Concrete, mortar, screed
Three products that look similar in the bag and are not interchangeable. The difference is the aggregate.
- Concrete
- Cement, sand and coarse aggregate — the stones. It carries load. Slabs, footings, posts, anything structural.
- Mortar
- Cement and sand only, no stone, often with lime. It bonds masonry. Strong in compression between bricks, useless as a slab.
- Screed
- A fine sand-cement layer poured over a structural slab to give a flat finish for tile or flooring. Not structural: it is a surface, and it will crack if asked to carry a load.
- Ballast
- The pre-mixed sand and stone with no cement, sold loose. You add the cement. Cheaper for large volumes and how a mixer job is usually bought.
Strength classes
European classes are written as C plus two numbers — the characteristic compressive strength in newtons per square millimetre at 28 days, measured on a cylinder and on a cube. C25/30 is a 25 N cylinder and a 30 N cube. American mixes are quoted as psi, where 3000 psi is roughly C20/25.
| Class | Typical use |
|---|---|
| C16/20 | Non-structural fill, blinding, bedding for kerbs |
| C20/25 | Shed and greenhouse bases, light foundations, fence posts |
| C25/30 | The general-purpose domestic mix: slabs, footings, paths |
| C30/37 | Driveways, garage floors, anything taking a vehicle |
| C35/45 | Structural work, heavy loads, exposed conditions |
Choosing a class above what is needed is not free, and it does not compensate for a thin slab or missing reinforcement. What thickness and what steel a pour needs is a structural question that depends on load and ground — this is a materials page, not a specification.
Yield and how it is ordered
Premixed bags state a yield: the volume of wet concrete one bag makes, which is well under the bag's own volume because water fills the voids. A 25 kg bag typically yields about 12.5 litres; an 80 lb American bag about 0.6 cubic feet.
bags = ceil(volume × (1 + over-order) ÷ bag yield)
Ready-mix is ordered in cubic metres in Europe and cubic yards in the United States — 1 m³ is 1.31 cubic yards. Suppliers usually have a minimum load and a part-load charge, so the price per unit rises sharply on very small orders.
| Volume | Practical answer |
|---|---|
| Under 0.2 m³ | Bags, mixed in a bucket or a barrow |
| 0.2 to 1 m³ | Bags with a hired mixer, or ballast plus cement |
| Over 1 m³ | Ready-mix: about 80 bags per cubic metre is not realistic by hand |
| Over 6 m³ | A full load; ask about part-load charges below this |
Order about 10% over the calculated volume. The sub-base is never level, formwork bulges, and some concrete always stays in the barrow. A short pour cannot be topped up once it has started to set — it becomes a cold joint, which is a weakness rather than an inconvenience.
Practical notes
- Ready-mix arrives with a clock running. Have the formwork, the barrow route and the labour ready before the truck.
- Cement is caustic. Gloves and eye protection, not a nicety.
- Do not add water to make the mix easier to work. It weakens the result more than any class upgrade can recover.
- Cure slowly: keep the surface damp and covered for several days. Concrete gains its strength over 28 days, not overnight.
- Bagged cement has a shelf life and hates damp. A bag that has gone lumpy in a shed is not worth the pour.
Strength classes follow EN 206. Bag yields vary between manufacturers — take the figure from your own bag. Nothing here is a structural specification; thickness, reinforcement and exposure class belong to whoever is responsible for the design.