Service life extension of existing precast concrete girders

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1 Concrete Repair, Rehabilitation and Retrofitting II Alexander et al (eds) 29 Taylor & Francis Group, London, ISBN Service life extension of existing precast concrete girders D.I. Banic, D. Tkalcic & Z. Banic Civil Engineering Institute of Croatia, Zagreb, Croatia ABSTRACT: Segmental bridges have been the most commonly built type of road structure in the last 3 years, as a part of modern roads and motorways in Croatia. Fast construction of road network was possible only with segmental concrete bridges. In most cases bridges were built as simple supported systems. During regular and preventive bridge inspections that were carried out on a great part of motorways, dangerous defects on overpasses were recorded. These defects were mostly found on one type of bridge structure. The paper shows a detailed description of this bridge structure, problems in inspection and maintenance, the results of damage inspections, characteristic defects as well as repair methods and proposals for future maintenance. These bridges were tested, using both static and dynamic full scale load tests, before and after rehabilitation. The paper also presents test results and conclusions. As a result, some improvements that would upgrade structural behaviour and load bearing capacity were suggested. 1 INTRODUCTION More than 14 km of modern roads have been constructed in Croatia in the last 3 years. Accelerated construction imposed the use of prestressed girders, connected with cross girders and a reinforced-concrete deck slab. This type of structure enabled the construction of around 2 bridges in the 197 s and 8 s. In the last fifteen years the construction was significantly accelerated due to the reconstruction of motorway network and other roads, after the war for independence, as well as due to the deteriorated condition of other bridges. Road and motorway owners invested very little in the regular maintenance and repair of existing bridges in that period. As the planned construction of 15 km of modern motorways and 3 km of roads of other classes is nearing its completion in 212, an increasing amount of attention is being given to the preventive inspections and maintenance of existing structures. In the period between 2 and 27, the Civil Engineering Institute of Croatia carried out detailed visual inspections, field investigation works and laboratory testing of specimens and based on that made a condition evaluation of over 2 bridges on Croatian motorways. The results of these inspections will be of great use to the owner in reaching decisions on maintenance measures as well as on the amount of necessary means that need to be invested. Conclusions reached in this study will help state authorities to make a modern system of bridge management on Croatian roads. The analysis of the results of investigation works showed that bridges with the same or similar structures suffered the same type of damage, but in various degree of progress. Since some structures no longer satisfied the required level of load bearing capacity and reliability, additional investigation works were performed on them, as well as control static and dynamic calculations. This referred especially to the overpasses across the motorway, which sustained damage due to the traffic of heavy vehicles under and over the structure. The majority of these overpasses was built with precast hollow core prestressed girders of SAN 75 type. 2 OVERPASSES WITH SAN TYPE GIRDERS Overpasses built with SAN girders have proved as very suitable for accelerated construction. The girders are manufactured in a factory, as precast elements and delivered to the site, where they are placed simply and quickly, by means of truck cranes, due to their small weight of approx 4 tons. Girders of SAN type can be made with voids or without them; they have different height and thickness, and a different amount of reinforcement, depending on the required bearing capacity. The width of individual girders is from 1. to 2.1 m (three voids). They bridge spans from 1 to 25 m (Fig. 1). The majority of overpasses across the motorway consist of four spans m = m. Their total length amounts to m, including abutment wing walls of 1.98 m. 69

2 Figure 1. Overpass cross section with SAN girders. Figure 3. View of a typical structure. Figure 2. New overpass cross section with SAN girders. Cross section of the structure (Fig. 1) consists of five precast girders, with three voids of 5 cm diameter. Precast girders are 21 cm in width and 75 cm in height, and they are marked VIADUCT «SAN 21/75». Girders are fabricated in different lengths, but their length is most often 246 cm on motorways. The girders are previously prestressed with adhesion in the plant with 48 wires of 12.4 mm diameter. They are made with concrete of class C 3/37 and additionally reinforced with structural deformed reinforcement. They are manufactured in production plants and delivered to the site by trucks or train. After placement in position, the girders are connected with liquid polymer cement mortar in the longitudinal direction (Fig. 1). In some