Stewart M. Verhulst, M.S., P.E., RRC
— Benjamin Franklin
Introduction
Public safety is of paramount importance for engineers and architects designing buildings. Most design professionals are diligent in their pursuit of safe structures, carefully calculating the loading conditions, detailing connections and support conditions, and preparing their drawings and specifications. Because of these efforts and with advancements in human understanding of nature’s forces, the safety of the built environment has improved over time, and large-scale structural collapses of modern buildings are rare. Some notable examples of such failures do exist, but buildings are generally better able to safely resist the forces of nature now than at any time in the past.
However, unsafe conditions still occur in the built environment. The potential for objects falling from overhead is one of the most common safety concerns related to the structural and exterior wall systems of buildings. Typical root causes of such falling objects include poor detailing, a lack of clarity in the drawings and specifications, errors in construction, and a lack of proper maintenance. Due to the potential devastating effects of building materials falling from a structure, significant care must be taken to ensure that such failures do not occur.
Most engineers and architects have likely observed some degree of spalling at the surface of a reinforced concrete structure or element. Spalling typically occurs as the result of stresses caused by differential movement of building materials that are in contact with one another, or due to a progressive deterioration mechanism, such as corrosion. As an example, concrete spalling from the corner of a foundation is common due to the different expansion and contraction characteristics of many building veneers when compared to the foundation concrete. However, this condition occurs near ground level and does not present a potential overhead safety hazard.
However, if a similar mechanism occurs on the upper floors of a downtown high-rise, the safety concern is much different. In this situation, a piece of concrete spalling from the edge of the building will gain speed as it falls and, because such a building is typically surrounded by public streets and spaces, it is very likely to fall into a public space with a large amount of occupancy or traffic. Such concrete debris can also impact the building as it falls, potentially causing damage to exterior glass elements, which can create falling glass debris in addition to the concrete.
To discuss some real-life examples of falling debris, including the causes and the safety hazards, the conditions at three structures are presented and discussed below. Two of these structures are residential high-rise buildings located in downtown areas. The third structure is a busy parking garage. By understanding the causes of these failures and by exercising care in design and construction, designers and contractors can avoid many of the common pitfalls that cause building materials to dislodge and fall from buildings, and can thereby greatly reduce the risk of significant injury from falling building debris.
Structure #1 – Downtown Residential High-Rise
A desirable feature for the occupants of any office or residential building is a great view. This is perhaps especially true for a high-rise residential structure located in a popular downtown area. In such a case, achieving the best views typically requires balconies projecting from the face of the structure. The view from an upper-floor balcony on one of these structures creates an awe-inspiring view of the city (and perhaps a vertigo-inducing sensation if you look down). You can feel like you are on top of the city and, in a way, you are. However, when you look over the edge of the balcony, you will most likely realize that you are aligned directly over a city street or public area. This makes any object falling from one of these balconies a significant public safety concern.
To remain attached to the building, concrete at the edge of a balcony relies on proper steel detailing, proper placement of the reinforcement, and good concrete bond characteristics. However, the most common cause of concrete spalling at balcony edges (and building exteriors in general) is corrosion of the reinforcing steel, which causes expansive internal forces within the concrete that can eventually result in a spall. Therefore, for a balcony on this type of structure, proper protection of the reinforcement during the life of the building is not only a structural concern, it also becomes a primary public safety concern.
Steel reinforcement at the balcony edge is protected from the elements by the concrete cover and by the quality of the encasing concrete. Inadequate cover or weak/porous concrete can allow water and other elements an easier or more direct path to the reinforcement, accelerating corrosion and increasing the likelihood of spalling.
Structure #1 was a high-rise residential structure, with balconies typical at each floor. Falling concrete debris was noted by occupants when the building was only a few years of age (some of the residential units had not yet been completed or occupied). Investigation revealed that a portion of the debris was related to corrosion of reinforcement. Concrete cover along the balcony edges was not in conformance with either the project specifications or the building code, including areas that had less than 25% of the code-required minimum cover and locations where the cover was less than 1/8″. Thus, the cover was deficient with respect to the project specifications and with respect to the requirements of the building code. This lack of protection resulted in corrosion of the reinforcing steel and localized spalling at multiple locations throughout the exterior of the structure where the cover was deficient.
Concrete spalling and falling debris also occurred as the result of poor reinforcement installation at the balcony edge. A portion of the reinforcement intended to be placed in the top portion of the cantilevered balcony slabs was not properly supported prior to concrete placement and had displaced such that portions of the reinforcement were ultimately located near the bottom of the slab. Along several balcony perimeters, this displaced reinforcement was loose and was “sagging” at the balcony undersides, with relatively large pieces of concrete delaminating along the reinforcement. Corrosion likely exacerbated the concrete fractures and delamination along the length of the reinforcement.
The combination of the corrosion spalling and the concrete delamination along the edges of the projecting balconies and edges of the building created conditions of falling debris at the building perimeter and unsafe conditions below. This necessitated an extensive repair protocol throughout the façade of the structure to identify and repair the potentially hazardous conditions.
Structure #2 – Downtown Hotel and Residential High-Rise
Structure #2 was a high-rise structure housing a hotel and residential suites. Similar to Structure #1, Structure #2 was located in a busy downtown area and had occurrences of spalled concrete falling from the edges and corners of the balconies. Upon evaluation, it was determined that the spalling was the result of corrosion of the reinforcing steel, which was in turn attributable to a lack of proper concrete cover.
As with Structure #1, the concrete cover at the balconies of Structure #2 was deficient with respect to the project specifications and with respect to the requirements of the building code. Conditions of insufficient cover were widespread throughout the building exterior, with potential safety hazards identified at many of the balconies and with two (2) locations identified as having immediate safety concerns, necessitating the prompt removal of spalled and loose concrete.
