control joints on monolithic slab foundation
control joints on monolithic slab foundation
(OP)
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.







RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
Dik
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
Dik
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
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%20...
I'm curious, is this product available elsewhere?
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
Concrete Slabs as Barriers to Subterranean Termites
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
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.
RE: control joints on monolithic slab foundation
Good advice!
RE: control joints on monolithic slab foundation
http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
The reality is various strategies have been developed to prevent termite infestation because termites do attack structures where concrete separates the ground and wood. Even if theoretically a concrete slab should stop termites, in practice they don't do it well enough. Perhaps it is because of a gap between what is specified and what is actually constructed, but that happens A LOT! A better method, or redundant methods, are needed.
RE: control joints on monolithic slab foundation
RE: control joints on monolithic slab foundation
If you read the study it was interesting that termites actually need cracks significantly wider than their size because they don't go through without surrounding mud tunnels so allowable crack sizes are actually even higher.
RE: control joints on monolithic slab foundation
Who told you that over half the new homes in Australia use only the slab?
RE: control joints on monolithic slab foundation
Wow, ok.
> I think the stainless mesh only has openings about 0.5 mm. Don't know why a slab with a 1.4 mm crack would be better.
There are several reasons for this. Typically mesh size is designed to be smaller than the head of a termite which is a pretty good specification. However, the space to allow movement of a termite is larger and comes into play over a longer crawl space like a slab which is also abrasive. Specifying concrete to have smaller cracks would simply add cost without benefits.
> Who told you that over half the new homes in Australia use only the slab?
Do a search.
RE: control joints on monolithic slab foundation