced axial forces, it caused tension or compression changes in the elastic steel chords, altering the vibration frequency of the steel chords. By measuring the frequency change using a frequency meter, the magnitude of the forces acting on the steel reinforcement could be determined. This, in turn, allowed for the estimation of the forces experienced by the concrete structure.

For the experiment, a common 1% reinforcement ratio was used as the baseline. Three types of specimens were prepared by adding different volume percentages of steel fibers: 0%, 1%, and 1.5%. These specimens were labeled as G0, G1, and G1.5, respectively. After molding, the specimens were demolded after 24 h and then moved to a standard curing room for a curing period of 28 days (with a temperature of 20 ± 2 °C and relative humidity of 60 ± 5%). Due to the relatively low water-to-cement ratio used in the design, while this design increased the strength of the concrete, it was not conducive to the performance of the expansion agent in the concrete. This significantly affected the self-stress values of the micro-expansion fiber-reinforced concrete. To address this issue, during the curing period, the specimens were observed, and water was appropriately replenished on the surface to keep the specimens moist. After completion, the JMZX-3001 comprehensive testing machine was used to track and measure the stress changes in the specimens.

The compressive and flexural strength were determined according to the standard GB/T 50082-2009 “Standard Test Methods for Mechanical Properties of Ordinary Concrete” [21]. The testing equipment is shown in Figure 3. The dimensions of the flexural and compressive test specimens were 150 mm × 150 mm × 515 mm and 150 mm × 150 mm × 150 mm, respectively. After molding for 24 h, the specimens were demolded and placed in a standard curing room for curing until the ages of 3 days, 7 days, and 28 days.

2.2.2. Field Test and Long-Term Observation of Steel-Fiber-Reinforced Self-Stressing Concrete

To analyze the long-term performance of steel-fiber-reinforced self-stressing concrete in prefabricated prestressed pavement joints, a prefabricated prestressed concrete pavement was laid at a certain intersection in Dezhou City, Shandong Province. The installation process involved the use of remote real-time monitoring equipment, as shown in Figure 4.

Figure 5a shows the design drawing of the prefabricated prestressed pavement panels. The connection between the panels was achieved through steel bar welding and concrete bonding, as illustrated in Figure 5b. The length of the joint was 7500 mm, with a width of 200 mm and a thickness of 240 mm. The designed reinforcement ratio was 1%. Concrete strain gauges were embedded in the joints for continuous observation, with data recorded at 10-min intervals. The positions of the strain gauge measurement points are indicated in Figure 5c, with four sensors embedded in two joints, two in the transverse direction (MR-01 and MR-04), and two in the longitudinal direction (MR-02 and MR-03), arranged at the 1/4 and 1/2 positions of each joint. The production of steel-fiber-reinforced self-stressing concrete involves on-site mixing, and after the assembly of the panels, the concrete is poured. The proportion of steel fibers in the mix was selected based on the 28-day self-stress values of the concrete measured in the experiment mentioned in Section 2.2.1 of this paper, using the optimal numerical proportion for pouring. After 24 h of casting, a geotextile was placed on the surface, and construction personnel arranged for a 10-day water curing. Subsequently, the pavement was reopened to traffic, and long-term monitoring was conducted.

3. Results and Discussion

3.1. Analysis of Self-Stress Influence of Micro-Expansion Fiber Concrete

Figure 6 shows the self-stress results of concrete with different steel fiber contents before and after 3 days under the same expansion agent content. The SFRSSC control sample (G0) showed a higher self-stress before and after 3 days (mainly due to the action of concrete on the steel plates at both ends under the constraint of steel bars). The self-stress increases rapidly before 3 d, the growth rate slows down after 3 d, and the concrete self-stress value begins to decrease slowly after reaching the first peak at about 7 d. When the curing specimen reached about 14 d, the concrete self-stress began to rise again and reached the maximum value of 7.4 MPa in the control sample (G0) after 28 d. This delayed self-stress growth phenomenon may be attributed to the current low water–binder ratio design, which makes the concrete lack water available for the expansion agent reaction. This makes part of the expansive agent unreacted in the early stage of the concrete hydration process. Since this experiment was conducted in a standard curing room, and the surface was properly sprinkled with water, the water entered the concrete through the pores and continued to hydrate the expansive agent to form a stable expansion element, so the concrete self-stress began to rise again.

