12" foundation mat - placement size?

[RHTPE]

Okay, we have a 12" foundation mat, reinforced T&B with #6@8" EW. Placement will be in the next month or so. The EoR is likely to refer back to his restrictions for slab on grade placement size limits of 60 ft or 3,600 SF. Of course, the contractor (my client) would like something greater for this slab. He will also have to factor in his winter protection issues.

Any thoughts on practical limits? on cold weather curing methods?


Ralph
Structures Consulting
Northeast USA

 

[RHTPE]

Also important to consider - this is NOT the finished slab - it gets covered with 6" of stone and then a 6" finished slab.


Ralph
Structures Consulting
Northeast USA

 

[conceng]

If there is no finish requirements, I'd allow them to go as large as they want so long as immediately after strike-off, they cover the concrete with styrofoam boards or insulated blankets. If you have reinforcing steel sticking out, blankets will need to be placed over the reinforcing steel too or it will act like a heat sink, pulling heat our of the concrete.

Greg

 

[beton1]

How big is this whole slab?

 

[RHTPE]

beton1 - Please see the attached. Note the list at the bottom of the sketch. My contractor client and I are preparing for a meeting with the CM, excavator, owner's rep & design team next Tuesday. While I have most of my concerns & suggestions organized, I'm open to any thoughts from the collective wisdom here.


Ralph
Structures Consulting
Northeast USA

 

[beton1]

You have more of a restriction due to weather than the EOR restriction. If it was good weather, probably you could pour in one shot or two, depending on the contractors set-up and delivery/pumping speed. Once the bulk excavation is done, and if trimming is done immediately, the whole area is subject to freezing. You might want to do three sections, based on the geometry. For example, the small area to the left that juts out, then at the bottom and then the rest. This way you are starting out with a smaller section that is easier to control, and progressing. The smallest section will likely be worked on in the most severe weather... maybe.
Insulating blankets are placed on non-work areas, or better yet the bottom foot and trimming of the excavation is left until you are ready to start there. Then, as mentioned, protection of fresh concrete is needed. Not an easy task. The membrane placement also complicates it all.

 

[conceng]

I don't understand why they need a gravel layer and a mud mat. What are the underlying soil conditions like? If the soils are soft, I can see them wanting the mud mat for rebar support, but a gravel layer may accomplish the same goal. I would argue for one or the other, but both should not be required. The mud mat will also need to be protected from freezing too.

I don't understand the need for the membrane. What kind of structure is it? Could that be moved to the top of the mat slab?

Can the reinforcing steel be prefabricated in mats, then lift into place? This would greatly help in minimizing the grounds exposure to heat loss and would probably better maintain the integrity of the membrane. Once the steel is in place, ground thaw heater lines can be draped over the steel with blankets on top to keep the ground below from freezing. During concrete placement, the blankets and ground thaw lines can be removed as the pour progresses.

Benton1 brought up a good point about placement capacity. What is the maximum pour rate or quantity the concrete producer can provide while maintaining concrete temperatures?

Based on the temperatures you give, it is possible that frost is already at the depth of the final excavation. The ground may need to be thawed before any construction could begin after excavation, or possibly before final excavation.

Bottom line...how many blankets does your contractor have and how many ground thaw heaters is he willing to provide.

Greg

 

[RHTPE]

To all who are following this thread: The construction is what it is. The CM and the concrete contractor (my client) are not in a position to deviate from the contract documents UNLESS there is a real benefit (time, $$, quality) AND the owner demonstrates a willingness to entertain value engineering.

I am not privy to the geotech report at this time, so I cannot comment on the existing soils at the foundation level.

This is a college classroom building. The basement contains all of the mechanicals.

Placement capacity, as well as finishing capacity will be discussed next week.

More data to follow.

Ralph
Structures Consulting
Northeast USA

 

[TDAA]

What is the reason for the max limit? The contractor should be able to place as much as he can reasonably finish (as needed) and protect. Keep in mind that this means protecting from freezing, and getting any required joints cut before cracking occurs.

Is the EOR looking to force joints? If that is the case, and you must have construction joints that often, I would say to form it in a checkerboard fashion, using column locations ac corners, and place alternate squares. This could get you down to two main (or total) pours with proper planning.

 

[hokie66]

Ralph,
This is not a slab on grade. It is a mat foundation. It should be placed in the largest practical sections, with joints if required to be located near midspan where the shear is miminal.

TDAA,
This is a structural mat slab, not something you are going to sawcut. Any joints should consider how the slab works, including whether it resists hydrostatic uplift, which I assume it does due to the layers described, including the membrane.

 

[JAE]

We had a mat foundation for a 42 story building placed in our city in one pour - 4400 cubic yards. Largest pour in state history.

The key is how you organize the flow of concrete, the placement, the vibration, and the finishing (if any).

If the trucks can't get in to the site at a rate to prevent cold joints from forming, then you've got a problem.

 

[SirAl]

I personally do not see why you can not pour the entire slab monolithically from one end to the other, providing you have the delivery and placing capability to handle the pour non-stop. I have seen it done both ways. I have been on some projects in which the design engineers have specified staged construction of large slabs - possibly to minimize shrinkage and/or curling issues.

 

[RHTPE]

Okay folks, here's where things stand:

Placement size - 5 sections, max 3,900 SF, max dimension 82 feet. It was like pulling teeth to get to that.

The entire basement is about 5' below the water table, so everyone is concerned about cracks occurring that would permit water intrusion. Even considering the waterproofing membrane underneath the mat.

Not meaning to cast a bad reflection upon a fellow engineering type, but the PM for the EoR is an EIT. I suspect he is being instructed to hold the line at what the contract documents require. Logical, practical or not.

The current hot issue is the protection & curing of the 3" mud slab below the membrane. Must achieve 300 day degrees before removing heat. Today's high is about 18 degrees F at 4:00 pm. Morning low was -1 degree F. The curious thing is I find no mention of day degrees as a criteria for curing a slab on grade in their specifications, only the statement: "Concrete shall be treated and protected immediately after concreting or cement finishing is completed, to provide continuous moist curing above 50 degrees F for at least seven days, regardless of ambient air temperatures." Day degrees requirement occurs only with regard to vertical & elevated concrete work.

This experience further confirms my decision to stay away from the estimating & sales side of construction. I could never second-guess the requirements in the contract documents and still price work so it would be competitive.

Ralph
Structures Consulting
Northeast USA