Keywords: RCC dams, birth and death technique, thermal analysis, field problems, schedule of construction, finite element.

Thermal variations in RCC dams have an important role in the structural behaviour of dams. Concrete temperature rising from the hydration of cement in RCC dams, coupled with the low conductivity of concrete can induce a high thermal gradient along the interior mass and the exterior surface of the dam. The interior restraints, due to this thermal gradient and the external restraints, such as foundation restraint, during the cooling of the dam can cause significant thermal stresses, which are sufficient to cause cracking. Hence the analysis and design of RCC dams necessitate some forms of thermal analysis to be performed [1]. Chemical reaction which occurs during the hardening of the fresh concrete i.e. hydration of cement, is a calorification reaction. This reaction causes an increase in the concrete volume. The hydration heat equation given by Ishikawa [2] for RCC has been used in this study.

This paper deals with the development of finite-element based computer code, called STARD, along with other necessary relationships for determination of temperature within the dam structures. The finite element code is then applied to a real full-scale problem of RCC dams, to determine the impact of placement schedule on the thermal changes in the dam structures. The finite element model of the dam is generated for every construction phase, by applying birth and death of elements technique. The entire procedure is repeated until the whole dam is constructed. Throughout the analysis, in addition to placement schedule, the impact of some other influencing factors, such as daily variations of temperature, hydration heat generation, initial temperature of rock foundation and placement temperature of each layer are incorporated.

The developed finite element code is applied to a real full-scale problem of roller compacted concrete dams obtained from the available literature [3]. The maximum dam height is 169m. The upstream and downstream facing are made of conventional concrete. The cushion layer on the bottom is also made of conventional concrete (3m thick).

The temperature distributions in the rock bedding just before the start of casting of concrete are evaluated. First it is assumed that the initial temperatures of all nodes corresponding to the rock bedding are the same. Second, changing the atmospheric temperature for two or three years using to observed data, then the heat transfer is analyzed between the atmospheric temperature and the rock ground. In this way, the temperature distribution of the rock bedding can be obtained [4]. Based on the observed data at the project site, the daily changes of temperatures are taken into account in the thermal analyses. The effects of solar radiation during construction were incorporated by allowing an increase in ambient temperature of 1.0C to account for solar radiation heating of the concrete surface [5].

Although the real RCC is placed in 25-30 cm thick horizontal layers, 5m thick layers with the placement time of 20 days are used in order to decrease the computer works [4]. Hence the section of the RCC dam is divided into 34 layers given a total number of 329 elements and 1029 nodes with one degree of freedom per node. In simulation of the sequence of construction, the birth and death of elements techniques have been used in the present study.

Based on the results obtained, it is concluded that for a given RCC dam, the starting date of RCC placement schedule has significant effect on the distribution of temperatures in RCC dam body. Starting the RCC placement in hot season leads to the development of higher temperature zones in the lower portion of the dam structures. Therefore, changing the placing schedule can optimize the location of maximum temperature zones which might lead to decreasing the tensile stresses at the critical zone.

1

S. Tatro, E. Schrader, M. Asce, "Thermal Analysis for RCC- A Practical Approach." Roller Compacted Concrete III, ASCE, New York, USA, 1992.

2

M. Ishikawa, "Thermal Stress Analysis of a Concrete Dam". J. Computers & Structures, 40(2), 347-352, 1991. doi:10.1016/0045-7949(91)90360-X

3

J. Noorzaei, M. Meherdadi "Thermal Analysis in Roller Compacted Concrete Dams" World Conference on Concrete Materials and Structures, Malaysia 2002.

4

A Saetta, R. Scotta, R. Vitaliani, "Stress Analysis of Concrete Structures Subjected to Variable Thermal Loads." J. of Structural Engineering, 121,(9), 446-457, 1995. doi:10.1061/(ASCE)0733-9445(1995)121:3(446)

5

ACI 207. 1R, "Mass Concrete for Dams and Other Massive Structures" ACI Committee, USA, 1987.

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