Remedial work on pond rill - Advice on wet-formed joint

[zimbo000]

I am doing some remedial work on my ponds. I will be re-building a rill (this is a shallow “canal� that lets water flow from one pond to another, it looks level but has a very slight slope from one pond to the other) and as part of this I want to create a sluice gate in the rill to stop the water flow at certain times.

The rill will be re-laid in concrete as a very shallow U-shape, approx 18 inches wide, sides 3 inches high.

The sluice gate will be a rectangular piece of oak that fits into slots in the sides and base of the new concrete rill channel. When in place, the water flow will be stopped – that’s the theory!

I therefore need to create these slots when I lay the concrete. The slots only need to be about an inch deep into the concrete, about half-inch wide (to match the ‘gate’).

Having looked at the Joints for Concrete section of the web site, I think what I need to create is a wet-formed joint, as shown in the Crack Control Joints (Dummy Joints) section – but when I remove the temporary former I would not place any polysulphide sealant in it.
[NB.1. I will however be using a special concrete sealant for ponds - but in normal pond use, the slot has nothing in it.]
[NB.2. Of course this is not a 'control joint' as such, I'm not joining 2 pads together nor is there a need to have an expansion joint in such a small piece of concrete - it's the technique of creating these 'slots' that I'm looking at.]

I have two questions:

1. What depth of concrete should I use for the rill base and sides? [There's only about an inch of water that flows over the rill, so no great weight.]

2. From the info on this site, I think I would use a piece of timber as the temporary former, to create the slots (might even use the actual ‘gate’ itself), and then remove this “once hardening has taken place� – can someone advise how this long this hardening is likely to take? The point here is that the profile of the slot (wet-formed joint) is quite critical as it needs to match the gate to form a waterproof seal i.e. I can’t have the slot ‘sagging’ when I take the former out; but if I leave it too long, I guess it gets stuck in! [NB. I realise I might have to resort to some sort of additional seal in the slot to make it absolutely water-tight, but if the slot profile matches the wood, it will help enormously.]

Thanks in advance.

[Tony McC]

1 - 100mm. Anything less and the concrete can't support its own weight

2 - initial cure depends on ambient temp, but should be somewhere around 2-6 hours. Make sure the former is coated with mould oil to aid release.

[zimbo000]

1 - 100mm. Anything less and the concrete can't support its own weight

2 - initial cure depends on ambient temp, but should be somewhere around 2-6 hours. Make sure the former is coated with mould oil to aid release.

Many thanks, and I just found the section on the web site about mould oil, so I'll see what I can find in a local builder's merchant.