Research on Existing Bridges Rapid Load-Lifting Technology with UHPC in Highways Reconstruction and Expansion Projects

Abstract

There are a large number of small and medium-span bridges in the highway reconstruction and expansion projects. These bridges have been used for at least 20 years. Due to the design according to the old code the ultimate bearing capacity is generally insufficient after the reconstruction and expansion. This paper selects hollow-core slab bridges that account for more than 80% of the reconstruction and expansion projects as the research object, carefully evaluates the specific reasons for the insufficient bearing capacity of small and medium-span bridges, designs a plan for rapid loading-lifting of existing bridges, and uses theoretical analysis. Then, the comprehensive method of finite element calculation analyzes and studies on the reinforced bridge’s bearing capacity. The research shows that the existing small and medium-span bridges in the reconstruction and expansion project are mainly manifested by insufficient shear-bearing capacity after the load is improved. Using the bridge deck to add the UHPC cast-in-place layer (participating in the structural internal force) can effectively improve the existing old bridge Shear resistance capacity, and UHPC cast-in-situ layer shear reinforcement efficiency is much greater than that of ordinary concrete.

1 Introduction

At present, a large number of small and medium-sized span concrete bridges are involved in domestic highway renovation and expansion projects, with a service life of about 20 years. After testing, most of the bridges have good overall performance. However, due to the old code used in the bridge’s initial design, the load standards could be higher. If the technical standards used in the reconstruction and expansion project are followed (Several Opinions on Handling Technical Issues in Highway Reconstruction and Expansion Project ([2013] No. 634), “Technical Standards for Highway Engineering” (JTG B01-2014), and Design Rules for Highway Reconstruction and Expansion (JTG L11-2014) [1], the ultimate bearing capacity cannot meet the requirements. Therefore, it is necessary to carefully select whether the existing bridge should be reinforced or demolished for reconstruction.

If the existing old bridge is demolished in situ and then newly built, it will cause significant financial waste; When using conventional reinforcement utilization schemes, there are drawbacks such as low reinforcement efficiency, complex construction techniques, and significant impact on existing high-speed traffic. In response to the above issues, mainstream China design units currently recommend using the method of adding ordinary concrete. However, adding ordinary concrete is inefficient in improving the cross-sectional bearing capacity and requires complex processes such as planting steel bars on existing bridge decks.

In order to effectively utilize existing old bridges and save engineering investment, it is urgent to study how to quickly strengthen existing concrete bridges, a practical problem faced in renovation and expansion projects. This article relies on a China highway renovation and expansion project to address the problem of insufficient ultimate bearing capacity of existing small and medium-sized span concrete bridges. It proposes a rapid load-bearing plan for pouring ultra-high performance concrete (UHPC) bridge deck layer (participating in structural internal stress).

2 Project Overviews

A certain Highway runs from east to west and is an essential economic artery transportation channel within the province. Since its opening, with the rapid development of the regional economy and society and the further improvement of the highway network, the traffic volume has overgrown. The existing road capacity of highways can no longer meet the needs of economic, social, and transportation developments. The project urgently needs to change the existing 4 lanes to 8 lanes, and the total route mileage of the renovation and expansion project is about 310 km. The bridges along the line include a large number of small and medium-sized span bridges designed based on the 1985 bridge code According to the original design and construction drawings and on-site field investigations of this project, the main types of bridge superstructure are: prefabricated reinforced concrete solid slabs, prefabricated reinforced concrete hollow slabs, prefabricated prestressed concrete hollow slabs, prefabricated prestressed concrete I-shaped composite beams and prefabricated prestressed concrete T-beams. The spans of hollow slab (solid slab) bridges with a length of 5–16 m account for 80% of the total spans of the bridge.