cases, girders were placed without the deck slab, and asphalt was placed directly on them, while in other a deck slab was cast on top. Precast girders made in this way had a very thin concrete cover (approx 2 cm), which affected the duration of their service life, especially since they are situated is a very aggressive environment. Static system (cross section) of these overpasses is either a simple supported beam, if girders are sealed with tricosal, or a continuous structure if deck slab was placed. Precast reinforced-concrete elements of sidewalks are connected to end girders by in situ cast kerbs at the anchorage point of reinforcement. Main girders are supported by precast head beams of piers and abutments, through elastomer bearings. Deck structure consists of two asphalt layers, each 3 cm thick. Waterproofing layer is placed directly on the upper girder surface. Extensive investigation works that were performed on all structures included a visual inspection, field and laboratory testing. Field methods included pull off, mapping of electrochemical potential, registering the position and width of cracks, reinforcement layout, and determination of concrete strength with rebound hammer. Laboratory testing was carried out on drilled concrete specimens. The following properties were determined: compressive strength of concrete, depth of carbonation, chloride profile, capillary absorption coefficient and specific gas permeability coefficient. 3 STRUCTURAL CONDITION Most of the damage in structural elements arose from a poorly made longitudinal joint between individual girders. Particularly severe damage was caused by leakage of salty rainwater and damaged waterproofing layers and expansion joints. Installed waterproofing functioned properly for a while, but because of the problematic vertical joints between girders, the system sustained damage. Based on the detailed analysis of the cause of waterproofing damage, it was concluded that damage occurred due to different displacements of main girders in the cross section, under the action of asymmetrical loading. This type of damage caused a great difference in vertical displacements between individual girders, which then led to waterproofing damage and long lasting leaking of seep water with chlorides. In parts of main girders, due to a very thin concrete cover of 1.5 cm, which was at that time considered adequate, it was assessed that 6% of prestressed cables in each girder no longer perform its function (Fig. 4). Moist stains are visible on joints between girders, which indicates that waterproofing is no longer functioning properly, and that concrete is moist in its entire height. Concrete cover 61

3 Figure 4. Corroded prestressed reinforcement. Since girders were prestressed with adhesion, they should be repaired segment by segment, using high quality mortars with high early strength and compensated shrinkage. It would be necessary to wait 5 to 7 days in order to move to the next phase, in order to prevent loss of prestress. At this rate, the duration of works on one span would be 45 days, which means that the repair of one structure would last 18 days in ideal conditions. Extension of works would reduce the traffic flow and increase the possibility of accidents. The cost of this rehabilitation would amount to 1.7 million euros. The second method would be a complete replacement of the structure, which means that it would be necessary to stop the traffic on the motorway completely, during disassembly of certain parts, especially during the construction of the new structure. The cost would be 8, euros. The third method would be the replacement of the entire overpass superstructure and the repair of head beams, piers and abutments. The evaluated cost of this repair method is around 3, euros. After consulting the owner about the repair costs, temporary traffic regime during the works, the duration of works and the designed service life of the structure, the third repair method was chosen. 4 STRUCTURAL REPAIR Figure 5. Damaged girder surfaces. on piers was breaking off to such an extent, that areas up to 1.5 m 2 were missing on some piers. The results of investigation works showed that the repair of individual parts of the structure is possible, including the removal of damaged concrete and local repair of concrete surfaces. When rehabilitation and repair methods were discussed in cooperation with motorway owners, one of the requirements was to minimize traffic disruptions and the duration of works. The first repair method would consist of hydrodemolition of damaged concrete surfaces, placement of additional reinforcing bars where necessary and the repair of damaged concrete surfaces. But, taking into consideration the significant increase in traffic load with respect to the designed load, the duration and complexity of individual work phases that would limit the traffic, it was decided to remove the entire superstructure. In this case, the repair of main girders would require the greatest amount of time. 4.1 Substructure Parts of the substructure were made with reinforced concrete, so it was necessary to perform standard repairs of individual elements, in order to remove damaged or carbonated concrete, repair or replace reinforcement bars, depending on the local damage. These works involved the removal of 1.5 to 2. cm thick layer of moist and chloride saturated surface concrete, in order to allow good adherence between the old and new concrete. It was reprofiled with repair mortar, which was applied manually or by shotcreting in one or two layers, depending on the thickness of the concrete layer. When works were finished, the entire visible concrete surface was protected with silane and siloxane impregnation liquid. Since the overpass was open to traffic during repairs, it was supported by scaffolding. 