Investigation into the Impact of Humidity on Early Age Cement Concrete Pavement Behavior in Hot and Humid Regions

Abstract

Cement concrete pavement is prone to early deterioration during the construction phase, and the early performance during the construction phase is significantly affected by the external temperature and humidity field. This article selects meteorological parameters in the Fuzhou area as a typical representative of a humid and hot climate and develops a three-dimensional humidity simulation program based on Fick’s law and the finite difference method to study the evolution behavior of the humidity field in early age of cement concrete pavement. It discusses the humidity distribution characteristics of road panels and analyzes the influence and sensitivity of cement type, construction conditions, and road panel structural parameters on road panel humidity. Research has shown that the evolution law of the humidity field of road panels shows a 24-h periodic change with the external environment. The environmental field has a significant impact on the surface humidity of road panels. The horizontal humidity of the panel is concentrated from the boundary to the middle of the panel, and the tangential humidity is concentrated from the top to the bottom of the panel. The humidity field of road panels is the most sensitive to environmental humidity and maintenance methods, but less sensitive to material and structural parameters. Therefore, during construction, it is possible to avoid the hot season and choose a time period when the environmental humidity increases to pour concrete. Appropriate maintenance methods are adopted to reduce the humidity stress of the panels, reduce early age deterioration, and improve their service life.

1. Introduction

It is now understood that once cement concrete pavement has been poured and completed, environmental factors can cause temperature and humidity gradients inside the concrete, resulting in slab surface shrinking, expansion, and warping, which cause large tensile stresses. Ultimately, these stresses can trigger cracking and even spalling [1]. Especially under the coupling effect of high temperature, high humidity, heavy rain, and strong wind, media such as water and erosion ions in the environment accelerate to penetrate into the interior of concrete along microcracks, causing further crack growth, increasing the probability of diseases such as concrete pavement peeling, affecting service life performance, and further reducing service life.

In recent years, much emphasis has been placed on temperature differences and their impact on cement concrete pavement panels domestically and abroad [2,3,4,5,6,7], Scholars have also conducted numerous studies on mechanics [8,9]. China’s pavement panel design specification now incorporates the influence of temperature variation on pavement panel design calculations.

Unlike the effect of the temperature field, the effect of the moisture field has been largely understudied and is not considered in current concrete pavement design and calculations [10]. Recent research has highlighted the significant impact of humidity on the deformation and stress of pavement panels, especially during the early stages of development [11,12]. Ziari et al. found that the humidity gradient along the board body can cause the board to warp [13]. Based on the coefficient of thermal expansion, Mateos et al. could linearly equivalent the humidity gradient to a temperature gradient and calculated the humidity stress through the equivalent temperature gradient [14,15]. Lyu et al. considered the influence of bending creep on humidity stress [16,17], and Li Zuzhong and Xing Xuemin analyzed the influence of temperature and humidity on the structural modulus of concrete pavement [18,19,20]. Rania E. Asbahan et al. [21] conducted experiments to measure the curvature of concrete panels due to humidity strain after 1 to 2 years. Wei, Qin et al. [22] carried out numerical simulations and found that the humidity warpage of pavement panels under extremely dry conditions was significant, with a magnitude of up to 2.52 mm and a corresponding equivalent temperature difference of −107 °C/m, which is comparable to the daily temperature difference. In addition, a significant moisture difference could cause a large moisture shear stress in the surface layer of concrete pavement panels, leading to delamination and early edge spalling of the panels [23].

At present, research on the moisture field is still primarily focused on a two-dimensional perspective [24], which limits the ability to consider the influence of the diffusion effect on the moisture field in multiple dimensions. Additionally, there is a lack of research on the moisture field in special climatic regions. To address the above problems, this article studies the early age humidity field behavior and the effect of cement concrete pavement panels from the perspective of a three-dimensional humidity field, combined with a humid and hot climate environment, in order to provide theoretical support for further improving pavement performance.

