The next step is to build the retaining walls that will enclose the pool on three sides. These walls are built upon the footings that were poured just before we broke for winter.
The walls are built using hollow concrete blocks, which will be filled with concrete, per the design discussed in a previous post.
Not entirely level
The first challenge encountered (aside from moving ~600 ~20kg blocks from the front of the house to the site) was that despite best efforts during the pouring of the footings, a three meter section of the long footing for the lower wall ended up around 25mm higher than it should have. Although it's not essential that the block wall is 100% level (the top 100mm of it will be poured concrete that will be level of on top), the stickler in me demanded that it was.
Varying the thickness of the mortar under the first course (i.e. row) of blocks can provide some offset for this, but the difference is too great for mortar alone. So I manually cut varying amounts off the bottom of around 12 blocks to compensate.
The first course is always the hardest
Although the blocks we're using are mortar-less above the first course, the first course must be mortared to provide a level base for the remaining courses. This is by far the most time consuming aspect of the wall construction.
In addition to ensuring consistent height, the walls must be perfectly straight, meet at the corners (in multiples of block length), and perhaps most importantly, be vertical. Since only the first course is mortared, it must be perfectly horizontal or the wall will will have a lean as the remaining courses are stacked on top of it. This can (and was) partially offset through the use of slivers of plastic inserted between the blocks, but the initial placement was crucial.
To provide tensile strength to the walls, vertical and horizontal steel reinforcement is required. The starter bars from the footing are continued all the way past the top of the wall to where the bond beam will be. Horizontal N12 bars are placed every 400mm with an N16 bar around the top perimeter.
Once the first course is complete, the remaining courses can simply be stacked atop the course below. At least that's the theory. The reality involves the use of small plastic chips to account for slight manufacturing differences between the blocks as well as occasional drilling of the blocks to account for less than stellar positioning of some of the starter bars.
The top course is left until later as we need room to backfill the gap between the wall and the pool shell. But before backfilling, we need to fill the walls with concrete.
Core filling the walls with concrete is a relatively simple task when you have a concrete pump.
When filling walls, it is essential to ensure that the entire wall is filled with no gaps. With the aid of a high range water reducer (AKA superplasticizer), a very high slump can be achieved without compromizing strength, allowing the concrete to flow into the wall cavity without forming voids and without requiring vibration.
Our concrete is superplasticized and contains a relatively small coarse aggregate (7/10mm mix), giving it excellent flow characteristics.
After filling all of the pool retaining walls and the rear retaining wall, the walls were wrapped up to cure in preparation for backfilling.