CROSS Safety Report
Inadequate billets used in precast concrete connections
This report is over 2 years old
Whilst completing a third-party design appraisal, a correspondent found that solid steel billets intended to form the primary shear transfer mechanism between concrete beams and columns were insufficient for this purpose.
Key Learning Outcomes
For civil and structural design engineers:
Careful attention should be given to connections of precast concrete structures, particularly ones that are subject to dynamic and fatigue loads
Consideration should be given to connections where poured grout may be susceptible to cracking and over time lead to water ingress and corrosion
Helpful guidance on such connections can be found in The Concrete Centre publication Design of Hybrid Concrete Buildings and The Temporary Works forum (TWf) publication Precast concrete - good practice and common issues in temporary works
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A reporter was carrying out a third-party (Category 3) appraisal of a large external precast concrete structure subject to dynamic and fatigue loads. During the appraisal it was identified that the beam to column connections were formed using solid steel billet pieces.
These were inserted into sleeves and pockets cast into the side of the columns. The billets were fabricated from untreated steel plate, which was then protected using grout poured into the beam/column interface pockets at completion of the works.
The billets were intended to form the primary shear transfer mechanism between the beams and columns. However, further analysis indicated they were insufficient for this purpose and additional support columns were introduced. These measures rendered the billet connection as a temporary support only. The concern is that in structures subject to dynamic or fatigue forces, the poured grout will be susceptible to cracking and over time will lead to water ingress and corrosion of the billet.
This raises associated long-term concerns of corrosion, spalling and in the extreme potential failure of the connected parts. Guidance from the precast concrete designer suggested this detail was commonplace in precast structures, hence there is a risk that this in-built corrosion trap will exist in other exposed precast structures.
Expert Panel Comments
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Off-site manufacture, including the use of structural precast concrete elements, places considerable reliance on connections. For major bridge works, a design life of 120 years is not uncommon so all elements must be capable of operating satisfactorily for that time span. Because of the embedded positions of billets, repair or replacement would be extremely difficult. As noted by the reporter, fatigue and corrosion resistance need to be considered at the design stage for all elements including billets and their immediate surroundings.
Off-site manufacture, including the use of structural precast concrete elements, places considerable reliance on connections
For some building structures, such as car parks, billets have been used for years in connections between precast joints. Whilst subject to cyclical thermal loading and vibration, joints in car parks are however usually given a reasonable amount of weather protection by virtue of location and waterproofing surfacing on roofs.
References to such connections can be found in the following texts:
The Concrete Centre publication Design of Hybrid Concrete Buildings
The Temporary Works forum (TWf) publication Precast concrete - good practice and common issues in temporary works
Any evidence of corrosion or other problems arising from the use of billets would be welcomed by CROSS.
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