” Hey...it’s bleeding!”
” Hurry up, call a doctor…who is bleeding?”
The c…concrete is bleeding? Oh yes, it’s bleeding “water””. This phenomenon occurs when the aggregates tend to "sit on the bottom" (watch out! Segregation is knocking on the door!) and free water very easily finds its way to the surface (due to its lower density in comparison with the other elements of the mix) ... .all normal then? Well, let's say that it is quite common to see this phenomenon on construction sites but the fact that it is common does not make it at normal at all, and absolutely not harmless.
First of all, this phenomenon is in fact what it seems, that is, an excess of water ... so pay attention to the true w/c ratio value with consequent lower values of compressive strength. Furthermore, too much water, as known, leads to segregation and premature cracking phenomena. The rising water (bleeding water) also creates micro-channels that exponentially increase the permeability of the concrete and simultaneously reduce its durability with consequent premature decay of the conglomerate. Since this is free water (water that is not used to hydrate the cement), this will form a film around the surface of the aggregates and also to the rebars. This phenomenon therefore reduces the bond between the cement paste and the aggregates/rebars.
Another serious problem is related to concrete for industrial flooring. Not only bleeding (as mentioned, clear demonstration of excess of water) will inevitably lead to a lengthening of the setting and hardening times, but real "puddles" will easily be created, clearly visible on the surface, these puddles indicate a slight difference in level in the flatness of the floor, so the hardener based on quartz and similar (where used) will not have a homogeneous thickness, moreover, coming into contact with water and not with fresh concrete, it will practically have no use in that area, as it will certainly tend to detach or delaminate from the concrete base.
And what about concrete pumped when bleeding occurs? It will be much easier to have problems of blocking or "jamming" in the pipeline due to concrete with a strong tendency to segregation, moreover "in situ" the concrete will necessarily be not very homogeneous (this both when a pump is used or not).
So must bleeding necessarily be avoided, and is it really possible? Well, yes it is possible to avoid it but… it is not always advisable. A small percentage of bleeding can help concrete not to shrink too quickly (plastic shrinkage, see related post) especially when the surface is exposed to sun and wind. So when we talk about horizontal structures a small percentage of bleeding can be a good ally.
But how much is "a small percentage", how muchi s not too much? It could be said that the percentage of evaporated water should be equal to the percentage of bleeding water ... ok ... a little empirical I know, the truth is that it takes a good deal of experience in the field and a lot of practice to recognize "healthy" bleeding and unless you have a trusted technician with such experience to guide you through the whole process, you will be forced to trust your foreman, until proven otherwise of course.
Is there anything else that can be done besides reducing the a/c ratio to contain the bleeding phenomenon? Normally, in addition to a good mix-design, the use of fillers and/or a certain percentage of added air (by means of aerating additives) effectively contribute in this direction. On site, always check that there is no water on the casting surface because this will inevitably rise to the surface, rising through the concrete and generating bleeding not from excess water from the mix but from water present on the casting surface. It should never be forgotten to mix the concrete properly before each casting and ... yes, I know, it's trivial, but I really have to write it: NEVER ADD WATER INTO CONCRETE!!!!