2. The 3D Moisture Simulation Program for Cement Concrete Pavement Panels

2.1. Principles and Equations

The variation of moisture in a certain area inside concrete can be expressed as humidity diffusion and cement hydration. Based on the fact that the phases of moisture inside the concrete are in thermodynamic equilibrium, the variation of the relative humidity inside the concrete and the variation of the moisture content are approximately in a linear relationship, as explained by the principle of adsorption and desorption, expressed as

$$\frac{\partial h}{\partial t}=\frac{\partial h_d}{\partial t}+\frac{\partial h_s}{\partial t}+\kappa \frac{\partial T}{\partial t}$$

(1)

In the Equation (1), $\partial h/\partial t$ is the total relative humidity change in the panel, $\partial h_d/\partial t$ is the humidity change due to diffusion, $\partial h_s/\partial t$ is the relative humidity change due to self-drying, and $\kappa \partial T/\partial t$ is the change in relative humidity under the influence of temperature. The following are the expressions and differential equations for the three components of Equation (1).

(1) Humidity diffusion model

Scholars have conducted numerical studies of humidity diffusion based on Fick’s law and the law of mass conservation [25]. As shown in Figure 1, Fick’s law states that the rate of transport of diffusing substances per unit area through a region is proportional to the concentration gradient in that region, such that $h_d=h-h_s-\kappa \cdot \partial T$ transforming Equation (1) into

$$\frac{\partial h_d}{\partial t}=div\left(D\cdot grad\cdot h_d\right)$$

(2)

In Equation (2), the humidity diffusion coefficient $D$ is a function of $h$, expressed as

$$D(h)=D_0\left\{{\alpha }_0+\raisebox{1ex}{$1-{\alpha }_0$}\!\left/ \!\raisebox{-1ex}{$1+{\left[\left(1-h\right)/\left(1-h_{c0}\right)\right]}^n$}\right.\right\}$$

(3)

In Equation (3), $D_0$ is the moisture diffusion coefficient of saturated concrete; n is the regression coefficient of the moisture nonlinear diffusion equation, indicating the rate of decline of $D\left(h\right)$, usually taken as 6–16; ${\alpha }_0=D_{\min }/D_0$, $D_{\min }$ is the diffusion coefficient at $h=0$; $h_{c0}$ is the relative humidity value of $D$ at $(D_0-D_{\min })/2$, usually taken as 0.75–0.8.

(2) Temperature correction model

When the water content inside the pores remains constant, the saturated vapor pressure will change with temperature, indirectly affecting the relative humidity $h$ and the humidity diffusion coefficient $D_0$ of saturated concrete. The influence of temperature on $D_0$ and $h$ can be expressed as [21]

$$\kappa =0.0135h\left(1-h\right)/\left(1.25-h\right)$$

(4)

$$\frac{D_T}{D_0}=\frac{T}{T_0}\exp \left(\frac{Q}{RT}-\frac{Q}{R{T}_0}\right)$$

(5)

In Equation (4), $\kappa$ is the moisture diffusion coefficient, T is the absolute temperature ($°K$); $T_0$ is the reference temperature, usually chosen as room temperature, $D_T$ and $D_0$ are the saturation diffusion coefficients of concrete at $T$ and $T_0$, respectively, $Q$ is the activation energy of hydration.

(3) Self-drying model

The relationship between the change in relative humidity due to self-drying $\Delta h_{as}$ and the degree of hydration $\alpha$ can be expressed as

$$\Delta h_{as}=\frac{a+\alpha }{b}\frac{1}{2}$$

(6)

In Equation (6), $\Delta h_{as}$ is the change in relative humidity due to self-drying; $\alpha$ is the degree of hydration; $a$ and $b$ are the shape parameters.

2.2. Conditions for the Fixed Solution of the Three-Dimensional Humidity Diffusion Differential Equation

(1) Initial moment

The default concrete is fully saturated when it is first formed, and the initial moment humidity is

$$h(x,y,z,0)=1, (t = 0)$$

(7)

(2) Surface boundary conditions

The evaporation rate model of concrete proposed by Al-Fadhala and Hover [26] was chosen to calculate the moisture exchange with the environment.

$$E_r=E_W\cdot \exp \left(-{(\frac{t}{a})}^{1.5}\right)$$

(8)

In Equation (7), $E_r$ is the evaporation rate of water on the concrete surface; $E_W$ is the evaporation rate of water on the free water surface; $t$ is the age period; $a$ is the time constant.

The equation for the free water surface evaporation rate proposed by Paul [20] was chosen:

$$E_W=0.313\left(e_{so}-h\cdot e_{sa}\right)\cdot \left(0.253+0.06w\right)$$

(9)

In Equation (9), $h$ is the relative humidity; $e_{so}$ is the vapor pressure of concrete surface, kPa; $e_{sa}$ is the vapor pressure of air, kPa; $w$ is the wind speed km/h.