Compared with G0, the specimens G1 and G1.5 with added steel fibers exhibited different phenomena before 3 days (3 d), and it is particularly noteworthy that G1 showed a significant negative self-stress before 3 d. This could be attributed to the fact that the steel fibers restricted the expansion of the hydration products of the expansion agent in the concrete. Previous research has indicated that nearly 50% of concrete’s self-induced shrinkage occurs before 3 d. The lower expansion in the early stages cannot compensate for the higher shrinkage of the concrete, resulting in an apparent negative self-stress. After 3 d, the trend of change in self-stress was the same as observed in the G0 group. Self-stress increased with time and around 14 days (14 d), the self-stress generated in the G1 group surpassed that of the G0 group. This can be attributed to the constraint effect of the steel fibers on the concrete. While the steel fibers restricted the concrete’s expansion in the early stages, they also partially limited its shrinkage, thereby improving the concrete’s pore structure and making it more compact in the early stages. When the expansion agent’s hydration products were formed, the volume of these products did not change under the higher constraints, resulting in more apparent macroscopic self-stress. The stress variation trend in the G1.5 group before 3 d was consistent with that of the G0 group, but the values were slightly smaller than those in the G1 group, which could be due to the increased amount of steel fibers providing higher constraints on the concrete. The higher constraints caused a significant slowing of the stress growth in the G1.5 group, and at 28 d, the self-stress value exhibited by the G1.5 group was the same as that of the G0 group. After 28 d, when the specimens were removed from the standard curing room, the self-stress values began to show a more apparent decrease, which may be attributed to the loss of moisture in the SFRSSC due to the lack of water supplementation in the ambient temperature environment. From the temperature change results obtained from the instrument, it was found that the internal temperature of the concrete stabilized after 3 d during the hydration process, and no significant correlation was found between the self-stress values and the temperature changes within the concrete.

In summary, the analysis of the concrete self-stress results shows that the addition of steel fibers in SFRSSC has no significant impact on the concrete’s deformation before demolding. Within 3 days after demolding, due to the concrete’s high shrinkage, increasing the amount of steel fibers would limit the concrete’s expansion. After 3 days, during the standard curing period, the deformation also shows the restricting effect of steel fibers on the concrete’s expansion. After leaving the curing room, the non-fiber-reinforced concrete undergoes rapid shrinkage, while the steel fibers, due to their limiting effect on the concrete, exhibit a significantly slowed rate of shrinkage. By comparing the rate of shrinkage between G1 and G1.5, it was found that the two rates were essentially the same. Therefore, compared to ordinary concrete, SFRSSC has a noticeable resistance to shrinkage, and using it at the joints between prefabricated prestressed concrete pavement panels can effectively avoid the risk of concrete cracking due to shrinkage. Furthermore, through detection, it was found that after the concrete leaves the curing room, there is still some residual stress, which can partially counteract the pavement arching caused by temperature gradients, thereby prolonging the durability of the joints.

3.2. Mechanical Response Analysis of Micro-Expansion Fiber Concrete

In the design of prestressed concrete pavement, according to the Highway Cement Concrete Pavement Design Code JTG D40-2011 [22], the compressive strength of concrete prefabricated blocks should not be less than 50 MPa. Therefore, the selected joint material should also have a compressive strength not lower than this value. Additionally, after the hydration of the joint material is completed, it typically takes on a rectangular prism shape, and when vehicles stop at the joint, they exert a certain force on both ends of the joint. This requires the selected joint material to have good flexural strength. Figure 7 shows the compressive and flexural strength values of SFRSSC with different steel fiber contents. After the addition of steel fibers, the compressive and flexural strengths of SFRSSC were improved at various ages. Compared to the blank control group, when the steel fiber content was 1%, the compressive and flexural strengths of SFRSSC after 28 days of curing increased by 11.5% and 32.6%, respectively. When the steel fiber content was 1.5%, the compressive and flexural strength of SFRSSC after 28 days of curing increased by 28.3% and 41.6%, respectively. The addition of steel fibers increased the flexural-to-compressive strength ratio of concrete, which had a beneficial effect on the development of concrete strength. It is worth noting that the increase in steel fiber content did not have a significant effect on the compressive strength of SFRSSC at 3 days. This is because the early concrete strength is not high, and under the action of the expansion agent, the concrete exhibits micro-expansion. After demolding, the surrounding constraints are reduced, resulting in some loosening of the early concrete structure. After 28 days of curing, the high content component (G1.5) showed an increase of approximately 9.44 MPa compared to the G1 group.

Based on the Highway Cement Concrete Pavement Design Code [21], it can be observed that SFRSSC (steel-fiber-reinforced self-stressing concrete) exhibits a significant increase in flexural performance due to the addition of steel fibers while maintaining excellent compressive strength. The mechanical behavior of SFRSSC enables it to serve as a stable joint material, thereby avoiding damage caused by insufficient strength at the joints.

