If you have ever looked around your home and spotted horizontal or vertical cracks in stem walls and wondered about them, this is the post that can help you.

In this article, we’ll explain:

• What causes them?
• Are they serious?
• How can they be fixed?
• Will they come back again?
• What the heck is a stem wall?

As you can see, these cracks start out looking very innocuous and, over time, can result in spalling of the concrete and eventual complete destruction.

Let’s first skip to question 5. 

What the heck is a stem wall?

Below is a typical house frame. The Trusses on top spread all the weight to the outside walls. Interior walls typically do not support any roof loads. The weight of those walls goes down and is supported by the stem walls and footings.

What causes them?

Steel rebar and other steel embedments have been used in concrete for almost 200 years… And generally, they perform pretty well. So what is the deal? Something does not add up. I have gone into neighborhoods and literally seen every house with deteriorating stem walls. Why is this different?

It is the triple-headed dragon of chlorides, sulfates, and carbonation. They act as a multiplier effect on each other. Separately, they can still be a problem, but it takes many years to become noticeable… sometimes longer than the life of the structure. But together in a wet environment, they amplify their effects on each other, causing the concrete to last only a few years. 

Chlorides

Iron and steel embedded in concrete are normally protected by a passive film — a thin layer of magnetite and hematite that forms spontaneously in the highly alkaline pore solution at pH 12.5–13.5. This film is stable and self-repairing as long as alkalinity is maintained. This film is only 2–10 nanometers thick (10,000 times smaller than a hair).

Chloride ions attack this thin film  protection through a specific electrochemical sequence: 

Once depassivation occurs at a local site, a micro-electrochemical cell forms between the active pit and the surrounding still-passive film-protected steel.

The critical variable is whether the chloride concentration at the steel surface reaches the threshold concentration of approximately 0.2–0.6% by cement weight before other protective mechanisms fail (although this can very widely depending on conditions). Once chloride concentration exceeds the threshold, Cl⁻ ions competitively adsorb onto the passive film surface, attacking it at weak points — grain boundaries, surface defects, mill scale discontinuities. The mechanism is electrochemical. [1]

Once depassivation occurs at a local site, the micro-electrochemical cell forms between the active pit and the surrounding still-passive steel. The electrochemical current is 0.1 μA/cm² to 10 μA/cm² in more advanced cases.

Carbonation

Carbonation affects concrete by lowering the pH level of concrete from high alkaline levels of 12.5 to 13.5 to 9 or below, which is considered acidic. This destroys the passive film barrier on the steel and initiates corrosion regardless of whether the chloride threshold has been met. It also simultaneously releases salts (chlorides) that would typically be bound up in the concrete. Thus, this multiplier effect from previous chloride exposure suddenly becomes critical.

Sulfates

Sulfates attack concrete chemically by reacting with calcium aluminate hydrates to form expansive ettringite and with calcium hydroxide to form gypsum — both reactions consuming the same Ca(OH)₂ alkalinity reserve that the carbonation front is simultaneously depleting, accelerating pH collapse at rebar depth and destroying passive film stability faster than either mechanism acting alone. 

The expansive micro-cracking from ettringite formation simultaneously opens direct convective pathways through the cover zone that bypass the tortuous pore network normally slowing chloride transport, multiplying the effective chloride delivery rate by 3–10 times in cracked zones and dramatically compressing the time to threshold concentration at the steel surface. 

When sulfate ions from sulfide-bearing  landscape rock are cyclically delivered against the stem wall by irrigation water alongside chlorides, all three mechanisms operate on the same chemical system simultaneously — each accelerating the others in a compounding cycle that produces corrosion initiation far earlier than any single or dual mechanism model would predict. [1]

Delivery Methods (How these chemicals make it to the rebar)

All fine and good. But how do these mechanisms make it into the concrete to attack the steel? And why is the stem wall environment different than steel in concrete everywhere?

Capillary Absorption

The first delivery mechanism is capillary absorption. Concrete has millions of microscopic capillaries that form during the initial hydration process. Moist soil next to it delivers water and its chemical ingredients to the relatively drier concrete, allowing those capillaries and pores to suck up the moisture. Similar to a paper towel that sucks up water it’s exposed to. 

Chlorinated landscape water seeps into the concrete, often picking up additional chlorides from the soil. 

Additionally, CO2-infused rainwater and additional CO2 from garden mulch and fertilizers are delivered by the same mechanism into the concrete, changing the pH balance from alkaline to acidic chemistry, with the multiplier effect discussed above. Sulfate is similarly absorbed initially this way as well. Sulfates are present in many in situ soils and are present in decorative landscaping rocks.

Diffusion

Once water is distributed through the pore network, the chlorides that are more concentrated in the water near the outside naturally distribute to areas with lower concentration… more toward the interior.

Cyclical Concentration

Landscape irrigation and irregular rains create wet and dry cycles. Each cycle deposits chlorides and sulfates. As the water evaporates, it leaves behind the chlorides, carbon, and sulfates. When the next cycle arrives with more sulfates, carbon, and chlorides, it encounters the previously left-behind chemicals, absorbs them, and carries them deeper along with the chemicals already in the water. Each cycle repeats and increases the concentrations and depths of the process.

Sulfate Cracking

The chemical attack by sulfates creates tiny cracks that grow larger and deeper over time. These cracks now allow the chemical-laden water to penetrate much faster than following the circuitous pore network… thus multiplying the speed of the other two mechanisms.

