A re-entrant corner is an internal corner of a concrete slab. Depending on prevalent conditions, such a corner may be subject to cracking and often requires some reinforcement. In this article, we will review the potential problems re-entrant corners pose, conditions for cracking, design solutions, and repair solutions for existing cracks.
Potential Problems of Re-entrant Corners
The fundamental problem of re-entrant corners in a structure is the susceptibility to cracking. After the initiation of a crack, it forms a point of stress concentration on the structure. Further loading heightens the possibility of more cracks and their penetration through the concrete. Some of the resultant problems other than cracks include:
- Unpredictable loading: The presence of cracks from re-entrant corners means that the loading on the concrete structure differs from the initial design. Also, as the cracks spread, they weaken several parts of the structure, resulting in unexpected failure.
- Susceptibility to damage from seismic loads: Structures with re-entrant corners are highly susceptible to damage from earthquakes. This is because their presence heightens the stress concentrations and torsional forces on the structure. Thus, common building practice avoids avoiding such corners in building designs, especially in regions that experience earthquakes.
Conditions for Re-entrant Corner Cracking
Although all re-entrant corners are internal corners – angles of 180° or less – there are other conditions that encourage crack initiation and propagation.
Plan Setback Geometry
For an internal corner to qualify as a re-entrant corner, its plan setback should be above 15% in both directions, as the figure below shows.
Thus, a building having this geometry increases the potential for damage, as each wing exhibits different dynamic properties. Moreover, the vibration period and subsequent displacement of each wing will be different, with larger magnitudes for the wing with less stiffness. As a result, the torsional effect can cause damage to the end columns and floor diaphragms at the junction of the wings. Generally, buildings of ‘L’, ‘T’ and ‘H’ shape, as the figure below contains, are prone to having re-entrant corners.
Shrinkage Due to Curing
When concrete slabs undergo the curing process, their volume shrinks as the chemical reaction of cement and water takes place. If the curing method is not appropriate, the top and bottom of surface of the slab will cure at varying rates. Thus, inducing stress and initiating cracks on the slab.
Concrete increases in volume with increasing temperature, and shrinks as temperatures drop. With restraints such as friction at the bottom of the slab, piers, and stiffening ribs, the alternating expansion and shrinkage process induces tension in the slab. Therefore, cracks initiate and progress.
Design Solutions for Re-entrant Corners
In practice, buildings or any structure should avoid having re-entrant corners. If this is unavoidable, engineers must make provisions to avoid the consequences of their presence. This section contains a review of some of these provisions.
Separation of Structures with Re-entrant Corners
The simplest solution to the re-entrant irregularity is to separate the structure at the notches. Thus, it converts them into smaller blocks of regular configurations. As a result, this eliminates all stress concentration and crack risks. In addition, there should be sufficient clearance between the blocks to avoid pounding effects during earthquakes.
Strengthening of Structures
For existing structures, where separation is not an option, strengthening the region of the re-entrant corner is a viable option. To achieve this, engineers may specify stiffening elements such as bracings or shear walls . Retrofitting these elements into the design can go a long way to ensuring that the building is structurally sound.
Lessening the Stress Concentration
Rather than using right-angle re-entrant corners on a structure, splay corners can serve. Consequently, this widens the internal angle, and reduces the stress concentration significantly.
In the case where there is already crack formation on a re-entrant corner, some measures can be taken to prevent further spread.
Builders embed these short steel rods into concrete slabs to stabilize the joint with other slabs. Moreover, the presence of rebars at re-entrant corners obstructs the propagation of cracks through the concrete. So, to achieve this, the process involves drilling holes into the concrete and pumping in epoxy, before placing the rebars.
Placing contraction joints at re-entrant corners creates intentional weak planes in the concrete slab. Thus, cracking occurs in a planned rather than an uncontrolled manner. Forming these joints requires tooling them while the concrete is still setting. If the concrete has already formed, a dry-cut saw or wet-cut saw can cut the joints.