CROSS Safety Report
Defects found in precast (prefabricated) concrete façades
The reporter found several issues when inspecting or reviewing buildings with precast concrete façades, particularly on older buildings, due to a lack of attention to durability and poor workmanship. Defects included the failure of connections due to corrosion, the breakdown of veneered layers of concrete, and the corrosion of reinforcement due to lack of cover and poor workmanship resulting in spalling concrete. In some instances, the reporter found precast concrete façades were in danger of falling off the building.
Key Learning Outcomes
For structural and civil design engineers:
- Pay close attention to the detailing of precast (prefabricated) concrete elements
- Specify hot-dipped galvanized dowels and inserts as a minimum, and consider specifying stainless steel items where elements are exposed to aggressive environments
- Ensure non-load-bearing panels are detailed and constructed in such a manner that no unintentional loads are transferred in either horizontal or vertical planes
- Include periodic inspections in the fabrication yard (or on site in the case of tilt-up construction) as part of the inspection regime for quality control
- Pay careful attention to the design of grouted joints, and specify accordingly
- Utilise suitably skilled labour for the grouting of load-bearing joints
- Grout joints to load-bearing elements as the work proceeds
- Do not allow any loading to load-bearing elements until grouting is complete and the grout has reached the specified strength
For asset owners and managers:
- Inspect precast panels during the building life, particularly on older buildings, taking account of those subject to potentially accelerated degradation.
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The reporter wishes to draw the attention of structural engineers to several issues when inspecting or reviewing buildings with precast concrete façades, particularly on older buildings. The reporter has inspected, or is aware of, several precast concrete façades which have been in danger of falling off the building, often due to the effects of corrosion.
Significant failures of precast concrete panels
The period 1960-1980 was an era of significant change to building envelope construction as designers in Australia and elsewhere developed precast concrete façades. With the improvements in craneage equipment both in the factory and on site, there was a substantial shift to using both load-bearing and non-load-bearing precast concrete panels for external walls in multistorey buildings.
Unfortunately, as is often the case with advances in technology, a lack of understanding of the behaviour of the overall structure and the durability of the precast cladding elements has resulted in some significant failures. This includes the effects of axial shortening in the main vertical structural elements (resulting in load being transferred to non-load-bearing precast panels), corrosion failure of connections due to poor durability, the breakdown of veneered concrete layers, spalling of concrete and the corrosion of reinforcement due to lack of cover and poor workmanship.
The failure of veneer construction involving an outer layer of more durable, and therefore more expensive, concrete and the underlying layer of lower grade concrete became evident in some precast concrete panels exposed to the weather. This form of construction requires care to be taken to ensure that the veneer concrete is poured before the base concrete has fully set, and that the two concretes have similar properties. Most of the early problems occurred because of significant time differences between the pouring of the two types of concrete used in veneer construction.
Other significant issues the reporter found included using ungalvanized ferrules, ungalvanized dowel bars, and ungalvanized J-bar lifting inserts (usually located in the top and sides of the prefabricated concrete for lifting purposes and for connections). In many cases, sealants were never replaced which allowed water to enter the façade causing corrosion in the ferrules and dowels. This led to local failures of the panels and possible failure of restraint, usually requiring expensive repairs. Proprietary lifting inserts, dowel bars and ferrules are now hot-dipped galvanized as a minimum and stainless steel should be considered for aggressive environments. However, proprietary lifting inserts are not readily available in stainless steel.
From the late 1970s, concerns were raised about the durability of concrete as the Concrete Code (Australian Standard AS1480-1974) provided little guidance to designers, and the required covers to reinforcement were generally inadequate. In 1979, Beresford and Ho identified the extent and cost of durability failures - approximately 10% of the expenditure of new buildings. In 1987, Marosszeky et al. studied 95 buildings in Sydney involving significant corrosion (indicating inadequate cover) as well as poor detailing and workmanship.
The Concrete Institute of Australia (CIA) also published Practice Note No 12 in March 1983, setting out some of the factors affecting durability using information from the draft AS3600. The Cement and Concrete Association of Australia (CCAA) published Technical Note TN57 on Durable Concrete Structures in 1989 and, in 1990, the CIA published Recommended Practice Durable Concrete.
Structural engineers ... need to be aware of these durability issues and ... need to understand the causes of cracking
Structural engineers need to be aware of these durability issues when inspecting precast concrete façades and, where cracking is found, they need to understand the causes of the cracking. This can often mean significant investigations to determine the reasons for cracking, the extent of corrosion and failure of connections, and the formulation of a suitable repair procedure.
Peyton J.J. and Wynhoven J.H., “Symposium on Concrete, Towards better concrete structures - Design and construction aspects of precast concrete façades, the evolution of the system”,1981.
Campbell-Allen D. and Roper H., “Towards better concrete structures - Durability of Precast Façades”, University of Sydney Symposium on Concrete, 1981.
Beresford F.D. and Ho DWS, “The repairs of concrete structures - a scientific assessment”, Concrete Institute of Australia, Biennial Conference, Concrete 79, Canberra.