In order to investigate the performance of existing small and medium-span bridges, the testing unit conducted load tests on several typical bridges in this project. The vehicle loads in the load tests were taken as Automobile – Class 20, Trailer -120, and the spans of typical bridges were selected as 5 m, 8 m, 10 m, 13 m, 16 m, and 25 m. The test results showed that the calibration coefficients of all bridges were less than 1, the relative residual displacement was less than 20%, the measured fundamental frequency was greater than the theoretical fundamental frequency, and the measured impact coefficient was less than the theoretical impact coefficient, No new cracks were found in the inspection results; The inspected bridge is in an elastic working state under the load of “car over 20 ton”, and the overall performance is good. In this renovation and expansion project, based on multiple plan reviews and expert discussions, it was unanimously agreed to continue to fully utilize or reinforce small and medium-sized span bridges with good performance. Because the proportion of hollow slab bridge beams in this project reaches 80%, research should focus on the rapid reinforcement scheme of existing hollow slab bridge beams.

In order to concise this article, which only evaluates the bearing capacity of reinforced concrete hollow slabs with typical spans of 10 m, 13 m, and 16 m, and studies the rapid reinforcement scheme. Typical hollow slab bridge section structure is shown as in Fig. 1.

Fig. 1.

Typical hollow slab bridge section structure (cm)

3 Assessment of Bearing Capacity of Existing Bridges Before Reinforcement

3.1 Technical Status of Bridges

According to Specification for Inspection and Evaluation of Load-bearing Capacity of Highway Bridges (JTGIT J21-2011) and the inspection report, the statistical analysis of the technical status of prestressed concrete solid (hollow) slab bridges along the entire line is shown in Table 1. From the statistical analysis results in Table 1, it can be seen that the prestressed concrete hollow slab bridge is mainly composed of 10 m and 16 m bridges of Class II and Class III, with the most representative bridges having a technical level of Class III.

Table 1. Statistics of technical status of prestressed concrete solid (hollow) slab bridge

3.2 Calculation of Bearing Capacity Before Reinforcement (According to JTG B01–2014 Code)

In the case of directly utilizing the old bridge, there is basically no lifting space for the old bridge. According to Highway Class I (JTG B01-2014 load), the bearing capacity of the old bridge after widening was checked, and the calculation results are shown in Tables 2 and 3.

Table 2. Calculations of flexural bearing capacity of 1/2 section in the middle of 10, 13, 16 m slabs

Table 3. Calculation of shear bearing capacity at support of 10, 13, 16 m slabs

The calculation results of whether the required section bending and shear resistance are met indicate that when the thickness of the old pavement layer does not participate in the section stress, the bearing capacity of all hollow slabs does not meet the requirements; When the thickness of the old pavement layer is involved in the force acting on the section, the bending bearing capacity of the section can basically meet the requirements of the new code, but the safety factor is low. At the same time, the increase in shear bearing capacity is limited and still does not meet the requirements of the new code.

Based on engineering experience, the bridge deck pavement layer in actual engineering is involved in bearing capacity of structure. Therefore, the bending bearing capacity of existing small and medium-sized span bridges can basically meet the requirements, but the safety factor is relatively low. On the other hand, the reserve of shear bearing capacity for existing small and medium-sized span bridges still needs to be increased even after considering the participation of the pavement layer in the bearing capacity. Therefore, when formulating the design plan for lifting and strengthening existing bridges, the primary consideration should be to enhance the shear-bearing capacity of existing bridges.

4 Research on Load Lifting and Reinforcement Scheme for Existing Bridges

4.1 Overall Principles and Objectives of Reinforcement

• (1) General principles of reinforcement

  • 1) Scientific and reasonable design with economic and environmental protection;

  • 2) Increasing the self-weight of the structure is not significant and does not cause damage or slight loss to the original components;

  • 3) Convenient and fast construction with minimal impact on the surrounding environment;

  • 4) The bridge reinforcement can meet the bearing capacity requirements of the new code.

