*3.3. Mock-Up Test for Semi-Adiabatic Temperature Monitoring*

#### 3.3.1. Preparation of Mock-Up Specimen

A mock-up test was performed to verify the heat of hydration of mass concrete. The two numbers of mock-up specimens were casted with the best optimum mixes for cryogenic concrete such as C40-2 and C40-4. Detailed sizes of Specimens were shown in Figure 13. The size of mock-up specimen was 2.0 m by 2.0 m by 1.5 m and the side surface of it was surrounded with insulation board of 200 mm thick. Concrete placing work and casting specimen had been taken for 30 min after produced. The vibrating works were carefully applied for good consolidation of poured concrete during concrete placing as shown in Figure 14. Temperature of fresh concrete was measured in accordance with AASHTO T 309 [56]. The initial and 30 min temperatures of concrete were controlled less than 32 ◦C because it was produced in summer season. Table 15 indicated that the slump (flow) and air content of both C40-2 and C40-4 mixtures were satisfied on the target requirement. A wooden form and polystyrene insulation were removed in 21 days after concrete placement. The top surface of concrete had cured with the moisture curing method such as wet blankets and plastic films.

#### 3.3.2. Semi-Adiabatic Temperature Monitoring

All temperature sensors were installed and mock-up test was pretested before concrete pouring. A total of five temperature sensors (if required, two more spare sensors at the surface and center of specimens) were installed and positioned in concrete specimen at various depths and locations as shown in Figure 15. Temperatures of concrete specimen were measured every 30 min for the first 48 h and then every 1–2 h for 21 days. Ambient temperature was also recorded.

**Figure 13.** Dimension of Mock-up specimen.

(**a**) Placing concrete (**b**) Vibrating concrete

#### **Figure 14.** Placing and vibrating of concrete.



**Figure 15.** Locations of temperature sensors.

The maximum temperature was captured in the center of the mock-up specimen and the temperature difference was measured in the center and surface of mock-up specimen. The maximum temperature of the hardened mass concrete usually occurred between 1 to 3 days after placement and then gradually decreased. According to ACI 308-16, ACI 207.1R and CS 163, in the case of blended cement, the maximum temperature should be controlled in the range of 70 ◦C and 85 ◦C, and the temperature difference should not exceed 19 ◦C [57–59]. Table 16 and Figures 16 and 17 show the measured temperature data. For C40-2 with Daracem 208 (naphthalene), the maximum temperature of the center location was 70.85 ◦C and the temperature difference between center and side surface was 21.85 ◦C. For C40-4 with Baxel PC 650, the maximum temperature of the center location was 70.8 ◦C and the temperature difference between center and side surface was 16.95 ◦C. The maximum temperatures of C40-2 and C40-4 were controlled by less than 75 ◦C, but the temperature difference of C40-2 did not satisfy the requirement with the exceed of 19 ◦C. Moreover, the binder amount of the C40-2 and C40-4 mixtures was applied with 495 kg/m<sup>3</sup> and 475 kg/m3, respectively. That meant that the amount of binder for the C40-2 mixture was 20 kg/m3 more than that of C40-4. With regard to the heat of hydration, C40-4 mixture was better to control the thermal cracks. Thus, the mix design of C40-4 (GGBS 65% with Baxel PC650) was more suitable for cryogenic concrete, in terms of workability, mechanical and thermal properties under cryogenic conditions and heat of hydration.

**Table 16.** Results of measured temperature data (replacement of GGBS 65%).


**Figure 16.** Cryogenic concrete with 65% GGBS and naphthalene type admixture.

**Figure 17.** Cryogenic concrete with 65% GGBS and Polycarboxylate type admixture.

#### **4. Conclusions**

The purpose of this study is to suggest the optimum mix design with a high volume of GGBS replacement and the procedure of the cryogenic test to consider mechanical and thermal properties, and durability performance.

Above all, many research efforts including ACI 376 were reviewed to define the investigation items about mechanical and durability properties under cryogenic environment. Following this, all raw materials were tested to compare the test results with requirements. Particularly in this study, to the control of heat of hydration, the high-volume of GGBS replacement was adopted. For the improvement of freeze-thaw resistance, air entrainer admixture was used. With respect to the emergency condition such as LNG leakage, two types of cryogenic test methods were employed under one-cycle cryogenic condition (Test A) and 50-cycles cryogenic condition (Test B). Next, a mock-up test was conducted to find out the productivity and semi-adiabatic properties. The test results were summarized as below:


binder for the C40-2 mixture was 20 kg/m<sup>3</sup> more than that of C40-4. With regard to the heat of hydration, the C40-4 mixture was better to control the thermal cracks.

Thus, the mix design of C40-4 (GGBS 65% with Baxel PC650) was more suitable for cryogenic concrete, in terms of workability, mechanical and thermal properties under cryogenic conditions and heat of hydration. This test procedure would be helpful to select the better cryogenic mix design and to define the trend of mechanical, thermal and durability properties and test methods. In the future, the long-term performance of cryogenic concrete needs to be investigated.

**Author Contributions:** Conceptualization, G.L. and O.N.; Data curation, O.N.; Formal analysis, O.N.; Funding acquisition, G.L.; Investigation, G.L. and O.N.; Methodology, G.L. and O.N.; Supervision, G.L. and O.N.; Validation, G.L. and O.N.; Visualization, O.N.; Writing—original draft, G.L.; Writing review & editing, O.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
