Investigators of cracking problems in concrete structures have from time to time noticed horizontal and vertical cracks in concrete columns. How does this happen when cracking indicates tension in the concrete whereas columns usually are in compression? Ruling out pure compressive overload, there are three possible explanations. First, the column may in fact be in tension due to an isolated settlement of the foundation. In this case, the column could be "hanging" from the floor system above, resulting in tensile cracking. Next, if the column is the end or exterior one in a structural bent or frame, it may have high enough bending to cause tensile forces in the outside face. This condition would produce horizontal cracks which would disappear on the inside face. The third and most likely explanation has to do with creep and shrinkage.
What really happens in a reinforced concrete column is that creep causes the concrete to unload its compressive stress and transfer it to the reinforcing steel. In other words, a redistribution of stress takes place which is directly proportional to the amount of reinforcing steel in the column the higher the percentage of steel, the more load the steel carries so that with about 8 percent reinforcement, the steel carries almost 100 percent of the load. With about 4 percent steel, the load distribution is approximately 90 percent on the bars and only 10 percent in the concrete. This explains why cracking can occur in the concrete "shell." Due to the unloading, there is little compressive stress left in the concrete to compensate for or overcome shrinkage stress or other factors such as temperature which can subject the columns to tension.