CONTROL OF THERMAL CRACKING USING HEAT OF CEMENT HYDRATION IN MASSIVE CONCRETE STRUCTURES

Transcription

1 CONROL OF HERMAL CRACKING USING HEA OF CEMEN HYDRAION IN MASSIVE CONCREE SRUCURES. Mizobuhi (1), G. Sakai (),. Ohno () and S. Matsumoto () (1) Department of Civil and Environmental Engineering, HOSEI University, okyo, Japan () Civil Struture and Materials Group, Kajima ehnial Researh Institute, okyo, Japan Abstrat In massive onrete strutures suh as dams or large bridge foundations, thermal stresses originating from the heat of ement hydration beome main reason of ausing the thermal raking. In order to ontrol the thermal raking, it is neessary to redue the temperature rise of the member inside and to equalize the temperature of the hole member as muh as possible. As speifi methods, it is effetive to disharge the heat of ement hydration aumulated in the member outside and to redue the temperature gradient beteen inside and outside in the member by temperature-ontrolled uring or heat uring in respet of the onrete surfae. In this study, results of the analytial study on the method for simultaneously arrying out redution of onrete temperature of the member inside and heat insulation in the onrete surfae by running ater, hih as armed by running in the pipe buried in the member, in the pipe buried in the onrete surfae are reported. 1. INRODUCION Pipe ooling is generally used in massive onrete strutures suh as dams or large bridge foundations. In most of the strutures here the system has been used until no, the primary objetive has been to redue the temperature of the members to the temperature around the struture. Cases here the primary objetive as to aggressively redue the temperature rise aompanying the heat of hydration are rare. Hoever, if the spaing beteen pipe installation loations is made small, the rate of flo of ater inreased and pipe arranged at loations here the temperature of the member is maximum then a onsiderably large amount of heat an be removed in a short time. Suh a measure an aggressively inhibit thermal stresses originating from the heat of ement hydration. In this study, the authors propose the system to ontrol thermal raking in onrete surfae by being redued internally restrained stress sine it is possible to redue temperature gradient of inside and surfae part of onrete by using the method to utilize ater armed in the onrete inside ith the pipe ooling method for arming onrete surfae part.

2 In this paper, results of the analytial study on onditions suh as ater temperature and flo veloity for seleting optimum onditions in this system are reported.. SYSEM O CONROL OF HERMAL CRACKING USING HEA OF CEMEN HYDRAION IN MASSIVE CONCREE SRUCURES hermal rak ours in ase hih the thermal stress ourred by restriting volume hange aused by the temperature hange during heat of hydration of the ement reahes over tensile strength of onrete. It is possible to divide the thermal stress into to types of internally restrained stress and externally restrained stress. hese thermal stresses are related to temperature rise of the onrete member inside generated by hydration of ement. herefore, it is important to redue temperature rise of the onrete member inside in order to ontrol the thermal raking. A lot of methods suh as reduing used unit ement ontent as muh as possible, used lo heat ement, used admixture materials suh as fly ash, pre-ooling method, et. are proposed and applied as the ountermeasure redued temperature rise. he pipe ooling method that heat of hydration during hardening onrete is ompulsorily removed by passing ater in pipes embedded the onrete member after plaing onrete is an effetive one in those ountermeasures. he pipe ooling method hih has been used mass strutures suh as dams until no has been applied to redue the temperature of the members to the temperature around the struture. herefore, pipe ooling has been done ith the purpose of ahieving a ontration joint, ith stabilised opening, in the ase of grouting joints beteen bloks rather than to ontrol thermal raking due to the hydration of ement. Conrete member Pa formork in Pipe in onrete member Pipe in Pa formork Figure 1: System to ontrol of thermal raking using heat of ement hydration In this study, it as studied to estimate the system for ontrolling surfae raking of mass onrete strutures as part of the development of method for ontrolling the thermal raking by applying the onventional pipe ooling method. In this system, as shon in Figure 1, the pre-ast board embedded a pipe onneted ith ooling pipe embedded the onrete member is used as a formork and it is possible to arm near surfae of the onrete member by running the ater armed by running the old ater in the ooling pipe into the pipe embedding the pre-ast board. By using this system, it is possible to redue the temperature rise by heat of hydration of the ement in the onrete member and hold the onrete surfae division arm simultaneously. herefore, internally restrained stress is not only redued as the gradient of inside and outside temperature in onrete member is redued, but externally restrained stress is also redued as temperature rise in the onrete member is redued. out

