Why consolidatate fresh concrete?

[dirtsqueezer]

I'm bracing myself for the impending flood of responses to this question.... The importance of consolidation using a vibrator just doesn't seem intuitive to me.
I am aware that consolidation of fresh concrete is as common at a concrete pour as the presence of forms or reinforcing steel, and that the contractor should be doing so if the specs tell them to. What's the difference in fresh concrete that has, for example, a sand grain at a certain location from that where the same grain has sunk a cm or so lower? And if the concrete is poured at, say, a 5 inch slump per approved specs, it seems to me it should be wet enought to saturate the re-bar to securely form the bond.
Is there any documentation that could help me strengthen my gumption on the subject?

 

[Rjeffery]

PCA (

http://www.portcement.org)

has many excellent references that will assist you in your search for information on the subject.

In general, Concrete can and will trap air that causes voids. These voids allow stress to develop in a concentrated area causing local weakness. Bad in beams worse in columns.

Also the vibration of the concrete adds 'energy' to the mix that 'helps' it flow over and around reinforcing steel and into the corners of form work.

 

[bluesman]

Placing and compaction are interdependent and carried out simultaneously as they are important to ensure the required strength, permeability and durability. The purpose of compaction isn't just to force the concret to flow and the particles into a closer configuration but also the elimination of the entrapped air to produce better quality concrete. With higher strength (less voids in the paste with lower stress concentrations), lower permeability (less air voids connecting the capillary voids system in the paste) and higher durability (comes along with the lowered permeability).

And yes, a very high slump workable concrete will eliminate the need for compaction which is a different type of concrete called self-compacting concrete which is very different in use and composition of the conventional concrete pored at sites (lower aggregate max. size, higher content of fine materials, higher cement content and especial viscosity agents) this type mof concrete will self compact and level when pored.

Try these:

http://www.ce.memphis.edu/1101/notes/concrete/everything about concrete/9_-_handling.html

http://155.217.58.58/cgi-bin/atdl.dll/fm/5-428/ch5.htm#s7p4

http://www.portcement.org/cb/concretebasics_placing.asp

More details??????????
A. Sadi

http://www.geocities.com/concretesite

 

[dirtsqueezer]

Well you guy have been good sports about my inquiry. Excellent resources. Thank you.