Shrinkage and Creep Properties of High-Strength Concrete Up To 120 MPa

Transcription

1 Seventh International Congress on Advances in Civil Engineering, October11-13, 26 Yildiz TechnicalUniversity, Istanbul, Turkey Shrinkage and Creep Properties of High-Strength Concrete Up To 12 MPa H. C. Mertol, W. Choi, S. Rizkalla, P. Zia North Carolina State University, Department of Civil Construction and Environmental Engineering, Raleigh, NC, USA A. Mirmiran Florida International University, Department of Civil and Environmental Engineering Miami, FL, USA Abstract Recently, the use of high-strength concrete has become more popular due to the availability and significant variety of the admixtures. High-strength concrete increases the load carrying capacity of the columns, therefore, reduces their sizes in buildings and bridges. In bridges, high-strength concrete could result in reduction of number of girders as well as an increase in the span length which significantly reduces the complexity of a project, construction time and overall cost. Due to lack of research data for the material characteristics of high-strength concrete, most of the design codes worldwide limit the applicability of high-strength concrete for concrete structures. This paper summarizes the research findings on the shrinkage and creep behavior of high-strength concrete. A total of 42 cylindrical specimens and 18 prism specimens were monitored for one year. The variables considered in this investigation were the concrete compressive strength (7 to 12 MPa), specimen size (cylindrical or prism), curing type (moist or heat curing), age of concrete at loading (1, 7, 14, 28 days) and loading stress level (.2f c or.4f c ). The creep coefficients and shrinkage strains were obtained for the range of concrete compressive strength, evaluated and compiled with the current predictions according to the Bridge Design Specifications. Research findings indicate that the current Specifications could be used to estimate the creep coefficient for moist cured high-strength concrete, however, overestimates the values for heat cured high-strength concrete. The current AASHTO LRFD Specifications predict the shrinkage behavior very well, therefore, can be used to estimate the shrinkage strain of moist and heat cured high-strength concrete. Keywords: Creep, Shrinkage, High-strength concrete. 1

2 Introduction Use of high-strength concrete became more popular recently due to the availability and significant variety of the admixtures. High-strength concrete increases the load carrying capacity of the columns, therefore, reduces their sizes in buildings and bridges. In bridges, high strength concrete could result in reduction of number of girders as well as an increase in the span length which significantly reduces the complexity of a project, construction time and overall cost. Due to lack of research data for the material characteristics of high-strength concrete, most of the design codes worldwide limit the applicability of high-strength concrete for concrete structures. Concrete is a time dependent material. In particular, concrete creeps under sustain load, and shrinks due to the changes in the moisture content of the surrounding environment. These physical changes increase by time. The information on creep and shrinkage of concrete can be used to determine the prestressing losses, long term deformations and cracking of the civil engineering structures. The evaluation of creep and shrinkage of concrete is very important especially for long-span and high-rise structures. The current code equations for creep and shrinkage predictions are based on normal-strength concrete; therefore, there is a need to evaluate these characteristics for high-strength concrete. This paper summarizes the research findings on the shrinkage and creep behavior of high-strength concrete. A total of 42 cylindrical specimens and 18 prism specimens were monitored for one year. The variables considered in this investigation were the concrete compressive strength, specimen size, curing type, age of concrete at loading and loading stress level. The results were compared to the predictions according to the Bridge Design Specifications (24). Test Specimens Experimental Program Test series consisted of 42 cylindrical specimens of 12x35 mm dimensions and 18 prism specimens of 76x76x286 mm dimensions. Thirty six cylindrical specimens were used to evaluate the creep behavior of high-strength concrete where two specimens were tested in each creep rack. Six cylindrical specimens and 18 prism specimens were used to evaluate the shrinkage behavior of high-strength concrete. The test matrix for this program is presented in Table 1 and 2. 2

