Half beam pour

[releky]

I always do beam pour all at one time or monolithic but I heard some engineers do it by stages especially those who don't use ready mix. For the case where the beam is poured one half first (left side) and then next day the other half (right side). There is a separation right in the middle. Wouldn't this have the same effect as flexural crack that goes above the neutral axis all the way to the compression zone splitting the beam in half?

 

[BAretired]

Sometimes you do not have a choice. If something goes wrong during a pour, you must decide where to place the pour break. For a beam, the best place is the point of maximum moment and minimum shear. Usually this occurs at or near midspan. It is a good idea to roughen the surface before placing fresh concrete. Some engineers add dowels across the pour break as well.

The answer to your question is yes, it has the same effect as a flexural crack but the effect is not harmful.

BA

 

[a2mfk]

For a simple span beam at least, directly in the center would be the best place for it in terms of shear. If we assume concrete is only in compression then in theory it seem OK.

 

[TDIengineer]

I like a shear key installed as well with shear dowels even though the shear is zero at midspan.

 

[releky]

I was asking this because once flexural cracks form in a fully placed beam and since concrete is fragile in tension, then what would prevent the crack from forming beyond the neutral axis even if it is in compression? Supposed you have a glass, and the neutral axis is at middle, once "flexural" cracks form below the glass, won't the crack propagate beyond the neutral axis even if the glass above is in compression?

And let's say your beam is such that the stress block in compression is in the T-beam flange. When the web flexural cracks form below, what would prevent the compression zone in the flange from being affected in the crack propagation (say hairline cracks). If this occurs, it's like having half beam pour occurring anywhere in the beam. Hasn't this happened anywhere as it's theoretically possible?

 

[JAE]

releky - it is not like glass - don't forget you have reinforcement on the bottom and it is not cracked.

Flexural beam design already assumes that the beam cracks (you neglect concrete in tension anyway). So a formed construction joint a midspan is no problem. Some type of shear key or roughened surface is preferred, but the formed joint is not problem.

 

[releky]

I was asking if cracks can form right in the compression zone from propagating flexural crack pressures from below the neutral axis at service load. I can't imagine the neutral axis able to suddenly stop propagating crack pressures from below especially if the stress block above the neutral axis is just 5 inches.

 

[Enhineyero]

Codes will suggest that the ideal location for a joint is within the middle third of the beam length (which is where the mid span is located). So basically this is allowed. Engineers view construction joints as a "bond" problem which is a primary concern in shear since shear is resisted by both the concrete and steel, and poorly bonded concrete will perform poorly under shear. While I recognize this fact, it is my own preference that I move my CJ near to the one third point of the beam since I prefer not to have a CJ to where a plastic hinge is likely to develop.

 

[BAretired]

I was asking if cracks can form right in the compression zone from propagating flexural crack pressures from below the neutral axis at service load. I can't imagine the neutral axis able to suddenly stop propagating crack pressures from below especially if the stress block above the neutral axis is just 5 inches.

We are discussing a cold joint at or near the midspan of a beam. There are no cracks to propagate. The new concrete in the compression block simply bears against the existing. What is your concern?

BA