Core Drilling: Survey of Practices
Drilling holes through concrete, masonry or wood for grouting, installation or testing
Drilling is a process of making a cylindrical hole, that is long in comparison to its diameter, into a surface (called a substrate). Drilling operated as a primary activity to retrieve a cylindrical core, such as for testing purposes from a substrate is called core-cutting. In applications other than core-cutting, drilling is a secondary activity associated with primary activities such as installation or grouting. Thus, drilling is done to install anchors such as rebar dowels, post-anchored bolts, screws and fasteners., and to install sleeves or cases for plumbing, electrical, pavement lightings or other services. Drilling is also required for grouting chemicals, cementitious mixtures, blasting detonators with fuses etc.
“Resources required for drilling: Drilling machine/rig, drill-bit, extension rods (if required)”
Most common technique used is rotary drilling, where a sharp tool called drill bit penetrates the substrate by continuous rotational abrasion. Drill bits are commercially identified by the diameter and kind of substrate as follows:
- Dry drilling core drill-bit for masonry substrate available in 20-150mm dia
- Vacuum dry drilling core drill-bit for masonry or plain concrete substrate available in 40-150mm dia
- Wet drilling handheld core drill-bit for reinforced and hard concrete available in 20-150mm dia
- Wet drilling core drill-bit for reinforced and hard concrete using non-handheld drilling machines available in 20-610mm dia
- Pancake bit for recessed drill hole such as runway light canister
- Carbide bit for rebar and steel substrate
- Retractable Carbide Walker Duct Bits for interchangeable use with concrete and rebar without changing the drill bit
- Step bit for Recessed step such as for drain cover
- Dry bit for precision drilling of tiles
- Marble/granite bit for precision drilling flagstones
- Non-coring bit without unbroken core collection spoon
- Pre-cast bit for precast or prestressed concrete substrate
- Wood carbide drill bit for precision drilling wooden substrates
- Specially designed or proprietary drill-bits with Variations in drill, chisel and rake shape and/material to improve performance
Wet drilling implies circulation of water on the substrate in order to flush the debris, clean and cool the drill bit. Drilling may sometimes require water-trap rings as stand-by as self-attaching ring for water containment during core drilling on all types of stone, tile and glass.
Resources associated with drilling are electric drilling motor, operator and required extension rods, in addition to the drill-bit, all resources being consumed in hours. DeWalt, Milwaukee and WEKA are leading manufacturers of electrical drilling motors. Makes vary based on the Amps, RPM, application, and the facilities provided on the control panel. Motors are also available with mechanical, hydraulic or pneumatic transmission. The motor should be provided with water and/or vacuum swivels. Wet drilling is also to be supported if required with portable water tank, say of 10L capacity; Vacuum drilling is supported by vacuum pumps. Electric power supply may be provided from grid supply or with generators.
A column-mounted drill-rig or stand is required for vertical or near-vertical drilling. Drill rigs vary based operation angles (vertical-only/inclined) and holding down mechanism (anchored/vacuum clamp). Drill-rig mast may also be specially designed for level difference between fixing and drilling, such as stairs. Heavy-duty applications and large drilling operations may use specially designed rigs supplemented with suitable hydraulic power packs.
|A column-mounted drill-rig||
A skid-mounted drill-rig
Dynatech manufactures a Universal Drilling Machine, that is skid-mounted, vacuum anchored, self-propelled portable drilling rig. Skid-mounted horizontal core drills are also manufactured by E-Z for concrete doweling in both towed and self-propelled models. These assemblies have inbuilt motors and dust collection arrangements with replaceable drill bits.
Measures & Quantification
Most core drills operate to a standard length of drill-hole about 230mm – wet drilling core drill-bit may operate to about 330mm without extensors. Length beyond this is achieved using extensors of 150/300mm length, to say 1220mm. Longer drill holes, as for example into rock can be achieved using threaded drill bits, threaded extensor barrels and threaded caps.
Most core drills operate to a standard length of drill-hole about 230mm. Short drills shall be quantified in numbers.
Long important drills shall be measured by the length of the hole.
Effort of drilling depends on the diameter and length of drill-hole and material of the substrate. As drill-bits are commercially identified by the diameter and kind of substrate, it is simplest to define the effort of drilling for a standard drill bit and extrapolate the effort for the selected drill-bit, using an equivalence ratio. Since, this may not take into account the length of drill-hole, which is achieved by extension rods or barrels, we shall classify drilling as short drilling that would be measured in numbers without extensors and long drilling that would be measured by the length of drill-hole, with required extensors. Drill bit consumption should accommodate for the wearing of drill-bit.
Baevr Activity Models
Screw Drill: Measured in numbers, screw drilling is modelled as screw + drill motor + operator. Effort is defined based on the type of screw as By-resource.
Short drill: Measured in numbers, short drill is modelled as drill bit + motor + operator. Composition is based on the type of drill-bit, by resource. Usually, hole length less than 230mm shall be considered as short.
Hand-held Long drill: Measured by length, this is modelled as drill bit + motor + operator + extension rods. Number (of hours) of extension rod is based on the length being drilled. Composition is based on the type of drill bit, by resource.
Rig-mounted Long drill: Measured by length, this is modelled as drill bit + motor + operator + extension rods + drill rig. Number (of hours) of extension rod is based on the length being drilled. Composition is based on the type of drill bit, by resource.
Dowel drill: Same as rig-mounted long drill, except that skid-mounted dowel drill-rig will be used instead of column-mounted drill-rig
Long earth drill: Measured by length, this is modelled as threaded drill-bit + motor + operator + helper + threaded extension barrels + drill rig.