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RedR TSS gravatar image

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 outsized product. The timber face may need replacing regularly.

Quality: Before pouring the concrete a slump test should be taken, again an ISO cone and rod needs to be used. This tells you before pouring if the concrete meets the spec.

Each unit should be visually inspected, to check it looks the shape it should, there is no honeycombing or mix separation/gradation of mix, and that it weighs about right and that the finish is that of the desired quality.

For every batch there should be at least 2 concrete cube units tested for strength at varying ages, it is usually 3, 7,14 and 21 and/or 28 days the results should give an early indication if there is something wrong with the batch if the 3 or 7 the day strength isn’t met, also confidence that the long term strength will be achieved if Ok. The strength test is a loading test rather than testing to destruction, a load is applied to the cube made from the same concrete batch as your precast units as described above, and the cube inspected for damage and deemed a pass or fail. The cube once tested cannot be used again.

Best wishes Pauline