As the name suggests, a hollowbar is a bar of metal with a central bore running along the entirety of its length. These bars are produced in the same way as seamless tubes, being extruded from a forged bar and then cut into the preferred shape.
Manufacturing Range:
Products
Thickness
Specification
Grade
32 mm to 250 mm OD
5 mm to 30 mm
ASTM A-312 & EN 10216-5
Standard Austenitic Grades (with special sulphur content)
There are three basic types of drill bits for use with hollow bars. Versatile cross cut bits in carbide or hardened steel, button bits for intact rock in carbide or hardened steel and steel stepped clay bits for cohesive soils. The selection of the drill bit type and size is based on the material that is being drilled through and the desired borehole diameter. A larger borehole diameter provides greater load carrying capacity and greater grout cover. Grout cover protects the anchor rod from corroding. Depending on the actual soil type, a 2.5 inch diameter bit can produce a 6 to 8 inch diameter grout column.
Production rates are increased through the use of hollow bar systems, as compared to traditional solid bar anchors. This is especially true when drilling through “difficult conditions”. In sites with low headroom, large scale drilling rigs and hole casing systems can be avoided. With drill rigs where “through the head grouting” is not available, grout swivels can be used to retrofit standard rotary percussion drills.
Hollow Bar Corrosion Protection
The level of corrosion protection is dependent on the anticipated service life of the anchor, installation methods, and the corrosion potential (aggressiveness) of the environment. The FHWA has studied the effects of installation on both galvanized and epoxy coated bars as reported in FHWA CFL/TD10-002. This study revealed epoxy coatings were both partially and completely removed at the leading edge of the screw profile and around the couplings.
The installations were constructed using hollow bars as both the drill rod and reinforcement. The effects of the removal reduced the service life of the bar substantially by creating concentrated locations for potential corrosion. As such, the designer should evaluate all possibilities when determining the level of corrosion protection required.