• (2) Reinforcement target

  • 1) The ultimate limit state calculation of the bearing capacity of the original bridge after reinforcement should meet the requirements of Highway – Class I (JTG B01–2014 load), and the ultimate limit state calculation of normal use should meet the load level requirements of “Automobile – Class 20, Trailer -120”;

  • 2) Damage that occur in bridge structures or components should be repaired or improved through reinforcement measures to meet the needs of expansion.

4.2 Comparison and Selection of Reinforcement Methods

Based on current engineering practice [2], the methods for strengthening the ultimate bearing capacity of bridges include: ① increasing the cross-section (increasing the primary reinforcement, increasing the beam ribs, thickening the bridge deck, etc.); ② pasting steel plates (carbon fiber, fiberglass, etc.); ③ external prestressing (steel bars, carbon plates, etc.) reinforcement; ④ adding auxiliary components or changing the structural system method. Among them, ②–④ reinforcement methods have high requirements for construction technology, require professional teams for construction, have high requirements for the construction environment, and have high construction costs.

In order to ensure the unity of the later management and maintenance of newly built and existing bridges in the renovation and expansion project, this project proposes to use secondary reinforcement technology for existing bridges, so that when constructing and expanding bridges, simple and rapid reinforcement method is used for existing old bridges; When the performance of the bridge structure deteriorates in the later stage, the secondary reinforcement methods described in ②–④ can be adopted to maximize the service life of the existing old bridge.

Most of the existing old bridges in this project can meet the requirements for flexural bearing capacity, but their shear bearing capacity needs to be significantly improved. Using simple and rapid reinforcement methods for shear reinforcement is a technical challenge faced by engineering.

In terms of shear-bearing capacity, the main factors affecting shear bearing capacity are shown in Fig. 2 mainly including Vc (shear bearing capacity of concrete in the compression zone at the top of the inclined section), Vs (shear bearing capacity of stirrups), and Vsb (shear bearing capacity of bent steel bars).

Since the shear reinforcement of existing old bridges cannot generally be supplemented or strengthened, it is essential to focus on strengthening the compression zone Vc of the inclined section. Based on the principle of rapid construction, the method of “increasing the beam section” is considered to increase the shear area of the section. Specifically, the technique of “thickening the bridge deck” can be selected to use the method of adding a concrete layer to the bridge deck.

Regarding selecting the cast-in-place concrete layer for the bridge deck, two schemes, C50 ordinary concrete and UHPC, have been preliminarily formulated.

Fig. 2.

The bearing capacity influencing factors of the oblique section

4.3 Basic Principles of Bridge Deck Reinforcement for Existing Bridges Based on UHPC

There are few cases and research results of using UHPC bridge deck to reinforce existing bridges in engineering practice [3]. This study draws on the relevant research ideas of the Swiss standard “Recommendation: Ultra High Performance Fiber Reinforced Cement based composites (UHPFRC) Construction Material, Dimensioning and Application” (SIA-2052) [4].

$$ \begin{gathered} \quad {\text{M}}_{\text{d}} \le {\upsigma }_{\text{c}} {\text{b}}\frac{{\left( {{\text{x}} - {\text{h}}_{\text{U}} } \right)}}{2}\left( {{\text{h}}_{\text{C}} - {\text{d}}_{{\text{sc}}} - \frac{1}{3}\left( {{\text{x}} - {\text{h}}_{\text{U}} } \right)} \right) + {\upsigma }_{{\text{sc}}} {\text{A}}_{{\text{sc}}2} \left( {{\text{d}}_{{\text{scc}}} - {\text{d}}_{{\text{sc}}} } \right) + ({\upsigma }_{{\text{Uc}}} + \hfill \\ \frac{{{\text{E}}_{{\text{UC}}} }}{{{\text{E}}_{\text{C}} }}{\upsigma }_{\text{c}} {\text{b}}){\text{b}}\frac{{{\text{h}}_{\text{U}} }}{2}\left( {{\text{h}}_{\text{U}} + {\text{h}}_{\text{C}} - {\text{d}}_{{\text{sc}}} - \frac{1}{2}{\text{h}}_{\text{U}} } \right) + {\upsigma }_{{\text{sU}}} {\text{A}}_{{\text{sU}}} ({\text{d}}_{{\text{sU}}} - {\text{d}}_{{\text{sc}}} ) \hfill \\ \end{gathered} $$

