1 Introduction
Due to good mechanical properties and durability, ultra-high-performance concrete (UHPC) has come to the forefront of research and application in the field of civil and structural engineering [
1]. In particular, maintenance techniques for UHPC and steel fibers have been subject to much research [
2,
3]. UHPC has been applied in composite structures including high-rise buildings and long-span bridges [
4,
5]. In recent years, reactive powder concrete (RPC) [
6] from France has gradually become a research focus in China, and technologies for paving steel bridge decks with UHPC have been developed [
7,
8]. Hunan Province, for example, has developed a local standard covering Technical Specification for Steel-STC Lightweight Composite Structure Decks [
9] to guide the use of steam-cured UHPC connected with rivets.
Pavements on orthotropic steel bridge decks have always posed technical problems during steel bridge construction and management [
10]. At present, the steel deck pavement materials mainly include pouring asphalt, epoxy asphalt (EA) and resin mixture [
11,
12]. These types of pavements, to some extent, are flexible pavement systems, and their elastic modulus is generally lower than 10 GPa. Furthermore, the temperature sensitivity of elastic modulus of asphalt materials is high and it has no effect on stiffness of orthotropic plates.
As service life of steel-structure bridges constantly increases in China, the fatigue damage to orthotropic plates and the early damage of the pavement structure are becoming increasingly serious under the repeated action of vehicle loads and environmental effects [
13,
14]. To overcome the difficulty in ensuring durability of orthotropic plates and pavements, since 2000 many scholars have assessed various pavement technologies used for high-performance concrete. These scholars attempted to reinforce orthotropic plates by use of concrete pavements with a high elastic modulus of 30 to 50 GPa to reduce the stress amplitude of steel plates and delay the onset of fatigue damage. For example, a steel fiber reinforced concrete (SFRC) pavement was developed and applied to reinforce steel bridge decks in Japan [
15]. A densely reinforced high-performance concrete (RHPC) overlay was proposed to reinforce steel bridge decks in The Netherlands [
16]. In China, a super-tough concrete (STC) pavement structure was formed by riveting steel plates to UHPC [
17].
The problem related to interface bonding between UHPC and steel plates is still not fully solved; interface bonding is not considered in some projects, and so cracks appear in service [
18]. Although the mechanical connection between concrete and steel plates is facilitated by riveting, the waterproofing at interfaces and the residual stress and damage caused by welding have not been completely addressed [
19], so attention still needs to be paid to long-term performance. In addition, cleaning and maintenance of rivet-connected concrete pavements in the later stage of their service life are key issues.
In the present study, a new epoxy resin (EP) bonded UHPC pavement structure system, i.e., resin bonding polymer concrete (RBPC) was investigated. The system offers the following advantages: 1) EP has waterproofing and corrosion resistant effects on steel plates and improves interfacial bonding between UHPC and steel plates; 2) After being bonded with EP, UHPC shows an elastic transition layer, which can improve the fatigue durability; 3) EP can coordinate with shrinkage strain of UHPC, thus reducing the generation of cracks; 4) It reduces the difficulties in later curing and maintenance compared with the rivet-connected system. It is expected that the research findings can be beneficial to the research and application of UHPC in pavements on steel bridge decks.
2 Materials and test methods
2.1 Test materials
2.1.1 Cement-based concrete materials
New UHPC (U-180) products provided by Jiangsu Sino Road Transportation Science and Technology Co., Ltd. were used, which comprised cement, mineral powder, fine aggregates, steel fibers, and additives. The mix proportions are listed in Table 1.
The performance of UHPC materials is tested by referring to the specification standard [
20,
21], as shown in Table 2. Meanwhile, for comparative research, the material and structural properties of SFRC were compared in this study.
2.1.2 Waterproof and bonding materials
RB-type high-toughness resin and EP provided by Jiangsu Sino Road Transportation Science and Technology Co., Ltd. were separately utilized as the waterproof layer of steel plates and the interface bonding layer. The RB waterproof layer was cured for 8 h at 23°C, while the EP bonding layer was cured for 48 h at 23°C. A tensile test was conducted according to ASTM D638-2014, while drawing strengths when bonding with steel plates at high temperature and room temperature were tested based on ASTM D4541-09. The test results are summarized in Table 3.
