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Published - Badovli Thermal Analysis

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Published - Badovli Thermal Analysis

  1. 1. New Progress on RollerCompacted Concrete Dams, Jia Jinsheng eta/. (eds) © 2007 China WaterPower Prees, ISBN 978-7-5084-4994-4 . Badovli Dam Thermal Analysis Hamidreza Araghian Project Manager& Concrete Specialist, LAR Consulting Engineers, Tehran, IRAN, E-mail: hra@hra.ir Abstract: Badovli Dam will be located in North West of IRAN, in West Azerbaijan province near the Turkey borders on the Aghsu River.The project is now under design and the main purpose of construction of Badovli Dam is providing the potable water for Shoot,Bazorgan and Maku cities and water for agricultural purposes of Bazorgan and some part of Poldasht.This dam is located in 16 km far from Maku city & 216 km from Urumia the capital city of West Azerbaijan porvince. The mean annual air temperature in the dam-site is 8.9 degrees of centigerade with minimum monthly average temperature of -3°C in the january.the maximum monthly average temperature is 19.4 °C in July. Badovli is a roller compacted concrete dam with a total volume of 350,000 m3 (RCC volume is 290,000 m3 ). Since the RCC will be placed in a short period of time, in order to choose a suitable time schedule for constructuion, it is necessary to to calculate the generated heat and resultant stresses. In this project the RCC will be placed in 0.3 m lifts that is conventional for RCC dams. The two major type of cracking in the RCC dams including surface gradient cracking and mass gradient cracking have been investigated. The surface gradient cracking specially have a great degree of importance because of very cold winters. Mass gradient cracking doesn't occur based on the analysis have been performed. Key words: Badovli Dam, thermal analysis, cracking 1 Introduction Based on ACI 207.1R, Mass concrete is any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking. Since the mass concrete structures (and RCC dams) are poured in many lifts and layers, the effect of the time interval between successive lifts , temperature of fresh concrete and the boundary conditions during construction should be taken in to account when the thermal regime in the structure is calculating. In a thermal analysis, the differential equation of heat is solved by considering the boundary conditions. Eq.l shows the transient form of heat differential equation. a~ a~ a~ aTkx;x-2 +kYY - 2 +kV.-2+w=pc- (1) ax ()y az at In this equation, k, w, p, c, T are conductivity, generated heat, density, specific heat and temperature of the body respectively. Boundary conditions should be defmed carefully for solving this equation. Contact of concrete with air (or water) and rock are examples ofthis boundary condition. When the concrete surface is in touch with a fluid (like air) concrete cools down via convection. The convection coefficient depends on the type of fluid and its velocity. Convection coefficient increases with fluid velocity increment. So continuous curing has and important role in cooling concrete and should not be neglected in thermal analysis. When the concrete is in contact with another infinite solid like rock or soil, the temperature of these boundaries may be considered constant and equal to the average temperature of the the air.During the process of cement hydration some heat liberates. The amount of produces heat depends on the cement type, chemical composition and size of cement grains. Since cement hydration is a thermo-activated reaction, the rate of hydration will be increased by temperature increment. (Fig. 1) Freiesleben and others (1977) developed the equivalent age maturity function, shown in (2), based on the Arrhenius rate theory for chemical reactions. The equivalent age maturity function converts the chronologie curing age t of a concrete cured at any concrete temperature Tc to an equivalent curing age te for a specimen cured at a specific reference temperature T,.. (2) 615
  2. 2. 3 Analysis assumptions The following assumptions have been applied in the current analysis: (1) Concrete is considered Isotropic & Homogeneous consequently all of the thermal properties are the same in all directions. (2) Thermal and structural properties of concrete are constant for all of temperature and time intervals. (3) The Nonlinear effect of temperature on heat generation rate has been considered in the analysis. This is an important factor in calculation of temperature in concrete. (4) The Analysis will be performed in the plane of the dam. (2 - dimentional thermal analysis) (5) The Compressive strains in the dam body will be neglected conservatively. may occure in the colder months of the first or second year after the construction. These type of cracks are nearly closed after reduction of the Surface thermal gradient . Mass gradient cracking of concrete can be visible after final cooling of the internal part of the mass and this may take even many years. 