LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE LOAD OF

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1 2006/2 PAGES RECEIVED ACCEPTED V. BORZOVIČ LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE LOAD OF ABSTRACT Viktor Borzovič, PhD. Research field: Concrete composite structures, prestressed bridge girders. Department of Concrete Structures and Bridges Slovak University of Technology, Faculty of Civil Engineering, Radlinského 11, Bratislava, Slovakia KEY WORDS The paper deals with an experimental investigation of a composite continuous system made from prestressed precast beams mutually connected with a cast in situ topping, where continuity was ensured by a reinforced concrete crosshead. The experiment focused on the long-term effects due to the shrinkage and creep in hyperstatic composite systems. The results were compared with measurements on isostatic composite members that were made together with members of continuous systems. Further, the effect of cracks over intermediate supports on the redistribution of internal forces was investigated. The long-term monitoring included measurement of the actual forces (losses) generated by the prestressing units, concrete strains, cambers, rotations and support reactions as well as the material properties of the concrete (modulus of elasticity, creep, shrinkage) and reinforcing steel. Precast prestressed girder, Cracking, Composite behaviour, Load test. 1. INTRODUCTION Decks made from precast prestressed beams and cast in situ toppings are frequently used solutions for bridges with shorter spans, i.e., a length of up to 30 m and rarely, 40 m. Composite precast decks are mostly designed as isostatic structures, even for multi-span bridges (e.g., the first continuous composite precast motorway bridges in Slovakia are under construction at present). Therefore, only limited experience exists with continuous solutions, particularly with systems where continuity is provided by non-prestressed RC crossheads. Continuous structures have a lot of advantages compared to their simply supported counterparts, e.g., fewer expansion joints, fewer bearings or a higher stiffness, the smoother passage of cars and lorries. The maintenance of continuous bridges is usually cheaper than that of multi-span isostatic structures. Why designers have preferred the application of simply supported solutions is hidden in the complexity of their designs. The design of continuous composite bridges is more complex than isostatic members. The calculation of the shrinkage and creep effects on the internal stresses (composite behaviour) and the internal forces due to restrained deformation in the redundant structures requires a lot of time and is burdened by a high level of uncertainty. Further uncertainties are connected with the redistribution of internal forces due to cracking in the transverse support beams - cross heads, which are cast at intermediate deck supports and provide the continuity of the structure. Therefore, experimental investigation of the behaviour of continuous composite girders has been performed in the laboratory of the Department of Concrete Structures and Bridges at the Slovak University of Technology in Bratislava. The experiment focused on two major problems that are connected with the above-mentioned type of structure: on the effects of shrinkage and creep on the stress state of the girders and on the effect of cracking at intermediate supports on the redistribution of internal forces SLOVAK UNIVERSITY OF TECHNOLOGY borzovic.indd :10:39

2 2. DESCRIPTION OF THE EXPERIMENTAL GIRDERS Six post-tensioned 200mm x 360mm and 4500 mm long rectangular beams were cast at the beginning of the year 2005 as shown in Fig. 1. They were cast using strength class C35/45 concrete, reinforced with steel of a characteristic strength of 500 MPa. Four straight monostrands were embedded in each beam. The prestressing units consisted of 7-wire low relaxation strands with a characteristic strength of 1800 MPa. A sectional area of a strand was mm 2. The first set of three beams (beams A1 to A3) was cast on March 9th and the second set (beams B1 to B3) on March 15th. Two beams from each set of A1, A2 and B1, B2 respectively were made continuous. Beams A3 and B3 were tested as simply supported girders, in order to compare the behaviour of the isostatic and hyperstatic structures cast from the same concrete batch. The beams were stored for 50 days and later placed on the supports and prestressed by a fourstrand jack. The reinforced concrete slab (topping) was cast after prestressing. The cast-in-place slab of C30/37 was 90 mm thick and 800 mm wide. Continuity was provided by casting-in-place a 400 mm wide and long deck diaphragm (crosshead), see Fig.2. Fig. 1 Experimental beams before casting the topping Fig. 2 Experimental beams after casting the topping LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :10:43

