Concrete filled brick retaining - wall max height

[demayeng]

Hi,

I've been told there is a maximum height of 1100mm (3ft something) on brick retaining walls (2 brick skins with a 100mm concrete filled cavity).

I think it has to do with deflection, however I'm not sure.

Does anyone else know why this is and if it's true?

I guess there is an issue with the wet concrete applying too much pressure over a certain height, although I imagine you could solve that with a slow pour rate or multiple pours.

I assume also with this type of wall that because you don't have the 'clean-out blocks' of concrete block walls, so you may get dodgy corefill at the bottom... Maybe that is why the height is limited..

Any thoughts..?

Thanks

 

[hokie66]

I see no reason for an arbitrary height limitation. The design should be based on engineering principles like any other retaining wall. Tying the two wythes together to resist the pressure of the grout is important for constructability...you don't want to blow out the brick with fluid pressure. There is no reason why cleanouts cannot be left in brick walls, but with a 100 mm cavity, they should not be required.

 

[concretemasonry]

It sounds like an arbitrary code requirement and not based on and engineering requirements.

the concept of filled grout space does little to improve the lateral strength that is required for a retaining wall. For that total wall thickness, a partially reinforced CMU wall or poured in place concrete with steel closer to the surface would be more efficient.

The sloppy unclean cores can affect the vertical capacity, but not the lateral strength to a great extent.

Original poster - where are you located?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

Dick,
I assume he has reinforcement in the grout filled cavity.

 

[demayeng]

Yes, the concrete will be reinforced as per a concrete block wall.

I'm in Australia.

Doing a number on the liquid pressure for a 1.8m high wall seems like it is very high for brick ties to resist..

 

[hokie66]

Use stronger brick ties, or require the grouting to be done in two stages.

 

[valleyboy]

I've used this solution for walls of up to 4.0m. I adopted Hokie66' s method and limited the height of concrete placement to 1.2m per lift.

Also worthwhile keeping an eye on your minimum concrete core thickness. At low wall heights, for design purposes you can get away with very thin cores. You may need to adopt a minimum thickness of say 150mm to accomodate your vertical reinforcement, laps with the starter bars, and still allow compaction.

VB

 

[concretemasonry]

The wall is only 1.1 m high and is made of two outer wythes/leafs of brick with an approximate 4" (100 mm) gout core in the center of the wall.

Since it is so low, the suggested limit of 1.2 m per lift could be applied (if you have a micrometer on the job).

The wall may be a true retaining wall if there was no load applied later, but reinforcement in the center would not add much and a 6"(150 mm)CMU would carry more load laterally and vertically. Clean-outs would be inane, silly and unprofessional for a wall only about 1 meter high. Further information could provide different picture.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

I do not know if this is what you are referring to, but any wall retaining over 4 feet of earth needs to be engineered. As to a structural limit, that would depend on the wall thickness, steel, footing size, lateral load applied, and allowable soil pressure.

There is for any project an economical limit to the wall height where other types of soil retention would be less costly. Is that whar you mean?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Dick,
He wants to build a wall 1.8 metres high, per his second post. I see no problem with building a brick retaining wall his way...probably what the owner or architect wants to see.

 

[demayeng]

Hi,

Mike - no, it is a structural limit, I think the limit was mainly governed by concrete liquid pressure. I think I have a much better understanding now.

Thanks for all the replies.

The wall will be significantly better than the 1.8m high, unreinforced double brick retaining wall that is there at the moment! It has quite a lean to it..

I am going to specify brick ties at 300 centres.. leave out every second brick at base of external skin for cleanout... brickie to minimise dags at rear of internal skin... corefill in two lifts

 

[hokie66]

demayeng,

Although it may work with central reinforcement, I would specify the bars to be toward the earth side. You know...design for central, tell the builder to put them on the earth side, hope he doesn't put them on the wrong side.

There are a lot of those leaning, unreinforced brick walls around.

 

[csd72]

hokie,

With a 100mm cavity, they have to be central to get the necessary cover.

Concretemasonry,

This is fairly standard practice in parts of Australia, though I agree that it does not seems to be as proctical as using some CMU.

msquared48,

Australian codes are generally not that specific and leave those sorts of decisions up to the designer.

 

[hokie66]

csd72,
I think 20 mm cover is enough in a grouted core, and AS3700 agrees, even for aggressive soil exposure. So assuming a 20 mm bar, you could have the starter bars at 70/30 within the cavity. I would tend to dimension it at 60/40, and wouldn't accept 50/50. Too little tolerance? Many would agree, but good builders can do it.

 

[csd72]

hokie,

The bricks add absolutely nothing to the durability of the bars. I would be surprised if 20mm would be enough, even in concrete masonry as these are quite porous.. But in this case it is really plain concrete with brick formwork as far as durability is concerned.

 

[hokie66]

We will have to disagree on that. Bricks are much less pervious than concrete block. The alkalinity of the concrete provides the protection for the steel.

 

[csd72]

There is no alkalinity in brickwork to provide the protection - thats my point.

With regards to porosity, I was referring to the porosity of a typical Australian concrete block - too high to properly count on for protection.

 

[hokie66]

AS3700 allows this type of construction with 20 mm or less distance between the face shell and the steel. A classic reference on masonry construction, the Masonry Design Manual used in California (I have the Second Edition, 1972, edited by concretemasonry's friend James Amrhein) has design examples of grouted cavity brick retaining walls. These details show 1/4" clearance between the brick and the vertical bars.

 

[csd72]

hokie,

Yes I saw it in the cmaa manual shortly after posting but did not want to double post (in case I got scalded by the fire and brimstone that would be sent my way).

Personally I would allow more than 20mm to concrete and deifnately more than 1/4" to clay masonry but that is a personal opinion.

 

[hokie66]

I certainly wouldn't specify 1/4" either...just relaying what the book said.