A term like “concrete rust” seems like a misnomer. After all, concrete is not metallic in any way, so how can it rust? In truth, it can’t. However, if exposed to water and oxygen, rebar and other metal items embedded in the concrete can be used. And if that happens, the rust from the metal within will find its way to the surface of the concrete, and voila! Concrete rust. The question then becomes: “how do I get rid of it?” Is pressure washing effective at eliminating concrete rust? Let’s find out.
Concrete rust is a common issue that pressure-washing professionals regularly encounter in their line of work. It manifests as reddish-brown stains on concrete surfaces, detracting from the appearance and potentially indicating underlying structural problems. While concrete itself doesn’t rust like metal, the term “concrete rust” refers to the discoloration caused by the oxidation of iron particles within or near the concrete. In the following guide, our pressure washers review the top causes of concrete rust and answer some of the most common questions we’ve received.
Six Top Causes of Concrete Rust
Understanding the causes of concrete rust is crucial for effective cleaning and maintenance strategies. Concrete rust typically stems from one (or more) of the following six common causes.
Cracks and Permeable Concrete
Skyscrapers aren’t the only structures that employ rebar to strengthen concrete. Most houses also have rebar in their foundation. Also, most concrete driveways and concrete slabs in garages have rebar within, helping them maintain their structural integrity.
However, even though the rebar may prevent the foundation, driveway, or slab from crumbling, it can’t prevent tiny cracks from forming that allow moisture to infiltrate. Cracks in concrete allow moisture and contaminants to penetrate deeper into the structure, reaching the reinforcement and exacerbating corrosion. Poorly compacted or permeable concrete exacerbates this issue by providing easier pathways for moisture ingress. When that happens, the rebar begins to rust, which bleeds onto the concrete surface, creating rust.
Inadequate Concrete Cover
Insufficient concrete cover over the reinforcement bars leaves them vulnerable to corrosion. Poor construction practices, including improper concrete mixtures or inadequate curing, can compromise the protective layer of concrete, hastening rust formation. As pressure washers, we often observe concrete surfaces where the rust stains are more pronounced due to inadequate cover over the reinforcement.
Reinforcement Corrosion
One of the primary causes of concrete rust is corrosion of the steel reinforcement bars (rebars) embedded within the concrete. When moisture seeps into the concrete and reaches the steel, it triggers a corrosion process. As rust forms on the steel, it expands, leading to cracks and surface deterioration. Professional pressure washers often encounter rust staining on structures like bridges, parking garages, and industrial buildings where reinforced concrete is prevalent.
Exposure to Water
Prolonged exposure to water, whether from rainfall, irrigation, or leaks, can accelerate rust formation on concrete surfaces. Water provides the necessary oxygen and electrolytes for corrosion, leading to unsightly rust stains. Pressure washers often encounter rust staining on outdoor surfaces such as sidewalks, driveways, and patios exposed to frequent moisture, requiring specialized cleaning techniques to restore their appearance.
Carbonation
Carbon dioxide from the atmosphere can penetrate concrete, gradually reducing its pH levels through a process known as carbonation. This reduction in alkalinity compromises the passive layer, protecting the steel reinforcement and making it susceptible to corrosion. Pressure-washing professionals frequently encounter carbonation-induced rust on older concrete structures, particularly in urban environments with high levels of atmospheric pollution.
Chloride Ion Ingress
Chloride ions from marine environments or deicing salts used on roadways can penetrate concrete pores and accelerate corrosion. Chlorides disrupt the protective oxide layer on the steel surface, promoting rust formation. Structures exposed to chloride-rich environments, such as coastal areas or highways, often exhibit severe rust staining, requiring specialized cleaning techniques from pressure-washing experts.