Concrete questions

A slab is priced per m² and a foundation per metre, for your job: the area and depth, the concrete grade, the steel, access for the truck and what's under the slab. Send the job and you get a written price.

A 4.8 × 2.4 m sheet is €79 to €104, depending on VAT.

A 25 kg bag of cement is about €9.65.

Revenue charges ready-to-pour concrete and concrete blocks at the reduced rate; other building materials are at the standard rate.

25 N/mm² cylinder and 30 N/mm² cube strength at 28 days, under I.S. EN 206. It used to be called 30N.

S.100 Mix B (C30/37); for yard slabs exposed to frost, Mix A (C35/45) or Mix B with at least 3.5% air entrainment.

Protect it for the first 7 days; its strength class is measured at 28 days. A standard drive is typically fit for parking after 5 to 7 days.

Not when the air is 5°C and falling, and never on frozen ground. Start at 3°C and rising, and keep the concrete at 5°C or above for 48 hours.

A standard domestic car driveway uses a minimum 100 mm to 125 mm slab of C30/37 concrete laid over 150 mm of compacted Clause 804 stone. Where oil lorries, delivery vans, or campervans will turn on the drive, stepping the slab up to 150 mm with A142 steel mesh prevents corner cracking.

You can usually walk carefully on a newly poured concrete slab after 24 to 48 hours depending on outdoor air temperature. Keep cars and light vans off for at least 7 days, and wait a full 21 to 28 days before allowing heavy oil delivery lorries or skips onto the driveway.

Concrete shrinks slightly as excess mix water evaporates during curing. Saw-cutting straight contraction joints to a quarter of the slab depth every 3.5 m to 4.5 m gives that natural tension a straight, neat relief line to follow instead of forming random diagonal surface cracks.

Resurfacing an existing driveway or laying up to 25 m2 of paving to the front or side of a house is generally exempt, provided rainwater drains into a permeable lawn border, gravel soakaway strip, or linear channel connected to a soakaway on your own property rather than flowing out onto the public footpath.

Brushed concrete is a clean grey slab textured with a stiff broom for high grip and the lowest cost per m2. Stamped concrete presses stone or cobble patterns into coloured concrete and seals it with acrylic lacquer. Exposed aggregate washes away the top millimetre of cement paste to leave natural decorative gravel showing at the surface.

Light passing mist is manageable with polythene covers on standby, but pouring in heavy rain washes cement out of the top surface and ruins the finish. In winter, concrete is not poured when ground temperatures are freezing or forecast to drop below 3C without frost blankets, as ice crystals inside fresh concrete permanently weaken the slab.