Post-Installed Rebar, What Are They?

A common and long-standing application of anchoring adhesives is the installation of deformed reinforcing bars (rebar) in holes drilled in concrete to emulate the behavior of cast-in-place reinforcing bars. These are commonly referred to as post-installed reinforcing bars. This application can be characterized as follow:

•  Post-installed reinforcing bars are embedded in adhesive in a hole drilled in existing concrete on one side of the interface and are usually cast into new concrete on the other side of the interface. The bars may be equipped with hooks or heads on the cast-in end, but are by necessity straight on the post-installed end;

• Post-installed reinforcing bars, in contrast to adhesive anchors, are often installed with small concrete cover (3f> c > 2f, where f is the reinforcement bar diameter and c is the concrete cover). 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);
• Post-installed reinforcing bars are typically not designed to resist direct shear loading in the manner of an anchor bolt and
• Post-installed reinforcing bars are generally embedded as required to “develop” their design stress σsd using the basic required anchorage length, design anchorage length and splice length provisions of Eurocode2 .  In order to achieve ductility of the structure the design stress will often be close to the design yield strength.

In some specific cases the tensile stress in post-installed reinforcing bars must be directly transferred into the concrete. In such cases they are designed as anchors taking account of the CCD-Method. The two design theories rebar theory and anchor theory, have peculiar differences which are summarized in the following Table;

                                                                                           
Application range

As noted above, post-installed reinforcing bars are typically used to develop a monolithic connection between new and existing concrete elements or structures. Post-installed reinforcing bars are used in both retrofit work and in new construction and are suitable for a wide range of applications.

One of the most common classes of applications for post-installed reinforcing bars is the extension of existing reinforced concrete (R/C) structural elements such as slabs, walls, and columns (as shown in below figures), either to facilitate expansion of floor space or to address other functional changes in the use of the structure.

Such applications usually involve the placement of large numbers of bars with close spacing. In some cases, the post-installed reinforcing bars are installed close to the surface of the concrete (e.g., using the minimum concrete cover according to Eurocode2 provisions, whereby the presence of existing reinforcing must be taken into account).

Where applicable, such as in a column, slab, or wall extension, it is generally preferable to place the post-installed reinforcing bars inside of the existing reinforcing bar cage, both to minimize spall during drilling and to ensure adequate cover. Avoidance of existing reinforcing is facilitated by the use of reinforcing detection equipment (please see below figure).

Since the required embedment to satisfy anchorage length provisions of the building code (typically -40 to 60 bar diameters) often greatly exceeds typical anchoring embedment (generally limited to 20 bar diameters), special precautions may be necessary to ensure that the holes are drilled straight, the drilling process does not spall or otherwise damage the existing concrete or the existing reinforcing or other embedment.  Another class of applications includes the strengthening of existing concrete structures, often to improve performance e.g. due to refurbishment.

A third application class with unique requirements is the extension, rehabilitation, and strengthening of existing concrete bridges and other civil engineering structures (Figure below). These applications are often distinguished by the need for enhanced resistance to corrosion and temperature extremes.

Compatibility of post-installed reinforcing bars with cast-in-place reinforcing

Post-installed reinforcing bars are designed to transfer tension loads only. Extensive research programs have been conducted to verify that post-installed reinforcing bars installed demonstrate load transfer and load vs. displacement behavior comparable to cast-in-place reinforcing.

When the value of minimum concrete cover is greater than 3ϕ (where ϕ is the bar diameter) (i.e., where splitting does not limit the bond strength), post-installed reinforcing bars typically exhibit higher pull-out strength than cast-in-place bars of equivalent bar diameter and anchorage length.

It should be noted that the method of load transfer between post-installed reinforcing bars and concrete in which they are anchored is bearing of the reinforcing deformations (lugs) on the adhesive surrounding them. These bearing stresses in turn are transferred from the adhesive to the surrounding concrete via adhesion and micro-friction, whereby the lateral dilation of the adhesive layer in response to the bearing stresses enhances the friction mechanism. The concrete in turn develops circumferential (hoop) stresses around the bars that can result in splitting cracks at certain load levels. This response is identical to that observed for cast-in-place reinforcing bars loaded in tension.

For near-edge bars subjected to tension loads the ultimate limit state behavior is characterized by splitting of the concrete along the bar or splice in response to the hoop stresses developed around the bar. Provided that the adhesive used can accommodate redistribution of stresses along the bar length in a manner similar to cast-in-place bars, post-installed reinforcing bars exhibit similar peak strengths to cast-in-place bars (for small values of concrete cover).

In 2006, the European Organization for Technical Approvals issued the technical report TR023 Assessment of Post-Installed Reinforcing Bar Connections. This document provides guidance for verifying that post-installed reinforcing bar connections built with a specific mortar system will exhibit comparable behavior to cast-in-place reinforcing bar connections in terms of load, and displacement behavior under several environmental conditions.

NOTE: Where it has been verified through appropriate qualification testing (in accordance with EOTA TR 023 or similar procedures) that a given post-installed reinforcing bar system results in similar bond strength and displacement behavior as cast-in-place reinforcing bars, the design of post-installed reinforcing bar connections employing that system can follow the provisions for cast-in-place reinforcing bars.

