Thermal curing of precast concrete is inalienable and crucial stage of production, especially in time limit conditions. There are a number of methods of its implementation using different energy carriers and heat transmission ways currently known. Steam curing is the most widespread type of it. The reason for this is that due to high heat transfer ratio from saturated steam to concrete its heating is accelerated and more intensive hardening is provided. When heated in an environment with humidity close to 100%, moisture evaporation from concrete and its drying are excluded. It should be noted that this technology is simple and versatile. However, steam curing is imperfect in terms of energy efficiency, which motivates manufacturers to search more cost-effective decisions. One of them is curing in gas combustion products environment. This method makes procedure much cheaper due to direct heat carrier generation in close to curing chamber, but lacks the mentioned features of steam using, which has a less favorable effect on the quality of the final product.
Another alternative way to improve energy efficiency and provide more favorable conditions for hardening is using of direct electric preheating of concrete mix (DEPC) applied generally during cold-weather concreting. The essence of the method lies in intensive introduction of heat into the mixture before it is compacted. The mixture is heated to a temperature of 70–90 ° C in 10–15 minutes, poured and compacted. DEPC application makes the pouring on frozen surface feasible, allows to transport the mixture for a long distance. The heat introduced in the mixture leads to intensification of cement exotherm and other physicochemical processes. Due to this phenomena the temperature of concrete preserves for a long time. This particular effect is the reason of DEPC feasibility to factory conditions. This is due to the strategy of precast manufacturing generally provides for positive ambient temperature and possibility to provide more qualitative moisture- and heat insulation of product severally, as well as using especial covers or chambers. The use of listed features makes possible to increase energy efficiency significantly, which is confirmed by the authors when approbating DEPC in similar (factory) conditions. After pouring and compacting the mixture and insulating the product there is little colling of concrete (as a result of heat transfer between liquid phase and less heated aggregates), then temperature rise again to maximal (sometimes even more), then a very slow natural cooling of the structure occurs, which is comparable to isothermal phase with conventional methods. For example, concrete grade B27,5 (according to Russian standard) heated to a temperature of 70°C, after a slight decrease in temperature and re-reaching the peak, cooled down at a rate of no more than 1,5 ⁰C per hour. This is more than enough to simulate a typical accelerated curing process. In this case, the energy consumption for heating the mixture was 155 MJ/m3, which is significantly below than conventional indicators (450 MJ/m3 and more).
DEPC applying provides for the use of thermal equipment just at the preheating phase, virtually excluded the use of ancillary equipment (fans, pumps, instrumentation and controls) at subsequent stages, which also leads to additional costs reduction.
|Position of speaker||graduate student|
|Affiliation of speaker||National Research Tomsk Polytechnic University|
|Publication||Impact Factor journals|