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# Horizontal construction joint in large mat9

## Horizontal construction joint in large mat

(OP)
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?

### RE: Horizontal construction joint in large mat

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

https://www.researchgate.net/publication/237711465...

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!!

### RE: Horizontal construction joint in large mat

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?

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

(OP)
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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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?

### RE: Horizontal construction joint in large mat

#### Quote:

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

2

#### Quote (canwesteng)

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.

### RE: Horizontal construction joint in large mat

Here is the marked up sketch:

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

#### Quote (ACI 318 26.5.6)

(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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

(OP)
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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

#### Quote (OP)

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.

### RE: Horizontal construction joint in large mat

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?

### RE: Horizontal construction joint in large mat

2
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.

### RE: Horizontal construction joint in large mat

(OP)
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.

### RE: Horizontal construction joint in large mat

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.

### RE: Horizontal construction joint in large mat

#### Quote (canwesteng)

...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.

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