Q: The architect on our current project has asked about engraving letters into architectural concrete after it is poured. The building is mostly underground, with an oval-shaped, domed glass roof that rises from a concrete ring beam just abovegrade. The ring beam is to be faced with architectural concrete, and the architect would like to have the building's name inscribed into the concrete. I've seen this done in granite and marble, but not in concrete. Would we need to use a special concrete mix for this? Are there other pitfalls we're likely to encounter?
A: You may be able to do what the architect has asked for, but we anticipate a number of potential problems. You'll want a concrete with the most uniform texture possible and you'll need to prevent any voids from forming during placement that could be apparent when the surface is carved. Using a self-consolidating concrete mixture could help in this regard.
The carving itself may be difficult to accomplish due to hardness differences between the cement paste and aggregates. The concrete should be allowed to cure for at least one, preferably two, weeks before carving to ensure sufficient bond has developed between the paste and aggregate. You also need to be concerned about maintaining sufficient concrete cover over reinforcement in the carved-out areas and about the increased potential for cracking at those locations.
It might be better to have panels precast and carved offsite under controlled conditions, then installed as facing on the ring beam. That way, you could be sure the carving looks right before the panels are set in place. You could even propose using carved marble or granite panels for the building name. Whichever way you go, we'd recommend building a full-scale mockup for the architect's approval before proceeding.