It's not uncommon to have finger-tip sized bug holes in rough-formed concrete. Most concrete specifications provide for pointing & patching the holes and rubbing them smooth with grout. Usually, this condition is a result of inconsistencies in slump & vibration, not mix or air entrainment or chemical reaction, as some members have mentioned. If the mix is workable and internal vibration is executed properly, you can usually avoid significant-sized openings like a few in the picture.
In this case, diagnosing the picture, to me, is very straight-forward: There are two causations. One is placement & vibration. Large voids are often-times caused by large lifts of plastic concrete being placed combined with inadequate vibration: The air pockets simply don't have the ability to be worked out of the mix. If you place too large of a lift, you inhibit the ability for trapped air to be exhausted. The second issue in this picture relates to the the form oil/release agent that was used. Look at the white pop-outs on some of the voids: The cement paste was torn off, which means the concrete adhered to the form, which of course, is why the forms are treated prior to closing.
A couple of suggestions: If you really need a smooth finish, using high-density overlaid plywood, which are constructed with laminated sheet on the formed surface whereby bonding does not occur, will aid in providing a smooth finish. Yes, it's a more-expensive system than regular formwork, but if your finished product requires it, it could be worth considering. You can also consider using a form vibrator which provides contact vibration between the face of the form and the concrete, which will provide more-uniform vibration at the contact surface than you might get with a spud vibrator. I've provided a couple of links below if you care to advance your knowledge of both. I hope this information helps!
Vibco Form Vibrators
HDO Concrete Formwork
Hildebranski.com is a site written for civil engineers, construction engineers, resident engineers and construction project managers. Each article focuses on real-world issues that civil & construction engineers face written by an industry veteran.
hildebranski.com
