Evaluation of the static and dynamic structural performance of segmental pre-stressed concrete box girder bridge after repairing and strengthening

Abstract

The objectives of this study are to explain the repairing and strengthening methods which are used to improve the structural performance of the bridge structure, to analyze the static and dynamic responses after strengthening, and to evaluate the performance of the bridge structure after repairing and strengthening. The methods of repairing and strengthening include reconstruction the deck of the bridge by casting 10 cm layer of concrete, strengthening the web and bottom floor of box girders of middle spans and side spans by sticking the steel plates, strengthening the whole bridge structure by using external pre-stressing tendons, and treatment the cracks. The results of theoretical analysis show that the values of tensile stress and vertical deflection are decreased and the compressive stress is increased after strengthening. There are not tensile stresses are appeared in the sections of the bridge structure. The modal analysis results show that the value of natural frequency is equal to 2.09 Hz which is more than the values before strengthening which is equal to 1.64 Hz, indicating that the stiffness of the bridge structure is improved and the strengthening process is effective to improve the cracks resistance and bearing capacity of the bridge structure.

Keywords

structural performance / steel plates / external pre-stressing / cracks / grouting method / static

Introduction

The strengthening of concrete structure includes improvement of the strength and stiffness of structural members, and the repairing process involves re-establishing the strength and function of the damaged members. The strengthening of the bridge structural members can be carried out by replacing poor quality or defective materials by using better quality materials, attaching additional load-bearing materials, and re-distribution of the loading actions through imposed deformation on the structure system [1-3].

The repairing process includes materials selection, method selection, support design, safety precaution, costs, and logistics. The performance requirements of concrete repair involve protection of re-bars, aesthetics, integrity and computability, carry loads, and waterproofing. Concrete structure repair can be classified either as cosmetic-repairs or rehabilitation-repair [4-6].

The selection of the suitable method for the repairing and strengthening of the bridge structural members depends on many factors. These factors include the type and age of structure, the importance of structure, the magnitude of the strength required which is need to increase, the type and degree of damage, available materials, cost and feasibility, and aesthetics [3,7].

External post-tensioning is defined as a system in which the pre-stressing tendons or bars are located outside the concrete section. The pre-stressing force is transferred to the member section through end anchorages, deviators or saddles. The main aim of the bridge structure strengthening by using additional external pre-stressing tendons is to fulfill all necessary serviceability criteria and not to extend its ultimate limit state. Strengthening by using external post-tensioning is simply to apply axial load combined with bending moment to improve the flexural and shear capacity of the bridge structural members [8-11].

The objectives of this study are to explain the repairing and strengthening methods which are used to improve the structural performance of the bridge structure, to analyze the static and dynamic responses after strengthening, and to evaluate the performance of the bridge structure after repairing and strengthening.

Description of the bridge structure

Sanguxian viaduct pre-stressed concrete bridge is type of a continuous segmental box girder T-shape rigid frame bridge and it was located in the Mudanjiang-Harbin Highway within Heilongjiang province in the east north of China. The total length of the bridge is 280 m and the width is 12 m. The spans of bridge are arranged as 35 m+ 60 m+ 90 m+ 60 m+ 35 m. The transverse section of the bridge consists of 10.5 m as a deck and 2 × 0.75 as sidewalk. It has slop 2.2% along the length of the spans. The bridge is constructed by using the cast-in-place cantilever method. There are two separate T-shaped cantilever beams. Each separate T structure is consisted of 10 segments for each side .The length of segment No. 0 is 7.0 m which is located on the top of pier. The segments number one and two are cast-in situ segments. The others eight segments are cast-in-place cantilever segments. The height of the box girder on the top of piers is 5.0m, and the height of the mid-span box girder is 2.0 m .The height of girder varies according to two parabolas along the length of the bridge. Bridge deck pavement has is 1 thickness which is equal to 10 cm. The bridge was open to traffic in 1997. Figure 1 shows the view of the bridge structure. Figure 2 shows the pier and span pre-stressed box girder layout.

