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Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/04/2015 4:32 PM

We will be pouring a 25cm thick mat foundation approximately 100M x 60M that has a number of offsets/etc following the exterior walls of the building out 1M (the actual area is 4,900 square meters). This will be on top of a sandstone aggregate that has been compacted (with a CAT vibrating roller) in several 25cm lifts to a thickness of 2M. This aggregate is on top of a very poor quality soil (we removed 50cm of this soil that was very rich in organic matter)

The foundation has two layers of rebar both tied in 15cm grids, one layer is 1/2" rebar, the other is 3/8" rebar.

We plan to do this as a continuous pour, using a batch plant setup at the site and requiring approximately 1,242 cubic meters of "240 ksc" strength concrete.

This is in the Dominican Republic, so very constant temperatures.

With a capacity of 35M3 per hour, this will require a minimum of 36 hours.

1) Anyone see a problem with a continuous pour and no construction or control joints?

2) Thinking Plastic under the concrete may cause the more problems than help with such warm temperatures here. Agree?

Thanks for your opinions.

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#1

Re: Mat Foundation - does it need any type of joints, plastic sheeting?

08/04/2015 4:37 PM

<...We...> need to ask the building's Designer, who will be familiar with local codes and regulations, together with the environmental conditions within which the pour will take place. This is not an issue for anonymous strangers on the internet!

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#5
In reply to #1

Re: Mat Foundation - does it need any type of joints, plastic sheeting?

08/04/2015 11:53 PM

"We" have asked. Due diligence requires confirming information provided.

So far I have received more comments on the worthiness of this post rather than professional opinions - which often times vary. Continuous pours, the need for joints and use of plastic are all issues with differing opinions on the pros and cons. Because opinions vary (and everyone has one) forums such as these I thought were meant to promote discussion and a place to offer professional opinions.

Large continuous pours are difficult and go against traditional methods, as does the use of plastic which can cause curling and uneven curing - some studies suggest plastic causes a lower final strength and more cracking. A mat foundation with joints is contrary to the concept of a mat foundation, so it is no simple issue.

If all engineers agreed upon everything there would be no need for these forums, and no need for engineers.

Based on responses in this forum so far there is no need to have asked, as everyone who has replied apparently knows all except answers to the questions posed and reasoning behind those answers.

Thanks anyway.

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#6
In reply to #5

Re: Mat Foundation - does it need any type of joints, plastic sheeting?

08/05/2015 2:58 AM

Don't give up yet. Not all engineers here have an answer to your proposed question. Simply because there is a lot of non-civil engineers here as well.

The common sense answer is: if you have to ask go look for a structural engineer and pay him for advice.

But you could as well wait for the civil engineers to come online and jump on the question. Those that posted is us regulars that have something to say for everything. This does not make use specialist in the field we respond to.

But trust it, your project sounds like it needs some paid advice to be believable.

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#2

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/04/2015 6:41 PM

In response to your title, yes, and yes.

Beyond that see #1.

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#3

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/04/2015 7:40 PM

It sounds like you are doing all the right kinds of things except one: Hire a reputable engineering or construction firm that has done a lot of warehouses or factories. That's a huge pour and fixing it later if it gets screwed up will cost many times the cost of expert help.

Good luck with your quest.

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#4

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/04/2015 9:15 PM

All it needs is an engineer that knows his stuff.

Don't save on the wrong end!

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#7

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/05/2015 9:27 AM

You have already made a big mistake by not putting a geotextile under the compacted gravel, especially if there is any cyclical ground water saturation and vadose zoning.

If the gravel can be kept surface saturated dry during casting of concrete, you do not want or need plastic for the reasons you described.

Control joints are essential, but should be more than just a grid pattern of saw cuts, meaning applied loads and supported features must be considered. Try to use a palette of water reducing and shrinkage reducing admixtures.

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#8
In reply to #7

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/06/2015 10:15 AM

Blinding or lean concrete in preference to plastic membrane.

Expansion joints required and location dictated by the load that this slab will support.

What was under the organic overburden, ie what is the compacted stone sitting on.