4.2 Superstructure The works on the superstructure included the replacement of main girders, removal and replacement of sidewalks, expansion joints, guard rail, kerbs and the deck structure. In order to prevent greater differential displacements, main girders were connected with a deck slab. It had changeable thickness from 611

4 Total duration of all repair works was 6 days, as it was anticipated. Overall repair costs exceeded the estimated costs in the study by somewhat more than 1%. 5 STRUCTURAL LOAD TESTING Figure 6. Pier repair. In order to confirm the efficacy of scheduled and performed works on rehabilitation of structures, load testing of rehabilitated overpass was performed. Testing procedure was carried out in accordance with Croatian regulations for bridge testing before opening to traffic. Test results were compared to the behaviour of the structure before rehabilitation and with results of testing carried out on similar structures Figure 7. New cross section of the structure. 5.1 Static structural testing Static testing included deflection measurement by high precision geometric levelling at bearing points and at mid-span, measurement of permanent deflections after load release at the same measuring points and deformation measurement at critical points in the structure. Static testing was performed according to a previously drafted testing programme, which defines the number of measuring phases, the number of truck corridors (loads) and measuring lines. The loads were in a different position in each phase. Figure 9 shows the course of testing according to phases, as well as Grad Motorway Figure 8. Removal of girders on the overpass. 1 to 2 cm, so as to attain sufficient fall for rainwater drainage. The deck slab and main girders were connected by placing reinforcement on the upper surface of girders. Such a stiff deck slab increases the transverse stiffness of the structure up to 4%, and contributes to the total bearing capacity of the structure under the increased traffic loading. Above deck slab new waterproofing were installed and in the next phase two asphalt layers 7 cm thickness. This solution represents improvement in durability of the whole structure since it decreases transverse movements of the girders. phase corridor I (2 trucks) corridor I + II (4 trucks) Figure 9. Testing according to phases. 612

5 Phase 2(1), load span I 2A 2B 2C 2 alfa-i alfa-t Figure 11. Deflections of the «old» structure in transverse direction. Figure 1. Position of trucks in transverse direction. the position of measuring lines and corridors. Four 3 ton trucks were used as test load. Analysis of the results, obtained by comparison of measured displacements of the load bearing structure on three longitudinal measuring lines A, B and C, showed that the maximum deflection of the «old» structure amounted to approx 8. mm, while in the rehabilitated structure they amounted to around 6. mm, under the same full load. It amounts to an average of 25% of deflection reduction for the same load. The greatest difference between these results is in deflections for the asymmetrical loading, i.e. when the structure is loaded only with one truck corridor. In this loading phase, it was noticed that transverse distribution of vertical displacements of existing overpasses is significantly poorer and it deviates from theoretical assumptions, which is not the case in rehabilitated overpasses. The effect of transverse distribution can be expressed by the coefficient of transverse distribution α, which represents a deviation of measured from theoretical deflection at a certain point, in relation to the mean value of deflection for asymmetrically loaded cross section. Figures 11 and 12 show the relation of theoretical and measured values of α. Based on Figures 11 and 12, it can be concluded that calculated transverse distribution coefficient in the overpass before repairs, differs significantly from the measured values. It can also be observed that there is discontinuity in the line i.e. when the load is located close to the measuring point 2A that girder takes over a greater part of the load, while the end girders are insignificantly loaded. This fact demonstrates that the theoretical assumptions on linear distribution in the cross section are not correct. Longitudinal joint made with tricosal mortar is not stiff enough to enable uniform transverse distribution of loads. At the same time, it is the proof of the main cause of damage to the deck waterproofing and leaking of water on the reinforcedconcrete