In the last picture you posted, from the Australian publication, there are no bars at the top to hold the concrete, so the entire beam depth settles under its own weight Without sufficient vibration, the cracks as shown can form due to the abrupt change in concrete thickness.
But in your case, there is a rigid group of bars to support the plastic concrete, and these bars also inhibit effective penetration of the vibrator, so the concrete below the bars settles under its own weight, causing the horizontal cracking below the beam bars, or probably in line with the bottom layer.
I have seen both types of cracks, both due to inadequate compaction.
In other words, the T-beam flange and beam web has separated already (full crack) as far as concrete is concern? And what's only holding them together is the stirrups? In your experience, this would cause reduction in ultimate capacity or nominal strengths of the members? Which of them would be reduced in the cases where such separation indeed occur? Truss action?
Can the cracks from plastic settlement form or become manifested later or does it appear immediately? Please see the picture of the beam below one year ago compared with at present. There is no cracks one year ago 3 weeks after removal of formworks. Could plastic settlement cracks be just inside and the normal movement of the beam made cracks manifest outside much later?
Im sure they are exactly the same beam (the latter one with crack is one already painted white that's why the stains in the original picture gone). So I wonder if it's settlement cracks.. it could just be inside virtual the whole year then it slowly cracks outside because of concrete tension
Let's say the neutral axis is one half of the beam depth at service load. So the cracked flange and web is in the middle. One can think of a column with vertical crack in its half. With ties that hold it together, won't there be transverse pressure as axial strain increase (or compression strain in the case of a beam)? This means the ties or stirrups would be more stressed than usual if there is concrete crack as they have to resist the tranverse strain, or do you believe that even in uncrack concrete, the ties are working to resist transverse strain (or the beam stirrups resisting transverse compression strain in the compression zone of the beam)?
We invited one of the country's top structural engineer to examine the horizontal cracks. He said it could be bleeding or cold joint or formworks related as there is no deflection in the beam. I told him whether I should chip the crack deeper to see extend. He said what's the point because cold joint can cause it. I told him about epoxy injection. He doubts it can even reach the inner part (if the inner part is indeed bridged).
Here's the weird part. In over hundreds of buildings he inspected. He has never seen horizontal crack in beam one inch below slab. Most of you haven't seen it too except Hokie66. So how come only I and my crew and Hokie has seen such cracks. Is it so unique in the annals of structural engineering history that we should let the photograph appear in structural books?
Curious, what country do you practice? The very short span slabs with very thin slabs with 2-way beams reminds me of framing concepts prevalent in the Philippines back before 2000.
I have repaired a lot of cracks on concrete structures, including several horizontal beams cracks caused by cold joints, plastic settlement, restraint to shrinkage, incorrect PT drapes, etc etc. For a RC beams that is only 12" wide, low viscosity resin via pressure injection with an experienced operator will indeed fill the crack width and length, if deemed structurally necessary.
One project that had significant horizontal beam cracking caused by a cold joint, "leaked" cement grout during the PT grouting operations!
I practice in the Philippines. Why, have you visited it prior to 2000? Can you repair the horizontal cracks in the project?
Have you seen horizontal cracks near slabs or is it in middle of beam? The one who inspected it said he only see horizontal cracks in middle of beam, not near slab.
What Hokie66 mentioned about plastic settlement forming beneath bars made me realize one thing. Why you mustn't place beam and column concrete at same time, because after the column settles, a gap would form between it and the beam bars concrete. And the building can suffer moment magnification. In the Philippines, most contractors do column and beam pour together even in 50-storey buildings. Isn't it scary. In my neighborhood, an 8 storey buildings just falls to the ground (it's good the tipping is noticed at morning and all evacuation), then falls in the afternoon hitting another building. See video at:
I think the bad effects of gaps in columns from beam-column placed together can only occur during strong earthquakes (as we don't have falling buildings often).
Does anyone know how to compute for settlement? Supposed the concrete has height of 400 mm and width of 300 mm.. what is the settlement distance in mm for a concrete of 4000 psi compressive strength, ready mix with admixture and fly ash. Does this settle more than normal cement mix? No one in my country knows how to compute for settlements so they don't separate column and beam placements but do them at same time.
Foundation failure is the most likely cause of the collapse in your video.
As to plastic settlement, adequate compaction of the concrete and adherence to proper construction practices will eliminate these concerns. Prediction of crack thickness, when you can prevent the cracks from occurring, is poor thinking and not what engineers do.
The practice of pouring a floor or roof together with its supporting columns is not done in my area and should never be permitted because of plastic settlement.
The Canadian concrete code, A23 uses a factor k[sub]1[/sub] of 1.3 when calculating development length of horizontal reinforcement with more than 300 mm of fresh concrete below the bar. This recognizes the fact that bond in such bars is reduced because concrete settles, leaving a small gap on the underside of each bar.
I agree with BA on this one, Always have a construction joint between the columns and floor plates,
"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976
The invited structural engineer suspects the formworks may also be a factor. In your country. How exactly do you do your formwork? The following is what is done in my country, leaks always occur at the interfaces. The second url below is the exact picture of the beams with formworks and leaking cement.
"The following is actual picture of the leaking formwork. Standard in my country. How do you handle the leaks?"
Steel bearers and bamboo joists for formwork. Been a while since I have seen its use. I was in Manila in 1999 and it was prevalent, and of course Hong Kong, but more so for scaffolding rather than formwork.
Those form "leaks" look about similar to what I have experienced in AU, US and CA.
I do note that is some parts of world, formwork chamfer strips for "external" edges are not as common as other areas. Correctly installed chamfer strips can assist in making the forms leak less.
For concrete members of depth of more than 300mm it is usual practice to re-vibrate concrete placement, to minimize cold joints, plastic settlement, etc. The concrete vibration task is often passed to the least qualified/experienced site laborer, and his/her duties are paramount to the success of otherwise of the hardened concretes properties.