3 3. MATERIALS Three concrete mixtures were used for the experiment. Concrete with a working designation of A was used for casting the first set of precast beams; concrete B for the second set of beams and concrete C for casting the toppings and crossheads. In order to evaluate the material models for an analysis of the experimental girders, the following material properties were tested: 28-day and 180-day compressive strength of the concrete, the modulus of elasticity, shrinkage and creep. 3.1 Compressive strength of the concrete Three standard cubes for each concrete mixture were tested on their strength at concrete ages of 28 days and 180 days. The test results are presented in Table 1. The three curves in Fig.3 represent the theoretical time development of the concrete s strength with normal hardening cement according to EN and Model Code 90. The first one is for concrete with a cube strength of 55 MPa at an age of 28 days (51 MPa at an adjusted age of 18 days). The curve matches the results of the strength test performed at the age of the concrete of 28 days. The second curve represents concrete with cube strength of 51 MPa at age 28 days; no adjustment was performed for the lower temperature during the concrete s hardening. The third curve for the concrete Table 1 Average cube strength of concrete at ages of 28 days and 180 days Strength according to concrete class cube s strength of 47 MPa at age 28 days (55 MPa at an age of 180 days) matches the test results of the concrete at age 180 days. The chart shows that the concrete s compressive strength growth was slower than model for prediction offered. 3.2 Modulus of elasticity EN Measured values f cm,cube f cm,cube (28) f cm,cube (180) (MPa) (MPa) (MPa) A C35/ ) B C35/ ) C C30/ ) The average ambient temperature during the hardening of the concrete was 11 C, which resulted in the slow growth of the strength. The secant modulus of elasticity of the concrete was tested on three mm prisms for each concrete batch shortly before the load test of the A1-A2 girder. The measured values are illustrated in Table 2. Fig. 3 Development of the concrete s strength 16 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :10:45

4 Table 2 Modulus of elasticity - comparison of measured and standardized values Modulus of elasticity according to EN concrete class 3.3 Shrinkage Age of concrete t (days) E cm, 28 (GPa) E cm (t) (GPa) Measured (GPa) A C35/ days B C35/ days C C30/ days The development of the shrinkage strains was tested on three mm prisms for each batch of concrete. The prisms were stored next to the beams in order to ensure the same ambient conditions as the experimental girders. The measurements were begun just after the prestress transfer for the specimens from concretes A and B and 18 hours after casting the topping for concrete C. The measured values of the strain were compared with the predicted ones in Fig.4 and Fig.5. The predicted values were calculated using the models for shrinkage prediction defined in EN and ČSN /93, which is standard for design of concrete bridges use in the Czech and Slovak republics. The charts show that all models for shrinkage prediction underestimated the rate of shrinkage strain development compared to the measured strains. Particularly, it is quite visible for concrete type C. A very good approximation was provided to the models with rapid hardening cement. On the other hand, the ČSN model underestimated not only the rate but also the magnitude of this phenomenon. The explanation for such differences (nearly 3 times lower than measured) is the very low notional size of the specimens, h 0 = 50 mm. The ČSN model was evidently adjusted for higher values of a notional size. The predicted values were determined based on the following parameters: notional size of the specimens h 0 = 50 mm, the mean value of the 28-day concrete strength f cm = 43 MPa (38 MPa), relative humidity RH = 70 %, and normal and rapid hardening cement. 3.4 Creep Strains due to creep and shrinkage were measured for concretes A and B simultaneously with shrinkage. Three mm prisms, cast from both the A and B concretes, were subjected to an axial force with a magnitude of 150 kn (compressive stress of 15 Fig. 4 Time development of the shrinkage concrete B LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :10:46

5 Fig. 5. Time development of the shrinkage concrete C Fig. 6 Time development of the creep coefficient concrete A 18 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :10:48

6 Fig. 7 Time development of the creep coefficient concrete B MPa). The specimens were loaded at the age of the concrete when the prestress was introduced. The measured strains were transformed into the experimentally determined creep coefficients. For both concrete mixtures, the measured values of the creep coefficient coincide with theoretical ones according to the EN MEASUREMENT DEVICES AND INSTRUMENTS The experimental girders were both exposed to long-term monitoring and the A1-A2 girder to short term measurements during the load test. The choice of measuring instruments was determined by the measured parameters and properties. Attached strain gauges with a base length of 400 mm (200 mm) were used for measuring the concrete strains. Five sections (#1 to #5), which were located in each span, were fitted and monitored, see Fig. 8. The sixth section was located at the centre of the intermediate support. Tensometers (50/120LY41 from Hottinger Messtechnik) were glued to the A1-A2 girder at sections #3 and #6, and they served for shortterm strain measurements during the load test. Mechanical deflection gauges with a precision of 0.01 mm were placed in the middle of each precast beam for measuring the midspan cambers and deflections. Additional deflection gauges were added to the edge and intermediate supports during the load test. Mechanical levels were placed at the support sections, and they served for measuring the rotations. In order to determine the effect of creep and shrinkage on the redistribution of the sectional forces in the composite hyperstatic structures, the edge reactions were measured by four dynamometers. Two optical and two mechanical dynamometers were used. The sensitivity of the optical dynamometers was 0.04 kn (4 kg) and the mechanical 0.02 kn (2 kg). The magnitude and time development of the prestressing force was measured using elasto-magnetic sensors placed on the strands at the live anchorages. The sensors were embedded in the steel tube, which transferred the prestressing force from the anchor s body to the anchorage plate embedded in the concrete, see Fig. 9. The precision of the measurement was ± 1 kn. 5. LONG-TERM MONITORING The long-term monitoring started with the removal of the slab formwork, 18 hours after casting the slab. The frequency of the readings was two days during the first week, one week during the LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :10:50