(1)

The height $x$ of the concrete compression zone is determined by the following formula:

$${\text{f}}_{\rm{sd}}{\text{A}}_{\rm{sc}1}={\upsigma }_{\text{cb}}\frac{(\text{x}-{\rm{h}}_{\text{U}})}{2}+{\upsigma }_{\text{sc}}{\rm{A}}_{\text{sc}2}+{(\upsigma }_{\rm{Uc}}+\frac{{\text{E}}_{\rm{UC}}}{{\text{E}}_{\rm{C}}}{\upsigma }_{\text{cb}}){\text{b}}\frac{{\text{h}}_{\rm{U}}}{2}+{\upsigma }_{\rm{sU}}{\text{A}}_{\rm{sU}}$$

(2)

• (1) Calculation of bending bearing capacity

Basic assumptions for calculation: a) The cross-sectional strain remains plane b) Do not consider the tensile strength of concrete in the tensile zone; c) Take the corresponding stress-strain for the UHPC section; d) The compressed area is distributed in a triangular shape.

The bending bearing capacity of the composite section is jointly provided by ordinary concrete in the compression zone, existing steel bars in the compression zone, UHPC and supplementary reinforcement in the compression zone. Refer to Fig. 3 and Eqs. 1 and 2.

Fig. 3.

Calculation of flexural bearing capacity of UHPC-Common concrete composite layer

Fig. 4.

Calculation of Shear Resistance of Cross Section of UHPC Concrete

The formula for calculating the shear bearing capacity of UHPC ordinary concrete composite section is as Fig. 4 and Eqs. 3 and 4.

$${\text{V}}_{\text{Rd}}={\text{V}}_{\text{Rd},\text{c}}+{\text{V}}_{\text{Rd},\text{s}}+{\text{V}}_{\text{Rd},\text{U}}$$

(3)

• a) Ordinary concrete part:

  $$ {\text{V}}_{{\text{Rd}},{\text{c}}} = \frac{{{\text{f}}_{{\text{cd}}} \cdot {\text{b}}_{\text{w}} }}{2}\left[ {\frac{{\text{x}}}{{\sin {\upalpha }_{\text{c}} }} \cdot (1 - \cos {\upalpha }_{\text{c}} )} \right] $$

  (4)

  The inclination angle of diagonal cracks caused by bending moment and shear in ordinary reinforced concrete ${\rm{\alpha }}_{\text{c}}$. There are the following boundaries: $20^\circ \ll {\upalpha }_{\text{c}} \ll 60^\circ$. The first approximation can be assumed to be ${\rm{\alpha }}_{\text{c}} = 35^\circ$. Calculate the height of the concrete compression zone according to the following expression: $x = 0.9 \cdot {\upomega }_{\text{M}} \cdot {\text{d}}_{{\text{Eq}}}$ . Among them: ${\upomega }_{\text{M}}$ is the content of composite section steel bars; ${\text{d}}_{\text{Eq}}$ is the equivalent height of the composite cross-section.

• b) Vertical shear reinforcement:

  $$ {\text{V}}_{{\text{Rd}},{\text{s}}} = {\text{A}}_{{\text{sw}}} \cdot {\text{f}}_{{\text{sd}}} \cdot \cot {\rm{\alpha }} $$

  (5)

• c) Reinforced UHPC part:

  $$ {\text{V}}_{{\text{Rd}},{\text{U}}} = \frac{{2 \cdot {\text{M}}_{{\text{Rd}},{\text{RU}}} }}{{{\text{l}}_{\text{z}} }} \;{\text{where}}\;{\text{l}}_{\text{Z}} = {\text{a}}_0 - \frac{{{\text{d}}_{{\text{sc}}} }}{{\tan {\upalpha }_{\text{c}} }} $$

  Among them, ${\text{M}}_{\text{Rd},\text{RU}}$ is the ultimate flexural bearing capacity of UHPC.

