Foster Lyons, an engineer and building-science consultant, responds: The short answer is, it does, but it happens so slowly and takes so long that it is insignificant to us. Properly placed rebar typically corrodes at a rate of about 1/10 micrometer per year across the thickness of the material.
Traditional methods to prevent corrosion
Some of the traditional measures used to combat the corrosion of reinforced concrete are: Cathodic protection; Corrosion inhibitor admixtures; and. Anti-corrosion coating.
Put the rebar on top of a raised non-metallic platform, such as wooden pallets. Both the ground and concrete areas absorb moisture and can lead to rusting. Cover the rebar with a thick protective tarp and secure it with cinder blocks (or something similarly heavy) at the corners and edges.
There is also fiber-reinforced polymer (FRP) rebar which is lighter than steel and doesn't corrode. It doesn't heat up in the sun and is around 4.5 times stronger than steel rebar.
So if such bars will be exposed for more than, say, a couple of weeks, they should be covered. Mortar or grout droppings that are adhering firmly to bars need not be removed. If they will not come off easily, leave them. The effect of rust on the bond between reinforcement and concrete is debated endlessly on site.
A.: Section 12 of ASTM A 615-96a, "Standard Specification for Deformed and Plain Billet Steel Bars for Concrete Reinforcement," says that rust shall not be cause for rejection provided the weight, dimensions, cross-sectional area, and tensile properties of a hand-wire-brushed test specimen aren't less than the ASTM ...
You don't want to expose the rebar because it becomes more susceptible to corrosion. As the material deteriorates, it is more likely to cause concrete to fall apart.
Properly placed rebar typically corrodes at a rate of about 1/10 micrometer per year across the thickness of the material. So, under normal conditions, a #5 bar (5/8-inch diameter) loses 2/10 micrometer from its diameter every year.
In terms of fabrication, excessive rebar can cause problems with consolidation and creating voids. Per ACI, the gaps between the rebar shouldn't be tighter than 133% of the largest size of aggregate in your concrete (ex. 3/4" max agg size, 1" openings).
1-3 The problem has been attributed to the number and size of breaks or defects in the coating and to reduction in adhesion between the epoxy coating and steel substrate.
Epoxy-coated rebar
The most common alternative to standard black steel rebar is epoxy-coated reinforcement (ECR). It is produced by subjecting standard steel reinforcement to a four-step process of blast-cleaning, heating to about 450 degrees F, spray-application of dry epoxy powder, and brief curing.
Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar, the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins.
In many instances, concrete's susceptibility to cracking therefore necessitates corrosion protection for rebar. While most rebar is uncoated – leaving it vulnerable to corrosion – some receives an epoxy coating that has well-documented benefits and drawbacks.
Preventive materials include high-quality aggregate and cement, epoxy grout, polymeric fibres, non-corrosive metals like stainless steel, and protective concrete coatings. Incorporating these materials and tools during the construction phase ensures greater resistance against corrosion in the long run.
Copper and copper alloys: Copper is not normally corroded by concrete, as is evidenced by the widespread and successful use of copper waterstops and the embedment of copper pipes in concrete for many years.
The rust that forms on the surface of the rebar can cause the steel to become weaker and more brittle, which can lead to a loss of strength and an increased risk of failure in the structure. However, if the rusting is minimal and the steel is still in good condition, it is possible to use rusted rebars in construction.
Concrete Driveway Weight Capacity
To summarize: A 4-inch concrete driveway can support weights up to approximately 6,000 pounds.
Concrete surfaces required to uphold large trucks, heavy machinery or nonstop traffic need concrete rebar reinforcement. On the other hand, if your driveway is only expected to uphold the family minivan, you might not need rebar reinforcement.
Early 20th-century engineers thought reinforced concrete structures would last a very long time – perhaps 1,000 years. In reality, their life span is more like 50-100 years, and sometimes less.
The general rule of thumb is to add Rebar when the pouring concrete is at least 5 inches or more than depth and when the location typically goes through the freeze-thaw cycle.
The pressure caused by rusting steel reinforcements exerted on the concrete will lead to cracks. The more severe the corrosion, the wider the cracks will be formed, and the spalling of concrete may occur due to the loss of adhesion between concrete and steel reinforcement.
In short, rebar installed into concrete bridges means there is the potential for corrosion due to exposure to oxygen and/or water as well as chlorides from de-icing salts or saltwater.
To be most effective for this purpose, the reinforcement needs to be near the top of the slab. For corrosion protection, however, most specifications require a minimum depth of concrete cover that usually ranges from 1 inch to 2 inches.
Once it's clean, apply a concrete bonding agent to the rebar and surrounding surface, followed by a concrete patching compound to fill in the area and cover the rebar. Smoothing the patch to match the existing pathway will help the repair blend in while protecting the rebar from further exposure.