The fluidity of grouts varies with the type of grout one uses. Most grouting of post-tensioning tendons is done with neat cement (that is, a mixture of cement and water) to which an admixture has in some cases been added.
MOST GROUTS BLEED
Since normally grouts are simply a suspension of portland cement in water sedimentation occurs, which is commonly called bleeding. Some grouts bleed more than others, again depending on the cement, the temperature, the mixing and the admixture.
Today, the grouted post-tensioning tendon most commonly used is made up of bundled 7-wire strands. High strength bars and parallel bundled wires are also used. The total amount of bleeding depends on both the height of the grout and the type of tendons. Grout around bar-type tendons bleeds the least. Bleeding is greatest when 7-wire strands are used.
To minimize and control bleeding, it helps to use a gelling agent which keeps the cement particles in suspension. Methyl or ethyl cellulose has been found to be useful. When gelling agents are used by themselves or with other admixtures, they impart a thixotropic consistency. Thixotropy is the characteristic that makes a somewhat stiff material become liquid when it is agitated--and then after agitation become a high viscosity liquid or even a solid again.
TEST OF BLEEDING
In order to determine the relative bleeding or water separation characteristic of a grout, a pressurized bleed test has been developed. This test makes use of a 200-milliliter-capacity Gelman pressure filter with a 47-milliliter-diameter disposable glass fiber filter that retains 99.7 percent of all particles greater than 0.3 micron. This test was found to serve as a reasonable model of the filtering action of a 7-wire strand.