3.3. Influence Analysis of Long-Term Changes in the Use of Micro-Expansion Fiber Concrete

Figure 8 shows the recorded graphs of joint strain variations and temperature changes detected by the on-site buried monitoring devices. Due to a delay in installing the real-time recording devices, there was no real-time recording with 10-min intervals for the first 3 days, and manual recording was performed instead. From the curves in the figure, it is evident that the two joints poured with SFRSSC (steel-fiber-reinforced self-stressing concrete) exhibit completely different behaviors in the transverse and longitudinal directions. The longitudinal measuring points, MR-01 and MR-04, experienced expansion, with their highest strain values, occurring at approximately 3 days, of 238.6 με and 206.7 με, respectively. On the other hand, the transverse measuring points, MR-02 and MR-03, consistently experienced shrinkage, reaching a minimum value of −59.2 με at around 6 days. This phenomenon is attributed to the outward expansion of the SFRSSC along the longitudinal direction, creating inward compressive stresses at the two ends of the concrete. These compressive stresses, combined with the shrinkage stresses inherent in the concrete itself, make it difficult for the expansion stresses induced by the expansive agent to compensate, resulting in a relatively obvious shrinkage phenomenon at the macroscopic level. Regarding the trend of changes, the longitudinal SFRSSC joints exhibited higher initial expansion at 3 days, which gradually decreased with curing time, stabilizing at ±25 με from 7 days onwards. Furthermore, in analyzing the temperature curve changes, it was observed that the ambient temperature in the surrounding environment decreased by 10 °C within a short period of 5 days during the 33-day inspection. However, the strain values of the SFRSSC did not show significant changes. This indicates that SFRSSC exhibits relatively high stability after completing its hydration process and is less influenced by temperature variations. As for the transverse trend, it was noted that the SFRSSC underwent continuous shrinkage during the first 6 days, but the shrinkage rate gradually reduced over time, with a shrinkage value of only −25.2 με at 120 days. It is worth noting that MR-03 showed a slight increase after 3 days, and MR-02’s slope also decreased slightly. This could be attributed to non-uniform reactions of the internal expansive agent within the concrete, where the expansion induced by the agent exceeded the natural shrinkage of the concrete at 3 days. Comparing the numerical values of MR-01 and MR-04, MR-02 and MR-03, it is evident that the strain variations in the SFRSSC joints were not significantly affected by the measurement point locations but rather by the measuring direction. This indicates relatively high overall integrity of SFRSSC joints, with relatively uniform strain behavior in the same direction.

In conclusion, long-term monitoring of SFRSSC has revealed that its shrinkage deformation is more stable compared to other types of concrete, with shrinkage values ranging from −210 με to −600 με at 28 days [23,24,25]. Particularly, SFRSSC exhibits relatively small shrinkage deformation, which helps to address the issue of concrete cracking caused by its shrinkage. This characteristic offers the potential for achieving seamless connections between concrete slabs and joints, providing a viable solution for minimizing cracks and achieving seamless integration in concrete structures.

4. Conclusions

This study proposes the use of steel-fiber-reinforced self-stressing concrete (SFRSSC) as an assembly-type prestressed pavement joint material to improve the connection between slab segments and precast panels, thereby avoiding joint-related problems and increasing the overall service life. The research investigates the trends of self-stress and mechanical strength variation of SFRSSC and evaluates its stability through long-term testing on experimental road sections. The specific research conclusions are as follows:

(1)

SFRSSC exhibits higher self-stress compared to ordinary concrete, and this self-stress is mainly influenced by the moisture content and the amount of added steel fibers. A higher moisture content allows the expansive agent in SFRSSC to fully hydrate, leading to significant early expansion. However, a significant portion of this expansion is ineffective and decreases noticeably as the concrete leaves the moisture curing stage. The addition of steel fibers primarily limits the development of self-stress in the early stage, and this restriction reduces with increasing steel fiber content. However, in the later stages of concrete development, this limitation on self-stress also partially restricts the shrinkage of the concrete, enabling higher self-stress to be retained in the concrete over the long term, thereby improving the pavement’s crack resistance. It should be noted that excessive steel fiber content is not ideal during usage, as a high content of steel fibers (1.5%) limits early self-stress development without retaining high self-stress in the later stages.

(2)

SFRSSC overcomes the brittle fracture characteristics of ordinary concrete and exhibits high resistance to deformation under compressive and flexural loads. At the age of 28 days of curing, the compressive strength and flexural strength of SFRSSC can reach 62.46 MPa and 9.55 MPa, respectively, meeting the design strength requirements for assembly-type prestressed pavement materials.

(3)

Through long-term monitoring of the experimental road section, SFRSSC demonstrates high stability in the joints. After 28 days, SFRSSC deformation remains in a relatively stable state, and its strain values do not vary significantly with changes in temperature. Even after 120 days, the longitudinal strain of the road remains in an expanded state. Although the transverse strain of the road is in a shrinking state, the shrinkage value slowly decreases, with only a −25.2 με variation observed at 120 days.