(3) Lower surface boundary conditions

Assuming that the lower surface of the concrete is sealed and there is no moisture exchange, the boundary conditions are

$$\frac{\partial h}{\partial z}=0,z=l,t>0$$

(10)

2.3. The 3D Humidity Field Diffusion Difference Derivation

The units inside the panel have variable moisture diffusion and can be classified into six types based on the number of their diffusion surfaces. An example of a six-sided diffusion unit with three-dimensional diffusion is used to establish the three-dimensional humidity diffusion equation. The simplification leads to the following equation:

$$\begin{array}{cc}\frac{h_{i,j,k}^{t+1}-h_{i,j,k}^t}{\Delta t}& =\frac{D_{i+\frac{1}{2},j,k}^t\cdot \left(h_{i+1,j,k}^t-h_{i,j,k}^t\right)-D_{i-\frac{1}{2},j,k}^t\cdot \left(h_{i,j,k,t}^t-h_{i-1,j,k}^t\right)}{\Delta x^2}\\ & +\frac{D_{i,j+\frac{1}{2},k}^t\cdot \left(h_{i,j+1,k}^t-h_{i,j,k}^t\right)-D_{i,j-\frac{1}{2},k}^t\cdot \left(h_{i,j,k}^t-h_{i,j-1,k}^t\right)}{\Delta y^2}\\ & +\frac{D_{i,j,k+\frac{1}{2}}^t\cdot \left(h_{i,j,k+1}^t-h_{i,j,k}^t\right)-D_{i,j,k-\frac{1}{2}}^t\cdot \left(h_{i,j,k}^t-h_{i,j,k-1}^t\right)}{\Delta z^2}\end{array}$$

(11)

Some parameters in Equation (11) can be calculated using the following equations:

$$\begin{gathered} D_{i+\frac{1}{2},j,k}^t=\frac{1}{2}\left(D_{i+1,j,k}^t+D_{i,j,k}^t\right);D_{i-\frac{1}{2},j,k}^t=\frac{1}{2}\left(D_{i-1,j,k}^t+D_{i,j,k}^t\right) \\ {D}_{i,j+\frac{1}{2},k}^t=\frac{1}{2}\left(D_{i,j+1,k}^t+D_{i,j,k}^t\right);D_{i,j-\frac{1}{2},k}^t=\frac{1}{2}\left(D_{i,j-1,k}^t+D_{i,j,k}^t\right) \\ {D}_{i,j,k+\frac{1}{2}}^t=\frac{1}{2}\left(D_{i,j,k+1}^t+D_{i,j,k}^t\right);D_{i,j,k-\frac{1}{2}}^t=\frac{1}{2}\left(D_{i,j,k-1}^t+D_{i,j,k}^t\right) \end{gathered}$$

(12)

2.4. The 3D Meshing

To establish a model for concrete pavement panels, the panel was divided into 20 units in the length and width directions and 6 units in the depth direction, each with a size of 0.02 m. For instance, a pavement slab of size 5 m × 4.5 m × 0.24 m had a total of 2400 cells sized at 25 cm × 22.5 cm × 40 cm, numbered from the bottom to the top of the slab. Each depth plane consisted of 441 nodes, with the entire model consisting of 7 layers and 3087 nodes in total. he specific grid was divided as shown in Figure 2.

3. Early Behavior of Cement Concrete Pavement Panels Moisture Field

3.1. Design of Working Conditions

Under high humidity and no maintenance construction conditions in summer, the impact of humid and hot environments on the humidity of cement concrete pavement can be more fully presented [10,27]. Therefore, the three-dimensional moisture field properties of cement concrete pavement panels were analyzed by taking summer construction without geotextile sprinkler maintenance as the benchmark condition and examining the influence of different factors on the humidity for the first 28 days of pavement panels. The benchmark conditions and parameters are shown in

Table 1. Road panel 3D moisture field sensitivity analysis conditions and parameter values.

