The Influence of Borehole Cleaning

Post-installed fasteners transfer applied loads to the base material through load transfer mechanisms identified as keying, friction and/or bond [1]. Injection bonded anchors used with threaded rods, internally threaded rods, or rebar, transfer the tension load to the base material by means of keying at the surface between the anchor element and the mortar and adhesion combined with micro-keying at the surface between the mortar and concrete (Fig. 1). 

The load transfer mechanism at the surface between the mortar and concrete can be further divided based on the mortar product type. Epoxy based products such as Hilti HIT-RE 500 utilize strong adhesion combined with micro-keying as a load resistance mechanism. Hybrid mortars, such as Hilti HIT-HY 200, utilize primarily micro-keying for effective load transfer along the borehole surface.

Several concrete conditions exist for which an injection bonded anchor can be installed (Fig. 2), which influence the resulting performance of the fastening. These conditions are dry concrete, water-saturated concrete, water-filled holes, and flooded holes.   Dry concrete refers to a concrete condition unaffected by rain or surrounding water moister. Wet concrete (or water-saturated concrete) refers to fully hardened concrete that is water saturated. Water-filled boreholes refer to wet concrete conditions, where instructions should be followed to evacuate the water from the hole. For flooded holes, it is not possible to remove the water from the borehole and the mortar is injected directly into the flooded holes. Without specific additional information, the published bond strengths within an ETA document apply to both dry and water-saturated concrete base material conditions. Water-filled holes, and flooded holes should be mentioned explicitly within the ETA document as additional approved concrete conditions where the anchor system can be used; in such case, the corresponding performance values applicable for these base material conditions are provided.

An exposed rough borehole surface is necessary for the micro-keying load bearing mechanism to function. Inadequate or lack of cleaning of drilled holes allows for the retention of dust on the borehole surface, resulting in a smoother surface. In addition, dust on the borehole surface provides a barrier preventing proper adhesion between the mortar and concrete.

As a result of inadequate or lack of cleaning, a high increase in the scatter of resulting bond strengths and a general reduction in performance is to be expected.
 

Therefore with injection systems it is important that the hole is mechanically cleaned with a suitable brush and subsequently blown clean per the manufacturer’s installation instructions. Larger diameter holes or deep embedment depths require increased efforts and additional cleaning equipment such as the use of compressed air with extension accessories. 
 

Effective cleaning is particularly important in wet concrete, because drilling dust tends to adhere more aggressively to the borehole surface. Fig. 4 shows schematically the influence of hole cleaning on the load displacement curves of threaded rod bonded anchor systems in wet concrete.

Fig. 4 compares load displacement curves for cleaned holes by using brushing and blowing action as per manufacturer’s installation instructions for those derived from anchors installed with improper borehole cleaning. If no or improper cleaning is performed, the bond strength can be reduced up to or even more than 60% of the value for a cleaned hole [1]. In addition stiffness is significantly influenced and the “ultimate” residual load is attained at unacceptable large displacement.

The Hilti HIT-Z rod for use with Hilti HIT-HY 200 mortar is a shaped rod with multiple conical surfaces (Fig. 5). Based on its functionality, the HIT-Z anchoring system in combination with the Hilti HIT-HY 200 mortar exhibits consistent performance regardless of the cleaning efforts during installation.

The HIT-Z anchor is set by torqueing the rod during the installation process. When the required installation torque is applied, the smooth surface of the helix combined with a special coating allow a minimal axial movement and the resulting expansion forces create cracks in the mortar shell. The mortar shell segments then act as wedges to transfer lateral forces to the surface of the borehole through friction. The result is a load transfer mechanism that is combination of friction and micro-keying (Fig.6). The friction forces in particular make the system robust against dust and dirt in the borehole compared to standard elements.

Due to the functioning principle of the HIT-Z rod in combination with Hilti HIT-HY 200, no cleaning is required in dry and wet concrete with hammer drilled holes, as confirmed within the corresponding ETA. Fig. 7 compares schematically the load displacement curves using the HIT- Z element in cleaned and non-cleaned boreholes: both stiffness and ultimate load are comparable.

When a hole is drilled with a water assisted diamond core bit, the surface of the borehole is smooth and geometrically regular (Fig. 8b). In addition, even after excess water has been evacuated from the hole, the sides of the hole are coated with drilling slurry and tend to remain wet for some time [1].

Based on the smooth hole surface conditions, the bond strength which can be developed under these conditions with standard elements is highly dependent on the adhesion properties of the mortar. Alternatively, the HIT-Z rod in combination with Hilti HIT-HY 200 can be used: the cone shaped rod anchoring system produces expansion forces that increase friction along the hole surface, making it the only Hilti chemical anchor with an ETA approval for diamond cored holes where the concrete is considered to be cracked. During the installation, a sequence to flush the borehole with water and blow out with compressed air shall be performed.

Borehole cleaning represents an important step in the installation of an injection bonded anchor and has significant influence on the anchor system performance. Engineers can now specify the Hilti HIT-Z and HIT-HY 200 anchor system and remove the possibility for improper borehole cleaning to affect an injection bonded anchor’s installed performance. In addition, for a contractor, the installation of an injection bonded anchor system with no cleaning simplifies the installation procedure, and reduces required accessories as well as installation time.

For more information about the use of the Hilti HIT-Z with HIT-HY 200 system, please contact your local field engineer. An example specification (taken from the Hilti PROFIS Anchor software) for this non-cleaning bonded expansion anchor system can be found as follows:

Hilti HIT-Z non-cleaning bonded expansion anchor with Hilti HIT-HY 200 injection mortar with [60] mm embedment, [M12], Steel galvanized, Hammer-drilled installation per ETA 12/0006.

References

[1] Eligehausen R.; Mallee, R.; Silva, J.F. (2006): Anchorage in Concrete construction, Ernst & Sohn, Berlin 2006