The impact of diamond-cored hole vs hammer-drilled in post-installed rebar application and the Hilti solution
Post-installed reinforcing bars are being used in different applications at jobsites. During design phase, structural engineers usually care about identifying all structural loads, ensuring a safe design and verifying the performance of the chemical anchor to be used. Ultimately, it is to ensure the structural stability of the building.
The impact of different drilling method – Bond Strength
One of the important factors to be considered on post-installed rebar design is the drilling method. A clear specification, not only to the chemical anchor, should also be made to the drilling method as a clear instruction to the installer. With an improper drilling method being used, it could lead to a potential structural risk as the performance of the installed rebar might not reach the design capacity.
As we all know, the working principal of chemical anchors are relying on bonding and micro-keying effect. Chemical anchor can be very sensitive to the borehole condition, which is linked to the borehole roughness created by different drilling methods (e.g. compressed air drilling, hammer drilling or core drilling) and the cleaning steps applied. Different parameters lead to performance variation.
The below diagram (see fig. 1) shows the impact of the different drilling method on the bond strength of chemical anchors in general. Since the diamond-cored holes have smooth surface compared to the rough hammer-drilled ones (See fig. 2) and chemical anchor relies on the micro-keying effect to take place between the interface of chemical anchors and concrete, it greatly reduce the chemical anchor performance in a diamond-cored hole
.
Fig. 1 Bond Strength-Displacement curves for chemical anchor with different drilling method
Fig.2 The borehole condition of hammer-drilled and diamond-cored hole
The impact of different drilling method – Amplification factor in ETA
It is important to know that not all chemical anchors are tested and approved to be used with diamond cored holes for post-installed rebar, and the wrong application could lead to a serve performance drop and a fail rebar connection.
According to the EN 1992-1-1, when designing for post-installed rebar using a chemical anchor which is approved to be used with diamond-cored holes, an amplification factor, listed in the product ETA, needs to be applied to both minimum anchorage length as well as minimum lap length.
Fig. 3 example extracted from a post-installed rebar ETA of chemical anchor
When an engineer did a rebar design with hammer-drilled hole, the installer must adhere to the same drilling method specified. However, if there are any challenges in the jobsite to go with the specified drilling method (e.g. hitting an existing rebar during drilling and hence diamond coring is used to drill through the rebar), re-design must be carried out and an engineering verification/approval (needless to say to get the approval for drilling through rebar!) must be received. This process ensures (1) the chemical anchor to be used is tested and approved according to the ETA for the use in a diamond-cored hole (2) a re-design with the consideration of the amplification factor and its impact to the minimum anchorage length and the minimum lap length.
However, a re-design with diamond-cored hole might lead to a deeper anchorage length which could be not feasible to the jobsite condition.
The Hilti solution to bring back the performance of diamond-cored hole
Understanding the challenge, Hilti developed a innovative and test-proven solution for specifiers and installers and it is the Hilti Roughening Tool (TE-YRT) (see fig. 4)
Fig. 4 Hilti Roughening Tool (TE-YRT)
Hilti Roughening Tool is approved and tested by third parties (EOTA) and is clearly listed in the Product - European Technical Assessment (ETA). With the roughening tool, designer can ignore the amplification factor and use it as 1.0 in your design with diamond-cored holes.
Fig. 5 Extracted from the Hilti RE500 V3 ETA-16/0142 for post-installed rebar with application factor 1.0 for diamond-cored hole with the use of Hilti Roughening Tool TE-YRT
Summary
With this solution, designer can optimize the design when diamond coring is required and stay with full code compliance.
To the contractors, the Hilti roughening tool ensures a safe installation at jobsite as well as increasing productivity through the overall design optimization of the connection by minimizing installation time and material used.
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