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?

 

[MIStructE_IRE]

I had this before and found this paper suggesting that a joint can in fact IMPROVE flexural behaviour.

https://www.researchgate.net/public...EXURAL_BEHAVIOUR_OF_REINFORCED_CONCRETE_SLABS

I wouldn't be worried about it but ensure they use a retarding agent and scabble the face prior to the second pour. I don’t believe the dowels are entirely necessary (in theory!) - but I would definitely put them in!!

 

[SlideRuleEra]

An unplanned (approximately) horizontal cold joint (not a construction joint) should be avoided at all costs and is absolutely, positively the very worst outcome of a concrete placement gone wrong... and that is for everyone, from the Owner to the Contractor. We can go into the reasons if you want to and some suggestions how to avoid it.

For 400 yd³ placement that has a 1200 ft² footprint, I assume, means the concrete is 9 feet thick, is that right?
What are the rebar mat details?

http://www.SlideRuleEra.net

 

[r13]

Avoid the horizontal cold joint with good planning is the best defense - capability of continued supply, crew size... But bad thing happens, in such case, keep the surface moist with covers. When come back the next day, pressure wash to expose the stone to a magnitude of at least 1/4" (roughen the surface as recommended by ACI). I usually specify a thin layer of cement/mortar, but not everybody supports that practice.

 

[oldestguy]

Do not do what retired 13 says above. A wet older surface will not bond to the new upper layer. Instead on the dry lower surface roughen and blow off the dust. Then apply a paste of Portland cement with brushes and work it in. A dry lower layer can take in some of that paste in the pores. If the pores are filled with water you get no bond.. Immediately after the paste goes on you apply your new concrete layer. This bond likely will be stronger than the original.
Why not fill to required final surface as far as you can and make a standard joint.

 

[r13]

OG,

We are coming from very different schools. Yes, we should avoid placing concrete over standing water, but reverse is true, should never place fresh concrete over dry surfaces, as the substrate will draw water from the fresh concrete mix. Similar to placing masonry unit, we wet the unit, and place the unit at a "saturated surface dry" condition. I wouldn't mind a little standing water, because the weight of concrete will push it away. The bond is produced by the chemical reaction of the cement, which requires water, not a good time to lose water rapidly.

But at least we agree on a thin layer of cement mortar/paste will help to bond.

 

[canwesteng]

MI - what is the purpose of the retarding agent? Would that be in the first pour? Great paper though.
SRE - #11 T+B, plus #5 side bars. It's on average maybe 7 feet thick.
retired - that is what I was thinking, minus the cement paste.
og - why dry? I thought all the papers they used to develop the shear friction parameters insisted on SSD. This is what I do for most joints.

In any case the plan is to go do it in one go, but I'm just preparing for an issue at the batch plant. This is in rural America, we have one plant that can do this for us and it's the clients, they haven't done a job this big.

 

[oldestguy]

All this old guy knows is when cores are taken of the joint I describe they show a strength higher than cores taken at the unjointed parts. As to water needed for hardening there is darn plenty and the actual water needed for hardening is even less than 0 slump concrete has. This isn't masonry work.

I'll go on. Many parking garages in northern US have had the surface zone of their decks deteriorate due to cars bringing in salt. A common treatment is jack hammer off an inch or two as needed to remove the deteriorating concrete. Getting bond to that surface is similar to the subject problem of adding a layer of concrete on hardened concrete. It has nothing similar to doing masonry units work. If someone needs proof, one job that comes to mind is the parking garage in the lower levels of the University of Wisconsin Library on the north end of Park Street, Madison , WI. Of the many such jobs our inspectors observed, I happened to be there for much of this job. I did the coring and later the compression tests.
The proof is in the actual tests.
This old memory missed one important test. I took several cores and used a chisel to try to make a break at the bond. It never broke there, but off in the old concrete or the new part.

A question from above. Why dry? Being dry there is a place for wet mortar to enter pores. If those pores in the cement znes, no new cement can get there. The object is to get the glue in where it will do the job. A wet surface is in the way of that.

 

[MIStructE_IRE]

The retarding agent stops the concrete going off too quickly so your second pour is still placed against relatively green concrete.

9 feet thick?! Wow. How are you going to stop that from overheating?

 

[r13]

A question from above. Why dry? Being dry there is a place for wet mortar to enter pores. If those pores in the cement znes, no new cement can get there.

This is something new to me, that fresh mortar would be able to get into the pore space of the concrete. If somehow it can get in, you have honeycombs, in such case, the paste will repel and displace the water as it is much denser and heavier.

 

[oldestguy]

That's the difference between experience and no experience on the subject..

