Introduction –

Post-installed anchors are widely used in the construction industry for fixing of steel structures to existing concrete members. These connections range from heavy duty structural applications such as waler beams, mezzanine floors, structural glazing, canopy, elevator guide rails, tower cranes, steel jacketing to medium and light duty applications such as equipment fixing, utility supports, cable tray bracket fixing, walkway fixing etc. In absence of a national standard over the past decades, the safety and reliability of these connections have always rested with the respective manufacturers. The Indian Standard IS 1946 – 2025 (Part 2) – “Design of Post-Installed Anchorage to Concrete - Code of Practice” by Bureau of Indian Standards is a welcome move which aims to eradicate the inconsistency across the industry with respect to design of post-installed connections.

Factors affecting performance of post-installed anchors –

The performance of a post-installed anchor is governed by multiple in-service and installation parameters.

The installation parameters include the following:

1.    concrete grade and type

2.    condition of the drilled hole surface

3.    diameter, position, and orientation of anchors (centre to centre and edge distance of anchors)

4.    hole clearance

5.    working and curing time of adhesive anchors

6.    applied torque (for mechanical expansion anchors)

In-service parameters include the following:

1.    environmental conditions (corrosion, effect of freeze & thaw, impact of chemicals, etc.),

2.    temperature of the base material

3.    load duration (significant impact for sustained loading).

4.    the condition of concrete - cracked or uncracked.

As per IS 1946 (Part 2), concrete shall be assumed to be cracked for design purposes. 

In some cases, uncracked concrete may be assumed if under service conditions the anchor with its entire anchorage depth is located in the compression zone. This will be satisfied if the following equation is observed.

= stress (taken as positive, if tension) in the concrete induced by external loads, including anchors loads;

= stress (taken as positive, if tension) in the concrete due to restraint of intrinsic imposed deformations or extrinsic imposed deformations such as displacement of support and temperature variations (refer IS 456).

Why this is important  –

In the design of reinforced concrete flexural or tension components, a cracked tension zone is assumed because concrete possesses relatively low tensile strength, which may be fully or partly used by internal or restraint tensile stresses not taken into account in the design.

Experience has shown that crack widths resulting from primarily quasi-permanent loads (dead load and live load) do not exceed the value of 0.3 mm to 0.4 mm (Schießl (1986), Bergmeister (1988), Eligehausen, Bozenhardt (1989)). These crack widths are generally acknowledged as permissible. Wider cracks are to be expected under maximum permissible service loads, which according to Eligehausen, Bozenhardt (1989) reach 0.5 mm to 0.6 mm. Even wider individual cracks can occur under conditions of restraint if no additional reinforcement has been included to limit crack widths (Schießl (1986)). The causes of cracking, as well as type, appearance and features of the various types of cracks are described in detail in Comité Euro-International du Béton (CEB) (1981) and Deutscher Betonverein 1991). Cracks can occur in one direction (e.g. in beams, one-way-spanning slabs or tension members, or in two orthogonal directions (e.g. in two-way spanning slabs and flat slabs. They may taper in width (flexural cracks) or transect the section with more or less constant width (cracks in tension members).

Cracks may run inclined to the axis of the component in case of shear or torsion, and they occur parallel to reinforcement as a result of transverse splitting stresses. It has been observed that when cracks form in a concrete member, there is a relatively high likelihood that they will intersect the anchor location either directly or tangentially. This occurs because higher tensile stresses exist around the anchor as a result of hoop stresses associated.

Lotze (1987) performed a study on a reinforced concrete beam subjected to uniform moment. The objective was to see if the crack pattern would follow the spacing of the transverse reinforcement or if the cracks would propagate to anchor locations. Several stress discontinuities were studied – externally loaded anchors, anchors set, or prestressed, but not externally loaded, and drilled holes without anchors. In all cases, the holes for the anchors acted as a stress concentration that attracted the cracks. This and other studies indicated that it is reasonable and conservative, to assume that the crack will directly intersect the anchor locations.

Figure 1 Crack pattern in a flexurally loaded slab at service load (dimensions in [mm]) (Lotze (1987)

Research has shown that anchor behavior in cracked concrete is significantly different from its behavior in uncracked concrete (e.g., Cannon, 1981; Rehm et al., 1968 & 1982; Eligehausen and Balogh, 1995; Eligehausen et al., 2006; Hoehler, 2006). When located in cracks, the anchors experience a reduction in strength and stiffness and in some cases a change in failure mode. In general, as crack width increases, the anchor strength and stiffness decrease.

