## 1 Introduction  
Solar energy is a promising sustainable energy source due to its abundant supply and the relatively low environmental impact of its harvesting. However, solar powers are unavailable at night or in poor weather conditions. A thermal energy storage (TES) system is usually integrated with solar power plants to continue production of electricity. TES systems using thermal oil or molten salt as energy storage media are widely implemented, but this type of TES has a disadvantage of high investment cost. Solid sensible heat storage material is an alternative. According to [PERSON] et al. [1], concrete and castable ceramics are the most promising candidates of solid sensible heat storage materials due to their good thermal and mechanical properties and low cost. [PERSON] and [PERSON] [2] also carried out a cost comparative analysis between concrete and other heat storage materials and came to the same conclusion. [PERSON] et al. [3] identified some common materials for solid state sensible heat storage in the range 500-750 \({}^{\circ}\)C and found high alumina concrete had the lowest cost.  
The vast majority of work incorporating concrete as a TES material has been reported by the group led by [PERSON] in German Aerospace Center (DLR). Her group has carried out testing of a concrete energy storage system incorporating an array of embedded heat exchangers and thermal oil as heat transfer fluid, and the energy storage temperature has achieved up to 400 \({}^{\circ}\)C [4; 5; 6; 7]. [PERSON] et al. [8] presented guidelines for designing a concrete storage module and for its integration into a solar plant, respecting constraints linked both to an adequate solar field operation and to the production system. [PERSON] et al. [9] developed a high-performance concrete as a heat TES material and conducted high temperature tests in the range of 400-500 \({}^{\circ}\)C. In the tests, molten nitrate salt was used as the heat transfer fluid, which was circulated though stainless steel tubes embedded in concrete prisms, to charge the TES system.  
In addition to concrete, geopolymer-based materials have been assessed to be potentially employed for storage applications due to their reliable mechanical and thermal properties [10; 11; 12]. [PERSON] et al. [13] applied the finite element method to simulate transient behavior of sensible heat thermal energy storage systems using geopolymers. They have taken various aspects into consideration: thermal properties at high temperatures, the actual geometry of the repeated storage element, and the actual storage cycle adopted.  
The above mentioned studies are mainly focused on thermal performance of the heat energy storage system during the energy charge and discharge processes. In addition to the thermal characteristics, mechanical characteristics are another key issue for TES material. Structural stability is requested for the TES system at high cycling temperatures. There have been many studies aimed at understanding thermal behavior of concrete at high temperatures, and it was found that there is a reduction for mechanical properties in terms of strength and elastic modulus of concrete when it is exposed to high temperatures. The works are reviewed in the reference [14]. However, effect of thermal cycling on mechanical change of concrete characteristics, which is a major concern for TES system, is not involved in these studies. [PERSON] et al. [15] carried out thermal cycle tests of concrete and examined their residual compressive strengths which represent the structural stability of concrete.  
The purpose of this paper is to investigate the thermal cycling stability of low strength concrete (C20) which is of possible use as a TES material. A series of thermal cycling tests are conducted. Concrete specimens are first subjected to different thermal cycles between 100 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, and then their residual compressive strengths are measured.