beam crack

Hi,
I noticed visible hair line crack (single or 2 max) protruding from the bottom of the beam at mid-span. My guess is flexure crack just before the reinforcement yields. The problem is the building is still under construction & hasn't reached service load yet.
Having checked my calculation & plans, I am quite confortable with my design & detailing. I checked the concrete test results & reinforcement certificates; they are ok as well. I suspect it was due to premature removal of props & forms since we served the contractor written warning in 2 occasions on this matter. Is there anything I can do to remedy or to check the integrity of the beam?
Should I be worry in the 1st place 'cause I am.

thanx

 

[jeg]

A flexure crack will show across the bottom of a beam and up the side towards the neutral axis - widest at the bottom and closing as it approaches the neutral axis.
Apply a strain gauge or any other tell-tale (a thread) across the cracks and apply a test load. Even under a small supplemental load the crack will open further. If it stays open the steel is in the plastic zone and is overloaded, if it closes the steel is still in the elastic zone and not yet overloaded.

Another possibility - A shrinkage crack will form across the bottom of a beam at the position of the links.

Thereafter, you must look to the contract to discover how best to proceed with the contractor.

 

[Ginger]

The early removal of props theory does sound plausible. Unfortunately, you don't know how much the rebar has strained under the load. You should put a test load onto the beam and monitor the deflrction and crack width. From this you can determine what capacity you have left in the rebar. Andy Machon

 

[Rowland33]

If, as I read this, you have one or two hair line cracks at center span and you have rechecked your calculations, I would not be unduly worried. Keep the beams(s) under observation, if the cracks develope or multiply, then perhaps a load test will be necessary.

 

[anandaganesh]

Since it is a premature crack, you can inject epoxy resin in the crack to seal it so that concrete can take the designed load.

 

[JAE]

I fully agree with Rowland33 on this one. If you've re-checked your calcs and are confident that your flexural reinforcing (both bottom and top-at-ends) are correct, and that you have not exceeded the maximum reinforcing for the section, you will, and should get a ductile failure should any overload occur. Keep observing the crack over the course of the construction.

I would only use the epoxy injection (per anandaganesh) if the beam is exposed to moisture or other corrosive environments. At midspan, epoxy injection of a flexural crack does nothing for flexural strength. Concrete healed with epoxy would NOT "take the designed load" as it is in tension and assumed to crack at design loads anyway.

 

hi all,
thanks for the input.
Honestly, I prefer if the cracks are due to shrinkage for obvious reason.
In fact, I checked the concrete mix design of the plant & I noticed the sand was pretty fine. I'll do a back calculation on the beam design based on the early strength of the concrete. Any other aspect of the concrete design I should pay attention to?
The best way perhaps is to carry out a load test rather than speculating.
Anyway, assuming the worst case senerio, what are the ways to repair the beam if the beam has flexure failure?

p/s: this is a turnkey project & the contractor is not an easy group of people to get along with.
Again thanks a whole bunch

 

[dlew]

stupidcupid,

If the props were removed before the concrete attained enough strength to resist the reduced moment due to the own weight of the beam, there should be signs of crushed concrete at the top of the beam, and an excessive deflection of the beam should noticeable.

I am inclined to believe that the crack is due to shrinkage.
Is the beam continuous over several supports, or is it restrained rigidly at both ends? Any of these would increase the chances that the crack is a shrinkage crack.

Regards

AEF

 

[rapt]

Dlew,

The normal situation with early stripping is that the member does not have sufficient tensile strength so it cracks. The compressive strength is normally still perfectly adequate, especially as there is probably a nice wide flange in the compression zone at midspan so the compressive stresses are very low. Even without that, the compressive strength is much higher than the tensile strength so concrete crushing should not be expected unless the stripping was very early, like less than 2-3 days or the concrete strength gain was very retarded.

StupidCupid,

If the cracks are due to shrinkage I would expect them to be full depth and show in the slab surface as well. You could also expect cracks at the supports in this case as well.

Have you checked the tensile stress in the concrete at this section due to self weight + construction loads. If it is greater than about 2-3MPa (290-435psi) (this value could be more or less depending on how heavily reinforced the beam is and the shrinkage properties of the concrete and the concrete strength at time of stripping) then the beam could be cracked in flexure.

A hairline crack does not indicate that yield of the rebar is imminent. This could occur with a stress in the rebar of significantly less than half the yield strength depending on the concrete strength at time of loading, rebar size an rebar spacing.

Monitor the crack over time and as loading is applied and see what happens.

 

[dharni]

Stupidcupid,

The first things you should do before any further calculation or test is you measure the deflection of that beam.
Depends upon the outcome you can conclude what could be the reason. If the deflection is not there or very very small Shrincage crack is the reason and if there is a deflection then you need to chec with either calculation or load test.

Good luck

 

[abhystr]

hi

there is a possibility that you might have some pvc conduits running vertically in the beam. on couple my projects in bombay i have seen such cracks occuring on one particular floor and not on any other floor for the beam in the same location on the other floors. most of the time due to the presence of the conduits the concrete is properly compacted.

abhay