Why You Need To Specify The Drilling Technique While Designing PI Rebar connections

Post installed reinforcing bars are being used in different applications at jobsites. Structural Designers who designs post installed rebar connections mostly care about identifying all structural loads, ensure safe design calculations and verifying the performance of the chemical anchor to be used, to ensure structural stability of the building.

However, one of the most important factors while designing is to analyze all potential risks associated with the environment of the installation at the job site and ensure proper design specification is being made. This will help in minimizing the impact of any identified risk and will ensure that what was designed for will be achieved when the building is constructed.

One of these important factors to consider while designing post installed rebar connections, is the drilling method or the drilling technique.  Specifying an anchor for post installed rebar need to have the drilling method defined within the specification to make it clear for the installer to use the proper drilling method as this could lead to a potential risk if the improper method has been used at job site.

As we know, chemical anchors can be sensitive to different drilling techniques used, whether it is compressed air drilling, hammer drilling or core drilling, it will have different performance. For example, it is important to know that not all chemical anchors are approved or tested to be used with diamond cored holes for post installed rebar, and in such case, if it is used with core drilling and is not tested and approved, that would lead to a high risk of failure of the structure connection.

Accordingly it is important to use those approved and tested anchors only with the drilling technique stated in the third-party approval document like the ETA “European technical Assessment document”

The Above diagram shows how different drilling techniques affect the bond strength of chemical anchors in general, and we can see that diamond drilling used with chemical anchors shows lower bond strength since core drilled holes have smooth walls compared to the rough hammer drilled ones. (See below figures)

According to the EN 1992-1-1, When designing for post installed rebar using a chemical anchor which is approved to be used with core drilled holes, you need to multiply both minimum anchorage length as well as minimum lap length (That of the hammer drilling stated in the ETA) by an amplification factor if the application requires core drilling at the jobsite.

Please see example below form a post installed ETA approval of one of the chemical anchors;

According to the above, if an engineer did a design with normal hammer drilling design factors and pass it to the contractor to proceed with installation, the installer then must adhere to the same drilling technique specified, however if he face any challenge at the jobsite while drilling through the concrete due to for example existing reinforcement, and he need to use core drilling method, the case must be re-designed again and need to be verified back by the designer to ensure first; that the anchor used is approved with core drilled holes and second; to ensure that he re-design the case again using core drilling technique, to be able to re-define minimum anchorage length needed to achieve same resistance as the original design while using the defined amplification factor stated in the ETA.

On the Other side, re-designing using core drilling might sometimes lead to a deep anchorage length which might not be visible to apply at job site from practicality point of view, for that Hilti developed a new innovative solution for specifiers and installers to avoid such situation, this solution is called the roughening tool,

To be able to use such system, it must be approved and tested by third parties with specific chemical anchor and clearly stated in the ETA “European Technical Assessment” for post installed rebar application.

Hilti Roughtining tool is approved and tested by third parties (EOTA) and is clearly listed in the European technical assessment where if used, you can ignore the amplification factor and use it as 1.0 in your design with core drilled holes, please see figure below as an example from ETA document 16/0142, RE500V3 for PI Rebar with amplification factor of 1.0 if roughening tools is used after coring;

With this solution, designer can optimize the design in case core drilling is required while still complying to safe design codes and regulations and ensure what they designed for will be achieved at job site.

Same for Contractors who will ensure by using this system, safe installation at job site as well as increasing productivity through the overall design optimization of the connection by minimizing installation time and material used.

Need a support or have a question related to this article?

•  If you have any question related to the article or related to PI Rebar design, please post your question on ask Hilti platform.
• For design, you can download our Profis Rebar software from your local Hilti website.
• You can Access our 3 tutorial videos on how to use the software here; Episode 1 , Episode 2, Episode 3
• For training services, Kindly contact your local Hilti representative

References;

• EN 1992: Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General rules and rules for building construction; EN 1992-1-1: 2004
• European Technical assessment ETA-16/0142