Retaining Wall cold Joint

[kieran1]

I have a situation on site were we are constructing a 9.0m high retaining wall. The stem is 650mm thick and the horizontal pressure is 430KN. The contractor proposed to construct the wall in lifts of 1.2m however the first lift took 3 days and there is two cold joints in the wall.
I should say the job is in Africa and pouring concrete can be a problem. I have since discovered that the joints were not scabbled after the previous pour.
The starter bar reinforcemnt is continuous through the joints. I also have the same reinforcement on the compression face to save confusion when tying on site.

All thoughts appreciated. I don't want to instruct them to break down the concrete if I can avoid it.



Kieran

 

[epitome1170]

Without getting into the design, it sounds like all of your reinforcement is continuous through the joints so the only issue that I could forsee would be water infiltration through the cold joints. This could leave to some corrosion issues (and waterproofing issues if that is a worry) down the road.

 

[Teguci]

Per ACI

0.75xVn = Avf fy mu

0.75xVn = Ultimate shear strength - divide by 1.6 for allowable

Avf = area of shear friction reinforcement - your compression bars and starter bars should be considered.

fy = rebar yield stress

Mu = 0.6 for concrete placed against unconditioned concrete (1.6 for monolithic concrete).

 

[epitome1170]

Teguci- I assumed that during the design phase he had sized the bars between the footing and the wall as if it was not a monolithic pour so the shear friction calculation was taken into account already. He stated the bars were the same on both sides and (assuming) continuous to the top, so, IMO the shear friction should have already been checked.

 

[Teguci]

epitome - he noted that they didn't scabble (british term?) the concrete before pouring against it. That means to me that he designed for a mu factor of 1.0 at construction joints (concrete joint intentionally roughened to 1/4" amplitude)but he now has a mu factor of 0.6 for casting against smooth concrete.

 

[JAE]

This has two issues of concern - already stated above:

1. Shear friction at the joints - using mu = 0.6 assuming a smooth finish between pours. You might be able to use a bit higher mu factor if it was somewhat rough vs. 1/4" amplitude rough. In any case, the shear higher up on the wall, where the joints are, is bound to be much lower so it may not even be an issue.

2. Water infiltration. - just have them install some kind of joint sealant along the cold joints on the earth side of the wall...assuming they can purchase something like that in their location.

 

[epitome1170]

Teguci - I did not know what scabble meant so then you definitely have a GREAT point.

 

[kieran1]

Many thanks for you quick responses.

Unfortunately the first cold joint is only 400mm up the wall (1'4")and I did indeed assume the concrete would be roughened after each pour.

I am not concerned about water infiltration as i will instruct the use of sealant.

I will have a look at the shear friction, didn't have the formula or the co-efficient to hand so again thanks.

If it fails, would roughening up the rear of the wall (tension face, dowelling in bars to the base and thickening the stem for the first 1.2m height to say 950mm be an alternative solution to breaking out all the wall. There is 21 linear metres poured to date.

Kieran

 

[concretemasonry]

From a practical standpoint, I would break the retaining wall into individual sections with a planned, formed joint and pour each section with a width to determined by the availability to supply, place considering the location. Tight specs that cannot be achieved are meaningless. I would imagine you are dealing with hand batching, placement by the bucket, so there is a limit of the rate of placement to maintain continuity and get the best possible wall (in sections). The width of the panels/sections would be determined by the available conditions, which is very common in many areas.

Sizing for anticipated construction restraints results in better construction results, since you feel you can handle any moisture problems (if they occur where this is).

This approach will at least allow a uniform retaining wall with no cold joints. An require waterproofing can be more controlled.

Dick

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

 

[kieran1]

Thanks Concrete Masonry, I had instructed the very same, but for some reason they bit off more than they could chew.



Kieran

 

[DTGT2002]

While most folks in the States would say roughen, a scabbler is a pneumatic tool available in the states to abrade or roughen a surface. It may also be called a scaler by some manufacturers and sometime called a needle scaler or scabbler as it basically cycles hundreds of needles against a surface.

Reading the responses above, the design methodology presented is with reference to ACI 318.

To what design code or design specification is your wall designed?
That can certainly make a difference for the validity of the shear friction references.


With that said, I would hope the extra mat of reinforcement (compression face) can make up for the penalty of the unroughened surface.


Good luck!

Daniel

 

[BigH]

We've called it "green cutting" - but if the wall is properly designed to ensure no hydrostatic pressure, there should be a designed drainage layer behind the wall leading to a positive outlet and I wouldn't see seepage through the joint being a problem.

 

[concretemasonry]

kieran1 -

I appreciate you situation since the wall is theoretically designed is terms id vertical strips, the logical approach is to build it within the bounds of what is available and practical at that location. I don't think scabblers or or scalers are readily available to be used in an attempt to force a solution to make all the parameters fall within the theoretical guidelines.

Designing and making decisions on a project is difficult when you make decisions based on the local opportunities and have them over-ruled by others that may not understand.

I had a similar construction situation, but reversed, when I received so very fine structural drawings for a partially reinforced 6" loadbearing masonry apartment complex (10-20 buildings concurrently with many different strength CMUs), and I prepared a list of questions regarding the inspection and quality control. I was concerned about the intermingling of different strength block, mortar and clean-outs - all of the classic items is a code or textbook. Within an hour, I immediately decided to shut up after seeing the video equipment for inspecting the cores and the testing and color coding of different strength block at the plant before delivery to the site. Since they had site plant mixed mortar, there was only one product to test and no problems about acceptability.

There are many different situations, conditions and applications across a wide range and I think you were correct in your situation, but over-ruled. Sometimes you are right or wrong since there is a wide range of what is best and the standards are just a guide for professionals to apply when the codes and situations are different.

Dick


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