# Meso-Scale Failure of Freezing-Thawing Damage of Concrete under Uniaxial Compression
## 2 Experimental Materials and Methods
### Uniaxial Compression Test of Concrete Damaged by Freezing-Thawing  
With the goal of determining the uniaxial compression failure mode and stress-strain curve of concrete damaged by freezing-thawing, 7 groups of specimens were tested under uniaxial compression on a microprocessor-controlled electro-hydraulic servo pressure tester. According to The Test Code for Hydraulic Concrete [21], the average value of peak stress of three specimens was taken as the compressive strength of the concrete specimens. During the test, the concrete specimen was preloaded at 2 KN, and then the lifting speed of the testing machine was set to 0.5 mm/s until the strain reached \(3.0\times 10^{-3}\), then the test stopped. The failure modes and stress-strain curves of freezing-thawing-damaged concrete under uniaxial compression were obtained as shown in Figures 3 and 4. The peak stress-strain curves are shown in Figure 5.  
Figures 3 and 4 show that the severity of the uniaxial compression damage to concrete increased with the increase in the number of freezing-thawing cycles. When the concrete specimens without freezing-thawing were damaged, diagonal cracks mainly occurred. However, with the increase in  
Figure 2: (**a**) Damaged layer distribution of freezing–thawing concrete section and (**b**) the curve of relationship between the number of freezing–thawing cycles and damage thickness.  
freezing-thawing cycles, concrete became more brittle and the risk of bursting became less serious. With 25 freezing-thawing cycles, the diagonal main cracks deepened and large breaks in flakes or bulk along the direction of the main cracks occurred. The situation became more obvious with the increase in the number of freezing-thawing cycles. So when the specimen was damaged, many vertical main cracks usually occurred. With increasing freezing-thawing cycles, the micro-cracks inside the concrete gradually increased [5], which further deformed the specimen transversely under compression, resulting in the vertical cracking of concrete for its transverse tension. With increasing the compressive load, the change law of stress-strain curve of freezing-thawing-damaged concrete under uniaxial compression was similar to the one shown above (Figure 4), with a linear elastic stage, pre-peak plastic stage, peak point, and post-peak decline stage. In the linear elastic stage, the elastic modulus decreased with the increase in the number of freezing-thawing cycles. The peak stress decreased with the increase in the number of freezing-thawing cycles while the peak strain increased with the increase in the number of freezing-thawing cycles. This occurred because the internal micro-structure of concrete specimens was damaged after freezing-thawing cycles. With the increase in the number of freezing-thawing cycles, the internal damage of concrete developed continuously, leading to a change in the concrete constitutive behavior.  
To study the change in the axial dimensions of concrete specimens before and after uniaxial compression under increasing number of freezing-thawing cycles, the axial height of specimens before and after compression was measured using Vernier calipers. Each specimen was measured three times in the axial direction, and the average value was used as the axial compression deformation of specimens. In each group, we measured three different specimens to determine the size change of  
Figure 4: Stress–strain curve of concrete under different numbers of freezing–thawing cycles.  
Figure 3: Failure modes of concrete under uniaxial compression with different numbers of freezing–thawing cycles.  
concrete specimens after uniaxial compression under different freezing-thawing cycles, as shown in Table 2.