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#9

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/09/2015 5:29 AM

We did a continuous slab - though not as such a continuous pour - on a 180m x 100m slab. This was for the reason of having no joints so as to support the gridshell structure for the roof.

The main obstacle was not the ground temperature variation (we were in the tropics too) but the concrete shrinkage, both short term and long term. Creep was also a problem but mainly unrelated to this aspect of the slab (though still with an interaction).

The short term shrinkage problem was dealt with by putting a stitch of about 2m between slabs of 20m x 20m interval which was poured at a later date. The day joint was made with Nergalto mesh so that there was good adherence for the final stitching pour. The 2m was to allow for the lapping of rebar. This stitch also eliminates the heat of hyydration problem (heating of the rebar and so temporary dilation) but your 250 thick slab probably won't create this problem as the heat would be dissipated.

The long term shrinkage is dealt with by increasing the amount of re-bar to prevent cracking due to that shrinkage. The cracking is effectively dispersed and so lots of smaller cracks (less than the minimum. crack width)

The plastic under the concrete is not normally necessary as the slab is 250 thick. The rebar may be sufficient to be water tight depending on your ground conditions and relative settlement. If it is a raft foundation, an FEA model with continuous elastic supports will enable you to evaluate these additional moments due to relative settlement. If you manage to get to 0,2mm crackwidth and depending on your usage for the building, then it can be considered water tight.

If you have local pad foundations, are these linked to the top slab ? Or do they go around them ? This could create localissed stresses. A bit of an unknown here I'm afraid.

The date for the second stitching pour would be dependent on the calc for the rebar and the calc for the shrinkage at the date of the second pour. Not very straight forward and good expertise required. Ours was 28 days later.

Given that you are doing something that is quite standard and that shrinkage will be your major problem, it is probably best to stick with normal construction joints for your area (guessing this is about 40m for construction joints as per many codes).

Hope this helps you see a bit more "behind the curtain" of the codes. Don't hesitate to go looking for more answers ; the internet is a very big library.

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#10

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/15/2015 7:17 PM

First of all, 25cm is less than 10 inches thick, so it's more of a thickened slab, and less of a mat foundation.

Second, if you did not excavate all the way down to a dry substrate below ALL the organic material, then you did not excavate deep enough.

It sounds like your structure might be located in a relatively lower area than it's surroundings. If so, then drainage conditions would aggravated by the seasons' first hurricane, and it would get worse as the (wet) season went on. So one of your highest priorities should be to provide more than adequate surface drainage.

Is it too late to relocate to a place with better than ''very poor quality soil''?

It might be much better cast the floor area as a series of square pads, possibly 6 to 10 m square, cast in a checkerboard fashion.

There should be dowels every meter of slab length apart, along the top and bottom layers of rebar, probably at least 12 to 15 inches long, each.

For the necessarily wet seasons, you should give serious consideration to using plastic-coated rebar, unless you budget allows you to replace the entire structure every 10 to 12 years.

The purpose of a simple plastic sheet will probably be defeated by use of non-rounded gravel and workman's boots trying to evenly spread the concrete over such a membrane.

Serious consideration should be given to specifying some variety bentonite-type of membrane assembly, if you must include a membrane (The bentonite will adsorb 8 to 10 times its volume in water. while Cortex is prohibitively expensive.)

Lastly, extend the perimeter foundation wall 2 to 3 feet above surface grade.

In any case, keep us posted on what you decide.

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#11

Re: Mat Foundation - Does it Need Any Type of Joints, Plastic Sheeting?

08/22/2015 6:46 PM

2 m x 100 cm/m = 200 cm, total depth

200cm / 25cm/lift = 8 lifts,

Well ok. So each of 8 (eight!) lifts is going to be 25 cm thick. Why???

35 M3/hr will cover one lift in less than 36 hrs, straight, but for only one lift.

8 lifts x 36 hrs /lift = 288 hrs, total, that's about 12 continuous days of total pour.

Why ''invite'' 7 horizontal striations between 8 separate lifts curing at different time rates?

(By the way, the more expensive membrane material is Corflex, not Cortex. Sorry for my memory lapse but it's been quite a while since I've inspected on such a project...)

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