slump for low shrinkage concrete 2

[cmrdata]

The concrete contractor is going to pump the low-shrinkage concrete (0.02%) and questioned if the maximum slump limit of 4" indicated in the spec should be the slump at the point of placement, or at the point of delivery.
Does anyone has any comments or suggestions on this matter?
Thanks.

 

[Ron]

Slump for official recording purposes is taken at the point of delivery, as this is what the mix design is based on. If you want to know what the contractor is doing to the mix in the placement process, take a slump at both locations.

 

[cvg]

pumped concrete will typically get significantly stiffer between the time of delivery and discharge at the end of the hose. Your contractor is rightfully concerned that if he gets a stiff mix delivered to the site, it may plug the pumper which causes big problems. The concrete needs to be mixed and designed precisely to come in at maximum allowable slump, and delivery times need to be kept as short as possible. A retarder may also be warranted to slow down the set.

 

[cmrdata]

Thank you both, Ron and cvg, for your thoughts on this one.

Designating the 4" slump as the limit at point of placement might entice the contractor to add too much water at the beginning and still meets the limit after the slump loss during the pumping. Designating the 4" slump as the limit at point of delivery might be over-restrictive as it might cause pump failure at point of delivery.

I am attempting to allow the 4" slump be the limit at the point of placement, if the concrete contains high-range water-reducing admixtures.
Do you think this a viable solution?

Thanks.

 

[Zambo]

I think the slump is low for a pump mix and would suggest 150mm +/- 25mm at the point of discharge from the truck.

I don't think it is realistic to use a spec that says 100mm slump and then to say that the testing point is after pumping. The trial mixes will have to be carried out at a batching plant and won't have been through a pump so I just don't think the idea is workable.

I suggest that the slump value stated in the spec should be flexible enough to accommodate a change due to the mix design and delivery method. Therefore it could be modified to 100mm slump for delivery by crane/skip and 150mm for delivery by pump (which means 2 different mixes for the same strength grade).

 

[Hoaokapohaku]

In my opinion, it is unworkable to specify a slump at the point of placement when a mix is pumped because the pumping operation itself changes the properties of the mix. The supplier does not have control of the pump operation (usually), not all pumps have the same effects, and means of pump operation contributes to how much the concrete properties change. Unless the placement is very slow, and assuming the pump and all pump hoses are properly primed, the stiffening of the mix between delivery and placement is primarily caused by loss of entrained air during the pumping process, usually by about 2% in my experience. If one tests the air content of the mix at the point of delivery, the amount of air entraining admixture can be increased to compensate for air lost. After adjustment for air, the slump at point of placement will be very close to slump at discharge was before the air adjustment. However, the slump at delivery will also be increased and allowances must be made in the specifications to allow for this, or at least in the inspection.

 

[7515325]

Need to design a slump that can be adjusted to a 4" at the pump. Lot of times I see guys check a slump and get a 5" then they let the truck sit until it is a 3" or 4" to meet spec WHAT, its a 5" slump that is only setting up, it is still a 5". Sounds like a lab or mix design thing to me. Good luck

 

[GeoPaveTraffic]

Ah... if I had a dollar for every time I've had this argument, I would be a rich man.

This illustrates the problem with specifing slump. We as engineers really don't care what the slump is, only that the concrete perforems as required.

The design engineer should list the properties of the concrete that are required, such as shrinkage, maxiumum w/c ratio, air content, and strength. The contractor should tell the supplier what slump (at truck discharge) he needs to pump the concrete. The supplier should then supply a mix that meets all of these requirements.

If this system is followed, the slump should be checked at the point of discharge from the truck and the air content should be checked after the mix has been pumped.

Mike Lambert