Variable Type	Parameter Name	Range of Values	Base Value
Mixes	Cement Type/ASTM	Type I–V	Type I
Component Variables	Water to ash ratio	0.33–0.45	0.39
Environmental parameters	Average daily temperature/°C	Actual weather measurements in Fuzhou in March, July, September, and December	July
	Average daily humidity/°C
	Ambient humidity/%
	Daily average wind speed/m/s	0–6	2
	Daily maximum solar radiation intensity/w/m2	0–1200	600
	Sunshine time/h	12
Construction parameters	Pavement panel thickness/cm	20–28	24
	Paving time/24 h	0–24	16
	Conservation status	Yes/No	None

. The baseline environmental field conditions are based on the meteorological conditions in Fuzhou in summer (July), including ambient temperature and humidity, solar radiation intensity, and wind speed, as shown in Figure 3. The measured values of environmental humidity in March, July, September, and December in Fuzhou were selected. Figure 4 and Figure 5 show the corresponding environmental humidity and temperatures in Fuzhou in March, July, September, and December.

The selected benchmark of Fuzhou summer climate was characterized by high temperature, humidity, wind speed, and solar radiation. The minimum temperature was 27 °C, the maximum temperature was 37 °C, the temperature difference between day and night was about 10 °C, the maximum daily solar radiation was 950 W/m², and the wind speed was 7.5 m/s. Although there was no rainfall, the maximum humidity was 100%, but it did not last long. In Fuzhou, the ambient humidity in March, July, September, and December exhibited diurnal changes but with different amplitudes. In March and July, there were fluctuations in the range of 50% to 100% due to rainy conditions, and the duration of high humidity was more variable. On the other hand, in September and December, when the weather was clearer, the ambient humidity ranged from 42% to 98%, with less fluctuation during high humidity periods.

The ambient temperature and humidity exhibited diurnal variations, with differences in the magnitude and pattern of change. In March and December, the temperature varied by 10 °C to 20 °C with fluctuations during the day. In December, the temperature was generally higher than in March. In contrast, the temperature variation in July and September was relatively high, ranging from 25 °C to 35 °C, with July generally hotter than September. The humidity also varied with diurnal changes, but the amplitude of variation was different. In March and July, the humidity ranged from 50% to 100%, with more fluctuations during periods of high humidity. In contrast, when the weather was clearer in September and December, the humidity varied from 42% to 98%, with fewer fluctuations in periods of high humidity.

3.2. Three-Dimensional Moisture Field Trait Distribution of Road Panels at Early Ages

Figure 6 and Figure 7 show the three-dimensional evolution of moisture field distribution of concrete pavement panels for summer construction without maintenance benchmark conditions at early ages.

The 3D humidity field evolution of cement concrete pavement panels during the first 28 days under the environmental field exhibited the following characteristics.

(1) The humidity varied significantly across the panel plane, with concentrations above and below the plate; this is consistent with the results of extensive research [27]. The maximum difference between the dividing line of 2 cm, 4 cm, and 6 cm in the first 28-day age period and the relative humidity in the plate was 1.3%, 2.3%, and 2.9%, respectively. Due to the rapid diffusion of the surrounding and surface humidity, which could drop to the equilibrium state in a short time, and the diffusion speed of humidity in the center of the panel, which was slow in the diffusion state for a long time and lower than the environmental humidity, and the humidity in the center of the plate was lower than that surrounding it.

(2) The surface humidity of the road panel exhibited obvious internal and external moisture regions, and the humidity boundary was about 2 cm on the four sides of the panel. Humidity within 2 cm of the edge of the plate changed obviously with the environmental fluctuations, while humidity in the middle of the panel fluctuated slightly. Under dry conditions, the humidity was less than that in the plate, and the humidity was higher than in a wet environment.

(3) The middle of the panel was less sensitive to environmental humidity than the boundary and surface parts. At 16 h after the pouring of the panel, the humidity at the boundary of the panel decreased significantly and began to fluctuate with the environment, while humidity in the middle of the plate did not fluctuate significantly with the environment.

(4) The humidity at the boundary was determined by humidity in the middle of the panel and the environmental humidity. The value of the humidity boundary represents a balance between the panel and environmental humidity, which increased with an increase in environmental humidity and panel humidity.

3.3. Influence of Different Factors on the Moisture Field of Pavement Panels at Early Ages

Although the road panel moisture field exhibited three-dimensional heterogeneity, this heterogeneity was primarily concentrated in the corners and edges of the panel position, which were closer to the boundaries. The main effect on the panel was caused by the moisture gradient along the depth direction. Based on the characteristics of the cement concrete road panel humidity field, a simulation program was developed to analyze the sensitivity of early age panel performance to changes in humidity field parameters.

