Posted on November 12, 2020
A crack is simply when the concrete matrix, or the “glue” for the rocks, cannot hold bond to itself, or the aggregate in the mix, due to volume changes, over-loading, and/or repeated loading. This creates a failure plane that extends to the exterior of the concrete that we can see in the form of a crack. The highest tensile stress at which concrete does not crack is the modulus of rupture, which is dependent upon the concretes compressive strength.
The crack location and shape can give clues as to its cause, as we could see in figure 1. The widest point of the crack usually indicates where the crack originated. This is the point of highest tensile stress. One can imagine which way to twist, push, pull, and bend a piece of concrete to start a crack at that point. Figure 2 has a crack that is widest at the bottom of the member, and narrows as it propagates towards the top of the member. Since the crack is at its widest at the bottom, then that is where the crack and the highest tensile stress originated. If we pushed the piece down too far, it could cause this flexural crack. In this case, the center of the trailer sagged, and the deflection of the member at midspan (the location of the crack) overloaded it in flexure. This created a tensile stress more than the member and strand could handle, thus creating a crack.
A crack in concrete may not necessarily mean imminent failure. Sometimes, it is supposed to crack, in order to engage the tensile steel. In figure 3, there are 3-½” 270 ksi Lo-Lax 7 wire pre-stressed strand crossing the crack, at about 2” from the bottom of the beam, each pulled to 31,000 lbs. The crack initiates under a specified loading and engages the steel. The steel holds the crack together using its tensile strength. Once the steel meets its yielding point, it begins to stretch. As the steel stretches, the crack propagates further towards the top of the beam. If these cracks were not part of the intended design, then this gives us a clue that something may be wrong. The beam in figure 3 formed a crack very close to the load expected, 20,000 lbs. (10,000 lbs. each cylinder). Once the crack engages the strand, it will propagate beyond the strand, stretching the strand, while take a loading up to 32,000 lbs., the expected failure point, i.e. strand breakage. The picture is loaded at approximately 28,000 lbs., to give you a feel for how close it is to failure. This beam cracked, took more loading, and then failed, as intended. Therefore, in this pre-stressed beam, the first indication of cracking was far from imminent failure.
Many mechanisms produce concrete cracks, some accidental, some on purpose. Once we are familiar with those mechanisms, we can determine the cause of the crack merely by its location and shape.
Concrete can be designed to crack, and still hold load. Here lies the beauty of precast pre-stressed concrete. We can customize our number of strand to a certain pattern, which adds compression, and enables us to design members as uncracked. This allows shallower members than cast in place members designed with mild reinforcement only. It also allows us to design members to take the full design service loads and remain uncracked.
Dustin Jones P.E.