structure. Figure 12 shows that the structure behaves better after repairs and rehabilitation. Calculated values coincide better with the measured values, so it can be concluded that connecting main girders Phase 2(1), load span I 2A 1 2 2B alfa-i alfa-t Figure 12. Deflections of the rehabilitated structure in transverse direction. by placing a reinforced-concrete slab improves not only the performance of the structure under asymmetrical load but overall stiffness as well. This follows from the fact that under the maximum load, the maximum deflection of the structure before rehabilitation amounted to the average 7.5 mm, and after rehabilitation to the maximum of 4.3. mm for the same test load. It can be concluded that overall stiffness for vertical loading increased by more than 5%. 5.2 Dynamic testing In dynamic testing vertical accelerations of the superstructure were measured at mid span, during the passage of a 34 ton truck across the overpass. The truck was driving along the overpass at different speeds (from 2 to 4 km/h). Measuring points were located at the deck structure, at mid-spans. Characteristics of individual instances of driving, during which the oscillations of the superstructure were measured, are given in the table. Directly or by data processing, the following is determined from the results: 1. the period of free oscillations T slob. 2. the maximum acceleration a max 3. damping coefficient ξ. Damping coefficient (ξ) is determined from the part of the record of free vibrations (oscillations of the structure) using the following expressions: a. δ = (1/m n) ln (A m /A n ) logarithmic decrement b. ξ = δ/2 Π where A m and A n are oscillation amplitudes in the mth and nth cycle of oscillations. 613

6 x mv 2 x=742ms 3 CONCLUSION s Average Standard Maximum Minimum cha:frequency(hz) Nov2412:42 Figure 13. Record of acceleration of the structure. mv x=-16,3mv,o=113,2mv,xo=219,6mv s Average Standard Maximum Minimum cha:frequency(hz).5774* Jul2512:12 Figure 14. Record of acceleration of the repaired structure. Based on detailed analysis, it was concluded that dynamic properties significantly improved in rehabilitated structures. The frequency of free oscillations was increased by 5% i.e. the period of natural oscillation of the first tone was reduced in comparison with the values before repair works. The maximum acceleration also decreased from.2 g to.11 g, and linear to that vertical displacements due to dynamic loading were reduced as well. Critical damping coefficient was increased by 6%, i.e. before the repair it amounted to 2.3, and later to 3.3. From the above mentioned it can be concluded that the applied repair measures improved the performance of structures and therefore we can assume that service life of the structure shall be extended in this way, with preventive maintenance. Due to errors repeated in construction, characteristic damage patterns with expected consequences occur on Croatian motorways. Under specific conditions, seeping of salty rainwater leads to severe and serious damage, which in turn significantly affects the safety and load bearing capacity of the structure. Design, repair and construction require the understanding of the problem, of the physical and chemical processes in concrete, steel reinforcement and repair materials. Extensive investigation works are necessary, especially if they are not performed frequently, in order to precisely determine the actual condition of the structure in terms of constituent materials, concrete, steel etc. Finally, the degree of damage should be established, as well as its fundamental cause, in order to determine the appropriate measures and repair method. Repair method and special requirements during the works shall be analysed in detail, in cooperation with the owner, and an agreed decision on future works and their dynamics shall be reached. Test loading of the structure is a very useful method for assessing the actual behaviour of the structure. It is also useful to perform it on a new structure before opening to traffic, in order to establish the reference condition of the new structure for condition assessment in the future. The owner can use data obtained by this testing in determining appropriate measures to ensure the extended service life of the structure. Results of this investigation were used to formulate a maintenance strategy for this and other types of overpasses on Croatian motorways. REFERENCES Mavar, K., Barisic E., Petrak I. International Conference on 4-9 in concrete bridge repairs. CEB-FIP Bulletin, 23, Monitoring and Safety Evaluation of Existing Bridges, May 21 24, Dubrovnik, Croatia. 26. Examples of use principles and methods according to HRN ENV 15 g Concrete Structures State of the Art Report, Task Group. Tkalcic, D., 24 Concrete Bridge Condition Assessment, Master s thesis, Bridge assessment system HRMOS. Manual for Condition and Evaluation of Bridges, 2nd edition, 2, American Association of State Highway and Transportation Officials. Balagija A., Mavar K., Skaric Palic S., 27, Repair of Overpass Novska on Highway A3, fib Symposium, Concrete Structures Stimulators for Development, May 2 23, Dubrovnik, Croatia. 614