Traditional concrete cavity walls

Concrete masonry cavity walls—particularly the mid-century type discussed in NCSEA/STRUCTURE—are a bit sneaky: they can look like modern brick veneer walls from the outside, but they do not behave the same structurally.

What an early cavity wall is

A common early cavity wall assembly consists of:

• an interior unreinforced concrete masonry unit (CMU) wythe, often 4 inches thick, sometimes 6 or 8 inches,

• an air gap,

• an exterior brick masonry wythe,

• and horizontal wire reinforcement embedded in the masonry bed joints tying the two wythes together.

These walls were commonly used in brick-clad buildings constructed from the 1950s through the 1970s.

How they were originally designed

These walls were often designed using empirical span-to-thickness requirements that considered:

• the combined brick and CMU thickness,

• minimum masonry material properties,

• and the gauge and spacing of the wire reinforcement connecting the wythes.

Common original features included:

• brick relieving angles at each story,

• very narrow or nonexistent horizontal expansion joints below those angles,

• limited water-management elements such as flashings and weeps at relieving angles and window heads,

• and generally no continuous air, water, vapor, or thermal barriers.

Connections from the CMU wythe to the building frame might have included:

• reinforcing dowels,

• dovetail anchors,

• or mortared bed joints,

but in some buildings these connections were omitted during construction.

How they behave structurally

This is the key point: unlike modern brick veneer construction, the brick and CMU wythes in these cavity walls resist out-of-plane lateral loads together as a combined assembly.

The horizontal joint reinforcement:

• ties the wythes together,

• distributes load between them,

• but usually is not stiff enough to make the wall fully composite in flexure.

So the load carried by each wythe depends on its relative stiffness.

At the wall supports, these systems were generally intended to transfer shear through the CMU wythe to the building frame, but the real load path can vary depending on how each wythe is actually attached.

Common problems

Three recurring issues are highlighted:

1. Distress from restrained volume change

Symptoms can include:

• cracks,

• spalls,

• bulges in the brick wythe.

Why it happens:

• brick absorbs moisture over time and undergoes irreversible growth,

• brick also expands and contracts with temperature changes,

• if horizontal expansion joints are too narrow or missing, the structure restrains that movement.

A common repair is to introduce properly sized horizontal expansion joints below relieving angles.

2. Corroded horizontal joint reinforcement

Symptoms can include:

• cracked or spalled mortar joints,

• corrosion staining on the wall face.

Why it happens:

• moisture and oxygen promote corrosion,

• poor water management increases susceptibility,

• rust expands, creating stresses that crack mortar and spall joints,

• long-term corrosion can reduce steel section and compromise the reinforcement.

Repairs may require localized or widespread brick removal to repair or replace the corroded reinforcement.

3. Envelope performance issues

Common concerns include:

• leakage to the interior,

• water dwelling in the cavity,

• missing or deficient flashings,

• poor thermal performance,

• lack of air barrier,

• occupant comfort issues.

Sometimes repairs are local, such as replacing brick near relieving angles or windows and adding flashing or weeps. In more extensive work, the entire brick wythe may be removed and replaced.

Why modifications can create structural issues

This is where engineers have to be careful: repairs that seem purely architectural or envelope-related can change the wall’s structural behavior.

Two major examples:

Adding horizontal expansion joints

This improves durability, but it can also alter the wall’s load path.

Without adequate joints, brick may have built-up compression from restrained expansion. That precompression can:

• reduce net flexural tension under out-of-plane loading,

• increase frictional resistance at connections.

If a new expansion joint is added, that precompression is released, which can:

• increase net flexural tensile stress,

• alter how shear is transferred to the building structure.

Some empirical cavity walls may have been relying on that precompression to help resist lateral loads.

Cutting embedded horizontal joint reinforcement

When major modernization is done—such as adding new waterproofing, air barrier, or insulation—the existing embedded joint reinforcement is often cut at the CMU face and replaced with post-installed brick ties.

That changes structural behavior because:

• post-installed brick ties do not behave the same as embedded joint reinforcement,

• load sharing between the brick and CMU wythes changes,

• the wall no longer conforms to the original empirical cavity wall assumptions.

When structural evaluation is needed

Under the 2018 IEBC definitions discussed in the article:

• repairs are maintenance/correction work,

• alterations are construction or renovation other than repair or addition.

Introducing expansion joints or cutting embedded joint reinforcement changes structural behavior, so these actions should be considered alterations.

For alterations, the IEBC requires lateral-load-resisting elements that experience more than a 10% stress increase to be analyzed for current code loads and strengthened if necessary.

Evaluation approach

If the designer cannot show that the modifications increase stress by no more than 10%, the modified wall should be analyzed for current code-prescribed lateral loads.

Important considerations mentioned:

• early cavity walls often use unreinforced CMU 8 inches thick or less,

• positive anchorage to the building structure is often lacking,

• material properties may need to be estimated using standards contemporary to original construction.

If analysis shows the wall is inadequate, a testing program may sometimes justify higher material properties, but that can be costly and may still not avoid strengthening.

Strengthening options

Several approaches are described:

Fiber reinforced polymer strips

• applied to the CMU wythe,

• generally required on both interior and exterior faces to resist out-of-plane loading in both directions,

• may also require new top and/or bottom wall attachments.

Where post-installed fasteners are used, in-situ testing is considered prudent because modern fastener tables may not reflect existing materials and workmanship.

Internal reinforcing bars

• open CMU cells,

• install vertical reinforcing bars,

• grout cells solid.

This can be done from either side of the wall and may include direct connection to the building structure or added wall attachments.

New cold-formed metal framed backup wall

• install a new interior backup wall,

• design it stiff enough to receive out-of-plane loads from the existing wall,

• connect it adequately to the CMU and building structure.

The article notes that using at least an L/600 deflection limit helps limit masonry cracking as load transfers occur.

This approach reduces occupiable floor area somewhat but creates a functionally new backup wall.

Wall replacement or overcladding

If strengthening is not preferred:

• demolish and replace the cavity wall, or

• leave it in place behind a new overclad wall.

These options allow broader rethinking of aesthetics and enclosure performance, but require review of the existing structure and foundations for support of the new wall system.

Practical takeaway

For older concrete masonry cavity walls, the big lesson is:

What looks like a simple facade or envelope repair may actually change the wall’s structural load path.

That means owners, contractors, and engineers should be especially cautious when:

• adding expansion joints,

• replacing brick extensively,

• cutting embedded reinforcement,

• adding insulation, waterproofing, or air barriers,

• or trying to modernize performance to meet current expectations.

A cavity wall may be part wall, part cladding, part historical surprise package.

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