# Experimental Evaluation of Thermal Performance and Durability of Thermally-Enhanced Concretes
## 2 Materials and Methods
### Test Methods
#### 2.2.2 Durability Performance  
Accelerated carbonation and cyclic freeze-thaw tests were performed to investigate the durability of thermally-enhanced concrete. Carbonation is defined as the chemical reaction where carbon dioxide gas diffuses into the concrete and reacts with calcium hydroxide in the hydrated cement paste to form calcium carbonate (Equation (1)) [29; 30]:  
\[\text{Ca(OH)}_{2}+\text{CO}_{2}\rightarrow\text{CaCO}_{3}+\text{H}_{2}\text{O} \tag{1}\]  
Carbonation reduces the pH of the pore solution in hardened cement paste from an average 13.5 to below 9.0, which causes the protective oxide film of reinforcement steel to degrade and corrosion may take place. It is known that the carbonation rate is a function of concrete strength, water-to-cement ratio, and curing conditions. For instance, low-strength concrete with a compressive strength lower than 30 MPa may experience relatively rapid carbonation within several years [31; 32]. The well-known accelerated carbonation testing method using a carbonation chamber is recommended by Reunion Internationale Laboratoires Experts Materiaux (RILEM) to determine the depth of carbonation by means of the phenolphthalein solution as an indicator [33]. However, careful consideration should be given to interpret the accelerated carbonation testing results, since carbonation in real concrete structures is greatly influenced by the exposure conditions, such as atmospheric relative humidity, periodic wetting and drying, and temperature. The control conditions of the accelerated carbonation chamber in accordance with BS EN 13295 [32] are listed as follows: temperature 20 \(\pm\) 2 \({}^{\circ}\)C, relative  
Figure 2: (a) General view of the calibrated hot box apparatus; (b) and installed large-scale specimens mounting panel (photograph courtesy of Fire Insurers Laboratories of Korea). humidity 60 \(\pm\) 5%, CO\({}_{2}\) level 5 \(\pm\) 0.2%. Specimens were installed for eight weeks after curing in water for 27 days. The average carbonation depth was recorded with a spaying of 1% solution of phenolphthalein at one, four, and eight weeks. Cyclic freezing and thawing attack, one of the physical attacks on concrete, causes progressive deterioration and leads to inadequate durability of concrete since freezing water in the fine cracks results in a 9% volume expansion. The main factors influencing the resistance of concrete to freeze-thaw attack are known as the degree of saturation, strength, and the pore system of the hardened cement paste. Since thermally-enhanced concretes have various insulation materials, they may increase the porosity and adversely affect the resistance to freezing and thawing. There are no standard methods for measuring the resistance of concrete to cyclic freezing and thawing, which have been developed on a practical basis [34]. The most widely used test method is ASTM C666 [35]. This method alternatively raises and lowers the temperature of a specimen from 4 to \(-\)18 \({}^{\circ}\)C within 2-5 h at a freezing rate of 4 \({}^{\circ}\)C/h-11 \({}^{\circ}\)C/h. Generally, the criteria of testing is 300 cycles with 4-10 cycles per day. Among many factors, including length and mass reductions or strength loss, the relative dynamic modulus of elasticity calculated from fundamental transverse frequencies is used to assess the resistance on freezing and thawing when it drops to 60% of its original values, as defined in Equation (2):  
\[P_{c}=(\frac{n_{1}^{2}}{n^{2}})\times 100 \tag{2}\]  
where \(P_{c}\) is the relative dynamic modulus of elasticity after \(c\) cycles of freezing and thawing in %; \(n\) is the fundamental transverse frequency at 0 cycles of testing; and \(n_{1}\) is the fundamental transverse frequency after \(c\) cycles of testing. However, this method is recognized as a relatively severe test since the degree of saturation of the concrete and aggregates at the early age of 14 days has a great effect on the deterioration of the concrete to freezing and thawing.