2.2 Test methods
2.2.1 Drawing test of the composite structure
By referring to the test method for bonding strength in Specifications for Design and Construction of Pavement on Highway Steel Deck Bridges (JTG T3364-02-2019), the vertical drawing test for bonding strength was performed using the BA-400D machine (Japan). The molding and test methods are illustrated in Fig. 1. First, a composite structure consisting of steel plates, waterproof and bonding layers and the UHPC pavement was molded and then the drawing test was conducted after standard curing for 28 d. The drawing specimen measured 300 mm × 300 mm × 50 mm and the drawing rate was 10 mm/min.
2.2.2 Bending test of the composite structure
By referring to Specifications for Design and Construction of Pavements on Highway Steel Deck Bridges (JTG T3364-02-2019), a comparative three-point bending test was conducted on the five pavement structures listed in Fig. 2. The test specimen measured 380 mm × 100 mm (length × width), in which reinforcing mesh with a diameter φ of 10 mm and dimensions of 50 mm × 50 mm was placed between layers in Type-IV and V composite structures (the areal reinforcement ratio was 3%). A universal material testing machine was used to conduct the bending test at a controlled cross-head displacement rate of 1−2 mm/min.
2.2.3 Fatigue test of the composite structure
By referring to the fatigue test method for three-point loaded composite beams recommended in Specifications for Design and Construction of Pavement on Highway Steel Deck Bridge (JTG T3364-02-2019), the composite structures of steel bridge decks with three different systems were prepared on the surface of steel plates with a thickness of 14 mm. It was composed of the UHPC structure directly wet-bonded with steel plates, the EP bonded structure, and the SFRC composite structure. The specimen dimension is 380 mm × 100 mm (length × width), as shown in Fig. 3. The fatigue test on the composite structure was carried out with a DTS fatigue test device by applying sinusoidal load at 10 ± 0.1 Hz at 20°C ± 2°C. The spalling of the bonding layer and cracking of the pavement layer were observed and the number of load cycles applied to the composite structure under different fatigue load amplitudes was recorded. Moreover, the fatigue performance of the composite structure under repeated loading was evaluated.
3 Results and discussion
3.1 Bonding performance of the structures
To study bonding performance of the composite structure bonded with EP, in this research we separately compared the trends in the drawing strengths of the composite structure at −10°C, 25°C, 40°C, and 60°C. The results in Fig. 4 show that drawing strength between layers of the RBPC pavement structure decreases with increasing temperature. At room and low temperatures, drawing strength is about 2.2 MPa, while it reduces by about 20% at 40°C. Moreover, at 60°C, the drawing strength is 1.26 MPa. This indicates that temperature sensitivity of organic resin materials affects the bonding performance of the structure.
Failure modes of the composite structures at different temperatures in the drawing test are shown in Fig. 5. The failure occurred between the waterproof bonding layer and the UHPC pavement layer, while only part of the UHPC cement slurry remains on the interface. This implies that the tensile strength of UHPC is much higher than the bonding strength of the interface. The freezing of pore water at −10°C increased the interface matrix bonding force in Fig. 6(a), but the freezing of pore water leads to the failure to participate in the matrix hydration reaction, resulting in the formation of partial pore structure at the interface. Figure 6(b) shows the matrix morphology of the pull-out fracture surface at 25°C at room temperature. It is obviously adhered with UHPC cement matrix that 80% of the surface of the waterproof adhesive layer. The adhesive amount of interface cement matrix at 40°C and 60°C is less in Figs. 6(c) and 6(d). It appeared partial softening or bonding failure at high temperature, which affected the interlaminar pull-out strength, as a result of the resin binder of waterproof adhesive layer had thermoplastic property.
Based on the analysis on the interface between cement and resin published elsewhere [
22], it is considered that neat cement paste is mainly composed of plate-like calcium hydroxide crystals and clusters of isogranular C-S-H, while the resin mainly consists of sheets of wave-shaped materials and a few scattered particles. At the resin-cement interface, two different materials are bonded together, but there are black, unfilled holes at the interface (Fig. 7). Considering that the holes at the interface can reduce the bond strength, we investigated the effects of different coating areal densities (1 and 2 kg/m
2) of bonding layer on the bond strength of the structure at room temperature; the drawing strengths separately are 2.21 and 2.1 MPa, showing no significant difference in strength (Fig. 6).
