Concrete Stitching 1

[RobPE]

Does anyone know of any guidelines to concrete stitching for large (25' x 0.3") residential slab on grade cracks? I'm looking for guidelines such as spacing of stitches, stitching material (rebar vs carbon fiber), length of stitches, and epoxy procedures such as injection method, epoxy types, surface prep, etc.

Thanks

 

[Ron]

I'm not sure why you want to "stitch" a residential slab. It generally has no load on it and the cracks are not usually structurally related...they are usually drying shrinkage cracks.

If you have a homeowner who is concerned about the cracks, they can be filled with epoxy. If you do that, I would also sawcut some joints in the slab, even at this late date, to take future shrinkage stresses off the epoxy repair. Others will argue the effectiveness of cutting joints at this point; however, shrinkage in concrete continues for a long time, and cracks will widen with time. If nothing is done, the original shrinkage stress buildup that caused the crack will continue (though to a lesser degree)and put stress on the epoxy/concrete bond on the face of the crack. Since you will not know the coverage and depth of epoxy penetration, add the sawcut joints as a precaution. When you cut the joints, cut them deeper than you would for new concrete joints. This will assure a reduced cross section and since the concrete has gained strength, give you a better chance to have the process work properly. I would cut the joints to at least 1/3 the depth of the slab (in new construction, the joints are cut 20 to 25 percent of the depth).

Use a gravity feed epoxy (one with a high viscosity)that will penetrate as deeply as possible into the crack. Since you have a relatively wide crack, that shouldn't be a problem. You can also use an acrylic material such as Simpson's AcrylTie that is normally used to set anchor bolts. This material bonds nicely to the concrete and is slightly less rigid than epoxy.

 

[RobPE]

The slab cracking is presumably due to foundation problems with unstable soils rather than concrete shrinkage as evidenced by the size of the crack (~0.3" wide) and a slumping corner. The area also has a long history of these types of problems. The remediation plan is to underpin the corner and stitch the crack before installing new flooring (slab is no longer subject to curing/shrinkage stresses). The homeowner has also addressed some drainage issues which probably exacerbated the problem. Ron, in this situation, do you think that epoxy without stitching as you described would be appropriate? Do you think stitching is overkill?? Any other thoughts?

 

[miecz]

Agree with Ron that epoxy with new control joints is a better solution. You may also consider filling the joints with a cement based grout, and using a flexible joint sealant, thereby eliminating the need for new control joints, (which should be sealed with a flexible sealant).

 

[Ron]

RobPE...if you have a corner settlement problem to the extent that you need underpinning, then stitching is probably a good idea. In this case, you will need to be concerned about the shear bond of the epoxy on the crack face between the stitches...again, some shrinkage will continue to occur, thus placing stress on that shear-bond interface. I would still do the joint cuts.

Small rebar in saw-cut slits, epoxy filled, will suffice for the stitching. Use No.3 or 4 rebar. Gravity feed the epoxy.

I would place 12 inch long stitches at 18 inches on center.