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Investigators of cracking problems in concrete struc-tures have from time to time noticed horizontal andve rtical cracks in concrete columns. How does thishappen when cracking indicates tension in the con-

crete whereas columns usually are in compression?

Tension a possibility

Ruling out pure compre s s i ve overload, there are thre epossible explanations. First, the columns may in fact bein tension due to an isolated settlement of the founda-tion. In this case, the column could be “hanging” fro mthe floor system above, resulting in tensile cra c k i n g .

Next, if the column is the end or exterior one in as t ru c t u ral bent or fra m e, it may have high enough bend-ing to cause tensile forces in the outside face. This con-dition would produce hori zontal cracks which woulddisappear on the inside face.

Creep behavior

The third and most likely explanation has to do withc reep and shri n k a g e. Both are related phenomena. Un-der constant compre s s i ve stre s s, concrete continues tod e f o rm with time. The rate of this time-dependent de-f o rmation (strain), which is called cre e p, depends onmany factors. He re are some of them:

• The magnitude of the sustained stress; the higher theload, the greater the cre e p.

• The strength of the concre t e. Creep is greatest whenthe strength of the concrete is low.

• The duration of the loading peri o d .

• The age of the concrete at the time of loading; creep isg reater when the concrete is loaded at an early age.

• The chara c t e ristics of the aggre g a t e s.• The quantity of cement paste. Creep is approx i m a t e l y 

p ro p o rtional to the amount of paste in the mix.

The rate of creep is greater for small members and inc o n c retes with high water-cement ra t i o s, high cementc o n t e n t s, large slumps or those cured in a dry atmos-p h e re. Creep is greatest immediately after load applica-tion but decreases rapidly asymptotically as shown inthe graph. For concretes cured in air, about one-quart e rof the total creep potential takes place during the first 2 weeks of loading, one-half during the first 3 months, andt h re e - q u a rters during the first ye a r. Co n c rete loaded af-ter 4 years or more creeps ve ry little and re c overs almostcompletely after being unloaded.

Stress redistribution

 With this as background, what really happens in a re-i n f o rced concrete column is that creep causes the con-c rete to unload its compre s s i ve stress and transfer it tothe re i n f o rcing steel. In other word s, a re d i s t ribution of 

 Why concr ete colum nscan cr ack 

Creep may be the most common cause

BY RAYMOND A. DIPASQUALE

ARCHITECT / STRUCTURAL CONSULTANT

Creep increases rapidly whenload is first applied, thenincreases more slowly astime goes by, and finallyshows lit t le or no change atextended durations ofloading. Data adapted fromHardened Concrete: Physicaland Mechanical Aspects, byAdam Neville, published bythe American ConcreteInstitute, Detroit, 1971;based on concretes stressedat 1000 psi.

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s t ress takes place which is directly pro p o rtional to theamount of re i n f o rcing steel in the column—the higherthe percentage of steel, the more load the steel carri e sso that with about 8 percent re i n f o rcement, the steel car-ries almost 100 percent of the load. With about 4 perc e n tsteel, the load distribution is approximately 90 perc e n ton the bars and only 10 percent in the concre t e.

This explains why cracking can occur in the concre t e“s h e l l .” Due to the unloading, there is little compre s s i ve

s t ress left in the concrete to compensate for or ove rc o m es h rinka ge stress or other factors such as tempera t u re which can subject the columns to tension.

Safety factor remains

It is interesting that in all of this, the factor of safety against ultimate failure of the column remains pra c t i c a l-ly unaltered. Although the effect of creep in pre s t re s s e dc o n c rete for example can be harmful in that it re d u c e s

the amount of pre s t re s s, and in tall buildings differe n-tial creep can cause movement and cracking of part i-tions or other stru c t u ral effects in beams and slabs, theove rall picture is that cre e p, unlike shri n k a g e, is benefi-cial in relieving stress concentrations and has con-t ributed considerably to the success of concrete as as t ru c t u ral materi a l .

Acknowledgement

Reprinted with permission from Building Failures Forum, Vol.1, No. 4, January 1981.

P U BL I C AT I O N # C 8 1 0 7 3 7

Copyright © 1981, The Aberd een Gro u p

All rights re s e r v e d