# Behavior of Airport Concrete Pavement Slabs
## 4 Temperature and Moisture Effects of Slab Behavior
### Real Strain of the Concrete Slab  
The real strain calculated using Equation (2) reflected the slab behavior caused by the environmental loading constrained by friction with the sublayer, the self-weight of the slab, dowel bars, and other factors. The trend of variations in the real strain was similar to that of variations in the concrete temperature at the same depths, as shown in Figure 10. The correlation between temperature and real strain was examined according to the slab depth to analyze the effects of temperature. Temperature predominantly affected the real strain for the first seven days and exhibited a linear correlation with \(R^{2}>0.8\) at every slab depth, as shown in Figure 11. The \(R^{2}\) at a depth of 50 mm was 0.8555, and it increased to 0.9741 at a depth of 450 mm. Therefore, factors other than temperature affected the real strain near top of the slab, such as the drying shrinkage [28].  
Figure 9: Ambient and concrete temperature for the first seven days.  
Figure 10: \(Cont.\)  
Figure 11: \(Cont.\)  
Figure 10: Concrete temperature and real strain according to slab position for the first seven days: (a) 50 mm depth, (b) 150 mm depth, (c) 250 mm depth, (d) 350 mm depth, and (e) 450 mm depth.  
Figure 11: \(Cont.\)  
The real strain measured at the corners, mid-edge, and center of the concrete slab were compared according to the slab depth, as shown in Figure 12. The expansion strain increased in the daytime when the ambient temperature was high, whereas the contraction strain increased at night when the ambient temperature was low, as reported elsewhere [7, 8]. The variations in the daily expansion and contraction strain cycles were more evident near the top of the slab, where they were affected directly by the ambient temperature. Accordingly, downward curling occurred in the daytime when the top of the slab is lower than the bottom of the slab.  
the slab expanded more than the bottom, and upward curling occurred at night when the top of the slab contracted more than the bottom, as shown in Figure 13.  
Figure 12: \(Cont.\)  
Figure 12: Real strain according to the slab depth for the first seven days: (**a**) corner, (**b**) mid-edge, and (**c**) center.  
Figure 13: _Cont._  
Immediately after the concrete was placed, the expansion strain increased at all positions and depths because of the heat of hydration of the concrete, as shown in Figure 10. The contraction strain increased at all positions and depths as the heat of hydration disappeared and the ambient temperature decreased. Figure 12b represents a conceptual diagram of the horizontal expansion and contraction of the slab due to the generation and disappearance of the heat of hydration and the decrease in ambient temperature. The expansion strain that occurred at the center of the slab due to the heat of hydration was approximately 50% smaller than those at the mid-edge and corner. The contraction strain caused by the disappearance of the heat of hydration and decrease in ambient temperature increased near the top of the slab at the corner, where the upward curling was the most evident. The contraction strain was similar throughout the slab depth at the mid-edge and center.