Shrinkage in Concrete 1

[dawn836]

Hi all
i ask a question ,i am afarid that it is a simple question and from basics of Concrete Design,(but anyway)
what is meant by shrinkage of concrete?
and when it happend?

 

[SperlingPE]

Concrete shrinks as it cures. Majority of shrinkage takes place very early in the cure cycle. Many things (mix design and amount of water among others) affect the amount of shrinkage.

 

[dbuzz]

Concrete consists of aggregate particles dispersed in a cement paste. Paste shrinkage is an order-of-magnitude greater than aggregate shrinkage, so the system finds equilibrium with the aggregate in compression offering restraint to the paste shrinkage. The paste is in tension and will crack in regions of stress concentration.

 

[casseopeia]

try the Portland Cemen Association's website for additional technical information about concrete

http://www.cement.org/

 

[dawn836]

Thanks all for your help, so shrinkage of concrete results tension cracks in concrete beams for example, and if the beam depth incrase the surrface(depth) subjected to shrinkage will have more lenght which will lead us to use some side bars (distrubted over beam depth) to resist this tension force
am i right in what i said?

 

[JedClampett]

Yes, this is correct. The reinforcement does tend to hold the cracks together. ACI 318-02, section 10.6.7 gives requirements for this reinforcement.

 

[dawn836]

well that is the commentry of ACI 318-02,section(R10.6.7)

—
For relatively deep flexural members,some reinforcement should be placed near the vertical faces of the tension zone to control cracking in the web.
Without such auxiliary steel,the width of the cracks in the
web may exceed the crack widths at the level of the flexural tension reinforcement.
Where the provisions for deep beams, walls,or precast panels require more steel,those provisions(along with their spacing requirements) will govern.

they have not mentionated shrinkage or temp. effect in this article.(but i think they mean this reinforcement is for shrinkage and Tempreature)

Does this article also apply to MAT Foundations and Other Footings?!
Does Shrinkage have big effect in Mat and Footings as in beams and slabs?

 

[JedClampett]

Temperature and shrinkage cracking happens to all concrete members. For other members see ACI 318, chapter 7 (I think, I'm home). For minimum reinforcing conservatively use .005 of the concrete gross area for reinforcing.

 

[rapt]

I will throw a spanner into the works here.

Linear Shrinkage of concrete does not induce any stresses or cracking in concrete.

Restraint to the shrinkage of concrete induces stresses in the concrete. This restraint can be caused by connection to other elements which will not allow the concrete member to shorten due to shrinkage or is may be induced by the presence of reinforcement in the concrete which in itself does not shrink when the concrete does thus causing tensile stresses in the concrete.

The reinforcement in a normal concrete slab can easily induce 1MPa (145psi) or more tensile stress depending on the percentage of reinforcement provided and its placement. Designers should take this into account for their cracking and deflection calculations.

 

[dawn836]

well rapt thank you so much for your reply
but i do not understand a point.

How can Reinforcement make cracks in the concrete?!

Does shrinkage occur on the sides of thick Footings and thick Mats?

 

[rapt]

dawn836

When the concrete shrinks the reinforcement that is bonded to it goes into compression because it does not shrink itself. The two materials are bonded together so if the concrete shortens due to shrinkage then the reinforcement must shorten by the same amount. To shorten the reinfrocement must experience a compressive stress. This compression induces an equal and opposite tension stress in the concrete. If the reinforcement is not arranged symmetrically in the section, it will also induce bending stress due to the eccentricity of the compression stresses.

While this tensile stress itself would not normally be sufficient to crack the concrete unless the reinforcement ratio is very large, when combined with other tensile stresses from loading or other effects it could be sufficient to cause a supposedly uncracked concrete section to crack.

 

[DaveFlax]

If you want concrete that does not shrink and crack, use Type K, shrinkage-compensating cement. It tries to expand a lot in the first 7 days, but most of the potential expansion is restrained by the reinforcement. This puts the reinforcement into tension and the concrete into compresssion which is ideal, because that is where each of these materials works best. Note that when Portland cement concrete shrinks just the opposite occurs. The reinforcement is in compression and the concrete is in tension. That is what causes the cracks.