A COMBINED EXPERIMENT-SIMULATION STUDY ON TEMPERATURE REGIME OF ROLLER-COMPACTED CONCRETE APPLYING FOR DAM CONSTRUCTION

Similar to most of the other developing countries, Vietnam has national programs for industrial waste recycling including fuel, ash, and slag. In which, fly ash (FA) has been used commonly as a pozzolanic additive in the roller-compacted concrete (RCC) mixture for the dam construction of hydropower projects. This usage allows reducing the concrete cost, the hydration heat, and the thermal cracking during the construction process of the RCC. In this study, the optimal concrete mixture and the maximum temperature of the RCC dam were determined using the experiment planning method, Matlab, Maple 13, and Midas Civil. In addition, the mathematical model has been used to adequately describe the influence of the intensity concreting (IC) and the initial temperature of the concrete mixture (ITC) on the temperature regime of the RCC dam. The calculation of the temperature regime during the construction of the RCC dam of 45 m high and 1 m thick in Vietnamese climate conditions was performed with considering the IC and the ITC. As the results, the maximum temperature of the RCC dam was determined depending on the IC and the ITC. Calculation found that at IC = 0.6 m/day and ITC = 20°C, the maximum temperature in the central dam zone reached 36.38°C after 1800 hours from the beginning of construction. The results of the present study further support the safe and durable construction of the RCC dam in the future.


INTRODUCTION
Dam, which is one of the oldest artificial structures, has a responsibility for retaining water in small and large hydropower projects. So far, roller-compacted concrete (RCC) plays an important role in the modern construction of the hydraulic structure. China, a world leader in small and large hydropower projects, is home to the planet's largest hydropower potential at approximately 384 gigawatts, with roughly 20,000 large dams. In China, more than 10 million m3 of the RCC mixture are used annually in the construction of the hydro-technical structure and hydroelectric plants [1,2]. While the first RCC usage for the construction of the hydro-technical dam dates back to 1980 in Japan and about 40 RCC dams have been built [2] after that. Besides, the RCC has been used for building gravity dams in the United States since 1982. In that year, the first RCC dam was completed namely the Willow Creek Dam near Heppner in Oregon [3].
Like other developing countries and emerging economies, there has recently been a great demand for electricity in Vietnam, leading to an unprecedented boom in the construction of hydropower dam, which is also facilitated by the great natural hydropower potential in large power plants. Therefore, the issues of hydropower project construction are very relevant, great economic, and social importance in Vietnam [4][5][6]. The main characteristics of the five hydroelectric RCC dams in Vietnam recently are given in Table 1. At the same time, the application of RCC technology in the construction of the hydroelectric power project in Vietnam is accompanied by a number of serious problems. One of the major problems is an uncontrolled heat release during the hydration In this paper, the preliminary proportion of the RCC mixture was calculated in accordance with the ACI 211.3R-02 standard. The optimal concrete mixture and maximum temperature of the 45 m high RCC dam were determined by the experiment planning method, Matlab computer program, Maple 13 and Midas Civil. Additionally, the mathematical model has adequately described the influence of the intensity concreting (IC) and the initial temperature of the concrete mixture (ITC). Further, the temperature regime of the RCC dam with the optimal proportion was also discussed.

EXPERIMENTAL DETAILS Materials
Characteristics of the raw materials used in this study were determined in the laboratory according to the guidelines of the related standards with the results as follows: Type CEM I 42.5 N Portland cement (PC) from But Son factory (Vietnam) was used as a major binder material in RCC mixture. The maximum heat of the cement hydration is 309 J/g at 28 days. The physical and mechanical properties of the PC are presented in Table 2. The class-F fly ash (FA) sourced from locally TPP Vung Ang (Vietnam) was used as a cement substitution. The volume of the natural porous state is 572 kg/m 3 . The chemical compositions and the physical characteristics of the FA are given in Tables 3 and 4, respectively.  Crushed limestone (CL) sourced from Vietnam with the particle sizes of 5-20 mm and 20-40 mm in a volume ratio of 60:40 and the density of 2.62 g/cm 3 and 2.65 g/cm 3 , respectively, was used as coarse aggregate in RCC mixtures.
Local tap water (W) was used for both mixing concrete and curing of the test specimens.

Experimental program
The fully experimental program planning for this study is depicted in Figure 1, in which, the optimal RCC composition was determined by the mathematical experiment planning method with second-order. Mathematical methods are among the most widely used models in operational research and management science. The mathematical-experimental planning method was very useful in performing relationship tests between objects of study and building mathematical programming models or even deciding when such a model is applicable. Furthermore, the Midas Civil computer program based on the finite-element method was used to analyze the thermal behavior of the RCC dam. The desired outcome of this numerical analysis was to determine the spatial distribution of temperature at early-ages and maximum temperatures during the construction process.

