Question: I have recently taken charge of a multistory construction building in Pakistan, which three floors are already built, including one basement. On the first floor, it has been observed that the proper mixing of the material has not been maintained and the coarse material could have contained mud particles. Now, the first floor columns seem to be crumbling from a minor blow with a hammer and nail. Could you provide some guidance? Are there any treatments for the problem? Will I have to tear down the columns one by one? A second floor already has been constructed over the first, and that floor does not have any problems. I have to take this building to another eight floors on top of these three.
Answer: The situation you describe does not sound good. Construction should be halted until a qualified engineer has determined the strength of the concrete columns. Strong concrete relies on strong aggregate as one of its primary ingredients, and clumps of mud are the exact opposite. If the concrete in your columns crumbles with so little effort, you won't want to count on it to hold up a multistory building. How you proceed once you know how serious the problem is will depend on several things, including the design assumptions. This again is an area where you'll benefit from an engineer's assistance.
Obviously you don't want to tear the structure down and start over, even though that may end up being your best option if the damage is severe. However, you can investigate some other alternatives. One option would be to treat it as a repair project, removing all unsound material from around the reinforcing steel and replacing it with good concrete. However, that would be a very tedious approach.
If the concrete is weak but structurally sound you might use external reinforcement to increase its loadbearing capacity—much like wrapping an ailing joint, such as a sprained ankle. One way to do that involves putting steel bars or plates on opposite sides of the column and squeezing them together with bolts. Repeat on the other two sides, boxing in the concrete.
A third alternative might be to place some additional high-quality concrete around the existing column. You'll lose floor space from the increased size of the columns, but that may be more acceptable than starting over. It also will be important to ensure that the load transfer is effective at the top and bottom of the column section.
There are many complex considerations that go into designing and constructing a multistory building, beginning with the assumption that the materials and construction meet minimum quality standards. It's unfortunate that you came across the poor quality you describe, but it's a good thing you discovered it now rather than later.