What Is the Load Bearing Capacity of Different Sized Concrete Blocks?

The physical properties of building materials determine how and why you use them. Take, for example, concrete blocks. These blocks assume numerous sizes and shapes, though all exhibit a high degree of strength relative to their size. Because of this, they commonly appear in load bearing operations such as support columns, footings, walls and foundations. Understanding the load bearing capacity of concrete blocks requires a full overview of the factors affecting their strength.

Woman sitting on sculpture of ball
credit: Polka Dot Images/Polka Dot/Getty Images
Not all concrete blocks look like cinderblocks.

The Problem

Determining the load bearing capacity of a concrete block proves more or less impossible without context. Concrete blocks come in myriad sizes, numerous shapes and contain a variety of ingredients. In addition to material considerations, the structural context of a concrete block heavily affects its load bearing capacity. To determine the load bearing capacity of a block, which can run a gamut of hundreds, if not thousands, of pounds, you must test the actual block in its context.

Structural Context

A number of structural factors affect how much weight a concrete block can hold. For instance, weight applied to a concentrated point on a block via a support column exhibits greater stress than weight spread over a wide area. Because of this, a block used to support a column can bear less weight than the same block used to support a slab. Furthermore, blocks in a weight distribution network can support more weight than those used individually as load bearing units. For instance, a wall of blocks, particularly one reinforced with steel beams, can bear heavier loads than the sum load bearing capacity of each individual block.

Material Considerations

The material used in the construction of a concrete block affects its load bearing capacity. For instance, a 10-by-8-by-16-inch concrete block made with stone dust may support more weight than a 15-by-12-by-24-inch block made with sand, because stone dust exhibits greater weight and strength than sand. Also, some concrete blocks exhibit holes, while others comprise solid slabs of material. A large block with large holes can bear less weight than a smaller block with no holes. A large block distributes weight over a great area, however, and potentially serves as a better means of weight distribution, despite its load bearing capacity.

Standards

As per standards published in 2003 by ASTM, an international building standards non-profit, all concrete blocks must support at least 1,700 pounds of weight per square inch (PSI). This standard only describes minimum requirements, however. A small block may support more weight than a large block if it exhibits a high enough pounds per square inch load bearing capacity. Determining a PSI rating requires applying weight to a block until it cracks and dividing the pressure that made the block break by the total square inches in the block. This is more complicated than it seems because three-dimensional shapes, such as concrete blocks, measure in cubed, not square, units.