Post-installed Reinforcing Bar Codes

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 and are designed using an extension of the concrete capacity design method (CCD-Method) used for headed anchors and post-installed mechanical anchors1.

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 in view of 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 follows:

ΦR n ≥ U         [1]

where U is the factored load, Φ is a strength reduction factor, and Rn is the nominal resistance (generally taken as the 5% fractile).

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 in-place reinforcing bars (Figure 2). These are commonly referred to as post-installed reinforcing bars. This application, which until 2013 was largely unregulated in theU.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 post-installed 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 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.

The assumption of bar development is perhaps the most fundamental distinction separating post-installed reinforcing bars from adhesive anchors. Although instances do arise where post-installed reinforcing bars are designed for an applied force as described by Eq. [1], the typical case conforms to the assumption of bar development.

Please visit our Rebar Design Center for information regarding post-installed rebar applications.

Hilti is the first manufacturer to offer a software solution for the post-installed rebar application. PROFIS Rebar is a web based design software that allows engineers to design post-installed rebar using the only two approved adhesive anchor systems on the market, HIT-HY 200 and HIT-RE 500 V3. PROFIS Rebar calculates tension and compression lap splice lengths, development lengths for starter bars, development lengths for special moment frames and special structural walls, and development lengths for shear dowels. PROFIS Rebar also calculates post-installed reinforcing bar embedment based on research conducted using shear friction theory. 

 
1 ACI 318-11, “Committee 318: Building Code Requirements for Structural Concrete (ACI 318-11) 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 criteriaAC308, June 2013 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.