Horizontal construction joint in large mat 9

[canwesteng]

I'm concerned about the possibility of a contractor failing to complete a large mat (400 yards) in one go)=. I'd need a back up plan in case this does happen. I'm thinking the best case is get good consolidation of the concrete, get it roughly level, make sure we get some wet burlap on it, then come back the next day, roughen, insert dowels, wet the surface and continue pouring. I imagine we'll have to have mister on hand to maintain surface saturated dry condition, as it ends up being around 1200 sf. Other than transverse shear at the joint, is there anything I'm not considering?

 

[r13]

Wouldn't the 28 days cylinder test tell something? I usually ask the lab guy to take sample from the placement, not from the truck.

 

[KootK]

1) I've intentionally done this with full depth stirrups on a few projects and have witnessed it being done on several others.

2) I do think that it's prudent to design for this contingency and commend you for it for whatever that's worth..

3) My solution would be the same as yours:

a) Roughen the joint to the right shear friction amplitude.

b) Install dowels to restore the one way shear capacity required of the mat even though I agree with MIStruct that you're probably in pretty good shape even without the dowels. We'll not rely on that though. Well.. I'll sort or rely on at as I'll describe later in my post.

c) Ensure that the two pours are at least thick enough that you can get your post installed dowels developed on either side of the joint as that is required for shear friction per code. Nobody really seems to know why this is required definitively but, for now, that's the rule of the road.

d) Give some consideration to the fact that the upper pour will be shrinkage restrained relative to the lower pour. So lots of top bar to limit restraint cracking and to act as compression steel in case shrinkage in the upper pour has an adverse impact on your flexural stiffness (gotta close those cracks to engage the concrete). It sounds as though you've already got this covered.

4) With respect to the technical design issues, I discused a very similar topic with ajk1 a while back that you might find informative: Link. It is, however, one of the many instances in which I probably took things a little too far technically. You know how I do.

5) As I see it, the technical issues for strength capacity are:

a) Got enough one way shear capacity to match the design?

b) Got enough two way shear capacity to match the design?

There's much, much more on the two way part in the thread that I referenced.

I currently elect to to not bother with the two way capacity because:

c) The demands get onerous.

d) I feel that the horizontal shear for two way capacity can be "spread around" similarly to how we handle one way shear on design strips when, clearly, one way shear varies hugely about the width of our strips.

e) Where there is two way punching demand, you will probably have gobs of clamping capacity at the joint simply because that's where your columns will be pounding a bunch of cross thickness compression into the slab. So, really, you probably don't need any dowels right near the column.

 

[canwesteng]

SRE -

2) The rebar top mat cannot just just be "disassembled", lap splices at the perimeter are embedded in concrete. Will have to cut the mat out with acetylene torches. Then the cold joint can be prepared for the overlay. However,the overlay cannot take place until a replacement top mat has been installed. Installing the the replacement top mat will NOT be a "piece of cake" either. Lap splices at the perimeter are likely not an option because to limited length available above the space above the lower portion of the mat. Each #11 will need a mechanical or Cadweld splice. Don't think we need to cut the bar, it's on 1'0" so they will hopefully be able to reach around it. Might bust some long drill bits doing

3) Has anyone checked to see if the concrete forms are structurally sufficient for a 7' deep placement... responsibility of Contractor as part of his "means and methods".
Contractor resp., we are checking their calcs though

4) Will a mud mat be used be ensure that work does not take place in real "mud"?
No, cast on fill

5) Depending on #11 rebar spacing, the top mat will may weigh many thousands of pounds. An internal structural system will be needed to properly support the top mat and keep it in place during concrete placement. Another of the Contractor's means and methods responsibilities.
Yeah, I'm thinking these could reduce the dowelling required, I may ask them to hook these and use smaller bars

6) Why should a contingency plan focus on ONLY the best case (about mid-depth) for an unplanned horizontal cold joint? The cold joint is just as likely to occur with either 6 inch or 6 feet of concrete in the forms which paint an entirely different (and daunting) picture about a solution.
If there's 6 inches at the bottom it's a tear out and start again thing. If there's 6 inches left to be poured, it's going to take more engineering or a lot more hammering

7) I'm not going to go into all details, but logistical considerations need to be addressed by the Contractor... in advance. Things like expected concrete delivery rate (yd3 per hour to jobsite), plan for where concrete is placed (the entire horizontal cross section of the mat needs to the raised at a uniform rate to ensure that the entire concrete working surface remains fresh), schedule for the day of the placement including how to control mass concrete temperature, backup equipment on site, etc.
Haven't selected the contractor yet, I'm planning to ask them about temp control

Also, a qualified CM should review the EOR's plans and specs for constructability. What is rebar spacing of the #11 rebar mats? Tell us, and FWIW I'll give you and opinion and a reason why the spacing is either "good" or "bad" from a construction point of view. 1' c/c, suggesting the contractor tops the top mat with wire mesh for walking on, but it's their call

Kootk - two way/one way shear capacity is wildly above demand. This is just a mass of concrete for vibrating equipment. Depending on how conservative you want to be with flexural demand it controls, I was very conservative to be quick in the calc.

