Concrete core drilling requires a different approach to brick and blockwork drilling. Concrete is harder, more abrasive, and frequently contains steel reinforcement or post-tensioned tendons that cannot be cut without consequences. The correct equipment, bit specification, pre-drill scanning protocol, and CDM compliance approach all depend on the specific type of concrete being drilled — and identifying that correctly before starting is the most critical step.
Concrete Core Drilling at a Glance
- Wet diamond core drilling required for all concrete — dry bits will glaze immediately on concrete
- Reinforced concrete: soft-bond rebar-rated bit; GPR or ferroscan scan before every core position
- Post-tensioned concrete: GPR scan mandatory — cutting a tendon can cause structural failure
- Ground-bearing slabs (C25/C30): medium-bond wet bit at 300–500 RPM for 107mm diameter
- High-strength structural concrete (C40+): soft-bond wet bit — harder material, softer bond
- CDM 2015: structural concrete penetrations on commercial sites require method statements and COSHH assessment
- Water supply required: mains connection or 100+ litre tank with recirculating pump
Concrete Types and Drilling Requirements
Plain Concrete (Ground-Bearing Slabs, Foundations)
Ground-bearing slabs in domestic and light commercial buildings are typically C25/C30 plain concrete on hardcore. A medium-bond wet diamond core bit is the standard specification. RPM should be set to approximately 400–500 for a 107mm bit, reducing proportionally for larger diameters. Floor coring through ground-bearing slabs is straightforward provided the core drill stand is anchored to prevent rotation as the bit exits through the base.
Reinforced Concrete (RC Walls, Suspended Slabs, Columns)
Reinforced concrete is the standard structural material in UK commercial buildings, bridges, and infrastructure. It contains steel reinforcement (rebar) at defined spacings and cover depths. Core drilling through reinforced concrete requires a rebar-rated wet core bit — a soft-bond matrix that sheds under steel hardness rather than glazing. Pre-drill scanning is strongly recommended: a ferroscan or cover meter identifies rebar positions and enables offset positioning to reduce or eliminate rebar contact. See: reinforced concrete drilling services and drilling through rebar: technique guide.
Post-Tensioned Concrete (PT Slabs, Beams, Transfer Structures)
Post-tensioned concrete contains high-tension steel tendons under active compressive load — grouted into ducts or running as unbonded PTFE-coated strands within the slab. It is common in UK commercial buildings from the 1960s onwards, particularly multi-storey flat slabs, carpark decks, and transfer structures. Cutting a post-tensioned tendon releases the stored prestress force and can cause immediate localised structural failure.
GPR scanning by a specialist operator familiar with PT structures is mandatory before any core drilling on a suspected PT element. Post-tensioned tendons have distinct GPR characteristics from conventional rebar — the scan must be interpreted by a trained operator. Core positions must be cleared by the scanning contractor and reviewed by the structural engineer before drilling begins. See: GPR scanning before core drilling.
Precast Concrete
Factory-produced precast elements — wall panels, floor planks (Bison flooring, hollowcore), staircase flights, and structural columns — are typically C40–C50 high-strength concrete. Hollowcore planks contain prestressed strand reinforcement running longitudinally; cores through the plank should avoid the voids where possible and must not cut the prestressing strands. Precast wall panels often contain two-way mesh reinforcement. A rebar-rated wet bit and ferroscan scan are appropriate for both.
High-Strength and Dense Concrete (C40–C60)
High-strength structural concrete (C40 and above) requires a soft-bond diamond bit. The counterintuitive rule: harder material, softer bond. Hard-bond bits designed for abrasive brick will glaze rapidly on dense concrete because the matrix does not release fast enough to re-expose fresh diamond. Soft-bond segments shed more quickly, continuously regenerating the cutting face. See: how core drilling works: diamond abrasion and bond matrix explained.
