CROSS Safety Report
Grouting of joints between load-bearing prefabricated concrete members
This report highlights the risk of failure if grouted joints between load-bearing prefabricated concrete members are not designed and installed correctly.
It demonstrates that design of grouted joints can be complex and that there are many factors to be considered.
It discusses the roles and responsibilities of the parties involved and the importance of adopting correct grouting procedures.
Key Learning Outcomes
For structural and civil design engineers:
Understand the roles and responsibilities of the In-Service Designer as defined in AS3850: Prefabricated concrete elements
There is good practical advice available such as the NPCAA Understanding Grouted Precast Joints: A guide for engineers and building contractors
Carefully consider the many factors that may affect the design and provide full details with accompanying procedure to the builder/contractor
For construction professionals:
Talk to the In-Service Designer to ensure that you have full details and understand the correct grouting procedure
Consider conducting trials to prove the adequacy of the grouting procedures
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A reporter has been concerned for some time about the design of grouted joints between loadbearing prefabricated concrete units. There appears to be much less effort applied to the design and installation of grouted joints when compared with connections between steel elements.
They note that there have been several failures, particularly in high-rise buildings in Australia over recent years, and that the cost of remediation generally far outweighs any extra cost and time in grouting the joints correctly in the first place. While all parties involved in the design, documentation, and inspection of grouted joints have a role to play in avoiding failures, the In-Service Designer (as defined in AS3850) has particular responsibilities as outlined below. Accordingly, it is important that contract documents, for example project technical specifications, align with AS3850 so that there is no confusion regarding which party is responsible for a potential failure.
While all parties involved in the design, documentation, and inspection of grouted joints have a role to play in avoiding failures, the In-Service Designer (as defined in AS3850) has particular responsibilities
Responsibility for design of joints
The National Precast Concrete Association of Australia (NPCAA) has produced a guidance document: Understanding Grouted Precast Joints: A guide for engineers and building contractors. While strongly recommending that all those involved with the design and erection of prefabricated (precast) concrete elements should adopt this guide, the reporter stresses that it is the In-Service Designer who must check the capacity of the joint and the adjoining concrete to be satisfied that all actions can be appropriately transferred through the joint to reduce the risk of failure.
It is the reporter’s experience that many grouted joints are not designed but are assumed to be satisfactory if they are the same size and strength as the prefabricated concrete they are joining. While this may be adequate for low-stressed joints, it is not adequate for high-stressed joints. Grout may have similar properties to concrete, but unlike concrete, grout is typically placed into spaces that cannot be inspected and it is often batched on-site by relatively unskilled labour with little verification of mixing proportions or consistency.
Detail design of joints
In the opinion of the reporter, it is not good enough simply to state on the drawings or in the specification words such as “Grout all connections” and leave it up to the prefabricated concrete manufacturer, erector, or the contractor to sort out what is required. That is not their responsibility, and nor should they accept it. In such circumstances, a prefabricated concrete manufacturer, erector or contractor should ensure that non-acceptance occurs in written correspondence.
It is the In-Service Designer’s responsibility to design, specify and document correctly the performance requirements for the grouted joints. They should select grout materials from a proprietary supplier, review the technical information and be satisfied that it is appropriate for the intended use.
It is the In-Service Designer’s responsibility to design, specify and document correctly the performance requirements for the grouted joints
The reporter notes the following items to be considered by the In-Service Designer:
The load to be transmitted through the joint;
The type of joint, e.g., if it is stepped externally then the lower step should not be grouted;
The joint may be reduced in width for sealants and have other constraints such as chamfering, adjoining slabs or offsets;
The brittle nature and low tensile strength of grout;
Allowance for all construction tolerances;
Joints need to be designed to facilitate air displacement to maximise bearing contact with the grout to be placed. Grouting of flat, horizontal surfaces without appropriate air venting must be avoided; and
Whether the joint has dowel bars passing through it with dowel ducts in the adjoining prefabricated concrete elements.
For load bearing joints the reporter has noted that the following key items are not always fully considered:
The sequence of construction and the stage when joints are to be grouted related to the application of load e.g., from floors above;
The implications of the use of high-strength concrete that may exhibit increased brittle behaviour; and
For highly-loaded joints there is likely to be an ineffective zone at the edges reducing the effective width of the joint.
Good grouting procedures are important, and the experience of the grouting company should be reviewed by the In-Service Designer. Some of the issues that concern the reporter are:
Insufficient use of testing of grouting procedures and lack of on-site inspections.
The use of levelling shims and ensuring these are removed at the correct time.
Expert Panel Comments
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Precast/prefabricated concrete construction can be used by engineers, architects, builders and contractors very effectively to achieve speed of construction with quality and aesthetically pleasing finishes. With medium and high-rise apartment construction, load-bearing prefabricated elements (walls and columns) are commonly used as the primary structure and the reporter raises valid concerns if correct grouting procedures are not followed.
With precast construction the crucial part of the system is the ‘Grouted Joint’ where all the design actions are transferred through a very small area, and the importance of this part appears not to be fully understood by many engineers. If it is not designed, detailed, and built correctly there is a high likelihood of failure similar to that which has been witnessed in recent years.
With precast construction the crucial part of the system is the ‘Grouted Joint’ where all the design actions are transferred through a very small area
The In-service Designer
The National Precast Association of Australia (NPCAA) document referenced in the report gives good guidance with respect to grouting of precast elements. As the reporter states it is the structural engineer who is responsible for the correct design and details of the grouted structural joints and this is now recognised in the recent edition of AS3850 with “In-service Engineer’s” responsibilities.
It is the In-Service Designer’s responsibility to identify the load-bearing and non-load-bearing prefabricated elements and to convey the correct details to the builder/contractor including dimensions of the grout seating (minimum structural requirements), the required grout strength at loading (compressive strength test on grout), and the grouting procedure.
It is important that the builder/contractor correctly follows the design and details by having a documented QA procedure and the experience of the grout supplier can be called upon to provide input and to train construction workers on correct grouting procedures.
It is important that the builder/contractor correctly follows the design and details by having a documented QA procedure
For highly-loaded joints, it is important to consider the shrinkage and creep properties of the grout and the potential for differential behaviour relative to the precast elements. Where there is doubt, the procedure could call for testing and monitoring of completed high-risk joints.
As the reporter notes, the sequence of construction and the application of loads from above are important, and this may affect the effectiveness of grouted joints. For example, the grout may be damaged even with the slightest movement of the precast elements, e.g., from adjacent construction activities. To mitigate this risk an additional temporary stabilising system could be used to prevent movement until the grout reaches sufficient strength.
Although this report is directed at prefabricated concrete elements, correct grouting procedures are equally important for steel structures, as noted in Recommendations for Structural Grouting by Mullins and Parker published in Structure magazine Sept 2019 that reports on some failures in the USA.
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