Mortar bonding to concrete slab top surface 1

[Alan CA]

Hi Everyone,

We've been having a long discussion at work about polymer modified mortar (p.m.m.) bonding and sticking to concrete slab top surface.
Do you think mortar can bond well with the concrete if the slab surface is roughened? Slab will be subjected to vehicular and pedestrian loads.
I appreciate sharing your opinion

 

[dhengr]

Alan CA:
The existing slab should be cleaned and roughened. Talk with the manuf’ers. of the bonding mortar and ask them for application instructions and a written guarantee. Take care in the listening, that they are not blowing smoke up your leg, and won’t stand behind their claims. You will still likely have problems where the overlay feathers out to zero thickness. The thin layer is not strong enough w.r.t. bending due to perpendicular loading and you will get seepage into the bond joint, weakening it. Maybe better to saw cut .25” deep +/- and then chip and roughen, then caulk/seal that vert. joint in some way.

Edit: HotRod10 is much more experienced at these details than I am, I certainly wouldn't argue with him on his .5" edge cuts.

 

[BridgeSmith]

For bridge decks, we specify either high-strength repair mortar patches with a minimum of 1/2" saw-cuts at the edges, or silica-fume modified concrete overlays with a minimum thickness of 1".

 

[epoxybot]

These mortars are all designed to bond but surface preparation is critical. An internet search will provide a number of links to individual company's spin on the International Concrete Repair Institute (ICRI) technical guideline #03732, “Selecting and specifying concrete surface preparation for sealers, coatings, and polymer overlays". They offer photographs of surface texture that is helpful for gauging the work done in the field. Examples: Link Link

Along with well cut edges, you do not want areas with deep pockets. These need to be filled & tined to the depth-overall before placing the overlay. Otherwise they will behave like an anchor when the overlay contracts as it sets. Assuming some priming work is required, this should entail some mechanical application, brooming and/or Kalsomine brush/masonry brush to work the primer into the surface and eliminate most standing primer.

Assuming polymer modified mortar means something along the lines of a jug of white liquid (moose milk) & a pre-mixed bag of mortar, the next issue is what your expectations are regarding esthetic results. If you have a large area, you want to be sure the contractor isn't going to try and accomplish the work with 5 gallon buckets and a paddle mixer. There are specialty contractors with inline mixers and still others that can get it done with a mortar mixer.

Polymer modified mortars have different setting characteristics and can form a polymer skin that isn't friendly to being reworked. The color of the overlay will most likely not match the surrounding concrete. Creating some aesthetically pleasing break lines, might require more material/area. The young's modulus of the mortar should be equal to/or somewhat less than the existing concrete. If it is in a freeze/thaw area, a lower modulus is more durable. Adding more moose milk to the batch to make it more workable, needs the manufacturers approval - PERIOD. Polymer modified mortars are more sensitive to weather, hot or cold and wind; more so when working a large area.

Even if your chose to work with a manufacturer qualified contractor who has the right mixing/dispensing equipment, ask them what percent guarantee they can give that the mortar won't have unsightly cracking.

 

[Alan CA]

Thank you all for your replies and the helpful information. I appreciate your help

 

[racookpe1978]

18 Feb 19 00:10
For bridge decks, we specify either high-strength repair mortar patches with a minimum of 1/2" saw-cuts at the edges,

Wouldn't those smoothed 1/2 inch saw-cut edges have to be somewhat roughed up themselves before pouring the filler liquid/mortar/grout?

 

[BridgeSmith]

racook, I don't think the edges need to be roughened, since it doesn't need to interlock along the edges. The bottom will generally be rough due to the removal being done by chipping, and that is considered adequate to keep it in place.