Repairing and strengthening of the bridge structure

The results of damage inspection, static and dynamic load test show that the bridge structure has large downward deflection in the center of the bridge. The values of measured downward deflection under static load test are more than the theoretical values. The load test coefficients are more than the allowable values, indicating that the stiffness and elastic working state of the bridge structure is not good. The bridge structure suffers from serious cracks in the parts of middle span and side spans. Therefore, the bridge structure needs to repair and strengthen to improve the rigidity and structural performance. The methods of repairing and strengthening include reconstruction the deck of the bridge by casting 10 cm layer, strengthening the web and bottom floor of box girders of middle span and side spans by sticking the steel plates, strengthening the whole bridge structure by using external pre-stressing tendons, strengthening the lower edge of bottom closure end segment of middle span by using high strength composite fiber, and treatment the cracks.

Reconstruction the deck of the bridge structure

The thickness of original box girders deck is increased by casting reinforcement concrete layer which is equal to 10 cm to enhance the bearing capacity and overall rigidity of the original box girders of the bridge spans. The construction process of new layer includes:

1) Remove the entire old deck pavement.

2) Roughening the exposed top of the box girders and chisel away some of concrete surface of box girders about 2 cm in order to make the surface rough and into the dentate shaped, make stirrup expose. Some appropriate treatments to the bonding surface, such as cleaning and drying are carried out.

3) To make the connection of old and new reinforced concrete is better, epoxy adhesive is applied as a layer on the surface of box girders concrete which has been roughened and implanted ϕ12 steel bars which is 20 cm in length The implanted depth is equal to 10 cm. The spacing between steel bars is equal to 20 cm in a plum-shaped on the top of the original box girders to strengthen the connection between old and new concrete.

4) The steel reinforcement layer (Φ12) of the new concrete deck is laid on the surface of box girders with mesh 10 cm × 10 cm.

5) Casting the new deck layer (10 cm) by using concrete type C-50. Figure 3 shows the bridge deck after reconstruction. Expansion joints are replaced by using good quality materials to reduce the vibration of the bridge structure.

Treatment of the cracks

The bridge structure suffers from many types of cracks. These types include inclined cracks in the web of box girders, longitudinal cracks in the top roof of box girders, vertical cracks in the diaphragm of the pier box girder, and transverse cracks in the bottom floor. Chemical grouting method is used to prevent further expand and intrusion of harmful substances to the box girder structure which will lead to damage the reinforcements and influence the security of structure.

For cracks have width less than 0.15 mm, the grouting method stages include cleaning and remove laitance and dust of the concrete surface, chisel a V-shaped groove which has width is equal to 3 cm and deep is equal to 2 cm along the cracks, cleaning the cracks by using high pressure air to blow, and sealing the cracks by using epoxy resin No. 6101 or No. 624 which is mixed based on the following percentage: Phthalic acid: ethylenediamine: cement= 100∶16∶10∶300. The compressive strength of sealing material should be greater than the compressive strength of concrete. After finishing the sealing process, the surface of cracks is smoothed by using a scraper.

For cracks have width more than 0.15 mm, the pressure grouting method stages include cleaning and remove laitance and dust of the concrete surface, surface cracks should be sealed to keep the grout from leaking out before it has set and the sealed width is 6 cm, install the entry ports, when the crack has been sealed, it is flushed with water to clean it and test the seal for leaks, prepare the grouting materials, using the compressed air to inspect the sealed effect after the curing of sealant and paint sealant again if there is a leakage, and finally, pouring glue is injected into the cracks by using grout injector. Grouting pressure should be controlled at about 0.4 MPa.

Strengthening the web and bottom floor of box girders

After treatment all the cracks and before construction of the anchor beams, the outside and inside of box girders webs and bottom floors are strengthened by sticking steel plates. For the inside and outside of webs, vertical steel plates are used as a type of A3 steel which is 100 mm in width and 10 mm in thickness. The spacing between two plates is equal to 150 mm. vertical steel plates are supported along the height of the box girders webs. After finish the sticking process, the steel plates are painted in two layers of red and gray colors to protect the plates from corrosion. Figure 4 shows the strengthening of the inside of the box girders webs. Figure 5 shows the strengthening of the outside of the box girders webs. For the outside and inside of bottom floors of box girders of the second span and closure segment of middle span, horizontal steel plates are used to strengthen and to improve the rigidity of bottom of box girders. The using of steel plates with size 1200 mm × 600 mm × 30 mm in the strengthening of box girders bottom floors of the second span and size 2700 mm × 600 mm × 30 mm which is used in bottom of closure end of middle span. The spacing between two plates is equal to 28 cm. Figure 6 shows the layout of steel plates in the strengthening of box girders bottom floors.

