kahir thermal analysis full paper

The 7th International Symposium on Roller Compacted Concrete (RCC) Dams

KAHIR RCC DAM THERMAL ANALYSIS

Araghian H.R.1, Hajialikhani M.R.2, Jafarbegloo M.3

1, Concrete & RCC dam designer & Specialist, ,[email protected]

2, Asossiate Director, Jahan Kowsar Co., [email protected]

3, Technical & Quality Manager, Jahan Kowsar Co., [email protected]

Abstract: Kahir Dam is an RCC dam in Iran which is located in South east of Iran in a semi dry area over a

wild / heavy flooded river. It is FSHD type with 54.5 m height and crest length of 380 m and with RCC

volume of about 500.000 m3.

In this paper, 2-D Kahir thermal analysis within construction by using ANSYS finite element software is

presented. Based on the dam construction methodology, right and left parts has different construction

methodology. So two diffetent construction priority has been analyzed and temperature histories have been

calculated in different nodes upto 6000 days after start of construction. Effect of internal heat of concrete on

the rate of hydration has been considered in the model.

Finally, according to the calculated temperatures in different points of the dam, the potential of thermal

cracks in concrete dam body are investigated.

Key words: KAHIR RCC Dam, Thermal Analysis, FSHD, Heat of hydration,

1 Introduction

Due to gradual construction of massive concrete structures and RCC dams, calculation of heat

distribution in mass concrete is a complicated matter. Also, additional to thermal properties of

concrete and its initial temperature, some other parameters like as time interval between lifts and

mean ambient temperature act on heat distribution in massive concrete structures.

Totally, solving the heat differential equation is necessary to obtain adequate and accurate results.

Nowadays, because of computing developments, these equations are solved by finite element or

finite difference methods. The equation of heat distribution is expressed like as Equation 1:

t

Tcw

z

Tk

y

Tk

x

Tk zzyyxx

2

2

2

2

2

2

(Eq.1)

In Equation 1, the heat distribution in a mass is a function of time; but if a steady heat distribution

is envisaged, Equation 1 changes to Equation 2 –where T is the mass temperature, k is coefficient

of heat transmission, is the mass density, c is specific heat and w is heat generation in the mass.

02

2

2

2

2

2

w

z

Tk

y

Tk

x

Tk zzyyxx

(Eq.2)

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mailto:[email protected]


2 Effective parameters on hydration

Hydration process depends on various parameters such as chemical composition of cement, water

to cement ratio, fineness of cement particles and cement particle size distribution.

According to scientific observations, hydration does not exceed more than 70 to 80 percent. Also

theoretically, the highest rate of probable hydration is about 80 percent.

Hydration process is accelerated by heat like the most of chemical and physical processes (Figure

1). This effect can be seen in cement hydration in temperatures above 20 ˚C. So, for correct

simulation of heat generation in concrete mass, it's necessary to model the temperature effect on

hydration. Majority of math models for determining the temperature effect are based on Fourier

differential equation.

Figure 1. Temperature effect on concrete hydration

Equation 3 indicates the temperature effect on hydration rate:

)1

1

(0)(

TTR

Ea

eTH

(Eq.3)

Where T is concrete temperature at the calculation time and T0 is the initial temperature of

reaction equal to 293 Kelvin. Ea is the activation energy of Portland cement which is equal to 33.5

kJ/mol and gas constant, R, is equal to 8.31 J/mol.K.In the Kahir thermal analysis the effect of heat

on hydration of cement is considered.

3 Calculation of cracking risk

In Kahir project, cracking risk calculation is based on strain method. ACI 207.2R expresses this

relation as a function of L/H which presents in Equation 4:

TKK FR (Eq.4)

Equation 4 shows the created strain in concrete influenced by temperature difference where,

is the coefficient of thermal expansion of concrete and ΔT is temperature difference. Also, K R is

the internal restraint degree as a result of structure and foundation geometry which differs from 1 to


100 percent and obtained from ACI-207.2R. It should be mentioned that the reduction of restraint

with heigh has been neglected in the calculation of thermal restraint and KR was considered equal

1.0 conservatively.

4 General Project information

KAHIR RCC dam was designed in south-east of Iran and is under construction. The foundation

level is 13.5 MASL and the foundation width is 83 m. KAHIR dam typical section has been shown

in Figure 2.

