Causes
It is a phenomenon that occurs from time to time, all over the country and internationally , and no single cause has been established despite investigations by most major industry players. It happens regardless of concrete company, location, season, cement, admixtures, aggregates, and in jobs both with and without added oxide colouring.
The only significant helpful evidence is that it only very rarely happens on house floors. House floors usually use a higher strength than exterior concrete, have polythene underneath and are power-floated.
Also, due to the shape and appearance of the defects, it is widely concluded that Pinto could also be caused by moisture movement within the slab.
So it is highly unlikely to be caused by the concrete supplied as jobs supplied before or after jobs that result in pinto are almost always unaffected.
PINTO
MANAGING BEFORE & AFTER
What is Pinto?
Pinto is a problem that occurs only from time to time but one that leaves all parties in a project disappointed. Unfortunately, despite being a global issue no single cause has been established and so it is difficult to prevent.
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Prevention
Because the cause is unknown, it is difficult to know exactly what to do to prevent Pinto, but based on the example shown by house floors, we suggest that:
1. Polythene be used underneath
2. At least 25Mpa concrete be used
3. Surfaces are power floated
Unfortunately even with the above measures, no guarantee can be made that pinto will not occur.
Fixing
Our advice if Pinto occurs is to either:
1. Give the concrete an acid wash, wait a few months and then give it another. This seems to open up the surface and facilitate some changes. Both time and using an acid wash have been shown to have a positive effect in many instances.
2. If time is not available then a tinted sealer can be used to cover up the issue.
Grinding: In some instances (but not all) grinding the surface has removed the issue.
White vinegar: for smaller areas, white vinegar has been applied and left overnight and has successfully reduced the effect.
Conclusion
Pinto is a very unfortunate situation for all parties, but some things can be done to reduce the chance of it occurring. If it does happen a light acid wash should be undertaken immediately as that can definitely help.