Masonry Magazine August 1989 Page. 17
Grout in Reinforced Masonry Walls
Grout is used to fill the spaces within reinforced masonry walls, and its properties and application are crucial for the structural integrity of the wall. The grout must have a slump from eight to 10 inches, depending upon the size of the grout space, the height of the grout lift, height of the grout pour, the absorption of the masonry units and the weather conditions. Following good practice for grouted masonry construction will result in a strong and reliable structure.
The masonry units should be pre-wetted as previously described in the absorbent molds and tested after 28 days. The absorption of the excess water into the masonry units reduces the water/cement ratio to a satisfactory level so that the strength of the grout prism will be as required.
Depending upon the size of grout space, the area of the cell or the width of the grout space, either fine grout or coarse grout can be used. Fine grout is cement, sand and water, and for the grout specified by the Uniform Building Code, which required a minimum strength of 2,000 psi, the proportions by volume are one part portland cement and two-and-one-quarter to three parts sand. The usual proportions are 1:3. Fine grout is used where the grout spaces are narrow, two inches or less, or in hollow block that is four inches or less in width.
Coarse grout contains pea gravel or even larger coarse aggregate, and the proportions for coarse grout by volume could be one part portland cement, two to three parts sand, and not more than two parts gravel.
Coarse grout that is to be placed by means of a grout pump should have proportions of approximately 70% to 80% sand and 20% to 30% pea gravel.
Coarse grout may be used in grout spaces in brick masonry, two inches or more in horizontal dimensions and the cells in block construction should be four inches or more in both horizontal dimensions.
If the grout space is wide, say eight or more inches, in horizontal dimensions between the brick wythes, a coarse grout using 4-inch aggregate might be used. This larger size aggregate takes up more volume, reduces shrinkage of the grout and requires less cement for high strength. Also, the slump can be reduced to seven or eight inches because of easier placement.
In order to insure that the grout flows into all crevices, around all reinforcing bars and completely fills the grout space, it must be consolidated by puddling or vibrating. Much of the water that functions as the transporting or placement vehicle for the grout is absorbed into the masonry units and, accordingly, there is a volume reduction of the grout.
The use of shrinkage-compensating admixture is recommended on all high lift grouted construction. Expanding admixtures, such as Grout Aide, cause a slight expansion, about 8 per cent, which counteracts the volume change due to water loss.
Proper and adequate use of a mechanical vibrator can also counteract the effects of water loss and shrinkage.
The water not only acts as the transporting vehicle for the cement, sand and pea gravel, it also acts as the curing agent for the hydration of the portland cement as it is absorbed into the masonry unit. Portland cement, a hydraulic cement, hydrates or hardens in the presence of water. Therefore, the moisture contained in the masonry units cures the portland cement, and thus the grout gains strength. Accordingly, masonry walls need not be wetted down after the grout is placed.
What is the function of grout and why is grout used in reinforced masonry walls? Grout in brick or block walls (1) increases the cross-sectional area to the wall for vertical load support and for lateral shear both parallel and perpendicular to the wall, (2) it bonds the wythes together, and (3) it transfers stress to the reinforcing bars when the wall is subjected to lateral forces due to wind, earthquakes or earth pressure. The masonry element is stressed in compression and through the grout; the reinforcing steel is stressed in tension when the wall is subjected to forces perpendicular to the plane of the wall.
Grout is not mortar, grout is not concrete; grout is a material unique to reinforced masonry systems, and the rules governing concrete placement should not be imposed upon grout.