Seite - 1292 - in Book of Full Papers - Symposium Hydro Engineering

2.1.1. control the highest temperature Temperature of concrete is related to several factors including the concrete grade, position and duration of the concrete. Controlling the highest temperature is to prevent overlarge difference in temperature of foundation, temperature of adjacent layers or internal and external temperature of concrete, and then to prevent cracking in concrete. The targeted value of highest temperature is determined as following: (1) it should be no higher than the sum of grouting temperature and the allowed difference, for controlling stress caused by temperature difference of the foundation. (2) it should be no higher than the maximum temperature difference between internal and external values. (3) it should be determined differently in confined and unconfined sections. In practice, these different sections include bank sections and riverbed sections. (4) it should vary in different seasons. The previous practice in China generally shows that it is 27℃ for confined section and 31 to 33℃ for the unconfined. As to large engineering like Xiluodu, the standard is more restricted. The controlled temperature can be determined by local construction practice and the design standard. If the concrete is weak in crack resistance, it should be under 27℃. 2.1.2. coordinate the temperature change Coordinating the temperature change contains three aspects: (1) keep the temperature curve smooth all along the period of cooling down, as in figure 1(a). In former practice, the temperature curve was manually designed in steps, which added difficulty and labor cost for actual procedure. Assuming that we can automatically adjust the limit of temperature change ratio according to the local highest temperature, then we can keep the temperature curve smooth. Practice in Xiluodu has showed that it is realizable. (2) make different designed curve in different concrete sections. The stepped time-varying temperature in one concrete section (figure 1(a)) is called time variable curve. It’s aimed at limiting the temperature difference between the upper and lower layers from the grouting section to the weighting section of one dam section (figure 1(b)), which is called space variable curve. In a dam, concrete at the bottom is strongly confined while the upper concrete is unconfined. To avoid cracking, it’s important to reduce the difference between upper and lower layers. In Xiluodu project, the difference in the range of 1/4 length of old aged concrete (over 28 days) section is limited in 15 to 18℃. In a word, only both time varying and space varying curve controlled can achieve intelligent control. By measures like stage cooling and coordinating temperature change, we can make the temperature gradient in different concrete sections suitable and reduce the concrete stress. (3) limit the difference in foundation and the difference between interior and exterior. In Xiluodu project, according to the concrete property, using three- dimensional FEM and the design standard method, the difference in confined concrete is 14℃, in base that the cracking-resistance safety factor is no less than 1.8. If the thermal insulation is well conducted, the superficial temperature is 2 ℃ 1292