EXPANSION JOINTS. Poured concrete slabs require stress relief to prevent surface deterioration in the form of cracks. An expansion joint is generally defined as a void that contains dividers made of felt, rubber, plastic, or wood and are attached during the framing stage of the concrete form and left in place until cured. Expansion joints are complete separators that leave 1/4" to 2" wide gaps between sections of the slab that may range from 10' to 40' in length or width, depending on the type of slab being installed.
CONTROL JOINTS. Control joints are a different kind of stress relief used for concrete slabs. The control joint is a "tooled" joint that is applied during the concrete pour and is typically 1/8" to 1/4" in width. The joint is created using a concrete tool and is usually 1" to 2" in depth. The control joint depth is actually a predetermined point of weakness in the slab which allows the slab to crack at the bottom of this joint, thus minimizing the possibility of unwanted cracking or deterioration in the slab surface. Control joints can be added to cured (dry) concrete using a circular saw and diamond-tip blade, or a stand-behind concrete saw, in the event stress relief is needed in older concrete.
Keeping the control joints free of decorative overlay material, such as PermaCrete, is important since PermaCrete would bond the two adjoining sections of the slab together and possibly allow future cracks in the finished surface areas. The following are procedures for keeping PermaCrete out of control joints are as they are made.
- Place strapping tape, vinyl clothes line, or backer rods (rolls of foam or expanded polystyrene used specifically for filling voids) into the control joint prior to application of the skim coat.
- Apply the PermaCrete skim coat and wait until cured.
- Remove the packing from the joints by pulling the tape up and away from the surface leaving a straight edge on either side of the control joint.
- Re-pack the control joint as before prior to applying the PermaCrete texture coat.
- Apply the texture coat.
- Allow the PermaCrete to cure.
- Remove the packing.
- Apply tape on both sides of the outside edges of the control joint at the lip of the void where the edge of the PermaCrete meets the edge of the control joint. Make sure the tape runs the full length of the joint on both sides.
- Pack backer rod into the control joint to within 1/4" depth of the top of the joint and run along the entire length of the joint.
- Apply PermaCaulk sealant throughout the joint and tool the joint surface smooth with a wet paint brush.
- Remove the tape.
- Seal the entire surface of the slab including over the newly-installed PermaCaulk in the control joint(s).
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