Wall ties for 200mm cavity retaining wall

[Moonshine]

For the retaining walls on site the design is a 200mm filled cavity block work wall as below.

 

I need to get the wall ties for this construction, which need to be 325mm to be embedded a min 50mm within each leaf.

 

I am looking for a cost effective tie to buy which doesn't need to be special as they are going to be embedded in the concrete fill.

 

They only need to hold the leaves together of the pressure when the concrete is poured. The ancon ST1 comes in 325mm but are expensive, and given the use in the filled wall they seem over kill.

 

Any ideas?

Edited April 28, 2021 by Moonshine

[Moonshine]

@joe90 didn't you build a 200mm cavity masonry wall? what ties did you use?

 

The SE spec says embedded a min 50mm into leaves, and its a 200mm cavity. So a 300mm tie would meet the spec and are available at a much more palatable price (£64 for 250)

 

https://www.lbsbmonline.co.uk/wall-ties-dt300-double-triangle-300mm-box-of-250-cavity-tie

Edited April 28, 2021 by Moonshine

[markc]

As they are embedded in concrete i would be using good quality stainless ties. The interface between concrete and blocks is a prime corrosion point. Re-bar is ok unprotected because it is fully encased in concrete at all times. The ties will still be holding the inner and outer block work together long after the concrete has set.

[joe90]

25 minutes ago, Moonshine said:

didn't you build a 200mm cavity masonry wall? what ties did you use?

Yes I did , builder supplied 300mm stainless wall ties. I did some rough calcs on losses through these rather than basalt stuff and it was minimal.

[PeterW]

Are you filling as you go..? You could cut A142 mesh into strips that you could embed in the wall as you go. It only has 6mm bars so you’d be able to bury it in the joints. 

[cwr]

For my 240mm cavities I found that Varty Engineering were by far the cheapest for unusually long wall ties.

 

http://vartryengineering.com/products/extra-long-wall-tie-large-cavities.html

[Moonshine]

12 minutes ago, cwr said:

I found that Varty Engineering were by far the cheapest for unusually long wall ties

 

Thanks, i have sent them an enquiry, i will probably need less than 250.

[George]

Normally you'd build the RC wall first with formwork and use stainless steel frame cramp type ties into the concrete.

 

Are you intending to use the brick/block as a permanent formwork? You will need to support the wall as the lateral force from the concrete would potentially burst the wall. 

 

I think you need to go back tot he designer and ask them how they intended for this to be constructed.

Edited April 30, 2021 by George

[Gus Potter]

On 28/04/2021 at 09:46, Moonshine said:

For the retaining walls on site the design is a 200mm filled cavity block work wall as below.

 

I need to get the wall ties for this construction, which need to be 325mm to be embedded a min 50mm within each leaf.

 

I am looking for a cost effective tie to buy which doesn't need to be special as they are going to be embedded in the concrete fill.

 

They only need to hold the leaves together of the pressure when the concrete is poured. The ancon ST1 comes in 325mm but are expensive, and given the use in the filled wall they seem over kill.

 

Any ideas?

Hi Moonshine.

 

What you have looks like a reinforced masonry retaining wall. I assume you are using dense concrete blocks not aerated.

 

Nice to see them being used, a bit like timber flitch beams which seemed to go out of fashion for a bit, like bell bottom / flared trousers.

 

Yes 325mm ties to allow for construction tolerance so you get your min 50mm embedment.

 

Peter has suggested (I think) using A142 on the flat to create the wall ties. I would maybe give this a swerve as the blockwork is more porous to moisture and air than well compacted concrete. I think this risks corrosion of the A142 in the blockwork courses. The issue here would be that when steel corrodes it expands and can exert a huge expansive force on the masonry which could damage it. This effect can be seen on historic buildings where iron straps, cramps etc have been built into the stonework resulting in disappointment.

 

I'm curious as the cavity fill is quite thick.. I wonder if the wall is fairly high for a reinforced masonry wall or if you have a clay soil that has a low shear strength. By that I mean a clay soil say that is pretty soft thus exerts a fair bit of lateral pressure on the wall. I can see that the ground slopes up from the wall so this tends to add lateral load to the wall too. I would be interested to see the detail of the rebar at the base of the wall and how the base is working.

 

For the curious these types of reinforced walls are quite clever in how they work, it's the simplicity that attracts.

 

To explain a bit of the theory. Imagine you have a reinforced concrete beam spanning between two walls subject to a downwards load. The bottom of the beam is in tension. Concrete is not so good in resisting tension so you add in rebar in the bottom which is good for the tension. The top of the beam is in compression and concrete / masonry is good at resisting compression.  Now, if you make the beam deeper the tension and compression forces at the top and bottom edges reduce as you have a longer lever arm between the bottom and top of the beam. A bit like using a longer spanner or car wheel nut brace.

 

Imagine you take said beam and cut an imaginary slice through it just off the middle of the span. In the very top you have a compressive force acting in one direction, a tension force acting in the opposite direction at the bottom of the beam. In the middle of the depth of the beam these opposing forces are at their maximum in the direction of the span. For the very keen these longitudinal forces are often called complimentary shear forces. In summary, they are most onerous in the middle of the beam and reduce to zero as you move towards the top and bottom of the beam. 

 

Now a reinforced masonry retaining wall wall works by recognising that these complimentary shear forces reduce as you move towards the inner face of the wall. There comes a point where the concrete has enough bond strength to the block to resist the complimentary shear. You hit a sweet spot where you can get the concrete and the block to work compositely, like a metal deck floor. The wriggly tin resists the tension, it is bonded to the concrete which resists the compression.

 

Again, great to see this kind of reinforced masonry detail. They can be a great addition to your design armoury. For the self builder they can deliver savings, particularly if you have lot's of masonry on the job anyway. Like many retaining walls they are often height /soil sensetive but they can be great given the right conditions and save you money.

 

Moonshine..One key thing is that you need to make sure that you keep the inside of the blocks clean, no chucking the washout from the mixer down the cavity, letting soil fall into it. Also make sure the brickie does not butter up the inside of the cavity as you want the inner face of the blocks as rough and clean as you can to make all work. Keep the blocks moist when pouring the concrete as you don't want them to suck the water out the concrete on a hot summers day which will encourage the concrete to debond from the blocks.