Test methods
The preliminary proportion of the RCC mixture was firstly calculated following the procedures as described by ACI 211.3R-02 standard. The workability of the fresh concrete mixture was measured by means of Vebe time in accordance with BS EN 12350-3: 2009. The Ø150 × 300 mm cylindrical concrete specimens were prepared for compressive strength test in according to the guidelines of ASTM C39/C39M-14. The RCC samples were de-molded 24 hours after casting and cured at 20 ± 5 o C in a water curing tank until testing time. The compressive strength test was performed at 28 days. The RCC composition was optimized by means of the mathematical experiment planning method with second-order for two factors. The maximum temperature of the RCC dam was calculated using the Midas Civil computer program. Evaluation of crack formation possibility in concrete at an early age was conducted basing on the magnitude of the maximum permissible temperature and maximum temperature arising inside. Effect of the IC and the ITC on the maximum temperature in dam during the construction process was evaluated by the mathematical experiment planning method with first-order for two factors.

RESULTS AND DISCUSSION Calculation of the preliminary concrete composition
In the case of this study, the RCC properties were selected from the application of RCC technology in the construction of the hydroelectric power project in Vietnam and must possess the following requirements: (i) The workability of the RCC that measured by means of Vebe time device ranges from 15 to 20 seconds; (ii) The 28-day compressive strength of the RCC is greater than 21 MPa; (iii) The RCC has the water-resistant ability in the aquatic environment; (iv) FA is used to partially replace PC in the RCC mixture (FA/PC = 1.5 by volume); and (v) The relative volume of entrapped air is no more than 1%.  Table 5.

Optimization of the RCC composition by mathematical experiment planning method
In this study, the achieved second-order regression equation of the objective function was the compressive strength with the application of the central composite rotatable design (CCRD) method for two factors. The CCRD method allows obtaining more accurate mathematical description by increasing the number of experiments at the center and special value of "star shoulder" 2 1 414  = = . [19,20].

Definition of the objective function and input factors for the experimental model description
The compressive strength of the RCC specimen at 28 days, denoted as R28 RCC (MPa), is considered as an objective function of this experimental model.
The input variable factors are influenced by the RCC compressive strength, including the weight of each material (PC, QS, CL, W, and FA) required for 1 m 3 concrete. In order to reduce the number of experiments, the weights of QS and CL were fixed to 633 kg/m 3 and 1553 kg/m 3 , respectively. Therefore, the input factors and their limitation were chosen as follows: x1 = FA/PC (FA-to-PC ratio) ranged from 1.25 to 1.75 by volume. x2 = W/B (water-to-binder ratio) ranged from 0.95 to 1.35 by volume. The input variable factors and their variation intervals are shown in Table 6. Hence, the number of experiments of the CCRD for the two factors were indicated by formula (1): where k is the number of factors (k = 2) and m is the number of trials that repeating at the center (m = 5 as suggested by Terwilliger et al. [21] and Williams [22]). Therefore, N = 22 + 2×2 + 5 = 13. The RCC compositions that calculated following the ACI 211.3 R-02 standard and central composite rotatable design method of the second-order for two factors are shown in Table 7 and the RCC compressive strength  values at 28 days are presented in Table 8.
The second-order regression, equation (2), was obtained from the Matlab computer program. The coefficient values, the response surface image, and the corresponding contour plot are shown in Figure 2. The Fisher criterion was used as part of the objective function in the mathematical-experimental method and was used to test regression coefficients for every model. The linear and non-linear parameters of the experimental model were estimated by the root mean square method. According to Wang et al. [23] and Tang et al. [24], the regression equation adequacy was checked by means of the Fisher criterion. The calculated value was indicated by equation (3)  Based on the above calculation, the concrete composition corresponded to the highest RCC compressive strength and optimal dosages of variable components was selected. The material consumption for one cubic meter, which was based on the regression equation (2), is presented in Table 9.

