Do you accept to add water to concrete mix at site to improve slump, if you dont have admixtures? if you agree what is the limit and reference?

in fact,  adding extra water is a problem which leads to the formation of capillary pores which seriously affect the concrete strength and the concrete durability.

moreover, during  the doing mix design of concrete ,

1.DOE method can calculate water content for both wokabilty requirements and strength and durability needs.

2.we can add extra water for the absorbation of aggreaget.

so that  we have a totall water content in fresh concrete .

on site,

if there is any problem  regaring to slump as your a quesion, we can incearse water content untill reaching the totall water content which mention above.

when we reach the full water conent , it is not accepted to add water any more.

NO! never. The mixture wanted depends on the purpose , quality and desired strength. There are enough additives to make it workable, but think in front not at the site

it is not acceptable , but if the slump hard to much use water in ratio with cement20lt-1kg cement

No, I can't accept. It decrease strength and durability of concrete. Think before by analyzing site conditions and you may add some admixtures at the time of concrete manufacturing .

It is not preferrable to add water after the design mix has been proposed. but in some cases due to difficulty in pumping the concrete it may be unavoidable. 

Note: try adding admix

water upto2%