1 Introduction
As urbanization, economic development, and high-rise buildings have increased due to insufficient land areas or various global demands, large-scale mass concrete construction has also increased, such as mat foundations of skyscrapers, bridge piers, and power plant structures. A critical issue related to mass concrete is that it is difficult to ensure its quality due to cracks caused by the thermal gradient between the center and the surface because concrete is exposed to various environmental effects. Due to the hydration of cement, concrete generates heat and the heat from members with small sections could be dissipated relatively easily, whereas the heat from members with larger sections accumulates inside, which is likely to increase the temperature significantly. This can lead to thermal stress and cracks resulting in a harmful effect on durability.
The reduction methods for the heat of hydration of mass concrete currently used in the field include the material-based method of using a binder that generates less hydration heat, construction-based methods of pre-cooling, in which materials are cooled to reduce placing temperature, and pipe cooling, in which pipes are installed inside mass concrete for cooling. However, construction-based methods are not widely used due to higher costs and limits in the dimensions of members; therefore, the material-based method of reducing the heat of concrete through mix design is primarily used (Choi et al.
2006). However, as material-based technology for reduction of hydration heat is not noticeably effective to control the thermal stress and cracks of massive structures, concrete is commonly cast over two or three layers. These layered casting method would cause problems such as requiring additional measures for bonding strength in the cold joints and delays in the construction period (Kim et al.
2008).
Phase change material (PCM) is a material whose phase changes depending on temperature change, keeping latent heat when the phase changes, and can absorb or release more energy than sensible heat (David et al.
2016). In the field of construction, research on PCM was primarily conducted for applying it to slab or the wall to improve energy saving. In the paper, it was reported that the energy required to control the internal temperature of the building could be saved by using PCM for the material of the slab or wall. However, in addition to the application of temperature and energy keeping, PCM can be used for suppressing temperature rise caused by materials (Park
2010). According to the paper, it was reported that the increase of temperature by heat conducted through slab or wall can be suppressed due to the energy storage of the phase change of PCM.
Mass concrete has high cracking probability due to the temperature difference between the inside and the outside of concrete structure by heat of hydration after a large amount of concrete is casted. These thermal cracks can be the main cause of the deterioration of structural performance and mitigation of durability. Therefore, reducing and controlling the heat of hydration are very important technique in mass concrete construction.
The use of thermal energy storage may serve to mitigate the temperature change. Latent heat storage is one of the most efficient methods of storing thermal energy, and offers higher storage density owing to the small temperature difference between storing and releasing heat. PCM is a representative example of a thermal energy storage using a latent heat, and can increase the thermal inertia. A paper published by Dinçer (
2002) dealt with thermal energy storage, being PCMs just a part of it, and not focused on the application in buildings. Since then, many researches have been conducted about PCM as thermal energy storage material. In 2004, the research group led by Farid et al. (
2004) published two papers on the PCM, and one of them covered the building application of PCM. Lee et al. (
2007) evaluated the thermal storage capacity of gypsum wall applied a micro-encapsulating PCM, and reported that thermal storage capacity is increased as the thickness of the PCM film is increased. In addition, many researchers have attempted to apply the PCM in the slab and wall as an energy saving way.
In some countries, researches using an encapsulating PCM has been carried out to reduce the heat of hydration (Lee et al.
2007; Fang et al.
2008; Maruoka and Akiyama
2003). These methods not only require advanced technology to manufacture the microcapsule, but is uncertain about its practical application in mass concrete despite an excellent performance in reducing thermal crack. Likewise, the larger the size of the capsule, dispersibility problem in concrete is raised.
Mihashi et al. (
2002) conducted test to apply a retarder containing PCM in a paraffin microcapsule in order to control the heat of hydration. In the study, some hypotheses were formulated; as the paraffin was melted, it derived the hydration heat of mass concrete, and retarder reduces the hydration rate of mixture and releases the heat. This paper reported that the maximum temperatures under the semi-adiabatic curing can be reduced in both small cement paste specimens and large concrete specimens.
Hunger et al. (
2009) investigated the fresh and the hardened properties of self-compacting concrete mixes using different amounts of PCM. Microencapsulation of PCM is effectively functional in fresh state concrete, but it detrimentally influences the mechanical strength of the concrete.
Choi et al. (
2014) evaluated the applicability of seven types of inorganic PCM under conditions that were similar to those used for concrete materials. In the study, a strontium-based PCM was selected as the most effective PCM for reduction of hydration heat in mass concrete. Based on the previous study test results, it was found to control thermal stress by reducing heat of hydration of mass concrete. To assessment the applicability of the strontium-based PCM for mixture of concrete, the mechanical properties of concrete were investigated.
Eddhahak-Ouni et al. (
2014) investigated a Portland cement concrete modified with organic microencapsulated Phase Change Materials (PCMs) by experimental and analytical methods. In the study, a loss of the compressive strength was noticed with the addition of PCMs. And they reported that the thermal conductivity proposed using a multi-scale approach well-predicted the approximation of the equivalent thermal conductivity of the PCM-concrete and the gain in the considerable experimental time.
This study aims to evaluate the method for reducing the hydration heat using PCM, which can absorb or release a large amount of heat by improving the conventional material-based method for reduction of hydration heat. In this study, PCM was used to control the hydration heat of mass concrete. To evaluate the effect of PCM on the heat of hydration of concrete, adiabatic temperature rise tests and thermal analysis were performed for various types of concrete mix. And mechanical properties of concrete in wet and dry condition were evaluated to find relationship between strength development and hydration heat of concrete.
Authors’ contributions
KLA, SJJ, BSK, and WSP made substantial contributions to conception and design, or acquisition of data, or analysis and interpretation of data. SWK and HDY analyzed the data and were involved in drafting the manuscript or revising it critically for important intellectual content. All authors read and approved the final manuscript.