Retarder in bridge deck concrete

[tmalik3156]

Good day.
We know retarders are used in concrete to delay the setting time.
The diagram above shows the deck pouring sequence of a slab-on-girder bridge. In one day, segments 1A, 1B and 1C will be poured. When the concrete gains at least 25 MPa strength, segments 2A and 2B will be poured.
Designer has asked that only for segments 1A and 1B, the concrete needs to have retarder up to the duration of the completion of 1B.
No retarder is needed for other segments.

Can anyone please explain the reason behind it? At first, I thought that the reason for using retarder is to avoid cold joints. But using it only for two positive moments regions indicate there might be other reason (deflection, unbalanced moment) behind it. So, I wonder if any one can explain.
Thank you

 

[BridgeSmith]

Typically, the reason for pouring the pier sections last is so that they don't get 'stretched' by the deflections resulting from the added weight of the concrete in the other regions. There would be some upward movement of the end spans when the weight of slab in the center span is added. I suppose it's possible it could be significant enough to cause cracking in the partially-set concrete in sections 1A and 1B.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bridgebuster]

Ditto what BridgeSmith said about stretching.

Typically, we like to pour the deck as quickly as possible. However, it doesn't always work out. Sometimes the placement a placement is large, or it gets slowed down, or it's delayed, which means some concrete may take the initial set before everything is placed. This leads to deflection cracks; to avoid/minimize the problem retarder is used. This way everything is (or is almost) placed before the initial set.

One question: Is it intended for 1A & 1B to be placed simultaneously? If not, look into uplift restraints at the opposite abutment from where the placement begins.

 

[BridgeSmith]

Our typical placement sequence would be 1C, then 1A & 1B, and 2A & 2B as a third placement.

The effect on 1A from the placement of 1B, would typically be negligible.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[tmalik3156]

Thank you gentlemen for your replies.
@bridgebuster: No, 1A and 1B are not simultaneous. There are no uplift restraints used to hold down girders.
@BridgeSmith: If 1C is poured first, as you suggested, then uplift at abutments may be a possibility. We don't pour 1C first because of limited reach of available concrete pumps.

 

[BridgeSmith]

We would provide a way for the pumps to access the 1C section, and provide hold down anchors for the girders at the abutments if there was a net uplift there. By the time 1B and 1A have cured sufficiently to drive the pumper truck onto them, we would assume they're too cured to 'stretch' them by adding load to the center span. We would allow them to place 1A, 1B, and 1C at the same time, depending on the total volume.

Rod Smith, P.E., The artist formerly known as HotRod10