To ensure the suitability of building stones for construction, they must undergo tests to evaluate essential properties like strength, durability, and quality. These tests are divided into two main groups: field tests and laboratory tests, each serving distinct purposes.
Field Tests: Field tests are simple assessments that can be performed on-site to quickly evaluate the stone’s basic characteristics. These tests provide a preliminary understanding of qualities such as appearance, structure, and hardness, giving immediate insights into the stone’s suitability for certain applications.
Laboratory Tests: Laboratory tests require specialized equipment and controlled conditions, allowing for a more thorough analysis of the stone’s properties. These tests include compressive strength, porosity, and specific gravity measurements, among others, which reveal the stone’s structural integrity and long-term performance potential.
As per standards such as IS: 1121 (1974) and IS: 1124 (1998), both field and laboratory tests play a critical role in evaluating the wide variety of stones available across the country. By conducting these tests, engineers can select stones that will provide reliable and durable performance for construction projects.
1. Field Tests
i. Absorption Test (Water Absorption Test)
The Absorption Test, commonly known as the Water Absorption Test, is a straightforward test performed on all types of stones to measure their absorption capacity. Here’s the process:
- Preparation: A cubical stone specimen of approximately 50 grams is immersed in water for 24 hours.
- Calculation: The amount of water absorbed by the stone is measured and expressed as a percentage of absorption by weight, which should not exceed 0.6%. This test is also referred to as the electrical resistance test.
Test Procedure
Sample Selection: The selected stone pieces are crushed or broken. Only the material passing through a 20 mm IS Sieve and retained on a 10 mm IS Sieve is used. Approximately 1 kg of the test piece is washed to remove dust and then immersed in distilled water at a temperature of 20-30°C for 24 hours.
Air Removal: Entrapped air is removed by gently agitating the water in a clockwise and counterclockwise rotation immediately after immersion and again at the end of the soaking period.
Drying and Weighing:
- The test piece is placed on a dry cloth and gently dried, then transferred to a second dry cloth until no further moisture is removed.
- The sample is then weighed (B) after being air-dried for at least 10 minutes, ensuring it appears completely surface-dry.
Volume Measurement:
- The dried sample is placed in a 1000 ml measuring cylinder, and distilled water is gradually added from a 100 ml cylinder until the water level reaches 1000 ml.
- The amount of water added, expressed in grams, is recorded as C.
Oven-Drying: The sample is dried in an oven at 100-110°C for at least 24 hours, then cooled to room temperature and weighed again as A.
Calculations
Using the following formulas, the test results are determined:
True Porosity Calculation
The True Porosity of the stone is calculated by:
Where:
- A = Weight of the oven-dry test piece (g)
- B = Weight of the saturated surface-dry test piece (g)
- C = Quantity of water added in the 1000 ml jar with the test piece (g)
This absorption test helps in assessing the stone’s porosity, durability, and quality, ensuring it meets the necessary standards for building materials.
ii. Smith’s Test
The Smith’s Test is used to identify the presence of earthy matter or any signs of deterioration in stones when immersed in water. This test helps determine the stone’s purity and durability.
Test Procedure
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Preparation: A specimen of the stone is placed in a glass container filled with water and stirred vigorously to dislodge any loose particles.
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Soaking: The stone remains immersed in water for at least 24 hours.
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Observation:
- After 24 hours, if the water in the container appears turbid or cloudy, it indicates the presence of earthy substances in the stone.
- For a simpler version, freshly quarried stone chippings about the size of a coin are placed in a glass filled one-third with clean water.
- If the water turns slightly cloudy, the stone is considered good and durable.
- If the water becomes dirty or murky, it suggests the stone contains excessive earthy or mineral matter, which may impact its durability and strength.
This test provides a quick assessment of a stone’s quality and composition, making it suitable for evaluating all types of building stones before use.
iii. Toughness Test
The Toughness Test is a simple field test used to assess a stone’s ability to withstand impact. Although it doesn’t follow a strict procedure, it provides a quick indication of the stone’s toughness.
Test Procedure
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Hammer Strike: A stone specimen is struck with a hammer to observe its resistance to breaking.
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Observation:
- The force required to break the stone gives an idea of its toughness.
- A high resistance to hammering indicates that the stone is tough and durable, making it more suitable for construction applications where impact resistance is important.
