Fundamentals of plastic settlement cracking in concrete

The durability of flat concrete elements such as floor slabs and bridge decks remains a concern. One of the main causes of durability issues is the corrosion of reinforcing steel in the concrete. Since the presence of cracks significantly accelerates the corrosion process it is of utmost importance to prevent cracking if the expected service life of concrete is to be reached. This paper investigates the crack formation due to plastic settlement, since plastic settlement cracking is the earliest form of cracking in concrete it arguably has one of the highest potentials to reduce the service life of a concrete structure. Plastic settlement cracking occurs once the concrete has been cast up to the initial setting time which indicates that the concrete is no longer in a plastic state. The mechanism of cracking is differential settlement of the concrete due to vertical restraints in the form of imbedded reinforcing steel or a non-uniform depth. Once these cracks have formed they act as locations for further crack growth due to volume changes such as drying shrinkage. Although plastic settlement cracks can be mitigated through the re-vibration or the sequential placement of the concrete, both methods are practically difficult to implement for large concrete pours. Due to the early occurrence and difficulty associated with mitigating these cracks it is imperative to fundamentally understand the behaviour of these cracks, if they are to be prevented. The results identified three distinctly different defects due to plastic settlement cracking described as follows: 1) Shear cracks starting from the bottom upwards at an angle at both the left and right outermost points of the reinforcing steel bar; 2) Tensile cracks starting from the surface downward; 3) Water pockets beneath the reinforcing steel bar. The results also showed that the severity of the cracking increases with a decrease in cover depth and spacing of reinforcing steel. The shear cracks tend to approach the behavior of tensile cracks as cover depth decreases.