January 25, 2021
By Nigel Parker, P.Eng., LEED AP
Parking garages and other concrete structures are often exposed to severe corrosive environments leading to deterioration that necessitate the need for evaluations, repairs, and rehabilitation. Owners, engineers, architects, and contractors involved in the operation, maintenance, and restoration of parking garages play a key role in ensuring the structures are maintained in a state of good repair. Failure to do so lead to increased repair costs, potential loss of revenue due to closure of the facility, and, in the absolute worst case, structural failure resulting in a loss of life.
With many different concrete structural framing system types, repair methods, and maintenance solutions, knowing how to recognize a problem and evaluate it; design and implement a repair strategy, and finally how to maintain the structure becomes increasingly difficult.
Structural framing systems
To understand how to repair and maintain concrete structures, an understanding of the types of concrete structures is required. In the Canadian market, there are three main types: cast-in-place, precast, and post-tensioned concrete.
Cast-in-place concrete structures consist of reinforcing steel and plastic concrete constructed onsite. Within the cast-in-place normally reinforced umbrella, several framing systems exist, including two-way, one-way, pan-joist, and waffle.
Precast concrete consists of concrete elements, either un-reinforced, normally reinforced, pre- or post-tensioned, constructed at a fabrication plant and then shipped to site. One of the most common precast structure types is the single- or double-tee parking structure.
Finally, post-tensioned concrete structures consist of concrete that is cast with an unbonded or bonded/grouted tensioning cable that is tensioned after the concrete cures to provide compressive forces to the member.
In addition to these three types, there are also structures utilizing structural steel framing to support the concrete slab or act compositely with concrete. The type of structural framing system will inform decisions on available repair solutions.
Concrete deterioration 101
Regardless of the type of concrete structure the potential for deterioration exists. Corrosion and carbonation are the two main types of concrete deterioration. Corrosion is an electrochemical reaction where electrons migrate from the anodic to the cathodic zone, releasing ferrous ions at the anode and hydroxide ions at the cathode. This process eventually leads to a potential difference between the anodic and cathodic areas at the surface of the steel reinforcement, resulting in the creation of rust as a by-product. Since rust occupies a larger volume than steel, it exerts internal pressure that causes the surrounding concrete to crack, delaminate (i.e. separate from the steel), or spall (Figure 1). As the concrete deteriorates, the reinforced-steel becomes exposed, thereby allowing more chlorides to penetrate the concrete and speed up the process of corrosion. The corrosion process is accelerated in structures, such as parking garages, where chloride ions (i.e. de-icing salts) are used. Concrete structures with a chloride ion by weight of cement of 0.20 per cent are generally considered to be above the threshold for corrosion to occur.
Unlike corrosion, which is an electrochemical reaction, carbonation is a chemical reaction between carbon dioxide (CO2) in the environment in the presence of moisture with hydrated cement minerals, specifically calcium hydroxide, to produce calcium carbonate (CaCO3). Carbonation of concrete is associated with the corrosion of steel reinforcement and shrinkage. However, it also increases the compressive and tensile strength of concrete, so not all of its effects on the material are bad.
In addition to corrosion and carbonation, concrete can also deteriorate through freeze-thaw damage, alkali aggregate reaction (AAR), sulphate attack, fire damage, overloading, and, in the case of post-tensioned structures, post-tensioning strand failure.
Recognize and evaluate
A deep understanding of the type of concrete structure and also the deterioration mechanisms enables the engineer to undertake a comprehensive assessment of the structure to document the existing conditions and provide recommendations for repair. A comprehensive assessment of a concrete structure would include a detailed review of all available documents, past reports, and original drawings, in conjunction with a site visit. The site visit component would include both visual and acoustical surveys
(i.e. chain drag and hammer tap) to fully assess the extent of deteriorated concrete and/or moisture protection systems.
A chain drag survey is a technique used to locate hollow sounding areas on the suspended slab surface. However, it does not provide information with respect to the type of deterioration detected (Figure 2). The hollow sounding areas detected may be a result of concrete delaminations caused by the corrosion of the embedded reinforcing steel in the structural slab or due to the separation of the moisture protection system from the surface of the suspended slab. Similar to the chain drag technique acoustical sounding of the vertical concrete elements and suspend slab soffit is undertaken to identify the extent of concrete delamination (Figure 3). The principle behind the use of tapping rods/hammers to detect delaminations is similar to that of the chain drag for detecting slab surface delaminations. A tapping rod, when struck against the slab soffit, or a hammer struck against a wall or column, gives off a high-pitched ringing sound if the concrete is not delaminated whereas a hollow sound is heard when delaminated concrete is struck.
In addition to site assessment, material and destructive testing can be undertaken to determine the in-situ properties of the concrete (strength, density, air voids, Petrographic Number [PN], etc.), the chloride ion content, rapid chloride permeability (RCP), carbonation, half-cell potential, and as-built conditions. As the assessment is tailored to each structure, an engineer specializing in the repair and protection of concrete structures will be able to provide recommendations on what material testing, if any, is needed for a given project. For example, a strand assessment is necessary for post-tensioned structures to understand the condition of the embedded post-tensioning system. This may require test openings to expose the strands.
Design and implement
The results of a detailed assessment of the concrete structure will inform the type, extent, and scope of available repair strategies. Protecting the structure and maintaining the waterproofing systems in a state of good repair allow for the uninterrupted safe use of the facility, and ensures stability in the value of the asset and reduced long-term capital expenditure costs.
