One cubic metre of concrete weighs about 2,400 kg and divides it up sharply: three quarters or more is aggregate, 12-16 per cent is cement, 7-8 per cent is water, plus a few litres of admixture. Mix design is the choice of these proportions. It starts from the required result (strength class, exposure class, consistence) and works back to the quantities, not the other way round.
The binder: the cements of EN 197-1
EN 197-1 divides common cements into five types. CEM I is Portland cement, with at least 95 per cent clinker. CEM II is a composite Portland cement, with 6-35 per cent of other constituents such as limestone (L, LL), blast-furnace slag (S), pozzolana (P, Q), fly ash (V, W) or silica fume (D). CEM III is blast-furnace cement, with 36-95 per cent slag. CEM IV is pozzolanic and CEM V is composite. Each type has a 28-day strength class (32.5, 42.5 or 52.5 N/mm2) and a letter for early strength development: N for normal, R for rapid.
The choice affects durability, site schedules and emissions. Cements rich in slag, such as CEM III/B (66-80 per cent slag), gain strength more slowly and must be cured more carefully, especially in cold weather. In return they resist chlorides and sulfates better and, according to the producers' environmental product declarations (which vary from plant to plant), emit less than half as much per tonne as a CEM I.
Water and the water/cement ratio
The water/cement ratio (w/c) governs strength and porosity. Complete hydration of the cement would require a ratio of about 0.40. The extra water only serves to make the mix workable and, as it evaporates, leaves capillary pores: these are the path by which carbon dioxide, chlorides and water reach the reinforcement. With common cements, lowering w/c from 0.60 to 0.45 raises the 28-day strength from about 30 to about 45 N/mm2 (indicative values, which depend on the cement and the aggregates).
Workability is measured by the Abrams cone slump and expressed in classes: S1 (10-40 mm), S2 (50-90 mm), S3 (100-150 mm), S4 (160-210 mm), S5 (over 220 mm). Adding water on site to make the mix more fluid is the most common cause of lost strength. In a mix with 350 kg of cement, 10 extra litres of water per cubic metre raise the w/c ratio by about 0.03.
The aggregates
Aggregates (EN 12620) are the most abundant part and the least visible. The maximum size is normally 16-32 mm, and it must not exceed a quarter of the smallest dimension of the member nor block the passage between the bars. The grain size distribution is compared with the Fuller curve, P = 100 √(d/D), where P is the percentage passing sieve d and D is the maximum size. The curve gives the maximum theoretical compactness and is then corrected to obtain the desired workability.
Cleanliness and reactivity matter too. Clays, chlorides and sulfates in the aggregate damage the mix, and some siliceous aggregates react with the alkalis in the cement (alkali-silica reaction, covered in the article on pathologies). The moisture of the aggregates must be measured every time, because the water they carry is part of the batch.
Admixtures and additions
Admixtures (EN 934-2) are dosed at less than 5 per cent of the cement mass. Water reducers remove at least 5 per cent of the water at equal workability, and high-range superplasticizers, today almost all polycarboxylate-based, at least 12 per cent. Air-entraining agents introduce micro air bubbles, normally 4-6 per cent by volume, to resist freeze-thaw cycles (XF classes). Retarders and accelerators regulate the setting time according to the season and the distance from the plant.
Additions are fine materials that enter into the calculation of the binder: fly ash (EN 450), silica fume (EN 13263), ground granulated slag (EN 15167). They serve to densify the matrix and, when they replace part of the cement, to reduce the clinker.
An example of volume calculation
A C32/40 is required in exposure class XC4, consistence S4, maximum size 25 mm. From UNI 11104:2025 we obtain a maximum w/c of 0.50 and a minimum cement content of 320 kg/m3. We choose a content of 370 kg/m3 and 175 litres of effective water, with superplasticizer, giving w/c = 0.47. The rest of the volume is occupied by the aggregates.
| Component | Mass (kg) | Volume (litres) |
|---|---|---|
| Cement CEM II/A-LL 42.5 R (density 3.10) | 370 | 119 |
| Effective water | 175 | 175 |
| Entrained air (2 per cent) | - | 20 |
| Sand 0/4 mm (38 per cent of the aggregates) | 690 | 260 |
| Fine gravel 4/12 mm (25 per cent) | 454 | 171 |
| Gravel 12/25 mm (37 per cent) | 672 | 254 |
| Total | 2,361 | 1,000 |
Curing
A correct mix can be ruined in the first few days. If the surface dries before hydration has closed the pores, the outer layer, which is precisely the concrete cover, remains porous and permeable. EN 13670 sets a minimum protection period that depends on the rate of strength development and on temperature. On site, at least a few days of moisture retention (often seven) are normally planned, using sheeting, misted water or curing compounds, and longer with slow-developing cements. Below 5 °C hydration slows down a lot, and below zero the pour must be protected from frost.