ABSTRACT
The simplest way to accomplish this, which is the one that is first thought of when concreting a canal lining, is to directly apply the concrete paste over the excavation of the canal box or transverse section and then compact it with rams and smooth it with a trowel before it hardens. This system should be totally rejected because if the concrete is very wet,
it will slide down towards the bottom of the canal slope, and if it is very dry, it will not compact well, leading to insufficient densities and even leaving significant cavities within. This lining will be permeable and have very little resistance against adverse weather conditions. In addition to this, it is very difficult for the outer surface of the lining to
be sufficiently flat and smooth due to the lack of a thickness reference for each point and that the thickness is adequate over the entire area. To eliminate this last problem, one system that has been very frequently
used consists of fitting wood ribs or masters, which adapt perfectly to the excavated canal section, with a thickness that is equal to the required lining thickness. The fresh concrete is spread and smoothed with a plank supported on
two wood masters, which are called so precisely because they direct the operation being carried out. The thickness is therefore that required, provided that the exactness of the ground excavation has been carried out with sufficient accuracy. This same plank is then employed to strike the fresh concrete mass in
order to compact it. However, the lack of plank weight and the very nature of the operation means that the concrete suffers from the same lack of compaction as in the previous case (Figure 5.1). However, this system provides one significant improvement. If the section
between two consecutive ribs is a slab, then the concreting is carried in alternative slabs; in other words, one is concreted and the next one is not. Once the concreting of two alternative slabs has been carried out, concrete, and the concrete has set and
hardened, the ribs are removed and the empty area is then concreted. The only difference is that the smoothing and compacting plank is supported on the two neighbouring hardened slab surfaces. It should be pointed out that this system allows the construction of canals
with any shape of transverse section, without it necessarily having to be trapezoidal. The only requirement is that the excavation and ribs have the necessary shape. The advantage turns out to be that without realizing it we have produced
a contraction joint, which is precisely the joint between two consecutive slabs. It is a contraction joint because it permits a reduction of slab length due to cooling or shrinkage, but not their dilation because they are in contact with each other. The joint is impermeabilized simply by painting the concrete surface that was in contact with the rib with a suitable material, whether bituminous or not, chosen from those already examined, before concreting the intermediate slab. From what has been said, it should be clear that the separation between
the ribs should be equal to the separation required between the contraction joints in accordance with the regulations described in a previous chapter. Previously, the use of a similar method was also very frequent, and this
consisted of constructing concrete ribs buried in the ground, also in the direction of maximum angle of slope, and separated by the same distance contraction joints. First of all, these ribs
were used to exactly define the profile of the excavation under the lining since it was only necessary to stretch a length of cord over two consecutive ribs in order to immediately establish whether further excavation was required or not. For this reason, maximum attention was paid to constructing these ribs
at their exact elevation since they were to define the final support base for the lining, which again was the reason why they were called “masters”. It was only then necessary to ensure that the canal lining thickness was sufficient to establish the canal lining surface at its correct elevation. All that was required was a plank employed as a mould or formwork set on top of the concrete ribs in accordance with the maximum canal slope angle, which would then laterally contain the concrete for the first series of slabs. This system concretes the lining by alternative slabs and this turns out to
be an advantage because the initial opening of the contraction joints is less since the previous slabs had already set and undergone shrinkage before the intermediate slabs are constructed. In addition to the poor degree of concrete compaction obtained with these
manual compaction systems, proof of which is sometimes given by the fact that plants sometimes grow in the lining cavities, there is also the problem in which the constructor gives way to the temptation of extending a layer of mortar over the lining in a form of roughing-in intended to cover up these cavities. However, this roughing-in operation does not bond sufficiently to
the underlying concrete and tends to loosen with temperature changes, and its use should be totally outlawed. These defects can be seen in Figure 5.2, which shows the Alberche Canal in Spain. Because of the poor quality it produces, manual compaction of concrete
should always be prohibited and only concrete that has been vibrated at speeds of 5000 vibrations per minute or higher (which leads to problems that we shall examine later) or vibro-compacted should be accepted.
