Masonry Magazine August 1989 Page. 16
GROUT
continued from page 15
built above without the mortar squashing or squeezing out and deforming. Although mortar has a low water/cement ratio, this ratio is further decreased after the mortar is spread on the bed and the masonry units are placed on it. Any excess water in the mortar is absorbed both down into the masonry unit on which the mortar is spread and up into the masonry unit that is placed on the mortar bed. This absorption helps create bond between the mortar and the masonry unit.
Grout, although having the same ingredients of concrete, has a fluidity or a plasticity far greater than normal concrete as mentioned above. Grout is placed in the cells of hollow masonry units and in relatively narrow grout spaces in brick walls in heights of anywhere from just a few inches to as high as 25 feet, as in the case of high lift grouting. Accordingly, grout must be fluid. it must completely fill the cells, the grout space and the joints between masonry units in order to provide a solid, homogeneous grouted masonry wall.
If the grout is stiff, it will not flow into the cells or grout space, but will hang up and leave large voids within the wall. Grout must have fluidity with a slump of eight to ten inches. This fluidity allows the grout to flow through the grout space, around the reinforcing bars and completely surround and bond to the steel and masonry unit.
Now what about the water/cement ratio? After all, the grout must have a minimum or specified strength, as will be discussed later. This is taken into account in the design of the grout mix. The excess water, which is a placement vehicle for the grout and helps it flow throughout the wall, is absorbed by the masonry units, whether brick or block, and thus the final water/cement ratio of the grout is reduced to a point where the strength of the hardened grout is in accordance with the specification or code requirements.
The high water content is important. To quote the Office of the State Architect, Structural Safety Section, Circular Number 9, "Filled Cell Concrete Masonry. High Lift Grouting Method." paragraph 4(b), Grout, states, "Sufficient water shall be added to make a workable mix that will flow into all joints of the masonry without separation or segregation. When grout is to be placed in masonry units with typical rates of absorption, the slump of the grout should be approximately five to ten inches, depending upon temperature and humidity conditions."
The Office of the State Architect, Structural Safety Section. Circular Number 10, "Clay Brick Masonry, High Lift Grouting Method," section 4(b) states, "Sufficient water shall be added to make a workable mix that will flow into all joints in the masonry tiers without separation or segregation. The slump of the grout should be varied depending upon the rate of absorption of the masonry units and temperature and humidity conditions. The range should be from eight inches for units with a low rate of absorption (30 to 40 grams per minute) up to 10 inches for units with a high rate of absorption (80 to 90 grams per minute)."
The recommendations of the Masonry Institute of America and other masonry organizations, provide the basis for allowing and needing grout of high fluidity so that it can be properly placed in walls. After all, if a very stiff grout were used, say similar to concrete with a slump of four inches and it hung up in the wall, there could be large void spaces in the wall. This would be unsatisfactory and the wall would not be a solid grouted masonry wall.
Grout, as required by the Uniform Building Code, must have a minimum 28-day compressive strength of 2,000 psi. Grout specimens for compression tests must be taken in accordance with the Uniform Building Code Standard No. 24-28 in which masonry units are placed together to form a square with side dimensions of the square one-half the height of the prism it forms. The mold is lined with paper towels or blotting paper to prevent bonding of the grout to the masonry units and to allow the excess water from the grout to be absorbed through the blotting paper or paper towels into the masonry units. This simulates the same conditions that would occur in placing the grout in the masonry wall and compression test results should be representative of the strength of grout in the wall.
Another sampling technique for hollow block construction is to place the grout in the clay or concrete block cell, allowing the excess water to be absorbed by the block, and after a period of time of several days, the block is broken away from the grout. The grout then is sawed into a prism with a height-to-width ratio of two. These grout prisms are cured in a moist room in accordance with curing concrete specimens and tested in compression at the end of 28 days. The grout must have a minimum strength of 2,000 psi in compression in accordance with the Uniform Building Code.
Many times, higher strength grouts than required by code are needed due to the design requirements for the masonry system. Accordingly, if an f of 3.000 psi is required for the wall, grout strengths should be specified of approximately 3,750 psi or at least 25 per cent more than the desired f... To attain grout of this strength, a laboratory designed mix should be made. This designed mix should have sufficient cement, sand, gravel and water to have a slump of eight to 10 inches. The test specimen should be made as