4 Dam section geometry Fig. 2 shows the largest block of the dam. The crest elevation is 1,664.5 m. the height of dam is 98.8 m. The lower part of main monolith, acts as cofferdam during construction. The embedded parts such as irrigation and water intake pipes have been located in this part. Since this part of the dam should be constructed in the diyersion stage and RCC mixes may not be available at that time. based on the proposal of the contractor this part may'be constructed as Conventional concrete. ~ = T2 >T1 .9 ~ .a £~ 0 v e00 v "0 t2 t1 time All J1 Where: te is equivalent age at reference curing temperature; dt is chronological time interval, h; T is average concrete temperature during time .interval dt; To is reference temperature, °C (20°C or 293 K for cement); Eo is activation energy, J/mol and R is universal gas constant. Heat generation of the concrete is also a function of time. This function can be expressed as below form: Q=Qm(l-e-m.tcr ) (3) In the above equation, Qm is total heat of concrete, m is a coefficient depends on the Type of cement, and te is the equal age which can be calculated based on Maturity of concrete. Tensile stress due to the concrete cooling can be expressed as Eq. (4): u=k . a . E . iYl' (4) In this equation, compressive stresses have been neglected conservatively. This is due to the higher creep in the early ages after concrete placement. k is the total restraint (foundation and geometry), E the elasticity modulus and aT the cooling degree of the concrete element in the dam. Fig. 1. Factors affecting the degree of hydration of concrete 2 Thermal cracking in RCC dams 7 +1,653.5 Two different types of Thermal cracking can be expected in Conventional Concrete and Roller Compacted concrete dams: Mass Gradient cracking & Surface gradient cracking. Surface gradient cracking caused by the tensile stresses induced due to the temperature difference between surface and internal part of the massive concrete structure. Mass gradient cracking is a dangerous the~ cracking that caused by restraint against the movement of total mass of the dam during cooling up to the fmal stable temperature. these type ofcracks can c~use large amount ofleakage in concrete dams. Normally Surface gradient cracks are narrower and 616 0.85 Rce g 1,58820 evc Fig. 2 Typical Cross Section of Badovli Dam The lower part modelled in 1 meter lifts with time
  3. 3. interval of 5 days. This is due to the lack of pre-cooled aggregates in that time. The RCC part will be constructed in 0.3 m lifts and 1 day between two successive list left. Upstream and downstream face concrete thicknesses are 1.5 & 1.0 meter of air entrained concrete to provide enough durability against freezing and thawing. 5 Environmental condition Continue Property Rce eve Thermal Expansion Coeff. 8.8xl0-6 8.8xI0-6 Elasticity Modulus (Gpa) 18 22 Tensile strain capacity-Rapid 80J.l 100 J.l Tensile strain capacity-Slow 110 J.l 150 J.l Poisson ratio 0.2 0.2 6 Concrete mix designs Based on mentioned results, the Qm and m will be 72 callgram and 0.206 respectively. Based on the Laboratory results, RCC and CVC mix designs consist of 160 kg and 220 kg of cementitious materials respectively. Cement Type is Natural Portland pozzolanic Cement (IP). the results of heat of hydration test (Based on ASTM CI86), is in the Table 1. Table 1 Heat of hydration of Uromia IP cement based on ASTM C186 8 Finite element model ANSYS finite element program was applied for modeling and analysis. This software has excellent capabilities of modeling concrete structures temperature & construction procedure effect on the internal temperature of the dam. The Model consists of 2,193 PLANE77 Elements and 6,871 Nodes. This type of element, with its great features, enables user to model temperature in the dam body. By using kill option, the user will be able to model construction effects .The geometry of the generated mesh for BADOVLI has been modified to present a reliable model for getting the required data's. BADOWLI THERMALANALYSIS 72 28 d Heat of hydration(caVg) 55 7d Badovli Dam site is a cold region; the average annual temperature of dam-site is about 8.9 °C. The sinusoidal function of the temperature in Badovli may be expressed as below: Temp(t) = 8.2 +11.2sin 27[(t - 95) (5) 365 Where: t is the temperature and t is time in days from beginning of the year. The maximum and minimum monthly average temperature is 19.4 °C and -3°C respectively. 7 Thermal and structural properties Fig. 3 Meshed dam body Thermal and structural properties are shown in the Table 2. . 9 Placing Temperatures 617 Property RCC cvc Thermal diffusivity(m2 /d) 0.113 0.113 Specific heat [J/(kg.oC)] 950 950 Density (kglm3 ) 2,420 2,420 Thermal Surface transmit ion- 60 60coeff [kcall(m 2 .h·C)] ..... , Table 2 Thermal & structural properties of RCC and evc According on US Army the placing temperature of the roller compacted concrete calculated as Table 3. Based on these temperatures, RCC shall be started in the February and ended in the November Before the temperature falling below 5 °C. during the winter RCC shall be protected form the low temperatures. in the next year RCC placing will be started in February again. Time interval between successive lifts is about 1 day, and a maximum volume of 1,500 m3 /d predicted to be placed.