In addition, the intended use of the technical report are applications which are allowed with straight rebar according to Eurocode2 (as shown in below figure).

This “limitation” is based on the fact that the tension forces from the rebar transferred to the concrete by bond should be balanced either by local (e.g. splices) or global compression struts to avoid brittle concrete failure. A tension loaded rebar may cause brittle concrete cone failure by using the tensile capacity of the surrounding concrete if not balanced by compression struts. Since this failure mode is prevented by means of global or local compressive struts, it is not considered in the national concrete code design rules.

As a consequence of what has been declared in the previous chapter, the application range of post-installed rebar is limited by EOTA TR 023 to:

(a) Overlap joint for rebar connections of slabs and beams and overlap joint at a foundation of a column or wall by means of a non-contact splice. In this case the tension loads are transferred between adjacent bars via compression struts. The tension forces generated by the hoop stresses are taken up by the stirrups or transverse reinforcement, respectively, in the splice area.

(b) Simply supported beams and anchoring of reinforcement to cover the line of acting tensile force Based on strut and tie modelling, at the end support of a simply supported beam the node consists of a diagonal strut, the force from the tie and the supporting force

                                                                                                        .

The equilibrium of this node is provided by bond between rebar, adhesive and concrete. The anchorage condition of the post-installed rebar can be assessed as a CCT node (compression-compression-tension node). The post-installed reinforcing bar is loaded against the horizontal component of the diagonal strut, while the vertical component of the diagonal strut is in equilibrium with the supporting force.

Due to the presence of the horizontal compression stresses in the area of the anchorage, the post-installed rebar may fail by pullout or steel failure but not by concrete cone failure. Similar conditions can be expected for anchoring of reinforcement to cover the line of acting tensile forces where post-installed rebar is used as reinforcement in tension areas. Therefore, these applications are also covered by EOTA TR 023.

Above figure shows also a typical application, connections subjected to a bending moment, of post-installed reinforcing bars explicitly excluded from the applications of post-installed rebar by EOTA TR023. Such cases must be considered with specific models going beyond the approval-based approach.

After an extensive experimental and numerical analysis Hilti developed a design method for moment resisting connections made out of straight bars. On contrary, in cast-in place construction, such nodes are realized with hooked bars according to the Eurocode2 provisions. (please refer to our webinars section to know more about Hilti rebar design method)

 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.

References;

• Fuchs, W.; Eligehausen R.; Breen, J. E. (1995/2): Concrete capacity design (CCD) Approach for fastenings to concrete. ACI Structural Journal, Vol. 92 (1995), No. 1, S-73-94
• Appl, J.: Tragverhalten von Verbunddübeln unter Zugbelastung; Dissertation, Lehrstuhl für Werkstoffe im Bauwesen, Universität Stuttgart, 2009
• DIN EN 1992: Eurocode 2: Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken – Teil 1-1: Allgemeine Bemessungsregeln und Regeln für den Hochbau; Deutsche Fassung EN 1992-1-1:2004
• Hamad, B., Hammoud, R., and Kunz, J. (2006). "Evaluation of Bond Strength of Bonded-In or Post-Installed Reinforcement," ACI Structural Journal V. 103, No. 2, pp. 207-218.
• Kunz, J. (2005). "Splitting Design for Anchorages and Splices with Post-Installed Reinforcement," Proceedings, ICF XI - 11th International Conference on Fracture.
• Kupfer, H., Münger, F., Kunz, J., and Jähring, A. (2003). “Nachträglich Verankerte Gerade Bewehrungsstäbe bei Rahmenknoten,” (Post-installed Straight Reinforcing Bars in Frame Joints), Bauingenieur, Band 78, 15 pp. (in German).
• Palieraki, V., Vintzileou, E., Silva, J. (2013). Discussion 109-S73/from the November-December 2012 ACI Structural Journal paper “Towards an Improved Understanding of Shear-Friction Behavior,” ACI Structural Journal Sept.-Oct. 2013, pp. 887-888.
• Randl, N. and Kunz, J. (2012). “Concrete Splitting for Rebars Post-Installed with High Bond Adhesives,” Proceedings of the 4th International Symposium on Bond in Concrete, Cairns, J., et al., eds., Manerbio, Brescia, Italy, pp. 521-526
• Simons, I. N. (2007). "Verbundverhalten von Bewehrungsstahl Stäbe unter Zyklischer Belastung,” (Bond Behavior of Reinforcing bar Rods under Cyclic Loading), doctoral thesis, University of Stuttgart (in German).
• Spieth, H., Ozbolt, J., Eligehausen, R., and Appl, J. (2001). "Numerical and Experimental Analysis of Post-Installed Reinforcing Bars Spliced with Cast-In-Place Reinforcing Bars," Proceedings, International Symposium on Connections between Steel and Concrete, RILEM Publications, Stuttgart, pp. 889-898.
• Spieth, H. (2002). "Tragverhalten und Bemessung von Eingemörtelten Bewehrungsstäben,” (Structural Behavior and Design of Bonded Reinforcing Bars), doctoral thesis, University of Stuttgart (in German).
• European Organisation for Technical Approvals (EOTA): Technical Report 023, Assessment of post-installed rebar connections, 2013