This test, although informal, is a useful preliminary assessment of stone toughness in the field.
iv. Acid Test
The Acid Test is conducted to determine the presence of alkaline or lime content in stones, which can affect their durability and weather resistance.
Test Procedure
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Sample Preparation: A stone cube weighing between 50 to 100 grams is placed in a 1% hydrochloric acid (HCl) solution for one week.
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Observations:
- If the corners of the cube become rounded or loose particles appear on the surface, it indicates the presence of alkaline content.
- Fluorescence on the stone’s surface in reaction to the acid reflects the presence of lime.
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Weather Resistance Check: To assess weather resistance, 100 grams of stone chips are immersed in a 5% solution of sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) for 3 days. After soaking, the chips are dried.
- Stones with sharp, firm edges and corners after drying are considered sound and weather-resistant, making them suitable for outdoor use.
This test is particularly useful for checking the cementing material in sandstone and identifying stones that are highly fire-resistant, as they do not react to sulfuric acid.
v. Crystallization Test
The Crystallization Test assesses a stone’s resistance to weathering by examining the effects of crystallization due to chemical reactions, which can lead to loss of weight and surface defects.
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Test Procedure
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Sample Preparation:
- A cubical stone specimen with a 40 mm side length is immersed in a sodium sulfate solution for 2 hours and then dried in an oven at 100°C.
- This process is repeated five times, and any loss in weight and surface cracking are recorded. Minimal defects and weight loss indicate high durability.
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Extended Testing for Durability:
- Three specimens, each with 50 mm diameter and 50 mm height, are dried for 24 hours and weighed (W₁).
- The specimens are then immersed in a 14% sodium sulfate solution (density 1.055 kg/m³) for 16-18 hours at room temperature (20°C to 30°C).
- After immersion, the specimens are left in the air for 4 hours and then oven-dried at 105°C ± 5°C for 24 hours before cooling to room temperature.
- This cycle is repeated 30 times to simulate prolonged exposure, and the weight is measured after every five cycles.
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Calculation of Durability:
- The change in weight between the initial weight (W₁) and the weight after 30 cycles (W₂) indicates the degree of decay. The average of three test results is reported as the durability value.
A low percentage weight loss suggests that the stone has high resistance to decay and is suitable for use in harsh environments where weathering resistance is essential.
vi. The Field Hardness Test
This test, also known as the Mohs’ Scale of Hardness Test, measures the hardness of a stone based on the abrasiveness of minerals. The stone’s surface is scratched using a pen knife, and the hardness is determined relative to the Mohs hardness scale (ranging from the softest, talc, to the hardest, diamond).
Procedure
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Scratch Test:
- The surface of the stone is scratched with a pen knife.
- If the knife leaves a mark, the stone’s hardness can be gauged based on Mohs’ scale.
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Hardness Scale:
- Mohs’ scale classifies minerals by hardness in ten levels:
- Talc
- Gypsum
- Calcite
- Fluorspar
- Apatite
- Orthoclase Feldspar
- Quartz
- Topaz
- Corundum (Sapphire)
- Diamond
- Mohs’ scale classifies minerals by hardness in ten levels:
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Interpretation:
- If the stone can be scratched by the knife, its hardness is likely around H = 3 (similar to calcite).
- If the stone resists scratching, it may be as hard as quartz (H = 7), often indicative of siliceous rocks.
This scale provides a quick and effective method for estimating the hardness of stone materials, helping assess their suitability for construction and wear resistance.
2. Laboratory Tests
i. Compressive Strength Test
This test assesses the compressive strength of stone specimens, a crucial indicator of a stone’s durability and suitability for construction purposes.
Sampling Requirements
- Sample Size: Stones weighing at least 25 kg should be obtained from an unweathered section of the quarry.
- Test Specimens: Stones are prepared into cubic or cylindrical pieces with a minimum lateral dimension or diameter of 50 mm.
- Dimension Ratio: The height-to-diameter (or lateral dimension) ratio should be 1:1.
- Number of Specimens: At least twelve pieces should be tested, with three specimens for each condition (saturated and dry) and orientation (load applied parallel and perpendicular to the stone’s rift).
Testing Conditions
- Saturated Condition:
- Specimens are immersed in water at 20–30°C for 72 hours.
- Dry Condition:
- Specimens are oven-dried at 105 ± 5°C for 24 hours, then cooled to 20–30°C in a desiccator.
Test Procedure
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Testing Machine: Specimens are tested using a universal testing machine.
