control joints on monolithic slab foundation 1

[pappy64]

I would like to place control joints along walls so they can be hidden and fit well with ceramic tile joints over them. However,I am concerned whether such control joints would activate because beneath walls of the slab is thick concrete foundation. The below image is a side view cross section of the slab showing the proposed 4 color coded cement truck pours. The 2 construction joints through the slab on either end of the green double as control joints. The plan is the slab is 4" thick, the 2 joints are grooved 1" deep, and no slab rebar will cross the joints though rebar deeper in the foundation will.

 

[Ron]

I realize your handle is "computer", so let's start with getting the terminology. The truck is a "concrete" truck, not a cement truck. It will deliver concrete, not just cement.

I'm not sure why you are thickening the slab edges when you have a separate footing underneath the edges. Monolithic generally means that the footing and slab are cast integrally, not in separate placements.

To answer your wall question, the weight of the wall will help restrain the slab; however, your control joints should activate long before the walls are up. Control joints are most effective in the high shrinkage time of the first 30 days or so of the slab's curing process.

Timing of the control joint placement along with proper spacing of the control joints will determine more about their success than the wall location. For instance, on a 4" thick slab, the joints should be no more than about 12 feet apart in either direction, should be 1" deep as noted, and should be cut the same day as placement of the concrete...don't wait until the 2nd day...some cracks will have already occured.

With your geometry, you are likely to get restraint cracks at the thickness transition of the slab...that is why I question your placement approach.

 

[dik]

I generally put joints at the edge of the slab thickening, not in the middle as shown. I've not tried that... have to think about it for a bit...

Sawcutting should be undertaken as soon as the saw can cut the concrete without it 'tearing up the surface with the aggregate' (ravelling). This is generally within 6 or 8 hours after finishing... earlier with a sofcut saw. YOu may be sawcutting in the evening/night, if slab is cast in the afternoon.

 

[pappy64]

Thanks guys. I plan to cut the control joints with a hand concrete groover, just after bull floating and when the watery sheen disappears. The edges of the slab are not actually thickened, the picture is just trying to illustrate the first pour brings the footings to 6" below the slab. Even though all 4 pours should be finished in one day making it truly monolithic, the idea is each pour will be performed in a fail safe way such that if there is an unexpected stop such as an equipment failure or bad weather the current pour can end at a suitable construction joint allowing proper continuation the next day.

I will reinforce the slab with heavy enough steel mesh that allows 20' spans between control joints. I prefer this longer span because I can get away with just two control joints that can mostly be hidden below trim boards and since the slab itself is 20' wide it means control joints are only needed in one direction. From what I have read, there will be cracks at this span indeed, but the specification I need to meet is the overall implementation (compaction, concrete quality, reinforcement, vibration, control joints, etc) will limit cracks to 1mm width maximum to prevent termite entry.

So the question in my mind is whether on a monolithic pour a control joint on the slab over a footing will activate properly.

 

[Ron]

Heavier reinforcement will not necessarily increase the distance between cracks. In fact, it can actually decrease the distance, though the cracks will generally stay tightly closed. In my opinion, 20-ft spacing on control joints is too much, reinforced or not.

The mix design of the concrete has a lot to do with how wide your cracks will ultimately open. If the water-cement ratio is more than 0.50, the shrinkage will be relatively high, thus opening the cracks more. If you are using small coarse aggregate for pumping expedience, more cement will be required, thus more shrinkage. Keep the water-cement ratio as low as possible and the coarse aggregate as large as practicable to mitigate shrinkage.

 

[jgailla]

If you use wire mesh, it will not do much good unless it is placed properly.
In almost every construction and demolition job I have seen, the wire mesh is at the bottom of the slab where it does little good.
For this reason, I think wire mesh is a waste of money and do not spec it.
For light use such as sidewalks, residential slabs, and driveways, you don't need wire mesh. Use plastic fiber in the concrete and, most importantly, cut your control joints as Ron says, no more than 12 feet apart. Some will disagree with me on the benefits of plastic fiber, but the control joint placement and timing is critical.

 

[dik]

With a 4" slab, I wouldn't normally go more than 10' or so without a sawcut...