The above analysis shows that after the combination of UHPC and ordinary concrete sections, the shear bearing capacity of the combined section exceeds that of VRD and U. The core of the UHPC reinforcement section is that the ultra-high strength UHPC layer limits the propagation of oblique cracks in the cracking zone.

4.4 Design of Reinforcement Plan for Existing Bridges

In this project, the elevation needs to be adjusted through asphalt concrete pavement. Hence, the reinforcement design plan also considers the thickness matching of the cast-in-place concrete layer and the asphalt concrete layer. The specific design scheme is shown in Table 4.

Table 4. Reinforcement plan for cast-in-situ layer of 10 m, 13 m, 16 m span hollow slab bridge deck

The main construction steps of using UHPC to reinforce the bridge deck include: (1) chiseling off the old asphalt concrete and concrete pavement layer of the bridge deck; (2) Using high-pressure water jet and other methods to chisel and remove debris; (3) The UHPC joint is connected with steel bars as auxiliary connections, and the anchoring length of the joint steel bars should be > 15d (d is the diameter of the overlapping steel bars), as shown in Fig. 5.

Fig. 5.

Joint treatment of cast in situ layer of UHPC bridge deck

Many studies have shown that [5,6,7,8,9,10,11] UHPC has good bonding performance with conventional concrete, so there is no need to use complex processes such as planting steel bars for interface connection. Due to the omission of the original bridge deck reinforcement planting process, construction efficiency can be significantly improved.

4.5 Effectiveness Evaluation of Reinforcement Schemes Based on Swiss Code (SIA-2052)

In the above reinforcement plan, due to the reliable interface connection between the cast-in-place layer and the existing old bridge, the UHPC (or C50 concrete) cast-in-place layer can participate in the stress as a part of the structure.

The bending and shear bearing capacity of hollow slabs with spans of 10 m, 13 m, and 16 m were calculated and analyzed using the UHPC reinforcement design theory in the Swiss code (SIA-2052) and combined with three-dimensional finite element analysis as an auxiliary analysis.

Only the calculation results of the middle board are displayed. The analysis results are shown in the following Figure.

Fig. 6.

Calculation result after reinforcement of 10 m span low slab (medium slab)

Fig. 7.

Calculation result after reinforcement of 13 m span low slab (medium slab)

Fig. 8.

Calculation result after reinforcement of 16 m span low slab (medium slab)

From the calculation results in Figs. 6, 7 and 8, it can be seen that compared to ordinary concrete C50, the UHPC layer does not significantly improve the bending bearing capacity of existing bridges, and the coefficient of improvement is about 1.0% to 2% under the same thickness of reinforcement layer; The UHPC layer significantly improves the shear bearing capacity of existing bridges, with an increased coefficient of about 15%–16% under the same thickness of reinforcement layer.

4.6 Evaluation of Reinforcement Effect Based on 3D Solid Finite Element Method

In order to fully reveal the influence of pavement thickness and materials (ordinary concrete, UHPC) on the shear bearing capacity of the section, Abaqus three-dimensional universal finite element method was used for precise elastic-plastic numerical simulation. A 10 m span hollow slab was taken as a specific research object.

• (1) Model establishment

  Establish two finite element models: ‘original hollow slab’, ‘hollow slab + 10 cmC50 reinforcement layer’, ‘hollow slab + 15 cmC50 reinforcement layer’, and ‘hollow slab + 10 cmUHPC reinforcement layer’.

  The concrete slab is simulated using solid unit C3D8R; Ordinary steel bars and prestressed steel bars are simulated using rod element T3D2.