The subject being the bond strength of new concrete on older concrete at a horizontal plane.

Edit: There is a saying about many situations. "If it can be done wrong, it will be done wrong." For that reason place a requirement on this job. "No water hoses allowed on the job of the new concrete placement." Otherwise some do-gooder worker will soak the hell out of the place. Ponded water doesn't get removed, etc.

 

[SlideRuleEra]

In any case the plan is to go do it in one go...they haven't done a job this big.

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.

Oh, Brother...
Appears there is no plan, just hope for the best.

The Owner should hire and empower a qualified, independent Construction Manager experienced in mass concrete placement. First thing the CM should do is require the Contractor to submit a detailed plan for review by the EOR and CM.

ACI has the so called 90 minute rule for concrete placement. Concrete, even with retarder or fly ash, does not go into suspended animation because it is kept wet. No one is going to be able to insert dowels into structural concrete that is 18+ hours old... and this raises plenty of other questions:

1) Focusing on how to perform a bonded overlay is a concern, but there are much bigger problems:

The cold joint, where the bonded overlay work would take place is inaccessible INSIDE the #11 rebar cage with limited overhead clearance (see the marked up sketch below).

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.

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".

4) Will a mud mat be used be ensure that work does not take place in real "mud"?

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.

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.

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.

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.

http://www.SlideRuleEra.net

 

[SlideRuleEra]

Here is the marked up sketch:

http://www.SlideRuleEra.net

 

[mfrad]

Follow SlideRuleEra's comments. Need to also control internal temperature of concrete (to be less than 160 degF)via circulating cold water through embedded piping. Important to avoid differential cracking. See ACI 301 and ACI 207 for more information. Anything over 3 feet is mass concrete per ACI let alone 9 feet if that is the case. Thermal monitoring if internal temperature required. keep delta T between outside surface and internal volume to 35 degree max difference.

 

[r13]

Please read this article about "surface preparation of concrete joints", Link, which refer to ACI 318-14 CH 26.5.6, that says:

(d) Construction joints shall be cleaned and laitance removed before new concrete is placed.
(e) Surface of concrete construction joints shall be intentionally roughened if specified.
(f) Immediately before new concrete is placed, construction joints shall be prewetted and standing water removed.

Since the existing concrete has not fully hardened, blasting by water is the best choice, as it serves to remove the laitance, roughen the surface, and pre-wet the joining surface, all in one shot.

OG, based on your excellent experience, you shall consider to join ACI to write the rules.

 

[ldeem]

If the batch plant hasn't done a job this big have you considered asking them what size job they typically do and then plan for cold joints accordingly?

I assume you know the shear requirements through the mat depth so if you can pin down from the contractor/batch plant the amount of concrete they have made in the past then you could locate joints and add rebar to the existing design for cold joints at those locations. This wouldn't cover you for unplanned problems but at least you would be covered for what they typically do.

I don't know anything about batch plant production but I assume there is some design limit to production and then there is what they typically run. It sounds like they will be running above the normal throughput so I can see where that is a cause for concern.

 

[oldestguy]

In my experience with so called standards, keep in mind who wrote them. I've been us against contractors that are the main guys on these committees. If they haven't experienced problems with something, they refuse to consider any improvement. Just because it is in writing, consider who wrote that. There is a better way and on my jobs it was done my way, not by those so called rules. Again, if it can be done wrong, it will be done wrong. There are numerous other places where the so called rules are not good, but major problems have not come up, especially years after the job is done. Then another engineer does the fix. Hopefully this job doesn't get fouled up by "do-gooders".

Edit: I'll add with question, how does the contractor remove that water in the form on this one? A vacuum cleaner, a mop? Ya. Re-bars n the way besides. $$'s that won't be wasted on that part.

 

[r13]

I don't know who said this - "If you don't like them, join them." I've never seen a water tight form works, except for underground/under water situation.

 

[r13]

To be fair, the OP has planned well and prudently think ahead for unintentional stoppage of concrete pouring. He deserve some credits for asking.

 

[oldestguy]

Ok anther story about things these days. My younger son on now 62 had been science teacher and left due to rules about getting kids to behave were nothing. For many years has been a construction inspector. Has been taking concrete cylinders on the jobs and later tests them, per specs. In the job the workers adhere to slump requirements and then he take samples in cylinder forms. It then is very common for the job supt to order "more water". Complaining has fallen on deaf ears. This even happens with the job engineer on the site observing. It is so common he takes the cylinders and takes no effort to complain to deaf ears. So on the subject job my saying will likely come true. "If it can go wrong it will".

My step son is a ready-mix truck driver He sees this all the time, but has no control on water, but follows orders.