Some major project data are as below:

Type of dam: RCC gravity dam (FSHD type)

Crest length: 380 m

Crest width: 5 m

Figure 2 . Typical cross section of Kahir RCC dam

Spillway width: 160 m

Dam height from foundation: 54.5 m

Reservoir Volume : 314 million m3

RCC volume: 500,000 m3

CVC volume: 180,000 m3

RCC required compressive strength: 70 kg/cm2 @ 180 days

Diversion system: One Tunnel with 6m diameter & 280m length

5 Ambient conditions

Kahir dam is located in a semi dry region. Sinusoidal curve of mean monthly ambient temperature

is shown in Figure 3. The site average annual temperature is 27.7 degrees celsius.


Figure 3. Mean monthly temperature in KAHIR dam region RCC mixture

At present, RCC mix proportion is designed at the dam local laboratory. Cement content in the

current mix design is equal to 110 kg/m3. The required specified strength is 70 kg/cm2.

6 Heat generating of RCC

Khash pozzolanic cement with 20% Natural pozzolan is utilized in preliminary mix program. The

heat of hydration of Portland pozzolanic cement is considered in the analysis. and the heat

hydration from 3 to 90 days is shown in Figure 4.

.

Figure 4. Heat of hydration of Khash pozzolanic cement

7 Coefficient of thermal expansion

The coefficient of thermal expansion of RCC is a function of expansion coefficient of the

aggregates and consequently, the petrology of concrete aggregates (Sandstone,Limestone and

Volcanic). Due to nature of RCC aggregates –in this project- and considering tables 2.9.1 and 2.9.2

of ACI-209, the coefficient of thermal expansion is determined equal to 10.2×10-6 (1/˚C).

15.0

20.0

25.0

30.0

35.0

40.0

Ap

r.

Ma

y

Jun

e

July

Au

g.

Sep

.

Oct

No

v

Dec Jan

Feb

Ma

rch

Tem

p (

C)

Monthly Temperature in KAHIR site

3, 51.7

7, 69.528, 74 90, 74

40

45

50

55

60

65

70

75

80

0 20 40 60 80 100

He

at h

ydra

tio

n (

kal/

gr)

Time (day)


8 Coefficient of heat diffusivity

This coefficient is dependent on the type of concrete aggregates. The greater the coefficient of heat

diffusivity, the more the transmitted heat per unit time within concrete. Considering the Kahir

borrow area investigations, the aggregates are a combination of Sandstone,Limestone and Volcanic

aggregates. Hence, the coefficient of heat diffusivity of concrete is identified equal to 0.117

(m2/day).

9 Coefficient of convection

When concrete is in touch with a fluid like as air, concrete is cooled through convection.

Coefficient of heat transmission between concrete and air is equal to 11.6 kcal/m2.hr.˚C

10 Specific Heat

The concrete specific heat is an amount of heat needed to raise the temperature of one unit of mass

of concrete by one degree of centigrade. The concrete specific heat increases with temperature rise

-which is not counted for confidence- and determined equal to 950 J/kg.˚C according to ACI

207.2R.

11 Structural Properties

The tensile strain capacity is an amount of strain which concrete can suffer without cracking and

indeed, is the quotient of division of tensile strength by modulus of elasticity. In slow-rate loading,

the creep effect is also considered and so, the capacity is increased. Supposing such a condition, the

tensile strain capacity of concrete is equal to 60 µstrain, which is considered due to the low cement

content of RCC.

The modulus of elasticity of RCC is considered 12Gpa. Foundation Rock deformation modulus is

about 1Gpa,but it is assumed to be 1.5Gpa conservatively for calculation of restraints.

12 Construction time schedule

Based on the dam construction methodology, right and left parts has different construction

methodology. So two diffetent construction priority has been analyzed. Due to the flood seasons,

right part of dam construct up to level +33 first, then left part construct up to this level, and finally

the whole part of the dam construct to the final level.

Furthermore, the rate of concrete pouring has been calculated for two different conditions. First, the

roller compacted concrete of dam will be constructed during 17 months and second, the roller

compacted concrete of dam will be constructed during 13 months. According to two time schedules,

the concrete volume at the end of each month in different levels is mentioned in Table 1.


Table 1. Two different construction time schedules

Month

Concrete Level Concrete Level

MASL MASL

Right Left Right Left

1 18.0 14.5 18.0 14.5

2 21.5 14.5 23.0 14.5

3 25.5 14.5 30.5 14.5

4 31.5 14.5 33.0 18.8

5 33.0 18.0 33.0 24.0

6 33.0 22.3 33.0 29.0

7 33.0 26.2 34.0

8 33.0 30.0 37.5

9 33.6 41.2

10 36.0 45.2

11 39.0 50.0

12 42.0 57.0

13 45.0 60.0

14 48.3 ---

15 52.5 ---

16 56.0 ---

17 60.0 ---

13 Calculation of Placing Temps

Placing Temperature of Fresh RCC has been calculated based on Usarmy method. So the

Sinusuidal equation of placing Temperatures is as equation 5:

T(t)=28.8+7.3Sin(π(t-27)/365) (Eq. 5)

Which T is placing temperature and t is the day number from beginning of the year (persian year).