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Load Application: Load is applied at a rate of 14 N/mm² per minute until the specimen fails.
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Compressive Strength Calculation: Compressive strength is the maximum load (in Newtons) divided by the area of the bearing face (in mm²).
- The average result of the three specimens for each condition should be recorded as the compressive strength.
Special Calculation for Non-Standard Dimensions
If the height-to-diameter ratio deviates by more than 25%, use the following formula:
Where:
- CcCc = Compressive strength of the standard specimen.
- CpCp = Compressive strength of the non-standard specimen.
- bb = Diameter or lateral dimension.
- hh = Height.
The crushing strength of stones usually varies between 15–100 N/mm².
ii. Transverse strength test
To determine the transverse strength of natural building stones, specimen blocks are prepared and tested in both saturated and dry conditions. The process, apparatus, and calculations are based on standardized procedures outlined by the Indian Standard.
Sample Preparation:
- Specimen Blocks: Each test piece should be a block with dimensions of 200 mm x 50 mm x 50 mm.
- Conditioning: Test specimens are tested under both saturated and dry conditions:
- Saturated Condition: Specimens are immersed in water at a temperature between 20°C and 30°C for 72 hours before testing.
- Dry Condition: Specimens are dried in an oven at 105 ± 5°C for 24 hours, then cooled in a desiccator to 20–30°C.
Apparatus:
The test requires two self-aligning bearers (supports) 40 mm in diameter, spaced 150 mm apart, which hold the specimen in place. A central load is applied through a third bearer positioned midway between the supports. The load application is gradual, at a rate of 2 kN per minute, until the specimen fails.
Testing Procedure:
- Positioning: Each specimen is placed horizontally across the two bearers, with one bearer (A) on two screws with hardened steel balls for self-alignment, and the other bearer (B) on a single screw with a steel ball.
- Load Application: The load is applied uniformly at the midpoint of the specimen block until failure occurs.
- Breaking Load Recording: The breaking load (W) at failure is recorded in Newtons (N).
Calculation:
The transverse strength RR of the specimen in N/mm² is calculated using the formula:
where:
- WW = Central breaking load in Newtons (N)
- LL = Span length between supports, in mm (150 mm for this test setup)
- bb = Width of the test piece at the midpoint, in mm
- dd = Depth of the test piece at the midpoint, in mm
Reporting Results:
- Average Transverse Strength: Calculate the average transverse strength from three specimen tests (in both conditions: saturated and dry).
- Defect Evaluation: If any result is 15% below the average, the specimen should be checked for defects, and the flawed specimen replaced if necessary.
- Documentation: Include sample identification, test date, stone type, dimensions of test pieces, and preparation method in the report.
This testing method provides a standardized approach to assessing the suitability of natural building stones based on their transverse strength in different conditions, which is crucial for construction applications.
iii. Split Tensile Strength
The procedure outlined here measures the tensile strength of building stones under saturated and dry conditions. This test is critical for evaluating stone suitability for construction purposes, ensuring consistent quality and performance.
Sample Selection and Preparation
Sample Collection:
- Stones should represent the true characteristics of the type or grade being tested.
- Samples must be collected either directly from quarry sites or the natural rock source, ensuring they are free from damage caused by external factors like blasting or heating.
Test Specimen:
- Samples are prepared into cylindrical specimens with a minimum diameter of 50 mm, maintaining a diameter-to-height ratio of 1:2.
- Preparation should follow the guidelines in IS 9179:1979, which specifies rock specimen preparation methods for laboratory testing.
Conditioning of Specimens
Saturated Condition:
- To achieve saturation, specimens are vacuum-saturated by immersion in water (20–30°C) under a vacuum of 50 to 100 mm Hg for 4–5 hours.
- Weights are checked hourly until a constant weight is reached, which confirms saturation.
Dry Condition:
- Specimens are oven-dried at 70 ± 5°C for 48 hours and then cooled in a desiccator to room temperature.
- A constant mass is considered reached when two consecutive weight measurements differ by no more than 0.1%.
Apparatus Required
The test requires a tensile testing machine with adequate load capacity:
- Equipped with two steel bearing plates, each at least 10 mm thick.
- The upper bearing plate is mounted on a spherical seat to ensure even distribution of load across the sample, while the lower plate remains flat and rigid.
- Plate surfaces must be flat within a tolerance of 0.0125 mm.
