THERMAL ENGINEERING FOR THE CONSTRUCTION OF LARGE CONCRETE ARCH DAMS

Concrete arch dams are typically constructed from individual cantilever blocks, as illustrated in Fig. 1, using approximately 10’ thick mass concrete placements or lifts to build up each block. During placement and the initial curing of a lift, water is circulated through embedded pipes, as illustrated in Fig. 2, to control the peak temperatures and thermal gradients. Once the individual blocks are completed to the crest height, water is again circulated to contract the blocks and open the contraction joints between the blocks. The open joints are then pressure grouted to form a monolithic arch dam. This type of mass concrete construction has two opposing design considerations for the thermal behavior due to the heat of hydration. During the placement of the lifts, excessive temperatures and thermal gradients must be controlled to prevent thermal induced cracking. However, during the final cooling period, an adequate drop in the mean concrete temperature must be available to allow the contraction joints to open sufficiently for grouting. This paper discusses the methodology and computer simulations currently used to evaluate the potential for cracking and the performance of the joints during the final cooling period for grouting. In particular, several important modeling features needed to simulate this type of construction are described and highlighted. A coupled thermal stress analysis is conducted to simulate the incremental construction process using nonlinear concrete material behavior for creep and aging. A cooling coil model is used to simulate the active removal of heat through the embedded cooling coils. A method for addressing the construction process of placing lifts in the low blocks against the previously placed high blocks and the effect of the associated compressive load across the contraction joints for very young concrete is discussed. To illustrate this methodology for the thermal engineering of large concrete structures, the results of a study for the Portugues Arch Dam, which is to be constructed near Ponce, Puerto Rico, is presented.