What's your action if you cast concrete at site and concrete truck reached at site with slump less than allowable and patch plant away from your site?

you need to apply a repreprotion for the mix. start by adding plastisizer equiped with the truck. in fact it should be carfully controlled by the site quality control.

Workability can be achieved with either, using plasticizer or adding water ( which is not preferred)

if the truck driver doesn’t have a pail of plasticizer (4 ltrs)  with him, well considering your circumstances, you don't have any chose but :

1- carefully add water ( know that adding water reduces the compressive strength and could leads to bleeding and segregation)

2- if the difference between of the needed slump and the actual slump is low ( like you asked for120 mm and you got80-90 mm) and you don't use pump , place the concrete and use vibrator), and that depends on what element you are casting.

3- reject the truck, and ask for another one - if that will not affect your workflow

(notice that, if you're using pump, with low slump you could break the pump down)

basically plasticizer is admixture  so already mixed in concrete. and slum is reflecting the strength so less slum will result low workability and for more workability no more water/admixture should be used. but if the nature of work is less important and pump is not needed then QC can allow but it is not technical. i will reject the mix straightaway.

I think the best solution is to use plasticizer to adjust concrete slump and little effect on concrete strength.