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I have just written an explanation for the pulverization of the concrete over at the STJ911 forum, which is a private forum, to address the exact doubts you express. The points I made there are:

1. The concrete in the twin tower floors contained little or no aggregate, to keep it light. It was essentially just powdered cement. There was no structural requirement other than the individual floor area loading, so it was feasible to do this to cut down on the weight of each floor. The lack of aggregate would allow it to be returned to a powder form when acted upon by explosive or high impulsive forces.

2. The twin towers were most probably demolished in a series of traditional controlled demolitions every third floor from the aircraft impact points down, with the upper stories also being demolished from the aircraft impact points up. It was essentially a series of thirty some controlled demolitions, one on top of the other all the way down. The lightweight concrete would have been acted upon many times during these continuous demolitions and I think it is easy enough to see how it could be pulverized.

how much destabilization of the building would take place from the thermate cutting of certain steel columns? Would there be a "shift" of the infrastructure, based on the thermate cutting alone, then followed by the explosives?

Loads could be increased "2000%" before failure occurred. This implies that there had to be very significant column loss for failure to occur. Read my essay about the building designers on my blog for more building designer claims.

“We're an empire now, and when we act we create our own reality."

Structural concrete Would have to contain a significant quantity of silica, at least in the form of silica sand, in a greater proportion than the cement used.

It is inconcievable that the concrete would not encase an intense rebar grid.

That lightweight aggregates could be used to some extent is true, but,

The concrete, as an amorphous crystalline mass, would have been immensely strong and require enormous amounts of energy to be shattered.

Your post seems to minimize these aspects of structural concrete.

You suggest, ("The lack of aggregate would allow it to be returned to a powder form when acted upon by explosive or high impulsive forces.")

that the floors were made this way so as to be be blown up easily. This is a an enormous cannard.

The post by Tony Szamboti appears to contain false information. Careful voting this stuff up.

Who said the floors were made this way to be blown up easily?

How could you possibly take that from what I said?

My point was only that due to its lightweight construction, and lack of aggregate, it would more readily pulverize under intense forces than regular concrete which contains aggregate..

Specify the "lightweight aggregate used and in what proportion.

"essentially concrete powder" is not lightweight.

"My point was only that due to its lightweight construction, and lack of aggregate, it would more readily pulverize under intense forces than regular concrete which contains aggregate.."

This statement seems extremely dubious. The strength and weight of the concrete was massive.

Pulled from a random supplier on Google:
http://www.escsi.org/New%20Web/Structural%20Lightweight%20Concrete.htm
"Structural lightweight concrete has strengths comparable to normal weight concrete, yet is typically 25% to 35% lighter. Structural lightweight concrete offers design flexibility and substantial cost savings by providing less dead load, improved seismic structural response, longer spans, better fire ratings, thinner sections, decreased story height, smaller size structural members, less reinforcing steel, and lower foundations costs."

Uses of Structural Lightweight Concrete
• Floors in steel frame buildings, (lightweight concrete on
fire-rated steel deck assemblies)
• Concrete frame buildings & parking structures (all types,
including post-tensioned floor systems)
• Bridge decks, piers & AASHTO girders
• Specified density concrete
• Lightweight concrete precast & prestressed elements
(beams, double-tees, tilt-up walls, raised access floor
panel planks, hog slats, utility vaults, pipes, ornamentals,
etc.)

The trusses actually do the work holding up the steel pan/ concrete floorbed, made as thin as possible for the reasons stated above. Rebar is minimized or absent in such contruction.

"more blast resistant, and has better shock and sound absorption, High-Performance
lightweight aggregate concrete also has less cracking" (http://www.escsi.org/New%20Web/Structural%20Lightweight%20Concrete.htm)

So is what Tony said still true ...

"The lack of aggregate would allow it to be returned to a powder form when acted upon by explosive or high impulsive forces."

Im not sure exactly which of Tonys ideas is accurate.

Also the article says the need for Rebar is "minimized" not eliminated.

I believe the foundations of Tonys argument are weak and its conclusions unsupportable.

Let's not stray too much from Tony's point that the aggregate helps to strengthen 'structural' concrete, which is to say, not the floor systems used in steel structures where the lightweight concrete is more frosting than anything else, and the steel trusses are distributing most of the loads to the columns (with the help of the steel pan/lightweight concrete diaphram). Without the aggregate, the concrete used in flooring systems would be more prone to pulverization IMO. Certainly, not much concrete remained in chunk form at ground zero- this was, of course, quite unexpected.

BMAC, do you know of a NYC building code that states that concrete used for floors must contain rebar? Just curious.

In the Port Authority film Building the World Trade Center you can see some guys tying off rebar, but it doesn't look like this is on top of a pan assembly. (10:24 mark)

At the 11:52 mark you can see guys pouring and smoothing off concrete, but you can't see if there is any rebar underneath.