Mathematical modeling for the effect of the IC and the ITC on the maximum temperature in the RCC dam during the construction process
The Midas Civil computer program was used for determining the maximum temperature of the RCC dam with the optimal concrete mixture as given in Table 9. Simultaneously, using the first-order planning method for two factors, the effect of both the IC and ITC on the maximum temperature in the RCC dam was assessed. According to Aniskin and Chyk [26], the IC values of the RCC dams were selected from 0.2 to 0.6 m/day.
In the RCC dams, the concrete temperature rises from the heat of cement hydration. The quick construction process and low concrete conductivity may be caused by the high thermal gradient in the interior mass and exterior dam surface, as well as thermal-stress. These thermally induced stresses may be significant enough to induce cracks in the RCC dam. The temperature distribution evolution in the dam by time and resulting thermal stress depends on the temperature analysis parameters, such as the environmental conditions, concrete properties, cement amount, lift thickness, temperature, and concrete pouring process [27][28][29][30].

RCC dam description
The case study object was an RCC dam in Vietnam with approximately 45 m high and 1 m laid concrete layer thickness. A typical cross-section of the dam is displayed in Figure 3. So far, this hydraulic project, built in the summer, has been playing important roles in flood prevention, sedimentation reduction, irrigation, water supply, and power generation in Northern Vietnam.
According to Aniskin and Chuc [31], the summer temperature in Northern Vietnam change according to equation (6): where tair is daily average air temperature ( o C) and  is time (hours). .
Journal of Thermal Engineering, Research Article, Vol. 6, No. 5, pp. 772-785, October, 2020 780 where q is total heat (W/m 3 ), c is specific heat coefficient (J/kg. o C), PC is cement amount (kg),  is concrete density (kg/m 3 ), t() is concrete temperature in adiabatic conditions at the age of  days ( o C), tmax is temperature rise in the exothermic process under adiabatic conditions ( o C), and α is a constant, depending on the concrete mixture casting temperature (α = 0.833, [27]). The thickness of the laid concrete layer (m) 1 -

Material properties and environmental conditions
The model attributes were primarily evaluated using the existing data and typical RCC attributes. Table 10 shows the RCC properties and the dam foundation, which were used as inputs to determine the temperature regime in the construction of the concrete dam, including Poisson's ratio, modulus, specific heat, density, and thermal conductivity. The air convection coefficient, which is consistent without the moderate wind speed, was analyzed. Heat generation rate was adopted for the 95.15 kg PC + 119 kg FA in 1 m 3 concrete (as shown in Table 9).

Description of mathematical experiment planning method with first-order
The objective function of this experimental model was the maximum temperature in the RCC dam The input variables were chosen from the experimental design input parameters as follows: z1 -the IC, ranged from 0.2 to 0.6 (m/days). z2 -the ITC, ranged from 10 to 20 ( o C). The input variable factors and their variation intervals are shown in Table 11.  The number of necessary experiments (denoted as N) in first-order planning was determined by equation (8): where k is the number of factors (k = 2) and 1 is the number of this experiment at the center. Hence, 2 2 1 5 N = + = Figure 4 shows the three-dimensional (3D) finite-element mesh model of the RCC dam. By using the Midas Civil computer program, the maximum temperature that related to the IC and ITC occurring in the RCC dam body during construction time was determined with the values as shown in Table 12 The first-order regression equation (9) of experimental design showed that the maximum temperature arising in the RCC dam body during construction time increased with the IC and ITC. However, the effect of the ITC was much significant than that of the IC. According to equation (9), the maximum temperature in the RCC dam body obtained during the construction process was 35.85 o C at input factors z1 = 1 and z2 = 1.

CONCLUSION
Based on the experiment-simulation results, the following conclusions can be drawn: The preliminary composition of the concrete mixture with a Vebe time of 18.5 seconds was calculated following the ACI 211.3R-02 standard and the concrete obtained compressive strength value of 21.45 MPa at 28-day.
The composition of the RCC mixture was optimized, based on the second-order regression equation (2) of the RCC, using mathematical planning method for two factors of the Matlab computer program and Maple 13.
The finite-element coding based on the Midas Civil computer program as performed in this study is capable to simulate the thermal response of the RCC dam of 45 m height and 1 m laid concrete layer thickness during well reasonable construction. The maximum temperature of 36.38°C was recorded at the central dam zone after 1800 hours from the beginning of the construction process. In addition, the effects of the IC (z1) and the ITC (z2) on the temperature regime of the RCC dam with the optimal concrete composition were simulated by the planning method for two factors and presented by the first-order regression equation (9). According to equation (9), the Tmax RCC of 35.85°C was found at input factors z1 = 1 and z2 = 1.
By using Midas Civil computer program in combination with the available and full laboratory data, it is possible to simulate the temperature distribution in the RCC dam body during construction time, which can be used in practical applications to avoid the crack appeared in the construction process and ensure the required safety and durability of the constructed hydraulic structures when they get into exploitation and use in the future.