Marosseky, M., Griffiths, D., Sade, D., “Site study of factors leading to a reduction in durability of reinforced concrete”, ACI SP-100, page 1703–1726, 1987.
CIA, “Durable Concrete, How to Specify and Construct”, Note 12, March 1983.
CCAA, “Durable Concrete Structures”, Technical Note TN57, March 1989.
CIA, “Recommended Practice Durable Concrete”, February 1990.
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Expert Panel Comments
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The reporter describes a systemic problem with precast concrete façades in older buildings.
Issue not limited to older buildings
The issue, however, is not limited to older buildings. We have seen more recently constructed apartments with precast panels experiencing problems such as poor joint control, lack of fire seals, inadequate panel support, corrosion, and water damage from leaks. All of these have the potential to lead to substantial future repair costs. It would appear that we have not learned from the failures of façades of apartment blocks in Europe and the USA in the 1960s.
Durability is a critical factor in building performance that requires detailed attention to product selection and specification to ensure that documentation adequately considers the design life of the building and the severity of its exposure conditions. This is of particular importance for external façade panels. With the current emphasis on sustainability where the extended life of buildings is being encouraged, a design life in excess of 50 years should be considered.
As noted by the reporter, lifting inserts, dowel bars and fasteners should be specified as hot-dipped galvanized as a minimum. In addition, stainless steel items should be used where building elements are exposed to aggressive environments, noting that lifting inserts may not be available in stainless steel. In accordance with good practice, connection of dissimilar metals should be detailed to avoid galvanic corrosion. Responsibility for durability requirements lies with the designer.
Attention should be given to the New Zealand Building Code, clause B2 which states: '...building materials, components and construction methods are required to be sufficiently durable. They must ensure that the building, without reconstruction or major renovation, continues to satisfy the other functional requirements of the Building Code throughout its life.'. Compliance in New Zealand is typically required to SNZ TS 3404-2018, Durability requirements for steel structures and components.
For new panel construction, consideration should be given to conducting periodic inspections at production facilities to confirm conformance with documented materials, inserts, cover and the like. Checking the cover on delivery to site can also be conducted by means of cover meters.
All parties involved with the design, manufacture and erection of precast concrete should be familiar with:
Notwithstanding the differentiation between In-service Designer and Erection Designer as noted above, it is important for the In-service Design Engineer of the structure to be aware of any temporary measures, such as lifting points, and their potential effect on the long term function of the concrete.
The reporter has drawn attention to the importance of correct detailing to ensure that panels are not subject to loading other than that for which they were designed. This applies equally to non-load-bearing panels, where the provision of isolation joints should prevent any load transfer into the panels in both horizontal and vertical planes. Deformations of buildings due to shrinkage, post-tensioning forces, temperature movements and the like should be taken into account. Some dowelled connections loaded by such movements can produce slip/stick noises if the connections cannot carry the imposed loading. The cost of addressing such noises can be very high.
Connections, fasteners and fixings
Connections, fasteners and fixings are particularly important in prefabricated concrete, and they have many roles which must be considered in design. Light-duty cast-in ferrules should not be used for structural connections between concrete elements other than fixing for lightweight steel structures or similar. Only headed anchors complying with AS3600:2018 should be used for structural connections between adjoining concrete members. Exposed connections require the same fire rating as adjoining prefabricated concrete elements. CROSS-AUS report 993 - The use of cast-in ferrules as structural connections highlights several problems that can arise with this type of connection.
Importance of grouting procedures
Recent experience has highlighted the critical nature of grouting procedures. Highly stressed load-bearing joints can have complex stress patterns depending on joint configuration, they need to be correctly designed, detailed and specified. It should be noted that the effective width of the joint resisting loads will be reduced by the fact that compressive stresses cannot occur at the edges of the joint. Typically, the width of the joint resisting compression is reduced by at least the depth of the joint on either side or by the presence of concrete chamfers. Packers should be removed after grouting. CROSS-AUS report 961 - Grouting of joints between load-bearing prefabricated concrete members covers this important procedure in more detail.
Inspection and checks
It is also important to remember that, even with a theoretical design life of 50 years, some repairs and maintenance may be required during the life of a structure. Sufficient periodic inspections are required to be able to identify such requirements.
When inspecting structures, the condition of all parts resisting load needs to be assessed. Regarding façade panels, these parts are often hidden from view and assessment may require additional effort and cost. It is important that asset owners appreciate the need for this and make provision for inspection and timely repairs, especially with older structures and structures subject to accelerated degradation. Examples include coastal high rise buildings and buildings subject to higher exposure to chemical attack, perhaps from industrial or vehicle emissions.
For an American perspective on similar issues dating back to the 1970s and 80s, the paper by Jenna Cellini The Development of Precast Exposed Aggregate Concrete Cladding: The Legacy of John J. Earley and the Implications for Preservation Philosophy again stresses the importance of learning from the past.