Aerated Concrete (Aircrete — Thermalite, Celcon)
Aerated concrete block is structurally different from cast concrete: compressive strength 2.5–7.5 MPa vs 25–60 MPa for structural concrete. A dry diamond core bit at low RPM with light feed pressure is correct. Wet coring is generally unnecessary and risks waterlogging the lightweight block. No scanning is required for non-structural Aircrete partitions.
Equipment for Concrete Core Drilling
Concrete core drilling requires dedicated diamond core drill machines — SDS adaptors are unsuitable for concrete drilling at any practical diameter because:
- SDS chuck geometry does not accept diamond core bits above 52mm
- SDS drills lack the torque to maintain RPM through dense concrete under load
- The hammer function must be completely disabled for diamond bits; SDS trigger mechanisms make accidental activation possible
Equipment requirements by application:
- Domestic floor slabs (up to 150mm diameter): 1,500–2,000W single-speed or variable-speed core drill machine with water feed; vacuum anchor kit for stand securing
- Commercial structural concrete (150–300mm): 2,000–3,000W variable-speed machine; anchor bolt or vacuum anchor; 100+ litre water tank with pump
- Large diameter (300–600mm): High-torque column-mounted rig; chemical or mechanical anchor; mains water feed; hydraulic or geared feed mechanism
See: core drill machines guide.
Pre-Drill Scanning
Scanning before drilling concrete is not optional on commercial structural elements — it is standard practice under CDM 2015 risk management obligations and the only reliable way to avoid PT tendon strike or structural rebar damage.
| Scan Method | What It Detects | When to Use |
|---|---|---|
| Ferroscan / cover meter | Rebar position, cover depth | Reinforced concrete, standard structures |
| GPR (Ground Penetrating Radar) | Rebar, PT tendons, conduits, voids | Suspected PT, unknown embedded services, commercial sites |
| Both (combined survey) | Full embedded element map | Complex structural elements, high-risk cores |
See: GPR scanning before core drilling: when it's required and what it finds.
CDM Compliance for Concrete Drilling
Structural concrete penetrations on commercial and civil construction sites fall under CDM 2015 risk management requirements. Key obligations for concrete drilling contractors:
- COSHH assessment for silica dust exposure from wet concrete coring (slurry splash and residual airborne RCS)
- Method statement for structural penetration work: scanning protocol, core position sign-off, drilling sequence
- CSCS card for all site operatives
- For notifiable projects (30+ days, 20+ simultaneous workers, or 500+ person-days): inclusion in the construction phase health and safety plan
See: core drilling permits and regulations: full guide and core drilling for construction contractors.
Concrete Core Drilling: Common Questions
Do I need a wet core drill for concrete, or can I use a dry core bit?
You must use a wet core bit for concrete. Dry diamond core bits are designed for brick and soft masonry — their hard-bond matrix glazes almost immediately on concrete, destroying the bit within seconds. Wet coring requires a dedicated core drill machine with a water feed through the barrel or an external shroud, a water supply (mains or tank), and a collection method for the slurry. Do not attempt to use a dry bit on concrete, even for a single short core.
How do I know if the concrete I'm drilling into is post-tensioned?
Post-tensioned concrete cannot be reliably identified by visual inspection alone. The most reliable method is GPR scanning by a trained operator — PT tendons produce a distinct, consistent depth profile on GPR radargrams that differs from rebar. Building age and type are indicators: flat slab commercial buildings post-1980, multi-storey carparks, and transfer structures are commonly PT. If structural drawings are available, they should specify the slab type. If there is any uncertainty, treat the element as post-tensioned until scanning confirms otherwise.
What size core drill do I need for a drainage connection through a concrete floor slab?
The core size depends on the pipe OD. For 110mm OD soil pipe, use a 117–120mm core bit. For 160mm OD drain, use a 175–180mm core bit. Add 10–15mm to the pipe OD to give clearance for fitting and sealing. For concrete floor slabs, a wet diamond core bit on a dedicated machine is required — the core drill stand must be anchored to the slab to prevent rotation as the bit completes the cut. On suspended or reinforced slabs, carry out a ferroscan or GPR scan before drilling to locate reinforcement.