Effect of Dry-Wet Circulation on Moisture Absorption of Autoclaved Aerated Concrete

Moisture absorbability is the characteristic of autoclaved aerated concrete that differs from other wall materials. For autoclaved aerated concrete, dry-wet circulation is the main actual service environment and can directly affect moisture absorbability, which influences cracking performance of structure. In this study, autoclaved aerated concrete with dry-wet circulation times of 0, 30, 60, 150, and 270 is selected.-e experiment is performed under the condition of temperatures 20°C, 30°C, 40°C, and 50°C and relative humidities (RH) of 40%, 60%, and 80%.-e temperature and humidity have significant effects on moisture absorption. When the dry-wet circulation times are increased, themoisture absorption performance improves; when comparing the specimen at the drywet circulation of 0 times with the specimen of dry-wet circulation of 270 times, the amount of moisture absorption content increased by 85.7%, at the temperature of 50°C and RH of 80%. Origin software is chosen to fit the moisture absorption kinetics model. SPSS software is used to analyse the linear regression and variance. -e results of hygroscopic kinetics showed that the fitting effect of the double exponential function was optimal, and the temperature and humidity were closely correlated with the specimens under dry-wet circulation, for R greater than 0.941.

Dry-wet circulation is the main service environment of autoclaved aerated concrete, for rainfall infiltration and drying evaporation [11][12][13][14].Autoclaved aerated concrete continuously carries out hygroscopic and dehumidifying process along with dry-wet circulation.And dry-wet circulation affects the stability and safety of the enclosure structure that results in fatigue effect in the enclosure structure [15][16][17].
Studies show that dry-wet circulation caused irreversible progressive damage to the autoclaved aerated concrete [18][19][20].Generally, the deterioration of autoclaved aerated concrete performance is relatively strong at the initial stage of the dry-wet circulation, and the degradation of autoclaved aerated concrete decreases with the increase in dry-wet cycle times [21][22][23][24][25].
ere are several reasons for the decline of autoclaved aerated concrete performance [26][27][28][29][30]. e most important one is that dry-wet circulation changing the pore structure.When its internal structure is porous, internal porosity increases from 70% to 80%, from 40% to 50% for closed pores, and from 20% to 40% for evaporation of wool stoma.ese unique pore structures slow down the speed of moisture absorption and moisture evaporation.And moisture absorption directly affects the thermal insulation performance of building structures.With hygroscopic expansion and drying shrinkage, moisture absorption also influences cracking performance of autoclaved aerated concrete [31][32][33][34][35].
In recent years, scholars have conducted a large number of experiments to evaluate autoclaved aerated concrete moisture absorption properties; Shinsakku tested the moisture absorption and verified the process of moisture transfer associated with the pore structure of autoclaved aerated concrete.Shujin Li compared the moisture of three types of autoclaved aerated concrete, the raw materials added were hydrophobic silica, potassium methyl silicate, and stearic acid calcium, and results showed that autoclaved aerated concrete with hydrophobic silica had the best effect on moisture absorption.Yingying Wang studied the effect of temperature and humidity on the thermal physical parameters of wall materials and built the heat transfer model coupling by the relative humidity and temperature.
is paper studies the moisture absorption of autoclaved aerated concrete under the dry-wet circulation times of 0, 30, 60, 150, and 270; the relative temperatures of 20 °C, 30 °C, 40 °C, and 50 °C; and relative humidities of 40%, 60%, and 80%.By employing Origin software to fit the moisture absorption kinetics model, the effects of temperature and humidity on the moisture absorption dynamics are studied.SPSS software is selected to analyse the linear regression and variance.

Materials and Methods
2.1.Materials.Autoclaved aerated concrete was obtained from Changle building materials Co., LTD in Changsha, China.Table 1 shows the properties of autoclaved aerated concrete.

Dry-Wet Circulation Test
Methodologies.Methods of the specimen processing and performance test were performed with reference to ASTM C1693-11(2017) "Standard Specification for Autoclaved Aerated Concrete."Dry-wet circulation of the specimen was prepared by using a sawing machine and classified as parts of above, middle, and below of the specimens; in preparation, two parts were removed, 30 mm from the top of specimens and 30 mm from the bottom.And the size of the specimen was 100 * 100 * 100 mm, and a group of specimens was of three pieces.
e specimen is chosen and dried using an electric drum dryer to maintain the constant quality; then, the specimen was cooled at the temperature of 20 ± 5 °C and kept for 20 min.Next the specimen was soaked in the water tank at the temperature of 20 ± 5 °C, Next the specimen was soaked in the water tank, water level exceeding the height of the specimen, at the temperature of 20 ± 5 °C for 30 min.
en, the specimen is taken out and kept for 30 min at the temperature of 20 ± 5 °C.Finally, specimens were dried for 7 h at a temperature of 60 ± 5 °C by an electric drum dryer and then cooled for 20 min at a temperature of 20 ± 5 °C.