Incidentally;

Throughout this film you can see lots of welding going on, and there is also visual proof of lateral supports on the floor pan assemblies.

Im uncomfortable with what Tony said. However I will consider further what he is presenting.

It is crucial to grasp that Rebar is an important feature of structural concrete because it enhances its strength.

One result of the use of Rebar is that less concrete has to be used ---- therefore reducing the weight of the structure.

Rebar is an essential feature of modern concrete structures.

There would have been no need for rebar in the concrete of the floors in the twin towers, as there was no structural requirement on them other than to take the individual floor area loading, which is compressive and quite low. If a floor is rated at 125 Lbs./sq. ft. then the compressive load on the concrete is less than 1 psi. The steel floor pans and trusses took any tensile stress which could have been imposed, due to any overall differential loading of the floor.

Rebar is only used when the concrete needs to take tensile stresses. The twin tower floor concrete, with steel pans and floor trusses under them, had no need to take tensile stresses as it would have been unnecessarily redundant and adding rebar would have worked against the desire to reduce weight.

Concrete floors in buildings like parking garages are rebar reinforced as they need to take tensile stresses. But those floors are made of reinforced concrete only. The same thing with reinforced concrete bridges. If steel floor pans and trusses are used to support the floors in a building rebar won't be used in the concrete.

In the case of the towers.

I hope you do.

Whatever doesnt destroy your argument will only make it stronger so back up what you say.

I dont buy it.

Floor construction typically consisted of 4 inches of lightweight concrete on 1-1/2-inch, 22-gauge non-composite steel deck. In the core area, slab thickness was 5 inches. Outside the central core, the floor deck was supported by a series of composite floor trusses that spanned between the central core and exterior wall. Composite behavior with the floor slab was achieved by extending the truss diagonals above the top chord so that they would act much like shear studs

More Detail

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Best wishes

Nothing presented supports the basic assumptions Tony made even leaving out his retraction.

His conclusion about the powderization being easier than if aggregate was in the mix needs attention.

How about the spec. on the conrete slab thickness. It looks like the slab is of varying thickness.

It is highly likely that some Rebar is present in the slab.

Was more surface concrete added once the slab was in place?

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"What does not destroy your argument makes it stronger".....BMAC

My initial reason for posting was to show someone why it was plausible for a controlled demolition to have caused the pulverization we witnessed.

I am thinking this through as we post on here so don't think I am being hardnosed about it.

I believe the lightweight concrete had something to do with the concrete being pulverized more easily but that doesn't have to be true to show there was enough energy in the continuous controlled demolitions which took down the towers. So the lightweight concrete is a side issue.

We have learned some things here though and that is there was only rebar in the concrete at the mechanical floors and the parking garages. That makes sense and validates for the most part what I was saying about the lack of need for rebar. The lack of rebar would allow the concrete to come apart easier.

Concrete is composed of portland cement, sand, and coarse aggregate. Aggregate is used for taking up space in the concrete and the portland cement is the binder or glue. It is the type of aggregate which is part of the lightweight concrete. However, there is another trick it seems and that is the use of a foaming agent which would reduce the density of the concrete..

The energy can be shown to be there if one understands the continuous demolition from the top down. The lack of rebar would also contribute to it. And finally if the density of the concrete was lower there would be less surface area binding everything together in the concrete, which would have been fine for the office floors in the towers, which over 90% of the floors were.

Special thanks to BMAC for questioning. I agree with your point that an assertion needs to stand up to scrutiny, as long as it is honest, which I believe your questions were. I think we did finally resolve things to some extent. Do you agree?

much information. I offer my complete support for your inquiries. I may offer a closing comment later.

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What does not destroy your argument makes it stronger......BMAC

look as though the were shaped not to use rebar...

The "teeth key-in" corrugated shape probably removed the need for it.

The rebar was probably used for the car parking and lower basement floors and sky lobbies.

See slurry wall behing "guys" assembling rebar, indicating low level (sub-basement position).

Best wishes

It appears there was some aggregate with the mix, rebar / reinforced concrete not visible in floors (i.e. treading in concrete, not poured in shuttering mould, shovelling freely etc, but some pictures of it in documentary (below) was used extensively in slurry wall...

Below are some pictures from the "Building the World Trade Center"

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View full 18 minute documentary on YouTube...

Link : http://www.youtube.com/watch?v=__gUjUv1vvw&feature=PlayList&p=E649807EBE8C67C3&index=0&playnext=1

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Hope this helps

I will stand corrected. In one of the pictures it looks like there is rebar at the bottom. The designers may have wanted rebar at the bottom of the floors since there could be a concern about large differential loadings over time.

However, this doesn't change my basic point that the concrete was lightweight which means low on aggregate as compared to standard concrete. There is no other way to get there.

It is the lower aggregate use which would have made the concrete more suspectible to pulverization.