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]

RE...agree. Here we try to get at least 6" separation, a bit less than your 200mm, but the point about separation is significant. The mud tunnels can be readily observed as it is easier to build a tunnel on the outside face of the foundation than burrow through tight cracks. Another impediment is to put a metal flashing under the wall termination and extend out slightly past the wall surface....they don't build tunnels very well around metal corners !

 

[Hoaokapohaku]

From Pappy's link to "Concrete Slabs as Barriers to Subterranean Termites,"concrete slabs constructed in accordance with AS 2870 are generally expected to have crack widths less than 1 mm." If it is so that slabs constructed according to the standard are "generally expected" to have cracks less than 1 mm, then it is also true that a population of slabs should be "exceptionally expected" to have cracks greater than 1 mm. Who wants to be the designer when exceptions to what is generally true take place and termites ring the dinner bell?

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.

 

[Hoaokapohaku]

P.S. Termites are better slab inspectors than engineers. Being absolutely relentless in their pursuit; they find the concrete deficiency that the engineer missed and then they mark the trail for their buddies to follow to the feast.

 

[pappy64]

It would seem the methodology to obtain a slab that is termite resistant is no different than designing a slab to meet a certain strength requirement. It's just a matter of staying within the margins of proven specifications. If you stay within the margins it works, if you don't then it may not. Today over half of new homes in Australia use the slab as the sole preventative measure against termites. And no wonder, who really wants to use poisons around their houses and the price of a stainless steel mesh barrier can be astronomical.

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.

 

[hokie66]

Count me as one who wants to use "poisons around their houses". By the way, 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.

Who told you that over half the new homes in Australia use only the slab?

 

[pappy64]

> Count me as one who wants to use "poisons around their houses".

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.

 

[BigH]

Now, fellas, what about roaches??