Moisture Absorption Performance Test Method.
In the specimens under dry-wet circulation, the middle part was selected to cut, and the size was 50 * 50 * 50 mm.Constant temperature and relative humidity were used to test the moisture absorption.Moisture absorption test temperatures were, respectively, 20 °C, 30 °C, 40 °C, and 50 °C, and relative humidities were 40%, 60%, and 80%.In total, the number of specimens was 180 pieces.
A specific test method of moisture absorption was conducted as follows.First, an electric drum dryer was used to dry the specimen at the temperature of 60 ± 5 °C, and the quality had reached constant weight when the change of weight was less than 0.1%.Next, the constant temperature humidity chamber was used to control the temperature and humidity which is needed for this experiment en, the data of the weight of the specimen were recorded every 2 h within 12 h and then every 6 h.Finally, the experiment was ended when the change of weight was less than 0.1%.e amount of moisture absorption is calculated as follows: where M t is the amount of moisture absorption (g), W t is the amount of moisture absorption in the time t (g), and W i is the weight before the hygroscopic (g).

Data Analysis Method.
Origin software was used to fit the double exponential model, Weibull model, first-order process, and zero-order process.By employing SPSS software to analyse the linear regression and variance, the model that fits the hygroscopic process is identified and verified.

Moisture Absorption of Autoclaved Aerated Concrete
under Dry-Wet Circulation.Figure 1 shows the moisture absorption curve of the specimen under 20 °C. Figure 1(a) describes the specimen with the relative humidity (RH) of 40%.When the specimens are with no dry-wet circulation, the speed of moisture absorption rises gently from 0 h to 80 h, and the specimen reaches the hygroscopic balance at the time of 80 h and remains balanced from 80 h to 500 h.e moisture absorption of the specimens under dry-wet circulation times of 30, 60, 150, and 270 were as follows: from 0 h to 40 h, the speed of moisture absorption was rapidly increasing and showed the same regularity with no dry-wet circulation while exceeding 80 h.e curve result indicates that the amount of moisture absorption increases with the times of dry-wet circulation upward; for example, when the dry-wet circulation time is 270, hygroscopic properties increased by 114.0%, compared to the no dry-wet circulation.
Figure 1(b) displays moisture the absorption curve at RH 60%.When RH was increased from 40% to 60%, for the specimen under dry-wet circulation times of 0 and 30, the properties of moisture absorption did not have any significant difference.While the specimen was under dry-wet circulation times of 60, 150, and 270, the equilibrium moisture absorption content increased by 22.1%, 45.8%, and 45.8%.With the increase of the humidity, the content of hygroscopic balance increased.Comparing the time of the specimens that reached the hygroscopic balance, the specimens under drywet circulation times of 150 and 270 require the time of 100 h, while the others require 80 h.
Figure 1 shows that, at the same conditions of temperature and humidity, when the dry-wet circulation times are increased, the moisture absorption content increases.For the specimens with the fixed number of dry-wet circulation times and higher humidity, the hygroscopic balance increases.At the humidities of 40%, 60%, and 80%, the specimens under dry-wet circulation times of 270 had the largest quantity of moisture absorption, and it increases by  Advances in Materials Science and Engineering 114.2%, 140.9%, and 88.8%, compared to the specimens with no dry-wet circulation.
Figure 2 shows the moisture absorption curve of the specimens at the temperature of 30 °C. Figure 2(a) presents the specimens with RH 40%.When the dry-wet circulation times were 0, 30, 60, and 150, the specimens did not show significant increase, compared to the specimens at the temperature of 20 °C.At the dry-wet circulation times of 270, the moisture absorption increased by 15.6%, compared to the specimens at the temperature of 20 °C.And with the increased dry-wet circulation times, the specimens acquired larger moisture.e specimens reach the hygroscopic balance at 80 h, which remains the same at the temperature of 20 °C.
