Concrete school

What C25/30 means

First number is cylinder strength, second is cube strength, in MPa. C25/30 is the usual class for a house slab or footing.

Higher is not always better. Exposure class often sets the minimum, not a wish for “stronger concrete”.

Common classes

C12/15

Non-structural, blinding, fill.

C16/20

Light indoor slab, simple footing.

C20/25

Many indoor floors and foundations.

C25/30

Standard slab, footing, column. Often the outdoor minimum.

C30/37

Higher load, harsher exposure, many frames.

C35/45 and up

Heavy, chlorides, aggressive ground. Order with an engineer.

w/c

Water-cement ratio = water / cement. Lower w/c means denser, usually stronger concrete. Do not add water on site. That raises w/c and cuts both strength and life.

The environment matters more than the C-class

Exposure class describes what the concrete faces. Outdoors in Sweden, freeze-thaw (XF) often decides, not only the C-class.

Several classes can apply at once, e.g. XC4 + XF3 on a patio.

Overview

X0

No reinforcement, very dry. Indoor, low humidity.

XC1

Dry or permanently wet. Indoor, low humidity. Permanently under water.

XC2

Wet, rarely dry. Many foundations.

XC3

Moderate humidity. Outdoor, rain-sheltered.

XC4

Cyclic wet and dry. Outdoor in rain.

XD1–XD3

Chlorides other than seawater: de-icing salt, car parks, bridges.

XS1–XS3

Seawater. Coast, splash, tidal.

XF1

Frost, moderate water, no salt. Vertical faces in rain.

XF2

Frost with salt, moderate water. Vertical faces by roads.

XF3

Frost, high saturation, no salt. Horizontal faces.

XF4

Frost, high saturation, with salt. Salted driveways.

XA1–XA3

Chemical attack. Ground, slurry, sewage. Often a special mix.

Home rule of thumb

• Indoor reinforced floor: XC1.
• Foundation against ground: XC2, often more.
• Patio, no salt: XC4 + XF1 or XF3.
• Salted driveway: XF4 and often XD.
• Unsure: tick “unsure” in Kubik or Kubik Pro. Customer service will help.

Slump (S)

300 mm cone. The number is how many millimetres the concrete slumps when the cone is lifted. Higher slump = wetter mix.

S1 · 10–40 mm

Stiff. Kerbs, dry packing, slopes.

S2 · 50–90 mm

Plastic. Footings, slabs that are vibrated.

S3 · 100–150 mm

Soft. Common slabs, often pumped.

S4 · 160–210 mm

Very soft. Pump, tight forms.

S5 · ≥ 220 mm

Flowing. Only when specified. Segregation risk.

Flow table (F)

Flow table. Diameter in mm after 15 jolts. Used for softer and flowing mixes. F does not replace S — two different tests.

F1 · ≤ 340 mm

Stiff.

F2 · 350–410 mm

Plastic.

F3 · 420–480 mm

Soft.

F4 · 490–550 mm

Very soft. Common for pump.

F5 · 560–620 mm

Flowing.

F6 · ≥ 630 mm

Very flowing. Near self-compacting.

Slump-flow (SF)

Spread for self-compacting concrete. The cone is lifted and the diameter is measured, no flow table. SF does not replace S — another test.

SF1 · 550–650 mm

Lower flow. Tight sections, little steel.

SF2 · 660–750 mm

Usual self-compacting. Most frames.

SF3 · 760–850 mm

High flow. Dense steel, long runs. Segregation risk.

Order it right, leave the water

• Say how it will be placed: chute, pump, skip. The plant sets consistence from that.
• Pump usually needs S3–S4 or F4–F5.
• Do not add water in the chute. Order the right slump, or a standby truck.
• Retarder if unloading will take time. Not extra water.

On site

• Allow for waste. 5–10 % extra is normal. An uneven base takes more.
• A 5 m³ truck is often emptied in 20–40 minutes. Say so if you are slower.
• Several trucks: minutes between loads ≈ your unload time, so the joint can still be vibrated.
• Vibrate until the air is out. Not so long that the stone sinks and slurry sits on top.
• Cover to bars: concrete must cover the steel. Too little cover means rust, whatever the C-class.
• Max aggregate (Dmax) must fit the form and the bars. Ask for 16 or 32 mm.
• Below about 5 °C: freeze damage before the concrete has strength. Winter measures or wait.
• Hot sun and wind: the surface dries and cracks (plastic shrinkage). Start curing at once.
• Rain on an open slab before set can wash the cement skin. Cover.
• Do not pour against frozen ground or standing water in the excavation.
• Plan construction joints. Two cold loads meeting become a crack.
• Standby truck if the volume is unsure. Better than watering down the last load.

