A study of prestress losses of post tensioned beams cast with self compacting concrete and conventional concrete

A STUDY OF PRESTRESS LOSSES OF POST TENSIONED BEAMS CAST WITH SELF COMPACTING CONCRETE AND CONVENTIONAL CONCRETE LIM KHENG GUAN NATIONAL UNIVERSITY OF SINGAPORE 2004 Founded 1905 A STUDY OF PRESTRESS LOSSES OF POST TENSIONED BEAMS CAST WITH SELF COMPACTING CONCRETE AND CONVENTIONAL CONCRETE LIM KHENG GUAN (B.Eng. (Hons.). UTM) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENTS I would like to take use of this opportunity to acknowledge various individuals for their guidance and encouragement in this research. First, I would like to express my appreciation to my supervisor, Professor Gary Ong Khim Chye for his constructive suggestions, invaluable advice and helpful guidance. Besides that, the suggestions and advice given by Professor Tam Chat Tim are also highly appreciated. I would like to thank the technical staff of the Concrete Technology and Structural Engineering Laboratory of the National University of Singapore, Department of Civil Engineering, especially Mr. Sit, Mr. Choo, Mr. Ang, Mr. Koh, Mr. Ow, and Mdm. Annie, for their kind help at all stages of the experimental programme. I would like to express my thanks to my family and friends especially, Ms. Lee S.C. and Ms. Aye Monn Monn Sheinn for their help. I would not have my achievement and complete this research work without their valuable moral support and encouragement. Finally, I gratefully acknowledge the National University of Singapore for the facilities to carry out this research and the award of research scholarship to pursue this study. July, 2004 Lim Kheng Guan i TABLE OF CONTENTS ACKNOWLEDGEMENTS……………………………………………………… . i TABLE OF CONTENTS………………………………………………………… ii SUMMARY………………………………………………………………………… .v NOMENCLATURE……………………………………………………… .………vii LIST OF TABLES………………………………………………… .……………….x LIST OF FIGURES……………………………………………………… xi CHAPTER INTRODUCTION 1.1 General……………………………………………………………………….1 1.2 Objectives and Scope of Research……………………………………………3 1.3 Structure of the Thesis……………………………………………………… CHAPTER LITERATURE REVIEW 2.1 Introduction………………………………………………………………… 2.2 Properties of Self-Compacting Concrete…………………………………….8 2.2.1 Fresh Concrete Properties………………………………………… 2.2.2 Creep and Shrinkage of SCC………………………………………14 2.2.3 Elastic Modulus……………………………………………………17 2.3 Time-Dependent Variables in Prestressed Concrete Beams……………… .18 2.3.1 Shrinkage of Concrete…………………………………………… 18 2.3.1.1 Mechanism of Shrinkage…………………………………18 2.3.1.2 Factors Influencing Shrinkage……………………………20 2.3.2 Creep of Concrete………………………………………………….21 2.3.2.1 Mechanism of Creep…………………………………… 21 2.3.2.2 Factors Influencing Creep……………………………… 22 2.3.3 Shrinkage and Unit Creep versus Time Curves………………… .23 2.3.4 Modulus of Elasticity of Concrete…………………………………26 2.3.5 Prestress Losses……………………………………………………28 CHAPTER THEORETICAL ANALYSIS 3.1 Empirical Expressions for Modeling Creep and Shrinkage …………… 34 3.2 Prestress Losses…………………………………………………………… 35 3.2.1 Immediate Prestress Losses……………………………………… 35 3.2.2 Time-dependent Prestress Losses….………………………………38 3.2.2.1 Introduction………………………………………………38 3.2.2.2 Modified Time-Step Method…………………………… 40 3.3 Assumptions……………………………………………………………… .43 3.4 Deflection of Prestressed Concrete Beams……………… ……………… 44 3.5 Cracking Moment………………………………………………………… .45 3.6 Ultimate Moment of Resistance …………………….………… .…………46 ii CHAPTER EXPERIMENTAL PROGRAMME 4.1 Concrete………………………………… …………………………………50 4.1.1 Concrete Mix……………………………………… …………… 50 4.1.2 Test Specimens……………………………………… ………… .52 4.1.3 Curing and Test Condition…………… …………… ……………53 4.1.4 Test Method……………………………………… ………………54 4.1.4.1 Compressive Strength Test…………………….…………54 4.1.4.2 Tensile Splitting Test…………………………………… 54 4.1.4.3 Creep and Shrinkage Test…………………………….… 55 4.1.4.4 Modulus of Elasticity Test………………………….