about the Boston prob. How inferior can cement be ? water, sand, rocks, sounds simple enough to me.?

Answer 1

I'm not sure what the 'Boston prob.' is, but concrete is what you're describing. Cement has no rocks or sand in it.

As far as concret quality, just remember thats all you have (water, sand, and rocks) if its not mixed together correctly in the right proportions.

Most cement and concrete is engineered to yield the suitable performance. Troubles occur when the specifications aren't adhered to.

Answer 2

It isn't. If you don't use correct proportions, the resulting concrete (not cement - cement is the gray powder that you use to make concrete) will be weak and can fail. A common failing is to use too little cement in the concrete mix because it is the most expensive ingredient. But even if you get the mixture right, you still have to get it to the site in good condition (well mixed, but not starting to set) and install it properly, e.g.by using a vibrator to settle it and work out the air bubbles.
Concrete on site is often graded by a "slump test". A cone with an open top is filled with concrete, and the concrete allowed to settle. The cone is then removed, and the concrete pile will slump down and spread out. If it slumps too far, there is too much water; if not far enough, there is too much aggregate. To really tell if the amount of cement is right, you have to make a concrete casting (usually a cylinder the size of a soup can), let it set, and then test it for strength by squeezing it in a press.
But the Boston fiasco may well not be a concrete problem at all. The slabs were supported over the roadway by an arrangement of beams and bolts, and this may have failed. The investigators are still looking, and it is too early to tell.

Answer 3

The 'inferior cement' actually referred to a controversy in which construction workers used cement that had been mixed for too long without being used. It had apparently been partially hardened, or otherwise had lost its ability to set in non-cracking formation.

Water leakage from the surrounding earth is also blamed for damage to the cement, and may have contributed to the collapse.

Answer 4

It's all in the proper mixture, how it's poured and how long it's allowed to set up. Also how well it is reinforced with rebar, post tensioning etc.

Answer 5

concrete is really complex chemistry. bad batches are common especially if they aren't made fresh, and disaster can result