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Rebar that is installed ‘as designed’ – Part 2: Boosting the safety of your concrete connection

Posted by The Ask Hilti Team8 months ago

Tackle your concrete design with adequate reliability


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Here, we continue with our series on installing post-installed rebar as per the intended purpose and design found on this page. In the previous article, we recommend methods that improve the reliability of your concrete-to-concrete connection design, by mitigating the risks arising during execution, particularly in relation to drilling and hole cleaning. In this article, we elaborate on other aspects that enable your rebar design to become truly “jobsite-proof”.

Specification Appendix 1: Leave no room for doubt

Although this may seem banal, writing an unambiguous specification for any application is essential for avoiding changes and mistakes on the jobsite. For example, the specification text should ideally include the material required for proper installation: the extended product name (e.g. Hilti HIT HY 200-R), the bar and bore hole diameters, the embedment depth, the drilling method (e.g. automatic drilling, hammer drilling, diamond coring), and injection method (e.g. piston plug, automatic dispenser). This is then followed by the approval document number (e.g ETA XX-XXX) and refers to the instruction for use (or simply IFU) or MPII (Manufacture Printed Installation Instructor). Ideally, this ought to be included in the project documentation.

This (and more in the above-mentioned previous article) is just half the battle for ensuring peace of mind.

Specification Appendix 2: Roughening the existing contact concrete surface

Post-installed concrete-to-concrete connections require that the shear friction at the interface between the two elements is transferred adequately through surface roughness. If you write the required mean amplitude of surface roughening and the technique that must be used – hacking, chipping, water-jet, sand-blasting, etc. – in the drawing or in another construction document, this makes the requirement quite clear to the contractor.

In addition to roughening the surface prior to casting new concrete against the existing concrete, checking whether the existing surface is carbonated or not influences whether it needs to be removed. But how much of it needs to be removed? A rule of thumb is to remove the carbonated concrete over a circular area, using a diameter based upon the diameter of the bar plus 60 mm. However, the project and local code requirements often dictate this, therefore they need to be specified.

Roughening the existing contact concrete surface

and carbonated area around the existing rebar

 

Specification Appendix 2: Drilling aid for small covers

As is the case with cast-in bars, post-installed reinforcing bars must be covered with sufficient concrete to avoid corrosion. The corrosion of reinforcing bars results in a volumetric expansion of the rebar that, in turn, leads to the spalling of the concrete cover. Naturally, this results in a reduction in the steel’s load-bearing capacity. Another reason why sufficient distance must be provided from the concrete face is to facilitate drilling without splitting and/or spalling the existing concrete.

Drilling alignment can be employed to improve accuracy, and this is especially useful in deep hole installations. In the European framework, the ETA assessment document of the product typically provides the minimum clear concrete cover of the post-installed rebar in relation to the drilling method and tolerance, as shown in the following table in the ETA-16/0142 for HIT-RE 500 V3 where:

  • lv is the drilling/anchorage length of the rebar in concrete in mm
  • Ø is the diameter of the reinforcing bar in mm

Extract from the ETA-16/0142 for HIT-RE 500 V3 regarding minimum concrete cover depending on the drilling method


 

Without using a drilling aid, the deviation to be considered is 6% (0.02 lv) for hammer-drilled holes with or without a hollow drill bit, and 8% (0.08 lv) for compressed air drilling. The deviation factor is smaller (2%, 0.02 lv) if special drilling aid devices are used. If drilling aids are to be used or not, it is advisable to mention this in the relevant construction document. Additionally, note that the minimum concrete cover must also satisfy other requirements – such as durability and fire resistance – in relation to your projects and in line with the code used for the concrete structural design.

Specification Appendix 3: Training of workers

Use of the correct tools and accessories as well as technical expertise are essential where concrete connections are to be made using post-installed rebar. This includes reliable drilling machines, suitably matched cleaning and injection accessories, comprehensive instructions and documentation of the processes and, last but by no means least, proper training of the relevant personnel. This is necessary to ensure the reliability of the fastening solution over the entire life of the structure.

This consideration in your rebar design documentation can be represented as: ‘Anchor installation carried out by appropriately qualified personnel’. Furthermore, this is stated in the product documentation (ETA approval). It obliges contractors to employ trained installers who know how to use the system in accordance with the right procedures to ensure a proper installation. Hilti provides rebar training for installers; see the link here [BS1] for more details.

 

Specification Appendix 4: Inspection

For safety-relevant applications, it is advisable to specify rules for inspections. For instance, you might specify that a special inspector must be present on the jobsite during the initial installation and must then undertake periodic inspections at random to verify the adherence to the manufacturer's installation instruction and the design specification (e.g. hole cleaning procedure, hole dimensions, bar size, embedment depth, concrete cover, etc.).

The specification should also include the requirement to test a certain percentage (2.5% or 3%) of the total installations of post-installed rebar, whereby the quality and safety of the setting after installation is inspected and documented. On-site testing is a valuable tool that can help to confirm the correct handling and setting of the system and encourage installers to pay more attention to the execution. You can find out more about our Onsite Testing Service under this link.

Please bear in mind that the product qualification document or codes may contain more or different provisions. You must always ensure that the adopted rules are applicable and do not contradict other requirements.

Do you need further explanations or advice? Just leave your comment below.

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