Strengthening of the bridge structure by using external pre-stressing tendons

The objectives of strengthening by adopting external pre-stressing tendons are to reduce the tensile stress of the box girders, reduce the positive bending moment of the center of the bridge section, reduce the negative bending moment of the piers box girders section, increase the structural resistance for increasing of loads due to increase the deck slab thickness and overloading of traffic load, and increase the cracking resistance and close the almost original cracks. In general, the strengthening of the bridge structure by using external pre-stressing tendons will improve the structural performance, stiffness, rigidity, durability, elastic working state, bearing capacity of the bridge structure.

Layout of external pre-stressing tendons

According to the additional loads of traffics, tensile stress of sections, and weight of strengthening of webs and tops of box girders, the external pre-stressing are designed. Steel strands are adopted as pre-stressing tendons which have high strength, good erosion-resistance, and low relaxation. They meet the standard (ASTMA416-90A- USA). The standard strength is equal to 1860 Mpa and the tension control stress is 1116 MPa (60% of the design strength). There are six tendons (9ϕ15.24 tendons) are installed in the side spans (first span and fifth span), eight tendons (12ϕ15.24 tendons) are installed in the second side spans (second span and fourth span), and eight tendons (12ϕ15.24 tendons) are installed in the middle span (third span). For side spans, the arrangement of external pre-stressing tendons is making three tendons in each side, and four tendons for each side of the second, fourth span, and middle span. These tendons are supported in the top and bottom of box girders by using concrete and steel anchor beams, steel deviators and steel damping devices. Figure 7 shows the transverse layout of external pre-stressing. Figure 8 shows the external pre-stressing after construction.

Anchor beams

There are six concrete anchor beams are constructed on the box girders of abutments and piers. The anchor beams are constructed by using new self-compacting concrete pouring technology. The design compressive strength of self-compacting concrete is equal to C60. To ensure the good connection between the concrete of new anchor beams and original box girders, steel bars planting method is used. Figure 9 shows the anchor beams after construction.

Deviators devices

Steel and concrete deviator devices are used to realize a smooth tendons profile without unacceptable angular changes and un-tolerable deformation of tendons shape, and to resist the transversal and longitudinal forces generated by the tendons and transfer the same safely to the structure. There are two steel deviator devices are located and fixed in the bottom floors of box girders of each side spans (span No. 1 and span No. 5). For the spans No. 2 and No. 4, there are five steel deviator devices in each span. For span No. 2, the first deviator device is fixed in the bottom floor of box girder at distance 7.05 m from the pier No. 1 and for span No. 4, the first deviator device is fixed in the bottom floor of box girder at distance 7.05 m from the pier No. 4. The four steel deviator devices are fixed in the lower part of box girders webs at distances 34.85, 30.75, 26.6, and 22.42 m respectively from the piers No. 2. The middle span contains on the eight steel deviator devices, four of them are fixed in the lower part of box girders webs at distances 34.85, 30.75, 26.6, and 22.42 m respectively from the piers No. 2 and the others four are located in the same distances but from pier No. 3. Figure 10 shows the deviator devices after construction.

Damping devices

Damping devices are used to reduce the effects of vibrations. The damping system of external pre-stressing tendons is depended on the damping bracket supporting method by using damping beam. There are two damping devices in the each side span (span No. 1 and span No. 5). They are fixed in the bottom floors of box girders. The first one is fixed at distance 14.08 m from abutment and the second is fixed at distance 13.3 m from piers No. 1 and No. 4 respectively. For the spans No. 2, there are two steel damping devices. The first one is fixed at distance 13.3 m from pier No.1 and the second one is fixed at distance 19.3 m from pier No. 1. For the span No. 4, the first one is fixed at distance 13.3 m from pier No. 4 and the second one is fixed at distance 19.3 m from pier No. 4. There are four steel damping devices in the middle span. The first one is fixed at distance 38.9 m from pier No. 2 and the second one is fixed at distance 42.5 m from pier No. 2. The third damping device is fixed at distance 38.9 m from pier No. 3 and the fourth device is fixed at distance 42.5 m from pier No. 3. Figure 11 shows the layout of damping devices.

Theoretical analysis of static responses after strengthening

SAP2000 software Ver. 14.2.0 is used to analyze the internal forces of the bridge structure after strengthening due to dead load, live load, internal pre-stressed load, external pre-stressed load, temperature load, and crowded load. In this analysis, the stresses and vertical deflection are considered to evaluate the structural performance after strengthening under load combinations I and II. The following requirements are used in the analysis of the bridge structure. These requirements include:

1) Concrete density= 26 kN/m³, poisson ratio (μ) = 0.2, concrete compressive strength= 40 MPa (C40).