Results of Corrosion (Rust Jacking)

Once the pH balance of the wetted concrete is lowered and chlorides and sulfates have penetrated the passive film, initiating microchemical/electrical currents, the steel rusts at an accelerated rate. Far more than just sitting out in the open. Iron oxide takes up more volume than steel. As it grows in volume, it puts several thousand pounds of tensile pressure on the concrete, easily splitting it. Eventually, it will cause complete failure of the concrete.

Rebar

Expanding rebar is easily seen and identifies the horizontal crack.

Straps and J-bolts

Straps and J-bolts oxidation leads to more vertical cracks, which can cause corners to fall off.

Time frames

Normal concrete with steel embedments can last many years, depending on concrete quality, environmental conditions, and other factors, and could last 50 years before the rebar is attacked. Chlorides, sulfates, and CO2 each lower the time to rebar corrosion by factors of 3 to 10 years. When combined, they have a multiplier effect on each other that lowers the life to between 4 and 7 years.

Once the pH levels have lowered and the concrete has high levels of chlorides and sulfates, any new steel embedments installed will be rapidly attacked, resulting in continued rust jacking within just a few years.

ICRI repair standards

The International Concrete Repair Institute is a subchapter organization of the American Society of Civil Engineers. They are the authority for concrete repair. They have a publication that outlines the steps for repairing concrete with oxidizing rebar:

ICRI 310.1R requires removal of all unsound concrete plus a minimum 25mm clearance behind the rebar on all sides — not just to the bar face. Saw cut all repair boundaries to at least 10mm depth.

Preferred removal methods in order: hydrodemolition (not practical for stem walls), pneumatic chipping, and jackhammer, last resort due to microcracking risk.

Step 2 — Rebar and Hardware Treatment

Rebar under 25% section loss — blast clean to SSPC SP-10 near-white and apply zinc-rich or epoxy primer coating. By the time the cracks are noticeable, this threshold has already been exceeded. At a minimum, deformations have been lost.

Rebar over 25% section loss — replace or supplement with new bar, lap spliced per engineer. (30 diameters)

Hold-down straps — replacement strongly preferred over treatment. Coating the critical interface zone effectively in the field is not reliably achievable.

J-bolts — replace in virtually all cases. Hook geometry makes cleaning and coating the embedded portion difficult.

Problems with this approach in carbonized, pH-lowered stem walls

ICRI 330.1R states clearly that concrete repair is not restoration to original condition — it is arresting deterioration and extending service life. For irrigated stem walls with combined chloride, sulfate, and carbonation exposure, repair without addressing the ongoing ingress mechanism (irrigation management, surface sealing, carbonation mitigation) will result in a repair life of 5–10 years or sooner in some cases.

Superiority of non-Ferrous replacements (NeveRust® System)

Non-ferrous rebar, composite hold-down straps, and j-bolt replacements completely bypass the tiny micro-electrical currents (0.1 to 10 μA/cm²) by removing the anode. There is nothing to oxidize, and therefore, no rust jacking can ever occur.

Fiberglass rebar has been used extensively as a replacement for steel rebar. It has greater tensile strength, is lighter, is easily obtained, and is comparable in price. Simply use zip ties in place of wire ties.

Hold-down straps are more difficult. They are typically installed underneath the stucco or siding. Replacing them requires removing the stucco and lath, and the replacement is difficult to match. Once placed back in the caustic stem wall environment, already at a pH-lowered state with chloride, sulfate, and CO2 saturating it… It will last only a few years.

A non-ferrous alternative is suggested that does not require the removal and replacement of the stucco or drywall.

This type of installation is fast and simple. Once the deteriorated concrete has been removed, simply screw the provided 8” screws into the base plate and upright studs above it. Then run the new rebar through the hole in the strap provided for it. 

J-bolts are also more difficult to replace. Another non-ferrous composite is available. 

Again, installation is simple and easy. Cut off the existing corroded bolt. Cut out the hole to 1/3/8” diameter. Push the toggle bolt through the hole and twist the bottom to tighten.

Other systems

There are other strap replacement and j-bolt replacement systems available; however, they are made of ferrous components. These products, placed in a carbon-, chloride-, and sulfate-saturated stem wall environment, will be rapidly attacked by microchemical electrical currents, resulting in continued rust jacking. Even if they were galvanized, this is still a temporary solution, as zinc coatings are sacrificial and will be removed by galvanic processes over time.  

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Footnotes:

[1] Fe²⁺ → Fe(OH)₂ → FeOOH → Fe₂O₃·nH₂O Volume expansion: Original iron = 1.0× Fe(OH)₂ = 3.8× Fe(OH)₃ = 4.2× Hydrated rust = 6.4×

[2] ANODE — active pit site: Fe → Fe²⁺ + 2e⁻ (iron dissolves — metal loss occurs here) CATHODE — adjacent passive steel: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (oxygen reduction — no metal loss here) ELECTROLYTE — concrete pore solution: Ion current flows through pore solution completing the circuit METALLIC PATH — the steel bar itself: Electron current flows from anode to cathode

[3] All Three Mechanisms Operating on the Same Chemical System Simultaneously

Mehta, P.K. and Monteiro, P.J.M. — Concrete: Microstructure, Properties, and Materials, 4th ed., McGraw-Hill, 2014