Evaluation of Bond Strength of Bonded-In or Post-Installed Reinforcement

This paper reports on a research program conducted at the American University of Beirut (AUB) to assess the effect of several variables on the performance of frame nodes with cast-in and post-installed reinforcement, and thus to evaluate the performance of different injection systems used in bonding-in reinforcement. Eighteen full-scale specimens were tested in five sets. For each of two different bar sizes (12 or 14 mm), two different nominal concrete strengths (15 or 20 MPa), and three different embedment lengths (150, 250, or 290 mm), three or four companion specimens were tested in each set. The variables within each set were the installment history (pre-installed or post-installed), the anchorage type of the pre-installed or cast-in bars (hooked or straight), and the adhesive mortar used in the post-installing procedure. The test specimen simulated the rigid connection of two cantilever beams to a wall or column. An evaluation and comparison of cracking patterns, load-deflection curves, and bond strength values indicated that post-installed straight bars using appropriate adhesive mortars developed, in general, greater bond strength than companion cast-in straight bars, especially in the case of relatively shorter anchorage length. Also, test results indicated that ultimate strength of post-installed straight bars can be predicted with good precision by careful strut-and-tie modeling of the node.

Keywords: anchors; bars; bond; strength; strut-and-tie modeling.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

The objective of bonding-in or post-installing steel reinforcement in an existing reinforced concrete structure is to provide a connection between a new concrete element and the existing structure that is similar in strength and stiffness as cast-in reinforcement. The technology of post-installed reinforcing bars is gaining increasing importance since these bars are being used frequently in horizontal, vertical, and overhead applications in rehabilitation and strengthening of existing structures. Post-installed bars are also used in specific situations in new construction to simplify construction procedures and provide flexibility in design and construction. In cases where the tensile forces have to be redirected, the cast-in connecting bars are bent in the pertaining direction. Bonded-in reinforcement, however, can only be installed with straight bar ends in drilled holes by means of a bonding agent, often a resin-based product (Fig. 1). Application examples include:

* Floor slab connections, including intermediate floor slabs, structural alterations, or reinstating temporary openings such as crane opening;

* Wall and beam connections, including construction joints, structural extensions, or horizontal starter bars;

* Vertical connections, including new columns or piers, pile caps, or adding reinforcement for structural enhancement of vertical elements;

* Major structural repairs, including concrete remedial works and structural upgrading of columns, slabs, or beams;

* Structural connections to existing reinforced concrete walls or columns, including staircases, corbels, and cantilever connections such as balconies, access platforms, and landings; and

* Concrete overlays, including bridge deck renovation and structural bonding across composite interfaces.

The efficiency of the bond between post-installed bars and the surrounding concrete at the interface of the drilled hole is dependent on the characteristics of the bonding adhesive used. A comprehensive study on the influence of drilled hole condition and environmental factors such as temperature and humidity has been performed by Cook et al.1 The response of reinforcing bars anchored by qualified adhesive mortar systems generally corresponds to that of cast-in bars.2 Typically, the bond stiffness is somewhat lower but the pure bond strength under normal conditions is considerably higher.3

Two different adhesive mortars were used in the tests: Type 1 is a hybrid system consisting of organic and inorganic binding agents. This hybrid formula ensures reliability through cement reaction that improves stiffness, especially at higher temperature, low material shrinkage in conjunction with fast curing reaction, and through bond between reinforcing bar and concrete similar to that of cast-in-place reinforcement. Type 2 is a high-performance adhesive for anchoring reinforcing bars and anchor rods in concrete. Its formula ensures reliability through very high loading values in a wide range of diameters (8 to 40 mm [0.315 to 1.575 in.]), negligible shrinkage, suitability for holes drilled using diamond coring equipment or rotary hammers or pneumatic drills, low sensitivity to oversized and wet holes, and through its red color, easy verification of proper setting on site.

BOND FAILURE OF POST-INSTALLED REINFORCEMENT

Two types of bond failure of cast-in deformed bars exist: a splitting failure and a pullout failure. If the concrete cover on the bar ribs or the spacing between the anchored bars is small, then concrete splitting will occur. If the cover and spacing between the bars is great enough, or if sufficient transverse reinforcement is provided, a splitting failure cannot develop and a pullout failure will occur or the bar will yield.