Masonry Magazine November 1963 Page. 7
CLAY MASONRY WALL-BEARING CONSTRUCTION
For single unit 8-in. walls, the so-called 8-in. "Jumbo" (Fig. 4) produced in the southeast section of the country is an economical unit to lay and their compressive strengths are sufficient for wall-bearing structures up to 6 stories in height with proper design.
While economical wall-bearing structures can now be designed and built with present knowledge and standard masonry units readily available all over the country, it is anticipated that even more economical construction can be achieved through the use of clay units designed specifically for such specialized applications. Studies are currently under way at the Structural Clay Products Research Foundation which, it is hoped, will lead to the development of new types of wall-bearing units which will make possible even more economical construction by utilizing to a greater extent the inherent compressive and tensile strengths of clay units.
MORTARS
The compressive strength of clay masonry becomes of prime importance when the masonry is used to support a multi-story building. That strength is governed by the strength of both the masonry units and the mortar. Contrary to the impression many people have, the quality and compressive strength of the mortar used has much to do with the compressive strength of the masonry wall. For example, most building codes specify the allowable compressive stresses the designer must see in masonry and these allowable stresses are based on both the masonry unit strength and the type of mortar. If a brick with an average strength of 8000 psi or over is to be used with a Type M mortar (1 part portland cement, 1/4 part lime and 3 parts sand by volume), the designer is permitted to use an allowable working stress in compression of 400 psi for the masonry. On the other hand, if the designer proposes to use that same brick, but laid in a Type N mortar 1 part cement, 1 part lime and 6 parts sand, or a Type II Masonry Cement, he can use an allowable working stress of 300 psi. The use of a Type O mortar (1 part cement, 2 parts lime and 9 parts sand) with the same brick would reduce this allowable stress to 200 psi.
In the design of a multi-story wall-bearing building, the designer may find that in order to develop the required compressive strengths in at least the lower stories of the structure, a high strength mortar may be required. While it will be his responsibility to select and specify the mortar, it will be the responsibility of the mason contractor to see that mortar of the quality specified is obtained. In engineered wall-bearing construction, proper supervision and control of the batching and mixing of the mortar will be of great importance. Any revision or change in the mix by the mason to adjust it to his own individual feeling cannot be tolerated. If the engineered design of the structure requires a specific strength of masonry unit and a specified type of mortar, deviation from those requirements without the knowledge of the designer could result in structural failure.
The masonry strength requirements for low rise wall-bearing buildings will not be as great, of course. Even if the calculated stresses in the masonry are based on a given combination of masonry units and mortar, any deviation from those requirements that would result in a wall with a lower compressive strength might, in some instances, create a safety problem.
When "high-bond" mortar is commercially available, it will have many uses in wall-bearing construction. The greater tensile-bond and compressive strength of this polymer-modified portland cement mortar greatly increases the transverse, compressive and shearing strength of clay masonry walls and will broaden the scope of potential uses of wall-bearing clay masonry construction. The use of high-bond mortar in the design of such structures will require an even greater measure of quality control in its batching and use.
The need for more rigid and careful control of mortar batching and mixing in the construction of wall-bearing structures cannot be overemphasized. The batching of all mortar materials must be accurate and close. Qualified supervision of this operation will be essential.
II. CONSTRUCTION TECHNIQUES AND METHODS
# BRICKLAYING
Research and Development have provided many building trades with new tools and equipment which have helped those trades to keep pace with modern building technology and to perform their skilled tasks more efficiently and economically with less fatigue. The use of such specialized tools and equipment has, in many cases, lowered construction costs despite increases in the cost of materials and in the wage scales of the craftsmen using them.
Until the development and introduction of the "SCR masonry process" (Ref. 3) by the Structural Clay Products Research Foundation in 1953, science had not done much of anything for the bricklayer in helping to make his work easier and more efficient. Attempts to stem rising costs of masonry construction were confined largely to the efforts of individual contractors to improve the efficiency of their overall operations by better job planning and organization, improved materials handling techniques, modern cost accounting systems and the use of large and lighter-weight masonry units.
There is a limit, however, beyond which such improved methods of operation and materials cannot offset increasing materials costs and wage scales. Also, it appeared somewhat incongruous to provide the mason's helper and the laborer with modern materials handling equipment in order to make their work easier, more efficient and less costly and expect the skilled mason to work with tools and equipment and under conditions little different or improved over those of his grandfather.
The "SCR masonry process" has helped provide the bricklayer with better equipment and working conditions. The system of corner poles (adaptable for both load-bearing and veneer work), continuously adjustable scaffolding and the nylon marked line is being used by more and more mason contractors all over the country.
The potential of wall-bearing multi-story construction, however, has pinpointed the need for a special corner pole differing from most poles now on the market. Most load-bearing poles are braced to the ground or, if the work is above one story, braced to the outside of the wall when the pole is moved up. With masons working from the inside several stories above grade, it is impractical to brace long poles to the exterior masonry.
A special corner pole (Fig. 5) has been developed for multi-story load-bearing work and is presently being field tested. This pole sets on the masonry and provides guide lines for 16 in. of masonry at one setting. It is then slipped out of the masonry and reset for the next 16 in. It is simply and quickly plumbed by adjusting a cam and reading a small magnetic level.
The marked "line" can be used to good advantage on multi-story wall-bearing work. It is particularly useful in laying out the work at each floor level. Considerable time (Continued on page 13)
Reg. T.M. Structural Clay Products Research Foundation