Geotextile/filter fabric/filter cloth as a blinding layer under a cast-in-place retaining wall

[user277418]

Hi comrades

Designing a retaining wall.
Structural layering from top to bottom: the cast-in-place retaining wall base > blinding (weak concrete thin layer or geotextile) if necessary > crushed rock pad 0,5 m mainly to avoid capillary effect, but have to be compacted properly to provide good base > bedrock. Peak underground water level about 10 m lower. Bearing pressure and settlement are not a problem.

1. Do I need a blinding layer to avoid concrete seeping to the crushed stone pad? I always used concrete blinding before, but right now designing it in different country and, it seems to me, no one use it here in such cases. Internet surfing gave a lot of controversial information. Someone never heard about the concrete seeping, someone horrified to don't have the blinding layer. Have a guess, that the concrete seeping depends a lot on concrete slump grade and concrete used for foundations isn't very slumpy usually.
2. Ok, if the blinding layer is still required. Is it acceptable to use geotextile/filter fabric/filter cloth instead of blinding concrete layer under the cast-in-place retaining wall base on top of the crushed stone pad? May geotextile reduce friction coefficient in the case? For retaining structures it is important.

PS: sorry for my English, just in case

Best regards

 

[HTURKAK]

The use of blinding layer is not a must but provide better working platform for rebar installation, chalking and curing of the concrete.




If you do not provide blinding layer , may be the use of PE film could be an alternative but check the sliding requirements. IMO, the use of geotextile/filter is not acceptable since the water in fresh concrete can migrate thru geotextile.

 

[user277418]

IMO, the use of geotextile/filter is not acceptable since the water in fresh concrete can migrate thru geotextile.

In the country, they always use geotextile under concrete floors on ground on top of insulation layer to prevent the concrete seeping. But in the case friction reduction is a favorable effect
My supervisor suggested to use finer aggregate packing/interlocking on top of the crushed stone pad to avoid the blinding layer. Is not a bad idea as for me

 

[HTURKAK]

Friction reduction is not favorable which will reduce the sliding resistance and so FS against sliding and may require the use of a shear key .
If you do not provide blinding layer or PE membrane, the water content of fresh concrete will migrate through crushed stone and adversely effect the curing .

If you provide more details together with summary of calculation, you may get better responds.

 

[user277418]

the water content of fresh concrete will migrate through crushed stone and adversely effect the curing.

Could you provide any reference to a book/guide/standard/instruction/manual where the topic is described in details? I could not find any
And how it is handled when concrete poured to a trench?

 

[HTURKAK]

Nowadays , there is a famous tool and just learned the verb some weeks ago: googling ..Just write (blinding concrete prevents the concrete from losing moisture to the ground ) and look to the outcomes.

In case of pouring concrete to a trench, if it is for filling purposes ( weak strength ) you may pour directly on the ground . If durability and strength are concern, the use of PE membrane together with tanking of side and top surfaces is recommended..

 

[user277418]

there is a famous tool

As I wrote initially, I did it before. Doing it already 2 days in a row either. Have not found any 'solid' publication where the problem even roughly mentioned. Only some opinions in blogs/forums without any proves. Try by yourself and you are going to be surprised

 

[kostast88]

Do I need a blinding layer to avoid concrete seeping to the crushed stone pad?

Concrete is more viscous than you think (slump grade? - Specify it!). If you are afraid, simply increase base thickness + cover by 5-10mm, to say you account for it. Sand would be more reliable though. You may even get better friction resistance due to interlocking with the stone.

Foundations are not slumpy, but piles (which are foundations) are S4/5 in UK.

I assume you are talking about a closed fabric/woven textile. I would talk directly with the company that provides the product. Friction values are usually specified in the datasheet. Chances are someone has already asked them that before.

Warning: depends on the country you are talking about. A hot Mediterranean climate is different that the freezing temperatures of Scotland. That affects the curing/hydration of concrete.

Finally, I'd use blinding, and since you don't want concrete, I'd say use sand. Needs to be thicker than concrete though (Eurocode compliant). Don't forget to wear safety googles though, it's pretty annoying on site.

 

[user277418]

Thanks for the reply

slump grade? - Specify it!

It is not a rocket science. S2-3 for spread footings is a common practice. Should be fine for the retaining wall base in my case. But in general use of coarse sand or stone chips on top of the crushed rock pad is the same idea I have stopped on.

 

[kostast88]

S2-3 is more than enough. If you are still worried about concrete seepage, crushed stone permeability would be higher than sand.

 

[HTURKAK]

I write (blinding concrete prevents the concrete from losing moisture to the ground ) and searched the web;
-Blind concrete ? thread507-20372
-

https://www.quora.com/What-is-blinding-concrete

-

https://engineeringbasic.com/what-is-blinding-in-construction/

This is an international forum and do not know applicable codes in your region . Below find some documents which i am familiar,

1- Reinforced Concrete Designer's Handbook (by Charles E. Reynolds ,James C. Steedman ) BS 8110
6.13.4 Blinding layer
For reinforced concrete footings or other construction where there is no mass concrete at the bottom forming an integral
part of the foundation, the bottom of the excavation should be covered with a layer of lean concrete in order to provide a
clean surface on which to place the reinforcement. The thickness of this layer depends upon the compactness and
wetness of the bottom of the excavation, and is generally from 25 to 75mm or 1 to 3 in. The safe compressive service
stress in the concrete should be not less than the maximum bearing pressure on the ground.


2- Bewehren nach DIN 1045-1(2008) (by Klaus Beer )

Baustoffe 1.1 Beton ............Beton der Güte C8/10 wird nur für die Sauberkeitsschicht verwendet. Eine Sauberkeitsschicht von 5 bis 10 cm sollte immer unter einem Gründungsbauteil eingebaut werden. Diese verhindert einen direkten Kontakt des Betons mit dem aggressiven
Erdreich.

Alle Außenfundamente müssen frostfrei mindestens 80 cm tief ins Erdreich einbinden. Unter allen Fundamenten ist eine
mindestens 5 cm starke Sauberkeitsschicht aus Beton der Güte C8/10 vorzusehen.

Good luck..

 

[user277418]

Thank you very much HTURKAK for the last post.

...Reinforced Concrete Designer's Handbook ... DIN 1045-1(2008)...

The quora or the forum threads without references to some respected publications are still just opinions. But the last two quotes are from such publications.
In principal they say the same about the blinding layer:
- levels uneven soil surface;
- protects foundation FROM soil moisture;
- evens bearing stresses on soil;
- provides stiff surface for workers;
- defends foundations from aggressive soil.

Only I don't see there any mention about the fresh concrete losing moisture problem.

PS: I am not saying that the problem is not exist, but maybe it is not so critical. As it is not described in standards. For example ACI 360R-06 doesn't say anything about that. Moreover, in dry soil conditions, it allows to cast concrete floors on top of gravel pads without anything in between.

 

[Settingsun]

The blinding will have whatever problems the structural concrete would otherwise have without the blinding. The question is whether transferring those problems to the blinding is worth the additional effort, both in construction and in overturning entrenched local practice. Or is an inch of extra cover and wetting the crushed rock before the pour preferable?

 

[user277418]

Or is an inch of extra cover and wetting the crushed rock before the pour preferable?

I have stopped on that for now