GFRP rebars float up ?

[tmalik3156]

Good day.
We are using Glass Fiber Reinforced Polymer (GFRP) reinforcing bars in a slab-on-girder bridge deck. Concrete will be cast in a few days. Someone told me that since GFRP is very lightweight, during concrete placement and vibration, the reinforcing bars will float up on the fluid concrete, and will mess up the required top concrete cover. He suggested that we tie down GFRP bars to the shear studs of the steel girder.
Does anyone have any experience about this? Does any DOT mention this issue?
We are doing a bridge, but the same might happen in building slabs with GFRP, too.
Any knowledge sharing will be appreciated.
Thank you

 

[SlideRuleEra]

Someone told me that since GFRP is very lightweight, during concrete placement and vibration, the reinforcing bars will float up on the fluid concrete...

Yes, GFRP can "float" in concrete during vibration, but it is more of a "problem" in theory than in practice:

Excerpt from page 4, "Handling & Placement" of "Fiberglass Rebar"

"Floating" not too much problem because:

1) Unit weight of GFRP is about 125 lb/ft[sup]3[/sup] versus 145+ lb/ft[sup]3[/sup] for concrete... not too much difference. For example, takes less than 3 lb. force to keep a #8 rebar 20 ft. long from "floating".

2) During most of the vibration for a slab, some of the rebar cage is above the concrete level... providing extra weight to keep the part of the cage in the vibrated concrete from "floating".

3) Typically, concrete in only part of a rebar cage is being vibrated at a given time. Other parts of the cage in the unvibrated concrete are not subject to flotation.

4) Flotation is usually not an issue in unvibrated, fluid concrete... the fluid is too viscus and the uplift force is too low to allow aggregate to "move" out of the way.

Do tie down the GFRP rebar cage, but a reasonable amount of care should be enough... the rebar cage is not going to come "popping" to the top of the concrete like a cork.

 

[hokie66]

Use your heaviest concreter on the vibrator. Wait...they are all heavy.

 

[tmalik3156]

Thank you SlideRuleEra for your detailed reply.
Our contractor has tied the rebar mats to the form hangers as shown in the photo.

 

[SwinnyGG]

In practical terms, the guy standing there vibrating the bar is typically standing on the cage.

The chairs tied to the bottom mat also act to help keep the bar from moving up through the slab as it's poured.