Testing Procedure
- Specimen Placement:
- The specimen is placed between two steel plates, with dimensions matching the specimen length and a thickness of 10 mm.
- Load Application:
- Apply the load gradually at a uniform rate of 200 N/s, continuing until the specimen splits and can no longer sustain the load.
- Record the maximum load sustained before the split, with a precision of 1%.
Calculation of Tensile Strength
The split tensile strength (S) is calculated using:
where:
- SS = split tensile strength (N/mm²)
- WW = applied load at specimen failure (N)
- dd = specimen diameter (mm)
- LL = specimen length (mm)
Reporting Results
Results Averaging:
- Conduct five tests for each condition (saturated and dry), and calculate the average split tensile strength for each.
- If any individual result deviates more than ±15% from the average, examine the specimen for defects and exclude it from the results if found defective.
Documentation:
- Record the average tensile strength in N/mm².
- Include sample identification details, collection date, stone type, and specimen dimensions.
iv. Shear Strength Test
This procedure outlines the standardized methods for measuring the shear strength of natural building stones, as per IS 1121 (Part IV): 1974. Shear strength is an important property to determine, as it assesses the stone’s ability to withstand shear forces when used in construction.
Apparatus
The test can be performed using:
- Johnson Shear Tool : Suitable for testing stone bars with a cross-section of 50 × 50 mm and a minimum length of 180 mm.
- Dutton Punching Shear Device : Used for testing stone slabs that are 30 mm thick, with a width of 100 mm and a minimum length of 100 mm.
The testing setup should include a standard testing machine with adequate capacity to apply load gradually, alongside a spherical bearing block to ensure even load distribution in the Dutton device setup.
Sample Selection and Preparation
Sample Collection:
- Stones should be collected from quarries, ledges, or natural rock sources, ensuring that they represent a true average of the stone type.
- Any variation in stone color, texture, or structure within strata must be observed, and samples should be taken from all visibly distinct areas.
Test Piece Preparation:
- Samples should be prepared using saws to produce test pieces with precisely finished surfaces. Hand tools, such as chisels, are not permitted.
- Load-bearing surfaces must be made parallel and smooth to allow accurate load application during testing.
- The direction of rift (natural cleavage in the stone) should be marked on each sample, as tests must be performed with loads applied both parallel and perpendicular to this rift.
Conditioning of Test Specimens
- Dry Condition:
- Test specimens are oven-dried at 105 ± 5°C for 24 hours, then cooled in a desiccator to room temperature (20–30°C).
- Saturated Condition:
- Specimens are immersed in water at room temperature (20–30°C) for 72 hours prior to testing, ensuring complete saturation.
Three test pieces should be tested under each condition (dry and saturated), with separate tests for each orientation (parallel and perpendicular to the rift).
Testing Procedure
Using the Johnson Shear Tool:
- Place the test piece centrally in the shear tool and tighten the bolts securely.
- Position the tool in the testing machine, ensuring the spherical bearing block is centered on the top of the shear tool’s plunger.
- Apply load at a rate not exceeding 1 mm/minute, ensuring the testing machine’s beam remains in a floating position during loading.
Using the Dutton Punching Shear Device:
- Mark centerlines on one surface of the slab and measure its thickness to the nearest 0.2 mm at three equidistant points around a 50 mm circle centered at the intersection of the lines.
- Center the slab between the upper and lower plates of the shear device, aligning the marked section under the plunger.
- Lower the upper plate to make contact with the slab, then center the device in the testing machine with the spherical bearing block touching the plunger’s top.
- Load is applied at a rate not exceeding 1 mm/minute, maintaining the testing machine’s floating beam position.
Evaluation and Reporting of Results
Calculating Shear Strength:
For Johnson Shear Tool:
where:
- SS = shear strength (kg/cm²),
- WW = maximum load applied (kg),
- AA = area of the center cross-section of the test piece (cm²).
For Dutton Punching Shear Device:
where:
- SS = shear strength (kg/cm²),
- W1W_1 = total maximum load applied (kg),
- W2W_2 = initial load to bring plunger in contact with the specimen (kg),
- DD = diameter of the plunger (cm),
- TT = thickness of the specimen (cm).
Result Calculation and Documentation:
- The average shear strength is calculated based on the results from three specimens tested under each condition and orientation.
- Document sample identification, collection date, stone type, test piece size and shape, and the type of shear apparatus used.
