Study on the Heat of Hydration and Strength Development of Cast-In-Situ Foamed Concrete

-is study aims to investigate the relationship between the heat of hydration and the strength development of cast-in-situ foamed concrete. First, indoor model tests are conducted to determine the effects of the casting density and the fly ash content on the hydration heat of foamed concrete in semiadiabatic conditions. Second, compression tests are carried out to evaluate the development of the compressive strength with the curing time under standard curing conditions and temperature matched curing conditions. -ird, the hydration heat development of the foamed concrete is tested in four projects. -e results showed that the peak temperature, the maximum temperature change rate, and the maximum temperature difference increased with the increase in the casting density at different positions in the foamed concrete. For the same casting density of the foamed concrete, the peak temperature, the maximum temperature change rate, and the maximum temperature difference decreased with the increase in the fly ash content. For the foamed concrete without the admixture, the early strength was significantly higher under temperature matched curing conditions than under standard curing conditions, but the temperature matched curing conditions had a clear inhibitory effect on the strength of the foamed concrete. -e strengths during the early stage and the later stage were both improved under temperature matched curing conditions after adding the fly ash, and the greater the fly ash content, the larger the effect. -e maximum temperature increments were higher in the indoor model test than in the field tests for the same casting density. Reasonable cooling measures and the addition of fly ash decreased the maximum temperature increments and increased the corresponding casting times.


Introduction
Cast-in-situ foamed concrete is composed of cement, an admixture, and a proportion of stable tiny bubbles, and it is cast, molded, and cured at the construction site [1][2][3]. is type of concrete is a new type of geotechnical material developed in recent years and possesses the advantages of low weight, stable performance, convenient construction, and so forth. [4][5][6][7]. e applications of cast-in-situ foamed concrete have expanded considerably in civil engineering as a result of scientific and technical advances in its production. According to the statistical results of the China Concrete and Cement Products Association (CCPA), the annual market size of foamed concrete was over 40 million m 3 in China during 2017, more than 80% of which was cast-in-situ foamed concrete. Due to construction growth in civil engineering, the applications of foamed concrete will also increase [8,9].
Most applications of foamed concrete are in large-volume concrete construction projects. e heat generated by the hydration affects the temperature field in the structure, resulting in three problems. First, the increase and decrease of the hydration heat lead to the expansion and contraction of the foamed concrete structures, which affects the structure itself and adjacent buildings. e maximum temperature is used as an indicator of this problem. Second, a temperature difference exists between the external and internal portions of the structure and the temperature stress results in cracks in the structure's surface. is reduces the strength of the structure and affects its integrity and durability ( Figure 1) [10,11]. ird, as studies on the impact of the hydration heat on the strength of concrete have shown [12][13][14] a lot of heat is produced by the hydration of foamed concrete, which may affect the strength of the material. Because the coefficient of thermal conductivity for this material is low, high temperatures are maintained for very long periods in the structure. Regarding these problems, Jones [11] stated that the heat evolution in foamed concrete is affected by a greater number of parameters than that in normal-weight concrete. Tarasov [15] studied the influence of the density and the volume of the castings and of fine aggregates on the temperature profiles in foamed concrete. However, the effect of hydration heat on the strength development of foamed concrete was not considered, and the experiments were carried out in the laboratory without considering external influencing factors.
Fly ash is a pozzolanic material and has been widely used as an admixture in concrete to address the problem of the hydration heat. Its application in concrete has been studied widely, but its application time in foamed concrete is short. Most studies on the addition of fly ash to foamed concrete have focused on the mechanical properties and durability [16][17][18][19]. If fly ash can be used satisfactorily in foamed concrete, it can be used to replace part of the cement. is lowers the construction costs and increases the performance of the foamed concrete and its application potential. e low-carbon economy is an important goal in China; therefore, the application of fly ash has a far-reaching significance [20,21].
e main goal of this study is to investigate the relationship between the hydration heat and the strength development of foamed concrete. First, six groups of indoor model tests with different casting densities and different fly ash dosages were conducted to study the effects of the casting density and fly ash content on the temperature profiles of the foamed concrete. Second, compression tests under two curing conditions (standard curing, temperature matched curing) were conducted to study the effects of the curing conditions on strength development. Finally, the changes in the temperatures of the foamed concrete were analyzed in four field tests.

Materials.
e foamed concrete was comprised of ordinary Portland cement, water, and bubbles. e cement was Type I Portland cement conforming to GB 175-2007, the fly ash was Class F Type I conforming to GB/T 1596-2005, and the water was tap water. e bubbles were created using a synthetic foaming agent, which was highly eco-friendly, and its air bubbles were strong [22,23]. Table 1 shows the mix proportions and major parameters of the foamed concrete used in the indoor test. A prefoaming method was used to produce the foamed concrete. First, bubbles with a density of 35 ± 5 kg/m 3 were prepared [24]. Second, the cement and water were weighed and mixed to produce the cement slurry. Finally, the prepared bubbles were incorporated into the cement slurry to create the foamed concrete slurry, which was then cast (Figure 2). In order to meet the requirements of the fluidity of the foamed concrete during construction, its flow value should be maintained between 160 mm and 180 mm according to the Specifications for the Design of Highway Subgrades (JTG D30-2015).