7 Fig. 8 Position of the measuring devices Fig. 9 Live anchorage with devices for accommodation of the EM-sensors first month, and one month until the end of the period of long-term measuring, which took 10 months overall. Three analytical models for prediction of the long-term behaviour of the girders were prepared, and the computed values were compared with the measured ones. All the models were analysed using a general incremental stepby-step method. The first model used material properties based on Model Code 90 values (strength, shrinkage, creep, relaxation). The second was based on ČSN /93 values. Measured material properties were used for the third model. A comparison of the measured and analytical values is presented in the next four plots. The time developments of the midspan deflection at Location #3 (mid-span) in the simply supported girder B3 and in the continuous girder B1-B2 are shown in Figs. 10 and 11. The edge support rotation versus the time at Location #1 for the same girders is shown in Figs. 12 and LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :10:53

8 Fig. 10 Midspan deflection vs. time Girder B3 Location #3 Fig. 11 Mid-span deflection vs. time Girder B1-B2 Location 3 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :10:55

9 Fig. 12 Rotation vs. time Girder B3 Location #1 Fig. 13 Time development of rotation Girder B1-B2 Location #1 22 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :10:57

10 Fig. 14 Prestress at transfer - girder B3 Fig. 15 Time development of prestressing force for beam B3 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :10:59

11 Fig. 16 Development of the concrete stresses in section #3 of the simply supported girder A3 Long-term monitoring included measurements of the prestressing forces using EM-sensors. The prestress was introduced shortly before casting the slab with a four-strand jack from one side. Each strand was stressed up to 208 kn (1470 MPa) force before anchoring, see Fig.14. The prestressing force dropped to 165 kn (1165 MPa) immediately after the anchoring due to the anchorage slip. The stresses due to the prestressing force were MPa at the bottom of the beam and -4.3 MPa at the top of the beam. The time development of the prestressing forces is shown in Fig. 15. The time dependent losses were approximately 7% during the monitored period. The measurements indicate that the long-term losses do not significantly depend on the type of structural system. They are similar for both the simply supported and continuous solutions. The stresses in concrete are an important quantity in the practical aspect of a design. The values of stress in concrete cannot be measured directly, especially stresses due to time-dependent effects. But they can be calculated using analytical models. If the model is well calibrated, obtained results usually provide a good image about the actual stress strain state in the analysed structures. The theoretical time developments of stresses in the concrete are presented in Figs. 16 and 17, using all three analytical models. 6. LOAD TEST A load test of the continuous A1-A2 girder was conducted in September 2005, with the configuration presented in Fig. 18. The main goal of the test was to investigate the behaviour of the girder under a service load after cracking over the intermediate support occurred. The load test procedure consisted of five loading cycles that applied a concentrated load at both spans simultaneously. The maximum applied force corresponded to the allowable stress of 280 MPa based on Slovak National Code in continuity reinforcement in the slab at the intermediate support. The required force was initiated using two jacks. The jack s force was divided into two concentrated forces by a special steel structure composed of steel rods and I-shaped cross-beams. The distance between the forces was 600 mm. The maximum applied force of 150 kn by each jack caused theoretical stresses in the reinforcement of 287 MPa if a non-linear analysis of the structure is assumed. The linear behaviour of the structural materials allowed several load cycles. Altogether, 5 loading cycles were carried out. The first #0 cycle was applied on the girder without cracks during the first several loading steps. The load was increased by 20 kn in several steps until the maximum force reached 150 kn. Further loading cycles were 24 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :11:02