2) Deck loads: deck weigh+ sidewalk weight+ railings weight= 31 kN/m, crowded load= 2.9 kN/m². The total weight per square meter= 31/12 (width of bridge) = 2.58 kN/m².

3) Load combination:

i) Combination I (COMB I) = dead load (structure weight) + deck load+ internal pre-stressed load+ external pre-stressed load

ii) Combination II (COMB II) = COMB1+ moving load (vehicle load) + crowded load+ temperature load.

4) Live load: according to Chinese code (JTG D62-2004) [12], P_k = 180 kN if the length of span≤5m, P_k = 360 kN if the length of span ranges from 5 to 50 m. q_k = 10.5 kN/m. The bridge consists of two lanes. Therefore, the reduction factor is 1. The maximum span length is 90 m. Therefore, the P_k is equal to 360 kN.

Description of bridge model

The bridge model consists of one shell element object which includes five spans. The lengths of spans are equal to 35 m+ 60 m+ 90 m+ 60 m+ 35 m. The bridge is type of segmental box girder. The width of the bridge model is equal to 12 m. There are four piers. The height of girder varies according to two parabolas along the longitudinal bridge. Bridge deck pavement has thickness about 10 cm. Figure 12 shows the bridge model.

Analysis of stresses and deflection due to load COMB I

Analysis of stresses

Figure 13 shows the distribution of stresses due to load combination I along the bridge length before and after strengthening. From this figure it can be noted that the maximum compressive stress of box girder is equal to -12.5 MPa which occurs in top right of box girder at distance 144 m of the bridge length. This value is less than the allowable values in the Chinese codes (JTJ023-85 and JTG D62-04) which are equal to -19.6 and -18.78 MPa. The value after strengthening is increased by 1.8 MPa. There are not tensile stresses appeared in the sections of bridge structure after strengthening, indicating that the strengthening process is effective to improve the cracks resistance and structural performance of the bridge structure.

For JTJ023-85 [13]:

Allowable compressive stress= 0.7 × 28= 19.6 MPa; allowable tensile stress= 1.15 × 2.6= 2.99 MPa

For JTG D62-04 [17]:

Allowable compressive stress= 0.7 × 26.8= 18.78 MPa; allowable tensile stress= 0.7 × 2.4= 1.68 MPa

Analysis of vertical deflection

Figure 14 shows the distribution of vertical deflection on longitudinal bridge structure due to load combination I. From this Figure it can be noted that the maximum value of vertical deflection is equal to -54 mm which takes place in the left side of the center of bridge structure. This value is less than the vertical deflection before strengthening which is equal to -57 mm.

Analysis of stresses and deflection due to load COMB II

Analysis of stresses

The results of stresses analysis before and after strengthening can be shown in Fig. 15. From this figure it can be seen that for the maximum values of box girder top stresses, the higher compressive stress is equal to -11.9 MPa which is less than the allowable values in the Chinese codes (-14 and -13.4 MPa) and it occurs in top right of box girder at distance 144 m of bridge length. For the maximum values of bottom of box girder stresses, the higher compressive stress is equal to -11.4 MPa which is less than the allowable values in the Chinese codes (-14 and -13.4 MPa) and it occurs in bottom left of box girder at distance 185 m of bridge length. There are not tensile stresses appeared after strengthening. For the minimum values of box girder top and bottom stresses, the higher value of compressive stress is equal to -12.5 and -11.8 MPa which are less than the allowable values in the Chinese codes (-14 and -13.4 MPa) and they occur in top right and bottom left of box girder at distance 144 and 185 m of bridge length respectively. All the values of stresses after strengthening are less than the values before strengthening, indicating the strengthening process leads to improve the cracks resistance and bearing capacity of the bridge structure.

For JTJ023-85: Allowable compressive stress= 0.5 × 28= 14 MPa; allowable tensile stress= 0.9 × 2.6= 2.34 MPa.

For JTG D62-04: Allowable compressive stress= 0.5 × 26.8= 13.4 MPa; allowable tensile stress= 0.7 × 2.4= 1.68 MPa.