3.2 Bending performance of the structures
The bending resistance of the composite structure was assessed through three-point bending test and five pavement structures were compared. The test results of load and deflection at bending cracking as well as crack width of different composite structures are summarized in Table 4. For the Type-I composite structure and UHPC based on an RB gravel waterproof layer, cracks appeared in the concrete at the bottom at a load of 20.94 kN. The cracking loads of the composite structures (Types-II and III) with EP bonding layers with the thicknesses of 1 and 2 mm separately improve by 56% and 66% compared with that of the Type-I structure and cracking deflections rise by 16% and 25%, respectively. This indicates that the increased thickness of the bonding layer has little influence on the flexural resistance of such composite structures. When reinforcing mesh is laid in the UHPC structural layer, the cracking load of Type-IV composite structure improves by about a factor of two compared with that of the Type-I structure and mid-span cracking deflection decreases by 26%, suggesting that the reinforcing mesh can double the flexural resistance of the structure.
Figure 7 shows failure modes of different composite structures in three-point bending tests, in which cracks in Type-I, II, and III structures develop vertically toward the steel plate into through-going cracks. This is mainly because the bending moment is maximum at the mid-span position, so that the cross-sections thereat are subjected to tensile failure and cracks in such a tensile zone develop vertically, with the maximum seam width of 0.9 mm. The bending failure of Type-IV and V composite structures is manifested as gradual propagation of multiple micro-cracks inclined to the ribs, with the seam widths of 0.7 and 0.8 mm (a typical diagonal-tension failure).
By further analyzing the load–deflection curves of different composite structures (Fig. 8), it is found that mid-span deflection and deformation of different structures gradually increase with increasing load. When a load of 3.5 kN is applied to the unreinforced UHPC structure, failure is accelerated. As the load on the reinforced UHPC structure reaches 3.5 kN, a moderate form of damage develops, typical of the yield failure of such composite materials. For Type-IV and V composite structures with reinforcement, under the same load, mid-span deflection and deformation of a Type-V structure decrease by about 1 mm compared with those of a Type-IV structure. This further indicates that the wet bonding layer increases the integrity of the composite structures by improving the bond strength of the structures and the strength increases by 3% to 6%.
3.3 Fatigue performance of the structures
The dynamic response and fatigue performance of the composite structure were further studied based on three-point bending fatigue test of beams. Furthermore, they were compared with the SFRC pavement structure and composite structure of EA concrete [
3] with the same thickness. Dynamic deflection can reflect the dynamic bending stiffness of composite structures under repeated effects of certain stress regimes and the test results are as shown in Fig. 9, where
x is the load,
y is the dynamic deflection, and
R is the correlation coefficient. Dynamic deflections of the three pavement structures all increase with increasing fatigue load, following as exponential trend with good correlation. Under the same fatigue load, the dynamic deflection of the RBPC pavement structure decreases by about 20% and 38% separately compared with the SFRC and EA pavement structures. This indicates that the RBPC pavement structure has the maximum dynamic bending stiffness, which is conducive to improving the fatigue capacity of the structure.
A bending fatigue test was conducted on the three composite structures under the fixed force of 9 kN, to investigate influences of the paving materials and the bonding layer on fatigue life of the concrete pavement structures. The results are summarized in Table 5. Rapid disengagement occurs after the UHPC structure without a bonding layer is subjected to 100 load cycles (Fig. 10(a)). Mid-span cracking occurs after the SFRC pavement structure with a bonding layer undergoes 3.3 × 106 load cycles (Fig. 10(b)), which is related to the lack of tensile strength of SFRC; however, after application of more than 1.2 × 107 load cycles, the UHPC pavement structure with a bonding layer remains undamaged and experiences no reduction in stiffness. Such fatigue test results meet the requirements specified in Specifications for Design and Construction of Pavement on Highway Steel Deck Bridges for the fatigue performance of a pavement on a steel bridge deck under special traffic conditions. Meanwhile, it has similar fatigue resistance to a flexible EA pavement.
