2 STEP CONCRETE CASTING DETAIL FOR THE SLAB AND BEAM 1

[mehmetserif]

As I know that it is preferred casting a slab in one time(one step).
But if you have to cast it more than one step(i.e. 2 or 3 step), where should be the cold joints on the slab and beams?

 

[Ginger]

Mehmetserif

Just to correct your wording, your joints should be "construction joints" and not "cold joints". You do not want a cold joint in your pour. This occurs when the time elapsed between successive mixer trucks is too large and the concrete begins to set before the next wagon arrives...BAD NEWS.

To answer your question: for beams, many engineers suggest construction joints are located at third points along the span. This is based on the assumption of a UDL on the beam and at these points you get neither maximum moment or maximum shear. However, this is only true for the one loadcase. In theory, you can put your beam construction joint anywhere along the beam if you have good workmanship and you construct your joint correctly.

For slabs, the joints can go anywhere assuming that you have good workmanship. I do recommend that you ensure that on ALL slab pours the slab has one free edge in each direction (ie top edge and one side edge free) to allow the slab to expand and contract freely during the hydration nd cooling period of the concrete. If you do not do this you will most likely generate cracking due to restrained thermal effects in the slab. Andy Machon

 

[mehmetserif]

You are right that i am talking about construction joint but not cold joints.

Thank you for your information but i have to tell about an idea answering my question.

In Turkey, for the beams, many engineers think that 1/4 of the span from the supports are good poits for construction joints.(This is nearly similar to your knowledge) but one of my friend who is engineer too, insist on choosing construction joints in the middle of the beam and in the middle of slab if the case is UDL. (He says this is the way in America)
This is mainly due to the assumption that in the middle where moment is max., one side of the concrete doesnt work. At that side,where concrete doesnt work, only steel works. And on the other side concrete works for compression absolutely. For the all cases nobody can oppose to the fact that workmanship is really important. Becouse where the concrete take compression the joint should be resistant to the edge cracks. So, for those types of cracks may be additional precoutions should be taken. And for the other side, reinforcment might be needed to increse.

According to this idea, your reply would be appreciated.
Thank you for your sharing indeed.

Mehmet serif

 

[Ginger]

Mehmet

Your colleague is correct when he states that placing the construction joint at the middle of the span will not rely on the shear strength of the concrete because the bending moment will be at its maximum (hence zero shear). This is a valid argument.

I do maintain that you really can put the beam joint wherever you like. I would argue that, provided your design is correct and well detailed, the factor which governs the position of the joint is "which is the easiest place to put the joint for the site engineers"? Which location will mean that the falsework and backpropping will be the least difficult? The easier you make things for your site team, the better the quality of the finished concrete, and, as we have already agreed, quality and workmanship are key factors in creating good construction joints.

Andy