12 Fig. 17 Development of the concrete stresses in section #3 of the continuous girder A1-A2 Fig. 18 Load test arrangement carried out on the cracked beam at the intermediate support. The length of the cracked zone was approximately one meter. Two analytical models were selected as follows: Model 1 a linear elastic model ignoring the cracking of the concrete. Homogenized properties were used for the analysis as it is common in design practice. Model 2 a non-linear model, taking into account the cracking. Due to the low stress level, material non-linearity did not occur. The stiffness of the cracked sections was based on moment-curvature behaviour suggested by Model Code 90. LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :11:04

13 Fig. 19 Load deflection relation, beam A1 Fig. 20 Load versus edge support reaction, beam A1 26 LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE... borzovic.indd :11:05

14 The non-linear behaviour of the continuous girder due to cracking over the intermediate support can be seen in Figs. 19 and 20. The load versus midspan deflection is shown in Fig. 19. The #0 cycle curve indicates that the first cracks occurred when the force reached a value of kn. The actual stiffness of the member after cracking is showed by the #1 cycle curve. Further loading cycles on the already cracked girder showed almost constant flexural stiffness for each loading increment as seen from the almost linear load-deflection behaviour. The stiffness did not depend on the stress level in the continuity reinforcement. Fig. 20 shows the load versus edge support reaction. It is an important relation for the assessment of the effects of cracking on the redistribution of internal forces in the girder. The reduced stiffness over the intermediate support caused a growth in the edge reactions compared to the values determined by the linear elastic analysis. This growth was connected to an increment of positive bending moments in the prestressed beam. The increment of reactions due to the non-linear behaviour was 4.5 kn, which is approximately 9 % compared with the linear elastic solution. Contrary to the edge s reaction, the hogging moment at the intermediate support decreased for about 15 %, which means that the ratio of the redistributed moment to the moment before redistribution was δ = CONCLUSIONS Bridges built from precast prestressed beams mutually connected with a cast-in-place slab belong to structures frequently used for shorter spans of up to 40 m. However, the design of such structures suffers from complexity and design uncertainties. This was also confirmed in our experimental study. Uncertainties are connected with the actual material models. Material properties defined in standards do not always reflect actual values. Even modern codes with very comprehensive models for prediction of e.g., shrinkage or creep, do not have to provide correct values. The differences were quite visible for the modulus of elasticity of the concrete. The measured values were 11 % below the values provided by EC2 and even 17 % by ČSN Our tests showed a rapid development of the shrinkage compare with the models for shrinkage prediction, particularly for the younger concrete (the mixture C), see Fig. 5. The prevailing shrinkage at the early stages caused an even different indicator of measured deformations compared with the results obtained from analytical models with material properties defined in the codes, see Figs. 10 to 13. Better results (closer to the measurements) were obtained when experimentally determined material models were applied in the analytical model 3. The stress differences due to the different material models applied in the analytical models for long-term analysis were small, as opposed to the substantial difference in shrinkage. The good concurrence of the predicted (model EC2) and measured creep coefficients is notable, see Fig.6 and Fig.7. The second objective of the study was an investigation of the behaviour of an experimental continuous girder (A1-A2) subjected to a service load. Performed load test was focused on the effect of cracking in the cast-in-place slab and the crosshead at the intermediate support. The effect of cracking on the stiffness is quite visible in Figs. 19 and 20. The experimental results indicate a higher level of redistribution in comparison with the recommendation in the European code for Precast concrete bridge elements, where the redistributed bending moment over an intermediate support (hogging moments) may be assumed to be 90 % of the moment obtained by linear elastic analysis. The difference between the measured edge reaction and the calculated one on the uncracked linear-elastic model was 4.5 kn, which made the hogging moment at the support 15 % lower than obtained from the linear-elastic analysis. ACKNOWLEDGEMENT The author gratefully acknowledge the technical support provided by the Doprastav, a.s. company and Projstar PK, Ltd. company. The experimental study was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences (VEGA 733). REFERENCES Pérez Caldentey A. (2004) Program Hiper linear step-by-step analysis of evolutive structures, Program Manual, Madrid, Spain CEB-FIP Model Code 90 (1993): Thomas Telford Ltd., Wiltshire, Great Britain Eurocode 2, EN-1992 (2004): Design of concrete structures Part 1.1: General rules and rules for buildings, European Committee for Standardization, Brussels, Belgium fib Bulletin 29 (2004): State of art report prepared by Task Group Precast Concrete Bridges ČSN (1993): Navrhování mostních konstrukcí z předpjatého betonu (Design of prestressed concrete bridge structures) (in Czech) LONG-TERM BEHAVIOUR COMPOSITE GIRDERS AND BEHAVIOUR UNDER SERVICE borzovic.indd :11:07