Properties of concrete: technical values and how they change over time

It is common to find a thirty-metre industrial slab, poured in summer without joints, with a transverse crack every four or five metres. No calculation error: the concrete shortened and friction with the sub-base prevented it from doing so freely. With a final shrinkage of 0.4 per mille, thirty metres of slab tend to shorten by 12 mm. A temperature change of 30 °C adds another 9 mm. The physical properties of concrete can only be read properly by remembering that none of them is constant over time.

Density

Ordinary concrete has a density between 2,000 and 2,600 kg/m³ (EN 206). Below 2,000 we speak of lightweight concrete, above 2,600 of heavyweight. In load calculations the NTC 2018 assume 24 kN/m³ for ordinary concrete and 25 kN/m³ for reinforced concrete. The reinforcement weighs: with 100 kg of steel per cubic metre the mass of the concrete grows by almost 3 per cent.

Strength and development over time

The class refers to 28 days, but hydration continues. Eurocode 2 describes the growth with a coefficient β_cc(t) = exp{s [1 - (28/t)^0.5]}, where s is 0.20 for rapid-hardening cements (42.5 R, 52.5 N and R), 0.25 for normal-hardening ones (32.5 R, 42.5 N) and 0.38 for slow ones (32.5 N). With s = 0.25, at seven days the concrete has about 78 per cent of its 28-day strength and at 90 days about 112 per cent. With a slow cement (s = 0.38), at seven days it is at 68 per cent: this is the figure to consider before striking the formwork.

Tensile strength is about one tenth of compressive strength and its scatter is higher. For this reason design neglects it in strength calculations and uses it for cracking and bond checks. Poisson's ratio is 0.2 for uncracked concrete and is taken as zero for cracked concrete.

Stiffness and creep

The secant modulus of elasticity E_cm rises from 30 to 37 GPa between C20/25 and C50/60, much less than steel (210 GPa). Stiffness also depends on the aggregate: at equal class, a concrete with basalt aggregates is stiffer than one with soft limestone.

Under a constant load, concrete keeps deforming for years. This phenomenon, creep, is described by a coefficient φ which for common structures is between 1.5 and 3. The final deformation equals the instantaneous one multiplied by (1 + φ): with φ = 2.5, a deflection of 5 mm just after striking becomes 17.5 mm in the long term. This is why floor deflections are checked with the reduced modulus E_c,eff = E_cm/(1 + φ) and why camber and stiffness must be thought through from the design stage.

Shrinkage

Shrinkage has two components. Autogenous shrinkage occurs while the cement hydrates and grows as the w/c ratio decreases, so it matters mostly in HPC. Drying shrinkage depends on ambient humidity, member size and water content. Final values for common structures are of the order of 0.2-0.6 per mille. If shrinkage is restrained, tensile stresses arise that the concrete cannot withstand, and the result is a crack. Control joints, minimum reinforcement, moist curing and low water content are the most effective measures on site.

Thermal and fire behaviour

The coefficient of thermal expansion of concrete is about 10 x 10^-6 per kelvin, very close to that of steel (12 x 10^-6). This is a lucky coincidence: if the two materials expanded very differently, reinforced concrete would not exist. Thermal conductivity ranges from 1.35 to 2.3 W/(m K) depending on density and the amount of reinforcement (EN ISO 10456), and specific heat is about 1,000 J/(kg K). The high mass and medium conductivity give good thermal inertia, useful against summer overheating.

Concrete does not burn and it protects the reinforcement, but it loses strength with temperature. According to EN 1992-1-2, with siliceous aggregates at 500 °C it retains about 60 per cent of its compressive strength. In very dense concretes (HPC, UHPC) trapped steam can cause explosive spalling of the surface layer, a risk that is reduced by adding polypropylene fibres which, on melting, create escape channels.