Structure #2 also exhibited failures of concrete patch repairs at the balcony perimeters. A portion of the repair material, reportedly installed during the original construction, had debonded from the concrete substrate and had become loose. The poor bond and the resulting failure of the repair patches was attributable to poor surface preparation during the application of the repair materials. Due to concern that these repair materials would completely debond and fall, it was necessary for these materials to be removed.
In addition to the removal of materials at areas of immediate concern, the extent of the distress required repairs throughout the building exterior and will require periodic evaluation and on-going maintenance and repairs for the life of the structure.
Structure #3 – Parking Garage
Structure #3 was a six-story parking garage that served as the primary location of staff parking for a major hospital. The garage experienced consistent daily use and periods of heavy traffic at the start and end of shifts for the hospital staff. The parking garage was a post-tensioned concrete structure, with post-tensioning tendons used as the primary reinforcement for the beams and slabs of the garage. This example illustrates some of the potential issues related to the installation and potential failure of post-tensioning tendons.
Within 5 years of service, hospital staff had noticed two (2) tendons that had visibly failed and were protruding from the structure. As a result, the garage was monitored over a period of years and additional tendon failures occurred. An investigation was performed and it was discovered that other tendon failures existed, including some that exhibited no outward evidence of failure.
In contrast to failures from the deterioration of conventional reinforcement, which generally progress over time, the failure of a post-tensioning tendon can appear rather sudden and explosive. Tendons that are exposed to corrosive elements will deteriorate until they can no longer carry the tension load, at which point they “snap.” This sudden release of load can cause displacement of the tendon end anchorage, with the failed tendon protruding from the exterior face of the structure. This may also cause displacement of the grout pocket at the tendon end, and may even result in projectile discharge of the steel wedges used to anchor the tendon.
Failure of an individual post-tensioning tendon may or may not significantly affect the structural integrity of a structure. However, if a tendon fails due to corrosion deterioration and a loss of cross section, it is likely that similar corrosive conditions have affected numerous other tendons. The failure or potential failure of multiple tendons can quickly become structurally significant, and tendon failures are likely to cause falling debris, as discussed above.
The primary function of the grout that fills the pockets at the ends of post-tensioning tendons is to prevent the ingress of water and to provide long-term corrosion protection from the elements. However, use of the wrong materials or poor mixing and application of the grout may result in separations along the perimeter of the grout, which are potential paths for water ingress. This can create a corrosive environment that is somewhat hidden from view or easy detection.
In the case of Structure #3, extensive corrosion was discovered at a portion of the post-tensioning tendons. However, due to the hidden nature of the tendon ends and the existence of exterior surface coatings, the extent of the corrosion deterioration was not readily apparent at the garage exterior. To diagnose the nature and extent of the problem, it was necessary to remove the grout and concrete at a portion of the tendon anchorages throughout the structure and to obtain a representative sampling of post-tensioning tendons (through removal) for observation and metallurgical testing.
Upon detailed evaluation and by performing destructive testing, it was discovered that the grout pockets protecting the tendon ends had been poorly installed. Separations along the edges of the grout pockets were common. At numerous grout pockets, the grout did not fill the entire pocket, rather there were large voids within the pocket. These voids were not detectable at the exterior surface of the garage. In some areas, the grout had been poorly mixed, resulting in an inconsistent grout product and contributing to poor grout consolidation within the pockets. Separations between the grout and the surrounding concrete, in combination with significant voids within the grout pockets, provided paths for excessive water ingress and created conditions conducive to corrosion deterioration at the post-tensioning tendons and anchorages.
Further testing and evaluation of the tendons and anchorages revealed insufficient end cover at many tendons and varying degrees of corrosion at the exposed face of tendon anchorages. Several of the post-tensioning tendons removed from the structure exhibited severe corrosion and cross sectional loss directly behind the anchorage. Due to the severity of the damage to the tendons, the long-term integrity and durability of the structure was compromised. However, as noted above, the extent of the deterioration was hidden from view until significant damage had already been done.
Debris had fallen from the perimeter of the structure, including pieces of grout and portions of the cementitious surface coating that had been applied over the exterior face of the structure. As a result, it became necessary to cordon off portions of the garage exterior to protect the public from falling debris.
This case study illustrates how the deficient installation of post-tensioning tendons and of the grout pockets protecting the tendon ends can cause significant damage and potential safety hazards. Significant repairs were necessary to restore the performance and durability of Structure #3, even though it was less than 15 years of age. Because of the nature of post-tensioning tendon installation, tendon replacement was difficult, and the estimated cost of repairs was significant when compared to the original construction cost for the parking garage.
Conclusions
The issues discussed herein are intended to illustrate the importance of proper specification, detailing, reinforcement placement, and material application in concrete structures. Owners, designers, and contractors should consider the potential implications of spalling concrete and the resulting falling debris before construction is underway. Proper detailing and quality installation of reinforcement, using proper reinforcement support and ensuring proper reinforcement cover and protection, are important for the prevention and mitigation of many of the issues discussed in this paper. The proper application of the materials protecting the reinforcement, including grout protecting post-tensioning tendons at tendon pockets, may also be critical to the long-term performance of the structure.
It is perhaps obvious, but important enough to note, that the costs of the necessary exterior repairs to these structures far exceeded the costs of proper installation during construction. For projects where issues related to reinforcement placement have the potential to create potentially unsafe conditions from falling debris, more stringent specifications and additional third-party pre-pour reinforcement inspections should be considered to reduce the risk of a failure. It is much easier and much less costly to address these issues before concrete is placed than after the building has been occupied.
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