3.3.1. Effect of Cement Type

It is well-established that different types of cement generate various amounts of heat during the hydration reaction, which affects the internal temperature of the panel and, thus, the humidity field inside the panel. This article selects five different types of Portland cement to analyze their impact on the humidity of cement concrete pavement panels. The cement components are shown in

Table 2. Mineral fraction content of cement clinker (mass %, %) and specific surface area.

Number	C3S (%)	C3A (%)	C2S (%)	C4AF (%)	Specific Surface Area (m2/kg)
Type I	51.36	9.59	23.82	9.70	404
Type II	53.28	9.31	21.80	10.59	408
Type III	55.94	11.94	16.91	10.18	377
Type IV	62.34	9.38	12.87	10.41	412
Type V	52.34	3.89	24.93	13.87	357

. Except for cement, all other materials, water-cement ratio, sand rate, etc., are the same in the concrete configuration. The silicate cement with different clinker fractions on the relative humidity at the top of concrete pavement panels $h$ and the difference in humidity at the top and bottom of panels $\Delta h$, as shown in Figure 8 and Figure 9. Table 2 shows the composition of the cement clinker.

Comparing Figure 8 and Figure 9, it can be observed that the effect of cement type on the humidity of concrete pavement slabs was minimal. The difference between the relative humidity h of the five types of cement on concrete pavement slabs was not more than 1%, suggesting that the type of cement has little effect on the moisture content of pavement slabs. The humidity of concrete pavement gradually increased along the depth direction, forming a positive humidity difference $\Delta h$, that is, the humidity at the top of the slab was generally smaller than the humidity at the bottom of the slab at the same age.

We found that the effect of cement type on concrete humidity was mainly due to the difference in clinker mineral content of the cement, which mainly affected the early internal temperature of the concrete panel due to the heat of hydration. However, since the internal and bottom humidity values of the early concrete were generally high and remained in a saturated state for a long time, the small difference in temperature had a relatively minor influence on the internal relative humidity of the panel.

We adopted the environmental data from Fuzhou in July for the benchmark working conditions. This data showed fluctuations with age and was able to rapidly exchange humidity with the concrete on the top surface of the panel. As a result, the temperature and humidity at the top of the panel varied greatly, leading to a smaller difference in hydration heat produced by the type of cement in the range of the top of the panel and a smaller humidity difference Δh.

3.3.2. Influence of Construction Environmental Conditions

(1) The effect of construction month.

We analyzed the effect of the month of construction on the relative humidity h and the humidity difference Δh at the top of the concrete pavement slab. Figure 10 and Figure 11 show the effect of the construction month on the humidity of the concrete pavement slab.

As shown in Figure 10 and Figure 11, the construction month significantly impacted the humidity and humidity difference of the top surface of the panel. On the panel surface of a 7-day age period, the relative humidity in March and December was 97% and 92%; the humidity difference in March and December was 3% and 8%, respectively.

The panel was exposed to an environment lower than the surface humidity due to the formation of humidity difference between the panel surface and the environment within 7 days of concrete placement. This resulted in reduced moisture exchange between the panel and the environment and humidity fluctuation at the top of the panel. However, this moisture exchange yielded a minor effect on the relative humidity of the concrete inside the panel during the early stage, where the internal concrete humidity remained saturated (h = 100%). Calculation of the humidity difference between the panel and the environment plays a major role in the humidity of the panel surface, increasing the humidity difference and decreasing the surface humidity in a similar trend.

It was found that the Fuzhou area experienced significant environmental changes, which also significantly impacted the humidity inside the panel, which changed according to the construction month.

(2) The effect of wind speed.

Figure 12 and Figure 13 show the effect of wind speed on the relative humidity h and humidity difference h at the top of the concrete pavement panel. Although higher wind speeds resulted in a faster drop in concrete humidity; their effect on the panel’s overall humidity and humidity difference was not significant. In this respect, the difference between the drop amplitude of panel humidity and the humidity difference amplitude under the influence of different wind speeds was less than 1%.

We found that wind speed could affect the evaporation rate of water. When the wind speed increased, the rate of decline of the water vapor pressure on the pane’s surface increased, increasing the steam pressure difference between the panel and the air. As water evaporation increased, the surface humidity of the panel decreased. However, when the surface humidity of the panel drops to a level with the ambient humidity, the vapor pressure between the panel and the atmosphere will remain flat, and the water exchange will be in equilibrium. In this case, the rate of decrease in concrete humidity is determined by the diffusion coefficient of concrete humidity, so under different wind speed conditions the panel humidity h is different from the humidity difference Δh; the difference in h is not significant.