3 Set No Set 1 Set 2 Set 3 Rack No Table 1 Testing Scheme for Creep Specimens Target Concrete Day of Concrete Curing Strength Loading Strength 28 days (days) (MPa) (MPa) Concrete Loading Day (MPa) 1Rack1 1 Heat Rack Rack Rack Moist Rack Rack Rack1 1 Heat Rack Rack Rack Moist Rack Rack Rack1 1 Heat Rack Rack Rack Moist Rack Rack Applied Stress (MPa) 13.8 (.2f c ) 27.6 (.4f c ) 19.3 (.2f c ) 38.6 (.4f c ) 24.8 (.2f c ) 49.6 (.4f c ) Set No Set 1 Set 2 Set 3 Table 2 Testing Scheme for Shrinkage Specimens Specimen Target Concrete Specimen Curing Age of Concrete when No Strength (MPa) Type Type Tests Started (days) 1SP1 1SP2 1 Heat 1 1SP3 Prism 1SP4 69 1SP5 1SP6 1SC1 1 Heat 1 Cylindrical 1SC2 14SP1 14SP2 1 Heat 1 14SP3 Prism 14SP SP5 14SP6 14SC1 1 Heat 1 Cylindrical 14SC2 18SP1 18SP2 1 Heat 1 18SP3 Prism 18SP SP5 18SP6 18SC1 1 Heat 1 Cylindrical 18SC2 3

4 Material Properties Mixture designs, for the three concrete target strengths of 69, 97 and 124 MPa were developed by Logan (25) after numerous laboratory and plant trial batches. Details of the concrete mixture design for each of the target strengths are given in Table 3. Table 3 Three Mixture Designs for Target Concrete Strength 69, 97 and 124 MPa Target Strengths Material 69 MPa (Set 1) 97 MPa (Set 2) 124 MPa (Set 3) Cement (kg/m 3 ) Silica Fume Type F (kg/m 3 ) Fly Ash (kg/m 3 ) Sand (kg/m 3 ) Rock (kg/m 3 ) Water (kg/m 3 ) High-Range Water-Reducing Admixture* Retarding Admixture* w/cm Day Compressive Strength (MPa) * ml per 1 kg cementitious materials The coarse aggregate was obtained from Carolina Sunrock Corporation. The selected aggregate was #78M crushed stone with a nominal maximum size of 9.5 mm. Two types of fine aggregate were used depending on the target compressive strength. The first type of fine aggregate was a natural sand used by the Ready-Mixed Concrete Company in all of their concrete mixtures. The second type of fine aggregate was a manufactured sand known as 2MS Concrete Sand produced by Carolina Sunrock Corporation. The cement used was a Type I/II cement produced by Roanoke Cement. The fly ash producer was Boral Material Technologies and the silica fume producer was Elkem Materials, Inc. Both the high-range water-reducing and the retarding admixtures were manufactured by Degussa Admixtures, Inc. The high-range water-reducing admixture (HRWRA) used was Glenium 33 and the retarding admixture was DELVO Stabilizer. Test Method and Test Set-Up The two different curing conditions used in this investigation were: 1-day heat curing and 7-day moist curing. The 1-day heat curing was selected to simulate the conditions of precast prestressed concrete plants. Half an hour after casting, specimens for 1-day heat curing were placed in an environmental chamber for 24 hours where the temperature was controlled to achieve internal concrete temperatures between 66 C and 71 C. The cylindrical molds were covered with plastic lids and the prism molds were covered with wet burlap and plastic sheets to prevent moisture loss throughout the heat curing process. At the end of 24 hours, the concrete specimens were demolded and stored in the laboratory where the temperature was maintained at approximately 22 C with 5 percent relative humidity. The 7-day moist curing regiment was selected to represent typical curing procedures for reinforced concrete members. These specimens were kept in molds at room temperature for 24 hours. The cylindrical molds were covered with plastic lids and the prism molds were covered with wet burlap and plastic sheets to prevent moisture loss. After 24 4

