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Casting H&MGR Tunnel Portals

By JRinTawa

The H&MGR has 3 tunnels and thus 6 tunnel portals.  Luckily you can't see them all at the same time as they are all cast from the same rubber mould made by Michael Hilliar.  This is them in place, the portals are blended into the surrounds with Hypa-Tuffa (cement/peat moss/sand mix) which grows moss well.

Premix trade mortar (3:1 sand/cement with plastersizer) is used with a little bit of concrete colouring oxides to take away the raw cement/mortar colour.
The first picture show the latex rubber mould and chicken mess wire reinforcing and the wooden box used to hold the mould in.

The mould is given a spray of wet water (water with a few drops of dishwashing liquid) then the first layer of very runny mortar mix (with colouring) is added....

One end of the mould is picked up a couple of inches the dropped several times and repeated on each side to "vibrate" the mortar fully into the mould so it levels out and looks like this....

Now add the reinforcing plus some extra 1.5mm dia galvanised wire (I used tie wire used in fabricating reinforcing cages for full size concrete work) as well as the chicken mesh reinforcing....

Then add the rest of the mortar and repeat the pick and drop "vibrating" during which you'll see air bubbles working up and the mortar gets a more fluid look.
I also add a couple of wire loops for future tying in/fixing in place.  Not panic about the colour, it dulls off some 75% when dry, and besides I'm colour blind so it looks okay to me, (gulp).

Now the hard part; set aside and leave for at least 3 days, ideally a week if it's not summer.  While the hardening of cement is an exothermic reaction the fact it is in a rubber mould slows the free water evaporation and the mix stays a lot "greener" and susceptible to breaking (and future cracking) if taken out of the mould to early.  (You have been warned).

Once in place I gave each a quick spray of matt black to represent soot etc above the portal, looked a bit obvious at first but weathered in well in a few months time.  The portals have been in place now for some 2½ years and are blending in well.