Masonry grout is essentially fluid concrete. It is used to fill spaces in masonry, and to surround reinforcement and anchors. It is specified using ASTM C476 (Grout for Masonry).
Grout for masonry is composed of portland cement, sand, and (for coarse grout) pea gravel. It is permitted to contain a small amount of hydrated masons lime, but usually does not. It is permitted to be specified by proportion or by compressive strength. Neither of these is the default.
Proportion Specifications for Grout for Masonry
The proportion requirements of ASTM C476 for grout for masonry are repeated in Table 2.8. Note that hydrated lime is permitted but not required.
Properties of Fresh Grout
The most important property of fresh grout is its ability to flow. Masonry grout should be placed with a slump of 8 to 11 in., so that it will flow freely into the cells of the masonry. Because of its high water-cement ratio at time of grouting, masonry grout undergoes considerable plastic shrinkage as the excess water is absorbed by the surrounding units. To prevent the formation of voids due to this process, the grout is consolidated during placement, and reconsolidated after initial plastic shrinkage. Grouting admixtures containing plasticizers and water-retention agents can also be useful in the grouting process.
Properties of Hardened Grout
The most important property of hardened grout is its compressive strength. If grout is specified by compressive strength, that strength must be at least 2000 psi. If it is specified by proportion, its compressive strength is controlled indirectly to at least that value, by the ingredients used.
Because of its high water-cement ratio at the time of grouting, masonry grout cast into impermeable molds has a very low compressive strength, which is not representative of its strength under field conditions, when the surrounding units absorb water from it. For this reason, ASTM C1019 (Sampling and Testing Grout) requires that the compression specimen be cast using permeable molds. The most common way of preparing such a mold is to arrange masonry units so that they enclose a rectangular solid whose base is 2 in.2 and whose height is equal to the height of the units. The rectangular solid is surrounded by paper towels or filter paper, so that the compressive specimens water-cement ratio is similar to that of grout in the actual wall.
Self-Consolidating Grout
Starting with the 2008 MSJC Code, self-consolidating grout is permitted to be used in masonry. Self-consolidating grout is a highly fluid and stable grout, typically with admixtures, that remains homogeneous when placed and does not require puddling or vibration for consolidation.
This type of grout has the ability to flow easily into even small voids in the masonry, and to surround reinforcement without the need for mechanical consolidation. This ability is imparted by a combination of super-plasticizing admixtures and aggregate characteristics.
Test methods associated with self-consolidating grout are provided in ASTM C1611.