…….57 4.2 Steel…………………………………………………………………… … 58 4.2.1 Prestressing Steel………………………… ………………………58 4.2.2 Steel Bars………… ………………………………………… … 58 4.3 Prestressed Beams………………………………………………………… 59 4.3.1 Beam Fabrication………………………………………………….59 4.3.1.1 Beam Specimens…………………………………………59 4.3.1.2 Preparation of Reinforcing Cages……………………… 60 4.3.1.3 Preparation of Tendons………………………………… .61 4.3.1.4 Preparation of Wood Mould…………………………… .62 4.3.1.5 Concrete Casting…………………………………………62 4.3.2 Prestressing Method……………………………………………….62 4.3.3 Loading…………………………………………………………….63 4.3.3.1 Service Load…….…………… …………………………63 4.3.3.2 Ultimate Load…………………………………………….65 CHAPTER RESULTS AND DISCUSSION 5.1 Material Properties………………………………………………………….78 5.1.1 Properties of SCC and Normal Concrete………………………….78 5.1.1.1 Compressive Strength…………………………………….78 5.1.1.2 Tensile Strength………………………………………… 80 5.1.1.3 Modulus of Elasticity…………………………………….81 5.1.2 Time-dependent Deformation of Concrete……………………… .81 5.1.2.1 Shrinkage versus Time Curves………………………… .81 5.1.2.2 Creep and Unit Creep versus Time Curves………………84 5.2 Monitoring of Prestressed Beams………………………………………… .87 5.2.1 At Transfer…………………………………………………………87 5.2.2 Time Dependent Losses in Tendon Strain……………….……… .88 5.2.2.1 Losses in Tendon Strain…………………………… ……88 5.2.2.1.1 After Transfer………………………………88 5.2.2.1.2 During Service…………………………… .90 5.2.2.2 Comparison of Monitored and Predicted Tendon Strains 91 5.2.2.2.1 After Transfer………………………………91 5.2.2.2.2 During Service…………………………… .94 5.2.3 Changes in Extreme Top and Bottom Fiber Strains with Age…… 96 5.2.4 Deflection of Prestressed Beams versus Age………………… .99 5.3 Load Test to Ultimate……………………………………………………101 5.3.1 Load versus Deflection………………………………………… .101 5.3.2 Load versus Strain……………………………………………… 104 5.3.3 Crack Pattern………………………………………………… …104 iii CHAPTER CONCLUSIONS AND RECOMMENDATIONS 6.1 Concrete Mixes………………………………………………………… 132 6.2 Prestressed Beams……………………………………………………… 133 6.3 Recommendations for Future Research.…………………………………134 REFERENCES………………………………………………………………….136 iv SUMMARY Self-compacting concrete (SCC) is a recent generation of material introduced in the late 1980s and has undoubtedly a great potential in replacing conventional concrete especially in highly reinforced members. The development of SCC has changed fresh conventional concrete from being a granular material needing vibration for compaction into a fluid, with ability to fill formwork and encapsulate reinforcing bars under its own self-weight without segregation and bleeding. This new material has a large impact on the precast and prestressed concrete industry because it reduces skilled manpower and increases productivity in the casting of durable prestressed or precast members without mechanical vibration. In the design of prestressed concrete structures, the immediate and time-dependent losses in tendon strains (stresses) are important parameters. However, most published works on the time-dependent loss in tendon strain have been conducted on conventional concrete prestressed members and only very limited data exists for SCC prestressed members. The main objective of this study is, therefore, to study the application of SCC in prestressed beams by investigating the loss in tendon strain of the SCC prestressed beams due to creep and shrinkage at transfer, after transfer and during service compared to that of the conventional concrete prestressed beams. Since the loss in tendon strain is dependent on the properties of concrete, it is necessary to understand the engineering properties of the SCC as compared to conventional concrete, including creep and shrinkage. Four beams, consisting of SCC high and low prestress beams (HS, LS), and conventional concrete high and low prestress beams (HC, LC) v were cast, subjected to sustained loading and monitored for a duration of months. The study showed that shrinkage of the SCC was only slightly higher than that of the conventional concrete under same ambient condition, although paste volume of the SCC was 