Curing

[edit] Introduction

Curing is a term applied to the chemical process that describes a change in the state of materials, usually as they turn from liquid (or semi-liquid) to solid. It can apply to cement, concrete, adhesives, plasters, thermosetting plastics and other materials. It usually has important implications for strength development and so its effects must be considered carefully in the construction industry.

[edit] Concrete

The curing time for cast-in-place concrete can be anything from three days to two weeks. The process involves an exothermic reaction, i.e heat is given off. Ensuring the correct conditions for curing requires protecting the concrete from both high and low temperatures.

If freshly-poured concrete is not cured properly, it may:

• Set incorrectly and suffer fine cracking (‘crazing’) from shrinkage.
• Have an ultimate strength lower than the required design strength.

Both consequences could be catastrophic as cracks can allow the ingress of humidity and water which can cause corrosion of steel reinforcement or frost damage; while failing to reach its design strength will mean a weaker concrete than designed with potentially serious consequences.

[edit] Hydration

The strength of concrete depends on the reactions in its matrix and in particular on the growth of crystals. These are the result of the hydration process – the reaction between water and Portland cement. This process will continue almost indefinitely as long as there is water in the matrix. But a deficiency in water content will mean the crystals will not develop as they should and the concrete will:

• Not reach its design strength.
• Not achieve volume stability.
• Not be resistant to freeze-thaw cycles.
• Have lower resistance to abrasion and scaling.
• May not achieve target durability.

[edit] Temperature

Maintaining the correct temperature of the concrete is critical. It is currently thought that below 10°C sees a slow-down in hydration, while 4°C signals a virtual stop. In contrast, a temperature that is too high speeds up the exothermic reaction causing temperature differentials within the concrete that can lead to cracking. This causes weaker strength development as the matrix crystals have not developed as they should have.

[edit] Methods of curing concrete

Curing aims to control the rapid loss of moisture from the setting concrete, so the aim is to provide a seal to prevent or slow down the rate of water evaporation. However, methods differ according to the type of structure or curing requirement. Some of the more common curing methods include:

• Covering concrete work with polythene sheeting, particularly for vertical elements e.g columns.
• Covering with damp sand or damp hessian – applied as soon as the concrete hardens otherwise surface damage may result.
• Sprinkling or spraying.
• Ponding – can be used for horizontal surfaces such as slabs, roads, pavements etc. Involves placing small ponds of clay or sand across the surface which are filled with water. But water can leak out of the ponds and cause staining, and it is impractical for large areas.
• Steam curing.
• Membrane curing.
• Spray-on compounds.
• Formwork – can provide a degree of protection but this will depend on when it is struck. If it is left in place for four days or more, there may be no need to provide further protection after the formwork is removed. Time will therefore be an important consideration, as will the degree of exposure of the concrete work. Even if formwork is used as a method of protection, the tops of walls and columns will still require curing.

[edit] Curing time

The length of time for proper curing to take place will depend on specific conditions, national codes, etc. Flat concrete (e.g pavements, driveways, car parks etc) and structural concrete (e.g beams, columns, slabs, walls, retaining walls etc) typically require a seven-day curing time when temperatures are above 4°C. This usually allows the concrete to attain 70% of the specified compressive strength.

Curing time will depend on:

• Ambient weather conditions.
• Mix proportions.
• Specified strength.
• Size and shape of concrete being cast.

See also: Carbon dioxide curing.

[edit] Related articles on Designing Buildings

• Admixture, additive or agent.
• Admixtures in concrete.
• Alkali-activated binder.
• Alkali-aggregate reaction (AAR).
• Alkali-silica reaction (ASR).
• Carbon dioxide curing.
• Compressive strength.
• Laitance.
• Portland cement.
• Precast concrete.
• Reinforced concrete.
• Screed.
• Testing concrete.
• The properties of concrete.