  Due to the connection between the reinforcement layer and the original hollow slab through planting bars, in the model analysis, the reinforcement layer and the top surface of the original hollow slab are constrained by binding constrain (tie), without considering the shear slip between the new and old concrete surfaces.

  The load is displaced, with a loading position of 0.8 m from the fulcrum and a maximum loading displacement of 80 mm. Apply simple support at the four corners of the bottom surface of the box beam, as shown in the Fig. 9.

Fig. 9.

Three dimensional solid finite element model of low slab

• In this simulation, numerical analysis was conducted on the shear bearing capacity of different overlay heights and materials as shown in Fig. 10.

Fig. 10.

Shear reinforcement plan for low slab

• (2) Main calculation parameters

  The original hollow slab C40 concrete and the reinforcement layer C50 both use the ordinary concrete damage plastic mode, and its constitutive model adopts the relevant provisions of the “Code for Design of Concrete Structures” (GB 50010-2010); Ordinary steel bars adopt a bilinear elastic-plastic model; The prestressed reinforcement adopts an ideal elastic-plastic model and is prestressed by the cooling method.

  The material indicators of UHPC adopt the relevant regulations of the Technical Code for Ultra High Performance Lightweight Composite Bridge Deck Structures (GDJTG/TA01–2015) and its detailed parameters are shown in Table 5.

Table 5. UHPC damage plasticity model parameters

• (3) Analysis results

  The calculation results are shown in the following Figure:

Fig. 11.

Influence of payment thickness and material on shear capacity

• From the above three-dimensional elastic-plastic finite element analysis results (Fig. 11), it can be seen that when using the same thickness of reinforcement layer, the shear bearing capacity of UHPC layer is increased by about 12% compared to C50 layer, which is close to the calculation results using Swiss standard (SIA-2052) in Sect. 4.3.

5 Conclusion

This article takes the hollow slab with small and medium-sized spans, which accounts for many existing highways, as the research object. In response to the problem of insufficient bearing capacity brought about by the renovation and expansion of highways, a rapid reinforcement scheme using a UHPC layer on the bridge deck is proposed. The feasibility of this scheme is preliminarily verified through theoretical analysis and three-dimensional finite element simulation results. The main research conclusions are as follows:

• (1) After the renovation and expansion of existing small and medium-sized span bridges, when using new standards to evaluate their ultimate bearing capacity performance, their insufficient bearing capacity is mainly manifested as insufficient shear bearing capacity.

• (2) According to the on-site construction characteristics of the renovation and expansion project, a scheme of cast-in-place UHPC layer on the bridge deck and planting reinforcement to form a structural whole with the existing bridge deck is adopted, which has significant advantages such as simplicity, speed, and minimal traffic impact.

• (3) The effect of the UHPC layer on improving the bending bearing capacity of small and medium-sized span bridges is not significant, and it only increases by 1% to 2% compared to ordinary C50 concrete of the same thickness; The main reason is that the bending bearing capacity of bridges is not only related to the concrete in the compression zone but also directly determined by the reinforcement configuration in the tension zone.

• (4) The UHPC layer has a significant effect on improving the shear bearing capacity of small and medium-sized span bridges, with an increase of 12%–16% compared to ordinary C50 concrete of the same thickness; The main reason is that UHPC, which is rich in steel fibers and has high compressive and tensile strength, can significantly limit the development of cracks in inclined sections under shear limit states.

• (5) When using a UHPC bridge deck to reinforce existing bridges, as only the compression area is reinforced and the tension area is not correspondingly reinforced, it should be noted that the thickness of the UHPC reinforcement layer should be manageable. Otherwise, it is easy to form a few reinforced beams so that the structural failure mode changes from ductile failure to brittle failure.

• (6) There are relatively few UHPC-based bridge deck reinforcement cases for existing bridges worldwide. When researching design schemes, relevant calculations and analysis can be made by referring to the Swiss standard (SIA-2052); In the later stage, systematic research should continue to be conducted through theoretical analysis and model experiments, ultimately improving the appropriate design methods.