Precast Hollow Core finish

[bsh117]

I went out the a job site and noticed that the concrete on the bottom side of the hollow core plank appeared to look like it was "stretched" or "ripped." I've attached a photograph of one area. Would you consider this finish "typical." It's the first time that I've seen it.

Thank you

 

[concretemasonry]

That is a photo of "zero or near zero" slump concrete plank that is common for most precast planks/flat slabs produced on long beds. Since the long beds (100s of feet long) are cycled daily after the sections are cut, often, the bottom surface may not be a clean enough or the vibration is not adequate to provide a smooth finish for the zero slump concrete to slip on during manufacturing.

Some systems do not use zero slump concrete, but produce a very smooth finish, but the beds are much shorter and are more acceptable for a smooth architectural bottom surface.

Fortunately, most (85% are PCI members) and the manufactures keep good records and logs of all production and have very acceptable testing procedures and quality control procedures for years.

Dick

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

 

[bsh117]

Thanks for the reply? In your opinion, does this finish meet the "standard grade" definition?

 

[hokie66]

In my opinion, those slabs are not acceptable. Hollow core slabs are extruded, and the bottom surface should not look like that.

 

[concretemasonry]

Not acceptable, because of the indication of very possible poor bond between the strands and the concrete where the cover is low. Something like that is probably a combination of dirty forms that do not allow "slip" as the vibration extruder goes over a length of bed. Moisture variation in the mix is also a factor. Aggregate variations (gradation and particle shape) are remote possibilities. Aggregate is not too likely since concrete products are "prime" customers that require better controls than a ready-mix producer and get it because of the consistent demand on an annual basis.

As a design engineer the reasons are just excuses, but the appearance it a clue/hint to a possible quality or reliability problem.

Dick

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