So leaking formworks as shown in the illustration is normal? What country or place where the practice put putty around the formworks?
Ingenuinity, could you confirm if you have seen horizontal cracks in beams just below slab (one inch below) or is it more near the bottom or center of the beam?
If the project specifications call for Class 1 architectural off-form finishes, then it is possible that all form edges and joints are sealed, but for a standard structural finish then sealing of joints etc is not undertaken.
Yes, we recently had a project where there was a horizontal crack about 50mm below the slab soffit on a PT beam that was 1000mm deep adjacent to a column - very congested top rebar and PT tendons and the concretor failed to adequately compact the plastic concrete. It was a localized crack to one beam of a total of 6. Was repaired via injection.
I do not believe the PT influenced the cracks in the situation Ingenuity has described. You have a large congestion of reinforcement, I would agree with several other posts here that it is lack of compaction and poor placement.
Regards,
"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
Exactly as stated by "aaronPTeng" - PT as such was not the cause. Just congestion and concrete placement technique.
For your stated case, 1) your beam is deep (therefore more probable to be subject to plastic settlement), 2) is narrow (300mm), and 3) the top rebar is congested over the beam width - then 4) add in some inattentive concrete placement and it results in your horizontal crack. $hit happens. Point this out to future field construction supervisor or concretors and make them appreciate the consequences.
Can plastic settlement be avoided by compacting below it or is plastic settlement inevitable in spite of all efforts in the case of the beam I shown with horizontal cracks one inch below slab?
Or to rephrase it. If you will construct the same beam using exactly the arrangement of bars and beam width and depth (I shown). Is plastic settlement inevitable or can you do something that can totally avoid the plastic settlement underneath the 8 bars?
It may be inevitable if you can't get access to compact the concrete. Some of those bars could have been located outside the beam web. Constructability should always be a consideration in the design, and if you can't compact, it is not constructable.
I'm a quality control engineer who checks finished construction.. so maybe I should take more part directly in the construction. Our ready mix concrete pumpcrete hose is 4" in diameter. We don't have smaller one. What smallest size have you use?
It is not the delivery hose size that is important. It is the space for the concrete to get through the reinforcement, and for the vibrator head to penetrate into the concrete.
From face to face of columns, closed stirrups spacing starts at 1 stirrup at 50mm from column, then 10 stirrups at 100mm (4") spacing, 8 stirrups at 150mm
(6)" spacing and middle at 200mm (8") spacing. The beam is 500mm in depth, 300mm in width, there are 3 200mm (8") spacing stirrups at center (see picture) where 6-meter secondary beams on both sides framed into it. My concern is the contractor move one of the midpsan stirrup closer resulting it in forming 200mm, 100mm, 300mm spacings (see picture)..
Since the depth of the beam from tension bars to compression is 460mm.. half of it is 230mm... so one of the beam spacings at center didn't satisfy D/2 or 460/2=230mm because it has one 300mm spacing.
The following shows the shear and moment diagram at that location at 1.2 DL + 1.6 LL.
The concrete section + 300mm spacing stirrup has shear capacity V(c+s) more than the Vu (factored ultimate load).. which fulfills the spacing requirement but it doesn't satisfy D/2. According to the designer team and head, no problem with it since shear at midspan is not much. But I still have worry because it doesn't satisfy D/2. I told him what if diagonal cracks form there. He said he hasn't seen it happened. Now the reason I asked this is because I can have the 2 inches floor topping removed and change it to mere 3/4" topping to remove some Superimposed dead load to get even bigger margin.. the designer team and head said I can decide whether to do it or not. Isn't not satisfying D/2 in one of the stirrup a problem especially it is on midspan?
Designers in my country are not experts in physics. Most of them forget lessons back in the school days. My name is one of the signatories for liability. All codes require d/2 for the following reason (from
The reason for d/2 is to intercept any possible cracks. In the case of the one 300mm spacing of the 460 effect beam depth where d/2=460/2=230mm.. 300 exceeds 230mm. What if steeper cracks from 45 degrees form inside the 300mm spacing like in the following:
How do you think when working with this problem? Do you put close stirrups near midspan for concrete load to intersect steeper shear cracks than 45 degrees?
The steep cracks are flexural cracks, not diagonal tension cracks. The bottom steel intersects those cracks. No stirrups are theoretically required, but code rules do apply.
Not sure if you are using ACI318, but does the required ultimate shear force exceed 1/2 the factored shear capacity?
i.e if Vu < 0.5*phi*Vc than shear reinforcement is not required.
Note the top layer is in the flange part and second layer is just below the flange like in the bar arrangement illustration shown earlier. There are one inch hole in between the bars as required by code so the aggregates should flow in between them.
RFReund, Vu is > Vc, that is why stirrups are required.
hokie66, Is it not that steep cracks are combination of flexural cracks and diagonal tension cracks called flexural-shear cracks. Near midspan, I can imagine the flexural cracks forming vertically then it gets at an angle signifying start of diagonal tension cracks. Have you ever used Respond-2000? It can model how the crack exactly form and even width but I'm preflexed why in the following with the sections and bars data inputted, it only shows up to 119 KN. Who has tried Respond-2000 before? (freeware at
