Rebar Problems In Concrete Foundations, Slabs, and Walls
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Rusting and exposed rebar can reduce the structural strength of concrete. This can result in cracks and weakened foundations and slabs, as well as leaks in basement walls.
Although concrete is a very durable material and is great for supporting huge amounts of weight; it’s not great for tensile strength unless it has reinforcing in it, such as rebar.
Crack and Rebar Problems
Problem #1 – Rusting rebar loses structural strength
As rebar rusts, it slowly loses its strength and deteriorates. As it rusts, it expands in volume and this places tremendous pressure on the concrete that is covering the rebar.
Rusting rebar can expand up to 4 times its diameter – resulting in cracked and damaged concrete
As the rebar rusts, the strength of the bond between the rebar and the concrete deteriorates which ultimately results in weaker concrete. Also, corrosion and pitting contribute to structural fatigue.
Problem #2 – Spalling: chunks of concrete breaking off
Spalling, or chunks of concrete popping or falling off, generally occurs because of:
- Mechanical damage – i.e. the concrete being hit by a hard metal object with a lot of force.
- Forces of rusting rebar pushing against the concrete – As we have noted, rusting rebar exerts tremendous pressure against the concrete which may cause cracks or pieces of the concrete to break loose.
Problem #3 – Shrinkage cracks that allow moisture to reach the rebar
Shrinkage cracks are probably the most common type of cracks in concrete. When concrete is first mixed and poured, it has excess water in it and in the hardening process, the concrete loses its excess water which causes shrinkage cracks.
If too much water was added, then the cracking may become an issue. First, the concrete will be weaker and secondly, the shrinkage cracks may be larger and allow moisture to reach the rebar.
Problem #4 – Inadequate concrete coverage over the rebar
Building codes have requirements on how close rebar may be to the earth (soil) as well as how close it may be to concrete forms. The distances vary upon the location and size of the rebar.
Common rebar sizes used in residential construction for houses are usually in sizes ranging from #3 rebar to #6 rebar. # 4 rebar has a diameter of 1/2 inch (4/8 of an inch) or #5 rebar is 5/8 of an inch in diameter.
Clearances and coverage on rebar
Rebar generally should be encased or covered with concrete and in most instances, there are code requirements that establish guidelines. At times rebar may shift out of position when the concrete is being poured and therefore it doesn’t get proper coverage.
In general, rebar in residential construction needs to have 3 inches of concrete cover or separation from soil when the concrete for footings and pads is poured against soil and if poured against forms, 1½ inch. If formed concrete not exposed to earth or weather like in slabs and walls, then ¾ of an inch is required. Note, that there are many requirements about conditions and clearances.
Rust stains or a pattern of cracks
If a concrete wall or floor has rust stains near cracks, the rebar is usually rusting. When this condition is observed it would be wise to determine the source of the moisture and do maintenance and repair.
If there is a pattern to the cracks (i.e. rectangle or square), then the rebar may be too close to the surface of the concrete. Again, maintenance and repair are wise.
Basically, if there is no coverage or improper coverage then the rebar may be exposed to excessive moisture and rust.
Problem #5 – Rock pockets may expose rebar to moisture
Concrete that was not placed or vibrated properly may have rock pockets and exposed rebar. Often this problem occurs when the concrete was poured too dry because not enough water was added to the concrete when it was being mixed. This condition may result in rebar rusting and damaging the concrete.
In the photo, the concrete was poured too dry and not properly vibrated.
Problem #6 – If no rebar in concrete, then one side of a crack may rise above the other side: i.e. in a garage floor
If there is no rebar in a concrete garage floor, then one side of a crack may be higher than the other side of the crack. Without rebar, there is a tendency for cracks to grow large.
Houses built before or during the 1950s and ’60s
In many areas of the country, homes that were built during or before the ’50s and ’60s may not have rebar in their concrete slabs. These homes may have cracks running through a number of floor tiles, mirroring cracks in the concrete below the tiles.
In these homes, it is not uncommon to pull up carpeting or other flooring materials and find cracks, and often many cracks. These cracks can be patched or repaired but other cracks will probably appear, especially in areas that have expansive soils or slope creep.
Cracks without rebar are more likely to become a trip hazard
As previously stated, a lack of rebar in a slab will be more likely to allow one side of a crack to rise above the other side. This condition often creates a trip hazard. Inspectors will often consider a height differential of a 1/4 inch or more to be a trip and safety concern.
Trip hazards may be found on garage floors, house floors, walks, patios, and driveways.
Why does rebar rust or corrode?
- When the passive protective layer over the rebar breaks down, i.e. the cementitious materials go past the surrounding rebar, then chemical, carbonation, and chloride issues start the rusting process.
- Various pollutants in the air, freezing and thawing, moisture in the air (especially in coastal areas), salt and de-icing compounds, and aggressive soils can also lead to corrosion and rusting rebar.
- Exposure to excessive moisture and various chemical compounds can cause damage to concrete and rebar under a number of circumstances.
Why is rebar put in concrete?
Two of the main reasons are:
- Reduce cracking in the concrete
- Add structural strength, especially tensile strength
Other reasons for rebar in concrete
- Helps to keep one side of a crack from rising above the other side
- Can tie two separate sections or pieces of concrete together (i.e. at cold joints)
- Ability to reduce the thickness of concrete. With rebar in a slab or wall, there may be less concrete required, and the concrete may not need to be as thick
- Can help distribute the weight or load on the concrete to a larger area
- Helps hold the concrete together when it expands and contracts
Why does rebar have ribs on it
The small ribs on rebar serve several purposes.
- They add more surface area to the rebar which gives the paste in the concrete a greater surface area for bonding.
- The ridges provide for stronger mechanical anchoring to the concrete.
- The ridges help hold different pieces of rebar in place when pouring the concrete so they do not slip out of position, even though they’re tied to one another.
Improperly placed rebar or rebar exposed to moisture may rust and this may weaken or damage the concrete. At times, significant damage can result from this and can be costly to repair.
Homeowners who have rusted or exposed rebar should perform maintenance and repairs. Occasionally an engineer may need to be consulted if there is significant rusting, spalling or damaged concrete. Fortunately, the majority of the time maintenance is the only thing necessary.