FIELD EXPERIMENTAL STUDY ON EXTERNAL PRESTRESSING REINFORCEMENT OF A 420M PC CONTINUOUS BEAM BRIDGE

In this paper, the practical engineering of a 420 m prestressed concrete （ PC ） continuous beam bridge is taken as the research object, and an external prestressing reinforcement method is proposed to reinforce the damaged and cracked girder. The paper is to study the structural performance of a PC continuous beam bridge before and after reinforcement. The heavy vehicle loading test of the reinforced PC continuous beam bridge was carried out. A total of three test spans were selected, and each test span selected seven deflection test sections and a strain test section. The corresponding finite element model was established and verified by the test results. Finally, it was concluded in this study that the external prestressing reinforcement method has a good effect on improving the loading capacity performance of damaged bridges.


INTRODUCTION Current status of research
In the late 20th century, many reinforced concrete bridges were built in China [1]. With the increase of traffic load and service time, there are also construction defects and environmental changes [2]~ [3]. Many bridges have problems such as excessive mid-span deflection, cracking of beam bottom and web [4]. Reinforcement or replacement of old bridges is necessary to ensure safe operation of bridges. If the main beam is replaced directly, it will cause economic losses and vehicle traffic problems [5]. In order to effectively improve the loading capacity and service life of bridges. Many effective reinforcement methods are proposed and applied to the bridge reinforcement [6]~ [9].
Before the reinforcement and maintenance of an old bridge, we should first examine and analyse the existing diseases of the old bridge itself, and then select an appropriate reinforcement measure according to the specific situation [10]. External prestressing is widely used to repair existing structures and build new bridges [11] to [14]. This study selected the most suitable external prestressing reinforcement method according to the specific engineering characteristics.
In order to study the performance of external prestressing reinforcement technology, the situation before and after external prestressing reinforcement is compared. The finite element analysis model of the whole bridge is established. The test results are compared with the calculation results of the finite element analysis model, and the performance of the external prestressing reinforcement technology is analysed and evaluated.

Defects of old bridge
A 420 m PC continuous girder bridge was completed in September 1995. Cracks appeared in the roof, floor, and web of the main girder of the bridge under long-term vehicle load and the surrounding environment. Overall, there were more cracks on the outer surface of the bridge. The majority of the crack width is concentrated in the range of 0.10 mm to 0.40 mm, and the length ranges from 0.3 m to 2.6 m. The width, length, and number of these cracks tend to increase over time. The bridge layout is shown in Figure 1. At span 9, 5.49 m from the top of pier 9, inclined cracks began to appear on the web. Two oblique cracks were found. The crack lengths were 1 m and 0.91 m, respectively. The crack widths were 0.06 mm and 0.08 mm, respectively. The inclined cracks of spans 10, 11, and 12 were mostly concentrated between 34 and 39.5 meters from the fulcrum. The crack length was 0.41 m to 1.34 m. The crack width is 0.12 mm to 0.42 mm. They are caused by the principal tensile stress. Bending cracks appeared over a distance of 17.8 to 12.4 m. The crack height was between 0.06 m and 0.23 m. The crack width was between 0.02 mm and 0.14 mm. At span 13, 4.43 m from the top of pier 14, the web began to appear inclined. The crack length was 1.15 m. The crack width was 0.12 mm.
Secondly, the box girder floor and roof were checked. Some of the floor cracks were inclined cracks on both sides of the web extending to the middle line of the floor, while others were parallel to the bridge direction. The number of cracks increased gradually from each pier to the middle span. The number of floor cracks in span 10, span 11, and span 12 box girders increased gradually from the pier to the midspan direction. The two sides of the middle line of the floor cracks were symmetrical. In the range of 20 m on both sides of each pier, the bottom plate of the box girder had mainly longitudinal cracks. In the L/4 ~ 3L/4 section, a number of inclined cracks appeared in the bottom plate near the edge of the web. The floor crack width was mainly concentrated between 0.1 mm and 0.4 mm. The length of the crack was mostly concentrated in 0.70 m ~ 4.60 m. The roof cracks were symmetrically distributed along the bridge deck midline and increased from the pier to the midspan direction. The longest crack was 5.34 m. The maximum crack width was 0.34 mm. In the range of 15.5 m from the piers on both sides (pier 10 and pier 11), dense intermittent longitudinal cracks appeared. The crack length was between 0.30 m and 2.60 m. The crack width was between 0.08 mm and 0.18 mm. The distribution of main cracks in box girders is shown in Figure 2 Finally, the crack distribution of the prestressed tooth plate and diaphragm was checked. There were many inclined cracks at the bottom of the roof at the top prestressed tooth plate of pier 9 and the top prestressed tooth plate of pier 14. The crack length was 0.15 m to 0.52 m. The crack width was 0.08 mm to 0.20 mm. The inclination angle was 30° ~ 60°. There were no stress cracks discovered in the diaphragms of piers 10, 11, 12, and 13. The diaphragm at the middle section of span 9 ~ 13 had vertical cracks on the top of the passing holes. On the one hand, the crack cracked horizontally downward along the direction of hole thickness. On the other hand, it extended vertically upward to the bottom of the roof. The width was between 0.14 mm and 0.30 mm. The length was between 0.35 m and 1.43 m. The distribution of specific cracks is shown in Figure 3.