Dik

 

[pappy64]

I will follow ACI 302.1 guidelines that the mesh will be in the top 1/3rd of the slab to keep surface cracks tight. Likewise the mesh will be placed on precast-concrete bar supports rather than laid on the ground and pulled up by hand during the pour.

Below is the control joint spacing specification I am referring to. 665 mesh is 5.3mm diameter on 6" centers. Since the slab will mostly be covered by ceramic tile, minimizing visible joint lines is preferable, but only so long as the design is sound. Still looking forward to any input on the original question regarding placing control joints over footings. But comments or suggestions on other areas are also welcome.

 

[Ron]

Pappy64...the table is fine...for perfection...something you're not likely to see. If you don't tighten up the control joint spacing, you'll get cracks you don't want.

 

[pappy64]

My understanding is selecting a suitable mesh for a given span will not eliminate cracks, but it will keep cracks sufficiently tight. I wish to use the slab as a termite barrier and one of the requirements to that end is cracks are no greater than 1mm wide. Is line 1 of the table not going to do achieve that?

 

[dik]

lay your joints out to match your ceramic tile joints... the table should have spacing based on slab thickness... rebar is added info... if it says you can go 600' max, it doesn't say you should not exceed 10'... For a 4" slab, I'd stick to about 10' max and space your tile to suit the joints, or vice versa...

Dik

 

[hokie66]

I don't worry as much about cracks as some other folks, but wouldn't rely on controlling cracks as a termite barrier method. On the other hand, joints would provide free access.

 

[Hoaokapohaku]

I agree with hokie, a concrete slab is not effective as a termite barrier. There will always be cracks that provide a pathway for the termites.

In Hawaii, Basaltic Termite Barrier (BTB) is often used. BTB is a material patented by the University of Hawaii and licensed for production and sale to Ameron HC&D. It is graded to very specific graded basalt aggregate product. I don't know if a similar product exists elsewhere. Detailed product information and design guidance using the product can be found at the following sites:

http://www.ameronhawaii.com/plagg.html

http://www.architecturaldiagnostics.com/pdf/BTB Instn.pdf

I'm curious, is this product available elsewhere?

 

[Ron]

Hey Hoa...I heard your termites eat the concrete!

 

[Hoaokapohaku]

Ron - I've heard that too, but don't know if it's true and have always assumed a more likely explanation is that they can find their way through even very narrow cracks. I don't know how it's possible, but I don't know how that big rat got in my desk drawer to open and eat my bag of potato chips either.

One thing is for sure, a concrete slab doesn't stop termites. I know of housing development consisting of about 50 homes all built slab on grade. Within about a year after construction was complete, every house in the development that was built on ground where the cut was less than 3 feet deep, and every lot filled with material excavated from the top three feet of material was devoured by termites to the point that the houses had be be demolished. Then the law suits ensued.

 

[pappy64]

A concrete slab is an excellent termite barrier *if* made to proper specifications. See below study:

Concrete Slabs as Barriers to Subterranean Termites

 

[BigH]

@pappy 64: But we must remember these are Aussie termites. Given the penchant for Fosters and rugged sports, they are probably a bit buff than ones from other countries, eh?

 

[hokie66]

I wouldn't count on all the termites being 1.4 mm or bigger. They are clever little devils. Most house slabs are placed without supervision, so I wouldn't depend on the slab alone.

Hoaokapohake, a similar product is used in Australia, called Granitgard. There is also a fine stainless mesh called Termimesh which is used a bit. Both types require meticulous installation and avoidance of damage, which in houses is often an unrealistic expectation.

Maintaining a chemical barrier, avoiding wet areas adjacent to the building, and being vigilant for signs of infestation are the best safeguards.

 

[Ron]

Maintaining a chemical barrier, avoiding wet areas adjacent to the building, and being vigilant for signs of infestation are the best safeguards.

Good advice!

 

[rowingengineer]

I would also say a visual area between the earth and slab helps min 200mm is good to see if termites are active. I would also suport hokies earlier comments in regards to cracks

http://www.nceng.com.au/

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