Steam Curing of Concrete: Ordinary Pressure and High Pressure Methods

01 Atmospheric regime

Steam Curing at Ordinary Pressure

Process data

Peak temp.≈ 90 °C

PressureAtmospheric

Strength gain28-day in 3 days

Chamber typeChamber / tunnel

Steam curing at ordinary pressure is the regime most commonly adopted for prefabricated concrete elements. The process is carried out in closed chambers or tunnel kilns, within which the temperature is raised progressively to a maximum of approximately 90 °C. Under these conditions, the accelerated hydration of cement allows the concrete to attain the 28-day strength of normally cured concrete within roughly three days — a substantial reduction in production cycle time.

Retrogression of strength A phenomenon in which concrete exposed to elevated temperature too early in the hydration process gains strength rapidly at first, but subsequently loses part of that strength. It results from the formation of a porous, poor-quality hydration gel, in contrast to the dense, compact gel produced by slow curing at lower temperature.

To guard against retrogression, an adequate delay period prior to steaming is essential. As a general guideline, the concrete temperature should not reach 49 °C within 2–3 hours of mixing, nor 99 °C within 6–7 hours of mixing. Curing at ordinary pressure is also associated with a modest increase in drying shrinkage and moisture movement relative to normally cured concrete, an effect that becomes more pronounced when rapid-hardening cement or richer mixes are used.

Curing cycle

Table 1 — Stages of a standard atmospheric steam-curing cycle

Stage	Function
Initial delay	Allows early hydration before heat is applied
Temperature rise	Gradual heating toward the maximum temperature
Holding period	Temperature is sustained to complete accelerated hydration
Cooling period	Gradual reduction of temperature before demoulding

This method is widely applied in the manufacture of precast elements, in particular prestressed concrete sleepers and precast prestressed bridge girders, both of which form a substantial part of railway and infrastructure construction programmes.

Figure 1. Summary of process parameters, curing cycle, and typical applications of steam curing at ordinary pressure.

02 High-pressure regime

Steam Curing at High Pressure (Autoclaving)

Process data

Peak temp.≈ 175 °C

Pressure≈ 8.5 kg/cm²

Strength gain28-day in ≤ 1 day

RetrogressionNone observed

High pressure steam curing, also termed autoclaving, is conducted in a closed chamber using superheated steam at elevated pressure — approximately 8.5 kg/cm² — and temperatures reaching up to 175 °C. In contrast to atmospheric steam curing, autoclaved concrete attains its 28-day strength within one day or less, and, notably, without the strength retrogression observed under atmospheric conditions. This is attributed to the combined effect of heat and pressure, which produces a more compact, crystalline gel structure rather than the porous gel formed under early high-temperature exposure at atmospheric pressure.

Concrete cured by this method exhibits several durability advantages: improved resistance to sulphate attack, freezing-and-thawing action, and chemical attack; a marked reduction in drying shrinkage, to between one-third and one-sixth of that of normally cured concrete; and an absence of efflorescence, owing to the depletion of free calcium hydroxide during curing.

The pozzolanic reaction

Mixes intended for autoclaving are typically proportioned with 20–30 per cent pozzolanic material, such as crushed stone dust. Under the accelerated conditions of high pressure curing, calcium hydroxide — Ca(OH)₂ — is liberated in substantial quantity within a matter of hours, and reacts rapidly with the pozzolanic material. Cements with a comparatively higher proportion of C₃S benefit most from this reaction, since they liberate a greater quantity of Ca(OH)₂; cements richer in C₂S yield correspondingly less benefit. Durability improvement is likewise found to be greater in mixes with a higher water/cement ratio than in leaner mixes.

Trade-off The gel formed under autoclaving is approximately twenty times coarser than that produced by normal curing. This coarser structure reduces the bond strength between concrete and reinforcement by roughly 30–50 per cent, and the resulting concrete tends to be somewhat brittle and lighter, whitish in colour.

Curing cycle

Table 2 — Stages of a typical high-pressure steam-curing cycle

Stage	Duration
Delay after moulding	Short, plant-dependent
Temperature rise	3–5 hours
Holding at peak temperature	5–8 hours
Pressure release	≈ 1 hour

Autoclaving is practised in the manufacture of cellular concrete products — such as Siporex and Celcrete — particularly for lightweight concrete elements, where its combination of rapid strength gain and superior durability is of particular value.

Figure 2. Summary of process parameters, curing cycle, and applications of high pressure steam curing (autoclaving).

Closing note

Both regimes exist to solve the same underlying problem in precast practice — reducing the interval between casting and usable strength — but they do so through different combinations of temperature, pressure, and mix design. A sound grasp of their governing parameters and mechanisms remains fundamental to specifying and controlling curing in precast concrete manufacture.