Bleeding and segregation
Bleeding is a form of segregation where some of the water in the concrete tends to rise to the surface of the freshly placed material. This arises due to the inability of the solid components of the concrete to hold all of the mixing water when they settle downwards (water being the lightest of all the mix constituents). Bleeding of the water continues until the cement paste has stiffened enough to end the sedimentation process.
Cement types can influence bleeding capacity, increased proportions of, for example, cement combinations containing ground granulated blastfurnace slag (ggbs), leading to an increase in the time for bleeding continue due to a longer setting time. The presence of an adequate proportion of very fine aggregate (smaller than 150µm) reduces bleeding. Similarly polypropylene micro-fibres are known to reduce bleeding.
If the bleed water is remixed during the finishing of the top surface, a weak top surface will result. To avoid this, the finishing operations can be delayed until the bleed water has evaporated. Conversely, if evaporation of the surface water is faster than the rate of bleed, plastic shrinkage cracking may occur. BS 8500-1: 2015 Table A.9 c) states that Cements or combinations containing more than a mass fraction of 55% ggbs might not be suitable for the wearing surfaces of pavement concrete due to the possibility of surface scaling in the top few millimetres.
In more severe cases, segregation of aggregates can also occur, with the heavier coarse particles moving towards the bottom of the concrete, leaving a cement sand paste layer on the top surface.
Excessive working of a concrete prone to bleeding can prolong bleeding and encourage further aggregate settlement.
Finishing an unformed surface can be adversly effected by bleeding