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Capping a Patio


Posted by David on March 24th, 2003 06:35 PM
In reply to raising a concrete patio by eureka on March 24th, 2003 01:45 AM [Go to top of thread]

Eureka,

The minimum thickness of the new material should be no less than 3" when "capping" an existing slab; 4" is better if you have the room. Then:

If the current slab does not drain properly, and tilts towards the home, but is not cracked;

- consider first removing the slab. If not;

-seal the joint between the existing slab & the home with a butyl sealant, using backer rod first if the gap is wider than 1/4".

-Drill the foundation wall of the home, if it's concrete, 3" to 4" deep for 18" long horizontal rebar pins to extend into the new slab 1" above the existing slab to prevent it from dropping in the future.

- clean the surface thoroughly, then just prior to placing the new slab dampen it & sift some portland onto the surface, or coat with concrete bonding agent.

If the current slab IS cracked, AND tilts towards the home;

- definitely consider first removing the slab. If not;

-seal the joint between the existing slab & the home with a butyl sealant, using backer rod if the gap is wider than 1/4".

-do not do anything to adhere the new slab to the old; it can cause the new slab to crack at the same point. Drill the wall of the home 3" to 4" deep for 18" long horizontal rebar pins to extend into the new slab, and put down a large bead of butyl sealant around the edges (1"-2" in from outer edge) of the original slab 24 hrs. before placing the new material; this will prevent water from traveling between the slabs.

If the current slab is not cracked, and drains away from the home, but simply needs to be higher;

-Clean the surface thoroughly, then just prior to placing the new slab dampen it & sift some portland onto the surface, or coat with concrete bonding agent.

If the curent slab IS cracked, but still drains away from the home, yet needs to be higher;

-again, consider removal first. If not;

-do nothing to adhere the new slab to the old. Drill the wall of the home for horizontal rebar pins to extend into the new slab, and put down a large bead of butyl sealant around the edges of the original slab 24 hrs. before placing the new material.

In all the above, put a control joint(s) in any slab larger than 8' X 8'. Use a fiber-reinforced concrete or 4" X 4" wire mesh in any slab larger than 8' in any direction.

Dig a shallow, narrow trench next to the existing slab, just enough to allow for 1X or 2X form boards to sit tightly to the edges of the existing slab & extending a few inches down the sides.

Place your form boards so the top edges of the boards are also the top edge of the new slab, and ensure you have a minimum drainage away from the home of 1/4" in 10'; secure the forms by driving form stakes tight to the outside of the forms, nailing the forms to the stakes from the inside of the form. Assuming your concrete pour will be 6" thick or less, stakes should be no farther apart than 3' for 1X forms, 4' for 2X.

Eureka...do you still want to do this?

Post back with your thoughts, and maybe a bit more info, such as the size of slab you're talking about, and look at what might be done to surrounding ground to allow drainage. Also, take note of the slope of the slab; if it doesn't drain towards the home, maybe brick pavers could be used instead of taking on what can be quite an experience for an unexperienced person.

If you still feel this is the right approach, say the word, and I'll continue with more info.

Regards, David

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