When that happens, the concrete starts to dry. Evaporationīut sooner or later the bleeding will slow down and eventually stop, and it is only a matter of time until water evaporates from the surface faster than it is replaced by bleeding. The bad news is you should hold off on finishing, but the good news is you don't have to worry about the surface drying-at least not yet. A shiny concrete surface, one that has a “bleed-water sheen,” is good news and bad news.

On the positive side, bleed water rising to the concrete surface replaces water lost to evaporation, and as long as the rate of bleeding equals or exceeds the rate of evaporation, the concrete surface stays moist. If the bleed water gets finished back in, the hardened concrete surface may be weak, porous, and vulnerable to abrasion and salt scaling. Note shutoff values on both supply lines.Įven with moderate bleeding, finishing steps beyond bull-floating are usually delayed until after the bleed water has evaporated so as to avoid re-incorporating that bleed water into a thin surface layer. One way to create fog spray is by mixing water and compressed air using a fogging wand such as this one, built by John Weissert. But it can be a problem when the bleed water rises so fast that it brings cementitious materials and fines to the surface, causing the build-up of a weak surface layer called “laitance.” Such mixes should be modified with a more well-graded blend of aggregate sizes or with adjustments to slump, water content, cementitious materials, or air content.

Vibrating encourages settlement of the aggregates, and therefore brings about more bleeding, or earlier bleeding, or both.Īs we will discuss shortly, bleeding is generally a good thing. The more the consistency of the concrete allows the aggregates to settle, the more bleeding. So as gravity pulls the aggregates downward, bleed water comes to the surface. Known as bleed water, it floats to the surface because, of all the components in concrete, water has the lowest density. The first stage of drying occurs shortly after concrete is placed, when water begins rising to the surface. To fully appreciate what a powerful technique fogging is and how important it can be, we have to understand three things: how concrete dries, what the effects of that drying can be, and how drying can be controlled. It can also make the difference between success and failure in controlling evaporation between placing and curing the concrete. Fogging, or fog spraying, is effective in reducing the chances of plastic shrinkage cracking in mixes with low rates of bleeding.

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