Concrete is a very hard and durable material, which makes it great for structures under heavy loading and structures in corrosive environments. However, concrete has a known susceptibility to chlorides, one of the most common being road salt (calcium chloride, NaCl).
Most concrete is constructed with some form of steel reinforcing. For low-strength applications and smaller structures, a welded-wire mesh is commonly used since it can be installed quickly. For larger structures and higher strength applications, welded wire mesh may not provide enough steel volume and individual reinforcing bars must be designed and placed in a specified configuration (typically rectilinear mats).
Most people are familiar with steel corrosion under salt-water type exposures. Iron oxide, commonly known as “rust,” is an expansive and strength-reducing biproduct of an electrochemical reaction taking place on the surface of the chloride-exposed steel.
The steel reinforcing inside of concrete, and the bond between the steel and concrete, is critical to the strength of a reinforced concrete structure. This is true for large highway beams as well as mesh-reinforced driveways and sidewalks. If the bond between steel and concrete is compromised, then the steel cannot engage and the concrete effectively becomes a brittle, unreinforced block of hardened cement and aggregate waiting to crack under an unexpectedly light loading.
The stage is now set to highlight the biggest problem with salt and concrete. If the steel reinforcing is exposed to water-borne chlorides from the environment, either by way of a surface crack or a slow process called “diffusion” where the chloride ions slowly migrate through the concrete matrix itself due to a positive chemical gradient, then expansive iron oxide will eventually form on the surface of that embedded steel reinforcing. Once this process progresses enough, then the concrete, which is weak in tension, between the reinforcing bar and the surface will spall off. Once this occurs, the spalled concrete is completely separated from the structure and the corroding reinforcing bar is fully exposed to the environment. Propagation of reinforcing bar corrosion and subsequent spalling of concrete then accelerates from this point.
It seems that that steel reinforcing is both the vital, strength giving element that gives concrete it’s vast structural versatility and one of its biggest “achilles heels” for long term performance. Protection of the steel is this a fundamental consideration in designing and constructing durable concrete.
Let’s now turn to the humble homeowner with a concrete driveway. Using a non-chloride ice melter is one way to reduce the chloride load reaching the steel mesh within the slab. If nothing else, this at least buys time. Sealing cracks is another way to protect the steel from exposure to chlorides. Since concrete is a naturally brittle material that also shrinks as it dries, a perfectly crack free concrete driveway is not a practical expectation. Sealing of larger cracks will go a long way in keeping the all important steel reinforcing protected from corrosion. Sealing cracks has the added benefit of protecting the concrete from freeze-thaw action in cold climates.
Concrete is a very hard and durable material, which makes it great for structures under heavy loading and structures in corrosive environments. However, concrete has a known susceptibility to chlorides, one of the most common being road salt (sodium chloride, NaCl).
Most concrete is constructed with some form of steel reinforcing. For low-strength applications and smaller structures, a welded-wire mesh is commonly used since it can be installed quickly. For larger structures and higher strength applications, welded wire mesh may not provide enough steel volume and individual reinforcing bars must be designed and placed in a specified configuration (typically rectilinear mats).
Most people are familiar with steel corrosion under salt-water type exposures. Iron oxide, commonly known as “rust,” is an expansive and strength-reducing biproduct of an electrochemical reaction taking place on the surface of the chloride-exposed steel. The steel reinforcing inside of concrete, and the bond between the steel and concrete, is critical to the strength of a reinforced concrete structure. This is true for large highway beams as well as mesh-reinforced driveways and sidewalks. If the bond between steel and concrete is compromised, then the steel cannot engage and the concrete effectively becomes a brittle, unreinforced block of hardened cement and aggregate waiting to crack under an unexpectedly light loading.
The stage is now set to highlight the biggest problem with salt and concrete. If the steel reinforcing is exposed to water-borne chlorides from the environment, either by way of a surface crack or a slow process called “diffusion” where the chloride ions slowly migrate through the concrete matrix itself due to a positive chemical gradient, then expansive iron oxide will eventually form on the surface of that embedded steel reinforcing. Once this process progresses enough, then the concrete, which is weak in tension, between the reinforcing bar and the surface will spall off. Once this occurs, the spalled concrete is completely separated from the structure and the corroding reinforcing bar is fully exposed to the environment. Propagation of reinforcing bar corrosion and subsequent spalling of concrete then accelerates from this point.
It seems that that steel reinforcing is both the vital, strength giving element that gives concrete its vast structural versatility and one of its biggest “Achilles heels” for long term performance. Protection of the steel is thus a fundamental consideration in designing and constructing durable concrete.
Let’s now turn to the humble homeowner with a concrete driveway. Knowing some of the mechanics at play, what can be done to maximize the life of a concrete driveway? Using a non-chloride ice melter is one way to reduce the chloride load reaching the steel within the concrete. If nothing else, this at least buys time. Sealing cracks is another way to protect the steel from exposure to chlorides. Since concrete is a naturally brittle material that also shrinks as it dries, a perfectly crack free concrete driveway is not a practical expectation. Sealing of larger cracks will go a long way in keeping the all-important steel reinforcing protected from corrosion. Sealing cracks has the added benefit of protecting the concrete from freeze-thaw action in cold climates. Freeze-thaw action refers to expansion of freezing water within cracks in the concrete (both micro and macro), causing those cracks to expand, propagate, and eventually spall the concrete.
While salt is a big contributor to concrete driveway degradation, it is not the only degradation mechanism at play. Poor water management in the yard around the driveway or broken drains under the driveway commonly lead to subsidence (loss of bearing soils) under the driveway. Left unchecked, subsidence can lead to sunken section of driveway, which will lead to large stress cracks after repeated loadings with vehicles and exposure to seasonal weather variations. Addressing subsidence typically starts with addressing drainage, and it is impractical to re-level a partially sunken driveway. However, sealing cracks that form around these sunken areas is an important maintenance step to ensure the driveway reinforcing continues to perform and keep the driveway from becoming a runaway deterioration headache.