 

[hokie66]

If you cannot provide security in casting this mat in one placement, the design should be changed to something that can be achieved. My main concern with this mat would be controlling the internal temperature.

 

[jayrod12]

This is just a mass of concrete for vibrating equipment.

If that is in fact the case, then you just need to ensure there's a bit of tie between the two pours. I don't think this needs to get as in depth as everyone is originally thinking.

 

[oldestguy]

Before finalizing any plan for construction I'd contact the consultants that have designed the plants for US Steel. There well maybe some features that if not allowed for may bite you later. I am thinking related to concrete shrinking, etc. Large blast furnace supports, etc may be in some way related. Is cooling required because of the heat of concrete curing?

 

[SlideRuleEra]

canwesteng - #11 @ 12". That is "good".
The reason: Concrete should pass between the top mat bars without coating the top mat bars with cement paste. Even if the placement goes exactly as expected it will be a few hours before concrete reaches elevation of the top mat. Dried cement paste coating the top mat rebar will severely degrade rebar/concrete bond.

To keep the top mat clean a concrete pump hose or tremie projects thru the top mat. The nominal 10 1/2" clearance between top mat bars is big enough to do this. The hose or tremie should be moved regularly to uniformly distribute concrete over the entire footprint. If the wire mesh is used, leave openings at specific locations for the hose / tremie. Pouring the concrete directly out of the truck into the forms is a no-no... concrete will contaminate the top mat.

From your latest description, I see this is an inertia block not a heavily loaded structural slab filled with rebar. Based on that, you can "get away" with cutting a lot of corners... such as a dedicated Construction Manager. Two pieces of advice:

1) Get the Contractor to provide a complete written plan. This is for two reasons, a "good" reason and a "real" reason.
The "good" reason, to give you advance notice of what to expect and possibly, together with the Contractor, solve some potential problems.
The "real" reason, to force the Contractor to think thru the placement and present a plan in an understandable format. You may be surprised how often a Contractor will neglect necessary advance planning.

2) Work with both the Contractor and concrete supplier on mass concrete temperature control. Here are some of the procedures we used for 15' thick generating station foundations in South Carolina, in the summer, without internal cooling:

a) Use shaved ice instead of water in the concrete mix.
b) Plan to begin concrete placement at sunrise. This could mean the Contractor actually starts getting setup for the pour well before sunrise.
c) On the day of placement, the concrete plant produces concrete for ONLY this project.
d) Concrete supplier has several extra truck in reserve, at the plant, and ready to go if needed.
e) Concrete supplier has (cold) water sprinklers operating on his aggregate stockpile continuously starting the day before the pour.
f) Use a concrete mix design with slow heat of hydration... probably lots of fly ash in it.

http://www.SlideRuleEra.net

 

[canwesteng]

Appreciate the comments all - SRE, the client has already ruled out ice in the mix. Right now, I've reduced the concrete in the mix (down to 5 bags/yd), going to have them hose down the forms, and I'm trying to see if I can find fly ash here. I bet we can hose down the aggregate. We are going from lows of 55 to highs of 75 though I'm sure we can get away without internal cooling if you managed to in SC.

 

[r13]

Do you have coal fired powerplant, or steel mill, around, they might have something you need - fly ash and furnace slag, both are pozzolans that reduces heat of hydration and thermal shrinkage. Also, if strength is not a problem, thin metal tubing with circulating water has been used in the past.

 

[SlideRuleEra]

...I'm trying to see if I can find fly ash here.

For best results, have the concrete supplier obtain and use Type IP cement, per ASTM C595, that contains Class "F" fly ash, per ASTM C618. Not all coal fly ash is suitable for use in concrete because of high levels of unburned carbon. The best comes from efficient base-load generating stations operating at steady state, using electrostatic precipitators for fly ash removal. We started selling bulk fly ash for concrete in the early 1980's.

http://www.SlideRuleEra.net