Shot Blast Concrete Finish

[mattwilliamso2]

I am looking to specify a shot blast finish for a new concrete pathway. We are specifying this to achieve the nice textured effect it produces. Could anyone help me with how to specify such a finish? I know we want a light shot blast finish but do not know what else I need to specify. Size of shot? Etc..

The concrete will be 5" thick slab on grade with a 2" polyfibre mixed in. Thanks for any help you can provide.

 

[concretemasonry]

Have a couple of slabs made as samples using the mix you are specifying from the same concrete supplier.

Then, try a couple of blasting techniques/materials to get the appearance you want. You may have to burn the surface with a flame to get rid of the remaining fibers. Preserve the remaining approved slab for comparison to the attempts during construction.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

I agree that you need test panels. You can't specify a required finish without testing. The other way is just to shot blast it and accept what you get. The first thing you should do is lose the plastic fibres.

 

[mattwilliamso2]

Thanks for the posts. I did plan on having a mock-up done on site for review and approval. Would you recommend wire mesh instead of polyfibre?

 

[hokie66]

Well, yes, welded wire fabric will control the width of shrinkage cracks. Not so with plastic fibres. Best to try to prevent the cracks from forming by providing control joints at close spacing, concrete with low w/c ratio, adequate curing, and all the other good concreting practices which are often ignored in things like pathways.

 

[bimr]

I don't think the plastic fibers are going to add anything. I agree with Hokeie66 that you should specify welded wire. The welded wire will help to minimize cracks.

Shot blasting is a method that does an excellent job for surface preparation. Different surface profiles can be achieved by varying shot size, shot flow rate and machine travel speed. Shot blasting uses steel shot, or small BB’s that are blasted onto the concrete surface. The shot comes in different sizes and depending on how much material needs to be removed and the type of profile needed on the concrete surface, will determine the size of the shot. Once the shot has been blasted onto the concrete surface the shotblasting machine has a vacuum system that picks up and separates the reusable shot and surface debris. Stray shot is picked up with a magnetic broom.

Here is a link to firms that does that type of work:

http://www.vanguardconcretecoating.com/shotblasting.htm

http://00000y8.previewcoxhosting.com/shotblasting.htm

http://www.hovertrowel.com/tools/ShotBlastCleaningSystems.pdf

You need to find a local contractor that does the type of work that you want. They will have reference installations that you can visit. If you have seen a type of finish that you desire, then you should enquire with the owner to see who did the finish.

One would think that you also would also be interested in coloring the concrete. The shot blasted concrete should also have a sealer applied over it to protect the concrete.

This type of decorative concrete is going to cost you 2 to 3 times more than standard flat concrete work.

 

[mattwilliamso2]

Thanks Bimr for the reply. I'm beginning to think this shot-blasting will be too costly and that a standard light broom finish would be better.

 

[bridgebuster]

If you can, take a look at ACI 303R-04 - "Guide to Cast in Place Architectural Concrete". It elaborates a bit on what bimr wrote.