14 Finite Element Model

The finite element program Ansys 5.4 and the finite element PLANE 77 is used to build a finite

element model for thermal analysis of Kahir dam body (Figure 5). Overall algorithm of the cited

model is like below:

1- Perform the 1st lift with the specified initial temperature and the upper surface in touch

with the air.

2- Increase the degree of heat of hydration considering the cement content.

3- Read the temperatures calculated in clause 2 and calculate the new heat generation.

4- Calculate the air temperature in respect of the elapsed time from beginning of the previous

lift construction.

5- Re-compute the clauses 2 to 4, considering that the upper lift is in touch with the air

during the lift construction cycle.

6- Remove the surface transmission of concrete with the air and make the next lift elements

alive.

7- Re-compute the clauses 2 to 6 for the other lifts.


The heat generation rate of concrete is dependent on the time and temperature. So, these effects are

taken into account in thermal analysis and a nonlinear analysis is performed. It must be mentioned

that the analysis time step is based on the results' convergence and heat changes inside the structure

and in all cases, the ANSYS program automatically calculates and checks the convergence of

results. Based on construction methodology and time schedules, there are four models in Ansys

containing left and right part and two priority.

Figure 5. Ansys Finite Element Model

15 Results of thermal analysis

Temperature distribution have been presented in different times up to 6000 days after start of dam

body placement (Figure 6). Also For better understanding of details of graphs, temperature history

of the nodes has been presented up to 6000 days (Figure 7).

Figure 6. Sample of isothermal contours- 400 days after start- Right part- Second priority


Figure 7. Sample of temperature history- Node 530- Left part- First priority

In Table 2, risk of cracking in different Nodes is shown for first priority of construction

methodology. Results show that there will be no crack in dam body caused by thermal stresses.

Generally parts of the dam which has been poured in the warm months (summer) are more

vulnerable to crack. In this months, both of RCC placing temperature and ambient temperature are

higher than the other months of the year.

Table 2. Calculation of risk of cracking type 2 for Kahir dam- For First priority

No. Priority-

Part

Node

Number

Max.

Temperature

change (C)

Produced

Strain (µ) Status

1

First

Priority-

Right

part

155 5.09 12.4 No Crack

2 310 5 12.1 No Crack

3 530 5.3 12.9 No Crack

4 903 9.42 22.9 No Crack

5 1630 2 4.9 No Crack

6 2026 10.15 24.6 No Crack

7 2422 21.77 52.8 No Crack

8

First

Priority-

Left part

155 11.84 28.7 No Crack

9 310 10.83 26.3 No Crack

10 530 9.7 23.5 No Crack

11 903 4.97 12.1 No Crack

12 1630 2.01 4.9 No Crack

13 2026 9.81 23.8 No Crack

14 2422 21.76 52.8 No Crack

25

27

29

31

33

35

37

39

41

0 1000 2000 3000 4000 5000 6000 7000

Tem

p(C

)

Time (days)


16 Conclusion

In this paper, thermal analysis has been performed for two construction priority of KAHIR FSHD

dam. In each priority, due to the construction time, right and left part seperately analysed so there is

four analysis for dam body.

Based on performed calculations, thermal cracks will not occur in dam body in both first

and second priority. This is manly du to the very low restraint which is come from

foundation properties.

Calculatios show that there is no need to a contraction joint. But it was advised to place

some movement joints in 36 meters spacing to prevent cracking due to the probable

differential settlement.

Because the performed analyses are based on continous placement of RCC.In condition of

stop of placing, two meters of RCC should be placed in a low rate.

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] UsArmy-ETL 1110-2-542- Appendix A: Techniques For Performing Concrete Thermal

Studies

[6] ASTM C1074- Standard practice for Estimating concrete strength by the maturity method

[7] ANSYS 5.4 ADPL User's Manual

[8] F.R.Andriolo,”The Use of Roller Compacted Concrete”

[9] Bentz and de Larrard, " Prediction of Adiabatic temperature rise in Conventional and high

Performance Concretes using a 3-D Microstructural Model " , Cement and concrete Research

[10] F. Rueda, N. Camprubí and G. García,”Thermal Cracking Evaluation for La Breña II Dam

during the Construction Process”

kahir thermal analysis full paper

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