EFFECT OF HEAT CURING METHODS ON THE TEMPERATURE HISTORY AND STRENGTH DEVELOPMENT OF SLAB CONCRETE FOR NUCLEAR POWER PLANT STRUCTURES IN COLD CLIMATES

Abstract

The objective of this study was to experimentally investigate the effect of heat curing methods on the temperature history and strength development of slab concrete exposed to -10℃. The goal was to determine proper heat curing methods for the protection of nuclear power plant structures against early-age frost damage under adverse (cold) conditions. Two types of methods were studied: heat insulation alone and in combination with a heating cable. For heat curing with heat insulation alone, either sawdust or a double layer bubble sheet (2-BS) was applied. For curing with a combination of heat insulation and a heating cable, an embedded heating cable was used with either a sawdust cover, a 2-BS cover, or a quadruple layer bubble sheet (4-BS) cover. Seven different slab specimens with dimensions of 1200 600 200 mm and a design strength of 27 MPa were fabricated and cured at -10℃ for 7 d. The application of sawdust and 2-BS allowed the concrete temperature to fall below 0℃ within 40 h after exposure to -10℃, and then, the temperature dropped to -10℃ and remained there for 7 d owing to insufficient thermal resistance. However, the combination of a heating cable plus sawdust or 2-BS maintained the concrete temperature around 5°C for 7 d. Moreover, the combination of the heating cable and 4-BS maintained the concrete temperature around 10℃ for 7 d. This was due to the continuous heat supply from the heating cable and the prevention of heat loss by the 4-BS. For maturity development, which is an index of early-age frost damage, the application of heat insulation materials alone did not allow the concrete to meet the minimum maturity required to protect against early-age frost damage after 7 d, owing to poor thermal resistance. However, the combination of the heating cable and the heat insulating materials allowed the concrete to attain the minimum maturity level after just 3 d. In the case of strength development, the heat insulation materials alone were insufficient to achieve the minimum 7-d strength required to prevent early-age frost damage. However, the combination of a heating cable and heat insulating materials met both the minimum 7-d strength and the 28-d design strength owing to the heat supply and thermal resistance. Therefore, it is believed that by combining a heating cable and 4-BS, concrete exposed to -10°C can be effectively protected from early-age frost damage and can attain the required 28-d compressive strength.