Most major North American cities have hundreds, if not thousands, of reinforced concrete parking structures. Parking garages may be free-standing structures, structures connected to other buildings, or underground facilities. Each type of parking structure poses a unique set of advantages and disadvantages, however, they all share one thing in common: regardless of design or type, all reinforced concrete parking structures eventually require repair.

The Nature of Reinforced Concrete (The Technical Stuff)

Concrete that is strengthened by adding steel or other materials is reinforced concrete. Normal weight (steel) reinforced concrete has a specific weight (gamma) of approximately 24 kN/m3. For the purposes of this paper, only steel is considered. Steel is the most commonly used reinforcing material for the following reasons:

• Concrete and steel have similar coefficients of thermal expansion (COE) allowing the two materials to expand and contract together during temperature changes.
  
• Concrete and steel bond well together to form an effective building material. Reinforcing steel is often deformed (ribbed) to improve the strength of the bond.

Causes of Corrosion in Reinforced Concrete (More Technical Stuff)

The two main factors that allow corrosion in reinforced concrete to occur are carbonation and the penetration of chloride ions. Carbonation is the chemical combination of carbon dioxide in the air and lime in the concrete. As the CO2 migrates through the concrete, the free lime in concrete is reduced by carbonation. The lower levels of free lime result in lower pH values. Typically, the pH will drop from a desirable value of around 12.5 to an unacceptable level of 8.3. The lower pH creates an environment that is conducive to steel corrosion.

Corrosion of the reinforcing steel also occurs when chloride ions are introduced to the metal in the presence of moisture and oxygen. The chloride ions are either naturally present in the concrete, which means a poor quality mix has been used, or they are introduced to the concrete through de-icing salts and salt air. The salt migrates through cracks in the concrete to the reinforcing steel and when combined with moisture and oxygen, corrosion soon follows.

Typical Concrete Deterioration (The Interesting Stuff)

Generally, steel does not corrode easily in a concrete environment. Steel corrosion in reinforced concrete, however, is a common problem. Clearly, these two statements contradict one another. The difference is between ideal conditions and actual conditions. Ideally, the steel is protected from chloride ions, water and oxygen thereby inhibiting corrosion. Realistically, concrete cracks as it dries. Materials and workmanship may be below standard and a host of other uncertainties can exist.

Four of the more common problems that occur with concrete are: cracking, scaling, delamination and spalling.

Cracking (It's Not All It's Cracking Up To Be):

Cracks in concrete are difficult, if not impossible, to avoid. Cracks in concrete allow water, oxygen and salts to access the reinforcing steel. Exposing the steel to the elements inevitably leads to deterioration of the steel and subsequently, deterioration of the concrete. Cracks are caused by a variety of known and unknown variables. As new concrete cures, it is susceptible to shrinkage cracking. Current building practices try to limit cracking through the use of control joints, expansion joints and construction joints. While the use of joints is helpful in controlling the cracking, the resulting crack is seldom watertight. Any crack, induced or not, is an invitation for water ingress and, consequently, deterioration.  Furthermore, cracks can occur in unpredictable areas caused by unexpected sources such as differential settlement or seismic activity.

Scaling (I'm Scaling Away)

Concrete scaling or crumbling is usually a result of poor surfacing or an improper concrete mix. Improperly finished concrete surfaces may be caused by moisture trapped beneath the surface. The excess moisture alters the water-cement ratio of the surfacing cement and creates a poor quality finish that crumbles away. An improper mix can be caused by several factors. Lack of quality control and testing at the point of delivery, reactive aggregates, and inadequate air entrainment are three examples of how the integrity of the concrete mix may be compromised. Scaling depth can vary from five to twenty-five mm.

Delamination (De Lam is Baaad)

Delamination is noticed as a hollow sounding section of concrete. An area will have lifted leaving a section separated from the main slab. This can easily break apart from "normal" use and stresses.

Delaminations are caused by hidden crack planes that run parallel to the surface of the slab. Delaminating generally occurs at the level or depth of the reinforcing steel and may not be visible on the surface. The expansion forces of corroding steel create the plane of delamination.

Spalling (Help, I'm Spalling And I Can't Get Up!)

Spalls are chunks of concrete that completely break away from the concrete slab or surface. Spalls may be small or large pieces of concrete. Smaller bits of concrete that spall off the surface are popouts. The combination of moisture and a reactive aggregate particle is the most common cause of popouts. Larger deeper spalls develop following delamination. The expansion of corroding steel imposes tensile forces on the concrete that exceed the tensile strength of the concrete, resulting in failure. As the concrete spalls, the surface becomes more porous and therefore, more permeable to chloride ions. This results in accelerated deterioration.

Inspection Programs (Where To Start)

Reinforced parking structures should be inspected every one to three years. The inspections should be conducted by a qualified engineer who is familiar with reinforced concrete and concrete restoration.

A preliminary visual inspection of the facility is conducted to determine its general condition. Any potentially unsafe areas requiring immediate attention may be identified at this time. Visible signs of distress such as cracking and spalling will also be noted during this visit. The inspection may also involve a chain drag test to determine if any concrete delamination exists. The initial visit also provides the engineer with an opportunity to assess the structural design details and the quality of construction.

After the preliminary inspection, the engineer is usually able to provide an assessment of the facility and a reasonable estimate of the cost of the repairs.

After the initial inspection is completed and a repair strategy is undertaken, future regular inspections can be conducted and recorded by the maintenance staff as instructed by the engineer. The results of future inspections can then be reviewed by the engineer.

Repair Strategies (What Will They Do?)

The exact nature of concrete repairs depend on the individual facility, however, a typical repair strategy may include the following:

Spalls:

• Remove loose concrete and chip the affected area down to sound concrete.
• Remove rust build up on reinforcing steel.
• Treat cleaned re-bar with protective coating such as epoxy.
• Resurface concrete with one or more layers of quality patching cement.

Crack Repairs:

Crack repairs can be quite diverse and the repair strategy will depend on the nature of the crack. The repair approach may include one or more of the following techniques:

Conclusions

Parking garages are subjected to a variety of potentially damaging elements. In addition to the salt air and road salts, the slab is also subjected to repeated abrasive forces from the vehicles. It is impossible for any concrete structure to resist deterioration but parking garages typically begin to experience some form of minor distress after about five years. Repairs are often required in structures ten years or older. Generally, the areas which show the first signs of distress are the sloped ramps. These areas are particularly vulnerable because the slope change often results in less concrete cover. Timely inspections and regular maintenance can help prolong the life of the facility and help minimize the cost and degree of repairs required.

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