Indoor Model Test.
e layout of the Pt-100 thermal resistance thermometers in the indoor model test, which was conducted to determine the heat of hydration of the foamed concrete, is shown in Figure 3. e size of the model was 500 mm long × 500 mm wide × 500 mm high. e bottom and the periphery of the model were covered with doublelayer insulating foam boards. In order to simulate the conditions at a construction site, the top surface was covered with a thin film to simulate the semiadiabatic boundary conditions after the casting of the foamed concrete. e construction time interval of each layer was about 24 h in construction projects, and the temperatures were acquired for 36 h after the casting. e tests were conducted at a room temperature of 20°C.

Compression Test.
For this test, thirty identical samples (100 mm long × 100 mm wide × 100 mm high) were cast for each mix proportion. After the casting, half of the samples were cured under standard curing conditions, and the others were cured under temperature matched curing conditions. For the standard curing condition, the samples were cured in a standard curing room after they were unmolded until the test time. For the temperature matched curing conditions, the samples in the molds were placed in a constant temperature and humidity box. After the samples were unmolded, they were wrapped in bags cured in the constant temperature and humidity box ( Figure 4). e temperatures for the different densities were collected in the same manner as for the indoor model test. e humidity value remained at 100%.      Advances in Materials Science and Engineering corresponding time is 14 h for the three densities. For the same position, the peak temperature increases with the increase in the casting density of the foamed concrete. For the same casting density, the peak temperature is highest for position 1 followed by position 2 and position 3, and the corresponding times are different. is indicates that there is a heat exchange between the surface of the foamed concrete and the surrounding air.

Temperature Change Rate as a Function of the Casting
Time. e relationship between the temperatures change rate and the casting time for the three casting densities in position 1 is shown in Figure 6. For the foamed concrete with a casting density of 400 kg/m 3 , the temperature change rate reaches the maximum value of 7.5°C/h at a casting time of 10.5 h. For the casting densities of 700 kg/m 3 and 1000 kg/m 3 , the maximum values of the temperature change rate are 11.7°C/h and 14.3°C/h, respectively, at a casting time of 9.5 h. e maximum value of the temperature change rate increases, and the corresponding casting time decreases with the increase in the casting density. For the three casting densities of foamed concrete, the values of the temperature change rates are below 0 at casting times longer than 15-16 h, and the values remain stable when the casting time exceeds 20 h. e temperature change rate is highest for the casting density of 400 kg/m 3 , followed by the casting densities of 700 kg/m 3 and 1000 kg/m 3 ; therefore, the temperature attenuation increases with the increase in the casting density. In general, the temperature change rate increases before reaching the peak and the rate of temperature attenuation after the temperature peak increases with the increase in the casting density.

Relationship between the Temperature Difference and the Casting Time.
e relationship between the temperature difference and the casting time for the three casting densities is shown in Figure 7. For the foamed concrete with the casting densities of 400 kg/m 3 , 700 kg/m 3 , and 1000 kg/m 3 , the maximum temperature differences ∆T1 are 8.05°C, 10.65°C, and 12.46°C and the corresponding casting times are 21 h, 20 h, and 17 h. e temperature differences ∆T1 first decrease and then increase to the maximum value for all the densities. After that, the values decrease slowly. For the temperature difference ∆T2 at the casting densities of 400 kg/m 3 , 700 kg/m 3 , and 1000 kg/m 3 , the maximum values are 27.53°C, 33.50°C, and 39.21°C and the corresponding casting times are 11 h, 17 h, and 20 h. is indicates that for the temperature difference ∆T2, the maximum values of the temperature difference increase, and the corresponding casting times increase with the increase in the casting density.

Changes in the Temperature for Different Casting
Times. Figure 8 shows the relationship between the temperature and the casting time of the foamed concrete with different fly ash contents. e relationship is similar for the same casting density with different fly ash contents in the same position. For the foamed concrete with the casting density of 700 kg/m 3 in position 1, at a fly ash content of 0%, 10%, 30%, and 50%, the peak temperatures are 81. 03°C

Temperature Change Rate as a Function of the Casting Time.
e relationship between the temperature change rate and the casting time of the foamed concrete with different fly ash contents in position 1 is shown in Figure 9. At fly ash contents of 0%, 10%, 30%, and 50%, the maximum values of the temperature change rate are 11.68°C/h, 9.09°C/h, 5.97°C/ h, and 4.12°C/h and the corresponding casting times are 9.3 h, 10.5 h, 13.5 h, and 14.5 h. e maximum value of the temperature change rate increases, and the corresponding time increases with the increase in the fly ash content. When the value of the temperature change rate is stable, the value increases with the increase in the fly ash content, indicating that the temperature attenuates slowly for the foamed concrete with high fly ash content.

