Product Technical Guides : US-EN Post-Installed Rebar Guide - Page 3

Post-Installed Reinforcing Bar Guide
1.0 POST-INSTALLED REINFORCING BARS — WHAT ARE THEY ?
1.1 POST-INSTALLED REINFORCING BARS AND THEIR APPLICATION IN CONSTRUCTION
Since the mid-1970s , adhesive anchors have been used extensively in construction . In the U . S . and Canada , they are regulated by a variety of codes and standards , including ACI 318 , ACI 355.4 , CSA A23.3 , as well as acceptance criteria issued by the ICC Evaluation Service . Adhesive anchor systems for concrete typically employ threaded steel rod as the anchor element ( see Figure 1 ) and are designed using an extension of the concrete capacity design method ( CCD-Method ) used for headed anchors and post-installed mechanical anchors 1 .
Another common and long-standing application of anchoring adhesives is the installation of deformed reinforcing bars in holes drilled in concrete to emulate the behavior of cast-inplace reinforcing bars ( Figure 2 ). These are commonly referred to as post-installed reinforcing bars . This application , which until 2013 was largely unregulated in the U . S . 2 , can be characterized as follows :
a . Post-installed reinforcing bars are embedded in adhesive in a hole drilled into existing concrete on one side of the interface and are cast into new concrete on the other side of the interface . ( Figure 3 ) The bars may be equipped with hooks or heads on the cast-in end , but are , by necessity , straight on the post-installed end ;
b . Post-installed reinforcing bars , in contrast to adhesive anchors , are often installed with minimal edge distance . In such cases , the strength under tension loading of the postinstalled reinforcing bar connection is typically limited by the splitting strength of the concrete ( as characterized by splitting cracks forming along the length of the bar );
c . Post-installed reinforcing bars are typically not designed to resist direct shear loading in the manner of an anchor bolt ( i . e ., exclusively via dowel action3 ); and
Figure 1 — Adhesive anchor installation ( using Hilti HIT-ICE for low temperature conditions , Säntis , Switzerland ). d . Post-installed reinforcing bars are generally embedded as required to “ develop ” their yield strength using the development and splice length provisions of the code .
In this design environment the focus ( for tension loads ) is on bond failure / pullout failure and concrete breakout . Splitting of the concrete is deemed not relevant because of the limitations placed on edge distances , concrete thickness , and anchor spacing . Shear loads are resisted by the body of the anchor with the associated concrete failure modes as defined by the CCD method . From a design perspective , adhesive anchors are typically addressed in the same manner as other structural elements ; i . e ., the required strength is established as shown in equation 1 :
ΦR n
≥ U ( Eq . 1 )
where U is the factored load , Φ is a strength reduction factor , and R n is the nominal resistance ( generally taken as the 5 % fractile ).
Figure 2 — Injecting Hilti HIT adhesive in a drilled hole with Hilti pneumatic dispenser P 8000 D to facilitate installation of post-installed reinforcing bars .
1 ) ACI 318-19 , “ Committee 318 : Building Code Requirements for Structural Concrete ( ACI 318-19 ) and Commentary ,” American Concrete Institute , Farmington Hills , MI [ 2 ]. 2 ) The suitability of an injectable adhesive for installing post-installed reinforcing bar applications should be verified by ICC Evaluation Service acceptance criteria AC308 ,
March 2018 revision [ 9 ] or equivalent . 3 ) In some cases , the use of dowel action alone may be appropriate for resisting shear ; however , it should be noted that this resistance is typically associated with larger displacements .
Dowel action may also lead to premature edge failure where bars are close to an edge .
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