Building stone tests are crucial evaluations conducted to assess the suitability, durability, and performance of various types of stones used in construction. These tests help ensure that the chosen building stones can withstand environmental conditions, load-bearing requirements, and other factors that may impact their longevity and structural integrity. Different types of tests are employed to evaluate aspects such as compressive strength, water absorption, abrasion resistance, and thermal performance. The results of these tests aid architects, engineers, and builders in selecting the most appropriate building stones for a given project, taking into account the specific demands of the intended application and the prevailing environmental conditions.

This blog shall discuss the different types of stone tests for checking the viability of building stones for construction projects. This topic is important for all the upcoming Civil Engineering Examinations, including SSC JE CE ഒപ്പം RRB JE Civil.

Significance of Building Stones Tests

Different types of stone tests are conducted on building stones to ascertain their properties and suitability for diverse construction applications. These tests yield valuable insights into the physical and chemical characteristics, as well as the strength and hardness properties of the building stones. Understanding these properties is essential in making informed decisions about the most fitting materials for construction projects, ensuring both durability and structural integrity. Moreover, Stone tests play a pivotal role in determining the stones' resilience to environmental factors and their ability to withstand the demands of specific construction contexts. 

Types of Building Stone Tests

Following are different tests on building stones:

• Acid test
• Attrition test
• Crushing test
• Crystalline test
• Freezing and thawing test
• Hardness Test
• Impact test
• Water absorption test
• Microscopic Test
• Smith’s Test

Acid test 

The Acid Test is a crucial stone test conducted to ascertain the existence of calcium carbonate in building stones, typically focusing on sandstones. This constitutes one of the important stone tests. A stone sample, weighing approximately 50 to 100 gm, is immersed in a one percent hydrophobic acid solution for a duration of seven days, with periodic agitation. During this period, a high-quality building stone maintains its sharp edges and remains devoid of any surface powder. Conversely, the presence of broken edges and surface powder formation indicates the existence of calcium carbonate, signaling potential shortcomings in weathering resilience. This specific stone test is commonly performed on sandstone varieties.

Attrition test

This stone test is conducted to determine the wear rate of stones utilized in road construction, providing insights into their resistance against grinding action under traffic conditions. The procedure is outlined as follows:

• Samples of stones are fragmented into pieces approximately 60 mm in size.
• These stone pieces, totaling 5 kg in weight, are placed in both cylinders of the Devil’s attrition test machine, with cylinder dimensions of 20 cm in diameter and 34 cm in length.
• The cylinders are sealed, and their axes are positioned at a 30-degree angle to the horizontal.
• Rotating about the horizontal axis, the cylinders undergo a 5-hour cycle at a rate of 30 rpm.
• Upon completion, the contents are extracted and sieved through a 1.5mm mesh.
• The weight of the material retained on the sieve is measured.
• Percentage wear is determined by the formula: Percentage wear = (Loss in Weight/Initial Weight) x 100. This calculation offers a quantifiable measure of the stones' resilience against wear and tear, crucial for assessing their suitability in road construction applications.

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Crushing test

The crushing strength of a stone is the maximum load it can withstand per unit area before failure. It is determined by dividing the maximum crushing load by the area of the stone's bearing face.

To assess the crushing strength of stone, samples are cut into cubes measuring 40 x 40 x 40 mm. These cubes undergo precision dressing and finishing. The testing protocol allows for a maximum of three specimens to be tested, each immersed in water for approximately 72 hours before the examination to ensure saturation.

Subsequently, the load-bearing surface is enveloped with a 5mm thick layer of plaster of Paris plywood. The crushing test is conducted using a crushing test machine, applying axial load to the cube at a rate of 140 kg/sq.cm per minute. The crushing strength of the stone per unit area is then determined by dividing the maximum load at which the sample crushes or fails by the area of the bearing face of the specimen. This systematic approach yields crucial data for understanding the stone's ability to withstand compressive forces in various construction applications.

Crystalline test

The Crystalline test assesses the change in weight over several repetitions. It is expressed as a percentage of the original weight to evaluate the stone's durability against crystallization-induced decay due to weathering. 

A minimum of four 40mm-side stone cubes undergo a series of cycles, including drying for 72 hours, immersion in a 14% \Na2SO4\Na2��4 solution for 2 hours, and subsequent drying at 100°C, with the change in weight noted. This process, repeated at least five times, assesses weight fluctuations expressed as a percentage of the original weight. Despite \CaSO4\CaSO4 crystallization in stone pores causing weathering decay, its low solubility in water renders it unsuitable for incorporation into this test.