  4. 4. Table 3 Ree and eve placing temperatures RCC Placing Temperature in BADOVLI Dam Jan.* Feb.* Mar. Apr. May June July Aug. Sep. Oct. Nov.* Dec.* - 8.9 10 10 10 10 10 10 8.5 - - * Indicates that in these months the placing should be stopped. 1.00 P""-lIlii~--r---,----r-------r---....,.--------. A =0.035,825 B =0.188,592 C =0.341, 359 D =0.494, 127 E =0 .646, 894 F =0.799, 661 G =0.952, 428 H = 1.105 I 1.258 and restraint is high. So it is reasonable to expect mass concrete gradient cracks are started from the middle part. Fig. 5 Restraint factor for Badovli Dam body (perpendicular to dam axis) Restraint factors of Fig. 4 & Fig. 5 .are calculated based on the overall cooling of dam, While in a real case, because of differential cooling of the dam, these restraints may be increased. This effect bas been considered in the current analysis for Badovli Dam. It sh?uld be noted that the mass gradient cracking sball be checked along dam axis and perpendicular to dam axis. Cracking perpendicular to dam axis (transverse cracks), will endanger the water tightness and longitudinal cracking may divide dam section in to two or more column that will oscillate separately during the earth quake and the dam may not be stable in this condition. 11 Surface gradient analysis 6040 50 Height above foundation (m) ---- Joint-dist=45m. -._. Joint-dist=9Om 3010 --+-Joint-dist=30m -Joint-dist=15m 0.90 tfr;;-~~---+---I----+---+-----I 6 ] 0.80 t-t-~~---+---+----+---1--~ .s 0.70 ~1f-T-+~--+---+----+---1-----I CI:S ~ 0.60 ~t"*-T+-~-+---I----+---+-----I ~ 0.50 t--....-t.....,...------.,;=v---+----+---I--~ 0.40 t---t:::-T-t'~-~T_-+----+---1--~ 0.30 t---t----r.:~--+--~+----+---+--___I 020 J----t-+r-~-+---~~-+---I--___I 0.10 I----,...-+-~r_i~--I---~---I--___I '''-0.00 1....-----.-~.....~~I:lI:::l1IIiI...~t__~. . . . ....._... o 10 Restraint Factors Restraint factor for badowli dam -along crest ( crown-cantilever) Restraint factors in dam body calculated by using a comprehensive 3d model. The results have been shown in the Fig. 4 & Fig. 5.Transverse joint spacing has a major effect on the restraint factor and tensile thermal stresses induced in dam body parallel to dam axis. When total volume of the dam body cools down simultaneously and homogenously, the foundation restraint effect may decrease the stresses. Because this is not true due to the variations in peak temperature of concrete in different points, so it is reasonable to neglect this effect. So the low modulus of the foundation rock may decrease the thermal stresses of dam body slightly, but in the analysis this effect has been neglected conservatively. 618 Rg. 4 Restraint factor for Badovli Dam with different contraction joint spacings (along dam axis) As shown in Fig. 4, when the joint spacing increases, the restraint against thermal movement of the dam also increases for a specified point in the dam, and restraint in the middle part is higher than 'that of upstream and downstream faces. So it is reasonable to e~pect that mass gradient cracks to be started from the . middle part of the dam where both of the temperature Because Badovli Dam has been located in a cold region, the temperature gradient between core and surface may cause cracking so it is necessary the strains shall be calculated in the analyses and the surface tensile strain shan be checked to be less than cr~ckiDg strain for the concrete with consideration of contraction joint effects. Fig. 6 shows induced strains in the dam body after placing 20 meters ofevc in the lower part of the dam.
