Maximizing Concrete Block Wall Strength for Hurricanes and Tornadoes

Concrete block walls, built right, offer one of the best wind resistances available for residential construction.

When it comes to protecting buildings from the devastating wind forces generated by hurricanes and tornadoes, concrete masonry might not top your list of super-strong envelopes. But the research shows that if you prepare them properly they outperform wood, poured concrete and unreinforced block. 

As I’ll spell out in more detail in this post, this means filling cavities in the blocks with the recommended psi of the filler, using the right mortar, and properly sizing and placing rebar. 

Importance of Filling Cavities in Blocks:

Filling cavities in concrete blocks has a big impact on the overall strength and stability of the wall. The cavities are typically filled with a high-strength grout or concrete mix, often including “pea gravel.” 

This filling process ensures that the blocks act as a solid unit, reducing the risk of failure under intense wind forces. It also prevents the accumulation of water and debris within the cavities, which could lead to invisible weaknesses and additional damage.

Recommended PSI of Filler:

To achieve optimal results, use a filler with a compressive strength of at least 3,000 pounds per square inch (psi). This high-strength filler enhances the overall structural integrity of the concrete wall, and, when used with rebar reinforcement, gives the wall a much higher resistance to tension.

Type of Mortar:

Choosing the right type of mortar is essential for connecting the concrete blocks together securely. For reinforcing walls against wind forces, a Type S mortar is often recommended. Type S mortar contains a higher proportion of cement compared to other mortar types, providing increased bond strength and durability.

Rebar Size and Placement:

Reinforcing bars, known as rebar, play a crucial role in increasing the tension strength of concrete walls. For an 8 ft. tall, 12 ft. wide wall, for instance, a minimum of two horizontal rows of rebar should be used. 

The rebar should have a minimum diameter of ½ inch. One row of rebar is placed horizontally at the bottom of the wall, near the foundation, while the second row is placed horizontally at the top of the wall. 

Additionally, vertical rebar should be installed at regular intervals along the height of the wall, typically spaced 4 to 6 feet apart.

The table above provides a comparison between an 8 ft. tall, 12 ft. wide wall built with reinforcement and one without reinforcement, including the difference in tension strength and approximate wind speed at failure

Beyond the Blocks

Concrete block walls, built right, offer one of the best wind resistances available for residential construction. Filling cavities in the blocks with high-strength grout or concrete mix, using the recommended psi of filler, and employing Type S mortar for strong connections are all vital aspects of reinforcing the wall. It’s also key to connect the whole wall via steel to the concrete footers.

Of course, a house is only as hurricane-proof as its weakest link. For fortress-like strength, follow local building codes in high risk areas, installing impact-resistant windows and doors and secure roofing systems. 

A reinforced concrete block home envelope, when all the rebar, mortar and labor are added up, will probably cost about as much as a wood-framed home. But it will be a little more wind resistant, and a lot easier to clean up if the property is inundated with flooding but survives the ordeal.

Bad News for Old Block Homes

What about existing block homes? Will they withstand a hurricane blast? Maybe not. As McGarry and Madsen point out: 

“Older concrete block homes, especially ones from the 1960s and earlier, do not have this strong connection between the tie beam and the foundation. While the suction created under the roof during a storm is pulling upward on the roof structure, it is also tugging violently on the tie beam via the truss straps into it. That uplift on the tie beam opens up cracks in the weaker horizontal mortar joints in the blocks directly below the beam, causing the structural connection from the beam to the blocks below to eventually fail. Without the steel-reinforced connection between tie beam and foundation found in post-2002 homes, the walls must face the lateral wind loads as separate components and on their own. At that point, only their weight is resisting the wind and storm surge pressure.”

That being said, it’s theoretically possible to retrofit an unreinforced block structure. You could drill and cut the existing blocks and install rebar, a costly and messy job. Here's a highly detailed step by step set of instructions on how to accomplish this.

Reinforcing an older block home is a labor intensive process. Source

A less costly, albeit somewhat less effective approach, may be to begin with securing openings and weak points in the shell, such as windows and doors. If wind can’t get indoors to produce powerful uplift, it may not have a chance to “test” the connection between blocks and roof.