Freezing and Thawing test

Freezing and thawing in stone refers to a test procedure where a stone specimen is subjected to repeated cycles of freezing at low temperature, often followed by thawing at ambient temperatures. This cyclic exposure helps evaluate the stone's ability to withstand the stresses associated with freezing and thawing, which can be critical in assessing its durability and resistance to weathering in cold climates. 

The stone specimen undergoes a 24-hour immersion in water, followed by placement in a freezing machine at -12°C for an additional 24 hours. Subsequently, it is thawed at atmospheric temperature, with all steps performed in the shade to mitigate the influence of external factors like wind, sun rays, and rain. This cyclic process is repeated multiple times, and the stone's behavior is meticulously observed for thorough assessment.

Hardness Test

The hardness stone test involves assessing the resistance of a stone sample to abrasion by subjecting it to specific conditions. To determine the hardness of a stone, the following procedure is conducted:

• A cylindrical sample, with a diameter of 25mm and height of 25mm, is extracted from the stone and weighed.
• The sample is then placed in Dorry’s testing machine, subjected to a pressure of 1250 gm.
• An annular steel disc machine is rotated at a speed of 28 rpm, with coarse sand of standard specification sprinkled on top of the disc.
• After 1000 revolutions, the specimen is removed and weighed.
• The coefficient of hardness is calculated using the formula: Coefficient of hardness = 20 - (Loss of weight). This calculation provides a measure of the stone's hardness based on the weight loss during the testing process.

Impact test

The impact test on stone involves assessing its toughness by subjecting a cylindrical stone specimen. To determine the toughness of a stone, it undergoes an impact test using an Impact Test Machine in the following manner:

• A cylindrical sample, with a diameter of 25 mm and height of 25 mm, is extracted from the stone.
• The sample is positioned on the cast iron anvil of the machine.
• A 2 kg steel hammer is released axially in a vertical direction over the specimen.
• The height of each successive blow is incremented by 1 cm, starting from 1 cm for the first blow, 2 cm for the second blow, and so forth.
• The test records the blow at which the specimen breaks, and if it is the nth blow, 'n' is representative of the toughness index of the stone.

Water Absorption Test

The water absorption test of stone involves immersing a stone cube in distilled water for a specified duration, measuring its weight changes and suspended weight. The test assesses properties such as percentage absorption by weight and volume after 24 hours, volume of displaced water, percentage porosity by volume, density, specific gravity, and saturation coefficient. The test procedure for this stone test is discussed below:

• A cube weighing approximately 50 gm is prepared from the stone sample, and its initial weight is recorded as W1 gm.
• The cube is immersed in distilled water for a 24-hour duration.
• After removal from water, surface water is wiped off, and the cube is reweighed (W2 gm).
• The cube is freely suspended in water, and its weight in this suspended state is recorded as W3 gm.
• Boiling water is used, and the cube is immersed for 5 hours.
• After removal and wiping off surface water, the final weight is recorded as W4 gm.
• From these observations, various stone properties are derived:

1. Percentage absorption by weight after 24 hours = (W2 - W1) x 100 / W1
2. Percentage absorption by volume after 24 hours = (W2 - W1) x 100 / (W2 - W3)
3. Volume of displaced water = W2 - W3
4. Percentage porosity by volume = (W4 - W1) x 100 / (W2 - W3)
5. Density = W1 / (W2 - W3) kg/m³
6. Specific Gravity = W1 / (W2 - W3)
7. Saturation Coefficient = (Water Absorption/Total Porosity) = (W2 - W1) / (W4 - W1)

Microscopic Test

The test sample undergoes a microscopic examination, where stone sections are extracted and placed under a microscope for the analysis of several properties. These properties include:

• Average grain size
• Presence of pores, fissures, veins, and shakes
• Mineral constituents
• Nature of cementing material
• Presence of any harmful substances
• Texture of stones, and more.

Smith’s Test

The purpose of Smith’s stone test is to identify the presence of soluble matter in a stone sample. A few stone chips or pieces are placed in a glass tube, which is then filled with clear water. After approximately an hour, the tube is vigorously stirred or shaken. If the presence of earthy matter causes the water to turn dirty, it indicates the existence of soluble components. Conversely, if the water remains clear, the stone is deemed durable and devoid of any soluble matter.