Specifying Light Sandblast Concrete Finish 2

[ajk1]

What is the proper technical description to include in a project specification for a light sandblast finish?
In a preliminary architectural finishing specification prepared by the architect for the project, that we (structural engineers) are reviewing, it describes the following (see below) for a "light sandblast finish". Is that an acceptable description for a light sandblast finis? I would have thought that a light sandblast finish would expose the fine aggregate, but not the coarse aggregate.

"After concrete is 21 days old and thoroughly cured, sandblast using a hard sharp sand until course aggregate is in uniform relief and light texture is achieved to match approved mockup sample".

 

[BAretired]

This may be helpful:

http://www.caltrain.com/assets/_engineering/engineering-standards-2/Specs/Div3/03350.pdf

BA

 

[ajk1]

to BARetired - thanks for this. You certainly seem to be an encyclopedia of information. Do you happen to know if the definitions for the various types of sandblast finish as given in the Caltrain spec that you attached, are accepted in general building Standards publications (such as CSA, ACI, etc) of the Canadian or American or Australian or British Specification Writers Associations, or federal government master specifications?

 

[hokie66]

The only way to assure a sandblasted finish which is acceptable to an architect is to prepare test panels, obtain the architect's approval of those, and use them at the site for comparison.

 

[OldBldgGuy]

Sandblasting the surface should not affect the structural properties in any way, no matter how challenged the guy on the blaster is. Hokie66 has the only real answer.

 

[BAretired]

ajk1,
Sorry, I can't say whether the spec I referenced is accepted by the various standards you mentioned, but I agree with hokie66 that test panels are the best way to ensure the architect is happy with the resulting finish.

BA

 

[dik]

Always do two test panels to show a range of finish... it may be difficult to provide a uniform finish that matches 1 panel... by showing a range... you have to be in between... a lot easier to do and reduces any arguing about the finish.

Dik

 

[ajk1]

Test panels are of course specified for the project. But they cannot replicate the very particular aspect of the sandblasting that I am mostly concerned about, but which I did not mention in my post, namely the emphasized appearance that sandblasting causes to the inevitable shrinkage cracks in long walls. A test panel of limited size is not likely to have any shrinkage cracks of significant width, or perhaps none at all. So a perfectly acceptable sandblast finish on the test panel with no cracks and therefore no sandblasting emphasis of the cracks, may not be at all acceptable on the actual wall with shrinkage cracks.

Thanks everyone for the helpful comments. Particularly to BARetired for the document.

 

[hokie66]

If that is your objective, I fail to understand how a sandblasting specification will help. Providing a high degree of crack control by reinforcement, combined with closely spaced control joints would seem to be the engineering solution to an architectural problem.

 

[Ron]

Read both of hokie66's posts again!!

Also, you cannot reasonably expose the fine aggregate without also exposing coarse aggregate. "Light" sandblast is related to depth of relief. The goal is to mechanically etch the surface paste for slight texture and uniformity.

 

[dik]

What is proposed is not a light sandblasted finish. Best approach for exposing course aggregate is to apply a surface retarder and sandblast to expose. A light sandblasted finish does not normally expose fine aggregate (not the intent), although, it is often exposed. I used to have a set of standard samples in a grey plastic material that showed the various surfaces, including broomed, etc. I don't recall what the set was called, but, it had a 'Standard' name.

As Ron and Hokie have noted, sandblasting brings out imperfections, too. These can be minimised by proper reinforcing, curing and jointing.

Dik

 

[ajk1]

Thanks everyone for the comments and admonitions. I think we are all saying basically similar things. My long experience tells me (assuming I am not by now totally senile) to go with the lightest sandblast finish (what I would call a "brush-off" sandblast) that the architect can live with. That is how I have successfully dealt with the issue on at least one project in the past. If the architect wants a heavier sandblast than that, as determined by the pre-construction trial panels that we do, then we should gently advise her of the possible undesirable effects. Yes we have of course done all the elementary things, such as significantly increased the amount of rebar, and decreased its spacing such that we have tight crack control. But these items control the width of cracks, they do not eliminate them.
Hokie66, you being in Australia, perhaps do not have the same range of thermal cycling and resulting concrete cracking as we do here in this part of Canada (if my memory is right, we had this discussion about a year or 2 ago!).

 

[hokie66]

True, no freezing temperature here in Brisbane. When it gets down to 10C (50F), the natives complain. Not me, I like it.

 

[ajk1]

What size should the preconstruction job mock-up of the wall be? The architect's preliminary spec says 600 mm x 600 mm. Seems rather small to me...

 

[ajk1]

Sorry...I withdraw my latest comment. I see that the 600 x 600 mm is just the sample size, not the preconstruction mock-up size.

 

[kipfoot]

In my experience, a roughened concrete surface makes shrinkage cracks less apparent.

As an aside regarding the mock up... include features that will be in the final structure, ie. elec outlets, returns, corners. It's also essential, in my opinion, to have a site superintendent who is on board and invested in providing the craftsmanship to make the final product meet approach approximate the architect's vision.

 

[ajk1]

to kipfoot - I agree with you entirely on all that you say, including about the rough surface, if you mean a formed rough surface (such as a "board formed" rough surface). But a sandblasted finish in my experience over the last five decades is that a sandblast finish makes the cracks more visible, probably because it knocks the surface corners off the cracks and makes them wider at the surface. If an extremely deep aggressive sandblast finish were done, then it would perhaps be similar to a rough surface finish...I don't know because I have never seen an architect want that...they usually want a light sandblast. Your points are well put and I agree with them.