30 % more than that of the conventional concrete. Creep of the SCC was 34 % more than that of the conventional concrete under a sustained stress of 9.34 N/mm2. This research also found that the SCC mix used is applicable for prestressed concrete construction as the loss in tendon strain of the SCC prestressed beams was lower that that of the conventional concrete prestressed beams after transfer for a duration of 22 days. The loss in tendon strain of the SCC and conventional concrete prestressed beams was not significant during service (application of service load) for a duration of months. Keywords: SCC (self-compacting concrete); conventional concrete creep; shrinkage; loss in tendon strain; high/low prestress beams vi NOMENCLATURE Ac A ps Area of beam cross section Total area of tendons cross section d d ps e eo Diameter of cylinder Depth to the centroid of tendons Ec E ce E ci E ps Base of Napierian logaritma, ( e = 2.718) Eccentricity of prestressing steel measured from the centroid of the beam cross section Modulus of elasticity of concrete Effective modulus of elasticity of cocnrete Modulus of elasticity of concrete at time of initial prestressing. Modulus of elasticity of tendon F fc f c , 28 Prestressing force Concrete cube strength 28-days concrete cube strength f cgs Stress in the concrete at the centroid of prestressing steel ( f cgs ) FJ f pb Stress in the concrete at the centroid of prestressing steel due to jacking force. Stress in the concrete at the centroid of prestressing steel due to selfweight of beam Stress in tendon f pJ Stress in the prestressing steel at end of jacking f ps Stress in tendon f py Specified yield strength of prestressing steel fr f 'c I k K CA Modulus of rupture Compressive strength of standard test cylinder Second moment of inertia Profile coefficient Correction factor for age at loading (creep) Correction factor for humidity (creep) Correction factor for shape and size (creep) Correction factor for humidity (shrinkage) Correction factor for shape and size (shrinkage) The distance from the centroid of concrete cross section to the upper limit of central kern Length of beam Span of beam Length of cylinder ( f cgs ) G K CH K CS K SH K SS kt L l lc vii M cr Cracking moment MD Mu N CR n pi Dead Load Moment Ultimate moment of resistance A constant which is equal to the time at which the creep strain becomes equal to half the ultimate creep strain A constant which is equal to the time at which the shrinkage strain becomes equal to half the ultimate shrinkage strain Initial modular ratio P Pe Po r rps Point load Effective prestressing force Prestressing force in the tendon at the jacking end Radius of gyration of beam cross section Radius of curvature s S1 S2 Distance of draw-in Stress corresponding to a longitudinal strain of 50 millionths (MPa) Stress corresponding to 40% of ultimate load (MPa) SS cgs Stress-strength ratio of concrete at the centroid of prestressing steel SS cy. Stress-strength ratio of creep cylinder s/a T t ti tj Sand-aggregate ratio Splitting tensile strength Time Beginning of a time interval End of a time interval to w wc w/c W /B W /P x xp Age of concrete at the time of application of loading Uniform load Water content in the concrete mix Water cement ratio Water-binder ratio Water-powder ratio The depth of neutral axis The distance from the jack to the point in which prestressing force (after N SH Zb ε2 ε ci ε CR ε CR,U ε cu εe considered friction losses) to be computed Section modulus with respect to extreme bottom fiber Longitudinal strain produced by stress S2 (stress corresponding to 40% of ultimate load) Instantaneous elastic strain of concrete due to loading Creep strain Ultimate creep strain Strain in the extreme compression fiber of concrete at ultimate Strain in concrete at the level of tendon due to effective prestressing viii CHAPTER RESULTS AND DISCUSSION 600 550 500 450 Load (kN) 400 350 300 250 200 150 100 50 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 10 20 Deflection (mm) Figure 5.31: Load-deflection curve for HC beam at midspan. 