(3) The effect of solar radiation.

Figure 14 and Figure 15 show the effect of solar radiation on the relative humidity h and the humidity difference h at the top of the concrete pavement panel slab.

As shown in the above figures, solar radiation had a significant impact on the humidity and humidity difference of the top surface of concrete panels. The greater the solar radiation, the greater the magnitude of fluctuations in the humidity of the road panel; the relative humidity and humidity difference at the top of the panel fluctuated with increased solar radiation and time. The magnitude of fluctuations in the top humidity and humidity difference was 4%; the maximum humidity and humidity difference under different working conditions was 1.5%. It was found that the road panel absorbed solar radiation from daylight and then heated up as a whole, and after the temperature of the concrete panel increased, the internal saturated vapor pressure decreased and the vapor strength of the panel surface decreased, thus affecting the humidity field inside the panel.

3.3.3. Influence of Structural and Construction Parameters

(1) The effect of pavement panel thickness.

According to the traffic and highway grades, the concrete panel surface thickness was generally between 180 and 300 mm. We next analyzed the influence of the panel thickness on the relative humidity h and humidity difference Δh of the concrete panel roof. Figure 16 and Figure 17 show the influence of different surface thicknesses on the humidity field.

As shown in Figure 16 and Figure 17, the effect of panel thickness on both panel humidity and humidity difference was not significant. The relative humidity at 2 cm was not affected by the pavement thickness, and the difference in relative humidity between different thickness cases did not exceed 1%. The humidity difference was positively correlated with the thickness of the pavement panel, but the difference in humidity between panels of different thicknesses was not significant, as it did not exceed 1%. Therefore, it was concluded that the variation in slab thickness had little impact on the moisture field distribution during the early age of concrete.

(2) The influence of paving time

It is widely thought that the initial environment to which concrete pavement panels are exposed can significantly impact the panel’s moisture diffusion behavior. Next, we analyzed the influence of paving time on the relative humidity of concrete panel roof h and humidity difference Δh. We selected four construction periods, 1:00, 7:00, 13:00, and 19:00, and analyzed the influence of paving time on concrete humidity (Figure 18, Figure 19, Figure 20 and Figure 21).

As shown in Figure 18, Figure 19, Figure 20 and Figure 21, the paving time had a significant impact on the humidity field of the panel during its early age period within the first 24 h, and the extent of the influence gradually decreased with age, mainly due to the following characteristics:

1) The maximum humidity difference at the top of the panels with different paving times at the same time is 1%. The humidity of the road panels decreased with age in all four construction periods, with fluctuations in a cycle. After reaching the peak, the humidity slowly declined. For the road panels constructed at 1:00, the humidity dropped sharply and then decreased slowly to reach the second peak point. For the road panels constructed at 7:00, the humidity decreased the fastest and reached the first peak point. For the road panels constructed at 19:00, the humidity dropped more slowly and reached the second peak point. For the road panels constructed at 13:00, the humidity dropped quickly to the first peak point, but the peak point was lower than that of the road panels constructed at 7:00.

2) Different paving times had a maximum impact of 1% on the panel’s humidity difference at the same intervals, and the influence gradually weakened with age. Typically, the humidity difference between panels constructed during the daytime was greater than those constructed during the nighttime.

3) There were significant differences in panel humidity and humidity differences across different paving times for the same age period, which could reach up to 3%. It could be concluded that the paving time indirectly impacted the internal humidity field of the panel through the environmental temperature, humidity, and solar radiation.

At 7:00, the hydration reaction and the environmental factors almost reached their strongest state, resulting in a coupling effect that caused the panel humidity to drop faster and peak at a higher level. At 19:00, the temperature and solar radiation were lower than during the daytime, and the panel moisture evaporation rate and humidity dropped slowly. At 13:00, the panel poured was only partly exposed to direct sunlight, resulting in a lower moisture evaporation rate than at 7:00 but higher than at 19:00. Finally, panels poured at 1:00 experienced no solar radiation and lower temperatures, resulting in the lowest moisture evaporation rate.

It was found that the panels with the lay-up time in the afternoon had higher overall humidity and a smaller moisture difference.

