Slate Technical Data
- Properties of slate dimension stone
- Thermal Expansion
- Factor Affecting Properties
- Safety Factors
- Seismic Considerations
- Efflorescence and Staining
Properties of slate dimension stone
In centuries past, relatively little importance was attached to the ultimate physical capabilities of most building materials. In present day construction, however, this is far from being true. Performance requirements are daily becoming more demanding. In striving for taller structures, greater spans, firmer foundations, thinner walls and floors, stronger frames and generally more efficient buildings with more usable space, today’s architects and engineers must get the most out of the materials with which they work.
Slate is a product of nature and not always subject to the rules of consistent behavior that may apply to manufactured building materials.
Final design should always be based on specific values for the slate variety ultimately to be installed. These values may be obtained from the slate supplier. In order to avoid mistaken selections, tests for material values should be made prior to final material selection.
The thermal expansion of slate is an important consideration where slate is used with dissimilar materials to form large units which are rigidly fixed.
Factor Affecting Properties
The ultimate test of a building material is its ability to have and maintain the necessary structural strength, as well as beauty of appearance and low cost of maintenance, over the useful life of the structure. Experience has proven that slate meets this test as few other building materials can. Studies have shown that the durability of most slates is little affected by cycles of weather. This is because of slate’s low rate of absorption.
Good engineering practice requires that allowable design stress must provide a margin of safety in any structural element. As a necessary precaution against some conditions as wind, ice, snow, impact, temperature changes and imperfect workmanship, these allowable stresses must be smaller then those which produce failure.
For a particular construction, the closer the allowable load is to the ultimate failure load, the more efficient is the use of the material and the less the cost of construction. Contemporary building design does not usually employ slate as part of the structural frame but rather as an independent unit, a certain wall or veneer. Therefore, the primary concern in such cases is with wind or seismic loads and a safety factor of 5.0 is recommended. Where the slate is to be subjected to concentrated loading, such as stair treads or lintels supported only at the ends, a factor of 10.0 should be used.
These safety factors may be adjusted using sound engineering principles and judgment.
As buildings become taller and individual slate slab veneer becomes larger in area, the lateral forces due to wind loads must be considered. Wind tunnel tests are often used on major structures to determine wind dynamics and force magnitude. Reinforcement is sometimes necessary for large dimension slab veneer in critical areas.
Seismic considerations generally require “low” buildings be stiff, and that “tall” buildings be relatively flexible. Design of connections must account for seismically induced horizontal loading. Local codes vary and must always be checked to determine specific requirements for each area.
Efflorescence and Staining
Efflorescence is a deposit, usually white in color that appears on exterior surfaces of masonry walls.
The water soluble salts causing efflorescence come from other materials in the wall. The salts exist in small amounts and are leached to the surface by water percolating through the walls. The most feasible means of prevention is to stop the entrance of large amounts of water. Absorption from the face will not cause efflorescence unless there are open joints.
Slate is seldom injured by efflorescence. However, some of the salt crystals may form in the pores near the surface. Crystal growth in the pores can cause stress on the walls of the pores and cause the stone to flake off. If the conditions bringing about the action persist, scaling may continue and flake off one layer after another.