  5. 5. As can be seen, the maximum tensile strain is 117micro in z direction, which is higher than short-term tensile strain capacity of the CVC (Table 2). The crack spacing will be more than 50 meters that is higher than maximum joint space, so the cracking doesn't occur. Internal part of the dam is under compressive strains that can be tolerated by RCC safely. After ending the frrst winter, surface of concrete will be covered by roller compacted concrete, so the fIrst winter is almost the worst condition for this concrete form thermal stresses view point. It is planned that the major part of the aggregate to be produced in the colder months. RCC should be placed with a temperature not more that 10°C. Since the most probable occurrence time of the surface gradient cracking is the first or second winter The Isothermal and strain contours have been shown in these two seasons (Fig. 7 and Fig. 8). As it can be seen the maximum tensile strain in the second winter due to temperature gradient is 160 micro that is more than tolerable strain by RCC (100 micro strain). Because the contraction joints in the dam, are designed to be located at a maximum 19 meters distance, and this tensile strain may produce a cracking distance about 33 meters so the cracking doesn't occur. So the dam is safe against surface cracking. BADOWLI THERMAL ANAL YSIS ANSYS5.4 DEC 12005 22:45:06- Nodal solution TIME=155 TEMP (AVG) RSYS=O Power graphics EFACET=1 AVRES=Mat SMN=- 3.307 SMX=24.989 A =-1.735 B =1.409 C =4.553 D=7.697 E =10.841 F =13.985 G =17.129 H =20.273 I =23.417 BADOWLI THERMAL ANAL YSIS ANSYS5.4 DEC 1 2005 22:51:56 Nodal solution STEP=1 SUB=1 TIME=1 EPTOZ (AVO) RSYS=O Power graphics EFACET=1 AVRES=Mat DMX =0.001,504 SMN =-0.882E-04 SlvIX =0. 160E-03 A =- 0.744E-04 B =-467E-04 C =-191E-04 D =0.851E-05 E =0.361E-04 F = 0 .638E-04 G =0.914E-04 H =0.119E-03 I =0.147E-03 Fig. 6 Isothermal contours (left) and thermal strains (right) in the lower part (CVC) of the dam body BADOWLI THERMAL ANAL YSIS BADOWLI THERMAL ANAL YSIS ANSYS5.4 DEC 2 2005 19:34:23 Nodal solution STEP=1 SUB=1 TIME=1 BFETEMP (AVG) DMX =0.003, 307 SMN =-3.887 SMX=29.442 A=- 2.036 B=1.668 C=5.371 D=9.074 E=12.777 F=16.48 G=20.148 H=23.887 1=27.59 ANSYS5.4 DEC 2 2005 19:34:37 Nodal solution STEP=l SUB=I TIME=1 EPTOZ (AVG) RSYS=O Power graphics EFACET=l AVRES=Mat DMX = 0 .003, 301 SMN =-0.143E-03 SMX =0.I64E-03 A=-0.126E-03 B =-0.917E-04 C =- O. 576E- 04 D =-0.235E-04 E =0.105E-04~.-:::...- --r- ....:::..-~ F =0. 446E-04 G =0.787E-04 H =0.1l3E-03 I =0.147E-03 Fig. 7 (Left) Isothermal contours at second winter (Right) thermal strain in the dam body in the second winter (at 540 days after starting the construction of the dam) 12 Mass gradient cracking In order to investigate the mass gradient cracking occurrence, the temperature history in different locations of dam body have been calculated by using the Finite element model. The cooling of dam body up to 4,500 days after start of the construction, have been modeled. The maximum temperature in the dam body is 30.4 °C. The maximum mass gradient in the dam body is 26.1 °c in the height of 52 meters of the dam. This Gradient produces a crack distance of 50 m that is more than maximum contraction joint distance of 619