3 3. FEAURES OF HIS SYSEM Features of this system are shon in the folloing. 1). Sine it is possible to ontrol ooling of inside of onrete and heat insulation of the onrete surfae part simultaneously, it is easy to equalize the temperature of hole onrete member. herefore, this system is very effetive for ontrolling of thermal stress of mass onrete. ). It is possible that this system is idely applied to the mass onrete strutures. 3). his system an make thermal energy irulate and effetively utilize the thermal energy. 4). Sine this system is based on the simple priniple, it is not neessary to prepare the speial equipment. 5). In order to use the pre-ast formorks, it is possible to install the formorks ithout requiring the proessing in the onstrution site. 6). Sine the pre-ast formork is a embedded frame, it is not only neessary to remove it, but also it has the harateristi of the heat insulation effet.. 4. OULINE OF HIS SYSEM his system uses the pre-ast formork (it ill be alled the Pa formork sine then) as a formork as shon in Figure. he pipe in hih it is possible to run the ater in the Pa board is buried as shon in Figure. For the pipe, thin all ERW steel pipe in usually about 5mm diameter is used. he pipe is plaed like teeth of omb in order to effiiently arry out heat exhange ith the outside. On the other hand, in the onrete member, the ooling pipe is plaed on the onstrution joint as shon in Figure 3. After plaing onrete, river ater or tap ater is run into the ooling pipe buried in the member from the pipe entrane. hen, the ooling ater hanges to the arm ater by absorbing heat of hydration of ement and onrete is ooled don. he arm ater is guided and run in the pipe buried on the Pa formork. he Pa formork ooled don by the air is armed by heat exhange ith arm ater. Water ooled don in the Pa formork again is disharged outside. form outside A-A setion form inside joint pipe joint pipe goose nek B-B setion joint pipe Pa edge form Pa intermediate form Pa edge form C-C setion D-D setion E-E setion Figure : Outline of Pa formork By ontinuously arrying out this series of ater ondution, it is possible to arm the surfae of onrete ithout asting thermal energy and keep heat insulation, hile plaed onrete is ooled don. herefore, it beomes possible to build the system hih ontributes

4 to redue thermal stress and ontrol thermal raking by equalizing the temperature of the hole onrete member. Hoever, as shape and dimension of Pa formork shon in Figure are examples, they have to hange aording to target of the size and onstrution onditions of strutures. Joint beteen onrete member and Pa formork Disharge of ater Cooling pipe in onrete member entrane of ooling pipe Figure 3: Layout of ooling pipe and flo of ater 5. PROCEDURE FOR ANALYZING HEA ELIMINAION BY PIPE COOLING Only the fundamental onepts applied and the basi equations used in the proedure for analyzing heat elimination by pipe ooling are desribed here sine details of suh proedures have already been reported earlier. 5.1 hermal analysis in the onrete field he three-dimensional unsteady heat ondution equations given belo ere used in the onrete field. x + y + z λ + Q () t = ρ t (1) Where, λ=thermal ondutivity (W/mºC), = temperature of onrete (ºC), Q ( t) = heat rate in onrete, ρ= density of onrete (kg/m 3 ), and = speifi heat of onrete (kj/kgºc). Boundaries assumed ere the onvetive boundary in ontat ith the outside air and the boundary in ontat ith the surfae of the ooling pipe all. he onvetive boundary in ontat ith the outside air is generally expressed by the equation belo. λ l + m + n + η x y z ( ) + q = 0 S () Where, l, m, n are diretion osines in the boundary surfae, η is the heat onvetion oeffiient, =outside air temperature (ºC), q s = heat flux in the onrete surfae (W/m ).

