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Pre cast concrete process

I am looking for specific guidance on the hand manufacture of pre-cast concrete for construction of improved shelter and latrines for flood prone areas. I have found good guidance on materials and concrete mixes but little on the process itself. Specific advice sought on: • Mould construction and maintenance • Curing process • Destructive and non-destructive testing for quality

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I can outline some points from my onsite precast experience, with no access to established precasting companies in Indonesia.

Moulds: We formed moulds from timber using the same approach as making shutters, this was useful for a variety of sizes, although a size limit was established due to practical reasons including striking the shutters, curing time, lifting and positioning into place, and cost/time when compared to constructing insitu. Bolts and wingnuts can be used to hold the mould together, and ease striking or a timber bolt that can be knocked out. (Plastic tubing is used in the same way as shutters if the tie bolts pass through the concrete.)

The finish could be rough, so we lined the mould with floor lino, which was readily available and gave a reasonable finish for our purposes. A better finish could be achieved using an angle grinder to smooth off and polish the surface, though time consuming.

To maintain uniformity of concrete we made several moulds in a batch, ensuring to compact properly to avoid air pockets and voids and bubbling showing on the surface. The moulds would be left to cure for 7 days before striking due to the delicate design. The concrete units then left for a further 10 days to 2 weeks before being moved. This was adjusted to suit the varying moulds, mixes and according to the strength results we were getting back. We calculated how many moulds we needed to pour a day/week and then how many moulds we needed to satisfy the construction demand.

Curing: Ideally a specific ISO cooling tank should be used for curing, and ISO cube moulds for casting the strength test moulds described later. The test cubes need to be filled in 3 layers and compacted with an ISO rod at each layer before the next layer is applied. and smoothed off at the top, ready for load testing.

Whilst curing the concrete precast moulds need to kept cool too, we used hessian saturated in water, and cover that with cardboard packaging to keep out of the sun. The moulds were raised off the ground on pallets. The material was kept wet by daily or twice daily soakings as required. We used a thermometer to ensure the temp was kept down to that specified in a cooling tank. The cooling tank was used for test moulds and precast moulds to maximum capacity, the remainder were kept under the hessian, where a set of test moulds were also kept for comparison, and fared well.

Mould maintenance: The moulds need to be maintained using shutter oil, it stops the concrete sticking to the timber and preserves lifespan. It is applied to all internal surfaces of the mould before the concrete is poured in. After the mould is struck, cleaning is important and any repairs made, before being reused, to maintain consistent product. All moving parts should move easily + fit together. A dirty bolt can prevent the mould from closing tightly enough producing an ... (more)

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I should say that the following has come from a colleague of mine Otto Ruskulis.

A series of building materials leaflets, focused on concrete products has been produced by SKAT - the Swiss Centre for Appropriate Technology.

http://www.skat.ch/publications/prarticle.2005-09-29.1982292338/skatpublication.2009-09-30.0248134508

Practical Action also produced a brief manual on a concrete vibrating table / press and moulds and concrete product fabrication in Kenya about 15 years ago. The attached is in draft and was never finalised. As the market for building materials at the time in Kenya was very depressed very few producers showed interest in buying the vibrating table and the development of the project had to be stopped.

Use of a vibrating table or plate or poker is recommended to consolidate the concrete ensuring that large air bubbles are forced out of the concrete and do not become a source of weakness. However, without the mechanical vibrator it can still be possible to get some degree of consolidation of the concrete using manual tamping with a flat plate or board and manually shaking the concrete in the mould, if the mould is not too heavy.

Moulds can be of wood, plastic or steel. Wooden moulds are usually the cheapest and can be made by any carpentry workshop. The problem with them is that they become worn very quickly, perhaps after only a few uses. Plastic moulds can be used at least 50 times and with care perhaps a few hundred times. Small-scale fabrication of plastic moulds is not really feasible and they have to be made at a specialist plastics factory or imported. Glass fibre reinforced plastic moulds may be fabricated on a smaller scale using more basic equipment. However, the epoxy resins used are hazardous to health so workers need to use proper protective clothing and equipment and be trained in their use. Steel moulds are the most durable, can last for years and be fabricated by a well-equipped local engineering or metalworking workshop, but they are also the most expensive.

There is now quite a lot of information on the internet about small-scale casting of concrete products for the homeowner or as a business opportunity. Using the following search terms together - do it yourself concrete casting mould, for example gives a number of relevant results. However, I do not have the time available to go through some pages of the entries and evaluate the information

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Very good information and guidance on mixing, placing and curing concrete, for in situ work, precast work and blockwork is given in "Engineering in Emergencies" by Davis and Lambert (Practical Action Publishing, 2nd Ed. 2002). Thoroughly recommended!

RedR TSS gravatar imageRedR TSS ( 2014-02-13 08:59:50 -0500 )edit
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2014-02-03 11:06:18 -0500
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Feb 13 '14