Relationship between the Temperature Difference and
the Casting Time. Figure 10 shows the relationship between the temperature difference and the casting time with different fly ash contents. At fly ash contents of 0%, 10%, 30%, and 50%, the maximum temperature differences ∆T1 are 10.65°C, 10.91°C, 9.35°C, and 7.79°C and the corresponding casting times are 20 h, 22 h, 25 h, and 32 h. After reaching the maximum temperature difference, the values decrease more slowly with the increase in the fly ash content. At fly ash contents of 0%, 10%, 30%, and 50%, the maximum temperature differences ∆T2 are 33.50°C, 30.38°C, 25.19°C, and 18.17°C and the corresponding casting times are 17 h, 16 h, 18 h, and 17 h. In conclusion, the maximum temperature difference increases for the same position for the same casting density with the increase in the fly ash content. Figure 11 shows the relationship between the compressive strength and the curing time for three casting densities. For the foamed concrete with the casting densities of 400 kg/m 3 ,    e relationship between the compressive strength and the curing time with different fly ash contents is shown in Figure 12. For the foamed concrete with the casting density of 700 kg/m 3 and fly ash contents of 0%, 10%, 30%, and 50% at curing times of 1 d, 3 d, and 7 d, the compressive strength values are larger for the temperature matched curing conditions than the standard curing conditions. For the foamed concrete with a casting density of 700 kg/m 3  Studies have shown that there are more hydration products during the early stage and fewer hydration products during the later stage in the temperature matched conditions compared with the standard curing conditions for pure cement [26,27]. is results in early strength increases and later strength decreases. e main reason is that high temperatures promote the hydration of the cement particles during the early stage, whereas the hydration products cover the surface of the cement particles and inhibit the hydration reaction during the later stage. For the foamed concrete with the fly ash under temperature matched conditions, the hydration reaction of the cement and the pozzolanic reaction of the fly ash are enhanced; the surface of the fly ash particles experience etching, the content of the gelling material increases, and the structure becomes denser (Figure 13).

Effect of Curing Conditions on the Strength Development.
In summary, when the casting density ranged from 400 kg/m 3 to 1000 kg/m 3 and no fly ash was used, the difference between the internal and external temperature of the cast-in-situ foamed concrete was significantly higher than 25°C, which does not meet the requirements of the standard for the construction of mass concrete. When the casting density was 700 kg/m 3 and the fly ash content was 30%, the temperatures met the 0% m 10% m 30% m 50% m 0% s 10% s 30% s 50% s Figure 12: Relationship between compressive strength and curing time (containing fly ash). Note: 0% s denotes the value of the test when the fly ash content is 0% under standard curing conditions. 0% s denotes the value of the test when the fly ash content is 0% under temperature matched curing conditions. requirements of the standard for the construction of mass concrete. erefore, it is necessary to consider field conditions and add a fly ash admixture or develop a system to contain moisture to meet the requirements.

Introduction to Field Tests.
e field tests consisted of four projects, and the details of the projects are shown in Table 2. Figure 14 shows an overview of the test sites. For project 3, the water was tap water, and for the other projects, the water was drawn from the rivers using pumps. e casting temperature was adjusted using water. e layouts of the components in the four projects are shown in Figure 15. For project 1, the height was 0.8 m and this distance was divided into two layers of 0.4 m. Because the base was rough, the foamed concrete was cast with a thickness of 0.1 m. For the other projects, the height exceeded 1.5 m.
e three bottom layers were tested and each layer was 0.5 m. e bases were identical for projects 2, 3, and 4, and the gravel layer was used for drainage, and a geomembrane was used to separate the water. Figure 16  measures and the addition of fly ash decrease the maximum temperature increments and increase the corresponding casting times. When it rains, the foamed concrete structure is affected because the permeability coefficient of the foamed concrete is large and water can penetrate into the foamed concrete [25]. However, the upper layer is more affected than the internal portion. A large temperature difference between the internal and external portions of the structure     Advances in Materials Science and Engineering may cause numerous crack fractures ( Figure 17) and reduces the quality of the project; therefore, rain or sudden cooling should be avoided. e maximum temperature increments are higher for the indoor model test than the field test for the same casting density (Figure 18), but the corresponding casting times are similar. is is caused by the contact of the upper layer with the air and the heat transfer of the lower layer. e temperature decreases more slowly in the middle layer. Taking into account the results of the compressive strength tests under different curing conditions, this indicates that the strength should be reduced or the mix proportions should be improved. e maximum temperature increment is affected by the casting density, the fly ash content, and external influencing factors (wind speed, casting temperature, etc.). An experimental equation may be put forward in future studies when a larger dataset will be used.    Data Availability e data in this article allow researchers to verify the results, replicate the analysis, and conduct secondary analyses.

Conflicts of Interest
e authors declare that they have no conflicts of interest.