600 550 500 450 Load (kN) 400 350 300 250 200 150 100 50 -100 -90 -80 -70 -60 -50 -40 -30 Deflection(mm) -20 -10 10 20 Figure 5.32: Load-deflection curve for HS beam at midspan. 126 CHAPTER RESULTS AND DISCUSSION 600 550 500 450 400 Load (kN) 350 300 250 200 150 100 50 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 10 20 Deflection (mm) Figure 5.33: Load-deflection curve for LC beam at midspan. 600 550 500 450 Load (kN) 400 350 300 250 200 150 100 50 -100 -90 -80 -70 -60 -50 -40 -30 Deflection (mm) -20 -10 10 20 Figure 5.34: Load-deflection curve for LS beam at midspan. 127 CHAPTER RESULTS AND DISCUSSION -400 -300 -200 -100 Strains (µ) 100 200 300 400 500 Beam Depth (mm) -200 25 kN 50 kN -400 75 kN 100 kN 125 kN 150 kN 175 kN -600 Figure 5.35: Distribution of experimental strains across the beam depth at a section at midspan for different load levels in HC beam. -500 -400 -300 -200 -100 Strains (µ) 100 200 300 400 500 600 Beam Depth (mm) -200 -400 25 kN 50 kN 75 kN 100 kN 125 kN 150 kN 175 kN -600 Figure 5.36: Distribution of experimental strains across the beam depth at a section at midspan for different load levels in HS beam. 128 CHAPTER RESULTS AND DISCUSSION Strains (µ) -400 -300 -200 -100 100 200 300 400 500 Beam Depth (mm) -200 -400 25 kN 50 kN 75 kN 100 kN 125 kN -600 Figure 5.37: Distribution of experimental strains across the beam depth at a section at midspan for different load levels in LC beam. Strains (µ) -400 -300 -200 -100 100 200 300 400 500 Beam Depth (mm) -200 -400 25 kN 50 kN 75 kN 100 kN 125 kN -600 Figure 5.38: Distribution of experimental strains across the beam depth at a section at midspan for different load levels in LS beam. 129 CHAPTER RESULTS AND DISCUSSION 600 600 Load (kN) HC BEAM 500 εy εy εy 400 200 first crack occurred 100 100 Strain (µ) 5000 10000 (tension) Bottom steel Top steel -10000 -5000 (compression) LC BEAM 400 εy εy 400 εy 300 200 200 first crack occurred 100 first crack occurred Bottom steel Top steel Bottom steel Top steel -5000 (compression) 10000 (tension) 500 300 -10000 5000 LS BEAM Load (kN) 500 100 0 Strain (µ) 600 600 Load (kN) εy first crack occurred Bottom steel Top steel (compression) εy 300 200 -5000 500 400 300 -10000 HS BEAM Load (kN) Strain (µ) 5000 10000 -10000 (tension) -5000 (compression) Strain (µ) 5000 10000 (tension) Figure 5.39: Load-strain curves for the top and bottom bars in the beams at midspan. 130 CHAPTER RESULTS AND DISCUSSION a) HC Beam b) HS Beam c) LC Beam d) LS Beam Figure 5.40: Crack pattern of the prestressed beams after failure. 131 CHAPTER CONCLUSIONS AND RECOMMENDATIONS CHAPTER CONCLUSIONS AND RECOMMENDATIONS 6.1 Concrete Mixes Based on the test results obtained in this study, the following conclusions can be made: 1. The compressive strength, tensile strength and modulus of elasticity for both concrete mixes viz. SCC and conventional concrete, tested were similar. 2. The shrinkage of the SCC mix was slightly higher than that of the conventional concrete mix. Although the paste volume of the SCC was considerably higher (30 % more than that of the conventional concrete) the shrinkage of the SCC mix was only 22 % higher after monitoring for a duration of months. 3. Under sustained stress, creep of the SCC mix was higher than that of the conventional concrete mix. The difference in creep strains between the SCC and conventional concrete was 34 % after monitoring for a duration of months. 4. The unit creep of the SCC and conventional concrete mixes were 83 µ and 62 µ, respectively after monitoring for a duration of months. 5. Predicted creep and shrinkage of the SCC and conventional concrete mixes using the proposed hyperbolic expressions agreed well with the creep and shrinkage 132 CHAPTER CONCLUSIONS AND RECOMMENDATIONS values obtained experimentally (for a duration of months). 