For some time, development workers have promoted the small-scale production of concrete products as an income generation activity. The main product has been roofing tiles. After casting, the concrete roof tiles are kept in curing chambers until they pass a strength test. Curing can be done by immersion in water or, where plastic tarps are available, in water vapour under a plastic tarp. This article focuses on the plastic covering for solar powered high humidity curing chambers and on the design of easy-access curing chambers.
 
Cement comes in a powder form. When it is mixed with water, a chemical reaction begins resulting in a strong binder. This chemical reaction can take many years to complete. Initially, the chemical reaction happens very fast; about 80% of the reaction takes place in the first 28 days after adding water. Usually, roof tiles are cured in a curing chamber for about a week. While curing, the chemical reaction needs a constant source of water. If the concrete dries out, the chemical reaction will stop.
 
When concrete tile production was first promoted (about 1978), the tiles were cured in tanks filled with water. The tiles had to be cured for about 3 weeks before they could pass the strength standard. After some years, it was discovered that curing could be done in a water tank with just a few centimetres of water at the bottom, as long as the top was covered. One surprising thing about this was that the roofing tiles passed the strength test much faster in about a week’s time. In most cases, the covering was a black plastic tarp.
 
When we consider that most chemical reactions happen faster at a higher temperature, we find this result not so surprising. Immersing in water is cold. Curing with a little water under a plastic tarp is hot and humid. In industrialised countries, concrete products are often cured in 8 hours in steam rooms fed by boilers. Boilers are very capital intensive. Having an efficient solar curing tarp that gives us a high temperature is an appropriate alternative.
 
Solar concrete curing tarps
A clear plastic tarp is much more efficient than a black plastic tarp. This is because a clear plastic tarp will let more light energy through to the concrete. With a clear plastic tarp, we will have hotter concrete and cooler plastic than with a black plastic tarp.
 
Some clear plastic tarps are better than other clear plastic tarps. Ideally, we want a plastic which will let in as much sunlight as possible and trap it inside as heat. In scientific terms, we might say we want a plastic that is transparent to ultraviolet light (predominant in the sunlight) and opaque to infrared light (which can’t be seen by the human eye, but can be felt; we usually call this light "heat."). We also want the tarp to be durable and capable of trapping the water vapours.
 
Several different plastics were tested on a curing chamber to see which one would trap the maximum heat. A composite of a typical day is shown here:
 
The conclusion we can draw from these tests is that a higher temperature will always result from a clear plastic.
 
Please note that roof tiles at the CVBT pass strength tests after 5 days (or less) at the above temperatures. We must ask how much effect temperature has on the time it takes to cure concrete. One concrete expert explains it this way: "If the temperature of the water is – let’s say – +250C and you have found out that the tiles should be stored in water for 5 days, you will have to store the tiles for 6 days if the water temperature is +200C, for 8 days if the water temperature is +150C, for 13 days if the water temperature is +100C and for 22 days if the water temperature is +50C1."
 
This is saying that the higher the temperature the lesser the effect on the curing time. So, it is not necessary to have the most efficient plastic. Even a black plastic can give a high temperature.
 
Reducing the curing time is one way to reduce the amount of capital investment and working capital required. Reduced curing time means fewer curing chambers are required and lesser stock is kept in the curing chamber. Having excess capacity of curing chambers is not necessary. There are other factors affecting the time it takes for a product to pass a strength test. Things that increase the curing time include: using a lot of water in the concrete mix and a cold temperature. Things that reduce the curing time include: using less water, using a superplasticizer (a chemical additive that reduces the amount of water required to pour the concrete), a mix with a high cement content, or a hot curing temperature.

Curing Cover Performance

Easy access curing chamber
It is useful to have a curing chamber that makes it easy to put products in and take them out. One which has walls all around forces workers to lift the products up and over. Sometimes the products will hit the wall and be chipped or break. Also, the plastic covering will sag or tear when it rains. It is better to have a curing chamber that has a place to lean products but does not have walls. Workers do not have to lift the products up high. The plastic rests directly on the concrete and will not sag or tear when it rains. It costs less too.
 
Materials and Fabrication of Equipment
 
Solar Concrete Curing Tarps
Polythene (also called polyethylene or PE) is best. It should have a UV stabiliser; if it doesn’t, it will last only a few months. PVC does not last as long as PE and is more affected by high temperatures. The plastic comes in rolls of 50 or 100 yards, and in various widths. A thickness of at least 0.25-mm should be used. It can be cut with scissors and sewn on a heavy-duty sewing machine. Nylon thread should be used. Cotton thread will deteriorate in the caustic cement environment.
 
The edges may be folded over, sewn and a plastic hose inserted. This provides some weight so that a wind will not easily blow the tarp open. Always use a new plastic hose; old hose turns black and gets very hot. Too much heat will make the plastic tarp brittle and crack.
 
Easy Access Curing Chambers
A curing chamber can be made from a concrete floor with a leaning wall made of concrete blocks or bricks or stabilised soil blocks. It must be waterproof. Make sure the floor is poured before the blocks are mortared. If the blocks are used for the formwork, the curing chamber may leak. The floor does not have to be reinforced with steel bars but it should not be too long or it may crack and leak. It should have a good base so that the chamber does not sink.
 
The trough is filled with stones; concrete blocks laid on top (not mortared); and then, water is poured. A small overflow channel is a good idea to prevent products from getting wet and getting white efflorescence on the edge.
 
Usage
 
Solar Concrete Curing Tarps
The tarps should usually be open only in the early morning. This is when the chambers are the coolest and will lose the least amount of heat. Opening them in the afternoon will cause more heat loss. Also, in the afternoon the products may be very hot to handle.
 
In the case of concrete blocks or stabilised soil blocks, it may be possible to stack the fresh blocks directly in the stock area and then cover them with a tarp. This means no curing chamber and lower cost. The moisture inside the blocks may be enough to allow curing. This will also reduce the need to move the blocks in and out of the curing chamber. There will be a limit on how high the blocks can be stacked because they are not yet strong and the ones lower down will have to support some weight.
 
Easy Access Curing Chamber
The water level should be checked daily. When there is only 2cm. of water left, more water should be added. Adding water sooner than necessary will cool down the curing chamber. If water has to be added frequently, the chamber should be checked for leaks.
 
If freshly poured concrete dries out too fast it will crack. The last tile in each row should be covered with a fully cured tile (a grade B tile is useful for this).
 
Limitations

Solar Concrete Curing Tarps
Most of the plastics will last for 3 to 5 years. They can be patched if torn.
 
Easy Access Curing Chamber
This type of chamber is good where a clear plastic tarp can be bought. If plastic tarps are not available then a water-curing tank is more appropriate.
 
Equipment suppliers
Contact a local plastic tarp supplier. One place to ask is at shops that make furniture, awnings or seat covers. These stores usually stock PVC tarps which do not make good curing chamber tarps, but these stores purchase plastic tarps from manufacturers which may produce PE tarps as well. Agricultural or nursery supply stores may also have some clear plastic tarps. q

References