Indiana Limestone Custom Cut Stone

The Use and Overuse of Testing in Specifying Dimension Limestone

Knowledge of the physical properties of stone used for the construction of building walls is a requirement for both architects and engineers. The Indiana quarries have historical and current test data available to confirm the strengths of the stone produced in their operations. 

The limestone properties required are those cited in the American Society of Testing and Materials (ASTM) Standard Specification for Limestone Dimension Stone. Identified as C-568, this specification lists five major physical property requirements: 

     Absorption (C-97)
     Density (C-97)
     Compressive Strength (C-170)
     Modulus Of Rupture (C-99)
     Abrasion Resistance (C-241)
     (The latter is needed only in those instances where limestone is used as
      flooring and paving.)

C-568 classifies limestones according to their density, with differing requirements for each category, identified as:

     Type 1 (Low Density)
     Type II (Medium Density)
     Type III (High Density)

Density is defined in the C-568 ASTM Table as "the weight in pounds per cubic foot, or kilograms per cubic meter." Indiana Limestone is classified a Type II (medium density) stone.

Table 1 Physical Requirements
Physical Property Test Requirement Classifications ASTM Method
Absorption by Weight, Max, % 7.5 II Medium Density C 97
Density, Min, Lb/Ft3 135 II Medium Density C 97
Comprehensive Strength, Min, PSI 4,000 II Medium Density C 170
Modulus of Rupture, Min, PSI 500 II Medium Density C 99
Abrasion Resistance, Min, Hardness* 10 II Medium Density C 241
*   Pertains only to stone subject to foot traffic. In stairways, floors and platforms subject to heavy foot traffic, a minimum abrasion hardness of 10 is recommended.

The use of these properties, particularly compressive strength and modulus of rupture (bending), is required in designing stone cladding and connections. For design purposes, these properties are factored for safety. The recommended safety factors for Indiana Limestone are listed in the Technote on Safety Factors published by the Indiana Limestone Institute (ILI). Further, ILI resists the use of Indiana Limestone in thicknesses less than 2 inches for wall cladding.

In spite of the known durability of Indiana Limestone and the conservative practices recommended by ILI, a number of testing firms promote extensive test programs involving multiple test cycles in which stone is alternately frozen and thawed while soaked in acid solutions. These procedures are said to predict the long-term performance of stone in building construction and to establish a "remaining strength" figure after 500 cycles or more. In some instances, test promoters claim that a certain number of cycles is equal to a certain number of years of natural weathering, although no laboratory test correlation has ever been established based on actual long-term performance of the stone in exterior walls. 

Test promoters commonly use freeze-thaw graphs and charts prepared by the lab to show the diminishing resistance of the laboratory specimen stones to bending and compression. Calculations and assumptions are then made to establish new, higher, arbitrary safety factors to meet the presumed needs of buildings after ten, thirty or fifty years. Often the laboratory specimens are found inappropriate for the intended use in any circumstance.

Cost aside (and such tests can run many thousands of dollars per series), these procedures take time. The delays they cause in approvals can create construction delays and cost extras. Schedules are scrapped, especially when a search for new stone sources must begin. The specifier who is unaware of these dangers to his timetable runs serious risks — unnecessary risks for a time-proven material.

Long experience demonstrates that these acid immersion/freeze-thaw test procedures are inaccurate in predicting the performance of Indiana Limestone for the following reasons:

  1. Freeze damage to Indiana Limestone wall panel stones does not occur. Stone in wall construction typically exudes moisture from precipitation almost as rapidly as it is absorbed. Thus the stone is never completely saturated during periods of prolonged freezing temperatures, and in fact it is rarely wetted at all prior to any freezing temperatures.
  2. Acid rain created by the assimilation of sulfur and nitrogen oxides in urban environments coexists with exterior limestone wall cladding only at its surface, and then only at the start of a rain. Continuing rainfall has the effect of cleansing both the atmosphere and the stone surface.
  3. Indiana Limestone is America's most-used wall cladding stone. Buildings using it exist in every American city, in most smaller towns and villages, in Canada, and in every type of atmosphere. Specimens taken from many such buildings, constructed as long as 120 years ago, invariably test within the required strengths of ASTM Specification C-568 for newly quarried Type II limestone.

    Long-weathered Indiana Limestone commonly shows a strength gain, rather than the loss predicted by these expensive and time-consuming tests.

SUMMARY

ILI is a strong advocate of conservative design. Indeed, ILI's publications are models of conservatism in practice and procedure recommendations. Where other stones or other products have no legitimate track record established, or where a product is being used in thin sections or in untried conditions, extensive testing may be advisable. 

However, Indiana Limestone is time-proven, and no better laboratory tests exist than American and Canadian cities where the stone has been in daily use on thousands of buildings — many over 100 years old.

Therefore, ILI recommends that users of Indiana Limestone require only the certification of properties listed in C-568, Type II.

However, if extensive testing programs are desired, specifiers are urged to avoid the trap of delays and cost extras by specification wording that allows the omission or termination of testing programs before significant damage is done to construction schedules and costs. The following paragraph is quoted, with permission, from an architectural specification:

4.   Preconstruction stone testing may be waived providing the stone supplier: 
a.   Submits copies of the following tests, performed within the last five years, which demonstrate compliance with specified performance requirements:
Physical Property Test Requirement Classifications ASTM Method
Absorption by Weight, Max, % 7.5 II Medium Density C 97
Density, Min, Lb/Ft3 135 II Medium Density C 97
Comprehensive Strength, Min, PSI 4,000 II Medium Density C 170
Modulus of Rupture, Min, PSI 500 II Medium Density C 99
Abrasion Resistance, Min, Hardness* 10 II Medium Density C 241
*Pertains only to stone subject to foot traffic.
b.   Provide the names and locations of two or more projects in the * area (or other area of similar atmospheric conditions) which demonstrate satisfactory performance of the material.
*INSERT CITY WHERE PROJECT IS TO BE BUILT.