Rebar that is installed ‘as designed’ Part 1: Tips to make your specification as safe as possible

Imagine a scenario where you design a post-installed concrete-to-concrete connection. You select a qualified adhesive system that is suitable for the project’s condition and requirements (e.g. static, seismic), and is consistent with the regulatory framework used to design the main concrete structure. In the drawing, you specify the rebar (reinforcing bars) layout, sizes and embedment depths calculated using an efficient and reliable software such as PROFIS Rebar, which allows for optimisations. All jobsite circumstances are considered thoroughly in the selection process and you are confident of the outcome.

The next day however, you begin to doubt that executing this vital connection will be as straightforward as how you designed it yesterday, which is an uncomfortably familiar feeling. You wonder: how can I ensure that the application is installed ‘as designed’? You start to recall the instances where previous similar connections were fraught with problems, the installers did not have the right experience and they installed them poorly. To counteract this, you start to doubt whether there is enough margin for error within your design, and you apply an additional safety factor that increases the depth but makes the work cumbersome and expensive.

It does not have to be this way! As an engineer, you do have control over the way in which your designs should be executed on site. This is achieved through specifications in the drawings and other construction documents that improve the installation quality and put your mind at ease.

Here are the first few tips to minimise the risk of a poor installation.

Specification Appendix 1: Hammer drilling with an automatic hole cleaning system

The condition of the drilled hole during adhesive injection greatly influences the bond between the adhesive and the concrete. Firstly, the dust lining the surface of the bore hole creates a barrier between the mortar and the concrete, thereby preventing proper adhesion. Although it is evident that the dust needs to be removed properly, this procedure is neglected by the installer or, at best, is a job half done. To mitigate this potentially disastrous situation, automatic hole cleaning using special-purpose drill bits attached to a vacuum cleaner solves two problems in one, by combining the drilling and cleaning of a hole. With the dust removed and the hole cleaned in the same step, the installer can save time and reduce the number of accessories needed to complete a rebar installation. The system connection behaves as per its intended design and purpose. If you specify this method in the “General Notes” section of your drawing and in the method statement, the installer will be obligated to apply it. Adhesive systems such as HIT-RE 500 V3 and HIT-HY 200-R are qualified to be used with Hilti hollow drill bit systems (simply check the ETA assessment for the suitable conditions). More information about Safeset® can be found here.

Example of drilling work with a hollow drill bit (HDB).

Dust is removed using a vacuum system while the bore hole is made.

Specification Appendix 2: Hole roughening procedure in wet core diamond drilling

If the design embedment depth for a rebar is large, which is typically the case in large diameter post-installed lap splice connections, wet diamond drilling is often used in combination with suitable adhesive mortars. The bond strength for certain rebar diameters is reduced in this drilling condition compared with hammer-drilled holes, which is due primarily to the smooth surface produced by the coring procedure. Increasing the surface roughness by roughening the cored hole allows a greater mechanical interlock between the concrete and mortar, thus negating the loss in bond strength if this procedure is not undertaken. This solution should be specified only for qualified systems (such as HIT-RE 500 V3) and considered in the design.

Specification Appendix 3: Cleaning procedure in long diamond core drilling

Staying on the subject of diamond core drilling, cleaning involves the sequential flushing of the hole until it is clear, not filled with debris, water exits, compressed air is blown into the hole to remove debris and water, and finally the hole wall is mechanically scoured using a wire brush. All cleaning procedures end with the use of compressed air. (It is important to note that the use of compressed air may produce flying debris: the installer must always wear eye protection).

To ensure that this procedure is executed effectively, it is essential that the construction documents specify the required accessories for long hole cleaning, such as a Hilti Profis rebar accessory set. The accessories include: matched-tolerance wire brushes, brush extensions, attachments to facilitate power brushing, air wands, hose extensions, couplers and air nozzles. This solution can also be used for hammer drilling (in the case of hammer drilling, wherever possible always use automatic systems with a hollow drill bit).

Professional accessories for hole cleaning should always be specified,

wherever automatic cleaning systems cannot be used.

Specification Appendix 4: Injection with matched-tolerance piston plug

The objective of adhesive injection is to achieve a void-free installation. Aside from reducing the contact bond area and inhibiting the curing procedure, air voids in the injected adhesive lead to increased effort for the bar installation, and can cause uncontrolled ejection of the adhesive from the hole during bar installation, as air is forced out of the adhesive matrix.

To inject the adhesive with minimal air voids, professional injection systems utilize matched-tolerance piston plugs. These piston plugs ensure uniform injection of the adhesive through even pressure distribution along the length of the bore hole, allowing the installer to control the injection process, thereby dramatically improving injection quality and efficiency.

What happens inside the hole? Using piston plug systems dramatically improves the injection quality by reducing air void risks.

 

Wait for Part 2 of this series on “Installed as designed: Maximizing the safety of post-installed rebar connections”, where we will cover the other aspects.

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