  6. 6. dam body and mass gradient cracking doesn't occur. The Temperature histories at different nodes are shown in the Fig. 9. Node 3,554 is in the middle part of the dam and in the height of 49 meters above the foundation. Node 3,830 is Near Upstream face at the same height. So due to the effect of the surrounding air, some temperature oscillation can be seen in this node. Node 4,031 is also in the middle part between upstream and downstream of the dam and 54 meters above the foundation. After almost 3,500 days, the dam body reaches to its final stable temperature that is 8.5 C for Badovli Dam. Because of 3 d nature of real temperature flow, the real temperature history might show some discrepancy from the analysis results. However, the maximum and minimum temperature will be the same. ANSYS5.4 DEC 2 2005 17:34:46 Nodal solution TIME =400 TEMP (AVG) RSYS=O Power graphics EFACET=1 AVRES=Mat SMN=5 SMX=30.22 A =6.401 B=9.203 C=12.006 0=14.808 E =17.61 F =20.412 G=23.215 H=26.017 1=28.819 ANSYS5.4 DEC 2 2005 17:34:11 Nodal solution TIME=lOOO TEMP (AVG) RSYS=O Power graphics EFACET=l AVRES=Mat SMN =3.483 SMX=23.841 A =4.614 B =6.876 C =9.138 0=11.4 E =13.662 F =15.924 G =18.186 H=20.448 1=22.71 Badowli thennal analysis Badowli thermal analysis Fig. 8 (left) Isothermal contours at 400 days after start (right) Isothermal contours at 1000 days after start of placing Temperature history ofdifferent nodes 35 r----..,-----,------,------r------, e30 1---.r.--+----4-----I-----+----I G) ·S 25 t--l ~+----4-----I-----+----I G ~ 20 ~ 15 1--t-t-'"'II'-ir+:PI--..~~----I-----+----I 10 I-JjL-Y-+J-U~~~~-i..£.rL_~ 51----+---=---JlL-Y~~~I:.f.-_t.:H___Ir¥_-___I o 1,000 2,000 3,000 4,000 5,000 TlJDe(d) -Node 35S4 .......Node 3830 ---Node 4031 Fig. 9 Temperature history at different nodes 13 Conclusions All of the analyses were done based on the RCC placement in 9. months. Based on analysis no major crack which endangers dam performance and its water tightness is expected in future. In this design stage analysis, the foundation restraint factor considered to be 100%. (1) The heat liberation rate of the cement has a great influence on the heat storage in the concrete . inass. So including the effect of temperature on cement hydration in the thennal analysis of RCC dams, especially in cold areas, results in higher peak temperature in the concrete. This is due to the retarding effect of low air temperature on hydration of concrete. (2) Controlling Surface gradient cracking especially in the cold regions has a great degree of importance. Because surface cracking may be harmful for water tightnes.s of the dam. (3) fu the lower part of the dam, restraints against movement of the dam due to the thermal shrinkage are high so the risk of thermal cracking will be higher in this part. In the Badovli Dam, the maximum temperature of lower part has been limited to 25°C to control cracking parallel to dam axis and between contraction joints. References [ 1] ACI 207.2R - 95. Effect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete [2] ACI 207.1R. Mass Concrete [3] ACI 207.4R. Cooling and Insulating. Systems for Mass Concrete [4] ACI 209.R. 92. Prediction Of Creep, Shrinkage and Temperature Effects in Concrete Structures [5] ASTM CI074. Standard Practice for Estimating 620
  7. 7. Concrete Strength by the Maturity Method [6J ANSYS 5.4ADPL. User's Manual [7 J F.R.Andriolo. The Use of Roller Compacted Concrete [ 8J Bentz and de Larrard, "Prediction of Adiabatic Temperature Rise in Conventional and High Performance Concretes Using a 3-D Micro structural Model". Cement and Concrete Research [9J P, Kumar Mehta & Paulo. J. Monteiro. Concrete: Microstructure, Properties, and Materials [ 10J Andrade, F.R.Andriolo, Thennal Analysis of Roller Compacted Concrete [ 11] Hamidreza Araghian, Thermal Analysis of RAEESALI DELVAJU dam 621