Flexural Strength of Paving: Why It Matters for Slabs and Flags
Flexural strength is particularly important when we are looking at paving slabs, flags, natural stone and porcelain because these materials are subjected to bending forces, not just compression.
What Exactly is Flexural Strength?
Flexural strength is the ability of a material to resist bending before it cracks or breaks. It is normally measured in megapascals (MPa), which is the same as N/mm².
When a paving slab is tested for flexural strength, it is supported and a load is applied to it. As the slab starts to bend, one face of the paving is placed into compression while the opposite face is placed into tension.
This is important because materials such as natural stone, concrete and porcelain are generally much stronger in compression than they are in tension. Therefore, when a paving slab fails under bending, the crack will normally start on the face that is under tension.
Why is this Important When Laying Paving?
In the real world, a paving slab is rarely supported perfectly and uniformly across its entire underside. There may be slight variations in the mortar or bedding layer, small voids beneath the paving, movement or settlement within the construction, or concentrated loads from pedestrians, garden furniture or vehicle wheels.
If part of the slab is not properly supported, the paving can effectively behave like a small beam spanning across the weaker or unsupported area.
The higher the flexural strength of the paving, the greater the bending stress it can withstand before it cracks or fractures.
Thickness is also extremely important.
Two paving slabs manufactured from the same material could have the same flexural strength when expressed in MPa, but a thicker slab can carry a substantially greater load because its section has much greater resistance to bending.
This is why we should not look at flexural strength in isolation. The strength of the material, thickness of the paving, dimensions of the unit, quality of the bedding and the loads it will receive all need to be considered.
A simple way of remembering the difference between compressive strength and flexural strength is this:
Compressive strength is the material’s resistance to being crushed.
Flexural strength is the material’s resistance to breaking when it is bent.
For paving slabs and flags, flexural performance can therefore be far more relevant to whether an individual paving unit will crack in service than simply quoting its compressive strength.
