Engineering
Influence of Aggregate Interlock on Shear Transfer in Cracked Concrete
Quick fact
The shear transfer capacity of cracked concrete through aggregate interlock can be more than half of the uncracked concrete's shear strength, even for crack widths of 0.5 mm.
Why this is interesting
Imagine a cracked concrete beam still carrying heavy loads. How can a material that is already broken continue to transfer forces across the crack? The secret lies in the microscopic roughness of the crack surfaces.
Read the full explanation
Understanding Influence of Aggregate Interlock on Shear Transfer in Cracked Concrete
When a concrete beam is subjected to shear, it can develop diagonal cracks. At first, the crack is very fine, and the two faces of the crack are still in contact. The surfaces are not smooth; they are rough due to the aggregate particles that fracture through or around them. When one face of the crack tries to slide past the other, the protruding aggregates on one side catch against the depressions on the other side. This is akin to trying to slide two pieces of coarse sandpaper against each other—they resist sliding. This resistance is called aggregate interlock. It effectively transfers shear forces across the crack, preventing the two parts of the beam from moving apart. The wider the crack, the less contact there is, and the weaker this interlock becomes. That's why controlling crack widths is crucial in reinforced concrete design.
A deeper explanation
The mechanism of aggregate interlock is based on the interaction of rough crack surfaces. As shear displacement occurs, the inclined faces of the aggregate particles generate a force that has two components: one normal to the crack (causing the crack to open) and one parallel to it (resisting the shear). This is similar to riding a bike over a rough gravel path—the stones push you upward and also slow you down. The effectiveness of aggregate interlock depends on several factors: the roughness of the crack (which is related to the maximum aggregate size, since larger aggregates create rougher surfaces), the compressive strength of the concrete, and the crack width. The narrower the crack, the more effective the interlock. This mechanism is a central part of the shear transfer in reinforced concrete, and it is why many design codes allow for concrete to carry a certain amount of shear even after cracking. In fact, a significant portion of the shear capacity of a beam without shear reinforcement comes from aggregate interlock. Understanding this helps engineers predict when a beam might fail and how to provide adequate shear reinforcement when the interlock alone is not sufficient.