Figure 2(b) displays the moisture absorption curve at RH 60%.Compared to the specimens at the temperature of 20 °C and RH of 60%, the specimens reached the hygroscopic balance at 80 h, which is lessened by 20 h. e absorption speed of the specimen with no dry-wet circulation slows down at 40 h and the others at 60 h.For the amount of moisture absorption, the specimens with dry-wet circulation times of 0 and 30 show no different, while the others increased by 21.2%, 16.7%, and 14.3%.
Figure 2(c) shows moisture absorption curve at RH 80%.Compared to the specimens at the temperature of 20 °C, the time of the specimens to reach the hygroscopic balance is shortened, decreasing from 100 h to 80 h.For the amount of moisture absorption, the specimens with dry-wet circulation times of 0 and 30 showed no difference, which is the same as that of Figure 2(b).When the dry-wet circulation times reached 60, 150, and 270, the amount of moisture absorption increased by 12.5%, 9.2%, and 7.4%, compared to the specimens at the temperature of 20 °C.
Figures 3(a)-3(c) show the moisture absorption curves of the specimens at the temperature of 40 °C at RH 40%, RH 60%, and RH 80%.Compared to the specimens at the temperature of 20 °C, the amount of moisture absorption of the specimens increased obviously.
Figure 3(a) shows that equilibrium moisture absorption content of the specimen under dry-wet circulation times of 0, 30, 60, 150, and 270 increased by 22.4%, 9.1%, 14.6%, 17.7%, and 20.0%, respectively, compared to the specimens at the temperature of 20 °C.When the specimens are under no dry-wet circulation, hygroscopic rises at 0 h to 100 h, and when the specimens are under dry-wet circulation times of 30, 60, 150, and 270, the hygroscopic becomes rapid at 0 h to 60 h, at 60 h to 80 h, the speed slows down, and at the time of 80 h, the hygroscopic balance is reached.
Figure 3(b) shows the moisture absorption of specimens at RH 60%.When the specimen is under dry-wet circulation times of 30, 60, 150, and 270, the speed of moisture absorption is rapid at 0 h to 40 h and then the process of stable moisture absorption is reached.Compared to specimens with RH 40% shown in Figure 3(a), the amount of moisture absorption content of specimens under dry-wet circulation times of 60, 150, and 270, respectively, increases by 21.7%, 21.0%, and 18.2%, and the other specimens show no difference.
Figures 4(a)-4(c) present the moisture absorption at the temperature of 50 °C, for RH 40%, RH 60%, and RH 80%.In Figure 4(a), for specimens under no dry-wet circulation, hygroscopic slowly rises at 0 h to 100 h, and for specimens under dry-wet circulation times of 30, 60, 150, and 270, the speed of moisture absorption slows down at 60 h, and the hygroscopic balance is reached at the time of 80 h.And compared to 20 °C, the specimens under dry-wet circulation times of 0, 30, 60, 150, and 270, the equilibrium moisture absorption content increased by 23.2%, 10.3%, 11.1%, 14.6%, and 15.0%, respectively.
Figure 4(b) shows that when the specimens are under dry-wet circulation times of 30, 60, and 150, the speed of moisture absorption is slow at 40 h and the balance is reached at the time of 60 h.In the specimen under the dry-wet circulation times of 270, saturation is reached at 80 h and the equilibrium moisture absorption content increased by 87.3%, compared to the specimens at 20 °C.Compared to specimens at RH 40% shown in Figure 4(a), in specimens under dry-wet circulation times of 150 and 270, the equilibrium moisture absorption content increased by 23.9% and 14.8%.
Figures 1-4 show that, with the increase of dry-wet circulation times, moisture absorption content of specimens increases.While increasing the humidity from 40% to 60%, the content of moisture for the specimens of dry-wet circulations of 0 and 30 did not changed, but the moisture content in the specimens of dry-wet circulations of 60, 150, and 270 increased.Temperature improved the speed of moisture absorption.Autoclaved aerated concrete is a kind of high porosity material.Moisture absorption depends on the pore content and pore distribution.When the dry-wet circulation times were increased, the pore structure was damaged seriously.
e dry-wet circulations lead to expansion and shrinkage.e specimens absorbed moisture when the humidity increases and lost contraction with the internal moisture decline.Pore structure changed with the repetitive process, for aperture size increased and porosity reduced.It results in the exceeding of destructive power 4 Advances in Materials Science and Engineering beyond the tensile strength of the specimens, which leads to the damage of core and pore structure.