(3) The impact of conservation methods

It is widely acknowledged that various conservation methods can be used to increase the moisture content of concrete pavement panels from the surface to the bottom and reduce moisture differences and their adverse effects. To explore the effects of different maintenance methods on the early age moisture field in actual pavement construction, four methods were used: no maintenance, geotextile maintenance, water sprinkling maintenance (moisture saturation time of 6 h), and geotextile sprinkling maintenance (moisture saturation of the panel during maintenance). Figure 22 and Figure 23 show the effects of different maintenance methods on the moisture field of the panels.

It was found that these maintenance methods had a significant effect on the moisture field of the pavement panels, and the concrete pavement panels under the above three maintenance methods mainly exhibited the following characteristics:

The humidity level of the concrete pavement panel subjected to geotextile sprinkler curing almost reached saturation (RH = 100%) on the surface of the panel surface during the curing period, and the humidity difference was less than 1%.

The humidity level on the top surface of the road panel subjected to water sprinkling maintenance exhibited a downward trend, while the humidity difference exhibited an upward trend. However, the humidity level was higher, and the humidity difference was smaller, compared to when no pavement maintenance was performed. The decrease in humidity could be attributed to the moisture inside the panel spreading to the air during the interval between two sprinklings. As a result, the second sprinkling could not replenish the moisture lost from inside the panel. The moisture content of pavement panels without any maintenance measures was lower with age and decreased faster than the overall moisture content of pavements with maintenance measures, with higher moisture differences and faster growth rates.

4. Parameter Sensitivity Analysis

Based on previous research results [27], this article conducted sensitivity analysis on various parameters, wherein each model parameter was changed while keeping the other parameters constant. The sensitivity levels were divided into three levels: high, medium, and low, according to the influence of each factor on the humidity calculated by the program, respectively (high: h ≥ 2%, medium: 1 ≤ h < 2, low: h < 1%) and the sensitivity levels of each parameter are summarized in

Table 3. Moisture sensitivity of each parameter to concrete panels analysis.

Parameter Situation	Operating Conditions Take Values	Baseline Working Conditions	Panel Relative Humidity/%			Difference of Humidity at the Top and Bottom of the Board/%
			Peak/%	Difference from Base Value/%	Sensitivity Level	Peak/%	Difference from Base Value/%	Sensitivity Level
Cement type	Type II	Type I	93	0	Low	7	0	Low
	Type III		93	0		7	0
	Type IV		93	0		7	0
	Type V		93	0		7	0
Water to ash ratio	0.33	0.38	92.6	−0.4	Low	7.4	0.4	Low
	0.41		93.1	0.1		6.9	−0.1
	0.45		93.3	0.3		6.7	−0.3
Ambient Humidity	3	7	95.5	2.5	High	4.5	−2.5	High
	9		90	−3		10	3
	12		91.5	−1.5		8.5	1.5
Wind speed	0	2	93.2	0.2	Low	6.8	−0.2	Low
	4		92.8	−0.2		7.2	0.2
	6		92.7	−0.3		7.3	0.3
Solar radiation	300	600	93.7	0.7	Medium	6.3	−0.7	Medium
	900		92.5	−0.5		7.5	0.5
	1200		92	−1		8	1
Panel thickness	0.18	0.24	93.1	0.1	Low	6.9	−0.1	Low
	0.2		93.1	0.1		6.9	−0.1
	0.28		93.1	0.1		6.9	−0.1
	0.30		93.1	0.1		6.9	−0.1
Pavement construction moment	1:00	13	92.8	−0.2	Low	7.2	0.2	Low
	7:00		93	0		7	0
	19:00		93.3	0.3		6.7	−0.3
Conservation methods	Sprinkler	None	95.3	2.3	High	4.7	−2.3	High
	Geotextile + Sprinkler		93	0		7	0

.

The sensitivity analysis results showed that the influence of different parameters on the early age moisture field of concrete varied within a given range of values. The sensitivity of the cement type is only 0, and the sensitivity of water cement ratio is all less than 0.4. The sensitivity level of concrete material factors is low; concrete material factors had little influence on the moisture field of concrete panels. Environmental factors, including temperature, humidity, and solar radiation, significantly influenced panel moisture, while wind speed had a smaller effect. Construction parameters yielded a greater effect on the concrete road panel, while the thickness of the road panel had a smaller influence on the humidity field.

Each parameter had a significant influence on the humidity field of the panel through two aspects, namely indirect and direct influence.

