Masonry Magazine September 1968 Page. 20
Low-lift grouting requires no special concrete block, shapes or equipment, and is probably the most common method of constructing reinforced concrete masonry. The vertical steel reinforcing bars are relatively short in length; when placed in the wall it is only necessary that they extend above the top course a distance equal to 30 bar diameters. This will provide sufficient overlap with the reinforcing in the next masonry lift. Grouting may be accomplished with mechanical equipment, but more often it is done by hand. Codes require that the grout pour be stopped 1½ inches below the top of the masonry course so as to form a key with the next lift. Typical reinforced masonry construction using low-lift grouting is shown in Fig. 1.
On larger projects, grouting may be delayed until the wall has been laid up story height. This procedure has been termed "high-lift" grouting and generally will employ a grout pump for placing of the grout. High-lift grouting requires that the grout be placed in the cavities in lifts not exceeding four feet with a 30 to 60 minute delay between successive lifts. Before grouting by the "high-lift" procedure, mortar waste is removed from the steel reinforced cavities by means of cleanout openings at the base of the wall. Grout then is poured to a height of one lift and rodded or vibrated to thoroughly fill all voids, spaces and interstices. After a 30-60 minute waiting period a second lift of grout is poured to the same depth and all cells again rodded. The waiting, pouring, and rodding steps are repeated until the top of the wall is reached.
The advantages of high-lift grouting on large projects are obvious. Placement of the vertical steel can be done after the wall is erected; its location can be checked by the engineer; and on a job of moderate size the grout can be transit-mixed and poured in a continuous operation. Its main disadvantages are the need for a grout pump, or other means of pouring grout rapidly, and the requirement for cleanout openings at the base of the wall since these can affect wall finish in some instances. In reinforced hollow unit masonry, high-lift grouting also requires a minimum cavity dimension of three inches, whereas a continuous vertical cavity of 2 x 3 inches can be used in low-lift grouting. Figure 2 shows the grouting of a reinforced concrete masonry wall using the high-lift techniques.
When design requirements result in a large amount of closely-spaced steel reinforcement, a slightly different procedure may be employed in conjunction with high-lift grouting. Vertical steel bars are tied into their proper position at the foundation or base of the wall prior to laying of concrete masonry units. Special open-end, A-shaped block are then threaded around the steel as the wall is erected. Figure 3 shows one of these block, which is basically a two-core unit made up without one of the end cross-webs.
FIG. 2. High-lift grouting using a grout pump.
Materials
The applicable materials specifications for reinforced concrete masonry construction are listed as follows:
1. CONCRETE MASONRY UNITS:
Hollow Load-Bearing Units should conform to the Standard Specifications for Hollow Load-Bearing Concrete Masonry Units (ASTM Designation: C 90-). Units having two or three hollow cells are used, the former being more popular because the larger cells better facilitate the placement of steel and grout in the spaces.
Solid Load-Bearing Units should conform to the Standard Specifications for Solid Load-Bearing Concrete Masonry Units (ASTM Designation: C 145-).
Concrete Brick should conform to the Standard Specifications for Concrete Building Brick (ASTM Designation: C 55-).
Sand-Lime Brick should conform to the Standard Specifications for Calcium Silicate Face Brick (ASTM Designation: C 73-).
FIG. 3. Typical Open-end Concrete Block.
2. MORTAR AND GROUT:
Mortar and Grout for use in reinforced concrete masonry should conform to the Standard Specifications for Mortar and Grout for Reinforced Masonry (ASTM Designation: C 476-). All grout should be of fluid consistency, meaning that it should be as fluid as possible without segregation of the constituent parts.
Aggregates should conform to Standard Specifications for Aggregates for Masonry Grout (ASTM Designation: C404-), or Standard Specifications for Aggregate for Masonry Mortar (ASTM Designation: C 144-).
FIG. 4. Masonry Prisms for compressive testing. Prisms duplicate the project construction and are grouted solidly if the structure is to be similarily grouted.
TABLE I
COMPRESSIVE STRENGTH OF MASONRY
Gross area for masonry of solid units; net area for masonry of hollow units
| Compressive Strength of the Units | Assumed Compressive Strength of Masonry, f'm |
|---|---|
| psi | psi |
| 1,000 to 1,500 | 900 to 1,150 |
| over 1,500 to 2,500 | 1,151 to 1,550 |
| over 2,500 to 4,000 | 1,551 to 2,000 |
| over 4,000 to 6,000 | 2,001 to 2,400 |
| over 6,000 | 2,400 |