Dynamic Equation Optimization.
e relationship between the adsorption of materials and structure by moisture absorption kinetics is built.e moisture absorption kinetics model mainly has the double exponential model, welbull model, first-order process, and zero-order process.Table 2 shows the specific formula.
e following model is selected for analysing the moisture absorption of the specimen.e data of content of moisture of the specimen at 30 °C and RH of 40% are chosen to the optimal range.According to the figure, R 2 of the double index model is above 0.95, R 2 of the Weibull model is in the range 0.90-0.97,R 2 of the first-order process and the zero-order process, respectively, are in the range 0.85-0.97and 0.83-0.95.R 2 distribution interval was concentrated in the double exponential model, and numerical value is more close to 1; the results show that the double exponential model is the optimal model.
Figure 6 presents the percentile figure with the analysis index of RSS.When the RSS is close to zero, the fitting effect is optimal.Figures 6(a)-6(d) show the analysis index of RSS for the double exponential model, the Weibull model, the first-order process, and the zero-order process, respectively.Figure 6 shows that RSS of the double exponential model is between 0 and 1.For the zero-order process, RSS is from 1 to 4, whose distribution interval is maximum.For the Weibull model and first-order process, RSS is from 0 to 2 and 1 to 2.5, Advances in Materials Science and Engineering respectively.e results show that the double exponential model is the optimal model.Figure 7 presents the percentile figure with the analysis index of RMSE.When the RMSE is close to zero, the fitting effect is optimal.Figures 7(a)-7(d) show the RMSE for the double exponential model, the Weibull model, the firstorder process, and the zero-order process.Figure 7 shows that RMSE of the double exponential model is between 0.05 and 0.12.For the Weibull model, the first-order process and zero-order process, RMSE values are in the range of 0.07-0.16,0.08-0.16,and 0.07-0.18,respectively.
Figures 5-7 show the analysis index of R 2 , RSS, and RMSE in the double exponential model having small variations, which indicates that the fitting effect of double exponential model is optimal.

Influence of Temperature and Humidity on Accuracy of Model Analysis
. SPSS software is used to analyse the linear regression and variance.e accuracy of the fitting model is determined by the determination coefficient (R 2 ) and mean absolute percentage error (MAPE).e higher accuracy of the model fitting is decided by the higher R 2 and lower MAPE.Generally speaking, MAPE < 10% is required.
Formula (2) shows MAPE calculation method: where M expt is the moisture absorption content obtained in the test, g H 2 O/g and M pred moisture absorption obtained in the model, g H 2 O/g.Table 3 shows the dynamic parameters of autoclaved aerated concrete under different temperatures and humidities.It can be seen from the table that the value of R 2 is greater than 0.9811 and MAPE is less than 10%, indicating that the double exponential model has higher fitting precision.
e double exponential model shows moisture migration in the specimen conform to Fick's theorem.e moisture absorption rate curve can be used to compare the hygroscopic character of the specimen.e model of moisture absorption kinetics for analysis the characteristics of physical property provides a good theoretical basis for moisture absorption.
When the temperature and humidity are increased, the quantity of moisture absorption increases.Once humidity reduces, the water in pore structure starts to evaporate, causing the shrinkage of the specimen.e characteristics of the pore structure of the specimen affects the moisture content.4 shows the relationship between moisture absorption and temperature and Advances in Materials Science and Engineering

Conclusion
(1) e temperature and humidity had a great effect on moisture absorption performance of the autoclaved aerated concrete; when temperature and humidity increase, the amount of moisture absorbed increased obviously.And with the increase of dry-wet circulation times, moisture absorption performance differs; for specimens at 50 °C, RH 80%, and with drywet circulation of 270 times, moisture absorption increased by 85.7%, when compared to those under no dry-wet circulation.(2) e double exponential model is the optimal model, which indicates that moisture migration in autoclaved aerated concrete conforms to the theorem of Fick.
(3) By analyzing the temperature and humidity impact on the moisture adsorption of autoclaved aerated concrete, the results showed that temperature and humidity have appreciable impact.

Figures 5 (
a)-5(d) show the percentile box figure with the analysis index of R 2 for the double exponential model, Weibull model, first-order process, and zero-order process.

Table 1 :
Main properties of autoclaved aerated concrete.

Table 3 :
Moisture absorption kinetic parameters affected by temperature and relative humidity.Advances in Materials Science and Engineering humidity, autoclaved aerated concrete under dry-wet circulation.It can be seen that there is a close correlation with temperature and humidity, when R 2 is greater than 0.941.With the increase in times of dry-wet circulation, the effect of fitting is better.

Table 4 :
Hygroscopic kinetic parameters affected by temperature and relative humidity.