(1) Indirect influence was achieved by affecting the temperature of the panel, which, in turn, raised or lowered the saturated vapor pressure inside the panel, resulting in changes in the relative humidity of the panel. For example, the water-cement ratio, ambient temperature, and solar radiation mainly affected the humidity indirectly through the temperature of the panel, and this effect only had a fluctuation effect on the panel’s humidity. It did not affect the moisture inside the concrete with the average humidity over a period of time.

(2) Direct influence occurred through the internal moisture of the panel directly affected by changes in panel humidity. Factors such as ambient humidity, ambient temperature, maintenance methods, and wind speed could reduce or increase the panel’s humidity and have a direct impact. Among these, the maintenance method and environmental humidity had a direct impact on the panel’s moisture replenishment and loss, while environmental temperature and wind speed had an indirect impact on humidity by affecting the diffusion rate of moisture within the panel.

5. Discussion and Conclusions

5.1. Analysis and Discussion

Based on the above studies, the basic characteristics of the 3D humidity field in the early age period of road panels can be summarized as follows.

(1) The environment significantly influences the surface humidity of the concrete panel. The humidity trend of the concrete panel at 2 cm depth after pouring and completion exhibited a similar pattern to the fluctuation trend of environmental humidity when it reached its lowest value during the early age period. As shown in Figure 24, the trend was not significantly affected by cement hydration in the first three years, but after three days, the fluctuation trend of humidity inside the board was completely consistent with that of environmental humidity.

(2) The humidity at the top of the early cement concrete panel decreased rapidly with age, and the humidity difference at the bottom of the panel was always negative, and this trend increased with age since the top of the panel was the main area of moisture loss while the bottom surface remained mostly saturated (with a humidity of 100%). The surface moisture loss caused the moisture difference between the top and bottom of the panel to remain negative and to gradually increase. This difference will cause horizontal shear stress in the board, which may induce horizontal interlayer peeling disease. Prolonging the moisturizing and curing time can reduce such diseases.

(3) The sensitivity of concrete panel surface humidity to the environment varied depending on the location. Based on simulation and humidity monitoring results, it was found that the corner of the panel was the most sensitive to the environment, followed by the edges, while the middle of the panel was the least sensitive. This conclusion is consistent with other studies [10,28].

The difference in sensitivity of concrete panel surface humidity to the environment could be attributed to variations in humidity on each diffusion surface. Due to diffusion, nodes closer to the surface experienced greater internal and external humidity gradients. The concrete water loss was faster when there was a large humidity gradient and the moisture balance between the concrete and the environment was maintained. This phenomenon was observed in areas with high humidity gradient diffusion surfaces. Specifically, there were three high humidity gradient diffusion surfaces in the corner of the board, two on the edge, and one at 2 cm depth of the plate. The number of high-humidity gradient diffusion surfaces in each position was directly proportional to the balance between the humidity of the concrete and the environmental humidity in that area.

(4) The humidity distribution in road panels was highly heterogeneous, with humidity concentrations in the middle and bottom regions of the panel. During the 28-day period, there was a significant difference in humidity between the top surface of the panel at depths of 2 cm, 4 cm, and 6 cm, and the maximum humidity difference occurred in the middle of the panel (1.3%, 2.3%, and 2.9%, respectively).

5.2. Conclusions

(1) The humidity at the top of the early cement concrete panel decreased rapidly with age, and the difference in humidity at the top and bottom of the panel was negative during the early stages, with a tendency to increase with age. This difference would cause horizontal shear stresses in the slab, which may induce horizontal interlaminar spalling disease; prolonged moisturizing and recuperating time may reduce such diseases.

(2) The factors affecting the early age humidity of the panel can be categorized into two aspects: temperature influence, which alters the saturated vapor pressure inside the panel to cause a change in relative humidity, and direct impact on the moisture inside the panel to cause a change in humidity. The parameters that directly hydrate or lose water to the panel significantly affect the panel’s humidity.

(3) The maintenance mode and construction season have the most significant impact on the humidity field of the panel, followed by environmental and meteorological conditions such as solar radiation and wind speed. Therefore, during construction, if it is possible to avoid the hot season, choose a period of increasing environmental humidity to pour concrete, and adopt appropriate curing methods to reduce the humidity gradient inside the board, reduce the humidity stress of the panel, reduce early age deterioration, and improve its service life.