Observation from study with BITS Pilani –

A very common application, beyond the regular structural connections, which require much attention, is the fixing of base plate for façade brackets. Generally, the façade brackets are fixed on the concrete members, either on the face or top of floor beams or slabs. Conventionally, it has been a practice to consider concrete as uncracked, with an assumption that the tensile stresses are induced only at the soffit of these members. However, this does not take into consideration the behaviour of concrete members with fixity at the end or the load reversal pattern under wind or seismic loads. Also, the development of cracks due to shear force is not considered in this assumption.

Figure 2 Development of tension and compression zone in a continuous beam or slab

A research project undertaken by BITS Pilani clearly spells out the requirement of considering cracked concrete for design purpose, specifically for fixing of façade brackets. As part of the work, a four storied building was analysed and designed, considering different seismic zones and considering different utility of the building. A pushover analysis as per FEMA 440, followed by finite element modelling, was performed to monitor the development and subsequent propagation of cracks across the members. The following figure indicates that beams were stressed along the entire cross-section and cracks were found to develop in the respective zones of flexural tension and further propagate along the cross-section under gravity load (see Fig. 2) and pushover analysis (see Fig. 3). It also depicts the material failure in the beam-column joint and the propagation of cracks through the concrete in the location of the façade anchor. It was observed that the cracks were transmitted throughout the member thickness and were prominent on the opposite face as well.

Additionally, it was also observed that the cracks normally propagated through the anchor locations (see Fig. 4). As the seismic zone and if the importance factor in- increased, cracks appeared more frequently and began to form at different areas across the beams (mid-spans of beams, beam-column joints, and anchor holes). The observations were consistent across all the cases. The above observations clearly indicate that the cracks can originate in any part of the structure under different load conditions, and it is not recommended to assume that the flexural cracks would be localized or concentrated when the structure would be loaded.

Figure 3 Development of cracks under gravity loads

Figure 4 Development of cracks under pushover analysis

Figure 5 Propagation of cracks through anchor locations

As a practical matter cracked concrete should be assumed for design in most cases because in practice it is difficult to differentiate between locations where the concrete will crack and where it will not crack over the life of the anchorage. Only through a careful statical analysis can a determination of the theoretical zones of cracking be made. Such an analysis would recognise that components subjected mainly to bending exhibit large zones of tensile stress under various loading cases, and hence could be expected to develop flexural cracking. However, even in vertical load-bearing elements such as walls, the anchorages themselves may introduce loads of sufficient magnitude to introduce local tensile stresses in the concrete which may cause cracks, at least transverse to the main load-bearing direction Such an analysis must also account for tensile stresses corresponding to restraint of deformations caused by shrinkage, temperature fluctuation, support settlement, etc., the magnitude of which may be very difficult to estimate. In such cases the estimation of cracking potential is made more complex.

Effect of concrete cracks on anchor performance –

The effect of cracking on anchor concrete breakout capacity is two-fold. First, the distribution of hoop stresses around the anchor gets modified to maintain equilibrium, which applies to all types of anchors. Consequently, the anchor load-displacement behaviour is significantly affected by the load acting on the anchor in any direction and the crack in which the anchor may be located. Studies have indicated that the resistance of a post-installed anchor is reduced significantly (20 to 30%) when located in concrete cracks.

Outcome of research work at CSIR-CRRI –

This has been resonated in one of the research works carried out at CSIR Central Road Research Institute (CRRI), wherein a comparative analysis was made between the behaviour of post-installed anchors installed in concrete cracks. The study was carried out on more than 80 samples of different diameters and types (i.e., mechanical and adhesive anchors). This included generation of hairline cracks in concrete member, mapping the crack propagation, installation of anchors at the location of the cracks, enhancing the crack width to allowable limit of 0.3 mm and finally carrying out failure tests on the anchors (see Fig. 5). The outcome was exactly in line with earlier findings, indicating a drop in resistance by almost 20-25% when located in concrete cracks.

Figure 6 Research work at CSIR-CRRI

Figure 7 Experimental results indicating drop in anchor resistance when located in concrete cracks for a typical mechanical anchor

The effect of concrete cracks become more pronounced in the event of an earthquake, which requires an even more stringent assessment and design for anchors used in high seismic zones, considering the effects of pulsating tension, alternating shear and crack cycling.