Adding super plasticizer at site 4

[foxview11]

Good day everybody,

I have some concern regarding adding super plasticizer at site just before discharge of ready mix concrete. Our footing designed with lot of rebars so we need a flowable concrete for workability. Our mix design includes plasticizer and retarder admixtures but when arrived at site, the concrete slump is a bit lower, so it needs additional SP. Is there adverse effect if applying overdosage of plasticizer? Please any expert on concrete works if I may have your comment on this..Thank you..

 

[JedClampett]

First of all, Super-P is somewhat of a crutch. It's not going to help much in congested reinforcing. Aggregate size and good vibration help much more. But contractors think it helps, so I let them have it.
Why do you think they will overdose the Super P? It's not free, so the concrete guys don't throw it around willy nilly. I suspect that you'd really have to overdo it to have an effect on the concrete.

 

[Ron]

foxview11....this indicates to me that your ready mix supplier is not controlling the mix as they should. If you are losing slump quickly after a SuperP addition at the plant, you will typically see that either the transit time is too high or the aggregates have a moisture content that is too low and they are "sucking" in the water and reducing the slump.

While plasticizers used to be added at the site because of their low effect time, better formulations have allowed plant addition for over 20 years. I do not recommend the addition of plasticizer at the site because of control issues and the inability to properly disperse the plasticizer in the mix.

 

[foxview11]

Thanks Ron..if necessary to add super P, does it not affect concrete strength? and also, what is the usual dosage to add super P at a certain slump, say 100mm?

 

[Ron]

foxview11.....dose rates are given by the SuperP manufacturer.....it varies by manufacturer and cement content. SuperP does not affect the concrete strength directly, but indirectly it increases the strength because usually less water is required, thus decreasing the water-cement ratio.