# Seismic Performance of Reinforced Concrete Short Columns Subjected to Freeze-Thaw Cycles
## 3 Test Results and Analysis  
### Damage Process and Failure Pattern  
Figure 7 shows the observed failure pattern of the three columns at the end of each testing. Although all of the columns exhibited flexural yielding followed by shear failure, the increment of freeze-thaw damage shifted the specimen response from flexure-dominated to shear-dominated failure.  
For the control specimen ZD-0, the horizontal cracks with a length of 20 mm to 50 mm began at the bottom of the column when loaded to a drift of 0.3%. As the drift level increased, the number of horizontal cracks, as well as the length of the cracks, increased. Then, the longitudinal bar yielded and shortly after that horizontal cracks developed obliquely with an angle of 60\({}^{\circ}\). A few inclined cracks were concentrated and intersected each other in the column end. When the applied lateral load reached the maximum, vertical cracks appeared in the compressive zone at the bottom corner of the column, and concrete crushed slightly at the bottom corners, i.e., the column was entering the failure stage. After that, the lateral load declined gradually. Steeply inclined cracks initiated at the upper portion of the column. With the continuing reversed drift, new cracks ceased to appear while previously formed inclined cracks widened and some of them propagated along the column height. At the same time, the lateral load declined gradually. Finally, a small portion of cover concrete spalling along the diagonal compression strut and well-developed inclined cracks indicated the failure of the tested column, as shown in Figure 7. The whole damage process was characterized as a typical flexure-shear failure.  
Figure 5: Test setup.  
Figure 6: Loading protocol.  
Compared with the sound specimen ZD-0, there are some differences in the damage process of the specimens subjected to FTCs. For the specimen ZD-1, both the distribution of initial cracks and the sequence of cracks' appearance were similar to the specimen ZD-0 until the vertical cracks appeared a little earlier at a drift amplitude of 1.44%. Then, flaking of concrete cover happened in the bottom region and in the vicinity of the crossed cracks. When the test was terminated, the core concrete crushed and the spalling region was relatively large. The reason behind these changes could be related to the effect of FTCs on the concrete material. As previously shown in Figure 3, micro cracks happened in the interface of the concrete after certain numbers of FTCs, resulting in a floccule-like appearance, and consequently, the freeze-thaw damaged concrete became loose.  
For the specimen ZD-2, lengthwise visible cracks were detected at the early stage of the loading due to the combination of initial flaws induced by increasing FTCs and reversed loading action. Considering the freeze-thaw process, the expanded volume of water to ice placed constant stress inside the concrete body, which caused the initiation and propagation of inner creep cracks as illustrated before. The initial horizontal cracking, located at the column end, occurred at a lower force than the other two specimens since the freeze-thaw attack led to apparent decay in concrete tensile stress. The distribution of the bending cracks was parallel to the undamaged specimen. However, two diagonal cracks formed through the entire height of the column when the imposed drift value was 0.84%. Significantly, one of the cracks widened rapidly, indicating shear failure. The maximum width of the diagonal cracks was up to 2.5 mm before the column reached the peak load. After that, the lateral resistance dropped, evidently with the continued cycling, more than the other two specimens. The phenomenon of continued spalling of the cover concrete occured at the onset of the second cycle at the applied drift of 1.44%, and covered a large portion of the specimen surface at the failure stage, which was similar to specimen ZD-1.