6.2 Prestressed Beams 1. At transfer, days after casting, it was observed that the immediate losses in tendon stress of the SCC prestressed beams were higher compared to the conventional concrete prestressed beams especially when the prestress level was high. The loss in the HS beam was almost twice that of the HC beam whereas the loss in the LS beam was only % higher than the LC beam. 2. Losses after transfer when the beams were not subjected to loading for a duration of 22 days were lower for the SCC prestressed beams. For the H and L beams tested, SCC prestressed beams registered only half the losses of the conventional concrete prestressed beams. 3. When the beams were subjected to their respective service loads for a duration of months, the loss in tendon strain was not significant averaging well below 10 % in most cases. However the SCC prestressed beams tend to register higher losses. 4. The load at first crack and ultimate load were similar for the two beams each in the H and L series. 5. All the beams failed by crushing of concrete at the extreme top fiber at midspan. The crack patterns of the SCC and conventional concrete prestressed beams tested were similar. 133 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 6. The theoretical ultimate strength of the beams predicted using BS 8110: 1997 was very close to the ultimate strength obtained experimentally with well within 10 % difference. 6.3 Recommendations for Future Research The ultimate shrinkage values of the SCC and conventional concrete mixes used in this research differed slightly from the findings obtained by some researchers, possibly due to different experimental conditions adopted, such as ambient temperature, relative humidity and concrete mixes (differences in materials, water-cement ratio and water-powder ratio). More research on time-dependent deformation of SCC used in Singapore is required, especially on shrinkage. Future work on the creep of concrete should consider the use of embedded electrical strain gauges to be connected to computer aided data acquisition system to improve the accuracy of creep measurements. This would enable greater reliability in the creep measurements. Besides, the maintenance of the applied load is also essential in a creep test. A hydraulic system with auto-servo control would be more accurate and convenient than the spring system, which needs frequent manual adjustment of the applied load. Future work on the loss in tendon stress of SCC and conventional concrete prestressed beams, should consider other parameters such as water-cement ratio, water powder ratio, concrete strength and stress-strength ratio of the concrete. These parameters may influence the loss in tendon stress at transfer, after transfer and during 134 CHAPTER CONCLUSIONS AND RECOMMENDATIONS service. In practice, most prestressed concrete sections are thin web sections because of the beneficial effect of prestressing. Therefore, other shapes of cross section should be used in the study of prestress losses such as single T, double T and I sections. 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Dissertation, Texas A&M University, College Station, Texas, August, 1966. 141 [...]... strength of concrete is high, in much the same manner for both SCC and conventional concrete 2.2.3 Elastic Modulus Pons et al (2003) reported that self- compacting concrete and conventional concrete have similar elastic modulus at the same age Elasticity modulus test was carried out 7 days and 28 days after casting Pons et al also reported that SCC exhibited a setting delay at an early age (1 day) compared... deformation of SCC and conventional concrete mixes used, arising from creep and shrinkage is of particular interest here as it helps to understand the prestress loss of prestressed beams cast with the same mix 1.2 Objectives and Scope of Research The objectives of this research are to: 1) investigate and compare the loss in