Layout of External Prestressed Steel Bundles
The externally prestressed tendons were added to the box girder to improve the cross-sectional strength, control the development of cross-section cracks, and effectively improve the stress of the girder. Six strands for span 9 and 13. Eight strands for span 10, 11 and 12, each consisting of 13 steel wires with a nominal diameter of 15.2 mm [15]. The two ends of the side span external prestressed steel beam were respectively anchored on the beam and the anchorage block. The middle-span external prestressed steel beam was cross-anchored on the anchorage block at the pier top. The detailed arrangement is shown in Figure 5. The externally prestressed steel beam was tensioned at both ends. The tension control stress was 1209 MPa. The steel bundle was wrapped in a corrosion-resistant casing. Adjustable cable force and a replaceable anchor were used to facilitate future bridge maintenance. The field layout of externally prestressed tendons is shown in Figure 6.

Test section and loading condition
This bridge's span combination was (60 + 3 x 100 + 60) m. Span 9, 10 and 11 were selected as the test spans. The control load was about 350 kN (according to Chinese design code JTG 021-1989 [16]). Static load test using the principle of least favourable arrangement principle [17].The main test content was the deflection and strain of the test span. Finally, the structural performance parameters of the bridge before and after reinforcement were obtained.
Specific test sections were selected as follows: Seven deflection test sections and one strain test section were arranged for each span. The deflection test sections were located at L/8, 2L/8, 3L/8, 4L/8, 5L/8, 6L/8 and 7L/8. The strain test section was located at 4L/8. The specific layout is shown in Figure 7.

Fig.7 -Deflection and stress test sections
The static load test was divided into six conditions. Eight tri-axle trucks were used to simulate moving loads. A total weight of about 35 tons per truck. It takes 15 minutes to load each time. Then collect the data from each measuring point. Information about the loading condition is shown in Table  1. Loaded vehicle weight information is shown in Table 2. The loading vehicle arrangement is shown in Figure 8.  The site vehicle arrangement is shown in Figure 9. measurement, the pier top was a fixed point. The tower ruler was placed on the test section. The precision level was placed on the pier top beam. The vertical deflection of the test section relative to the pier top was measured. The arrangement of measurement points is shown in Figure 10. The picture of the field test is shown in Figure 11.

Deflection
Deflection is an important parameter to characterize the bearing capacity of a bridge. The vertical deflection data and theoretical finite element model data of spans 9, 10 and 11 before and after reinforcement are shown in Figure 12. According to the above curve, the bearing capacity of the No. 9 span increased by 18 %. The bearing capacity of the No. 10 span increased by 15%. The bearing capacity of No. 11 span increased by 18%. These results demonstrate how external prestress can greatly increase the structure's stiffness.
Comparison of deflection data before and after reinforcement. Most measured deflection data had exceeded theoretical data before reinforcement. After reinforcement, the measured deflection data has decreased to varying degrees. The maximum measured deflection after reinforcement is 28.49 mm, which is 41% lower than that before reinforcement of 48.52 mm.

Strain
Stress is another key data point reflecting bridge performance. The measured strain data before and after reinforcement and the theoretical data of the finite element model are shown in Figure 13.

Fig.13 -Measured strain curve under static loading test
The overall performance of the three test spans improved after reinforcement. No. 9 span the strain calibration coefficient between 0.74 and 0.85, the average carrying capacity increased by 24 %, and the average calibration coefficient increased by 22 %. No. 10 span the strain calibration coefficient between 0.71 and 0.83, the average carrying capacity increased by 27 %, and the average calibration coefficient increased by 25 %. No. 11 span the strain calibration coefficient between 0.70 and 0.85, the average carrying capacity increasing by 26%, and the average calibration coefficient increasing by 23%.
From the loading position, the bearing capacity under the central loading condition is increased by 25% on average, and the verification coefficient is increased by 23% on average. Under eccentric loading, the bearing capacity is increased by 23%, and the calibration coefficient is increased by 24%.
At the top of pier 10, the bearing capacity increased by 14% on average. The calibration coefficient increased by 16% on average. The measured strain data are shown in Figure 14.

Fig.14 -Measured strain data under static loading test
After external prestressed reinforcement, the stiffness and bearing capacity of the bridge were significantly improved. The safety reserve of the bridge was increased. Which provides security for the continuous service of the bridge.

CONCLUSION
This paper describes the external prestressing reinforcement of a damaged PC continuous girder bridge. Then the static load tests of span 9, span 10, and span 11 are carried out. By comparing the measured data with the theoretical data of the finite element model. It is discovered that the bridge's overall performance has improved. The deflection and strain data after reinforcement are lower compared with those before reinforcement.
Overall, span 9, span 10, span 11 and pier 10 bearing capacity increased by 24%, 27%, 26%, and 14%, respectively. From the loading position analysis, the bearing capacity of central loading increased by 25% on average. The bearing capacity of eccentric loading increased by 23% on average. From the above data, the external prestressing reinforcement method proposed in this paper is very effective for improving the bearing capacity and overall performance of a PC continuous beam bridge with aging damage. In subsequent studies, the external prestressing reinforcement method can be applied to other types of old bridges or can be used in conjunction with some other reinforcement methods.