3.4 Engineering application and tracking observation
Runyang Bridge entered service in May 2005 and comprises a suspension bridge with a main span of 1490 m and a cable-stayed bridge with a main span of 406 m. Two layers of EA with a thickness of 55 mm were placed (as a pavement) on the steel bridge deck. Over time, the traffic flow has increased, with the average daily traffic volume reaching nearly 60000 vehicles in 2019, of which heavy goods vehicles account for about 20%. To prevent fatigue cracking in the steel bridge deck, the most unfavorable position (the quarter-span) on the suspension bridge was maintained through paving with RBPC on the steel bridge deck in 2018. The pavement structure is shown in Fig. 11. HRB400 reinforcing mesh with a bar diameter of 10 mm at a spacing of 50 mm × 50 mm was laid in the pavement layer. The construction was mainly carried out according to the following procedures: de-rusting by sandblasting on the steel bridge deck, laying the RB gravel waterproof layer, spraying the EP bonding layer, and then paving with UHPC. To improve the skid resistance of the paved surface, a high-toughness resin-gravel wearing layer with a thickness of 3 to 5 mm was adopted. After a year’s service the pavement layer of Runyang Bridge appeared to be working well.
By utilizing finite element software ABAQUS, simulation of a partial beam segment of the orthotropic plate was conducted (Fig. 12). Lateral (x-direction) and longitudinal (z-direction) displacement constraints were applied along the driving direction. The design load was a standard load of 100 kN from a single axle and double wheelset, and the tire pressure was 0.7 MPa. In addition, an impact coefficient of 1.3 and stress of 0.91 MPa were applied. By referring to the boundary parameters and the most unfavorable load-position as evinced by previous reference, the effects of different structural parameters on the stress state in the orthotropic plate were analyzed and the elastic modulus of the paving materials was varied within the range of 0 to 50 GPa. The elastic moduli of the RB waterproof layer and the EP bonding layer were 1 GPa and that of the orthotropic steel plate was 210 GPa. The mechanical parameters of the model structure are summarized in Table 6.
The changes in weld strain in the orthotropic plate and relative deflection between ribs under load were analyzed. The results in Fig. 13 illustrated that, with the increase in elastic modulus of the pavement layer, the strain in the steel plate and relative deflection between ribs gradually decreased. As the modulus of the pavement layer increased from 10 GPa (equivalent to that of EA concrete) to 40 GPa, the weld strain in the steel plate decreased by about 32% and the relative deflection between ribs decreased by about 52%.
4 Conclusions
1) This study proposed a UHPC pavement structural system on steel bridge decks based on resin bonding, which included an RB gravel waterproof layer with a thickness of 3 to 5 mm, an EP bonding layer with a thickness of 1 to 2 mm, and a UHPC pavement structural layer with a thickness of 40 to 60 mm. This system provides a new direction for application of UHPC materials in the engineering of steel bridge deck pavements.
2) It showed that the interface was a weak plane in the structure as evinced by data from a vertical drawing test. The drawing strength of the pavement interface could be nearly doubled from 1.3 MPa through use of an EP bonding layer and this bonding performance affected by temperature sensitivity of the organic material, EP. The bond strength of RBPCdecreases with the increase of temperature.
3) As evinced by static bending test data, use of the EP bonding layer can improved bending resistance of the UHPC pavement structure on steel bridge decks by about 50% and the bending resistance after reinforcement in the UHPC structure was twice that of the UHPC structure without reinforcement: this changed the mode of failure from sudden tensile failure to a slower yield.
4) Dynamic deflection of the UHPC pavement structure increased exponentially with increasing fatigue load. The fatigue life was about 1.2 × 107 cycles of loading under a fixed force of 9 kN and a dynamic deflection of 0.35 mm, which satisfied the requirements specified in Specifications for Design and Construction of Pavement on Highway Steel Deck Bridge for fatigue performance of steel bridge deck pavements under special traffic conditions.
5) Through finite element simulation and analysis, the UHPC pavement structural system used on steel bridge decks, and based on resin bonding, could reduce weld strain in the steel plate by about 32% and relative deflection between ribs by about 52% under the standard axial load compared with the EA pavement system. It significantly improves fatigue performance of the orthotropic plate.
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