tendon stress of full-scale SCC prestressed beams to that of the conventional concrete. .. modulus of elasticity 4) study the creep and shrinkage of the SCC and conventional concrete mixes used For this study only Grade 40 MPa concrete was tested To study and compare the prestress losses of SCC and conventional concrete prestressed beams, only four full-scale 6 meter span prestressed beams were cast Due to time constraints, the loss in tendon strain and deflection of prestressed beams were monitored... concrete prestressed beams due to creep and shrinkage at transfer, after transfer and during service 2) understand the behavior of the SCC and conventional concrete prestressed beams such as deflection, first cracking load, crack pattern and failure mode when loaded to ultimate 3) compare the material properties of the SCC and conventional concrete, viz compressive strength, tensile strength and modulus of. .. important to study prestress losses when SCC is used in prestressed concrete construction as the properties of selfcompacting concrete in the fresh and hardened state are known to be different from that of conventional concrete In view of the direct influence that creep and shrinkage of concrete (conventional concrete and self- compacting concrete) have on prestress losses, this chapter will review available... compacted conventional concrete is also essential in order to provide a comprehensive understanding for better utilization of SCC in prestressed concrete structures This research is undertaken to study and compare the prestress losses of SCC prestressed beams with that of conventional concrete prestressed beams at transfer, after transfer and during service Estimation of prestress losses in the prestressed... of the concrete Arching cannot occur if the particles are too small compared to the dimension of the opening To achieve a suitable passing ability of SCC, it is a necessity to enhance cohesiveness to reduce aggregate segregation and ensure compatible clear spacing and coarse aggregate characteristics 13 CHAPTER 2 LITERATURE REVIEW 2.2.2 Creep and Shrinkage of SCC Self- compacting concrete contains a. .. MPa concrete, SCC and conventional concrete with silica fume have an analogous behavior Therefore, Pons et al (2003) concluded that SCC and conventional concrete creep behaviors are similar SCC mixes without silica fume exhibited 36 % higher total creep deformation when compared with SCC mixes with silica fume According to Persson (1999), creep and shrinkage of self- compacting concrete are similar... self- compacting concrete (SCC) Self- compacting concrete is concrete which has the ability to fill formwork and encapsulate reinforcing bars through the action of gravity and compacts under its self- weight without segregation Self- compacting concrete was developed in Japan and the necessity of it was advocated by Okamura (Midorikawa, 2001) in 1986 SCC has generated significant interest worldwide As construction... shrinkage, the fundamental cause of shrinkage must be sought in the physical structure of the gel rather than in its chemical and mineralogical character (Neville, 1995) Figure 2.4 shows the relationship between the shrinkage and the mass of water lost The shrinkage and mass of water loss for neat cement pastes is proportional to one another because no capillary water is present in the pastes and only adsorbed . A STUDY OF PRESTRESS LOSSES OF POST TENSIONED BEAMS CAST WITH SELF COMPACTING CONCRETE AND CONVENTIONAL CONCRETE LIM KHENG GUAN NATIONAL UNIVERSITY OF SINGAPORE. that of the conventional concrete prestressed beams due to creep and shrinkage at transfer, after transfer and during service. 2) understand the behavior of the SCC and conventional concrete prestressed. deformation of SCC and conventional concrete mixes used, arising from creep and shrinkage is of particular interest here as it helps to understand the prestress loss of prestressed beams cast with