PITTING ON POLISHED CONCRETE

Source:www.stonediamondtool.com Date:2018/7/30 Read:831

Question:

My company was the contractor on a retail remodeling project that had an existing concrete floor requiring a grind and polish finish. We had to sawcut and remove portions of the floor for new underground mechanicals and then pour these sections back. Our job was to then grind and polish the entire floor to the level of gloss required. On the existing concrete areas, we had no problems. However we experienced problems from the start with the new concrete areas. The engineer involved with the project gave us a method of grinding that allowed us to achieve the correct sheen on gloss tests. Now less than six months later, these areas have failed miserably and the surface is visibly porous and is trapping dirt to the point where floor looks black in areas. The polishing contractor is claiming that this is a concrete finishing issue and the only way to take out the highs and lows of the concrete surface is to grind deep and expose the aggregate, but this would not be acceptable to the owner.

Part of me agrees with the polishing contractor, but during a preconstruction meeting with the polishing company and project engineer, concrete finishing was a non-issue, and the finish did pass initial gloss testing. There have also been some comments about how the store has maintained the floor and their use of chemicals to clean it, which may have caused the concrete finish to fail. Any input you could give us would be great. We are a large general contractor in the Midwest, but our experience in grinding and polishing concrete is limited and we are hoping that this is not going to turn into an expensive lesson.

Answer:

Concrete, polished or not, should not pit and be weak on the surface. This usually indicates a mix design or curing issue. With situations like this involving many players, I like to go back to the start and focus on the key issue. In this case, the main issue seems to be that the replaced concrete sections are not holding gloss and polish because of surface 

deterioration that occurred soon after the floor was put into service – within the first six months.

From the details you have provided, it appears that the replacement sections of concrete are failing and soft, either due to substandard mix design and concrete placement or post-applied chemicals. Polished concrete needs to be properly chemically hardened during the polishing process. This increases the surface hardness and provides better gloss. Lithium or sodium silicate hardeners are most commonly used. If applied properly, these should harden the concrete to a point where soft pitting does not occur, unless the concrete was so substandard that it was really soft and weak to start with.

To zero in on what's causing the floor deterioration, the following questions need to be addressed:

· Is there anything different about the replacement areas in regard to mix design and how the concrete was placed and finished? Did the concrete freeze after placement? What you need to look for is anything that might have created a weak surface.

· What is the store using to clean the concrete? If a harsh cleaner is pitting the concrete, it would do it to some extent on all the concrete, not just the replacement sections.

In regard to the finish, it is probably one of the most critical parts of preparing the concrete when polishing. The concrete mix design, placement, finish, flatness, and curing are all critical. If you get a soft slab, you end up with what you are experiencing. As for the repair, you can regrind to go deeper and get rid of the pitting, but this ignores the real problem in hopes of a quick fix. If the concrete is weak, the new grind will last another six months and then you're right back to where you started. I suggest taking a core sample or performing a surface hardness test. Also, do some research on the hardener that was used and make sure it was applied correctly and at the right coverage rate.