Masonry Magazine January 1976 Page. 24
Prefabrication of Masonry
The fabrication operation of this method can be and has been performed either at an off-site plant or an on-site plant.
Casting
The casting method of fabrication involves the combining of masonry units and grout into a prefabricated element similar to precast concrete. The casting method is performed with the element either in a vertical or a horizontal position. This method in general lends itself to automated equipment which requires a form, some method of placing units and a system for grouting. This method of prefabrication usually takes place in an off-site plant.
The usual practice is to place the units, either by hand or machine, in a form or mold, either horizontal or vertical, and fill the form with a grout at atmospheric pressure or under moderate pressure.
Equipment
The equipment used in prefabrication as practiced today varies widely. It ranges from simple hand tools to highly sophisticated automated machinery. The hand-laying method will usually employ the conventional mason's tools: trowel, jointing tool, etc. In addition, the hand-laying method may also utilize corner poles, jigs and templates for special shapes. The casting method also will use jigs and forms which provide the spacing for joints. The hand-laying method will usually employ some type of adjustable scaffolding. This scaffolding may be manually adjustable, although there are available some motor-driven scaffolds which provide horizontal and vertical movements. Adjustable scaffolding can greatly increase mason productivity and reduce fabrication costs.
In addition to the more or less conventional equipment listed above, there are specialized tools used in prefabrication work. The casting method requires that the face of the unit be protected from contamination by the grout, which is usually done by pressure. Pressure, at the contact surface of the brick face and form, is created by either an inflated form face or by applying a load to the brick. There are available for use in the hand-laying method mechanized or pneumatic mortar spreaders for distributing mortar for the bed joints. Some prefabrication has been done using the casting method and automated unit placing machinery. This equipment places the unit with proper joint width by machine in lieu of hand placement of units in jigs or forms. Pressurized grouting systems have also been widely used in the casting method of prefabrication.
Masonry Units
There have been specially manufactured units made for prefabrication; however, this text will deal primarily with standard sized and shaped units. Both solid brick and hollow brick have been used in prefabrication. Solid brick masonry units are those which have coring of less than 25 per cent of the bedding area. Hollow brick units are cored in excess of 25 per cent but less than 40 per cent of the bedding area. The hollow units are suitable for and used in applications where vertical reinforcement may be required.
Large face size units have been used in prefabrication work, both in solid and hollow unit coring patterns. These units, having a finished face larger than the standard modular unit, 24 by 7% in., provide certain advantages. The main advantage of large face size units is the economy gained by increased productivity. In a single wythe application, placement of a 4 by 12-in. face unit gives an equivalent face area of 2.25 times that of the standard modular unit, and an 8 by 8-in. face unit provides three times the face area of a standard modular unit. Single and multiple wythe elements of the units outlined above have been used in prefabrication work.
Mortar and Grout
Prefabrication may use either conventional mortar or mortar with one of the high-bond mortar additives. Prefabricated panels usually are subjected to higher stresses during handling than when in service in a structure. The high-bond mortars give the needed higher tensile bond strength which may permit earlier handling of the finished panels. Extreme care must be taken to determine compatibility of the brick with the high-bond mortar. Some brick may not perform as well with high-bond mortars as others. Tests should be performed with the mortar and brick to determine if the combination will indeed produce the required flexural strengths for the project. (See Technical Notes 38 and 39.)
ADVANTAGES OF PREFABRICATION
There are several advantages in prefabrication which conventionally laid masonry does not have. By using panelized construction, the need for on-site scaffolding is virtually eliminated. This can be a significant cost savings in masonry construction. If an off-site plant is used, the work area and storage area for masonry materials at the job site are kept to a minimum. If proper scheduling of delivery is maintained, the panels can be erected as they are delivered, eliminating any need for panel storage at the site.
The use of panelization makes possible the fabrication of complex shapes. These shapes can be accomplished without the need for expensive falsework and shoring necessary for in-place laid masonry of the same shapes. Complicated shapes with returns, soffits, arches, etc., are accomplished by using jigs and forms. Repetitive usage of these shapes can lower costs appreciably; the more re-uses of the jigs and forms, the lower the per panel cost will be.
One of the distinct advantages of prefabrication is the factory setup. This allows for year round work and multi-shift work days. It permits the labor force to work under conditions not affected by weather. The use of prefabricated masonry may eliminate the need for or provide the means of winterizing the structure.
Prefabrication requires stringent quality control. However, this may be more easily attained under factory conditions. Mortar batching systems can be tightly controlled by automation or sophisticated regular labor force can be employed.