5 hours, the specimens were removed from their molds and were submerged in water in curing tank. The water temperature in the curing tank was maintained at 23 C ± 2 C using specially designed heaters equipped with adjustable thermostats. The water was saturated with lime to prevent leaching of calcium hydroxide from the test specimens. The curing tanks also contained pumps to circulate the water for the purpose of maintaining a constant temperature and concentration of calcium hydroxide throughout the tank. On the 7 th day of curing, the specimens were removed from the curing tanks and stored in the laboratory where the temperature was maintained at approximately 22 C with 5 percent relative humidity. Creep tests were performed using 12x35 mm cylindrical specimens. The test set-up is shown in Figure 1. Two identical cylindrical specimens were stacked and concentrically loaded in each creep rack equipped by a 55 kn hydraulic jack. Two different stress levels equivalent to.2f c and.4f c were used where f c is the target concrete strength. The applied load in each creep rack was monitored by a pressure gage connected to the hydraulic jack at the time of loading and by the strain gages attached to the three threaded rods of each rack at the time of monitoring. Six demec inserts were embedded in each concrete specimen on three 12 angle planes along the height to measure the concrete strain by 23 mm Demec gage. One-day heat cured specimens were loaded at the end of curing period, whereas the 7-day moist cured specimens were loaded at the 7 th, 14 th and 28 days. The creep racks were continuously monitored by a datalogger. Disk springs were used in the creep racks to compensate for the load drop due to creep and shrinkage of concrete. In case of a load drop in any rack more than 5% of the specified load, the load was adjusted using the hydraulic jack to the specified value. The creep specimens had companion cylindrical shrinkage specimens from which the shrinkage strain of the 12x35 mm cylinders was measured. These shrinkage strain readings were deducted from the Demec readings to obtain the creep strain of the specimens. The two ends of the cylindrical shrinkage specimens were sealed with epoxy to simulate the same surface/volume ratio of the loaded creep cylinders. Prism specimens, 76x76x286 mm, used to measure shrinkage in accordance to ASTM C 157. The test set-up is presented in Figure 2. Two inserts were embedded at the top and the bottom of each specimen in order to monitor the shrinkage strain by using a dial indicator. The tests for 1-day heat cured specimens were started at the end of the first day, whereas 7-day moist cured specimens were started at the 7 th day. The measurements from the creep and shrinkage specimens were recorded at the predetermined time interval throughout the duration of the test. These intervals were more frequent at the beginning of the tests. 5

6 Figure 1 Test Set-Up for Creep Tests Figure 2 Test Set-Up for Shrinkage Tests Test Results and Discussions Both the creep and shrinkage specimens were monitored for more than one year. Test results were compared to the predicted creep and shrinkage according to the AASHTO LRFD Bridge Design Specifications (24) which are derived based on the research conducted by Tadros (23). The creep and shrinkage prediction equations given by the Bridge Design Specifications (24) are shown in Table 4. Table 4 Creep and Shrinkage Prediction Equations by the Bridge Design Specifications Shrinkage Creep Equation ε 6 = 48 1 k k k k Ψ ( t, t ) = 1.9k k k k k k (time development factor) td hs hc sh td s hs f k td t = f ' + t ci i td la s hc f k, (humidity factor) k hs = H k hc = H k (size factor) s k (concrete strength factor) f k (loading age factor) la V / S ks = k f = f ' ci.118 la = t i In Table 3, ε sh is the shrinkage strain, Ψ is the creep coefficient, t is the age of concrete after loading in days, t i is the age of concrete when load is initially applied for accelerated curing or the age minus 6 days for moist curing in days, f ci is the specified compressive strength at prestress transfer for prestressed members or 8% of the strength at service for non-prestressed members in MPa, V/S is the volume to surface ratio in mm and H is the relative humidity of the ambient air. k 6

7 The temperature of the surrounding environment was kept constant throughout the testing period. However the relative humidity of the ambient air was varying for this duration. Therefore, the creep and shrinkage strain measurements were normalized by using the appropriate humidity factor presented in Table 3. An incremental procedure was used to normalize the measured data to 7 percent relative humidity for comparison purposes with the prediction relationships specified by the Bridge Design Specifications (24). Creep Behavior The creep strain was determined based on the measured total strain reduced by the measured shrinkage strain of the unloaded companion cylinders and the initial elastic strain of each creep cylinders. The creep coefficients, defined as the ratios between the creep deformations at time t and the instantaneous elastic strain, were calculated to evaluate the creep behaviour for high-strength concrete. The average creep coefficients presented for each concrete strength and each stress level are based on average normalized values using two cylinders in each rack. The measured creep strains were adjusted for 7 percent relative humidity as explained previously. The comparisons of the average creep coefficients of the creep specimens to the creep coefficient predictions provided by the Bridge Design Specifications (24) are presented in Figure 3 to 5. The dark lines represent the code relationship. Only the typical behavior for each concrete strength is presented in these figures due to space limitation of the paper Rack Rack Heat Cured, Loaded 1 st day Moist Cured, Loaded 8 th day Figure 3 Average s for Specimens of Set Rack Rack5 14Rack Heat Cured, Loaded 1 st day Moist Cured, Loaded 14 th day Figure 4 Average s for Specimens of Set 2 7

