Constructing the railway II - the trackbed

As I wanted a strong, somewhat raised, weatherproof trackbed (or base if you will) that can withstand summer heat, winter cold and (lots of) rain I choose for a concrete construction. I had have very good experiences with concrete track beds on my former layout. These where often a combination of concrete (Portland cement mixed with sand) and bricks, pavement tiles or other stones.
However, this made it very solid but also very heavy be. Only remembering the removal of it (when I moved out of the house) gave me an instant pain in the back again...

So, it had to be a track bed that is robust as concrete but lighter in wait and easier to remove if necessary.  I once read on a forum on a mixture of Portland cement, sand and Perlite. Perlite is however not easy to come by so I came up with a mixture with little stryropor balls to reduce weight.
Also I decided to use plastic bottles as "spacers" in the trackbed, to reduce the amount of concrete used. The bottles from hollow spaces in the concrete. This was especially useful for the bed on the back side because this had also a function of retaining wall.

A first start was made in fall 2017 in the form of a curve at the south side of the layout. It was a sort of a test to see how it works with the balls and bottles and to see how it holds.

Than in March 2018 the rest of the layout was constructed and I made some pictures of the process:.
 
The first step is to place the plastic bottles as spacers:

I than made a sort of formwork in the desired shape. I used some woodstrips I had laying around for it. This was not ideal and could not be re-used. I later used plastic / foam sheets for formwork. I the past I also used thin hardboard with good results.

Mixing the concrete with styropor balls. I am not sure anymore about the mixture but I guess it must be something about 1 part portland cement - 2 part sand - 2 parts styropor balls. (I mix more on feeling than on measurement I'm afraid).

At first I poured all parts with this mixture but I noticed the balls are often visible. I eventually poured the mixture to just above the top of the bottles (or actually, the bottom of the bottles) and finished it with a thin layer (topping) of non-mixed concrete. This gave a smooth surface

The trackbed is all level and most of the trackbed is about 10 inch in height. Depending on the part of the landscape it sticks out of the ground the full 10 inch or less.

On this last picture you can see at the left how the sides of the trackbed looks after removing the formwork. The balls are visible. On places where the sides are visible in the landscape you can put rocks against it or add a layer of cement for a better look. While doing so it is possible to carve stonework in the wet cement to represent a stone wall (as seen right on picture). 

Now, when the trackbed is all in place it is time to ballast the tacks!