Fresh concrete burns

Fresh concrete is strongly alkaline (high pH). It causes cement eczema and chemical burns, especially inside wet boots and gloves. Rinse skin with plenty of water. Eyes: rinse for a long time, get care.

• Gloves that stand cement, not soaked cotton gloves.
• Eye protection at the chute, pump and spray.
• Long sleeves, tight boots, trousers over the boot.
• Change wet clothes. Cement on skin for hours means sores.

Dust

Dry mix and cutting hardened concrete release quartz. Quartz in the lungs is serious. Cut and drill wet. Vacuum with the right filter. Longer work: P3 respirator.

During the pour

• Reversing mixer truck. Bank it. Keep children and animals off.
• Do not stand under the pump boom. Hoses kick.
• Spill is slippery. Wash the chute and the bay when you are done.
• Heavy chutes and pokers. Swap hands.
• Power and water together: earthed tools, sound leads.

Why

Cement needs water to harden. If the surface dries too soon it becomes weak, dusty and cracked. Curing keeps moisture in, keeps off sun, wind, rain and frost, and densifies the skin — where durability lives.

When to start

Start as soon as the surface is matt-damp, not ponded. Late is too late. In heat and wind that can be within an hour. In cold it is later — still do not leave a slab bare overnight.

Methods that work

• Leave the form on. Best cover for walls and beams.
• Plastic sheet, at least 0.2 mm. Overlap, seal edges, no draught under the sheet. Lay it while the surface is still damp.
• Wet hessian or mats, kept wet, plus plastic on top against drying.
• Spray so the surface stays damp. No ice shock: use tempered water. Do not start so early that you damage the surface.
• Curing compound, sprayed full cover when the water sheen goes. Not in construction joints. Not if the face will be coated or painted unless the compound is removed.
• Combine: sheet for the first days, then watering in heat.

How long — rule of thumb

Longer in cold, with slow cement, and in harsh exposure (everything except X0/XC1). Shorter in heat. Field figures, not a certificate.

Surface ≥ 25 °C

At least 1–2 days.

15–25 °C

At least 2–4 days.

10–15 °C

At least 4–7 days.

5–10 °C

At least 6–10 days. Protect from frost.

Below about 5 °C

Hardening almost stops. Insulate, cover, wait to load. Do not freeze the fresh concrete.

After the first days

• Do not pull the sheet in midday sun. The face can crack from the shock.
• Walk lightly after one to two days in heat, longer in cold. Full load and furniture much later — ask the engineer.
• Striking form is not the same as a slab carrying load. Wall form can come off earlier than a slab can take traffic.
• Keep edges covered. Edges dry first.
• Winter: insulating blankets over the sheet. Keep the heat in the concrete.
• Summer: shade and sheet. Water under the sheet if it balloons.
• Early visible cracks: plastic shrinkage. Cover tighter next time, start earlier, cut the wind on the face.

Two jobs that get mixed up

Curing keeps the surface wet for the first days so the cement can harden. Drying is what comes after: the moisture inside the concrete has to fall to the RH the floor can take. Do not leave the slab covered for months and expect it to dry. And do not lay the covering just because the surface feels dry.

RH

RH is relative humidity in the pores of the concrete, in percent. It is not the humidity of the room air. Tight coverings, adhesive and timber usually tolerate 85% RH. Some finishes allow 90%. 80% is a stricter demand and takes longer. The limit comes from the flooring supplier, not from the plant.

Two ways the moisture leaves

Self-desiccation: the cement binds water chemically. At a low w/c, about 0.40 and below, RH falls on its own even if the surface is covered. At a high w/c that effect is small.

Diffusion: surplus water moves out through open faces. It depends on thickness, how many sides are open, and on the air temperature and RH.

What sets the time

• w/c. Lower w/c, less free water, shorter time. A high w/c can take many months.
• Thickness. Time follows the square of the thickness. Double the thickness is about four times as long, when diffusion is in charge.
• One side or two. A slab on ground dries only upward. A suspended slab or a wall dries both ways and is about four times faster.
• Temperature. About 20 °C is normal indoors. Colder air slows the drying.
• Air RH. 60% is a common indoor value. Above 70% the time gets clearly longer. Rain on an open slab puts water back in.

How it goes

• Keep the surface wet for the first days. That is curing, not drying.
• Take the cover off when the surface should start to dry. Do not leave the sheet on the whole time.
• Keep the room warm and the air from getting too humid. Do not trap construction moisture.
• Wait. A thick slab on ground with a high w/c is not a one-week job.
• Measure before the floor goes down. The surface can be dry while it is still damp further in.

Where to measure

RBK measures at the equivalent depth: 40% of the thickness for one-sided drying, 20% for two-sided. That is the moisture the floor will meet. Kubik gives a guide in weeks, a range. It does not replace a measurement, and it is not a date the plant promises.