5 he boundary onditions of the ooling pipe all surfae ere set by the equations given belo assuming that heat transfer takes plae in the ater in the ooling pipe and the pipe all surfae in ontat ith the onrete. λ l + m + n + h x y z ( ) = 0 (3) Where, h= heat onvetion oeffiient on pipe all surfae and = temperature of ater in the pipe (ºC). By using Galerkin's method in equation (1) and the boundary onditions of equations () and (3), the equations an be expressed in matrix form as given belo. [ K ]{ } [ K ]{ } + [ C] = { F } t [ K ] = [ B] [ D][ B] dv + η [ N ] [ N ] ds1 + qs [ N ] ds1 + h[ N ] [ N ] ds V S1 S1 [ K] = h[ N] [ N ] ds, [ ] [ ] [ 1 C = ρ N N ]dv, { F } Q( t) [ N ] dv [ N ] ds V = S1 S V S (4) Where, [B]= temperature gradient matrix, [D]= oeffiient of heat ondution matrix, [N ]= shape fator matrix related to onrete temperature and [N ]= shape funtion matrix related to temperature of ater in ooling pipe. Also, S 1 = boundary surfae in ontat ith outside air and S =surfae area of pipe all. 5. Equations governing the temperature of ater in the pipe For deriving the basi equations related to temperature of ater in the pipe, the ooling pipe as assumed to be a single pipe from the entrane to the exit. he flo of ater in the pipe as assumed to be a one-dimensional flo by taking the polar oordinates along the pipe. he equation governing the temperature of ater in the pipe as derived as belo for the ase hen ater flos from diretion S i to diretion S i+1 at the flo rate u over an infinitesimal distane ds, here S is the distane from the pipe entrane. ρ Cr S t πu ds + πrdsh( ) + ρ C ds ( ) = C u + + ρ C = 0 h ρ (5) S r t Hereρ :density of ater (kg/m 3 ), C : speifi heat of ater (kj/kg ºC), and r: radius of the pipe (mm). he flo rate u in equation (5) as assumed to be uniform ithin the pipe. Sine the range of flo rates during the pipe ooling is generally beteen l/min. to 0 l/min., a state of turbulene an be assumed to prevail ithin the pipe, and heat exhange assumed to be performed adequately. he Galerkin method is applied to equation (5) similar to the onrete field, to obtain the equations for temperature of ater in the pipe in matrix form, as given belo. t [ ]{ } [ ]{ } [ ] { } K K + W = 0 (6)

6 [ K ] = ρ C u [ N ] [ N ] h r S [ ] [ N] ρ C u [ N ]ds i + 1 Si S S h + r [ K ] = [ N ] [ N ]ds, [ W ] = ρ C [ N ] [ N ]ds S S [ N ] [ N ]ds S Even if the pipe layout is omplex or if the flo rate or the temperature of ater hanges sequentially, the heat elimination effet by pipe ooling an be analyzed and evaluated by ombining equations (4) and (6). hese equations have been applied in the study of measures for ontrolling thermal raking in dam spillays until no. It as onfirmed that the results of analysis using the present proedure agreed ell ith the results of temperature measurements. Furthermore, the equations have also been used for evaluating primary ooling temperatures of arh dams. Although further studies on the oeffiient of heat onvetion of the pipe all surfae are neessary, the results of analysis using these equations agreed ell ith the results of measurements near the pipe. 6. ANALYSIS CASE he raising onstrution of dam raises about m in the existing dam of 40m height and in addition, the gravel onrete dam as onstruted by digging don 8m further than the existing level. he dam after extending beomes 50m height and 0m rest length. Figure 4 shos outline of the dam. m Raising part 40m Existing dam body Objet of analysis 8m EL-78m EL-69m Figure 4. Outline of raising onstrution of dam and objet of analysis In this study, analysis as arried out to estimate the effet of ontrolling thermal raking in assuming that this system as applied to the dam onstrution. he part of depth diretion 8m, rest length diretion 15m and 9m height of the dam from EL-69m to EL-78m as an analysis model in hih four lifts (thikness of a lift is 0.75m) plaed on hardened onrete ere onsidered. Figure 5 shos layout of the analysis model. Cooling pipes for the purpose of the heat removal is buried in the struture inside and Pa formork of 0.15m thikness

