Identifying and Managing Structural Life Safety Issues
INTRODUCTION All buildings have a life expectancy. With the proper maintenance and rehabilitation, its life expectancy can be extended indefinitely. When it comes to Capital planning, we have found that a top priority for Building Owners and Managers is the category of Life Safety. One of the main items regarding Life Safety is the STRUCTURAL INTEGRITY of the building.
Our presentation will touch upon the following items: • Common types of structural issues for: ➢ Parking garages ➢ Balconies • Why do these structural issues occur? • The OWNER’S PERSPECTIVE: ➢ When should I be concerned? ➢ What affects my options? • Preserving your investment once a structural repair program has been completed • Conclusion
WHERE DO COMMON STRUCTURAL ISSUES PRIMARILY OCCUR? 1. Suspended roof decks and intermediate slabs 2. Columns and walls 3. Ledge beams 4. Balcony slabs
TOPSIDE DELAMINATION Occurs on top surfaces of the concrete slab. The concrete spalls off, to expose reinforcing steel bars. Topside delaminations can be identified by sounding with a hammer or chain on the concrete surface. This condition affects the structural integrity of the slab. Topside deterioration at balconies are similar to those found in garages. Cracks in the balcony slabs and through unsealed penetrations (ie. anchor bolts) allow moisture to penetrate into the slab to contribute to corrosion.
SOFFIT DELAMINATION Occurs at the underside of a concrete slab. The concrete spalls, to expose reinforcing steel. These areas are hollow-sounding when hit with a hammer. You may also notice a rust stain from these spalled areas, which is created by the actively corroding reinforcing steel bars. This condition indicates that there is either a failure in the waterproofing membrane, and/or extensive topside delaminations above. As with the topside delamination this condition affects the structural integrity of the slab.
THROUGH SLAB DETERIORATION Normally occur when you have a topside and soffit delamination in the same location. This condition indicates aggressive corrosion at both the top and bottom steel bars. It has a huge effect on the structural integrity of the slab.
COLUMN & WALL DETERIORATION It affects the loading capacity of the structure at vertical members. It occurs when the waterproofing membrane on the exterior side of the foundation wall fails, and when column bases are exposed to salt. Occurs at exterior structural wall elements due to carbonation and poor reinforcing steel cover.
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LEDGE BEAM DETERIORATION Normally occurs at expansion joint locations. Once contaminated water penetrates through a failed joint seal, it can reach the reinforcing steel bars in the ledge beam, and corrode the steel.
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BALCONY SLAB EDGE DETERIORATION Slab edge deterioration does not have a significant impact on the structural integrity of the balcony slab itself. However, it is important because the proper anchorage of balcony posts can be directly dependent on the condition of the slab edge.
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WHY DO THESE STRUCTURAL ISSUES OCCUR? Before asking this question, we need to first ask: What typically makes up the structure in parking garages and balconies? The two main materials in a conventionally reinforced structure are: 1.) concrete (which is good in compression), and 2.) steel bars (which is good in tension). These components, working together, are very effective in providing the structural support requirements in parking garages and balconies. There are also structures that rely on post tensioned steel cables instead of steel bars.
GETTING TO THE ROOT CAUSE: • Structural issues arise out of the deterioration of concrete. • The most common cause of concrete deterioration that leads to significant structural issues is: • Corrosion of embedded reinforcing steel bars, due to: ➢ Exposure to chlorides (salt) ➢ Carbonation Other types and causes which are not common include: Alkali-aggregate reactivity, dissimilar metals, freeze thaw, chemical attacks, fire and heat, overload and impact, loss of support, and restraint to volume changes due to shrinkage and thermal stresses.
Corrosion of embedded steel bars is the leading cause in the deterioration of concrete. The corroding bar expands, and spalls the concrete.
2 metals (i.e. 2 steel bars at different energy levels) + An electrolyte (i.e. concrete) + Metallic connections (i.e. wire ties, chairs, the bar itself) = CORROSION PROCESS
So if steel corrodes, and corrosion leads to concrete deterioration, why bother using this material? … Because on its own, the combination of these materials DO NOT induce the corrosion process. There is a passive “protective film” that forms on the steel bars and reduces the corrosion to an insignificant level. Without this passive film, the rate of corrosion will increase by 1000 times and higher! What destroys this protective film? SALT ( Chloride ions )
Chloride ions (from deicing salts ) are essentially the primary cause of premature corrosion in reinforcing steel bars. How? Chloride ions penetrate the “protective film” around reinforcing steel bars, leaving the steel vulnerable to corrosion. Chloride Ions + Oxygen + Moisture = INITIATION OF CORROSION
For Balconies , the process of ‘carbonation’ induces corrosion (by lowering the PH of the concrete and affecting the “protective film”) of the reinforcing steel bars/OWSJ (open web steel joists) chords. Essentially Carbon dioxide (from the air) penetrates the concrete to react with chemicals within the concrete. Carbonation-induced corrosion often occurs at areas exposed to rainfall, shaded from sunlight, and have low concrete cover over the reinforcing steel. For example, at balcony slabs and exposed floor slab edges.
In structural restoration of PARKING STRUCTURES and BALCONIES, the threat of future potential corrosion in newly repaired areas is met with a number of options which are designed to address the ROOT causes of the issue : • Protect the reinforcing steel bars - Use epoxy coated steel bars - Proper concrete coverage of the bars • Deter the corrosion process - Use a corrosion inhibitors in the concrete mix - Placement of galvanic anodes • Prevent exposure of Chlorides (salt) - Use a waterproofing membrane/traffic deck systems
THE OWNER’S PERSPECTIVE - • During the acquisition of a building or as part of an ongoing Capital Management Plan, it is common practice to retain a Consulting Engineering Firm to perform a Building Condition Evaluation (BCE). This BCE serves as a high level snapshot of the overall Capital requirements of the building. • Based on this BCE, if the Engineering Firm determines that there are structural issues of concern regarding the parking structure and/or the balconies, they will recommend that a more intensive detailed investigation be done to determine the “cause and extent of deterioration”. • The “extent of deterioration” and the “risk of safety”, as a result of structural deterioration, will govern their recommendation(s).
THE OWNER’S PERSPECTIVE, cont’d When should I be concerned? The answer is ‘when SAFETY becomes an issue’. However, it is not always ideal to simply “replace everything”. Based on several project constraints and the unique condition of each building, there are various options that address: • Immediate action • Short term course of action • Long term course of action
THE OWNER’S PERSPECTIVE, cont’d What affects my options?
THE OWNER’S PERSPECTIVE - What affects my options? – con’t Based on project constraints, your options will normally fall into 3 main categories, depending on your overall Capital planning: 1. DEFER 2. LONG TERM REPAIR 3. REPLACE
DEFER
DEFER
LONG TERM
LONG TERM
LONG TERM
LONG TERM
LONG TERM
REPLACE
REPLACE
REPLACE
REPLACE
PRESERVING YOUR INVESTMENT ONCE A STRUCTURAL REPAIR PROGRAM HAS BEEN COMPLETED? Once a long term solution has been implemented, it is important to protect the investment that you have made to renew the life cycle of your building’s structural elements in the parking garage and/or balconies. A common and worthwhile option is to provide a waterproofing system for the parking decks/foundation walls and balcony top surfaces, that will address cracks and prevent the penetration of moisture and chlorides (salt). Another option is to replace the existing “sealant” or “looped” expansion joints in the parking garage with a pre-formed expansion joint seal, which are more durable and do a better job of preventing chloride-contaminated water from reaching the newly coated and/or replaced reinforcing steel.
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