Analysis of vertical deflection

The distribution of vertical deflections on the bridge structure length due to load combination II are shown in Fig. 16. From this Figure it can be noted that the maximum value of vertical deflection is equal to -62 mm which takes place in the left side of the center of bridge structure. This value is less than the vertical deflection before strengthening which is equal to -65 mm, indicating that the external pre-stressing tendons loads can resist the additional load system and reduces the vertical deflection.

Theoretical analysis of natural frequency

The dynamic performance of the bridge structures is an important indicator to evaluate the carrying capacity and operation state of the bridge structure. Shell element model is used in the dynamic analysis. To evaluate the dynamic performance, natural frequency analysis is adopted by using modal analysis. Figure 17 shows the modes shape of the bridge model. From this figure it can be noted that the value of natural frequency is equal to 2.09 Hz which is more than the values before strengthening which is equal to 1.64 Hz, indicating that the stiffness of the bridge structure is improved.

Conclusions

The conclusions of this study are listed as follows:

1) The results of damage inspection, static and dynamic load test show that the bridge structure has large downward deflection in the center of the bridge structure. The values of measured downward deflection under static load test are more than the theoretical values and the load test coefficients are more than the allowable values, indicating that the stiffness and elastic working state of the bridge structure is not good. The bridge structure suffers from serious cracks in the parts of middle span and side spans. Therefore, the bridge structure needs to repair and strengthen to improve the rigidity and structural performance. The methods of repairing and strengthening include reconstruction the deck of the bridge by casting 10 cm layer, strengthening the web and bottom floor of box girders of middle and side spans by sticking the steel plates, strengthening the whole bridge structure by using external pre-stressing tendons, strengthening the lower edge of bottom closure end segment of middle span by using high strength composite fiber, and treatment the cracks.

2) The results of theoretical analysis of static forces show that the values of compressive stresses, tensile stress, and vertical downward deflection are improved. The compressive stresses are increased, and the tensile stresses and vertical downward deflection are decreased, indicating that the strengthening methods are effective to improve the stiffness and bearing capacity of the bridge structure.

3) The results of theoretical analysis of dynamic responses show that the value of natural frequency is equal to 2.09 Hz which is more than the values before strengthening which is equal to 1.64 Hz, indicating that the stiffness of the bridge structure is improved.

References

[1] Naser E. Repair and Strengthening of Reinforced Concrete Structure. Toronto, Canada: R. V. Anderson Associated Limited, 2005

[2] Arthur H. Design of Pre-stressed Concrete. 2nd ed. New York: Wiley, 1987

[3] Ali F N, Wang Z L. Experimental analysis and performance evaluation of Fu Feng highway prestressed concrete bridge after strengthening in China, A. M. Research, 2011, 189-193: 2346–2352

[4] Brandon R A.Method of strengthening monitored deficient bridge. Master Thesis, Manhattan: Kansas State University, 2007

[5] Edward C. Concrete Repair. Department of Real Estate and Construction, RECO, Construction IV, University of Hong Kong, 2006

[6] Raina V K. Concrete Bridge: Inspection, Repair, Strengthening, Testing, Load Capacity, and Evaluation. 2nd ed. USA: Quebecor Book Press, 1996

[7] Hai L, Wei L, Daved J, Ai H. Strengthening an in-service concrete with CFRP bars. Journal of Zhejiang University, Science A, 2009, 5: 635–644

[8] Thiru A, Tim H. Strengthening of bridge headstocks with external post-tensioning: Design issues and strengthening techniques. In: Proceedings of the Road System and Engineering Technology Forum. Queensland, Toowoomba, 2005

[9] Ahmed E, Sherif Y. Use of external pre-stressing to improve load capacity of continuous composite steel girder. In: Proceeding of the Structure Congress. Structural Engineering and Public safety, ASCE, USA, 2006

[10] Suntharavadivel T, Thiru A. Overview of external post-tensioning in bridges. In: Proceedings of the Southern Engineering Conference. Toowoomba, Australia, 2005, 1–10

[11] Ali F N, Wang Z L. Monitoring of external prestressing tendons construction process of Jiamusi highway prestressed concrete bridge during strengthening in China. Advanced Materials Research, 2011, 163-167: 2873–2879

[12] JTG D62-04. Code for design of reinforced concrete and prestressed concrete highway bridges and culverts, 2004 (in Chinese)

[13] JTJ023-85. Design code for highway reinforced concrete and prestressed concrete bridges and culverts, People's Republic of China, 1985 (in Chinese)

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