7 クーリングパイプburied a ooling pipe beforehand as installed in the donstream surfae. hese to pipes ere oupled. In this analysis, it as assumed that the ooling pipe on eah lift as buried. Figure 6 shos pipe arrangement in the onrete member and in the surfae part. able 1 shos ondition of the analysis. Existing dam body Raising part EL-78.00m 9m EL-75.00m Existing part 8m 15m Figure 5. Analysis model Joint 表面部へ beteen onrete member and Pa formork Cooling pipe Pa formork パイプ間隔 Pipe interval Existing part クーリング水出口entrane of ooling pipe クーリング水入り口 entrane of ooling pipe donstream front elevation Figure 6. Cooling pipe layout of analysis model in raising dam In analysis ondition of pipe interval 0.5m in the Pa formork and ooling pipe interval 1.0m in the onrete member, figure 7 shos analytial results as flo rate in the pipe is made to hange at m/s and 15m/s. As shon in Figure 7, onrete temperature of the member inside dereased depending upon inreasing in the flo rate of the ooling ater. In the meantime, as shon in figure 8, onrete temperature in the Pa formork inreased 5 to 7ºC by running arm ater. Moreover, as shon in figure 9, the proportion of temperature rise in surfae part and temperature redution in the lift inside by running ater as influened by the hange of onrete temperature in starting the ater ondution. In this ase, analyses ere arried out on ase in hih ater temperature as varied ºC and 15ºC in order to onsider the differene beteen the plaes from hih ater as dran. It is neessary to rise

8 the initial running ater temperature so that temperature of the surfae part may rise. As results of the analysis, it seems that it is neessary to set the initial running ater temperature at about 15ºC to rise temperature in the onrete surfae to about 6ºC. On thermal stress in onrete surfae by the differene of ater temperature and flo rate of the ooling ater, Figure shos analytial results under the ondition of pipe interval 0.5m in the Pa formork and pipe interval 1.0m in the onrete member. From Figure, thermal stress of the onrete surfae dereased depending upon dereasing in ater temperature. As a result of analysis on effet of thermal stress redution by the running arm ater, as shon in Figure 11, in ase of the initial running ater temperature ºC, more than 1.3N/mm thermal stress as redued in omparison ith the ase ithout running ater. On the effet to the thermal raking index by the running ater in the onrete surfae, as shon in Figure 1, the thermal raking index inreased about 1.0 on average in ase of the initial running ater temperature ºC and even in ase of the initial running ater temperature 15ºC, the thermal raking index inreased about 0.5 on average. able 1: hermal and strength harateristis Input data Charateristis Raising onrete Existing orete Pa formork External onrete Internal onrete hermal ondutivity (W/mºC) Speifi heat of onrete (KJ/kgºC) Density of onrete (kg/m3) Adidbati temperature rise equation Q Q = 0. 9 C Q = 0. 5 C β Q() t = Q ( ) 1 e γ γ = C γ = C C.:Cast emperature β β = 0. 0 C β = 0. 0 C Donstream side Heat onvetion oeffiients (W/m ºC) Upper side untill an age of 7 days 8-14 afterl an age of 7 days Flo u (l/min) -15 Heat onvetion oeffiients on the pipe all surfae; h (W/m ºC) Outside air temperature (ºC) Compressive strength (N/mm ) M: Maturity (hr ºC) ensile strength (N/mm ) h = 0.06u u 01 Average temperature of eah month in onstrution site ( M ) = log M ( M ) = 13. 3log M f ( ) 0. f = f M 901 ( ) 1. f = f M t f t Young's modulus (N/mm ) Redution fator for Young's modulus by effet of reep; φ Coeffient of thermal expansion Pisson's ratio ( 6 ο C) ( ) 0. E = φ 6830 f M 498 E = φ 3400 log f ( M ) untill an age of 3days after an age of 3 days emperature (ºC) No ooling Flo:m/s, Water temp.ºc Flo:15m/s, Water temp.ºc ooling exeution Age (day) Figure 7: Conrete temperature of the member inside emperature (ºC) No ooling Flo:m/s, Water temp.15ºc Flo:15m/s, Water temp.15ºc ooling exeution Age (day)