8 Rack Rack2 18Rack Heat Cured, Loaded 1 st day Moist Cured, Loaded 14 th day Figure 5 Average s for Specimens of Set 3 In general, test results of the three concrete strengths considered in this investigation indicate that as concrete gets older and stronger, the creep of concrete decreases. The creep behavior of 1-day heat cured cylinders is less than that of 7-day moist cured cylinders. The creep for high-strength concrete is proportional to the applied stress provided that the applied stress is less than the proportional limit. The creep coefficient predictions specified by the Bridge Design Specifications (24) produce closer predictions for moist cured high-strength concrete specimens. The creep coefficients for heat cured high-strength concrete specimens are overestimated by the Bridge Design Specifications (24). The equation predicts higher early creep coefficient when concrete the compressive strength is over 8 MPa at the time of loading. The creep prediction relationship specified by the Bridge Design Specifications (24) must be used with discretion for early stages of structures with concrete compressive strengths over 8 MPa. Shrinkage Behavior Shrinkage strain were monitored at the same time and under the same conditions used for the creep specimens. The measured shrinkage strains were adjusted for 7 percent relative humidity as explained previously. The comparison of the adjusted shrinkage strains of the cylindrical and prism specimens to the shrinkage strain prediction provided by the Bridge Design Specifications (24) are presented in Figure 6 to 8. The dark lines represent the code relationship. Only the typical behavior for each of the concrete strength is presented in these figures due to space limitation of this paper SP2 1SP1 1SP SC Heat Cured, Prism Moist Cured, Cylindrical Figure 6 Shrinkage Strains for Specimens of Set 1 8

9 SP1 14SP2 14SP SC Heat Cured, Prism Moist Cured, Cylindrical Figure 7 Shrinkage Strains for Specimens of Set SC SP5 18SP4 18SP Heat Cured, Cylindrical Moist Cured, Prism Figure 8 Shrinkage Strains for Specimens of Set 3 Test results indicate that in general heat cured specimens has less shrinkage compared to the moist cured cylinders. The ultimate shrinkage strain does not vary significantly for high-strength concrete ranging from 7 to 12 MPa. The collected data indicate that shrinkage strain predictions according to the Bridge Design Specifications (24) produce good predictions for both heat and moist cured highstrength concrete specimens. Conclusions A total of 42 cylindrical specimens and 18 prism specimens were monitored for one year to evaluate the shrinkage and creep behavior of high-strength concrete. The variables considered in this investigation were the concrete compressive strength (7 to 12 MPa), specimen size (cylindrical or prism), curing type (moist or heat curing), age of concrete at loading (1, 7, 14, 28 days) and loading stress level (.2f c A g or.4f c A g ). The creep coefficient and shrinkage strain were obtained for the range of concrete compressive strength, evaluated, compiled and compared to the predictions specified by the Bridge Design Specifications (24). The test results indicate that: 1. Creep characteristics of high-strength concrete follow the same trend as that of normal-strength concrete. 2. The creep behavior of 1-day heat cured cylinders is less than that of 7-day moist cured cylinders. 9

10 3. The creep coefficients for moist cured high-strength concrete specimens are slightly overestimated by the Bridge Design Specifications (24). 4. The creep coefficients for heat cured high-strength concrete specimens are significantly overestimated using the Bridge Design Specifications (24). 5. The creep prediction relationship specified by the Bridge Design Specifications (24) predicts higher early creep coefficient when the concrete compressive strength is higher than 8 MPa at the time of loading. 6. Heat cured specimens have less shrinkage compared to moist cured specimens. 7. The ultimate shrinkage strain does not vary significantly for high-strength concrete ranging from 7 to 12 MPa. 8. The current Bridge Design Specifications (24) predict the shrinkage strain of heat and moist cured high-strength concrete quite well. Acknowledgements The authors would like to acknowledge the support of the NCHRP through project and the Senior Program Officer, David Beal. The authors also thank the contributions of Henry Russell of Henry Russell, Inc. and Robert Mast of Berger/ABAM Engineers, Inc. who served as consultants for the project. The contribution of Ready Mixed Concrete Company and the personnel of the Constructed Facilities Laboratory are greatly appreciated. The authors would also like to acknowledge the helpful efforts provided by the graduate research assistants, Andrew Logan, SungJoong Kim and Zhenhua Wu. References Logan, A. T. (25) Short-Term Material Properties of High-Strength Concrete. M.S. Thesis, Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, NC, USA. American Association of State Highway and Transportation Officials (24) AASHTO LRFD Bridge Design Specifications. 3 rd Edition including Interims for 25 and 26, Washington, DC, USA. Tadros, M. et al. (23) Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders. NCHRP Report 496, Transportation Research Board, Washington, DC, USA. 1