Strategies for repairs range from a do-nothing approach to complete replacement or demolition of the structure. The most common repair strategies have one thing in common: deteriorated material is removed and replaced with new, good quality concrete (Figure 4). As noted above, the full extent of any repair or rehabilitation program will be informed by a detailed condition survey assessment and the structural framing system for the structure in question. Included in the decision matrix on any repair program is the cost of the repairs and the anticipated service life of the structure and repairs themselves. An owner that intends to demolish a building within two years is not inclined to spend significant capital on long-term repairs, but may be more interested in installing temporary structural shoring to support the structure until the demolition is undertaken. Conversely, a wholesale rehabilitation approach, although more costly than localized repairs, may be more desired by an owner with an asset that is anticipated to be in use for the long term. In any restoration program, the corrosion process is typically reduced but not stopped, as the contamination within the concrete will remain in the unrepaired areas. Therefore, budgeting for periodic concrete repair and maintenance of the waterproofing system is required for the life of the asset.
To understand the required repairs, one needs to understand the structure type, extent of deterioration, available repair solutions, and the owner’s needs. Although most often used for civil projects, an asset depreciation curve is an exceptional model for concrete structures. Similar to roadways, investing capital in repairs and protection at the early signs of deterioration has a lower cost than repairing the structure when the asset is in poor condition. Investing $1 in localized repairs when the asset is in a fair condition eliminates the need to spend $4 to $15 on rehabilitation or reconstruction (Figure 5). Undertaking a $1 per square metre repair, protection, and maintenance program to a structure in the good to fair condition eliminates the need for a future $4 per square metre rehabilitation program when the structure is in poor condition, or when the structure is in very poor condition, a $15 or more per square metre structure replacement cost.
Figure 5 and the associated approximate costs are based on case study parking garage projects undertaken by RJC Engineers, the author’s firm, in the Toronto market within the last six years. The case study projects used to obtain the costing data are all normally reinforced parking structures with membrane protection systems repaired or replaced as part of the project. Although parking structures with different framing systems and varied protection systems will have diverse repair and rehabilitation costs and timelines, the asset depreciation curve is similar.
On average, a new freestanding parking structure in Canada costs approximately $6.50 (above grade) to $16.50 or more (below grade) per square metre or approximately $25,000 to $65,000, or more, per stall to construct. Costs for new construction vary greatly depending on the structure’s location (above or below grade), the structural system utilized (i.e. precast, cast-in-place, post-tensioned, etc.), site location, efficiency of parking (i.e. square metres per stall), and site-specific constraints/conditions. Given the costs to construct or replace the asset, repair and maintenance of the property in a timely fashion become critical. However, without a protection system installed on the structure, it will continue to deteriorate.
Protect and maintain
The Canadian Standards Association (CSA) S413, Parking Structures, which forms part of the building code, outlines various protection systems that can be installed on parking garages. Although a wide range of systems exist, it is important to note the use of a protection assembly is required. Broadly, there are four classes of protection systems: membranes, corrosion inhibitor, C-XL concrete, and sealer. The use of a membrane system is suitable for all types of framing systems. However, the use of any combination of corrosion inhibitor, C-XL concrete, and sealer is limited to only bonded post-tensioned, precast, and pre-tensioned structures.
For a detailed comparison of the different types of membrane systems for concrete parking structures, refer to the article, “Waterproofing for concrete parking structures: A Comparison,” published in the June 2018 issue of Construction Canada. In summary, different membrane technologies have varied rheological properties, which, in turn, lead to diverse resistances to damage, different effective service lives, and varying installation and maintenance costs. Given the number of waterproofing systems available, it is critical the designer know the system limitations prior to specification.
A maintenance program must be implemented to protect the asset repair or rehabilitation investment costs. Although the Canadian Parking Association Technical Bulletin No. 2: Parking Facility Maintenance Manual and CSA S413 are excellent resources for schedules of recommended maintenance, a maintenance schedule customized for the specific structure developed by an engineer in conjunction with the ownership team is strongly recommended. This customized maintenance program should include a schedule (i.e. monthly, quarterly, annually, or other) for reviews and specific maintenance items as follows: structure, moisture protection systems, parking control/security equipment, elevators, electrical systems, mechanical/plumbing systems, fire protection systems, surface finishes/line painting, cleaning, and snow removal.
There are limitless tools and techniques to repair and protect concrete structures. A team of design and industry professionals that know how to recognize a problem, evaluate it, design and implement a repair strategy, and maintain the structure in a state of good repair are the best asset an owner can have.
Given that on average a new freestanding parking structure in Canada cost approximately $25,000 to $65,000, or more, per stall to construct, undertaking a $1 per square metre repair and maintenance program, when the structure in good to fair condition, or a $4 per square metre rehabilitation program, when the structure is in poor condition, is capital spent well.
Nigel Parker, P.Eng., LEED AP, is an associate at RJC Engineers. He is responsible for managing projects from start to finish, including assessments, design, tendering, construction contract administration, and project close-out. Parker has spent his entire career in the fields of structure rehabilitation, repair, and new construction. He is also the former chair of the Canadian Society for Civil Engineering Toronto Section. He can be reached at firstname.lastname@example.org.
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