9 emperature (ºC) No oling Flo:m/s, Water temp.ºc Flo:15m/s, Water temp.ºc ooling exeution Age (day) Figure 8: Conrete temperature in the Pa formork emperature gradient (ºC) Surfae, Water temp.ºc Center, Water temp.ºc Flo;m/s Surfae, Water temp.15ºc Center, Water temp.15ºc Progress days (day) Figure 9: Proportion of temperature rise in surfae part and temperature redution in the lift inside by running ater Stress (N/mm ) Flo: m/s No oling Water temp. ºC Water temp. 15ºC Progress day (day) Figure : hermal stress of the onrete surfae Differene in stress ithout running ater (N/mm ) Water temp. ºC Water temp. 15ºC Flo: m/s Progress day (day) Figure 11: Effet of thermal stress redution by the running arm ater emperature (ºC) No ooling Flo:m/s, Water temp.15ºc Flo:15m/s, Water temp.15ºc ooling exeution Age (day)

10 Differene in Craking index ithout running ater Flo:m/s, Water temp. ºC Flo:m/s, Water temp. 15ºC Progress day (day) Figure 1: Effet to the thermal raking index by the running ater in the onrete surfae he thermal raking index is defined as the ratio of the tensile strength of the onrete to the tensile stress in the onrete generated by heat generated by the hydration of ement. he larger the thermal raking index, the lesser the possibility of rak ourrene, and vie versa. Generally, as the index beomes smaller, the number of raks inreases and the rak idth tends to gro larger: Ir(t) γ r (7) Here, I r (t): raking index, I r (t)=f tk (t)/σ t (t), f tk (t): tensile strength of onrete at age of t days, σ t (t): maximum prinipal tensile stress in onrete at age of t days, and γ r : fator of safety for probability of ourrene of raking. 7. CONCLUSIONS In the mass onrete strutures, as results of the analytial study on the method for simultaneously arrying out redution of onrete temperature of the member inside and heat insulation in the onrete surfae by running ater, hih as armed by running in the pipe buried in the member, in the pipe buried in the onrete surfae, it as possible to evaluate the effet of ontrolling thermal raking in onrete surfae by running armed ater. Hereafter, this study ill be driven for the appliation in existing strutures. REFERENCES [1] Gallagher,R.H Finite Element Analysis Fundamentals. okyo: Maruzen [] Kokubu, K Neest tehnique on mass onrete. okyo: JSCE [3] Kunimatsu,Y Study on the effetive range of temperature ontrol using pipe ooling method. Proeedings of the Japan Conrete Institute : [4] Mizobuhi, he analysis of the heat elimination by the ooling pipe system of massive onrete. Proeedings of the Japan Conrete Institute.7: [5] Mizobuhi, An Analytial Study on hermal Stress Using Pipe Cooling in Arh Dam Conrete. Proeedings of the Japan Conrete Institute.19: [6] Straus, M. W Cooling of onrete dam, Denver: U.S.Boulder Canyon Projet. [7] Inoue, I Study on ontrolling of thermal raking using pipe ooling method. Proeedings of the 58 th JSCE Annual Meeting, Vol.5: