Essential for Good Concrete

Curing is the process or operation which controls the loss of moisture from concrete after it has been placed in position, or in the manufacture of concrete products, thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time, days, and even weeks, rather than hours, curing must be undertaken for some specified period of time if the concrete is to achieve its potential strength and durability. Curing may also encompass the control of temperature since this affects the rate at which cement hydrates.

This period will depend on the properties required of the concrete, the purpose for which it is to be used, and the ambient conditions, that is the temperature and relative humidity of the surrounding atmosphere.

Since curing is designed primarily to keep the concrete moist by preventing the loss of moisture from the concrete during the period in which it is gaining strength, it may be done in two ways:

• by preventing an excessive loss of moisture from the concrete for some period of time, eg by leaving formwork in place, covering the concrete with an impermeable membrane after the formwork has been removed, or by a combination of such methods; or
• by continuously wetting the surface thereby preventing the loss of moisture from it. Ponding or spraying the surface with water are methods typically employed to this end.

Methods

There are three basic ways of curing concrete. 

1. The first - involves keeping the surface of the concrete moist by the use ponding, spraying/sprinkling, damp sand or damp hessian.
2. The second prevents the loss of moisture from the concrete by covering it with polythene sheeting or leaving the formwork in place.
3. The third involves the use of spray or roller applied curing compounds.
   

Water Curing

Water curing, if properly carried out, can be the most efficient - and the most appropriate for some types of work, e.g. floors, and include ponding, sprinkling, and wet coverings.

Ponding

On flat surfaces such as pavements, footpaths, and floors, concrete can be cured by ponding. Earth or sand dykes around the perimeter of the concrete surface retain a pond of water within the enclosed area. An efficient method for preventing loss of moisture from the concrete, ponding is also effective for maintaining a uniform temperature however, the method is often impractical except for small jobs.

Figure 1.1 Ponding the concrete surface

Sprinkling

Figure 1.2 Sprinkling the concrete surface

Continuous sprinkling with water is also an excellent method of curing. If sprinkling is done at intervals, care must be take to prevent the concrete from drying between applications of water. A fine spray of water applied continuously through a system of nozzles provides a constant supply of moisture. This prevents the possibility of "crazing" or cracking caused by alternate cycles of wetting and drying. A disadvantage of sprinkling may be its cost. The method requires an adequate supply of water and careful supervision.

Wet Coverings

Wet coverings such as hessian or other moisture retaining fabrics are extensively used for curing. Such coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. Care should be taken to cover the entire surface, including the edges of slabs such as pavements and footpaths. The coverings should be kept continuously moist so that a film of water remains on the concrete surface throughout the curing period.

Figure 1.3 Hessian requires frequent wetting

Sheet Curing

Sheet curing methods may not be so efficient but are usually satisfactory for all except very special work, and they have the big plus point that they can be carried out more easily on both horizontal and vertical surfaces.


Polythene Sheet

Figure 1.4 A roll of 125 micron polythene, 4m wide x25m long

A simple, yet effective, way of keeping moisture in the concrete is to cover it with 125-micron (500-gauge) polythene sheeting. Polythene sheet is a material that is easy to handle and store, and afterwards it can be re-used for so many other purposes on site including, or course, curing other concrete at a later date. Sheeting should not be used over pigmented or pattern stamped concrete as it can cause discoloration of the surface.

Using Polythene

The polythene should be placed in position as soon as possible - on vertical surfaces within half an hour of the removal of the formwork; on slabs as soon as the concrete is hard enough to prevent surface damage or marking. Wait until the water sheen has evaporated, but not so long that the concrete starts to dry out. When the surface of the slab has a special finish - such as a texture - the polythene should be spread over light frames above the concrete. With such finishes there is no need to wait for any bleed water to evaporate before you position the polythene; the curing can start immediately the slab is finished.

Figure 1.5 Polythene sheet is lapped, taped and tied around beams and columns

Figure 1.6 Protect the finish of a textured slab by raising the polythene on a frame

It is important that the exposed edges and corners of the sheet should be well fixed down. Otherwise wind and draughts get underneath and create a wind-tunnel effect that would dry the surface of the concrete.Formwork gives some protection, but tops of walls and columns still need to be cured. Good fixing also, of course, stops the polythene sheet from blowing about. If you have to use more than one sheet, overlap them by at least 300mm, andtape the joins. Use tapes or string for tying around beams and columns. If there is no chance of frost, and the concrete is hard enough, it will assist the curing if you spray on water before placing the polythene.

Formwork Protection

While it is in position, formwork protects concrete against loss of moisture, and thus allows curing to proceed. It is only after it has been struck that further curing - of columns, walls and beams, for instance - may be necessary. Concrete that is in an exposed position must be thoroughly cured even after prolonged protection by the formwork. But in a mild, protected environment, and provided that the formwork has been in position for at least four days, there might be no need for further curing even in dry weather. However you should check this point with the engineer or clerk of works. If the formwork is struck in less than four days, and the surfaces are designed to be a uniform colour - as in the case of a row of columns, or a continuous wall thorough and uniform curing is essential.

Figure 1.7 Formwork gives some protection, but tops of walls and columns still need to be cured.

Even where the formwork is left in place, exposed top surfaces must be cured in the normal way. The top of a wall, for example, will be exposed to more severe weather conditions than the face. The top will prove more durable if it is sprayed with a high grade-curing compound.

Where steel reinforcement projects from the top of a wall or column - through a horizontal construction joint - the top surface of the concrete should be covered with polythene sheet pressed into place over the ends of the bars. The polythene should be well taped to the top of the formwork.

When concrete will receive a final applied decorative treatment, such as rendering, plaster or paint, in conditions of mild exposure (inside a building, for instance) further curing may not be necessary, no matter how early the formwork is struck, but this is something you should check first.

Membrane Curing

Membrance curing methods are likewise, not as efficient as sheet curing methods, but have the advantage over the previous two methods in that they do not need any further supervision once applied.

Spray-on Compounds

Figure 1.8 Paved areas are cured with a super grade white, or luminised, spray-on pigment.

Curing compounds are sprayed on the surface - hand-held garden spray is suitable for most jobs - and can be used on both vertical and horizontal surfaces. It is essential always to choose the correct type of compound. Mostly, the compounds consist of a resin in a solvent that evaporates after it has been applied, leaving behind a thin, continuous film or membrane that seals-in most of wind and sun after about for weeks. Most of the compounds come in two grades, a standard grade having a curing efficiency of 75%, and a super grade with 90% efficiency. Both are usually available with either a white or aluminised pigment, or with a fugitive dye. The white or aluminised pigmented compounds are for external paved areas because they reflect he sun's rays, thus keeping the concrete cooler.

Those with a dye allow you to check at a glance that a uniform, all-over layer has been applied. They dye then quickly disappears without causing stains, provided it is not applied to a dry surface.

A super-grade compound with white or aluminised pigment it advised for external paving, but a lower, non-pigmented grade is satisfactory for structural concrete. In tropical climates, the higher efficiency grade should be used in every case.

Surfaces that will be exposed to bright daylight should not be treated with a curing compound if they are to receive any additional material that requires a positive bond. This includes screeds, rendering, paint type finishes or a further layer of concrete. A range of compounds that, it is claimed, can be used in these situations, has recently been developed. However, these compounds are not of highest standards of workmanship, so at this stage caution in their use is advised.

Figure 1.9 Fugitive dye shows whether the application has been thorough.

Curing compounds are generally non-toxic but approval should be obtained before using them on structures that will be holding drinking water.

When to Spray

The compounds should never be applied to dry surfaces since these will absorb the compound, and staining or discoloration may result. If the surface is dry, that is a sign that the curing has been left too late. 

On vertical surfaces, apply the compound - standard or high-grade-almost immediately after the formwork has been stuck. If the surface has dried out, wet it down with clean water then spray while it is still moist.

On slabs, apply the compound as soon as possible after any bleed water has evaporated, and certainly no later than half an hour after the sheen caused by the water has gone. Curing compounds do not work if sprayed on top of water.

In damp conditions the water may take a few hours to disappear, but much less on a dry day.

Using the Compounds

There are various points to watch when you are applying a curing compound. Before you start, make sure that you have enough stock in for the job in hand- you will need about a litre for each 4 of surface area - and sufficient cleaning solvent, too.

Figure 1.10 Equipment for applying a spray-on compound.

Always wear protective goggles, gloves and mask. You should avoid getting the compound on your skin, in your eyes or in your mouth, for it might cause harm. Make sure you read and understand the manufacturer's health and safety notes so that you will know what to do if anything goes wrong.Stir the compound before use, especially if it is a pigmented one, since the pigments tend to settle at the bottom of the drum. Fill the spray reservoir with the compound, then you can start work. You should hold the spray about 300 to 500 mm from the surface, preferably even closer when there is a wind.

In windy conditions, the roller application illustrated in Figure 1.11 is the preferred option. It is vital that you cover all the concrete, and with a uniform layer. This is best ensured by first moving the nozzle back and forth to apply half the quantity required, then applying the remainder by moving the nozzle at right angles to the initial strokes.	Figure 1.11 Roller application
Figure 1.12 Hold the spray close to the surface and work in two directions.	Clean out the equipment with solvent after you have finished - and whenever you break off spraying for more than, say half an hour - paying particular attention to the nozzle and hose.

Exposed Concrete

All concrete that will be permanently exposed to the weather needs to be cured with extra care, and for at least seven days. This is particularly important for visual concrete, including white and coloured concrete, plus special architectural finishes created by abrasive blasting or tooling.

When using water spray techniques on special finishes it is very important to avoid water streaking. This is best achieved by using a fine fogging mist spray with the concrete enclosed in a tent-like structure. This is more appropriate for special pre-cast concrete panels.

Wet-cured concrete will be more impermeable and better able to withstand freezing and thawing, wetting and drying. It will also have a better long-term appearance, since dirt will not collect so easily.

Polythene sheeting can be used, but a spray-on curing compound is usually more convenient, although its use may be subject to approval in the case of special visual requirements for colour and uniformity.

Colour Uniformity

The colour of concrete can vary according to how long the formwork is left is position after placing, and whether the day on which it is struck is dry or wet.

Where uniformity of colour is important, for example with as-struck, fair-faced and textured surfaces, you should either leave the formwork in position for four days, or, where it is struck in less than that time, cover or wrap the concrete in polythene sheeting for at least another three days. Curing compounds might discolour the concrete, even if only temporality, and might leave some uneven staining.

White and Coloured Concrete

The polythene sheeting method of curing white or coloured concrete needs to be used with care since direct surface contact at an early stage can lead to discoloration. Polythene has the advantage that, if left in place, it will protect the concrete from dirt caused by activities on the site. Removing stains and dirt at a later stage is both time-consuming and expensive. You could use a curing compound but only if you are sure from site trials that it will not cause staining of the concrete.

Rust Staining

The action of rain on reinforcement left projecting from the tops of walls and columns - for example, for continuity with concrete that will be placed later on - can create rust. This will form stains as it is washed down the face of the concrete. You can prevent these rust stains by applying a neat cement grout wash, or taping plastic sheaths, to the exposed reinforcement.

Figure 1.13 Cement grout or plastic sheaths prevent rust stains.

Paving and Floor Slabs

It is essential for most horizontal surfaces to be well cured, especially if they will have to stand up to foot or vehicular traffic.

Always start the curing soon after the water sheen has disappeared. However, if you are using polythene sheeting, allow the concrete to stiffen slightly first, so that the surface will not be marked or damaged as the sheet is placed on top of it.

Major concrete roads are usually cured by a membrane sprayed from a machine that is part of the paving train. They are not considered in this article.

Minor Roads, Paths and Drives

A curing membrane, applied by a hand-operated garden-type spray, is most convenient for small paved areas built by semi-manual methods of construction.

Always use a white pigmented or aluminised super grade of compound, if available, because this will reflect the suns rays. You must also take care to ensure that an even, uniform coat of the membrane is applied, particular care being needed in windy weather, when the stream of compound could easily be blown "off course"

Figure 1.14 In order to cover the entire surface you might need a moveable walkway.

If the road is fairly wide, you will not be able to reach far enough to spray the middle. You will therefore need to work from a movable walkway that spans the road. A stout plank well supported at both ends will be sufficient. However, you might find it more convenient to use polythene sheeting, and this is just as good. Keep the sheet in place for at least seven days, and make sure it is well fixed at the edges, where there is a tendency for the concrete to dry out more quickly.

As discussed before, when coloured surfacing has been used, a special spray-curing compound should be used rather than polythene.

Concrete Base Slabs

It is best to choose polythene sheet for curing a slab that is later to receive a screed of cement sand, or a granolithic or other topping. Do not use a curing compound, for this might affect the bond.

Cover the concrete as soon as any water sheen has evaporated, and the surface is stiff enough not to be damaged or marked by the polythene.

Direct Finishes and Toppings

Special attention to curing is essential for power floated and granolithic finishes, which have to be so hard wearing. Immediately after the final trowelling, the surface should be firm enough to be covered with polythene sheeting or similar, or treated with a curing compound. Polythene should be kept in place for at least seven days.

Some loss of moisture may occur at the edges and joints of the sheets, and it will then be necessary to turn them back and spray the surface with water every other day.

Cement-Sand Screeds

Curing compounds are not recommended for cement-sand screeds. The screed should be kept continuously damp for seven days, preferably by being covered with polythene.


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